[Show abstract][Hide abstract] ABSTRACT: Muscular dystrophies caused by defects in various genes are often associated with impairment of calcium homeostasis. Studies of calcium currents are hampered because of the lack of a robust cellular model. Primary murine myotubes, formed upon satellite cell fusion, were examined for their utilization as a model of adult skeletal muscle. We enzymatically isolated satellite cells and induced them to differentiation to myotubes. Myotubes displayed morphological and physiological properties resembling adult muscle fibers. Desmin and myosin heavy chain immunoreactivity in the differentiated myotubes were similar to the mature muscle cross-striated pattern. The myotubes responded to electrical and chemical stimulations with sarcoplasmic reticulum calcium release. Presence of L-type calcium channels in the myotubes sarcolemma was confirmed via whole-cell patch-clamp technique. To assess the use of myotubes for studying functional mutation effects lentiviral transduction was applied. Satellite cells easily underwent transduction and were able to retain a positive expression of lentivirally encoded GFP up to and after the formation of myotubes, without changes in their physiological and morphological properties. Thus, we conclude that murine myotubes may serve as a fruitful cell model for investigating calcium homeostasis in muscular dystrophy and the effects of gene modifications can be assessed due to lentiviral transduction.
[Show abstract][Hide abstract] ABSTRACT: Muscle weakness and exercise intolerance are hallmark symptoms in mitochondrial disorders.
Little is known about the mechanisms leading to impaired skeletal muscle function and
ultimately muscle weakness in these patients. In a mouse model of lethal mitochondrial
myopathy, the muscle-specific Tfam knock-out (KO) mouse, we previously demonstrated an
excessive mitochondrial Ca2+ uptake in isolated muscle fibers that could be inhibited by the
cyclophilin D (CypD) inhibitor, cyclosporine A (CsA). Here we show that the Tfam KO mice
have increased CypD levels, and we demonstrate that this increase is a common feature in
patients with mitochondrial myopathy. We tested the effect of CsA treatment on Tfam KO
mice during the transition from a mild to terminal myopathy. CsA treatment counteracted the
development of muscle weakness and improved muscle fiber Ca2+ handling. Importantly, CsA
treatment prolonged the lifespan of these muscle-specific Tfam KO mice. These results
demonstrate that CsA treatment is an efficient therapeutic strategy to slow the development of
severe mitochondrial myopathy.
Human Molecular Genetics 09/2015; DOI:10.1093/hmg/ddv361 · 6.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In skeletal muscle, intracellular Ca(2+) is an important regulator of contraction as well as gene expression and metabolic processes. Because of the difficulties to obtain intact human muscle fibers, human myotubes have been extensively employed for studies of Ca(2+)-dependent processes in human adult muscle. Despite this, it is unknown whether the Ca(2+)-handling properties of myotubes adequately represent those of adult muscle fibers.
To enable a comparison of the Ca(2+)-handling properties of human muscle fibers and myotubes, we developed a model of dissected intact single muscle fibers obtained from human intercostal muscle biopsies. The intracellular Ca(2+)-handling of human muscle fibers was compared with that of myotubes generated by the differentiation of primary human myoblasts obtained from vastus lateralis muscle biopsies.
The intact single muscle fibers all demonstrated strictly regulated cytosolic free [Ca(2+)] ([Ca(2+)]i) transients and force production upon electrical stimulation. In contrast, despite a more mature Ca(2+)-handling in myotubes than in myoblasts, myotubes lacked fundamental aspects of adult Ca(2+)-handling and did not contract. These functional differences were explained by discrepancies in the quantity and localization of Ca(2+)-handling proteins, as well as ultrastructural differences between muscle fibers and myotubes.
Intact single muscle fibers that display strictly regulated [Ca(2+)]i transients and force production upon electrical stimulation can be obtained from human intercostal muscle biopsies. In contrast, human myotubes lack important aspects of adult Ca(2+)-handling and are thus an inappropriate model for human adult muscle when studying Ca(2+)-dependent processes, such as gene expression and metabolic processes.
[Show abstract][Hide abstract] ABSTRACT: In addition to the primary symptoms arising from inflamed joints, muscle weakness is prominent and frequent in patients with rheumatoid arthritis (RA). Here, we investigated the mechanisms of arthritis-induced muscle dysfunction in rats with adjuvant-induced arthritis (AIA).
