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Ex vivo measures of muscle mitochondrial capacity reveal quantitative limits of oxygen delivery by the circulation during exercise

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Abstract

Muscle mitochondrial respiratory capacity measured ex vivo provides a physiological reference to assess cellular oxidative capacity as a component in the oxygen cascade in vivo. In this article, the magnitude of muscle blood flow and oxygen uptake during exercise involving a small-to-large fraction of the body mass will be discussed in relation to mitochondrial capacity measured ex vivo. These analyses reveal that as the mass of muscle engaged in exercise increases from one-leg knee extension, to 2-arm cranking, to 2-leg cycling and x-country skiing, the magnitude of blood flow and oxygen delivery decrease. Accordingly, a 2-fold higher oxygen delivery and oxygen uptake per unit muscle mass are seen in vivo during 1-leg exercise compared to 2-leg cycling indicating a significant limitation of the circulation during exercise with a large muscle mass. This analysis also reveals that mitochondrial capacity measured ex vivo underestimates the maximal in vivo oxygen uptake of muscle by up to ∼2-fold. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

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... Maximal oxygen consumption is defined by the integrated capacities of convective oxygen (O 2 ) transport, diffusion and muscle oxidative capacity during dynamic exercise involving large muscle groups. 1 Several lines of evidence indicate that cardiac output and thereby O 2 delivery is a major limiting factor for muscle VO 2 during exercise, 2 whereas muscle oxidative capacity is in excess of O 2 delivery in healthy individuals. 3,4 Diffusive capacity also plays a role in limiting O 2 transport to mitochondria, 5 and this is supported by the finding that O 2 extraction is only 85%-90% at VO 2peak in well-trained athletes. 6 Despite evidence for cardiac output being the major limiting factor in the O 2 cascade and the existence of diffusion limitation, some findings are difficult to explain. ...
... 34 However, a divergence in mitochondrial physiology is that mitochondrial respiration assessed ex vivo is significantly lower than what can be achieved in vivo using exercise with a small muscle mass. 3 The underlying mechanisms for the low ex vivo respiration are so far unresolved but is likely caused by altered mitochondrial function due to mitochondria isolation procedures, muscle fibre permeabilization, 35,36 as well unknown factors in the in vivo milieu. 37 This pattern was also observed in this study where muscle VO 2 obtained from ex vivo OXPHOS measurements extrapolated to leg muscle mass engaged in KE NORM and KE HYPER, respectively, was 38% and 44% lower than the in vivo muscle VO 2 obtained with Fick method. ...
Article
Aim We examined the Fick components together with mitochondrial O2 affinity (p50mito) in defining O2 extraction and O2 uptake during exercise with large and small muscle mass during normoxia (NORM) and hyperoxia (HYPER). Methods Seven individuals performed two incremental exercise tests to exhaustion on a bicycle ergometer (BIKE) and two on a one‐legged knee extension ergometer (KE) in NORM or HYPER. Leg blood flow and VO2 were determined by thermodilution and the Fick method. Maximal ADP‐stimulated mitochondrial respiration (OXPHOS) and p50mito were measured ex vivo in isolated mitochondria. Mitochondrial excess capacity in the leg was determined from OXPHOS in permeabilized fibers and muscle mass measured with magnetic resonance imaging in relation to peak leg O2 delivery. Results The ex vivo p50mito increased from 0.06±0.02 to 0.17±0.04 kPa with varying substrate supply and O2 flux rates from 9.84±2.91 to 16.34±4.07 pmol O2·s⁻¹·μg⁻¹ respectively. O2 extraction decreased from 83% in BIKE to 67% in KE as a function of a higher O2 delivery, and lower mitochondrial excess capacity. There was a significant relationship between O2 extraction and mitochondrial excess capacity and p50mito that was unrelated to blood flow and mean transit time. Conclusion O2 extraction varies with mitochondrial respiration rate, p50mito and O2 delivery. Mitochondrial excess capacity maintains a low p50mito which enhances O2 diffusion from microvessels to mitochondria during exercise. This article is protected by copyright. All rights reserved.
... Des résultats contradictoires ont été observés concernant l'élévation de la fréquence de contraction musculaire lors d'exercices d'extension du genou ou de flexion plantaire. Il convient de noter que la masse musculaire mise en jeu lors des exercices utilisés dans ces études était relativement faible et les réponses lors d'exercices mobilisant une plus grande masse musculaire pourraient être différentes (Boushel et Saltin, 2013). D'autre part, l'approvisionnement sanguin au niveau cérébral pourrait être affecté par la cadence de pédalage en raison d'une modification de la relation entre la pression artérielle et la circulation cérébrale due à un changement de la sympathoexcitation (Ogoh et al., 2008), ou en permettant une redistribution du sang entre la circulation systémique et la circulation cérébrale (Ogoh et al., 2005). ...
... Les modalités d'exercice utilisées dans ces travaux étaient des exercices d'extension du genou ou de flexion plantaire. La masse musculaire mise en jeu lors de ces exercices est donc relativement faible et les réponses lors d'exercices mobilisant une plus grande masse musculaire pourraient être différentes (Boushel et Saltin, 2013). D'autre part, une augmentation de la sympathoexcitation à cadence élevée pourrait avoir une incidence sur l'approvisionnement sanguin cérébral en modifiant la relation entre la pression artérielle et la circulation cérébrale (Ogoh et al., 2008), ou en permettant la redistribution entre la circulation systémique et la circulation cérébrale (Ogoh et al., 2005). . ...
Article
Choosing the pedalling cadence during the cycling exercise, in the laboratory as well as on the field, is a crucial element in fulfilling an exercise. Many studies have examined the effect of pedal cadence on various aspects such as performance, cardiorespiratory parameters, the participation of the “anaerobic” metabolism and muscle recruitment. However, few studies have investigated the effect of pedal cadence on the O2 availability and its utilization in the muscle as well as in the brain. This is why the main objective of this thesis was to understand this subject which is underdeveloped. The aim of our three experimental procedures was on one hand to study the effect of pedal cadence on the heterogeneity of the muscle’s deoxygenation during moderate exercise. On the other hand, to study the effects of pedal cadence on muscle and cerebral oxygenation and also on the performance during heavy exercise in untrained subjects, as well as in endurance-trained subjects.This work allows us to show that at moderate-intensity exercise, whole body V ̇O2 and the heterogeneity of muscle deoxygenation were higher at high cadence than at a lower one, even if the deoxygenation was not altered by the pedalling cadence in non-endurance-trained subjects. On the other hand, during intense exercise performed until exhaustion, the performance improved at 40 rpm than at 100 rpm in untrained subjects, while no significant difference was observed between the two cadences among triathletes. In addition, the O2 extraction in the vastus lateralis depended on the pedal cadence in untrained subjects and the opposite in endurance-trained subjects. Finally, we observed an effect of pedal cadence on cerebral oxygenation and in particular a possible rise in the availability of O2 in the brain on a lower cadence in both population levels. In conclusion, this work has allowed us to highlight the differences in the aerobic fitness of the subjects and in the intensity of the exercise in brain and muscle oxygenation responses and performance during exercises performed at different cadences.
... 9,32,48 Even in "untrained" leg skeletal muscle, the reserve in vasodilatory capacity is very high and supports 2-3 times larger blood flow per unit of mass, as observed during dynamic one-legged knee extension. 72 Simply increasing Q max (for instance, by training), without any peripheral adaptations, may increase the systemic O 2 extraction fraction by two mechanisms. First, the recruitment of a larger portion of the already existing capillary network may reduce diffusion distances and thereby increase the O 2 extraction. ...
... During whole-body maximal exercise, the oxidative capacity of skeletal muscle exceeds the O 2 delivery, as illustrated by the twofold higher V O 2 per unit of muscle mass during dynamic one-legged knee extension compared to cycling exercise (approximately 2.5 vs 20 kg active muscle mass, respectively). 10,72 Therefore, the leg muscles possess an oxidative reserve capacity at V O 2max during whole-body exercise, which has frequently been used as an argument to indicate that the large improvements in mitochondrial and capillary networks after endurance training are likely only crucial for improvements in endurance performance and do not affect the limiting factors to V O 2max . 83 In support of this view, the calculated O 2 extraction fraction is maintained or increases after prolonged bed rest (3-6 weeks), although a substantial reduction in mitochondrial volume density occurs. ...
Article
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We analysed the importance of systemic and peripheral arteriovenous O2 difference (a‐vO2 and a‐vfO2 difference, respectively) and O2 extraction fraction for maximal oxygen uptake (VO2max). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion vs perfusion limitations vary with VO2max. Articles (n=17) publishing individual data (n=154) on VO2max, maximal cardiac output (Qmax; indicator‐dilution or Fick method), a‐vO2 difference (catheters or Fick equation) and systemic O2 extraction fraction were identified. For the peripheral responses, group‐mean data (articles: n=27; subjects: n=234) on leg blood flow (LBF; thermodilution), a‐vfO2 difference and O2 extraction fraction (arterial and femoral venous catheters) were obtained. Qmax and two‐LBF increased linearly by 4.9‐6.0 L·min‐1 per 1 L·min‐1 increase in VO2max (R²=0.73 and R²=0.67, respectively; both P<0.001). The a‐vO2 difference increased from 118‐168 mL·L‐1 from a VO2max of 2‐4.5 L·min‐1 followed by a reduction (second‐order polynomial: R²=0.27). After accounting for a hypoxemia‐induced decrease in arterial O2 content with increasing VO2max (R²=0.17; P<0.001), systemic O2 extraction fraction increased up to ~90% (VO2max: 4.5 L·min‐1) with no further change (exponential decay model: R²=0.42). Likewise, leg O2 extraction fraction increased with VO2max to approach a maximal value of ~90‐95% (R²=0.83). Muscle O2 diffusing capacity and the equilibration index Y increased linearly with VO2max (R²=0.77 and R²=0.31, respectively; both P<0.01), reflecting decreasing O2 diffusional limitations and accentuating O2 delivery limitations. In conclusion, although O2 delivery is the main limiting factor to VO2max, enhanced O2 extraction fraction (≥90%) contributes to the remarkably high VO2max in endurance‐trained individuals.
... Of note, the remaining O 2 in the venous drainage is likely to reflect perfusion of less active tissues such as the skin and bone as well as a diffusion limitation across the capillary wall, interstitium, and sarcolemma given the expected low capillary PO 2 in regions of very high metabolic activity (Skattebo et al., 2020). In line with this central limitation, ex vivo measurements have demonstrated a 2-fold higher mitochondrial capacity relative to maximal in vivo O 2 uptake (Boushel and Saltin, 2013). Moreover, 3-8 weeks of endurance training leads to improvements in maximal O 2 uptake that are driven primarily by increases in systemic O 2 delivery (Montero et al., 2015). ...
Article
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Skeletal muscle is one of the most dynamic metabolic organs as evidenced by increases in metabolic rate of >150-fold from rest to maximal contractile activity. Because of limited intracellular stores of ATP, activation of metabolic pathways is required to maintain the necessary rates of ATP re-synthesis during sustained contractions. During the very early phase, phosphocreatine hydrolysis and anaerobic glycolysis prevails but as activity extends beyond ∼1 min, oxidative phosphorylation becomes the major ATP-generating pathway. Oxidative metabolism of macronutrients is highly dependent on the cardiovascular system to deliver O2 to the contracting muscle fibres, which is ensured through a tight coupling between skeletal muscle O2 utilization and O2 delivery. However, to what extent O2 delivery is ideal in terms of enabling optimal metabolic and contractile function is context-dependent and determined by a complex interaction of several regulatory systems. The first part of the review focuses on local and systemic mechanisms involved in the regulation of O2 delivery and how integration of these influences the matching of skeletal muscle O2 demand and O2 delivery. In the second part, alterations in cardiovascular function and structure associated with aging and heart failure, and how these impact metabolic and contractile function, will be addressed. Where applicable, the potential of exercise training to offset/reverse age- and disease-related cardiovascular declines will be highlighted in the context of skeletal muscle metabolic function. The review focuses on human data but also covers animal observations.
... Accordingly, circulatory adaptations to exercise result in a more efficient partitioning of the cardiac output to more activated muscle vascular beds mediated by the sympathetic nervous system to elicit a precise balance between vasodilation and perfusion pressure to optimize blood flow (Clausen 1977). This coordination is exemplified during combined arm and leg exercise where the sympathetic nervous system mediates the distribution of a finite cardiac output to both arms and legs (Clausen et al. 1973, Secher et al. 1977, Calbet et al. 2006, while excess capacity of muscle mitochondria serves to maximize the PO 2 gradient (Boushel & Saltin 2013). These relationships are also observed with detraining and bed rest where cardiac output, muscle oxidative capacity and VO 2 max are decreased, while O 2 extraction is maintained (Saltin et al. 1968. ...
Article
Aim: It is an ongoing discussion the extent to which oxygen delivery and oxygen extraction contribute to an increased muscle oxygen uptake during dynamic exercise. It has been proposed that local muscle factors including the capillary bed and mitochondrial oxidative capacity play a large role in prolonged low-intensity training of a small muscle group when the cardiac output capacity is not directly limiting. The purpose of this study was to investigate the relative roles of circulatory and muscle metabolic mechanisms by which prolonged low-intensity exercise training alters regional muscle VO2 . Methods: In nine healthy volunteers (seven males, two females), haemodynamic and metabolic responses to incremental arm cycling were measured by the Fick method and biopsy of the deltoid and triceps muscles before and after 42 days of skiing for 6 h day(-1) at 60% max heart rate. Results: Peak pulmonary VO2 during arm crank was unchanged after training (2.38 ± 0.19 vs. 2.18 ± 0.2 L min(-1) pre-training) yet arm VO2 (1.04 ± 0.08 vs. 0.83 ± 0.1 L min(1) , P < 0.05) and power output (137 ± 9 vs. 114 ± 10 Watts) were increased along with a higher arm blood flow (7.9 ± 0.5 vs. 6.8 ± 0.6 L min(-1) , P < 0.05) and expanded muscle capillary volume (76 ± 7 vs. 62 ± 4 mL, P < 0.05). Muscle O2 diffusion capacity (16.2 ± 1 vs. 12.5 ± 0.9 mL min(-1) mHg(-1) , P < 0.05) and O2 extraction (68 ± 1 vs. 62 ± 1%, P < 0.05) were enhanced at a similar mean capillary transit time (569 ± 43 vs. 564 ± 31 ms) and P50 (35.8 ± 0.7 vs. 35 ± 0.8), whereas mitochondrial O2 flux capacity was unchanged (147 ± 6 mL kg min(-1) vs. 146 ± 8 mL kg min(-1) ). Conclusion: The mechanisms underlying the increase in peak arm VO2 with prolonged low-intensity training in previously untrained subjects are an increased convective O2 delivery specifically to the muscles of the arm combined with a larger capillary-muscle surface area that enhance diffusional O2 conductance, with no apparent role of mitochondrial respiratory capacity.
... Other authors have shown that muscle mitochondrial oxidative phosphorylation capacity is in excess to systemic O 2 delivery during cycling (Boushel & Saltin, 2013). This finding further support our results, because a higher O 2 delivery capacity of the bloodstream, together with the increased myoglobin concentration and the higher mitochondrial respiration induced by the erythropoietin treatment, did not increase maximal whole body fatty acid oxidation, despite a higher VO 2max . ...
