Events of the excitation-contraction-relaxation (E-C-R) cycle in fast- and slow-twitch mammalian muscle fibres relevant to muscle fatigue

Modeling the mechanism of Ca2+ release in skeletal muscle by DHPRs easing inhibition at RyR I1-sites

Ca2+ release from the sarcoplasmic reticulum (SR) plays a central role in excitation-contraction coupling (ECC) in skeletal muscles. However, the mechanism by which activation of the voltage-sensors/dihydropyridine receptors (DHPRs) in the membrane of the transverse tubular system leads to activation of the Ca2+-release channels/ryanodine receptors (RyRs) in the SR is not fully understood. Recent observations showing that a very small Ca2+ leak through RyR1s in mammalian skeletal muscle can markedly raise the background [Ca2+] in the junctional space (JS) above the Ca2+ level in the bulk of the cytosol indicate that there is a diffusional barrier between the JS and the cytosol at large. Here, I use a mathematical model to explore the hypothesis that a sudden rise in Ca2+ leak through DHPR-coupled RyR1s, caused by reduced inhibition at the RyR1 Ca2+/Mg2+ inhibitory I1-sites when the associated DHPRs are activated, is sufficient to enable synchronized responses that trigger a regenerative rise of Ca2+ release that remains under voltage control. In this way, the characteristic response to Ca2+ of RyR channels is key not only for the Ca2+ release mechanism in cardiac muscle and other tissues, but also for the DHPR-dependent Ca2+ release in skeletal muscle.

Greater Neuromuscular and Perceptual Fatigue after Low versus High Loads in the Bench Press: A Preliminary Study Applying Frequentist and Bayesian Group Analyses with Subject-by-Subject Case Series Reports

Background/Objective: This study investigated the differences in acute fatigue following resistance training performed with low versus high loads in the bench press (BP). Methods: Trained males (n = 5, 21.2 ± 2.77 years; 81.86 ± 6.67 kg; 177 ± 7.52 cm) undertook three protocols with 50%RM and three with 85%RM with volume equalized between protocols: muscular failure protocols (TF, RTP1 and 2), half-maximum repetition protocols (RTP3 and 4), and cluster set protocols (RTP5 and 6). Mechanical performance, lactate, and perceptual responses were analyzed during protocols and at post 0, 24, and 48 h using frequentist (p < 0.05) and Bayesian approaches. Results: Moderate to large (ES ≥ 0.3) and trivial to moderate (ES < 0.3) effects were observed at 0 and 24 h post-session, respectively, across all protocols. TF protocols, particularly RTP1, showed the greatest impairments when compared to the other RTP (ES ≥ 0.3). The Bayesian analysis supported the frequentist results, showing strong-decisive evidence for our data under the model that included protocols as predictors for mechanical, metabolic, and perceptual variables during protocols. Inter-individual variability in responses was observed in the neuromuscular tests, potentially related to the strength level and perceptual responses. Conclusions: In summary, TF generates greater fatigue, while reducing set volume to half of maximum repetitions or including intra-set rest that helps to mitigate fatigue symptoms.

Maximizing plyometric training for adolescents: a meta-analysis of ground contact frequency and overall intervention time on jumping ability: a systematic review and meta-analysis

Plyometric training boosts adolescents' jumping ability, crucial for athletic success and health. However, the best total ground contact frequency (TGCF) and overall intervention time (OIT) for these exercises remain unclear. This meta-analysis aims to identify optimal TGCF and OIT in plyometric training for adolescents, focusing on countermovement jump (CMJ) and squat jump (SJ) outcomes. This systematic review encompassed five databases and included 38 studies with 50 randomized controlled experiments and 3347 participants. We used the Cochrane risk assessment tool for study quality and Review Manager 5.4 for data analysis. The current meta-analysis incorporated a total of 38 studies, comprising 50 sets of randomized controlled trials, to investigate the influence of different TGCFs and OITs on plyometric training. The Cochrane risk assessment tool indicated that all the included studies were classified as low risk. Various TGCFs in plyometric training positively affected CMJ and SJ heights in adolescents. The TGCF of less than 900 was ideal for enhancing CMJ, whereas more than 1400 was effective for SJ. The optimal OIT was 400-600 min, specifically, 500-600 min for CMJ and 400-500 min for SJ. Plyometric training improves jumping ability in adolescents. Lower ground contact frequency (< 900 contacts) enhances CMJ, while higher ground contact frequency (> 1400 contacts) is more effective for SJ. Optimal intervention time ranges from 400 to 600 min, with 500 to 600 min benefiting CMJ and 400 to 500 min improving SJ.

