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ABSTRACT: Lactate is an important intermediate metabolite in human bioenergetics and is oxidized in many different tissues including the heart, brain, kidney, adipose tissue, liver and skeletal muscle. The mechanism(s) explaining the metabolism of lactate in these tissues, however, remains unclear. Here, we analyze the ability of skeletal muscle to respire lactate using an in situ mitochondrial preparation that leaves the native tubular reticulum and subcellular interactions of the organelle unaltered. Skeletal muscle biopsies were obtained from the m. vastus lateralis in 16 human subjects. Samples were chemically permeabilized with saponin, which selectively perforates the sarcolemma and facilitates the loss of cytosolic content without altering mitochondrial membranes, structure, and subcellular interactions. High-resolution respirometry was performed on permeabilized muscle biopsy preparations. Using four separate and specific substrate titration protocols the respirometric analysis revealed that mitochondria were capable of oxidizing lactate in the absence of exogenous LDH. The titration of lactate and NAD(+) into the respiration medium stimulated respiration (p ≤ 0.003). The addition of exogenous LDH failed to increase lactate-stimulated respiration (p = 1.0). The results further demonstrate that human skeletal muscle mitochondria cannot directly oxidize lactate within the mitochondrial matrix. Alternately, these data support previous claims that lactate is converted to pyruvate within the mitochondrial intermembrane space with the pyruvate subsequently taken into the mitochondrial matrix where it enters the TCA cycle and is ultimately oxidized.
AJP Endocrinology and Metabolism 02/2013; · 4.75 Impact Factor
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ABSTRACT: The etiology of mammalian senescence is suggested to involve the progressive impairment of mitochondrial function; however, direct observations of age-induced alterations in actual respiratory chain function are lacking. Accordingly, we assessed mitochondrial function via high-resolution respirometry and mitochondrial protein expression in soleus, quadricep, and lateral gastrocnemius skeletal muscles, which represent type 1 slow-twitch oxidative muscle (soleus) and type 2 fast-twitch glycolytic muscle (quadricep and gastrocnemius), respectively, in young (10-12 weeks) and mature (74-76 weeks) mice. Electron transport through mitochondrial complexes I and III increases with age in quadricep and gastrocnemius, which is not observed in soleus. Mitochondrial coupling efficiency during respiration through complex I also deteriorates with age in gastrocnemius and shows a tendency (p = .085) to worsen in quadricep. These data demonstrate actual alterations in electron transport function that occurs with age and are dependent on skeletal muscle type.
The Journals of Gerontology Series A Biological Sciences and Medical Sciences 01/2013; · 4.60 Impact Factor
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ABSTRACT: There is disagreement whether i) differences in mitochondrial function exist across skeletal muscle types, and ii) mouse skeletal muscle mitochondrial function can serve as a valid model for human skeletal muscle mitochondrial function. The aims of this study were to first compare and contrast three different mouse skeletal muscles from one another, and second to identify the mouse muscle that most closely resembles human skeletal muscle respiratory capacity and control. Mouse quadricep (QUADM), soleus (SOLM), and gastrocnemius (GASTM) skeletal muscles were obtained from 8-10 week old healthy mice (n = 8) representing mixed, oxidative, and glycolytic muscle, respectively. Skeletal muscle samples were also collected from young, active, healthy human subjects (n = 8) from the m. vastis lateralis (QUADH). High-resolution respirometry was used to examine mitochondrial function in all skeletal muscle samples and mitochondrial content was quantified with citrate synthase (CS) activity. Mass-specific respiration was higher across all respiratory states in SOLM versus both GASTM and QUADH (p < 0.01). When controlling for mitochondrial content, however, SOLM respiration was actually lower than GASTM and QUADH (p < 0.05 and 0.01, respectively). When comparing respiration capacity across mouse muscle to human, QUADM only exhibited one different respiratory state when compared to QUADH. These results demonstrate that qualitative differences in mitochondria function exist between different mouse skeletal muscles types when respiratory capacity is normalized to mitochondrial content, and that skeletal muscle respiratory capacity in young, healthy QUADM does correspond well with that of young, healthy QUADH.
Experimental physiology 11/2012; · 3.17 Impact Factor
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ABSTRACT: Modifications of skeletal muscle mitochondria following exposure to high altitude (HA) are generally studied by morphological examinations and biochemical analysis of expression. The aim of this study was to examine tangible measures of mitochondrial function following a prolonged exposure to HA. For this purpose, skeletal muscle biopsies were obtained from 8 lowland natives at sea level (SL) prior to exposure and again after 28 d of exposure to HA at 3454 m. High-resolution respirometry was performed on the muscle samples comparing respiratory capacity and efficiency. Exercise capacity was assessed at SL and HA. Respirometric analysis revealed that mitochondrial respiratory capacity diminished in complex I- and complex II-specific respiration in addition to a loss of maximal state-3 oxidative phosphorylation capacity from SL to HA, all independent from alterations in mitochondrial content. Leak control coupling, respiratory control ratio, and oligomycin-induced leak respiration, all measures of mitochondrial efficiency, improved in response to HA exposure. SL respiratory capacities correlated with measures of exercise capacity near SL, whereas mitochondrial efficiency correlated best with exercise capacity following HA. This data demonstrate that 1 mo of exposure to HA reduces respiratory capacity in human skeletal muscle; however, the efficiency of electron transport improves.-Jacobs, R. A., Siebenmann, C., Hug, M., Toigo, M., Meinild, A.-K., Lundby, C. Twenty-eight days at 3454-m altitude diminishes respiratory capacity but enhances efficiency in human skeletal muscle mitochondria.