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Abstract The present investigation was performed to elucidate if the non-erythropoietic ergogenic effect of a recombinant erythropoietin treatment results in an impact on skeletal muscle mitochondrial and whole body fatty acid oxidation capacity during exercise, myoglobin concentration and angiogenesis. Recombinant erythropoietin was administered by subcutaneous injections (5000 IU) in six healthy male volunteers (aged 21 ± 2 years; fat mass 18.5 ± 2.3%) over 8 weeks. The participants performed two graded cycle ergometer exercise tests before and after the intervention where VO2max and maximal fat oxidation were measured. Biopsies of the vastus lateralis muscle were obtained before and after the intervention. Recombinant erythropoietin treatment increased mitochondrial O2 flux during ADP stimulated state 3 respiration in the presence of complex I and II substrates (malate, glutamate, pyruvate, succinate) with additional electron input from β-oxidation (octanoylcarnitine) (from 60 ± 13 to 87 ± 24 pmol · s-1 · mg-1 P < 0.01). β-hydroxy-acyl-CoA-dehydrogenase activity was higher after treatment (P < 0.05), whereas citrate synthase activity also tended to increase (P = 0.06). Total myoglobin increased by 16.5% (P < 0.05). Capillaries per muscle area tended to increase (P = 0.07), whereas capillaries per fibre as well as the total expression of vascular endothelial growth factor remained unchanged. Whole body maximal fat oxidation was not increased after treatment. Eight weeks of recombinant erythropoietin treatment increases mitochondrial fatty acid oxidation capacity and myoglobin concentration without any effect on whole body maximal fat oxidation.
... There are no studies measuring blood flow in BAT using NIR TRS . However, during the peak of exercise, muscle perfusion increases approximately 20-fold compared to perfusion during rest (29,30), and [total-Hb] (blood volume) measured by NIR TRS increases only 1.1-fold (31). Thus, the magnitude of the increase in blood volume detectable by NIR TRS is presumably much smaller than that of blood flow velocity during highly metabolic activity. ...
Article
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Objective Human brown adipose tissue (BAT) activity has been typically evaluated by 18F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) combined with computed tomography (CT). However, FDG-PET/CT has serious limitations (e.g., radiation and cold exposure). This study evaluated BAT density using near-infrared time-resolved spectroscopy (NIRTRS), a simple and noninvasive method of measuring the indices of tissue hemoglobin concentration [total-Hb] and mitochondrial density (µs′).Methods The NIRTRS parameters at 760, 800, and 830 nm in the supraclavicular region potentially containing BAT were evaluated. First, the NIRTRS parameters were compared at 27°C and during a 2-h cold exposure (19°C) in 18 men. Then, NIRTRS parameters at 27°C were compared with mean standardized uptake values (SUVmean) assessed by FDG-PET/CT after the 2-h cold exposure (19°C) in 29 men.ResultsThere was no significant difference between the NIRTRS parameters at 27°C and 19°C. The [total-Hb] and µs′ were significantly correlated to SUVmean (r = 0.73 and r = 0.64, respectively). A receiver operating characteristic analysis revealed that the sensitivity (75.0-82.4%), specificity (91.7-100%), and accuracy (82.8-86.2%) of the NIRTRS parameters were all good to determine the NIRTRS reliability.Conclusions Our novel NIRTRS method is noninvasive and simple and can reliably assess human BAT density in the supraclavicular region.
... Accordingly, circulatory adaptations to exercise result in a more efficient partitioning of the cardiac output to more activated muscle vascular beds mediated by the sympathetic nervous system to elicit a precise balance between vasodilation and perfusion pressure to optimize blood flow (Clausen 1977). This coordination is exemplified during combined arm and leg exercise where the sympathetic nervous system mediates the distribution of a finite cardiac output to both arms and legs (Clausen et al. 1973, Secher et al. 1977, Calbet et al. 2006), while excess capacity of muscle mitochondria serves to maximize the PO 2 gradient (Boushel & Saltin 2013). These relationships are also observed with detraining and bed rest where cardiac output, muscle oxidative capacity and VO 2 max are decreased, while O 2 extraction is maintained (Saltin et al. 1968, Ferretti et al. 1997). ...
Article
In humans, arm exercise is known to elicit larger increases in arterial blood pressure (BP) than leg exercise. However, the precise regulation of regional vascular conductances (VC) for the distribution of cardiac output with exercise intensity remains unknown. Hemodynamic responses were assessed during incremental upright arm cranking (AC) and leg pedalling (LP) to exhaustion (Wmax) in nine males. Systemic VC, peak cardiac output (Qpeak) (indocyanine green) and stroke volume (SV) were 18%, 23%, and 20% lower during AC than LP. The mean BP, the rate-pressure product and the associated myocardial oxygen demand were 22%, 12%, and 14% higher, respectively, during maximal AC than LP. Trunk VC was reduced to similar values at Wmax. At Wmax, muscle mass-normalized VC and fractional O2 extraction were lower in the arm than the leg muscles. However, this was compensated for during AC by raising perfusion pressure to increase O2 delivery, allowing a similar peak VO2 per kg of muscle mass in both extremities. In summary, despite a lower Qpeak during arm cranking the cardiovascular strain is much higher than during leg pedalling. The adjustments of regional conductances during incremental exercise to exhaustion depend mostly on the relative intensity of exercise and are limb-specific.
... One example is that when adding arm exercise to leg exercise during maximal exercise, the increase in leg blood is attenuated. 108 Another example is that the partial pressure of oxygen of working skeletal muscle at maximal effort is 3-4 mmHg. 109 In this regard, the regulation of skeletal muscle metabolism is directly influenced by the state of the vascular network and the muscles ability to efficiently exchange substrates to and from the vasculature. ...
Chapter
Almost a half century ago, regular endurance exercise was shown to improve the capacity of skeletal muscle to oxidize substrates to produce ATP for muscle work. Since then, adaptations in skeletal muscle mRNA level were shown to happen with a single bout of exercise. Protein changes occur within days if daily endurance exercise continues. Some of the mRNA and protein changes cause increases in mitochondrial concentrations. One mitochondrial adaptation that occurs is an increase in fatty acid oxidation at a given absolute, submaximal workload. Mechanisms have been described as to how endurance training increases mitochondria. Importantly, Pgc-1α is a master regulator of mitochondrial biogenesis by increasing many mitochondrial proteins. However, not all adaptations to endurance training are associated with increased mitochondrial concentrations. Recent evidence suggests that the energetic demands of muscle contraction are by themselves stronger controllers of body weight and glucose control than is muscle mitochondrial content. Endurance exercise has also been shown to regulate the processes of mitochondrial fusion and fission. Mitophagy removes damaged mitochondria, a process that maintains mitochondrial quality. Skeletal muscle fibers are composed of different phenotypes, which are based on concentrations of mitochondria and various myosin heavy chain protein isoforms. Endurance training at physiological levels increases type IIa fiber type with increased mitochondria and type IIa myosin heavy chain. Endurance training also improves capacity of skeletal muscle blood flow. Endurance athletes possess enlarged arteries, which may also exhibit decreased wall thickness. VEGF is required for endurance training-induced increases in capillary-muscle fiber ratio and capillary density.
... There are no studies measuring blood flow in BAT using NIR TRS . However, during the peak of exercise, muscle perfusion increases approximately 20-fold compared to perfusion during rest (29,30), and [total-Hb] (blood volume) measured by NIR TRS increases only 1.1-fold (31). Thus, the magnitude of the increase in blood volume detectable by NIR TRS is presumably much smaller than that of blood flow velocity during highly metabolic activity. ...
... Therefore, we assume that SDH activity of arm musculature is 80% of vastus lateralis SDH activity in healthy controls and CHF patients and 65% in cyclists. Here, arm muscle mass accounts for 20% of total skeletal muscle mass (i.e., ϳ6 kg; see Ref. 10). ...
Article
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VO2max during whole-body exercise is presumably constrained by oxygen delivery to mitochondria rather than by mitochondria's ability to consume oxygen. Humans and animals have been reported to exploit only 60-80% of their mitochondrial oxidative capacity at VO2max However, ex vivo quantification of mitochondrial overcapacity is complicated by isolation or permeabilization procedures. An alternative method for estimating mitochondrial oxidative capacity is via enzyme histochemical quantification of succinate dehydrogenase (SDH) activity. We determined to what extent V̇O2max attained during cycling exercise differs from mitochondrial oxidative capacity predicted from SDH activity of m. vastus lateralis in chronic heart failure patients, healthy controls and cyclists. VO2max was assessed in 20 healthy subjects and 28 cyclists and SDH activity was determined from biopsy cryosections of m. vastus lateralis using quantitative histochemistry. Similar data from our laboratory of 14 chronic heart failure patients and 6 controls were included. Mitochondrial oxidative capacity was predicted from SDH activity using estimated skeletal muscle mass and the relationship between ex vivo fiber VO2max and SDH activity of isolated single muscle fibers and myocardial trabecula under hyperoxic conditions. Mitochondrial oxidative capacity predicted from SDH activity was related (r2=0.89, p<0.001) to VO2max measured during cycling in subjects with VO2max ranging from 9.8 to 79.0 ml kg-1 min-1 VO2max measured during cycling was on average 90±14% of mitochondrial oxidative capacity. We conclude that human VO2max is related to mitochondrial oxidative capacity predicted from skeletal muscle SDH activity. Mitochondrial oxidative capacity is likely marginally limited by oxygen supply to mitochondria.
... Our results imply that in metabolic syndrome patients, the increase in the maximal capacity to oxidize fat during exercise is not explained by an increase in resting mitochondrial FA oxidation capacity or basal activation of key proteins governing skeletal muscle metabolic regulation (AMPK-ACC). It can be alternatively explained by an increased oxygen carrying and delivery capacity of the blood (Boushel & Saltin 2013), or increased capacity of FA uptake or release from the lipid droplets for oxidation. We acknowledge that phosphorylation of AMPK-ACC in response to acute exercise after a training intervention could result in substantial changes in the degree of pThr 172 -AMPKα and pSer 221 -ACCβ levels, and such adaptations could also influence the results presented in the study. ...
Article
Aerobic interval training (AIT) improves the health of metabolic syndrome patients (MetS) more than moderate intensity continuous training. However, AIT has not been shown to reverse all metabolic syndrome risk factors, possibly due to the limited duration of the training programs. To assess the effects of 6 months of AIT on cardio-metabolic health and muscle metabolism in middle aged MetS. Eleven MetS (54.5 ± 0.7 yrs old) underwent 6 month of 3 days a week supervised AIT program on a cycle-ergometer. Cardio-metabolic health was assessed and muscle biopsies were collected from the vastus lateralis prior and at the end of the program. Body fat mass (-3.8%), waist circumference (-1.8%) systolic (-10.1%) and diastolic (9.3%) blood pressure were reduced whereas maximal fat oxidation rate and VO2peak were significantly increased (38.9 and 8.0%, respectively) (all p < 0.05). The remaining components of cardio-metabolic health measured (body weight, blood cholesterol, triglycerides and glucose) were not changed after the intervention, and likewise insulin sensitivity (CSi) remained unchanged. Total AMPK (23.4%), GLUT4 (20.5%), endothelial lipase (33.3%) protein expression and citrate synthase activity (26.0%) increased with training (p < 0.05). six months of AIT in MetS raises capacity for fat oxidation during exercise and increases VO2peak in combination with skeletal muscle improvements in mitochondrial enzyme activity. Muscle proteins involved in glucose, fat metabolism and energy cell balance improved, although this was not reflected by parallel improvements in insulin sensitivity or blood lipid profile. This article is protected by copyright. All rights reserved.
... Thus, diffusion of insulin and glucose to the muscle fibers may be a key component in peripheral insulin action, as discussed almost 35 years ago (8) and more recently as well (45). In this context, one may also conclude from our results that VO 2 max is not limited by the oxidative capacity of muscle but rather by the delivery of oxygen to the muscle, as has been reviewed earlier (46,47). The relationship between GSIS and the percentage of area occupied by type II muscle fibers was unexpected and, to our knowledge, has not been reported previously. ...
Article
Context Muscle fiber composition is associated with peripheral insulin action. Objective We investigated whether extreme differences in muscle fiber composition are associated with alterations in peripheral insulin action and secretion in young, healthy subjects who exhibit normal fasting glycemia and insulinemia. Methods Relaxation time following a tetanic contraction was used to identify subjects with a high or low expression of type I muscle fibers: group I (n=11), area occupied by type I muscle fibers = 61.0 ± 11.8%; group II (n=8), type I area = 36.0 ± 4.9% (P<0.001). Biopsies were obtained from the vastus lateralis muscle and analyzed for mitochondrial respiration on permeabilized fibers, muscle fiber composition and capillary density. An intravenous glucose tolerance test was performed and indices of glucose tolerance, insulin sensitivity and secretion were determined. Results Glucose tolerance was similar between groups, whereas whole-body insulin sensitivity was decreased by ~50% in group II vs group I (P=0.019). First phase insulin release (area under the insulin curve during 10 min after glucose infusion) was increased by almost 4-fold in group II vs I (P=0.01). Whole-body insulin sensitivity was correlated with % area occupied by type I fibers (r=0.54; P=0.018) and capillary density in muscle (r=0.61; P=0.005), but not with mitochondrial respiration. Insulin release was strongly related to % area occupied by type II fibers (r=0.93; P<0.001). Conclusions Assessment of muscle contractile function in young healthy subjects may prove useful in identifying individuals with insulin resistance and enhanced glucose stimulated insulin secretion prior to onset of clinical manifestations.
... Loss of mitochondrial mass and loss of function are common consequences of muscle disuse (1,36). We developed an incremental single leg cycling protocol to evaluate muscular aerobic function (6). Along with the commonly observed decreases in strength and power, the present study shows a decline in aerobic power that was not fully recovered after 2 wk of AR. ...
Article
Muscle disuse results in the loss of muscular strength and size, due to an imbalance between protein synthesis (MPS) and breakdown (MPB). Protein ingestion stimulates MPS, although it is not established if protein is able to attenuate muscle loss with immobilization (IM) or influence the recovery consisting of ambulatory movement followed by resistance training (RT). Thirty men (49.9 ± 0.6 yr) underwent 14 days of unilateral leg IM, 14 days of ambulatory recovery (AR), and a further six RT sessions over 14 days. Participants were randomized to consume an additional 20 g of dairy protein or placebo with a meal during the intervention. Isometric knee extension strength was reduced following IM (−24.7 ± 2.7%), partially recovered with AR (−8.6 ± 2.6%), and fully recovered after RT (−0.6 ± 3.4%), with no effect of supplementation. Thigh muscle cross-sectional area decreased with IM (−4.1 ± 0.5%), partially recovered with AR (−2.1 ± 0.5%), and increased above baseline with RT (+2.2 ± 0.5%), with no treatment effect. Myofibrillar MPS, measured using deuterated water, was unaltered by IM, with no effect of protein. During AR, MPS was increased only with protein supplementation. Protein supplementation did not attenuate the loss of muscle size and function with disuse or potentiate recovery but enhanced myofibrillar MPS during AR. NEW & NOTEWORTHY Twenty grams of daily protein supplementation does not attenuate the loss of muscle size and function induced by 2 wk of muscle disuse or potentiate recovery in middle-age men. Average mitochondrial but not myofibrillar muscle protein synthesis was attenuated during immobilization with no effect of supplementation. Protein supplementation increased myofibrillar protein synthesis during a 2-wk period of ambulatory recovery following disuse but without group differences in phenotype recovery.
... Strong evidence exists for the positive physiological effects of hyperoxia when a large muscle mass is recruited e.g. during cycling and running (Cardús et al., 1998;Knight et al., 1996). This pattern is consistent with a blood flow limitation to V̇O 2max and a concomitant excess capacity of muscle mitochondria during exercise with a large muscle mass (Boushel et al., 2011;Boushel & Saltin, 2013). However, the extent to which acute hyperoxia elevates muscle V̇O 2 and performance during maximal exercise with small muscle masses is less clear. ...
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Recent technological developments have made it possible to use hyperoxia as an enhancement aid during training. Athletes wearing a mask can breathe a higher fraction of oxygen from a stationary or portable apparatus while exercising. A large body of evidence indicates that the oxygen transport capacity, lactate metabolism, power output and work tolerance (endurance) are improved when breathing hyperoxia. The physiological mechanisms underlying these performance improvements, although still not fully elucidated, are based on higher oxygen delivery and reduced central fatigue. Although much is known about the acute effects of hyperoxia, the effect of hyperoxic-supplemented endurance training on performance and the mechanisms beneath training adaptations are not very well understood, especially in well-trained endurance athletes. The few studies on the physiological effects of hyperoxia training have been conducted with conflicting results, discussed in this paper. Potential detrimental effects have not yet been shown experimentally and warrant further investigation.