AIA was induced in the knees of rats by injection of complete Freund's adjuvant and was allowed to develop for 21 days. Muscle contractile function was assessed in isolated extensor digitorum longus (EDL) muscles. To assess mechanisms underlying contractile dysfunction, we measured redox modifications, redox enzymes and inflammatory mediators, and activity of actomyosin ATPase and sarcoplasmic reticulum (SR) Ca(2+)-ATPase.
EDL muscles from AIA rats showed decreased tetanic force per cross-sectional area and slowed twitch contraction and relaxation. These contractile dysfunctions in AIA muscles were accompanied by marked decreases in actomyosin ATPase and SR Ca(2+)-ATPase activities. Actin aggregates were observed in AIA muscles, and these contained high levels of 3-nitrotyrosine and malondialdehyde-protein adducts. AIA muscles showed increased protein expression of NADPH oxidase 2/gp91(phox), neuronal nitric oxide synthase, tumor necrosis factor α (TNF-α), and high-mobility group box 1 (HMGB1). Treatment of AIA rats with EUK-134 (3 mg/kg/day), a superoxide dismutase/catalase mimetic, prevented both the decrease in tetanic force and the formation of actin aggregates in EDL muscles without having any beneficial effect on the arthritis development.
Antioxidant treatment prevented the development of oxidant-induced actin aggregates and contractile dysfunction in the skeletal muscle of AIA rats. This implies that antioxidant treatment can be used to effectively counteract muscle weakness in inflammatory conditions.
[Show abstract][Hide abstract] ABSTRACT: The contractile performance of skeletal muscle declines during intense activities, i.e. fatigue develops. Fatigued muscle can enter a state of prolonged low-frequency force depression (PLFFD). PLFFD can be due to decreased tetanic free cytosolic [Ca2+] ([Ca2+]i) and/or decreased myofibrillar Ca2+ sensitivity. Increases in reactive oxygen and nitrogen species (ROS/RNS) may contribute to fatigue-induced force reductions. We studied whether pharmacological ROS/RNS inhibition delays fatigue and/or counteracts the development of PLFFD. Mechanically isolated mouse fast-twitch fibres were fatigued by sixty 150 ms, 70 Hz tetani given every 1 s. Experiments were performed in standard Tyrode solution (control) or in the presence of: NADPH oxidase (NOX) 2 inhibitor (gp91ds-tat); NOX4 inhibitor (GKT137831); mitochondria-targeted antioxidant (SS-31); nitric oxide synthase (NOS) inhibitor (L-NAME); the general antioxidant N-acetylcysteine (NAC); a cocktail of SS-31, L-NAME and NAC. Spatially- and temporally-averaged [Ca2+]i and peak force were reduced by ∼70% and ∼20% at the end of fatiguing stimulation, respectively, with no marked differences between groups. PLFFD was similar in all groups, with 30 Hz force being decreased by ∼60% at 30 min of recovery. PLFFD was mostly due to decreased tetanic [Ca2+]i in control fibres and in the presence of NOX2 or NOX4 inhibitors. Conversely, in fibres exposed to SS-31 or the anti ROS/RNS cocktail, tetanic [Ca2+]i was not decreased during recovery so PLFFD was only caused by decreased myofibrillar Ca2+ sensitivity. The cocktail also increased resting [Ca2+]i and ultimately caused cell death. In conclusion, ROS/RNS-neutralizing compounds did not counteract the force decline during or after induction of fatigue.This article is protected by copyright. All rights reserved
The Journal of Physiology 11/2014; 593(2). DOI:10.1113/jphysiol.2014.279398 · 5.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Desmin, being a major intermediate filament of mature muscle cell, interacts with mitochondria within the cell and participates in mitochondria proper localization. The goal of the present study was to assess the effect of aggregate-prone and non-aggregate-prone desmin mutations on mitochondrial calcium uptake. Primary murine satellite cells were transduced with lentiviruses carrying desmin in wild type or mutant form, and were induced to differentiate into myotubes. Four mutations resulting in different degree of desmin aggregates formation were analyzed. Tail domain mutation Asp399Tyr has the mildest impact on desmin filament polymerization, rod domain mutation Ala357Pro causes formation of large aggregates composed of filamentous material, and Leu345Pro and Leu370Pro are considered to be the most severest in their impact on desmin polymerization and structure. For mitochondrial calcium measurement cells were loaded with rhod 2-AM. We found that aggregate-prone mutations significantly decreased [Ca(2+)]mit, whereas non-aggregate-prone mutations did not decrease [Ca(2+)]mit. Moreover aggregate-prone desmin mutations resulted in increased resting cytosolic [Ca(2+)]. However this increase was not accompanied by any alterations in sarcoplasmic reticulum calcium release. We suggest that the observed decline in [Ca(2+)]mit was due to desmin aggregate accumulation resulting in the loss of desmin mitochondria interactions.