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

Many neuromuscular disorders can have a differential impact on a specific myofibre type, forming the central premise of this review. The many different skeletal muscles in mammals contain a spectrum of slow- to fast-twitch myofibres with varying levels of protein isoforms that determine their distinctive contractile, metabolic, and other properties. The variations in functional properties across the range of classic 'slow' to 'fast' myofibres are outlined, combined with exemplars of the predominantly slow-twitch soleus and fast-twitch extensor digitorum longus muscles, species comparisons, and techniques used to study these properties. Other intrinsic and extrinsic differences are discussed in the context of slow and fast myofibres. These include inherent susceptibility to damage, myonecrosis, and regeneration, plus extrinsic nerves, extracellular matrix, and vasculature, examined in the context of growth, ageing, metabolic syndrome, and sexual dimorphism. These many differences emphasise the importance of carefully considering the influence of myofibre-type composition on manifestation of various neuromuscular disorders across the lifespan for both sexes. Equally, understanding the different responses of slow and fast myofibres due to intrinsic and extrinsic factors can provide deep insight into the precise molecular mechanisms that initiate and exacerbate various neuromuscular disorders. This focus on the influence of different myofibre types is of fundamental importance to enhance translation for clinical management and therapies for many skeletal muscle disorders.

The relationship between single muscle fibre and voluntary rate of force development in young and old males

PURPOSE

It is suggested that the early phase (< 50 ms) of force development during a muscle contraction is associated with intrinsic contractile properties, while the late phase (> 50 ms) is associated with maximal force. There are no direct investigations of single muscle fibre rate of force development (RFD) as related to joint-level RFD METHODS: Sixteen healthy, young (n = 8; 26.4 ± 1.5 yrs) and old (n = 8; 70.1 ± 2.8 yrs) males performed maximal voluntary isometric contractions (MVC) and electrically evoked twitches of the knee extensors to assess RFD. Then, percutaneous muscle biopsies were taken from the vastus lateralis and chemically permeabilized, to assess single fibre function.

RESULTS

At the joint level, older males were ~ 30% weaker and had ~ 43% and ~ 40% lower voluntary RFD values at 0-100 and 0-200 ms, respectively, than the younger ones (p ≤ 0.05). MVC torque was related to every voluntary RFD epoch in the young (p ≤ 0.001), but only the 0-200 ms epoch in the old (p ≤ 0.005). Twitch RFD was ~ 32% lower in the old compared to young (p < 0.05). There was a strong positive relationship between twitch RFD and voluntary RFD during the earliest time epochs in the young (≤ 100 ms; p ≤ 0.01). While single fibre RFD was unrelated to joint-level RFD in the young, older adults trended (p = 0.052-0.055) towards significant relationships between joint-level RTD and Type I single fibre RFD at the 0-30 ms (r² = 0.48) and 0-50 ms (r² = 0.49) time epochs.

CONCLUSION

Electrically evoked twitches are good predictors of early voluntary RFD in young, but not older adults. Only the older adults showed a potential relationship between single fibre (Type I) and joint-level rate of force development.

Sarcoplasmic reticulum calcium handling in unbranched, immediately post-necrotic fast-twitch mdx fibres is similar to wild-type littermates

NEW FINDINGS

Central question: What are the early effects of dystrophin deficiency on SR Ca²⁺ handling in the mdx mouse?

MAIN FINDING

In the mdx mouse, Ca²⁺ handling by the SR is little affected by the absence of dystrophin when looking at fibres without branches that have just regenerated following massive myonecrosis. This has important implications for our understanding of Ca²⁺ pathology in the mdx mouse.

ABSTRACT

There is a variety of results in the literature regarding the effects of dystrophin deficiency on the Ca²⁺ -handling properties of the SR in mdx mice, an animal model of Duchenne muscular dystrophy. One possible source of variation is the presence of branched fibres. Fibre branching, a consequence of degenerative-regenerative processes such as muscular dystrophy, has in itself a significant influence on the function of the SR. In our present study we attempt to detect early effects of dystrophin deficiency on SR Ca²⁺ handling by using unbranched fibres from the immediate post-necrotic stage in mdx mice (just regenerated following massive necrosis). Using kinetically-corrected Fura-2 fluorescence signals measured during twitch and tetanus, we analysed the amplitude, rise time and decay time of Δ[Ca²⁺ ]_i in unfatigued and fatigued fibres. Decay was also resolved into SR pump and SR leak components. Fibres from mdx mice were similar in all respects to fibres from wt littermates apart from: (i) a smaller amplitude of the initial spike of Δ[Ca²⁺ ]_i during a tetanus; and (ii) a mitigation of the fall in Δ[Ca²⁺ ]_i amplitude during the course of fatigue. Our findings suggest that the early effects of a loss of dystrophin on SR Ca²⁺ handling in mdx mice are subtle, and emphasise the importance of distinguishing between Ca²⁺ pathology that is due to lack of dystrophin and Ca²⁺ pathology that is due to muscle degeneration. This article is protected by copyright. All rights reserved.