... The fact that no changes in haemoglobin concentration were observed in the supraclavicular region, where BAT depots are mainly located and activated after an acute cold exposure [7,39], might be explained by the superficial skin layer influence and the underestimation of BAT perfusion. Such underestimation could occur because concentrations of tHb, O 2 Hb, and HHb measured by NIRS are sensitive to changes in blood volume, but not to perfusion flow velocity during high metabolic activities [22,36,40]. ...
Article
Purpose: Near-infrared spectroscopy (NIRS) has recently been proposed as an indirect technique to assess brown adipose tissue (BAT) in young men. NIRS arises as a novel technique to avoid the limitations of the "gold-standard" 2-deoxy-2-[18F]fluoro-D-glucose ([18F]DG) positron emission tomography combined with X-ray computed tomography (PET/CT). The aim of this study was to examine the association between near-infrared spatially resolved spectroscopy (NIRSRS) parameters and BAT volume and activity estimated by [18F]DG-PET/CT in 18 young healthy women. Procedures: NIRSRS parameters [tissue saturation index and concentrations of total haemoglobin, oxy-haemoglobin, and deoxy-haemoglobin] were continuously measured in the supraclavicular and forearm regions, in both warm and cold (2 h of personalised cold exposure) conditions. Then, the NIRSRS data were analysed as an average of 5 min in 4 different periods: (i) warm period as the baseline record, (ii) cold period I, (iii) cold period II, and (iv) cold period III. The data were then correlated with BAT volume and activity (SUVmean and SUVpeak) estimated by [18F]DG-PET/CT. Results: There was no association between the NIRSRS parameters in the supraclavicular region in warm conditions (no previous cold exposure) and BAT volume and activity (P > 0.05). Similarly, the cold-induced changes of the NIRSRS parameters in the supraclavicular region were not associated with BAT volume and activity (P > 0.05). Conclusions: NIRSRS does not seem to be a valid technique to indirectly assess BAT in young healthy women. Further research is needed to validate this technique against other methods such as PET/CT using different radiotracers or magnetic resonance imaging.
... 15 During arm cycling, the active muscle mass is small (~6 kg) and the mass-specific blood flow is large. 40 Thus, our results combined with previous studies strongly suggest that O 2 extraction during small muscle mass exercise is improved after endurance training, particularly when the exercise intensity is close to maximal. ...
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When exercising with a small muscle mass, the mass-specific O2 delivery exceeds the muscle oxidative capacity resulting in a lower O2 extraction compared to whole-body exercise. We elevated the muscle oxidative capacity and tested its impact on O2 extraction during small muscle mass exercise. Nine individuals conducted six weeks of one-legged knee extension (1L-KE) endurance training. After training, the trained leg (TL) displayed 45% higher citrate synthase and COX-IV protein content in vastus lateralis and 15-22% higher pulmonary oxygen uptake (VO2peak ) and peak power output (Wpeak ) during 1L-KE than the control leg (CON; all P<0.05). Leg O2 extraction (catheters) and blood flow (ultrasound Doppler) were measured while both legs exercised simultaneously during 2L-KE at the same submaximal power outputs (real-time feedback-controlled). TL displayed higher O2 extraction than CON (main effect: 1.7±1.6%-points; P=0.010; 40-83% of Wpeak ) with the largest between-leg difference at 83% of Wpeak (O2 extraction: 3.2±2.2%-points; arteriovenous O2 difference: 7.1±4.8 mL·L-1 ; P<0.001). At 83% of Wpeak , muscle O2 conductance (DMO2 ; Fick law of diffusion) and the equilibration index Y were higher in TL (P<0.01), indicating reduced diffusion limitations. The between-leg difference in O2 extraction correlated with the between-leg ratio of citrate synthase and COX-IV (r=0.72-0.73; P=0.03), but not with the difference in the capillary-to-fibre ratio (P=0.965). In conclusion, endurance training improves O2 extraction during small muscle mass exercise by elevating the muscle oxidative capacity and the recruitment of DMO2 ; especially evident during high-intensity exercise exploiting a larger fraction of the muscle oxidative capacity.
... 77 Previous studies comparing in vivo and ex vivo skeletal muscle OCR have not considered glycolytic repression of skeletal muscle respiration. 28,65,78,79 It is important to note: (i) IC-derived estimates of maximal The slope of paired in vivo (IC) and ex vivo (HRR) correlates differ significantly (F = 42.6, R 2 = 0.29, P < .0001) ...
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Aim This study sought to provide a statistically robust reference for measures of mitochondrial function from standardized high‐resolution respirometry with permeabilized human skeletal muscle (ex vivo), compare analogous values obtained via indirect calorimetry, arterial‐venous O2 differences, and ³¹P magnetic resonance spectroscopy (in vivo), and attempt to resolve differences across complementary methodologies as necessary. Methods Data derived from 831 study participants across research published throughout March 2009 to November 2019 was amassed to examine the biological relevance of ex vivo assessments under standard conditions, i.e. physiological temperatures of 37 °C and respiratory chamber oxygen concentrations of ~250‐500 μM. Results Standard ex vivo‐derived measures are lower (Z ≥ 3.01, p ≤ 0.0258) en masse than corresponding in vivo‐derived values. Correcting respiratory values to account for mitochondrial temperatures 10 °C higher than skeletal muscle temperatures at maximal exercise (~ 50 °C): i.) transforms data to resemble (Z ≤ 0.8, p > 0.9999) analogous yet context‐specific in vivo measures, e.g. data collected during maximal 1‐leg knee extension exercise; and ii.) supports the position that maximal skeletal muscle respiratory rates exceed (Z ≥ 13.2, p < 0.0001) those achieved during maximal whole‐body exercise, e.g. maximal cycling efforts. Conclusion This study outlines and demonstrates necessary considerations when actualizing the biological relevance of human skeletal muscle respiratory control, metabolic flexibility, and bioenergetics from standard ex vivo‐derived assessments using permeabilized human muscle. These findings detail how cross‐procedural comparisons of human skeletal muscle mitochondrial function may be collectively scrutinized in their relationship to human health and lifespan.
... Healthy women may also respond differently to moderate elevations of circulating testosterone compared to men since physiological sexual-dimorphism in human skeletal muscle mitochondria has been previously reported (Cardinale et al., 2018b). The increased mitochondrial oxidative capacity of the skeletal muscle in this study induced by testosterone supplementation may appear of low importance since the mitochondrial oxidative capacity has been shown to be in excess of the maximal capacity for oxygen delivery of the cardiorespiratory system (Boushel and Saltin, 2013). Instead, the excess oxidative capacity of skeletal muscle mitochondria has been shown to directly affect Frontiers in Physiology | www.frontiersin.org ...
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Background: Recently, it was shown that exogenously administered testosterone enhances endurance capacity in women. In this study, our understanding on the effects of exogenous testosterone on key determinants of oxygen transport and utilization in skeletal muscle is expanded. Methods: In a double-blinded, randomized, placebo-controlled trial, 48 healthy active women were randomized to 10 weeks of daily application of 10 mg of testosterone cream or placebo. Before and after the intervention, VO2 max, body composition, total hemoglobin (Hb) mass and blood volumes were assessed. Biopsies from the vastus lateralis muscle were obtained before and after the intervention to assess mitochondrial protein abundance, capillary density, capillary-to-fiber (C/F) ratio, and skeletal muscle oxidative capacity. Results: Maximal oxygen consumption per muscle mass, Hb mass, blood, plasma and red blood cell volumes, capillary density, and the abundance of mitochondrial protein levels (i.e., citrate synthase, complexes I, II, III, IV-subunit 2, IV-subunit 4, and V) were unchanged by the intervention. However, the C/F ratio, specific mitochondrial respiratory flux activating complex I and linked complex I and II, uncoupled respiration and electron transport system capacity, but not leak respiration or fat respiration, were significantly increased following testosterone administration compared to placebo. Conclusion: This study provides novel insights into physiological actions of increased testosterone exposure on key determinants of oxygen diffusion and utilization in skeletal muscle of women. Our findings show that higher skeletal muscle oxidative capacity coupled to higher C/F ratio could be major contributing factors that improve endurance performance following moderately increased testosterone exposure.
... Healthy women may also respond differently to moderate elevations of circulating testosterone compared to men since physiological sexual-dimorphism in human skeletal muscle mitochondria has been previously reported (Cardinale et al., 2018b). The increased mitochondrial oxidative capacity of the skeletal muscle in this study induced by testosterone supplementation may appear of low importance since the mitochondrial oxidative capacity has been shown to be in excess of the maximal capacity for oxygen delivery of the cardiorespiratory system (Boushel and Saltin, 2013). Instead, the excess oxidative capacity of skeletal muscle mitochondria has been shown to directly affect Frontiers in Physiology | www.frontiersin.org ...
Article
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Background: Recently, it was shown that exogenously administered testosterone enhances endurance capacity in women. In this study, our understanding on the effects of exogenous testosterone on key determinants of oxygen transport and utilization in skeletal muscle is expanded. Methods: In a double-blinded, randomized, placebo-controlled trial, 48 healthy active women were randomized to 10 weeks of daily application of 10 mg of testosterone cream or placebo. Before and after the intervention, VO2 max, body composition, total hemoglobin (Hb) mass and blood volumes were assessed. Biopsies from the vastus lateralis muscle were obtained before and after the intervention to assess mitochondrial protein abundance, capillary density, capillary-to-fiber (C/F) ratio, and skeletal muscle oxidative capacity. Results: Maximal oxygen consumption per muscle mass, Hb mass, blood, plasma and red blood cell volumes, capillary density, and the abundance of mitochondrial protein levels (i.e., citrate synthase, complexes I, II, III, IV-subunit 2, IV-subunit 4, and V) were unchanged by the intervention. However, the C/F ratio, specific mitochondrial respiratory flux activating complex I and linked complex I and II, uncoupled respiration and electron transport system capacity, but not leak respiration or fat respiration, were significantly increased following testosterone administration compared to placebo. Conclusion: This study provides novel insights into physiological actions of increased testosterone exposure on key determinants of oxygen diffusion and utilization in skeletal muscle of women. Our findings show that higher skeletal muscle oxidative capacity coupled to higher C/F ratio could be major contributing factors that improve endurance performance following moderately increased testosterone exposure.
... During whole-body exercise, the oxidative capacity of skeletal muscle exceeds the oxygen (O 2 ) delivery, as illustrated by the two-fold higher mass-specific O 2 delivery and peak O 2 uptake ( V O 2peak ) during dynamic one-legged knee-extension compared to cycling exercise (approximately 2.5 vs 20 kg active muscle mass, respectively) (Boushel and Saltin 2013;Cardinale et al. 2019). Yet, endurance training (ET) induces remarkable increases in mitochondrial enzymes and capillary density, commonly improving these by ~ 40% and ~ 10-20% after a few months of ET, respectively (Granata et al. 2018;Klausen et al. 1981). ...
Article
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Purpose: The endurance training (ET)-induced increases in peak oxygen uptake (VO2peak) and cardiac output (Qpeak) during upright cycling are reversed to pre-ET levels after removing the training-induced increase in blood volume (BV). We hypothesised that ET-induced improvements in VO2peak and Qpeak are preserved following phlebotomy of the BV gained with ET during supine but not during upright cycling. Arteriovenous O2 difference (a-vO2diff; VO2/Q), cardiac dimensions and muscle morphology were studied to assess their role for the VO2peak improvement. Methods: Twelve untrained subjects (VO2peak: 44 ± 6 ml kg-1 min-1) completed 10 weeks of supervised ET (3 sessions/week). Echocardiography, muscle biopsies, haemoglobin mass (Hbmass) and BV were assessed pre- and post-ET. VO2peak and Qpeak during upright and supine cycling were measured pre-ET, post-ET and immediately after Hbmass was reversed to the individual pre-ET level by phlebotomy. Results: ET increased the Hbmass (3.3 ± 2.9%; P = 0.005), BV (3.7 ± 5.6%; P = 0.044) and VO2peak during upright and supine cycling (11 ± 6% and 10 ± 8%, respectively; P ≤ 0.003). After phlebotomy, improvements in VO2peak compared with pre-ET were preserved in both postures (11 ± 4% and 11 ± 9%; P ≤ 0.005), as was Qpeak (9 ± 14% and 9 ± 10%; P ≤ 0.081). The increased Qpeak and a-vO2diff accounted for 70% and 30% of the VO2peak improvements, respectively. Markers of mitochondrial density (CS and COX-IV; P ≤ 0.007) and left ventricular mass (P = 0.027) increased. Conclusion: The ET-induced increase in VO2peak was preserved despite removing the increases in Hbmass and BV by phlebotomy, independent of posture. VO2peak increased primarily through elevated Qpeak but also through a widened a-vO2diff, potentially mediated by cardiac remodelling and mitochondrial biogenesis.
... Compared to visible light wavelengths, NIR wavelengths (700 to 3000 nm) are scattered less and, consequently, they show better penetration in biological tissue. However, light absorption by water limits tissue penetration above a limiting wavelength of about 900 nm; thus, the 650-850-nm range is suitable for measurements [27]. Accordingly, we used NIR wavelengths of 760, 800, and 830 nm to evaluate [oxy-Hb], [deoxy-Hb], and [total-Hb], respectively. ...
Article
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Brown adipose tissue (BAT) may potentially be used in strategies for preventing lifestyle-related diseases. We examine evidence that near-infrared time-resolved spectroscopy (NIRTRS) is capable of estimating human BAT density (BAT-d). The parameters examined in this study are total hemoglobin [total-Hb]sup, oxygenated Hb [oxy-Hb]sup, deoxygenated Hb [deoxy-Hb]sup, Hb O2 saturation (StO2sup), and the reduced scattering coefficient in the supraclavicular region (μs'sup), where BAT deposits can be located; corresponding parameters in the control deltoid region are obtained as controls. Among the NIRTRS parameters, [total-Hb]sup and [oxy-Hb]sup show region-specific increases in winter, compared to summer. Further, [total-Hb]sup and [oxy-Hb]sup are correlated with cold-induced thermogenesis in the supraclavicular region. We conclude that NIRTRS-determined [total-Hb]sup and [oxy-Hb]sup are useful parameters for evaluating BAT-d in a simple, rapid, non-invasive manner.
... In addition, the primary limitation tȯ VO 2max may differ between exercise modes, and e.g., the reliance on O 2 -utilization with a relatively small amount of exercising muscle mass (e.g., arm cranking) is shown to be higher than for leg and whole-body exercise . This is explained by the lower systemic blood flow leading to sufficiently high blood flow per unit muscle mass (Boushel and Saltin, 2013). Furthermore,VO 2max appears to be unaffected when O 2 -supply is further increased during exercise with a small amount of muscle mass (Pedersen et al., 1999;Hopman et al., 2003;Mourtzakis et al., 2004). ...