[Show abstract][Hide abstract] ABSTRACT: In cells, such as neurones and immune cells, mitochondria can form dynamic and extensive networks that change over the minute timescale. In contrast, mitochondria in adult mammalian skeletal muscle fibres show little motility over several hours. Here, we use a novel three channelled microflow device, the multifunctional pipette, to test whether mitochondria in mouse skeletal muscle connect to each other. The central channel in the pipette delivers compounds to a restricted region of the sarcolemma, typically 30 µm in diameter. Two channels on either side of the central channel use suction to create a hydrodynamically confined flow zone and remove compounds completely from the bulk solution to internal waste compartments. Compounds were delivered locally to the end or side of single adult mouse skeletal muscle fibres to test whether changes in mitochondrial membrane potential were transmitted to more distant located mitochondria. Mitochondrial membrane potential was monitored with tetramethylrhodamine ethyl ester (TMRE). Cytosolic free [Ca2+] was monitored with fluo-3. A pulse of carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP, 100 µM) applied to a small area of the muscle fibre (30 µm in diameter) produced a rapid decrease in the mitochondrial TMRE signal (indicative of depolarization) to 38% of its initial value. After washout of FCCP, the TMRE signal partially recovered. At distances greater than 50 µm away from the site of FCCP application, the mitochondrial TMRE signal was unchanged. Similar results were observed when two sites along the fibre were pulsed sequentially with FCCP. After a pulse of FCCP, cytosolic [Ca2+] was unchanged and fibres contracted in response to electrical stimulation. In conclusion, our results indicate that extensive networks of interconnected mitochondria do not exist in skeletal muscle. Furthermore, the limited and reversible effects of targeted FCCP application with the multifunctional pipette highlight its advantages over bulk application of compounds to isolated cells.
PLoS ONE 09/2014; 9(9):e108601. DOI:10.1371/journal.pone.0108601 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Purpose: Calcium (Ca2+) plays key role in muscle cell physiology, e.g. takes part in the contraction and acts as co-factor in Krebs cycle. Mitochondria are capable to uptake calcium due to close proximity to SR Ca2+ releasing microdomains. Mitochondrial calcium balance is crucial for cell destiny, increased calcium leads to apoptosis, when decreased drives autophagy. On the other hand mitochondria within the muscle cells tightly connected to desmin. In desmin knock-out mice mitochondria were non-functional. Desmin mutations are associated with developing of myopathies, however machinery remains to be elucidated. We proposed that mitochondrial failure in desmin-compromised cells might result in disturbed calcium balance. We aimed to assess impact of non-aggregate prone (D399Y) and aggregate prone (L345P, A357P, L370P) desmin mutations on mitochondrial calcium.
Materials and methods: Satellite cells were obtained from soleus muscle of young C57BL/6 males. Cells were transduced with lentiviral suspension, encoded desmin wild type or mutant and GFP as reporter gene. Cells were grown for one week, then differentiation was induced and continued two weeks more. Obtained myotubes were loaded with rhod-2 AM for 20 min, after that dye was washed out for 20 min. Electrical stimulation was applied for up to 30 s as follows: 1 Hz, rest, 10 Hz, rest, and 100 Hz, rest. Changes in rhod-2 fluorescence were detected using laser confocal microscopy. Intensity of fluorescent signal was measured in baseline (without stimulation (F0)) and during stimulation (F). Ratio F/F0 was calculated and represented peaks amplitudes of mitochondrial calcium transients.
Results: After electrical stimulation the peak amplitude of mitochondrial calcium uptake was similar in non-transduced and DesWT cells being 2.61 ± 0.19 and 3.12 ± 0.29 respectively. However, in DesL345P myotubes and DesA357P myotybes, the mitochondrial calcium uptake was markedly decreased by 50% and 44% respectively in comparison with DesWT cells. Interestingly, mitochondrial calcium uptake in the DesD399Y myotubes was similar to that seen in DesWT cells and was equaled 3.45 ± 0.53. DesL370P cells didn't decrease peak amplitude as much as DesA357P and DesL345P and showed transitional peak, F/F0 was 2.85 ± 0.31.