The Influence of Supplemental Dietary Linoleic Acid on Skeletal Muscle Contractile Function in a Rodent Model of Barth Syndrome

Barth syndrome is a rare and incurable X-linked (male-specific) genetic disease that affects the protein tafazzin (Taz). Taz is an important enzyme responsible for synthesizing biologically relevant cardiolipin (for heart and skeletal muscle, cardiolipin rich in linoleic acid), a critical phospholipid of mitochondrial form and function. Mutations to Taz cause dysfunctional mitochondria, resulting in exercise intolerance due to skeletal muscle weakness. To date, there has been limited research on improving skeletal muscle function, with interventions focused on endurance and resistance exercise. Previous cell culture research has shown therapeutic potential for the addition of exogenous linoleic acid in improving Taz-deficient mitochondrial function but has not been examined in vivo. The purpose of this study was to examine the influence of supplemental dietary linoleic acid on skeletal muscle function in a rodent model of Barth syndrome, the inducible Taz knockdown (TazKD) mouse. One of the main findings was that TazKD soleus demonstrated an impaired contractile phenotype (slower force development and rates of relaxation) in vitro compared to their WT littermates. Interestingly, this impaired contractile phenotype seen in vitro did not translate to altered muscle function in vivo at the whole-body level. Also, supplemental linoleic acid attenuated, to some degree, in vitro impaired contractile phenotype in TazKD soleus, and these findings appear to be partially mediated by improvements in cardiolipin content and resulting mitochondrial supercomplex formation. Future research will further examine alternative mechanisms of dietary supplemental LA on improving skeletal muscle contractile dysfunction in TazKD mice.

Maximal muscular power: lessons from sprint cycling

Maximal muscular power production is of fundamental importance to human functional capacity and feats of performance. Here, we present a synthesis of literature pertaining to physiological systems that limit maximal muscular power during cyclic actions characteristic of locomotor behaviours, and how they adapt to training. Maximal, cyclic muscular power is known to be the main determinant of sprint cycling performance, and therefore we present this synthesis in the context of sprint cycling. Cyclical power is interactively constrained by force-velocity properties (i.e. maximum force and maximum shortening velocity), activation-relaxation kinetics and muscle coordination across the continuum of cycle frequencies, with the relative influence of each factor being frequency dependent. Muscle cross-sectional area and fibre composition appear to be the most prominent properties influencing maximal muscular power and the power-frequency relationship. Due to the role of muscle fibre composition in determining maximum shortening velocity and activation-relaxation kinetics, it remains unclear how improvable these properties are with training. Increases in maximal muscular power may therefore arise primarily from improvements in maximum force production and neuromuscular coordination via appropriate training. Because maximal efforts may need to be sustained for ~15-60 s within sprint cycling competition, the ability to attenuate fatigue-related power loss is also critical to performance. Within this context, the fatigued state is characterised by impairments in force-velocity properties and activation-relaxation kinetics. A suppression and leftward shift of the power-frequency relationship is subsequently observed. It is not clear if rates of power loss can be improved with training, even in the presence adaptations associated with fatigue-resistance. Increasing maximum power may be most efficacious for improving sustained power during brief maximal efforts, although the inclusion of sprint interval training likely remains beneficial. Therefore, evidence from sprint cycling indicates that brief maximal muscular power production under cyclical conditions can be readily improved via appropriate training, with direct implications for sprint cycling as well as other athletic and health-related pursuits.