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RationaleThe main purposes of this study were to compare mitochondrial respiration in M. triceps brachii and M. vastus lateralis between elite cross-country (XC) skiers and physically active controls (CON), and to explore the associations between mitochondrial respiration in these muscles and peak oxygen uptake (V˙O2peak) in arm- and leg-dominant exercise modes.Methods Thirteen male elite XC skiers (age: 25 ± 4; peak oxygen uptake (V˙O2peak): 75.5 ± 4.2 mL⋅kg-1⋅min-1) and twelve CON (age: 26 ± 3; V˙O2peak: 57.2 ± 6.4 mL⋅kg-1⋅min-1) had microbiopsies taken from M. vastus lateralis and M. triceps brachii, which were analyzed for various measures of mitochondrial respiration using high-resolution respirometry. Thereafter, all participants tested V˙O2peak in both running (RUN) and upper body poling (UBP).ResultsXC skiers had generally higher mitochondrial respiration in M. triceps brachii compared to CON (P < 0.001), whereas no significant group-differences in mitochondrial respiration in M. vastus lateralis were revealed. XC skiers had higher mitochondrial respiration in M. triceps brachii compared to M. vastus lateralis (P = 0.005–0.058), whereas in CON, most mitochondrial respiration measures were higher in M. vastus lateralis than in M. triceps brachii (P < 0.01). When all athletes were pooled, there was a strong positive correlation between V˙O2peak in UBP and mitochondrial respiration in M. triceps brachii on several measures (P < 0.01), whereas no correlation was found for RUN.Conclusion The higher mitochondrial respiration found in M. triceps brachii compared to M. vastus lateralis among our elite XC skiers demonstrates the potential for the arm muscles to adapt to aerobic endurance training. The opposite pattern found in CON, clearly showed lower mitochondrial respiration in M. triceps brachii compared to XC skiers, whereas respiration in M. vastus lateralis did not differ between groups. The strong positive correlation between mitochondrial respiration in M. triceps brachii and V˙O2peak in UBP indicate that arm muscles’ respiratory function may be a limiting factor for V˙O2peak in arm-dominant exercise modes.
... 10). In contrast, others have claimed that each step in the O 2 cascade is important and, in particular, that the mitochondrial level in skeletal muscle is crucial for the determination of endurance performance, and V O 2max (52,57,58). ...
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The effects of mitochondrial uncoupling on skeletal muscle mitochondrial adaptation and maximal exercise capacity are unknown. In this study, rats were divided into a control group (CTL, n=8) and a group treated with 2-4-dinitrophenol, a mitochondrial uncoupler, for 28 days (DNP, 30 mg/kg/day in drinking water, n=8). The DNP group had significantly lower body mass (p<0.05) and a higher resting oxygen uptake (VO2, p<0.005). The incremental treadmill test showed that maximal running speed and running economy (p<0.01) were impaired but that VO2max was higher in the DNP-treated rats (p<0.05). In skinned gastrocnemius fibers, basal respiration (V0) was higher (p<0.01) in the DNP-treated animals, whereas the acceptor control ratio (ACR, Vmax/V0) was significantly lower (p<0.05), indicating a reduction in OXPHOS efficiency. In skeletal muscle, DNP activated mitochondrial biogenesis pathway, as indicated by changes in the mRNA expression of PGC1-α&β, NRF-1&2, and TFAM, and increased the mRNA expression of cytochrome oxidase 1 (p<0.01). The expression of two mitochondrial proteins (Prohibitin and Ndufs 3) was higher after DNP treatment. Mitochondrial fission 1 protein (Fis-1) was increased in the DNP group (p<0.01), but Mitofusin-1 and -2 were unchanged. Histochemical staining for NADH dehydrogenase and succinate dehydrogenase activity in gastrocnemius muscle revealed an increase of oxidative fibers after DNP treatment. Our study shows that mitochondrial uncoupling induces several skeletal muscle adaptations, highlighting the role of mitochondrial coupling as a critical factor for maximal exercise capacities. These results emphasize the importance of investigating the qualitative aspects of mitochondrial function in addition to the amount of mitochondria.
Chapter
Research over the last 20 years has led to a new understanding of the central role of mitochondrial function in the physiology of aging and the pathogenesis of cardiovascular diseases. Sarcopenia, the gradual loss of skeletal muscle mass, is the most vivid result of the progressive, global decline in mitochondrial DNA and energy production. This deterioration in cellular bioenergetics is implicated in many diseases such as heart failure and diabetes. It carries a significant economic burden, and portends decreased functional capacity and increased mortality. This chapter reviews recent research on emerging biomarkers to better diagnose mitochondrial dysfunction and sarcopenia.
Thesis
Les mécanismes impliqués dans les adaptations du phénotype métabolique musculaire au cours de l’exercice physique restent imparfaitement connus. Nous nous sommes intéressés au concept d’hormèse mitochondriale qui se définit comme un stress métabolique activant les voies de signalisation menant à une activation mitochondriale.En première partie, nous avons validé l’utilisation d’un nouveau système de mesure des échanges gazeux chez le rat au cours de différents exercices sur tapis roulant, et démontré que pour une vitesse de course sous maximale, un exercice en descente sollicite le système cardiovasculaire de façon modérée sans altérer la fonction mitochondriale musculaire, ni augmenter la production de radicaux libres oxygénés.En deuxième partie, nous avons montré qu’un découplage mitochondrial provoqué par un traitement des rats au 2,3-dinitrophénol (DNP) pendant 3 semaines engendre des adaptations métaboliques menant à l’augmentation de la masse mitochondriale du muscle squelettique. Ces animaux ont une capacité à l’exercice diminuée, malgré une augmentation de leur VO2max.Pour finir, nous avons montré qu’un préconditionnement par l’exercice protège la mitochondrie musculaire squelettique des effets délétères de l’ischémie-reperfusion. L’exercice semble activer le métabolisme via un phénomène d’hormèse mitochondriale permettant la protection musculaire. En conclusion, cette thèse nous montre d’une part l’importance de la mitochondrie (aspect quantitatif mais surtout qualitatif) en terme de limitation à l’exercice, et d’autre part nous suggère que l'optimisation du fonctionnement mitochondrial pourrait être une bonne garantie pour pouvoir lutter efficacement contre les stress, notamment oxydatifs, auxquels l'organisme est soumis en (quasi)permanence.
Thesis
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To understand what makes an athlete a champion, we aimed to unravel the physiological determinants of physical performance and to implement these insights into training strategies to improve physical performance. Whereas physical performance is commonly distinguished in endurance or sprint performance, most sports require a combination of sprint and endurance (such as cycling, rowing, hockey, and speed-skating). So far, the critical physiological determinants for achieving both a high sprint and high endurance performance remain unknown. Moreover, maximization of (combined) sprint and endurance performance is complex, because their physiological determinants exist at different biological levels, which interact, and as (determinants of ) sprint and endurance performance have shown to be mutually exclusive. In this thesis, we focused on athletes, who have optimized their physical performance. The following aims were addressed: 1) testing and improving technological tools to characterize skeletal muscle determinants non-invasively 2) obtaining insight in the key physiological determinants of the athlete’s physical performance using a comprehensive physiological profile, and 3) assessment of skeletal muscle adaptations to a training strategy with high potential for improving both sprint and endurance.
Article
We recently reported the circulatory and muscle oxidative capacities of the arm after prolonged low-intensity skiing in the arctic (Boushel et al., 2014). In the present study, leg VO2 was measured by the Fick method during leg cycling while muscle mitochondrial capacity was examined on a biopsy of the vastus lateralis in healthy volunteers (7 male, 2 female) before and after 42 days of skiing at 60% HR max. Peak pulmonary VO2 (3.52 ± 0.18 L.min−1 pre vs 3.52 ± 0.19 post) and VO2 across the leg (2.8 ± 0.4L.min−1 pre vs 3.0 ± 0.2 post) were unchanged after the ski journey. Peak leg O2 delivery (3.6 ± 0.2 L.min−1 pre vs 3.8 ± 0.4 post), O2 extraction (82 ± 1% pre vs 83 ± 1 post), and muscle capillaries per mm2 (576 ± 17 pre vs 612 ± 28 post) were also unchanged; however, leg muscle mitochondrial OXPHOS capacity was reduced (90 ± 3 pmol.sec−1.mg−1 pre vs 70 ± 2 post, P < 0.05) as was citrate synthase activity (40 ± 3 μmol.min−1.g−1 pre vs 34 ± 3 vs P < 0.05). These findings indicate that peak muscle VO2 can be sustained with a substantial reduction in mitochondrial OXPHOS capacity. This is achieved at a similar O2 delivery and a higher relative ADP-stimulated mitochondrial respiration at a higher mitochondrial p50. These findings support the concept that muscle mitochondrial respiration is submaximal at VO2max, and that mitochondrial volume can be downregulated by chronic energy demand.
Article
Purpose: This study investigated the effects of ischemic preconditioning (IPC) on the ratings of perceived exertion (RPE), surface electromyography (EMG), and pulmonary oxygen uptake (V̇O2) onset kinetics during cycling until exhaustion at the peak power output attained during an incremental test (PPO). Methods: A group of 12 recreationally trained cyclists volunteered for this study. After determination of PPO, they were randomly subjected on different days to a performance protocol preceded by intermittent bilateral cuff pressure inflation to 220 mm Hg (IPC) or 20 mm Hg (control). To increase data reliability, the performance visits were replicated, also in a random manner. Results: There was an 8.0% improvement in performance after IPC (Control: 303 s, IPC 327 s, factor SDs of ×/÷1.13, P = 0.01). This change was followed by a 2.9% increase in peak V̇O2 (Control: 3.95 L·min(-1), IPC: 4.06 L·min(-1), factor SDs of ×/÷ 1.15, P = 0.04) owing to a higher amplitude of the slow component of the V̇O2 kinetics (Control: 0.45 L·min(-1), IPC: 0.63 L·min(-1), factor SDs of ×/÷ 2.21, P = 0.05). There was also an attenuation in the rate of increase in RPE (P = 0.01) and a progressive increase in the myoelectrical activity of the vastus lateralis muscle (P = 0.04). Furthermore, the changes in peak V̇O2 (r = 0.73, P = 0.007) and the amplitude of the slow component (r = 0.79, P = 0.002) largely correlated with performance improvement. Conclusion: These findings provide a link between improved aerobic metabolism and enhanced severe-intensity cycling performance after IPC. Furthermore, the delayed exhaustion after IPC under lower RPE and higher skeletal muscle activation suggest they have a role on the ergogenic effects of IPC on endurance performance.
Thesis
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This thesis includes four research papers, based on four separate studies aiming to elucidate the importance of O2 extraction and blood volume (BV) for maximal O2 uptake (VO2max). In study I, twelve untrained subjects (VO2max: 44 ml · kg-1 · min-1) completed ten weeks of supervised endurance training (three sessions per week). VO2max and maximal cardiac output (Qmax) were measured during upright and supine cycling before and after training, as well as immediately after the training-induced gain in BV was reversed by blood withdrawal. The supine position increases venous return to the heart and may thus counteract potential adverse effects of blood withdrawal. The BV increased by 4% (~2 dl) with training. After reversing BV to the pre-training level, VO2max and Qmax remained 11% and 9% higher than before training, respectively, regardless of exercise position. By using the Fick principle (VO2 = Q × a-v ̅O2diff), it was calculated that 30% and 70% of the increase in VO2max was attributed to increased O2 content difference between arterial and mixed venous blood (a-v ̅O2diff) and increased Qmax, respectively. These improvements coincided with increased protein content of mitochondrial enzymes, a small increase in the capillary-to-fibre ratio (m. vastus lateralis) and an increased left ventricular mass (echocardiography). Thus, VO2max may increase with endurance training independent of BV expansion, caused by combined central and peripheral adaptations. In study II, thirteen subjects (VO2max: 63 ml · kg-1 · min-1) performed maximal exercise on a cycle ergometer in three experimental conditions: with normal BV and immediately after acute BV reductions of 150 ml and 450 ml, representing 2.5% and 7.6% of the total BV (6.0 l), respectively. After the 150 ml reduction, VO2max was preserved compared with the control test (non-significant reduction of 1%), likely caused by a rapid plasma volume (PV) restoration (calculated from changes in haematocrit and haemoglobin concentration). After the 450 ml BV reduction, VO2max was reduced by 7% despite partial PV restoration, increased maximal heart rate and increased leg O2 extraction as indicated by near-infrared spectroscopy. The reduction in VO2max was 2.5-fold larger after withdrawing 450 compared with 150 ml blood after normalising to the BV removed. Therefore, the body may cope with small but not moderate blood loss to preserve VO2max. These data may enhance our understanding regarding the impact of, e.g., acute BV manipulations, PV reduction following dehydration induced by prolonged exercise or hyperthermia, or daily oscillations of PV. In study III, the muscle oxidative capacity in one leg was increased by six weeks of one-legged endurance training (3-4 sessions per week) in nine subjects (VO2max: 56 ml · kg-1 · min-1). The impact on leg O2 extraction fraction (arterial and femoral venous catheters) vs the untrained control leg was investigated during dynamic two-legged knee extension exercise with both legs performing the same power output. This exercise model involves a small muscle mass, does not tax Qmax and is thus not perfusion limited. Therefore, the muscle oxidative capacity may potentially be the principal limiting factor for O2 extraction and VO2 before training. At low to moderate exercise intensities, O2 extraction fraction was similar in both legs. At higher exercise intensities, which are associated with greater mitochondrial activation and lower time for haemoglobin-O2 off-loading, the O2 extraction fraction was increased in the trained leg. The between-leg difference in O2 extraction correlated with the between-leg difference in mitochondrial protein content (m. vastus lateralis). Therefore, our data suggest that endurance training improves O2 extraction in exercise models where the mitochondria do not possess an apparent excess oxidative capacity over O2 delivery, particularly when the exercise intensity is close to maximal. In study IV, the relationships between pulmonary VO2max and systemic and leg O2 extraction fractions were investigated by statistically analysing data from 43 previously published catheterisation studies, comprising 377 subjects. It was observed that a-v ̅O2diff (mostly calculated by the Fick principle, and Qmax measured by the indicator-dilution method) increased curvilinearly and reached its maximum at ~4.5 l · min-1 in VO2max (moderately trained subjects), and was, if anything, slightly lower in those subjects with the highest VO2max (> 5 l · min-1). However, after accounting for the hypoxemia-induced lowering of arterial O2 content (CaO2) with increasing VO2max, the calculated systemic O2 extraction fraction (a-v ̅O2diff / CaO2) increased with VO2max up to ~4.5-5.0 l · min-1 and approached a plateau at ~90%. This pattern was strengthened by the direct measurements using arterial and femoral venous catheters, with leg O2 extraction fraction increasing progressively with VO2max until reaching ~90-95%. These analyses emphasise that a-v ̅O2diff and systemic O2 extraction fraction cannot be used interchangeably, and that the systemic and peripheral O2 extraction fractions improves with increasing VO2max and training status. By using the theoretical model of Piiper and Scheid, it appeared that the limiting factors to VO2max change with increasing VO2max: untrained, but healthy individuals display mixed perfusion-diffusion limitations, and this diffusion limitation reduces as VO2max increase.
Article
Whilst endurance training (ET) commonly augments maximal oxygen consumption (VO2max), it remains unclear whether such increase is associated with that of maximal cardiac output (Qmax) alone or along with arteriovenous oxygen difference (a-vO2diff). Herein, we sought to systematically review and determine the effect of ET on and the associations among VO2max, Qmax and a-vO2diff at maximal exercise in healthy young subjects. We conducted a systematic search of MEDLINE, Scopus and Web of Science, since their inceptions until September 2014 for articles assessing the effect of ET lasting ≥3 weeks on VO2max and Qmax and/or a-vO2diff at maximal exercise in healthy young adults (mean age <40 years). Meta-analyses were performed to determine the standardized mean difference (SMD) in VO2max, Qmax and a-vO2diff at maximal exercise between post and pre training measurements. Subgroup and meta-regression analyses were used to evaluate the associations among SMDs and potential moderating factors. Thirteen studies were included after systematic review, comprising a total of 130 untrained/moderately trained healthy young subjects (mean age=22-28 years). ET programs ranged from 5 to 12.9 weeks of duration. After data pooling, VO2max (SMD=0.75; P<0.0001), Qmax (SMD=0.64; P<0.0001) but not a-vO2diff at maximal exercise (SMD=0.21; P=0.23) were increased after ET. No significant heterogeneity was detected. With meta-regression, the SMD in Qmax was positively associated with the SMD in VO2max (B=0.91, P=0.007). The SMD in a-vO2diff at maximal exercise was not associated with the SMD in VO2max (B=0.20, P=0.40). On the basis of a relatively small number of studies, the improvement in VO2max following 5 to 13 weeks of ET is associated with an increase in Qmax, but not in a-vO2diff, in previously un- to moderately trained healthy young individuals.