Conclusion: Therefore mitochondrial capacity to uptake calcium depended on conformational form of desmin protein and aggregate prone mutations markedly decreased calcium uptake in comparison with non-aggregate prone mutation. We assumed that desmin aggregates broke mitochondria localization within the calcium release microdomains.
Cardiovascular Research 07/2014; 103(suppl 1):S73. DOI:10.1093/cvr/cvu091.80 · 5.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effects of the general antioxidant N-acetylcysteine (NAC) on muscle function and metabolism were examined. Isolated paired mouse extensor digitorum longus muscles were studied in the absence or presence of 20 mM NAC. Muscles were electrically stimulated to perform 100 isometric tetanic contractions (300 ms duration) at frequencies resulting in ∼85 % of maximal force (70-150 Hz at 25-40 °C). NAC did not significantly affect peak force in the unfatigued state at any temperature but significantly slowed tetanic force development in a temperature-dependent fashion (e.g., time to 50 % of peak tension averaged 35 ± 2 ms [control] and 37 ± 1 ms [NAC] at 25 °C vs. 21 ± 1 ms [control] and 52 ± 6 ms [NAC, P < 0.01] at 40 °C). During repeated contractions, NAC maximally enhanced peak force by the fifth tetanus at all temperatures (by ∼30 %). Thereafter, the effect of NAC disappeared rapidly at high temperatures (35-40 °C) and more slowly at the lower temperatures (25-30 °C). At all temperatures, the enhancing effect of NAC on peak force was associated with a slowing of relaxation. NAC did not significantly affect myosin light chain phosphorylation at rest or after five contractions (∼50 % increase vs. rest). After five tetani, lactate and inorganic phosphate increased about 20-fold and 2-fold, respectively, both in control and NAC-treated muscles. Interestingly, after five tetani, the increase in glucose 6-P was ∼2-fold greater, whereas the increase in malate was inhibited by ∼75 % with NAC vs. control, illustrating the metabolic effects of NAC. NAC slightly decreased the maximum shortening velocity in early fatigue (five to seven repeated tetani). These data demonstrate that the antioxidant NAC transiently enhances muscle force generation by a mechanism that is independent of changes in myosin light chain phosphorylation and inorganic phosphate. The slowing of relaxation suggests that NAC enhances isometric force by facilitating fusion (i.e., delaying force decline between pulses). The initial slowing of tension development and subsequent slowing of relaxation suggest that NAC would result in impaired performance during a high-intensity dynamic exercise.
Pflügers Archiv - European Journal of Physiology 08/2013; 466(3). DOI:10.1007/s00424-013-1331-z · 4.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Double discharges (doublets) of motor neurones at the onset of contractions increase both force and rate of force development during voluntary submaximal contractions. The purpose of this study was to examine the role of doublet discharges on force and myoplasmic free [Ca(2+)] ([Ca(2+)]i) during repeated fatiguing contractions, using a stimulation protocol mimicking the in vivo activation pattern during running. Individual intact fibres from the flexor digitorum brevis muscle of mice were stimulated at 33°C to undergo 150 constant-frequency (five pulses at 70 Hz) or doublet (an initial, extra pulse at 200 Hz) contractions at 300-ms intervals. In the unfatigued state, doublet stimulation resulted in a transient (~10-ms) approximate doubling of myoplasmic free [Ca(2+)] ([Ca(2+)]i), which was accompanied by a greater force-time integral (~70%) and peak force (~40%) compared to constant-frequency contractions. Moreover, doublets markedly increased force-time integral and peak force during the first 25 contractions of the fatiguing stimulation. In later stages of fatigue, addition of doublets increased force production but the increase in force production corresponded to only a minor portion of the fatigue-induced reduction in force. In conclusion, double discharges at the onset of contractions effectively increase force production, especially in early stages of fatigue. This beneficial effect occurs without additional force loss in later stages of fatigue, indicating that the additional energy cost induced by doublet discharges to skeletal muscle is limited.
The Journal of Physiology 05/2013; 591(15). DOI:10.1113/jphysiol.2013.257188 · 5.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Objectives:
Polymyositis and dermatomyositis are characterised by muscle weakness and fatigue even in patients with normal muscle histology via unresolved pathogenic mechanisms. In this study, we investigated the mechanisms by which high mobility group box protein 1 (HMGB1) acts to accelerate muscle fatigue development.