Calcium Imaging in the Zebrafish

The zebrafish (Danio rerio) has emerged as a widely used model system during the last four decades. The fact that the zebrafish larva is transparent enables sophisticated in vivo imaging, including calcium imaging of intracellular transients in many different tissues. While being a vertebrate, the reduced complexity of its nervous system and small size make it possible to follow large-scale activity in the whole brain. Its genome is sequenced and many genetic and molecular tools have been developed that simplify the study of gene function in health and disease. Since the mid 90's, the development and neuronal function of the embryonic, larval, and later, adult zebrafish have been studied using calcium imaging methods. This updated chapter is reviewing the advances in methods and research findings of zebrafish calcium imaging during the last decade. The choice of calcium indicator depends on the desired number of cells to study and cell accessibility. Synthetic calcium indicators, conjugated to dextrans and acetoxymethyl (AM) esters, are still used to label specific neuronal cell types in the hindbrain and the olfactory system. However, genetically encoded calcium indicators, such as aequorin and the GCaMP family of indicators, expressed in various tissues by the use of cell-specific promoters, are now the choice for most applications, including brain-wide imaging. Calcium imaging in the zebrafish has contributed greatly to our understanding of basic biological principles during development and adulthood, and the function of disease-related genes in a vertebrate system.

Intrauterine growth restriction affects diaphragm function in adult female and male mice

BACKGROUND

In utero diaphragm development is critically important for postnatal respiratory function and any disturbance to fetal development may lead to diaphragm dysfunction and respiratory complications in the postnatal period. Intrauterine growth restriction (IUGR) has been shown to affect respiratory function in a sex-dependent manner; however, the effect of IUGR on diaphragm function is unknown.

AIM

This study used a maternal hypoxia-induced mouse model of IUGR to investigate the impact of IUGR on diaphragm function and structure in male and female adult offspring.

MATERIALS AND METHODS

Pregnant BALB/c mice were housed under hypoxic conditions (10.5% O₂ ) from gestational days 11 to 17.5 and then returned to normoxic conditions. Control mice were housed under normoxic conditions throughout pregnancy. At 8 weeks of age, offspring were euthanized and diaphragms isolated for functional assessment in organ bath experiments and for histological analysis.

RESULTS

IUGR offspring were lighter at birth and remained lighter at 8 weeks of age compared to Controls. While diaphragm force (maximal or twitch) was not affected by treatment or sex, the IUGR group exhibited a longer half-relaxation time after twitch contractions compared to Control. Female offspring had a lower maximum rate of force development and higher fatigue resistance compared to males, independent of IUGR. There was no difference in the diaphragm myofibre cross-sectional area between groups or sexes.

CONCLUSION

Sex and IUGR independently affect diaphragm contraction in adult mice without changes in structure. This study demonstrates that IUGR affects diaphragm contractile function in later life and could impair respiratory function if exacerbated under conditions of increased respiratory load.

Measurement of force and calcium release using mechanically skinned fibers from mammalian skeletal muscle

The mechanically skinned (or “peeled”) skeletal muscle fiber technique is a highly versatile procedure that allows controlled examination of each of the steps in the excitation-contraction (EC)-coupling sequence in skeletal muscle fibers, starting with excitation/depolarization of the transverse tubular (T)-system through to Ca2+ release from sarcoplasmic reticulum (SR) and finally force development by the contractile apparatus. It can also show the overall response of the whole EC-coupling sequence together, such as in twitch and tetanic force responses. A major advantage over intact muscle fiber preparations is that it is possible to set and rapidly manipulate the “intracellular” conditions, allowing examination of the effects of key variables (e.g., intracellular pH, ATP levels, redox state, etc.) on each individual step in EC coupling. This Cores of Reproducibility in Physiology (CORP) article describes the rationale, procedures, and experimental details of the various ways in which the mechanically skinned fiber technique is used in our laboratory to examine the physiological mechanisms controlling Ca2+ release and contraction in skeletal muscle fibers and the aberrations and dysfunction occurring with exercise and disease.

The Muscle Fiber Profiles, Mitochondrial Content, and Enzyme Activities of the Exceptionally Well-Trained Arm and Leg Muscles of Elite Cross-Country Skiers