Article
Since ancient times, the health benefits of regular physical activity/exercise have been recognised and the classic studies of Morris and Paffenbarger provided the epidemiological evidence in support of such an association. Cardiorespiratory fitness, often measured by maximal oxygen uptake, and habitual physical activity levels are inversely related to mortality. Thus, studies exploring the biological bases of the health benefits of exercise have largely focused on the cardiovascular system and skeletal muscle (mass and metabolism), although there is increasing evidence that multiple tissues and organ systems are influenced by regular exercise. Communication between contracting skeletal muscle and multiple organs has been implicated in exercise benefits, as indeed has other inter-organ "cross-talk". The application of molecular biology techniques and 'omics' approaches to questions in exercise biology has opened new lines of investigation to better understand the beneficial effects of exercise and, in so doing, inform the optimisation of exercise regimens and the identification of novel therapeutic strategies to enhance health and well-being.
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Mitochondria form a reticulum in skeletal muscle. Exercise training stimulates mitochondrial biogenesis, yet an emerging hypothesis is that training also induces qualitative regulatory changes. Substrate oxidation, oxygen affinity, and biochemical coupling efficiency may be regulated differentially with training and exposure to extreme environments. Threshold training doses inducing mitochondrial upregulation remain to be elucidated considering fitness level. Copyright © 2014 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.
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For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.
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Abstract  Mitochondria are complex organelles constantly undergoing processes of fusion and fission, processes that not only modulate their morphology, but also their function. Yet the assessment of mitochondrial function in skeletal muscle often involves mechanical isolation of the mitochondria, a process which disrupts their normally heterogeneous branching structure and yields relatively homogeneous spherical organelles. Alternatively, methods have been used where the sarcolemma is permeabilized and mitochondrial morphology is preserved, but both methods face the downside that they remove potential influences of the intracellular milieu on mitochondrial function. Importantly, recent evidence shows that the fragmented mitochondrial morphology resulting from routine mitochondrial isolation procedures used with skeletal muscle alters key indices of function in a manner qualitatively similar to mitochondria undergoing fission in vivo. Although these results warrant caution when interpreting data obtained with mitochondria isolated from skeletal muscle, they also suggest that isolated mitochondrial preparations might present a useful way of interrogating the stress resistance of mitochondria. More importantly, these new findings underscore the empirical value of studying mitochondrial function in minimally disruptive experimental preparations. In this review, we briefly discuss several considerations and hypotheses emerging from this work.
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The permeabilized cells and muscle fibres technique allows one to study the functional properties of mitochondria without their isolation, thus preserving all of the contacts with cellular structures, mostly the cytoskeleton, to study the whole mitochondrial population in the cell in their natural surroundings and it is increasingly being used in both experimental and clinical studies. The functional parameters (affinity for ADP in regulation of respiration) of mitochondria in permeabilized myocytes or myocardial fibres are very different from those in isolated mitochondria in vitro. In the present study, we have analysed the data showing the dependence of this parameter upon the muscle contractile state. Most remarkable is the effect of recently described Ca(2+)-independent contraction of permeabilized muscle fibres induced by elevated temperatures (30-37°C). We show that very similar strong spontaneous Ca(2+)-independent contraction can be produced by proteolytic treatment of permeabilized muscle fibres that result in a disorganization of mitochondrial arrangement, leading to a significant increase in affinity for ADP. These data show that Ca(2+)-insensitive contraction may be related to the destruction of cytoskeleton structures by intracellular proteases. Therefore the use of their inhibitors is strongly advised at the permeabilization step with careful washing of fibres or cells afterwards. A possible physiologically relevant relationship between Ca(2+)-regulated ATP-dependent contraction and mitochondrial functional parameters is also discussed.
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Maximal ADP-stimulated mitochondrial respiration depends on convergent electron flow through Complexes I + II to the Q-junction of the electron transport system (ETS). In most studies of respiratory control in mitochondrial preparations, however, respiration is limited artificially by supplying substrates for electron input through either Complex I or II. High-resolution respirometry with minimal amounts of tissue biopsy (1–3 mg wet weight of permeabilized muscle fibres per assay) provides a routine approach for multiple substrate-uncoupler-inhibitor titrations. Under physiological conditions, maximal respiratory capacity is obtained with glutamate + malate + succinate, reconstituting the operation of the tricarboxylic acid cycle and preventing depletion of key metabolites from the mitochondrial matrix. In human skeletal muscle, conventional assays with pyruvate + malate or glutamate + malate yield submaximal oxygen fluxes at 0.50–0.75 of capacity of oxidative phosphorylation (OXPHOS). Best estimates of muscular OXPHOS capacity at 37 °C (pmol O2 s−1 mg−1 wet weight) with isolated mitochondria or permeabilized fibres, suggest a range of 100–150 and up to 180 in healthy humans with normal body mass index and top endurance athletes, but reduction to 60–120 in overweight healthy adults with predominantly sedentary life style. The apparent ETS excess capacity (uncoupled respiration) over ADP-stimulated OXPHOS capacity is high in skeletal muscle of active and sedentary humans, but absent in mouse skeletal muscle. Such differences of mitochondrial quality in skeletal muscle are unexpected and cannot be explained at present. A comparative database of mitochondrial physiology may provide the key for understanding the functional implications of mitochondrial diversity from mouse to man, and evaluation of altered mitochondrial respiratory control patterns in health and disease.
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Protocols for high-resolution respirometry (HRR) of intact cells, permeabilized cells, and permeabilized muscle fibers offer sensitive diagnostic tests of integrated mitochondrial function using standard cell culture techniques and small needle biopsies of muscle. Multiple substrate-uncoupler-inhibitor titration (SUIT) protocols for analysis of oxidative phosphorylation improve our understanding of mitochondrial respiratory control and the pathophysiology of mitochondrial diseases. Respiratory states are defined in functional terms to account for the network of metabolic interactions in complex SUIT protocols with stepwise modulation of coupling and substrate control. A regulated degree of intrinsic uncoupling is a hallmark of oxidative phosphorylation, whereas pathological and toxicological dyscoupling is evaluated as a mitochondrial defect. The noncoupled state of maximum respiration is experimentally induced by titration of established uncouplers (FCCP, DNP) to collapse the proton gradient across the mitochondrial inner membrane and measure the capacity of the electron transfer system (ETS, open-circuit operation of respiration). Intrinsic uncoupling and dyscoupling are evaluated as the flux control ratio between nonphosphorylating LEAK respiration (electron flow coupled to proton pumping to compensate for proton leaks) and ETS capacity. If OXPHOS capacity (maximally ADP-stimulated oxygen flux) is less than ETS capacity, the phosphorylation system contributes to flux control. Physiological Complex I + II substrate combinations are required to reconstitute TCA cycle function. This supports maximum ETS and OXPHOS capacities, due to the additive effect of multiple electron supply pathways converging at the Q-junction. Substrate control with electron entry separately through Complex I (pyruvate + malate or glutamate + malate) or Complex II (succinate + rotenone) restricts ETS capacity and artificially enhances flux control upstream of the Q-cycle, providing diagnostic information on specific branches of the ETS. Oxygen levels are maintained above air saturation in protocols with permeabilized muscle fibers to avoid experimental oxygen limitation of respiration. Standardized two-point calibration of the polarographic oxygen sensor (static sensor calibration), calibration of the sensor response time (dynamic sensor calibration), and evaluation of instrumental background oxygen flux (systemic flux compensation) provide the unique experimental basis for high accuracy of quantitative results and quality control in HRR.
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Human endurance performance can be predicted from maximal oxygen consumption (Vo(2max)), lactate threshold, and exercise efficiency. These physiological parameters, however, are not wholly exclusive from one another, and their interplay is complex. Accordingly, we sought to identify more specific measurements explaining the range of performance among athletes. Out of 150 separate variables we identified 10 principal factors responsible for hematological, cardiovascular, respiratory, musculoskeletal, and neurological variation in 16 highly trained cyclists. These principal factors were then correlated with a 26-km time trial and test of maximal incremental power output. Average power output during the 26-km time trial was attributed to, in order of importance, oxidative phosphorylation capacity of the vastus lateralis muscle (P = 0.0005), steady-state submaximal blood lactate concentrations (P = 0.0017), and maximal leg oxygenation (sO(2LEG)) (P = 0.0295), accounting for 78% of the variation in time trial performance. Variability in maximal power output, on the other hand, was attributed to total body hemoglobin mass (Hb(mass); P = 0.0038), Vo(2max) (P = 0.0213), and sO(2LEG) (P = 0.0463). In conclusion, 1) skeletal muscle oxidative capacity is the primary predictor of time trial performance in highly trained cyclists; 2) the strongest predictor for maximal incremental power output is Hb(mass); and 3) overall exercise performance (time trial performance + maximal incremental power output) correlates most strongly to measures regarding the capability for oxygen transport, high Vo(2max) and Hb(mass), in addition to measures of oxygen utilization, maximal oxidative phosphorylation, and electron transport system capacities in the skeletal muscle.
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Assessment of mitochondrial ADP-stimulated respiratory kinetics in PmFBs (permeabilized fibre bundles) is increasingly used in clinical diagnostic and basic research settings. However, estimates of the Km for ADP vary considerably (~20-300 μM) and tend to overestimate respiration at rest. Noting that PmFBs spontaneously contract during respiration experiments, we systematically determined the impact of contraction, temperature and oxygenation on ADP-stimulated respiratory kinetics. BLEB (blebbistatin), a myosin II ATPase inhibitor, blocked contraction under all conditions and yielded high Km values for ADP of >~250 and ~80 μM in red and white rat PmFBs respectively. In the absence of BLEB, PmFBs contracted and the Km for ADP decreased ~2-10-fold in a temperature-dependent manner. PmFBs were sensitive to hyperoxia (increased Km) in the absence of BLEB (contracted) at 30 °C but not 37 °C. In PmFBs from humans, contraction elicited high sensitivity to ADP (Km<100 μM), whereas blocking contraction (+BLEB) and including a phosphocreatine/creatine ratio of 2:1 to mimic the resting energetic state yielded a Km for ADP of ~1560 μM, consistent with estimates of in vivo resting respiratory rates of <1% maximum. These results demonstrate that the sensitivity of muscle to ADP varies over a wide range in relation to contractile state and cellular energy charge, providing evidence that enzymatic coupling of energy transfer within skeletal muscle becomes more efficient in the working state.
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Adenosine is a widely used pharmacological agent to induce a "high-flow" control condition to study the mechanisms of exercise hyperemia, but it is not known how well an adenosine infusion depicts exercise-induced hyperemia, especially in terms of blood flow distribution at the capillary level in human muscle. Additionally, it remains to be determined what proportion of the adenosine-induced flow elevation is specifically directed to muscle only. In the present study, we measured thigh muscle capillary nutritive blood flow in nine healthy young men using PET at rest and during the femoral artery infusion of adenosine (1 mgmin-1l thigh volume -1), which has previously been shown to induce a maximal whole thigh blood flow of ∼8 1/min. This response was compared with the blood flow induced by moderate- to high-intensity one-leg dynamic knee extension exercise. Adenosine increased muscle blood flow on average to 40 ± 7 ml,min -1,100 g muscle-1 with an aggregate value of 2.3 ± 0.61/min for the whole thigh musculature. Adenosine also induced a substantial change in blood flow distribution within individuals. Muscle blood flow during the adenosine infusion was comparable with blood flow in moderate- to high-intensity exercise (36 ± 9 ml.min-1.100 g muscle -1), but flow heterogeneity was significantly higher during the adenosine infusion than during voluntary exercise. In conclusion, a substantial part of the flow increase in the whole limb blood flow induced by a high-dose adenosine infusion is conducted through the physiological non-nutritive shunt in muscle and/or also through tissues of the limb other than muscle. Additionally, an intra-arterial adenosine infusion does not mimic exercise hyperemia, especially in terms of muscle capillary flow heterogeneity, while the often-observed exercise-induced changes in capillary blood flow heterogeneity likely reflect true changes in nutritive flow linked to muscle fiber and vascular unit recruitment.
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The proportion of "perfused" capillaries was evaluated in rat tibialis anterior at rest and during two different types of contraction after timed injection of thioflavine S. Capillary/fibre (C/F) ratio was estimated for "perfused" capillaries--those filled with fluorochrome--(Cp) from photomicrographs. Sections were subsequently stained for alkaline phosphatase and C/F ratio was estimated for all capillaries (Ct). At 7.5 sec after injection of fluorochrome, Cp:Ct at rest was 0.32 +/- 0.092 in the oxidative core and 0.43 +/- 0.058 in the glycolytic cortex (means +/- SEM). This increased to 0.83 +/- 0.045 and 0.88 +/- 0.026, respectively, during selective activation of glycolytic fibres. Activation of all fibres led to a modest further increase (0.92 +/- 0.040 in the core and 0.91 +/- 0.035 in the cortex). Blood flow (measured by radiolabelled microspheres increased to a similar extent (fivefold) in both regions of the muscle during activation of glycolytic fibres; the further increase during maximal activation was much smaller in the cortex (from 4 to 41 ml/100 g/min) than in the core (from 7 to 196 ml/100 g/min). Increased capillary perfusion during muscle contractions was thus independent of the type of activity, while muscle blood flow increased more in oxidative than in glycolytic regions during maximal activation. Thus the increase in muscle blood flow with maximal activation cannot be accounted for by further recruitment of "unperfused" capillaries and must result from a significant increase in the velocity of capillary blood flow.
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The oxidative capacity of cat skeletal muscles (soleus, gracilis, and gracilis chronically stimulated for 28 days) was derived from the total mitochondrial content in the muscle, the surface area of mitochondrial inner membranes, and respiratory activities of isolated mitochondria. Mitochondrial content was estimated by standard morphometry. The surface area of mitochondrial inner membranes per unit volume of mitochondria was estimated by a stereological method. The respiratory activities of isolated mitochondria were measured biochemically, using pyruvate/malate, glutamate/malate, succinate, or cytochrome c as substrate. Structurally and functionally, mitochondria from the three muscle types showed nearly identical characteristics. Oxidative activity was dependent on substrate; with succinate, 5.8 ml of O2 per min per ml of mitochondria was the rate most likely to represent physiological conditions. Oxidative activities of 3.1 ml.min-1.ml-1 with pyruvate/malate and 14.5 ml.min-1.ml-1 with cytochrome c as substrates were theoretical lower and upper bounds. The oxidative capacity of each of the three muscles was thus in direct proportion to the total volume of mitochondria in the muscle. The respiratory capacity of isolated mitochondria was very near to the maximal oxygen uptake rate of mitochondria that is commonly estimated in intact muscles of a wide variety of animals.
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Flux through the tricarboxylic acid cycle was calculated from oxygen consumption in hearts perfused near the physiological work load. Activities of citrate synthase, 2-oxoglutarate dehydrogenase and succinate dehydrogenase were measured in the same hearts. Only the activities of 2-oxoglutarate dehydrogenase correlated with calculated fluxes through the cycle.