The FASEB Journal 09/2012; · 5.71 Impact Factor
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ABSTRACT: With hypoxic exposure ventilation is elevated through the hypoxic ventilatory response. We tested the hypothesis that the resulting hypocapnia reduces maximal exercise capacity by decreasing (i) cerebral blood flow and oxygenation and (ii) the ventilatory drive. Eight subjects performed two incremental exercise tests at 3454m altitude in a blinded manner. In one trial end-tidal [Formula: see text] was clamped to 40mmHg by CO(2)-supplementation. Mean blood flow velocity in the middle cerebral artery (MCAv(mean)) was determined by trans-cranial Doppler sonography and cerebral oxygenation by near infra-red spectroscopy. Without CO(2)-supplementation, [Formula: see text] decreased to 30±3mmHg (P<0.0001 vs isocapnic trial). Although CO(2)-supplementation increased MCAv(mean) by 17±14% (P<0.0001) and attenuated the decrease in cerebral oxygenation (-4.7±0.9% vs -5.4±0.9%; P=0.002) this did not affect maximal O(2)-uptake. Clamping [Formula: see text] increased ventilation during submaximal but not during maximal exercise (P=0.99). We conclude that although hypocapnia promotes a decrease in MCAv(mean) and cerebral oxygenation, this does not limit maximal O(2)-uptake. Furthermore, hypocapnia does not restrict ventilation during maximal hypoxic exercise.
Respiratory Physiology & Neurobiology 08/2012; · 2.24 Impact Factor
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Paul Robach,
Christoph Siebenmann, Robert A Jacobs,
Peter Rasmussen,
Nikolai Nordsborg,
Dominik Pesta,
Erich Gnaiger,
Víctor Díaz,
Andreas Christ,
Julia Fiedler,
Nadine Crivelli,
Niels H Secher,
Aurélien Pichon,
Marco Maggiorini,
Carsten Lundby
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ABSTRACT: It remains unclear by which mechanism 'live high-train low' (LHTL) altitude training increases exercise performance. Haematological and skeletal muscle adaptations have both been proposed. To test the hypotheses that (i) LHTL improves maximal oxygen uptake (VO(2)max) and (ii) this improvement is related to hypoxia-induced increases in total haemoglobin mass (Hb(mass)) and not to improved maximal oxidative capacity of skeletal muscle, we determined VO(2)max before LHTL and after LHTL, before and after the altitude-induced increases in Hb(mass) (measured by carbon-monoxide rebreathing) had been abolished by isovolumic haemodilution. We obtained skeletal muscle biopsies to quantify mitochondrial oxidative capacity and efficiency. Sixteen endurance-trained athletes were assigned (double-blinded, placebo controlled) to ≥16 h/day over 4 weeks to normoxia (placebo, n=6) or normobaric hypoxia equivalent to 3000 m altitude (LHTL, n=10). Four-week LHTL did not increase VO(2)max, irrespective of treatment (LHTL: 1.5%; placebo: 2.0%). Hb(mass) was slightly increased (4.6%) in 5 (of 10) LHTL subjects but this was not accompanied by a concurrent increase in VO(2)max. In the subjects demonstrating an increase in Hb(mass), isovolumic haemodilution elicited a 5.8% decrease in VO(2)max. Cycling efficiency was altered neither with time nor by LHTL. Neither maximal capacity of oxidative phosphorylation nor mitochondrial efficiency was modified by time or LHTL. The present results suggest that LHTL has no positive effect on VO(2)max in endurance-trained athletes because (i) muscle maximal oxidative capacity is not improved following LHTL and (ii) erythrocyte volume expansion after LHTL, if any, is too small to alter O(2) transport.
British journal of sports medicine 07/2012; 46(11):822-7. · 2.55 Impact Factor
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ABSTRACT: Hypoxia-stimulated erythropoiesis, such as that observed when red blood cell volume (RCV) increases in response to high-altitude exposure, is well understood while the physiological importance is not. Maximal exercise tests are often performed in hypoxic conditions following some form of RCV manipulation in an attempt to elucidate oxygen transport limitations at moderate to high altitudes. Such attempts, however, have not made clear the extent to which RCV is of benefit to exercise at such elevations. Changes in RCV at sea level clearly have a direct influence on maximal exercise capacity. Nonetheless, at elevations above 3000 m, the evidence is not that clear. Certain studies demonstrate either a direct benefit or decrement to exercise capacity in response to an increase or decrease, respectively, in RCV whereas other studies report negligible effects of RCV manipulation on exercise capacity. Adding to the uncertainty regarding the importance of RCV at high altitude is the observation that Andean and Tibetan high-altitude natives exhibit similar exercise capacities at high altitude (3900 m) even though Andean natives often present with a higher percent haematocrit (Hct) when compared with both lowland natives and Tibetans. The current review summarizes past literature that has examined the effect of RCV changes on maximal exercise capacity at moderate to high altitudes, and discusses the explanation elucidating these seemingly paradoxical observations.