As one of the most physically demanding sports in the Olympic Games, cross-country skiing poses considerable challenges with respect to both force generation and endurance during the combined upper- and lower-body effort of varying intensity and duration. The isoforms of myosin in skeletal muscle have long been considered not only to define the contractile properties, but also to determine metabolic capacities. The current investigation was designed to explore the relationship between these isoforms and metabolic profiles in the arms (triceps brachii) and legs (vastus lateralis) as well as the range of training responses in the muscle fibers of elite cross-country skiers with equally and exceptionally well-trained upper and lower bodies. The proportion of myosin heavy chain (MHC)-1 was higher in the leg (58 ± 2% [34-69%]) than arm (40 ± 3% [24-57%]), although the mitochondrial volume percentages [8.6 ± 1.6 (leg) and 9.0 ± 2.0 (arm)], and average number of capillaries per fiber [5.8 ± 0.8 (leg) and 6.3 ± 0.3 (arm)] were the same. In these comparable highly trained leg and arm muscles, the maximal citrate synthase (CS) activity was the same. Still, 3-hydroxy-acyl-CoA-dehydrogenase (HAD) capacity was 52% higher (P < 0.05) in the leg compared to arm muscles, suggesting a relatively higher capacity for lipid oxidation in leg muscle, which cannot be explained by the different fiber type distributions. For both limbs combined, HAD activity was correlated with the content of MHC-1 (r² = 0.32, P = 0.011), whereas CS activity was not. Thus, in these highly trained cross-country skiers capillarization of and mitochondrial volume in type 2 fiber can be at least as high as in type 1 fibers, indicating a divergence between fiber type pattern and aerobic metabolic capacity. The considerable variability in oxidative metabolism with similar MHC profiles provides a new perspective on exercise training. Furthermore, the clear differences between equally well-trained arm and leg muscles regarding HAD activity cannot be explained by training status or MHC distribution, thereby indicating an intrinsic metabolic difference between the upper and lower body. Moreover, trained type 1 and type 2A muscle fibers exhibited similar aerobic capacity regardless of whether they were located in an arm or leg muscle.

Impaired muscle relaxation and mitochondrial fission associated with genetic ablation of cytoplasmic actin isoforms

While α-actin isoforms predominate in adult striated muscle, skeletal muscle-specific knockouts (KOs) of nonmuscle cytoplasmic β_cyto - or γ_cyto -actin each cause a mild, but progressive myopathy effected by an unknown mechanism. Using transmission electron microscopy, we identified morphological abnormalities in both the mitochondria and the sarcoplasmic reticulum (SR) in aged muscle-specific β_cyto - and γ_cyto -actin KO mice. We found β_cyto - and γ_cyto -actin proteins to be enriched in isolated mitochondrial-associated membrane preparations, which represent the interface between mitochondria and sarco-endoplasmic reticulum important in signaling and mitochondrial dynamics. We also measured significantly elongated and interconnected mitochondrial morphologies associated with a significant decrease in mitochondrial fission events in primary mouse embryonic fibroblasts lacking β_cyto - and/or γ_cyto -actin. Interestingly, mitochondrial respiration in muscle was not measurably affected as oxygen consumption was similar in skeletal muscle fibers from 12 month-old muscle-specific β_cyto - and γ_cyto -actin KO mice. Instead, we found that the maximal rate of relaxation after isometric contraction was significantly slowed in muscles of 12-month-old β_cyto - and γ_cyto -actin muscle-specific KO mice. Our data suggest that impaired Ca²⁺ re-uptake may presage development of the observed SR morphological changes in aged mice while providing a potential pathological mechanism for the observed myopathy.

NO-sGC Pathway Modulates Ca2+ Release and Muscle Contraction in Zebrafish Skeletal Muscle

Vertebrate skeletal muscle contraction and relaxation is a complex process that depends on Ca²⁺ ions to promote the interaction of actin and myosin. This process can be modulated by nitric oxide (NO), a gas molecule synthesized endogenously by (nitric oxide synthase) NOS isoforms. At nanomolar concentrations NO activates soluble guanylate cyclase (sGC), which in turn activates protein kinase G via conversion of GTP into cyclic GMP. Alternatively, NO post-translationally modifies proteins via S-nitrosylation of the thiol group of cysteine. However, the mechanisms of action of NO on Ca²⁺ homeostasis during muscle contraction are not fully understood and we hypothesize that NO exerts its effects on Ca²⁺ homeostasis in skeletal muscles mainly through negative modulation of Ca²⁺ release and Ca²⁺ uptake via the NO-sGC-PKG pathway. To address this, we used 5–7 days-post fecundation-larvae of zebrafish, a well-established animal model for physiological and pathophysiological muscle activity. We evaluated the response of muscle contraction and Ca²⁺ transients in presence of SNAP, a NO-donor, or L-NAME, an unspecific NOS blocker in combination with specific blockers of key proteins of Ca²⁺ homeostasis. We also evaluate the expression of NOS in combination with dihydropteridine receptor, ryanodine receptor and sarco/endoplasmic reticulum Ca²⁺ ATPase. We concluded that endogenous NO reduced force production through negative modulation of Ca²⁺ transients via the NO-sGC pathway. This effect could be reversed using an unspecific NOS blocker or sGC blocker.

Central activation, metabolites, and calcium handling during fatigue with repeated maximal isometric contractions in human muscle

PURPOSE

To determine the roles of calcium (Ca²⁺) handling by sarcoplasmic reticulum (SR) and central activation impairment (i.e., central fatigue) during fatigue with repeated maximal voluntary isometric contractions (MVC) in human muscles.