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We studied O2 transport in the leg to determine if hyperoxia will increase the maximal rate of O2 uptake (VO2max) in exercising muscle. An increase in inspired O2 fraction (FIO2) from 0.21 to 1.00 was postulated to have the following effects: 1) increase the leg VO2max by approximately 5-10%, 2) increase the maximal O2 delivery [arterial O2 concentration.flow (CaO2.Q] by approximately 10%, and 3) raise the leg VO2max in proportion to both the femoral venous PO2 and mean leg capillary PO2. To test these hypotheses, 11 men performed cycle exercise to the highest work rates (WRmax) they could achieve while breathing 100% O2 (hyperoxia), 21% O2 (normoxia), and 12% O2 (hypoxia). Leg VO2 was derived from duplicate measurements of femoral venous blood flow and CaO2 and femoral venous blood O2 concentrations (CVO2) at 20, 35, 50, 92, and 100% WRmax in each FIO2. Femoral venous leg Q (Qleg) was measured by the constant-infusion thermodilution technique, and leg O2 uptake (VO2) was determined by the Fick principle [VO2 = Qleg(CaO2-CVO2)]. Leg VO2max was the mean of duplicate values of VO2 at 100% WRmax for each FIO2. Hyperoxia increased leg VO2max by 8.1% (P = 0.016) and maximal O2 delivery by 10.9% (P = 0.05) without changing Qleg. There was a significant increase in femoral venous PO2 (P < 0.001) that was proportionally greater than the increase in leg VO2max. The results support our first and second hypotheses, providing direct evidence that in normal subjects leg VO2max is limited by O2 supply during normoxia.(ABSTRACT TRUNCATED AT 250 WORDS)
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This chapter reviews the historical record distinguishing the major contributors to the knowledge in this area of the oxygen transport system. The ability to study the oxygen transport system in exercising humans depended on many fundamental discoveries. These began with the isolation of oxygen independently in 1774 by Joseph Priestly (1733-1804) in England and Carl Wilhelm Scheele (1742-1786) in Sweden, the latter named this fraction of the air "fireair." Lavoisier made the first attempt to measure pulmonary gas exchange at rest along with the measurements during exercise. Most of the important intellectual concepts and hypotheses in the understanding of the oxygen transport system and its limitations were proposed by A.L. Lavoisier, E. Smith, N. Zuntz, E.G. Benedict, A. Krogh, G. Liljestrand, A.V. Hill, R. Herbst, H.L. Taylor, S. Robinson, and R.O. Astrand. Subsequent discoveries have solidified these positions and provided better quantification of the important factors or links in the process. In order to reach a more fundamental understanding of the molecular and integrative aspects of the movement of oxygen from inspired air to energy-yielding mitochondria, major contributions still are to be made. Contributions may range from identifying the genes of importance for VO2max and their activation to the very subtle and precise interplay between central nervous factors and reflexes to match and distribute the available cardiac output optimally to active muscle and other central organs at maximal exercise. © 2003 American Physiological Society Published by Elsevier Ltd All rights reserved.
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The origin of significant differences between the apparent affinities of heart mitochondrial respiration for exogenous ADP in isolated mitochondria in vitro and in permeabilized cardiomyocytes or skinned fibres in situ is critically analysed. All experimental data demonstrate the importance of structural factors of intracellular arrangement of mitochondria into functional complexes with myofibrils and sarcoplasmic reticulum in oxidative muscle cells and the control of outer mitochondrial membrane permeability. It has been shown that the high apparent Km for exogenous ADP (250-350 µM) in permeabilized cells and in ghost cells (without myosin) and fibres (diameter 15-20 µm) is independent of intrinsic MgATPase activity. However, the Km may be decreased significantly by a selective proteolytic treatment, which also destroys the regular arrangement of mitochondria between sarcomeres and increases the accessibility of endogenous ADP to the exogenous pyruvate kinase-phosphoenolpyruvate system. The confocal microscopy was used to study the changes in intracellular distribution of mitochondria and localization of cytoskeletal proteins, such as desmin, tubulin and plectin in permeabilized cardiac cells during short proteolytic treatment. The results show the rapid collapse of microtubular and plectin networks but not of desmin localization under these conditions. These results point to the participation of cytoskeletal proteins in the intracellular organization and control of mitochondrial function in the cells in vivo, where mitochondria are incorporated into functional complexes with sarcomeres and sarcoplasmic reticulum.
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• Ten subjects performed incremental exercise up to their maximum work rate with the knee extensors of one leg. Measurements of leg blood flow and femoral arteriovenous differences of oxygen were made in order to be able to calculate oxygen uptake of the leg. • The volume of the quadriceps muscle was determined from twenty-one to twenty-five computer tomography section images taken from the patella to the anterior inferior iliac spine of each subject. • The maximal activities of three enzymes in the Krebs cycle, citrate synthase, oxoglutarate dehydrogenase and succinate dehydrogenase, were measured in biopsy samples taken from the vastus lateralis muscle. • The average rate of oxygen uptake over the quadriceps muscle at maximal work, 353 ml min−1kg−1, corresponded to a Krebs cycle rate of 4.6 mol min−1 g−1. This was similar to the maximal activity of oxoglutarate dehydrogenase (5.1 mol min−1 g−1), whereas the activities of succinate dehydrogenase and citrate synthase averaged 7.2 and 48.0 mol min−1 g−1, respectively. • It is suggested that of these enzymes, only the maximum activity of oxoglutarate dehydrogenase can provide a quantitative measure of the capacity of oxidative metabolism, and it appears that the enzyme is fully activated during one-legged knee extension exercise at the maximal work rate.
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7 young, healthy, male subjects performed exercise on bicycle ergometers in two 20 min periods with an interval of 1 h. The first 10 min of each 20 min period consisted of arm exercise (38–62% of Vdot;o2 max for arm exercise) or leg exercise (58–78% of Vdot;o2 max for leg exercise). During the last 10 min the subjects performed combined arm and leg exercise (71–83% of Vdot;o2 max for this type of exercise). The following variables were measured during each type of exercise: oxygen uptake, heart rate, mean arterial blood pressure, cardiac output, leg blood flow (only during leg exercise and combined exercise), arterio-venous concentration differences for O2 and lactate at the levels of the axillary and the external iliac vessels.Superimposing a sufficiently strenuous arm exercise (oxygen uptake for arm exercise 40% of oxygen uptake for combined exercise) on leg exercise caused a reduction in blood flow and oxygen uptake in the exercising legs with unchanged mean arterial blood pressure. Superimposing leg exercise on arm exercise caused a decrease in mean arterial blood pressure and an increased axillary arterio-venous oxygen difference. These findings indicate that the oxygen supply to one large group of exercising muscles may be limited by vasoconstriction or by a fall in arterial pressure, when another large group of muscles is exercising simultaneously.
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The effect of high-intensity exercise on the respiratory capacity of skeletal muscle was studied in horses which ran five 600-m bouts on a track with 2 min of rest between exercise bouts, or once to fatigue on a treadmill at an intensity that elicited the maximal oxygen uptake. Venous blood and biopsy samples of the middle gluteal muscle were collected at rest, after each exercise bout, and 30 and 60 min post-exercise. Blood samples were analyzed for lactate concentration and pH and muscle samples for metabolites, pH, and respiratory capacity. Venous blood and muscle pH declined to 6.910.02 and 6.570.02, respectively, after the fifth track run and to 6.980.02 and 6.710.07, respectively, after treadmill running. Muscle metabolite changes were consistent with the metabolic response to high-intensity exercise. Muscle respiratory capacity declined >20% (P
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 The applicability of H2 15O-positron emission tomographic (PET) imaging for the assessment of skeletal muscle perfusion during exercise was investigated in five healthy subjects performing intermittent isometric contractions on a calf ergometer. The workload of the left calf muscles was kept constant in all exercises, while that of the right calf muscles was varied. During exercise H2 15O distribution in the calf muscles was measured by PET. Radioactivity measured in the left calf muscles was used as a reference for the radioactivity measured in the right calf muscles. In all studies, muscles were delineated by uptake of radioactivity. Four subjects demonstrated high radioactivity in the gastrocnemius medialis muscle, in one subject high radioactivity was distributed over the triceps surae muscles. The observed muscles demonstrated also local foci of radioactivity indicating regionally enhanced tissue perfusion. The right-left ratio of radioactivity in the active muscles increased as a function of the load. We conclude that inter- and intramuscle perfusion differences can be measured during exercise by H2 15O-PET imaging.
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This paper introduces a series of reports on the structure and function of the respiratory system of mammals. We propose and justify the hypothesis that structural design is a limiting factor for O2 flow at each level of the respiratory system. The background, the reasons, and the plan for the studies are described. The main approach is to compare the size of respiratory structures with maximal O2 consumption in a series of mammals spanning several orders or magnitude in body size. The papers that follow present the methods and results for maximal O2 consumption, pulmonary diffusing capacity, mitochondrial volume, and capillary density in muscles.
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The current problems of regulation of myocardial energy metabolism and oxidative phosphorylation in vivo are considered. With this purpose, retarded diffusion of ADP in cardiomyocytes was studied by analysis of elevated apparent Km for this substrate in regulation of respiration of saponin-skinned cardiac fibers, as compared to isolated mitochondria. Recently published data showing the importance of the outer mitochondrial membrane were compared with new experimental results on the proteolysis of skinned fibers and tissue homogenetes. In both cases 10min incubation and 0.125 mg/ml of trypsin resulted in a decrease of apparent Km for ADP from 297±35 and 228±16 to 109±2 and 36±16, respectively. Thus, the permeability of the outer mitochondrial membrane for ADP may be controlled by some unknown cytoplasmic protein(s), probably related to the cytoskelton, which are separated from mithochondria during their isolation. The extent of expression of this protein(s) depends on the energy state and type of muscle. Activation of mitochondrial creatine kinase reaction coupled to oxidative phosphorylation overcomes the diffusion difficulties of ADP by amplifying the stimulatory effect of ADP on respiration. It is concluded that both cytoplasmic and mitochondrial creatine kinases, adenylate kinase and cytoplasmic factor controlling outer membrane permeability may participate in metabolic feedback regulation of respiration in muscle cells.
Article
Endurance and strength training are established as distinct exercise modalities, increasing either mitochondrial density or myofibrillar units. Recent research, however, suggests that mitochondrial biogenesis is stimulated by both training modalities. To test the training "specificity" hypothesis, mitochondrial respiration was studied in permeabilized muscle fibers from 25 sedentary adults after endurance (ET) or strength training (ST) in normoxia or hypoxia [fraction of inspired oxygen (Fi(O(2))) = 21% or 13.5%]. Biopsies were taken from the musculus vastus lateralis, and cycle-ergometric incremental maximum oxygen uptake (VO(2max)) exercise tests were performed under normoxia, before and after the 10-wk training program. The main finding was a significant increase (P < 0.05) of fatty acid oxidation capacity per muscle mass, after endurance and strength training under normoxia [2.6- and 2.4-fold for endurance training normoxia group (ET(N)) and strength training normoxia group (ST(N)); n = 8 and 3] and hypoxia [2.0-fold for the endurance training hypoxia group (ET(H)) and strength training hypoxia group (ST(H)); n = 7 and 7], and higher coupling control of oxidative phosphorylation. The enhanced lipid oxidative phosphorylation (OXPHOS) capacity was mainly (87%) due to qualitative mitochondrial changes increasing the relative capacity for fatty acid oxidation (P < 0.01). Mitochondrial tissue-density contributed to a smaller extent (13%), reflected by the gain in muscle mass-specific respiratory capacity with a physiological substrate cocktail (glutamate, malate, succinate, and octanoylcarnitine). No significant increase was observed in mitochondrial DNA (mtDNA) content. Physiological OXPHOS capacity increased significantly in ET(N) (P < 0.01), with the same trend in ET(H) and ST(H) (P < 0.1). The limitation of flux by the phosphorylation system was diminished after training. Importantly, key mitochondrial adaptations were similar after endurance and strength training, regardless of normoxic or hypoxic exercise. The transition from a sedentary to an active lifestyle induced muscular changes of mitochondrial quality representative of mitochondrial health.
Article
Increased production of mitochondrial reactive oxygen species (ROS) by hyperglycemia is recognized as a major cause of the clinical complications associated with diabetes and obesity [Brownlee, M. (2001) Nature 414, 813–820]. We observed that dynamic changes in mitochondrial morphology are associated with high glucose-induced overproduction of ROS. Mitochondria undergo rapid fragmentation with a concomitant increase in ROS formation after exposure to high glucose concentrations. Neither ROS increase nor mitochondrial fragmentation was observed after incubation of cells with the nonmetabolizable stereoisomer l-glucose. However, inhibition of mitochondrial pyruvate uptake that blocked ROS increase did not prevent mitochondrial fragmentation in high glucose conditions. Importantly, we found that mitochondrial fragmentation mediated by the fission process is a necessary component for high glucose-induced respiration increase and ROS overproduction. Extended exposure to high glucose conditions, which may mimic untreated diabetic conditions, provoked a periodic and prolonged increase in ROS production concomitant with mitochondrial morphology change. Inhibition of mitochondrial fission prevented periodic fluctuation of ROS production during high glucose exposure. These results indicate that the dynamic change of mitochondrial morphology in high glucose conditions contributes to ROS overproduction and that mitochondrial fission/fusion machinery can be a previously unrecognized target to control acute and chronic production of ROS in hyperglycemia-associated disorders. • DLP1/Drp1 • mitochondrial fission • dynamin • diabetes • obesity
Article
The oxygen affinity of the enzyme system involved in mitochondrial respiration indicates, in relation to intracellular oxygen levels and interpreted with the aid of flux control analysis, a significant role of oxygen supply in limiting maximum exercise. This implies that the flux control coefficient of mitochondria is not excessively high, based on a capacity of mitochondrial oxygen consumption that is slightly higher than the capacity for oxygen supply through the respiratory cascade. Close matching of the capacities and distribution of flux control is consistent with the concept of symmorphosis. Within the respiratory chain, however, the large excess capacity of cytochrome c oxidase, COX, appears to be inconsistent with the economic design of the respiratory cascade. To address this apparent discrepancy, we used three model systems: cultured endothelial cells and mitochondria isolated from heart and liver. Intracellular oxygen gradients increase with oxygen flux, explaining part of the observed decrease in oxygen affinity with increasing metabolic rate in cells. In addition, mitochondrial oxygen affinities decrease from the resting to the active state. The oxygen affinity in the active ADP-stimulated state is higher in mitochondria from heart than in those from liver, in direct relationship to the higher excess capacity of COX in heart. This yields, in turn, a lower turnover rate of COX even at maximum flux through the respiratory chain, which is necessary to prevent a large decrease in oxygen affinity in the active state. Up-regulation of oxygen affinity provides a functional explanation of the excess capacity of COX. The concept of symmorphosis, a matching of capacities in the respiratory cascade, is therefore complemented by 'synkinetic' considerations on optimum enzyme ratios in the respiratory chain. Accordingly, enzymatic capacities are matched in terms of optimum ratios, rather than equal levels, to meet the specific kinetic and thermodynamic demands set by the low-oxygen environment in the cell.
Article
Across a wide range of species and body mass a close matching exists between maximal conductive oxygen delivery and mitochondrial respiratory rate. In this study we investigated in humans how closely in-vivo maximal oxygen consumption (VO(2) max) is matched to state 3 muscle mitochondrial respiration. High resolution respirometry was used to quantify mitochondrial respiration from the biopsies of arm and leg muscles while in-vivo arm and leg VO(2) were determined by the Fick method during leg cycling and arm cranking. We hypothesized that muscle mitochondrial respiratory rate exceeds that of systemic oxygen delivery. The state 3 mitochondrial respiration of the deltoid muscle (4.3±0.4 mmol o(2)kg(-1) min(-1)) was similar to the in-vivo VO(2) during maximal arm cranking (4.7±0.5 mmol O(2) kg(-1) min(-1)) with 6 kg muscle. In contrast, the mitochondrial state 3 of the quadriceps was 6.9±0.5 mmol O(2) kg(-1) min(-1), exceeding the in-vivo leg VO(2) max (5.0±0.2 mmol O(2) kg(-1) min(-1)) during leg cycling with 20 kg muscle (P<0.05). Thus, when half or more of the body muscle mass is engaged during exercise, muscle mitochondrial respiratory capacity surpasses in-vivo VO(2) max. The findings reveal an excess capacity of muscle mitochondrial respiratory rate over O(2) delivery by the circulation in the cascade defining maximal oxidative rate in humans.