Intact single fibres were dissociated from flexor digitorum brevis (FDB) of wild type, receptor for advanced glycation endproduct (RAGE) knockout and toll like receptor 4 (TLR4) knockout mice and cultured in the absence or presence of recombinant HMGB1. A decrease in sarcoplasmic reticulum Ca(2+) release during a series of 300 tetanic contractions, which reflects the development of muscle fatigue, was determined by measuring myoplasmic free tetanic Ca(2+). TLR4 and major histocompatibility complex (MHC)-class I expression in mouse FDB fibres were investigated by immunofluorescence and confocal microscopy. Immunohistochemistry was used to investigate TLR4, MHC-class I and myosin heavy chain expression in muscle fibres of patients.
Our results demonstrate that TLR4 is expressed in human and mouse skeletal muscle fibres, and coexpressed with MHC-class I in muscle fibres of patients with myositis. Furthermore, we show that HMGB1 acts via TLR4 but not RAGE to accelerate muscle fatigue and to induce MHC-class I expression in vitro. In order to bind and signal via TLR4, HMGB1 must have a reduced cysteine 106 and a disulphide linkage between cysteine 23 and 45.
The HMGB1-TLR4 pathway may play an important role in causing muscle fatigue in patients with polymyositis or dermatomyositis and thus is a potential novel target for future therapy.
Annals of the rheumatic diseases 11/2012; 72(8). DOI:10.1136/annrheumdis-2012-202207 · 10.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mitochondrial dysfunction can drastically impair muscle function with weakness and exercise intolerance as key symptoms. Here, we examine the time course of development of muscle dysfunction in a mouse model of premature ageing induced by defective proofreading function of mitochondrial DNA (mtDNA) polymerase (mtDNA mutator mouse). Isolated fast-twitch muscles and single muscle fibres from young (3-5 months) and end-stage (11 months) mtDNAmutator mice were compared to age-matched control mice. Force and free myoplasmic [Ca2+] ([Ca2+]i) were measured under resting conditions and during fatigue induced by repeated tetani. Muscles of young mtDNA mutator mice displayed no weakness in the rested state, but had lower force and [Ca2+]I than control mice during induction of fatigue. Muscles of young mtDNA mutator showed decreased activities of citrate synthase and β-hydroxyacyl-CoA dehydrogenase, reduced expression of cytochrome c oxidase; and decreased expression of triggers of mitochondrial biogenesis (PGC-1α, PPAR α, AMPK). Muscles from end-stage mtDNA mutator mice showed weakness under resting conditions with markedly decreased tetanic [Ca2+]i, force per cross-sectional area and protein expression of the sarcoplasmic reticulum Ca2+ pump (SERCA1). In conclusion, fast-twitch muscles of prematurely ageing mtDNA mutator mice display a sequence of deleterious mitochondrial-to-nucleus signalling with an initial decrease in oxidative capacity, which was not counteracted by activation of signalling to increase mitochondrial biogenesis. This was followed by severe muscle weakness in the end-stage. These results have implication for normal ageing and suggest that decreased mitochondrial oxidative capacity due to a sedentary life style may predispose for muscle weakness developing later in life.
The Journal of Physiology 09/2012; 590(23). DOI:10.1113/jphysiol.2012.240077 · 5.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dietary inorganic nitrate has profound effects on health and physiological responses to exercise. Here, we examined if nitrate, in doses readily achievable via a normal diet, could improve Ca(2+) handling and contractile function using fast- and slow-twitch skeletal muscles from C57bl/6 male mice given 1 mm sodium nitrate in water for 7 days. Age matched controls were provided water without added nitrate. In fast-twitch muscle fibres dissected from nitrate treated mice, myoplasmic free [Ca(2+)] was significantly greater than in Control fibres at stimulation frequencies from 20 to 150 Hz, which resulted in a major increase in contractile force at ≤ 50 Hz. At 100 Hz stimulation, the rate of force development was ∼35% faster in the nitrate group. These changes in nitrate treated mice were accompanied by increased expression of the Ca(2+) handling proteins calsequestrin 1 and the dihydropyridine receptor. No changes in force or calsequestrin 1 and dihydropyridine receptor expression were measured in slow-twitch muscles. In conclusion, these results show a striking effect of nitrate supplementation on intracellular Ca(2+) handling in fast-twitch muscle resulting in increased force production. A new mechanism is revealed by which nitrate can exert effects on muscle function with applications to performance and a potential therapeutic role in conditions with muscle weakness.