METHODS

Contractile performance was assessed during 3 min of repeated MVCs (7-s contraction, 3-s rest, n = 17). In ten participants, in vitro SR Ca²⁺-handling, metabolites, and fibre-type composition were quantified in biopsy samples from quadriceps muscle, along with plasma venous [K⁺]. In 11 participants, central fatigue was compared using tetanic stimulation superimposed on MVC in quadriceps and adductor pollicis muscles.

RESULTS

The decline of peak MVC force with fatigue was similar for both muscles. Fatigue resistance correlated directly with % type I fibre area in quadriceps (r = 0.77, P = 0.009). The maximal rate of ryanodine-induced Ca²⁺-release and Ca²⁺-uptake fell by 31 ± 26 and 28 ± 13%, respectively. The tetanic force depression was correlated with the combined reduction of ATP and PCr, and increase of lactate (r = 0.77, P = 0.009). Plasma venous [K⁺] increased from 4.0 ± 0.3 to 5.4 ± 0.8 mM over 1-3-min exercise. Central fatigue occurred during the early contractions in the quadriceps in 7 out of 17 participants (central activation ratio fell from 0.98 ± 0.05 to 0.86 ± 0.11 at 1 min), but dwindled at exercise cessation. Central fatigue was seldom apparent in adductor pollicis.

CONCLUSIONS

Fatigue with repeated MVC in human limb muscles mainly involves peripheral aspects which include impaired SR Ca²⁺-handling and we speculate that anaerobic metabolite changes are involved. A faster early force loss in quadriceps muscle with some participants is attributed to central fatigue.

Estimating the minimum stimulation frequency necessary to evoke tetanic progression based on muscle twitch parameters

The summation of the muscle force caused by an increase in the firing rate is named a tetanic contraction (tetanus), and the minimum stimulation frequency necessary to evoke an unfused/fused tetanus is related to the contraction time (CT) and relaxation time (RT) of the twitch. In particular, the fusion index (FI) is a very useful indicator, and it is used to evaluate the change in the muscle fiber component ratio. However, the measurement of the FI is invasive, because most patients experience pain during the electrical stimulation for tetanus. We expect that the twitch parameters CT and RT can substitute for the FI in the future. We found that the minimum stimulation frequency necessary to evoke the unfused/fused tetanus can be estimated from the twitch parameters as a first step. The results showed that (1) the minimum stimulation frequencies calculated from twitch parameters during unfused/fused tetanus were very similar to those calculated from FI parameters, and (2) they were also strongly correlated with FI parameters regardless of fiber components. The basic characteristics of tetanic progression in different fiber types could be estimated from twitch parameters.

Sodium bicarbonate ingestion and individual variability in time-to-peak pH

This study determined variability in time-to-peak pH after consumption of 300 mg kg^-1 of sodium bicarbonate. Seventeen participants (mean ± SD: age 21.38 ± 1.5 years; mass 75.8 ± 5.8 kg; height 176.8 ± 7.6 cm) reported to the laboratory where a resting capillary sample was taken. Then, 300 mg kg^-1 of NaHCO₃ in 450 ml of flavoured water was ingested. Participants rested for 90 min and repeated blood samples were procured at 10 min intervals for 60 min and then every 5 min until 90 min. Blood pH concentrations were measured. Results suggested that time-to-peak pH (64.41 ± 18.78 min) was variable with a range of 10-85 min and a coefficient of variation of 29.16%. A bimodal distribution occurred, at 65 and 75 min. In conclusion, athletes, when using NaHCO₃ as an ergogenic aid, should determine their time-to-peak pH to best utilize the added buffering capacity this substance allows.

Bone and skeletal muscle: Key players in mechanotransduction and potential overlapping mechanisms

The development and maintenance of skeletal muscle and bone mass is critical for movement, health and issues associated with the quality of life. Skeletal muscle and bone mass are regulated by a variety of factors that include changes in mechanical loading. Moreover, bone mass is, in large part, regulated by muscle-derived mechanical forces and thus by changes in muscle mass/strength. A thorough understanding of the cellular mechanism(s) responsible for mechanotransduction in bone and skeletal muscle is essential for the development of effective exercise and pharmaceutical strategies aimed at increasing, and/or preventing the loss of, mass in these tissues. Thus, in this review we will attempt to summarize the current evidence for the major molecular mechanisms involved in mechanotransduction in skeletal muscle and bone. By examining the differences and similarities in mechanotransduction between these two tissues, it is hoped that this review will stimulate new insights and ideas for future research and promote collaboration between bone and muscle biologists.(1).