Article
In recent years, the dynamic nature of mitochondria has been discovered to be critical for their function. Here we discuss the molecular basis of mitochondrial fusion, its protective role in neurodegeneration, and its importance in cellular function. The mitofusins Mfn1 and Mfn2, GTPases localized to the outer membrane, mediate outer-membrane fusion. OPA1, a GTPase associated with the inner membrane, mediates subsequent inner-membrane fusion. Mutations in Mfn2 or OPA1 cause neurodegenerative diseases. Mouse models with defects in mitochondrial fusion genes have provided important avenues for understanding how fusion maintains mitochondrial physiology and neuronal function. Mitochondrial fusion enables content mixing within a mitochondrial population, thereby preventing permanent loss of essential components. Cells with reduced mitochondrial fusion, as a consequence, show a subpopulation of mitochondria that lack mtDNA nucleoids. Such mtDNA defects lead to respiration-deficient mitochondria, and their accumulation in neurons leads to impaired outgrowth of cellular processes and ultimately neurodegeneration.
Article
Whether alterations in mitochondrial morphology affect the susceptibility of the heart to ischemia/reperfusion injury is unknown. We hypothesized that modulating mitochondrial morphology protects the heart against ischemia/reperfusion injury. In response to ischemia, mitochondria in HL-1 cells (a cardiac-derived cell line) undergo fragmentation, a process that is dependent on the mitochondrial fission protein dynamin-related protein 1 (Drp1). Transfection of HL-1 cells with the mitochondrial fusion proteins mitofusin 1 or 2 or with Drp1(K38A), a dominant-negative mutant form of Drp1, increased the percentage of cells containing elongated mitochondria (65+/-4%, 69+/-5%, and 63+/-6%, respectively, versus 46+/-6% in control: n=80 cells per group; P<0.05), decreased mitochondrial permeability transition pore sensitivity (by 2.4+/-0.5-, 2.3+/-0.7-, and 2.4+/-0.3-fold, respectively; n=80 cells per group; P<0.05), and reduced cell death after simulated ischemia/reperfusion injury (11.6+/-3.9%, 16.2+/-3.9%, and 12.1+/-2.9%, respectively, versus 41.8+/-4.1% in control: n=320 cells per group; P<0.05). Treatment of HL-1 cells with mitochondrial division inhibitor-1, a pharmacological inhibitor of Drp1, replicated these beneficial effects. Interestingly, elongated interfibrillar mitochondria were identified in the adult rodent heart with confocal and electron microscopy, and in vivo treatment with mitochondrial division inhibitor-1 increased the percentage of elongated mitochondria from 3.6+/-0.5% to 14.5+/-2.8% (P=0.023). Finally, treatment of adult murine cardiomyocytes with mitochondrial division inhibitor-1 reduced cell death and inhibited mitochondrial permeability transition pore opening after simulated ischemia/reperfusion injury, and in vivo treatment with mitochondrial division inhibitor-1 reduced myocardial infarct size in mice subject to coronary artery occlusion and reperfusion (21.0+/-2.2% with mitochondrial division inhibitor-1 versus 48.0+/-4.5% in control; n=6 animals per group; P<0.05). Inhibiting mitochondrial fission protects the heart against ischemia/reperfusion injury, suggesting a novel pharmacological strategy for cardioprotection.
Article
In this review we integrate ideas about regional and systemic circulatory capacities and the balance between skeletal muscle blood flow and cardiac output during heavy exercise in humans. In the first part of the review we discuss issues related to the pumping capacity of the heart and the vasodilator capacity of skeletal muscle. The issue is that skeletal muscle has a vast capacity to vasodilate during exercise [approximately 300 mL (100 g)(-1) min(-1)], but the pumping capacity of the human heart is limited to 20-25 L min(-1) in untrained subjects and approximately 35 L min(-1) in elite endurance athletes. This means that when more than 7-10 kg of muscle is active during heavy exercise, perfusion of the contracting muscles must be limited or mean arterial pressure will fall. In the second part of the review we emphasize that there is an interplay between sympathetic vasoconstriction and metabolic vasodilation that limits blood flow to contracting muscles to maintain mean arterial pressure. Vasoconstriction in larger vessels continues while constriction in smaller vessels is blunted permitting total muscle blood flow to be limited but distributed more optimally. This interplay between sympathetic constriction and metabolic dilation during heavy whole-body exercise is likely responsible for the very high levels of oxygen extraction seen in contracting skeletal muscle. It also explains why infusing vasodilators in the contracting muscles does not increase oxygen uptake in the muscle. Finally, when approximately 80% of cardiac output is directed towards contracting skeletal muscle modest vasoconstriction in the active muscles can evoke marked changes in arterial pressure.
Article
The aim of this review is to analyze the results of experimental research of mechanisms of regulation of mitochondrial respiration in cardiac and skeletal muscle cells in vivo obtained by using the permeabilized cell technique. Such an analysis in the framework of Molecular Systems Bioenergetics shows that the mechanisms of regulation of energy fluxes depend on the structural organization of the cells and interaction of mitochondria with cytoskeletal elements. Two types of cells of cardiac phenotype with very different structures were analyzed: adult cardiomyocytes and continuously dividing cancerous HL-1 cells. In cardiomyocytes mitochondria are arranged very regularly, and show rapid configuration changes of inner membrane but no fusion or fission, diffusion of ADP and ATP is restricted mostly at the level of mitochondrial outer membrane due to an interaction of heterodimeric tubulin with voltage dependent anion channel, VDAC. VDAC with associated tubulin forms a supercomplex, Mitochondrial Interactosome, with mitochondrial creatine kinase, MtCK, which is structurally and functionally coupled to ATP synthasome. Due to selectively limited permeability of VDAC for adenine nucleotides, mitochondrial respiration rate depends almost linearly upon the changes of cytoplasmic ADP concentration in their physiological range. Functional coupling of MtCK with ATP synthasome amplifies this signal by recycling adenine nucleotides in mitochondria coupled to effective phosphocreatine synthesis. In cancerous HL-1 cells this complex is significantly modified: tubulin is replaced by hexokinase and MtCK is lacking, resulting in direct utilization of mitochondrial ATP for glycolytic lactate production and in this way contributing in the mechanism of the Warburg effect. Systemic analysis of changes in the integrated system of energy metabolism is also helpful for better understanding of pathogenesis of many other diseases.
Article
We investigated whether the greater degree of exercise-induced diaphragmatic fatigue previously reported in highly trained athletes in hypoxia (compared with normoxia) could have a contribution from limited respiratory muscle blood flow. Seven trained cyclists completed three constant load 5 min exercise tests at inspired O2 fractions () of 0.13, 0.21 and 1.00 in balanced order. Work rates were selected to produce the same tidal volume, breathing frequency and respiratory muscle load at each (63 ± 1, 78 ± 1 and 87 ± 1% of normoxic maximal work rate, respectively). Intercostals and quadriceps muscle blood flow (IMBF and QMBF, respectively) were measured by near-infrared spectroscopy over the left 7th intercostal space and the left vastus lateralis muscle, respectively, using indocyanine green dye. The mean pressure time product of the diaphragm and the work of breathing did not differ across the three exercise tests. After hypoxic exercise, twitch transdiaphragmatic pressure fell by 33.3 ± 4.8%, significantly (P < 0.05) more than after both normoxic (25.6 ± 3.5% reduction) and hyperoxic (26.6 ± 3.3% reduction) exercise, confirming greater fatigue in hypoxia. Despite lower leg power output in hypoxia, neither cardiac output nor QMBF (27.6 ± 1.2 l min−1 and 100.4 ± 8.7 ml (100 ml)−1 min−1, respectively) were significantly different compared with normoxia (28.4 ± 1.9 l min−1 and 94.4 ± 5.2 ml (100 ml)−1 min−1, respectively) and hyperoxia (27.8 ± 1.6 l min−1 and 95.1 ± 7.8 ml (100 ml)−1 min−1, respectively). Neither IMBF was different across hypoxia, normoxia and hyperoxia (53.6 ± 8.5, 49.9 ± 5.9 and 52.9 ± 5.9 ml (100 ml)−1 min−1, respectively). We conclude that when respiratory muscle energy requirement is not different between normoxia and hypoxia, diaphragmatic fatigue is greater in hypoxia as intercostal muscle blood flow is not increased (compared with normoxia) to compensate for the reduction in , thus further compromising O2 supply to the respiratory muscles.
Article
In two groups of young healthy subjects who performed arm training (N = 5) and leg training (N = 5), respectively, the respiratory adaptation to submaximal exercise with trained and nontrained muscle groups was compared by measurement of the ventilatory equivalent (Ve/Vo2, pH, and blood gases (Pco2, Po2, and So2) in arterial blood and in venous blood from exercising extremities. After training Ve/Vo2 was significantly reduced during exercise with trained muscles, but unchanged during exercise with nontrained muscles. The reduction in Ve/Vo2 was closely related to a less pronounced increase in heart rate and in arterial lactate content, but showed no quantitative correlation to changes in arterial adaptations in trained muscles are mainly responsible for the reduction in Ve/Vo2. After training during exercise with trained as well as nontrained muscles a shift to the right of the blood oxygen dissociation curve occurred as extremities was lower while corresponding Po2 was higher.
Article
It can be shown theoretically and experimentally that the maximum activities in vitro of enzymes that catalyse near-equilibrium reactions in vivo must be considerably higher than the maximum flux through that pathway. Consequently, the activities of such enzymes cannot provide quantitative information on the maximum possible flux through a pathway. On the other hand, the maximum activity of an enzyme that catalyses a non-equilibrium reaction in vivo may provide quantitative information. Such possibilities must be tested experimentally. Thus the maximum flux through a given metabolic pathway is measured (or calculated) and compared with the maximum in vitro activities of enzymes that catalyse non-equilibrium reactions in that pathway. Catalytic activities similar to the flux suggest that such enzymes may be useful as flux indicators. For example, phosphorylase or phosphofructokinase activities provide a quantitative indication of maximum flux through glycolysis-from-glycogen (i.e. anaerobic glycolysis); hexokinase activities provide a quantitative indication of maximum flux through glycolysis-from-glucose; 2-oxoglutarate dehydrogenase activities provide a quantitative indication of maximum flux through the citric acid cycle. The advamtages of the use of enzyme activities in this manner include simplicity, general applicability to pathways, tissues and animals, and minimum intervention (particularly in larger animals including the human species). One disadvantage is that the properties of the enzyme must be known in detail before an assay that gives maximum activities can be developed, and the properties of enzymes that catalyse non-equilibrium reactions may be complex. These considerations emphasize the dangers of quantitative interpretation of the maximum flux through pathways from 'near-equilibrium' enzymes or from 'non-equilibrium' enzymes whose properties have been inadequately studied.
Article
1. The activities of 2-oxoglutarate dehydrogenase (EC 1.2.4.2) were measured in hearts and mammary glands of rats, mice, rabbits, guinea pigs, cows, sheep, goats and in the flight muscles of several Hymenoptera. 2. The activity of 2-oxoglutarate dehydrogenase was similar to the maximum flux through the tricarboxylic acid cycle in vivo. Therefore measuring the activity of this enzyme may provide a simple method for estimating the maximum flux through the cycle for comparative investigations. 3. The activities of pyruvate dehydrogenase (EC 1.2.4.1) in mammalian hearts were similar to those of 2-oxoglutarate dehydrogenase, suggesting that in these tissues the tricarboxylic acid cycle can be supplied (under some conditions) by acetyl-CoA derived from pyruvate alone. 4. In the lactating mammary glands of the rat and mouse, the activities of pyruvate dehydrogenase exceeded those of 2-oxoglutarate dehydrogenase, reflecting a flux of pyruvate to acetyl-CoA for fatty acid synthesis in addition to that of oxidation via the tricarboxylic acid cycle. In ruminant mammary glands the activities of pyruvate dehydrogenase were similar to those of 2-oxoglutarate dehydrogenase, reflecting the absence of a significant flux of pyruvate to fatty acids in these tissues.
Article
To test the applicability of a dye-dilution method to quantitate total arm blood flow at rest and during arm exercise, indocyanine green was infused at a constant rate into the brachial artery. Eight subjects performed continuous 30-min arm exercises with an increase in intensity every 10 min (30, 60, and 90 W). The loads corresponded to 29 +/- 1, 48 +/- 2, and 78 +/- 4% (means +/- SE) of the maximal O2 uptake (VO2max 2.13 +/- 0.08 l/min) during arm exercise. VO2max during arm exercise was 61 +/- 1.7% of that during leg exercise. The dye concentration was analyzed in blood samples from three arm veins, two ipsi- and one contralateral, at shoulder level. Corresponding dye concentrations in both ipsilateral veins and a stable concentration difference between ipsi- and contralateral veins were achieved. Total arm blood flow was calculated to be 0.21 +/- 0.04 l/min at rest and 2.43 +/- 0.14 l/min at 90 W. Arm O2 uptake rose from 9 +/- 2 to 323 +/- 21 ml/min. Arm blood flow and O2 uptake each correlated linearly with both work load (r = 0.98) and pulmonary O2 uptake (r greater than or equal to 0.98). Mechanical efficiency for the arm and body was 34-44 and 16-19%, respectively. We conclude that arm blood flow can be determined by continuous infusion of indocyanine green.
Article
The incomplete O2 extraction in stimulated muscle preparations as well as in exercising muscles has been attributed to diffusion limitation. In particular, the constancy of the ratio maximum O2 uptake-venous PO2 with varied O2 supply conditions is in agreement with predictions on the basis of simple perfusion-diffusion models. But several methods (inert gas washout, local xenon clearance, microsphere embolization) have revealed presence of a considerable degree of inhomogeneity of muscle blood flow and shunting both in supramaximally stimulated and in naturally exercising muscle. This inhomogeneity must be taken into account when estimating diffusion limitation in O2 supply.
Article
Spatial variations in microvascular function are described at two tissue sites in hamster cremaster muscle (pentobarbital sodium, 70 mg/kg ip). Arterioles observed include terminal arterioles and their feeding vessels, termed capillary network controllers (CNC). Although terminal arterioles at both sites had similar maximum diameters and cell flows, those at site I were significantly more constricted at rest (2.7 +/- 0.3 vs. 5.1 +/- 0.3 microns at site II) and showed lower resting flows (19.0 +/- 5.5 vs. 174 +/- 34 cells/s at site II). There were no spatial differences in CNC maximal parameters or CNC resting tone, yet CNC resting flow at site II (798 +/- 118 cells/s) significantly exceeded the value at site I (460 +/- 85 cells/s). At rest, median capillary cell flow at site I (3.3 cells/s) was half that at site II (6.3 cells/s). During hyperemia, perfused capillary segment length per unit volume was 84% greater at site I and estimated tissue erythrocyte content nearly double that at site II. Thus significant spatial differences in microvascular function exist in cell flow and vessel tone among terminal arterioles, in cell flow among CNC, and in capillarity and indices of capillary exchange.
Article
In 1985 both Pendergast et al. and Piiper et al. described a major regional heterogeneity in blood flow within single skeletal muscles both at rest and during exercise. Based on the microsphere method they described large variations in blood flow between muscle samples as large as 1 g each. The aims of the present study were: (1) To test this notion of regional heterogeneity in microsphere deposition within single skeletal muscles both at rest and during exercise. (2) To compare the distribution of microspheres with other blood flow tracers. (3) To test whether or not any heterogeneity was due to vasomotion in small arteries or arterioles. Microspheres were infused into anaesthetized rabbits over either 10, 30 or 120 s, or 10 min. Exercise was mimicked by tetanic contractions obtained by electrical stimulation of the motor nerves. Three hindleg muscles were divided into samples of 0.25 g each. Regional heterogeneity was expressed as the coefficient of variation corrected for statistical distribution of microspheres (CVc). The CVc at rest was about 0.34. The CVc was unaffected by the various infusion periods and did not change during exercise. Simultaneous infusions of microspheres and 86Rb+ or antipyrine gave high correlations between the two blood flow tracers, with all r values exceeding 0.83 (n = 18). We conclude that the microsphere method provides reliable estimates for regional blood flow within single skeletal muscles. The distribution of blood flow was markedly heterogeneous both at rest and during exercise. The heterogeneity in blood flow was apparently not a result of vasomotion.