Sports Medicine 06/2012; 42(8):643-63. · 5.16 Impact Factor
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ABSTRACT: Studies regarding mitochondrial modifications in human skeletal muscle following acclimatization to high altitude are conflicting, and these inconsistencies may be due to the prevalence of representing mitochondrial function through static and isolated measurements of specific mitochondrial characteristics. Therefore the aim of this study was to investigate mitochondrial function in response to high altitude acclimatization through measurements of respiratory control in the m. vastus lateralis. Skeletal muscle biopsies were obtained from ten lowland natives prior to and again after a total of 9-11 days of exposure to 4,559 m. High-resolution respirometry was performed on the muscle samples to compare respiratory chain function and respiratory capacities. Respirometric analysis revealed that mitochondrial function was largely unaffected, as high altitude exposure did not affect the capacity for fat oxidation or individualized respiration capacity through either complex I or complex II. Respiratory chain function remained unaltered, as both coupling and respiratory control did not change in response to hypoxic exposure. High altitude acclimatization did, however, show a tendency (p=0.059) to limit mass specific maximal oxidative phosphorylation capacity. This data suggests that 9-11 days of exposure to high altitude does not markedly modify integrated measures of mitochondrial functional capacity in skeletal muscle despite significant decrements to enzyme concentrations involved in the tricarboxylic acid (TCA) cycle and oxidative phosphorylation.
Experimental physiology 05/2012; · 3.17 Impact Factor
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Peter Rasmussen,
Nikolai Nordsborg,
Sarah Taudorf,
Henrik Sørensen,
Ronan M G Berg, Robert A Jacobs,
Damian M Bailey,
Niels V Olsen,
Niels H Secher,
Kirsten Møller,
Carsten Lundby
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ABSTRACT: Erythropoietin (EPO) preserves arterial oxygen content by controlling red blood cell and plasma volumes. Synthesis of EPO was long thought to relate inversely to renal oxygenation, but in knockout mice, brain and skin have been identified as essential for the acute hypoxic EPO response. Whether these findings apply to humans remains unknown. We exposed healthy young subjects to hypoxia (equivalent to 3800 m) and measured EPO in arterial and jugular venous plasma and in cerebrospinal fluid. To examine the role of the skin for EPO production during hypoxia, subjects were exposed to 8 h of hypobaric hypoxia with or without breathing oxygen-enriched air to ensure systemic normoxemia. With 9 h of hypoxia, arterial EPO increased (from 6.0±2.2 to 22.0±6.0 mU/ml, n=11, P<0.0001) and jugular venous EPO displayed a similar response (to 22.2±6.0 mU/ml, n=11). Thus, the arterio-jugular venous EPO difference was unaffected by hypoxia and also in cerebrospinal fluid EPO remained stable following hypoxic exposure (0.33±0.15 mU/ml, n=9 in normoxia vs. 0.41±0.20 mU/ml, n=9 in hypoxia, P=0.40). No change in plasma EPO was observed when only skin was exposed to hypobaric hypoxia (n=8). Thus, neither dermal oxygen exposure nor cerebral EPO production appears to be important for the systemic EPO response to acute hypoxia in healthy humans.
The FASEB Journal 02/2012; 26(5):1831-4. · 5.71 Impact Factor
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ABSTRACT: The combination of living at altitude and training near sea level [live high-train low (LHTL)] may improve performance of endurance athletes. However, to date, no study can rule out a potential placebo effect as at least part of the explanation, especially for performance measures. With the use of a placebo-controlled, double-blinded design, we tested the hypothesis that LHTL-related improvements in endurance performance are mediated through physiological mechanisms and not through a placebo effect. Sixteen endurance cyclists trained for 8 wk at low altitude (<1,200 m). After a 2-wk lead-in period, athletes spent 16 h/day for the following 4 wk in rooms flushed with either normal air (placebo group, n = 6) or normobaric hypoxia, corresponding to an altitude of 3,000 m (LHTL group, n = 10). Physiological investigations were performed twice during the lead-in period, after 3 and 4 wk during the LHTL intervention, and again, 1 and 2 wk after the LHTL intervention. Questionnaires revealed that subjects were unaware of group classification. Weekly training effort was similar between groups. Hb mass, maximal oxygen uptake (VO(2)) in normoxia, and at a simulated altitude of 2,500 m and mean power output in a simulated, 26.15-km time trial remained unchanged in both groups throughout the study. Exercise economy (i.e., VO(2) measured at 200 W) did not change during the LHTL intervention and was never significantly different between groups. In conclusion, 4 wk of LHTL, using 16 h/day of normobaric hypoxia, did not improve endurance performance or any of the measured, associated physiological variables.
Journal of Applied Physiology 01/2012; 112(1):106-17. · 3.75 Impact Factor