Ca2+-dependent regulations and signaling in skeletal muscle: from electro-mechanical coupling to adaptation

Calcium (Ca2+) plays a pivotal role in almost all cellular processes and ensures the functionality of an organism. In skeletal muscle fibers, Ca(2+) is critically involved in the innervation of skeletal muscle fibers that results in the exertion of an action potential along the muscle fiber membrane, the prerequisite for skeletal muscle contraction. Furthermore and among others, Ca(2+) regulates also intracellular processes, such as myosin-actin cross bridging, protein synthesis, protein degradation and fiber type shifting by the control of Ca(2+)-sensitive proteases and transcription factors, as well as mitochondrial adaptations, plasticity and respiration. These data highlight the overwhelming significance of Ca(2+) ions for the integrity of skeletal muscle tissue. In this review, we address the major functions of Ca(2+) ions in adult muscle but also highlight recent findings of critical Ca(2+)-dependent mechanisms essential for skeletal muscle-regulation and maintenance.

Sodium bicarbonate supplementation and ingestion timing: does it matter?

Although a considerable amount of literature exists on the ergogenic potential of ingesting sodium bicarbonate (NaHCO3) before short-term, high-intensity exercise, very little exists on optimal loading times before exercise. The purpose of this study was to determine the influence of NaHCO3 supplementation timing on repeated sprint ability (RSA). Eight men completed 3 (randomized and counterbalanced) trials of ten 10-second sprints separated by 50 seconds of active recovery (1:5 work-to-rest) on a nonmotorized treadmill. Before each trial, the subjects ingested 0.3 g·kg(-1) body weight of NaHCO3 at 60 (H1), 120 (H2), or 180 (H3) minutes before exercise. Additionally, the subjects were assessed for any side effects (gastrointestinal [GI] discomfort) from the NaHCO3 ingestion via a visual analog scale (VAS). Blood buffering was assessed using a 2-way analysis of variance (ANOVA) with repeated measures, whereas repeated sprint performance and GI discomfort were assessed via a 1-way ANOVA with repeated measures. Blood-buffering capacity was not different at preexercise times (HCO3(-) [millimoles per liter] H1: 30.2 ± 0.4, H2: 30.9 ± 0.6, H3: 31.2 ± 0.6; p > 0.74). Average speed, average power, and total distance covered progressively declined over the 10 sprints; however, there was no difference between conditions (p > 0.22). The incidence of GI discomfort was significantly higher (p < 0.05) from preingestion at all time points with the exception of 180 minutes, whereas severity was only different between 90 and 180 minutes. Ingestion times (between 60 and 180 minutes) did not influence the blood buffering or the ergogenic potential of NaHCO3 as assessed by RSA. However, VAS scores indicated that at 180 minutes postingestion, an individual is less prone to experiencing significant GI discomfort.

Truncated dystrophins reduce muscle stiffness in the extensor digitorum longus muscle of mdx mice

Muscle stiffness is a major clinical feature in Duchenne muscular dystrophy (DMD). DMD is the most common lethal inherited muscle-wasting disease in boys, and it is caused by the lack of the dystrophin protein. We recently showed that the extensor digitorum longus (EDL) muscle of mdx mice (a DMD mouse model) exhibits disease-associated muscle stiffness. Truncated micro- and mini-dystrophins are the leading candidates for DMD gene therapy. Unfortunately, it has never been clear whether these truncated genes can mitigate muscle stiffness. To address this question, we examined the passive properties of the EDL muscle in transgenic mdx mice that expressed a representative mini- or micro-gene (ΔH2-R15, ΔR2-15/ΔR18-23/ΔC, or ΔR4-23/ΔC). The passive properties were measured at the ages of 6 and 20 mo and compared with those of age-matched wild-type and mdx mice. Despite significant truncation of the gene, surprisingly, the elastic and viscous properties were completely restored to the wild-type level in every transgenic strain we examined. Our results demonstrated for the first time that truncated dystrophin genes may effectively treat muscle stiffness in DMD.