Article
A major heterogeneous distribution of blood flow has been described on a non-microvascular level within single skeletal muscles at rest and during exercise hyperaemia both in the dog and in the rabbit. The heterogeneity in blood flow distribution could be composed of both a steady-state region-to-region variability (spatial) and a time-dependent variability (temporal) in blood flow to each region. In the present study we estimated their relative contributions to the variations in blood flow within the muscles. Furthermore, we determined whether sympathetic nerve activity contributed to and whether pharmacologically induced vasodilation affected the heterogeneous blood flow pattern. Regional blood flow measurements were based on microsphere infusions into anaesthetized rabbits. Blood flow was determined under both resting conditions and during exercise hyperaemia in regions weighing 0.25 g each within hind leg muscles. The coefficient of variation for the spatial variability was twice that of the temporal one: 0.32 and 0.16 (mean) respectively. Neither stimulation of the sympathetic nerves, sympathectomy nor vasodilation affected the heterogeneity in blood flow. When exercise hyperaemia was induced, blood flow increased in all regions so that a positive (P less than 0.05) correlation was present between resting and exercising blood flow values in the individual regions. Although regional variation in vascularization could explain the observations during exercise hyperaemia, we have at present no fully satisfying explanation for the observed regional heterogeneity in blood flow.
Article
We compared the microcirculation of the predominantly glycolytic (cortex of tibialis anterior, TA) and purely oxidative (soleus) muscles of the rat. The TA has wider (3.4 +/- 0.1 microns diameter compared to 2.7 +/- 0.05 microns), longer (405 +/- 29 and 205 +/- 17 microns), and straighter capillaries. Velocity of RBCs at rest is higher in TA (0.30 +/- 0.02 mm/sec) and reaches a higher value during contractions at 1 Hz (0.38 +/- 0.04 mm/sec) more quickly than in soleus (0.21 +/- 0.02-0.28 +/- 0.03 mm/sec). The number of continuously perfused capillaries in TA increased during contractions, but there was little change in soleus. A computer program was devised to estimate the proportion of time spent stationary by RBCs in the capillaries. This was greater in soleus than in TA at rest and was reduced in TA only during contractions. The transit time (TT) through capillaries was much reduced in TA during contractions (from 1.69 +/- 0.17 to 0.78 +/- 0.13 sec) but remained unchanged in soleus (1.17 +/- 0.21 and 0.97 +/- 0.13 sec). The lack of functional hyperemia in soleus may be a direct consequence of this invariability in the TT.
Article
It is not currently known whether central hemodynamic or peripheral (vascular or metabolic) factors limit maximal oxygen uptake. By measuring the blood flow and oxygen uptake of exercising muscles when only a small fraction of the total muscle mass is engaged in exercise, it has been demonstrated that the skeletal muscle of man could accommodate a blood flow of at least 200 ml/100 g min, and consume 300 ml O2/100 g min at exhaustive exercise. Thus, in whole body exercise the limiting factor is the capacity of the heart to deliver oxygen, not the muscle. It has also been observed that at high perfusion of the muscle the arteriovenous O2 difference is small (14 to 15 vol %), and that the low extraction of oxygen is related to the mean transit time (MTT) of red blood cells passing through the capillaries. It has been concluded that the primary importance of enlargement of the capillary bed with endurance training is not to accommodate flow but to maintain or elongate MTT. It has also been concluded that, in whole body exercise, the capacity of the muscles to receive a flow exceeds by a factor of 2 to 3 the capacity of the heart to supply the flow. Thus, vasoconstrictor tone must also be present in the arteries that "feed" exercising muscles.
Article
Five subjects exercised with the knee extensor of one limb at work loads ranging from 10 to 60 W. Measurements of pulmonary oxygen uptake, heart rate, leg blood flow, blood pressure and femoral arterial-venous differences for oxygen and lactate were made between 5 and 10 min of the exercise. Flow in the femoral vein was measured using constant infusion of saline near 0 degrees C. Since a cuff was inflated just below the knee during the measurements and because the hamstrings were inactive, the measured flow represented primarily the perfusion of the knee extensors. Blood flow increased linearly with work load right up to an average value of 5.7 l min-1. Mean arterial pressure was unchanged up to a work load of 30 W, but increased thereafter from 100 to 130 mmHg. The femoral arterial-venous oxygen difference at maximum work averaged 14.6% (v/v), resulting in an oxygen uptake of 0.80 l min-1. With a mean estimated weight of the knee extensors of 2.30 kg the perfusion of maximally exercising skeletal muscle of man is thus in the order of 2.5 l kg-1 min-1, and the oxygen uptake 0.35 l kg-1 min-1. Limitations in the methods used previously to determine flow and/or the characteristics of the exercise model used may explain why earlier studies in man have failed to demonstrate the high perfusion of muscle reported here. It is concluded that muscle blood flow is closely related to the oxygen demand of the exercising muscles. The hyperaemia at low work intensities is due to vasodilatation, and an elevated mean arterial blood pressure only contributes to the linear increase in flow at high work rates. The magnitude of perfusion observed during intense exercise indicates that the vascular bed of skeletal muscle is not a limiting factor for oxygen transport.
Article
Average blood flow (q) was determined by trapping of 15 micron radioactive microspheres in the vastus lateralis, the gastrocnemius-flexor digitorum superficialis and the triceps brachii of five 18 kg untrained mongrel dogs at rest and during graded treadmill running. Oxygen uptake (VO2) and cardiac output (Qco) were simultaneously determined. q leveled off in all investigated muscles at 60-100 ml X 100 g-1 X min-1 when VO2 was ca. 70% of peak VO2. Qco increased linearly with VO2 up to peak VO2. The regional blood flow (qR) distribution pattern within the muscle was found to be extremely scattered around q, both at rest and at heavy exercise. qR ranged from approximately 5 to approximately 55 ml X 100 g-1 X min-1 at rest and from approximately 10 to approximately 200 ml X 100 g-1 X min-1 at maximal exercise. No significant topographic pattern was observed in the qR distribution of the gastrocnemius muscle which was essentially similar to that previously found for the isolated-perfused muscle preparation. The results indicate that maximal limb muscle blood flow and/or its uneven distribution may be the primary limiting factor to peak VO2 in untrained running dogs.
Article
1. The venous outflow from the slow soleus muscle, at rest and during exercise, was compared with that of fast muscles. The blood flow through the soleus at rest was found to be, on average, 52 ml./100 g.min, which is about 4 times that of fast muscles.2. On stimulation of soleus through its motor nerve at low frequencies, up to 8/sec, hardly any increase in flow was observed, whereas fast muscles stimulated at the same rates showed a marked increase, the maximal functional hyperaemia being reached at 8/sec. Even when the soleus muscle was stimulated at frequencies of 40/sec the post-contraction hyperaemia was very small and sometimes absent.3. The relative absence of functional hyperaemia in the soleus does not appear to be due to low vascular tone, for small amounts of acetylcholine, injected close arterially, produced a considerable increase in blood flow. Further, in experiments in which the vascular tone was increased by lumbar sympathetic stimulation, no functional hyperaemia was seen. It is concluded that a contracting soleus does not release in adequate amounts the substance causing functional vasodilatation in fast muscles.4. No vasodilator effect of adrenaline could be demonstrated in soleus, and the vasodilator effect of isoprenaline was much smaller than that seen in fast muscles.5. The vasoconstrictor effect of lumbar sympathetic stimulation on the resistance vessels of soleus was much smaller than the effect seen in fast muscles. However, the responses of the resistance vessels in soleus to close arterial injections of noradrenaline were not very different from those of fast muscles, and it is suggested that the density of the terminal sympathetic innervation of the vessels of soleus differs from that of fast muscles.
Article
Blood flows to fast-twitch red (FTR), fast-twitch white (FTW), and slow-twitch red (STR) fiber sections of the gastrocnemius-soleus-plantaris muscle group of sedentary and trained rats were determined using radiolabeled microspheres during the 1st and 10th min of in situ contractions at frequencies ranging from 7.5 to 90 tetani/min. Treadmill training increased the cytochrome c content of both FTW (6.0 +/- 0.13 nmol/g to 12.2 +/- 0.27) and FTR (22.2 +/- 0.32 to 26.7 +/- 0.25) muscle. Loss of tension, evident at 15 tetani/min and above, was less (P less than 0.001) in trained animals. Although steady-state blood flows (10th min) to FTR and STR fibers were not altered by training, initial flows (1st min) to the trained FTR section were greater (P less than 0.025). Overall initial flows to both red fiber types were excessively high at the easier contraction conditions, but subsequently declined to values more reflective of the expected energy demands. This time-dependent relative hyperemia was not found in either sedentary or trained FTW muscle. However, training increased the maximal blood flow in the FTW sections [60 +/- 3.2 (n = 36) vs. 88 +/- 5.2 ml X min X 100 g-1 (n = 36)]. This 40-50% increase in FTW blood flow would produce only a modest 10% increase in blood flow to a whole mixed-fiber muscle, since the flow capacity of the FTW muscle is only one third to one fourth that of FTR muscle. This overall increase in blood flow, however, is similar to changes in VO2max found in trained rats.(ABSTRACT TRUNCATED AT 250 WORDS)
Article
Muscle blood flow (BF) was measured using the radiolabeled microsphere technique within and among nine major muscles of rats before exercise and during treadmill walking or running at speeds of 15, 30, 45, 60, and 75 m/min. Measurements were made during exercise after 1 min of steady walking or running. Male Sprague-Dawley rats were chronically instrumented with 2 Silastic catheters, one in the ascending aorta via the right carotid artery for microsphere infusion and one in the left renal artery for arterial reference blood sample withdrawal. The preexercise results demonstrated that 1) BF to deep slow-twitch muscles was three to four times that to peripheral fast muscles (e.g., soleus and gastrocnemius BFs were 138 and 33 ml . min-1 . 100 g-1, respectively); 2) BFs to red portions within mixed muscles were three to four times those to white portions (e.g, red and white gastrocnemius BFs were 54 and 18 ml . min-1 . 100 g-1, respectively; and 3) there was a direct relationship (P less than 0.05) between BFs to muscles and their slow-twitch oxidative fiber populations. The results obtained during exercise demonstrated that 1) at the slowest speed studied (15 m/min) BFs to the red portions of muscles increased, whereas BFs to the white portions of the same muscles decreased; 2) BFs to all muscles (except soleus) were increased during running at 75 m/min when there was a range of flows of 30 ml . 100 g-1 . min-1 (white gastrocnemius) to 321 (vastus intermedius), 3) at all running speeds the increases in BF to muscles were directly related to the fast-twitch, high-oxidative fiber populations of the muscles; and 4) BFs to visceral tissues and fat were decreased during exercise.
Article
Recent studies have demonstrated that single-leg knee extensor (KE) exercise elicits high mass-specific blood flow (Q) which, if incremented toward maximum, in the presence of additional muscle recruitment would soon outstrip the heart's pumping capacity and blood pressure would fall. Thus incremental KE exercise provides the opportunity to determine the intensity at which, if at all, quadriceps muscle hemodynamics are altered during incremental exercise that involves a substantially greater muscle mass. Leg Q was measured during incremental KE exercise and again with superimposed incremental two-legged knee extensor exercise with incremental arm cranking (A+L) in trained subjects (n = 5). Leg Q and vascular conductance (VC) increased with work rate (WR) to reach high levels [Q = 385.7 +/- 26 and 342.3 +/- 15 ml.min-1.100 g-1 for KE and A+L exercise, respectively; VC at 90% of maximum WR (WRmax) = 79 +/- 5 and 75 +/- 6 ml.min-1. mmHg-1 for KE and A+L exercise, respectively], but the Q/WR and VC/WR relationships in KE and A+L exercise were not different. Maximum O2 consumption (VO2max) and the VO2max/WR relationship of the quadriceps were also unaffected by the additional muscle mass recruited. Despite a significantly greater net femoral venous norepinephrine (NE) outflow at both 90 and 100% of WRmax in A+L exercise (WRmax = 4,216 +/- 1,601 and 901 +/- 99 ng/ml for A+L and KE exercise, respectively; P < 0.05), leg Q continued to rise linearly with WR.(ABSTRACT TRUNCATED AT 250 WORDS)
Article
In striated muscle, capillaries occur as independent network groups. Network architecture was quantitated in cremaster muscle of pentobarbital sodium-anesthetized hamsters in three age groups [51 +/- 1 (SE), 65 +/- 1, and 79 +/- 1 days old]. We observed that networks consist of independent subgroups of capillaries, called modules. Module architecture did not vary significantly with age during maximal dilation (Max, 10(-4) M adenosine): tissue area (A) = 1.87 +/- 0.16, 1.19 +/- 0.13, and 1.81 +/- 0.3 x 10(5) microns 2; total segment length (sigma L) = 3.07 +/- 0.23, 2.15 +/- 0.21, and 3.02 +/- 0.45 x 10(3) microns; and segment number (N) = 14.5 +/- 1.1, 9.9 +/- 1.1, and 12.6 +/- 1.6, at ages 51, 65, and 79 days, respectively. Max module architecture was not different at 65 days of age at a second site of known higher O2 inflow: A = 2.03 +/- 0.32 x 10(5) microns 2; sigma L = 3.07 +/- 0.47 x 10(3) microns; and N = 13.1 +/- 2.1. During control conditions (Rest) there was also no significant difference with age: A = 1.68 +/- 0.18, 1.15 +/- 0.19, and 1.64 +/- 0.24 x 10(5) microns 2; sigma L = 2.31 +/- 0.23, 1.91 +/- 0.33, and 2.82 +/- 0.37 x 10(3) microns; N = 10.2 +/- 1.0, 7.8 +/- 1.3, and 11.0 +/- 1.4, at ages 51, 65, and 79 days, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)
Article
Maximum oxygen uptake (VO2max) is affected by hemoglobin concentration ([Hb]). Whether this is simply due to altered convection of O2 into the muscle microcirculation or also to [Hb]-dependent diffusive transport of O2 out of the muscle capillary is unknown in humans. To examine this, seven healthy volunteers performed four maximal cycle exercise bouts at sea level immediately after 8 wk at altitude (3,801 m, barometric pressure 485 Torr), a sojourn designed to increase [Hb]. The first two bouts were at ambient [Hb] of 15.9 +/- 0.7 g/100 ml breathing 21 or 12% O2 in random order. [Hb] was then decreased to a prealtitude level of 13.8 +/- 0.6 g/100 ml by venesection and isovolemic replacement with 5% albumin in 0.9% saline, and the exercise bouts were repeated. At whole body VO2max, PO2, PCO2, pH, and O2 saturation were measured in radial arterial and femoral venous blood. Femoral venous thermodilution blood flow was determined for calculation of leg VO2. Mean muscle capillary PO2 and muscle diffusing capacity (DO2) were computed by Bohr integration between measured arterial and femoral venous PO2. Averaged over both fractional concentrations of inspired O2, leg VO2 at maximum decreased by 17.7 +/- 4.3% as [Hb] was lowered while leg O2 delivery decreased by 17.5 +/- 2.6% and DO2 decreased by 10.7 +/- 2.7% (all P < 0.05). The relative contributions of decreases in leg O2 delivery and DO2 to the decrease in VO2max were computed to be 64 and 36%, respectively. These findings suggest that [Hb] is an important determinant of O2 diffusion rates into working muscle in humans. Possible mechanisms include 1) dependence of DO2 on intracapillary red blood cell spacing, 2) changes in the total rate of dissociation of O2 from [Hb], and 3) increased red blood cell flow heterogeneity as [Hb] is reduced.