The Journal of Physiology 06/2012; 590(Pt 15):3575-83. DOI:10.1113/jphysiol.2012.232777 · 5.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Measurements of free cytosolic Ca(2+) concentration ([Ca(2+)](i)) or free Ca(2+) concentration in cellular organelles have become more routine. The primary reason for this is the availability of membrane permeant forms of Ca(2+) indicators that can easily enter cells. In this chapter, the properties required of an ideal Ca(2+) indicator are identified and the advantages and disadvantages of available Ca(2+) indicators are pointed out. The pitfalls associated with usage of Ca(2+) indicators together with the clear advantages of ratiometric over non-ratiometric indicators are discussed. The excitation of Ca(2+) indicators and detection of the emitted fluorescence light require dedicated equipment; epifluorescence or confocal microscopes are most frequently used for this purpose and the advantages and disadvantages of these are discussed. Calibration experiments are required to translate changes in the fluorescence of Ca(2+) indicators into real [Ca(2+)](i) changes, but this procedure is non-trivial and potential sources of error are identified. Future developments in the field of Ca(2+) detection are discussed.
Advances in Experimental Medicine and Biology 01/2012; 740:27-43. DOI:10.1007/978-94-007-2888-2_2 · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We showed previously that force development in frog and FDB mouse skeletal muscle fibres is preceded by an increase of fibre stiffness occurring well before crossbridge attachment and force generation. This stiffness increase, referred to as static stiffness, is due to a Ca(2+)-dependent stiffening of a non-crossbridge sarcomere structure which we suggested could be attributed to the titin filaments. To investigate further the role of titin in static stiffness, we measured static stiffness properties at 24 and 35°C in soleus and EDL mouse muscle fibres which are known to express different titin isoforms. We found that static stiffness was present in both soleus and EDL fibres, however, its value was about five times greater in EDL than in soleus fibres. The rate of development of static stiffness on stimulation increased with temperature and was slightly faster in EDL than in soleus in agreement with previously published data on the time course of the intracellular Ca(2+) transients in these muscles. The present results show that the presence of a non-crossbridge Ca(2+)-dependent stiffening of the muscle fibre is a physiological general characteristic of skeletal muscle. Static stiffness depends on fibre type, being greater and developing faster in fast than in slow fibres. Our observations are consistent with the idea that titin stiffening on contraction improves the sarcomere structure stability. Such an action in fact seems to be more important in EDL fast fibre than in soleus slow fibres.
Journal of Muscle Research and Cell Motility 11/2011; 32(6):403-9. DOI:10.1007/s10974-011-9274-5 · 2.09 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Non-technical summary Prolonged muscle activity leads to a reduction of mechanical power and force output which is commonly indicated as muscular fatigue. The development of fatigue during repetitive stimulation of a skeletal muscle consists of an initial phase during which force decreases by 10–15%. This is followed by a second phase where force remains almost constant and finally a phase during which force drops precipitously to low levels. We show here that the initial fall in force is due to a reduction of the force generated by the individual molecular force generator, the cross-bridge, whereas in subsequent phases the force decrease is caused by a reduction in the number of molecular force generators. These results increase our understanding of muscular fatigue mechanisms.
Abstract Fatigue occurring during exercise can be defined as the inability to maintain the initial force or power output. As fatigue becomes pronounced, force and maximum velocity of shortening are greatly reduced and force relaxation is prolonged. In principle, force loss during fatigue can result from a decrease in the number of cross-bridges generating force or a decrease of the individual cross-bridge force or to both mechanisms. The present experiments were made to investigate this point in single fibres or small fibre bundles isolated from flexor digitorum brevis (FDB) of C57BL/6 mice at 22–24°C. During a series of 105 tetanic contractions, we measured force and fibre stiffness by applying small sinusoidal length oscillations at 2.5 or 4 kHz frequency to the activated preparation and measuring the resulting force changes. Stiffness data were corrected for the influence of compliance in series with the cross-bridge ensemble. The results show that the force decline during the first 20 tetani is due to the reduction of force developed by the individual cross-bridges and thereafter as fatigue becomes more severe, the number of cross-bridges decreases. In spite of the force reduction in the early phase of fatigue, there was an increased rate of tetanic force development and relaxation. In the latter stages of fatigue, the rate of force development and relaxation became slower. Thus, the start of fatigue is characterised by decreased cross-bridge force development and as fatigue becomes more marked, the number of cross-bridges decreases. These findings are discussed in the context of the current hypotheses about fatigue mechanisms.
The Journal of Physiology 05/2011; 589(Pt 13):3371-81. DOI:10.1113/jphysiol.2011.209874 · 5.04 Impact Factor