How well do muscle biomechanics predict whole-animal locomotor performance? The role of Ca2+ handling

It is important to determine the enabling mechanisms that underlie locomotor performance to explain the evolutionary patterns and ecological success of animals. Our aim was to determine the extent to which calcium (Ca2+) handling dynamics modulate the contractile properties of isolated skeletal muscle, and whether the effects of changing Ca2+ handling dynamics in skeletal muscle are paralleled by changes in whole-animal sprint and sustained swimming performance. Carp (Cyprinus carpio) increased swimming speed by concomitant increases in tail-beat amplitude and frequency. Reducing Ca2+ release from the sarcoplasmic reticulum (SR) by blocking ryanodine receptors with dantrolene decreased isolated peak muscle force and was paralleled by a decrease in tail-beat frequency and whole-animal sprint performance. An increase in fatigue resistance following dantrolene treatment may reflect the reduced depletion of Ca2+ stores in the SR associated with lower ryanodine receptor (RyR) activity. Blocking RyRs may be detrimental by reducing force production and beneficial by reducing SR Ca2+ depletion so that there was no net effect on critical sustained swimming speed (Ucrit). In isolated muscle, there was no negative effect on force production of blocking Ca2+ release via dihydropyridine receptors (DHPRs) with nifedipine. Nifedipine decreased fatigue resistance of isolated muscle, which was paralleled by decreases in tail-beat frequency and Ucrit. However, sprint performance also decreased with DHPR inhibition, which may indicate a role in muscle contraction of the Ca2+ released by DHPR into the myocyte. Inhibiting sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) activity with thapsigargin decreased fatigue resistance, suggesting that SERCA activity is important in avoiding Ca2+ store depletion and fatigue. We have shown that different molecular mechanisms modulate the same muscle and whole-animal traits, which provides an explanatory model for the observed variations in locomotor performance within and between species.

Corticospinal output during muscular fatigue differs in multiple sclerosis patients compared to healthy controls

Background:

In multiple sclerosis (MS), fatigue is a common and often disabling symptom. It has multiple causes with central motor fatigue playing an important role.

Objective:

The objective of this study was to analyse the central motor conduction changes in relation to muscle contraction force during muscle fatigue and recovery in MS patients compared to healthy controls.

Methods:

A total of 23 MS patients with fatigue and 13 healthy subjects were assessed during 2 minutes of fatiguing exercise of the abductor digiti minimi muscle of the hand and the subsequent 7 minutes of recovery. Central motor conduction was quantified by transcranial magnetic stimulation using the triple stimulation protocol and calculating a central conduction index (CCI).

Results:

Force declined to 36% of the pre-exercise level (SD 16%; p < 0.01) in MS patients and to 44% (SD 9%, p < 0.01) in healthy subjects (group differences, not statistically significant). The decline of the CCI was significantly less marked in patients (–20%, SD 26%, p < 0.05) than in healthy subjects (–57%, SD 15%, p < 0.05; group differences, p < 0.05). The decline of force and CCI were not correlated in either group.

Conclusions:

During a fatiguing exercise, the decline in central motor conduction is significantly less pronounced in MS patients than healthy subjects, although the reduction of force is similar.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Slowed relaxation and preserved maximal force in soleus muscles of mice with targeted disruption of the Serca2 gene in skeletal muscle

Sarcoplasmic reticulum Ca(2+) ATPases (SERCAs) play a major role in muscle contractility by pumping Ca(2+) from the cytosol into the sarcoplasmic reticulum (SR) Ca(2+) store, allowing muscle relaxation and refilling of the SR with releasable Ca(2+). Decreased SERCA function has been shown to result in impaired muscle function and disease in human and animal models. In this study, we present a new mouse model with targeted disruption of the Serca2 gene in skeletal muscle (skKO) to investigate the functional consequences of reduced SERCA2 expression in skeletal muscle. SkKO mice were viable and basic muscle structure was intact. SERCA2 abundance was reduced in multiple muscles, and by as much as 95% in soleus muscle, having the highest content of slow-twitch fibres (40%). The Ca(2+) uptake rate was significantly reduced in SR vesicles in total homogenates. We did not find any compensatory increase in SERCA1 or SERCA3 abundance, or altered expression of several other Ca(2+)-handling proteins. Ultrastructural analysis revealed generally well-preserved muscle morphology, but a reduced volume of the longitudinal SR. In contracting soleus muscle in vitro preparations, skKO muscles were able to fully relax, but with a significantly slowed relaxation time compared to controls. Surprisingly, the maximal force and contraction rate were preserved, suggesting that skKO slow-twitch fibres may be able to contribute to the total muscle force despite loss of SERCA2 protein. Thus it is possible that SERCA-independent mechanisms can contribute to muscle contractile function.

Kinetic changes in tetanic Ca²⁺ transients in enzymatically dissociated muscle fibres under repetitive stimulation

We used enzymatically dissociated flexor digitorum brevis (FDB) and soleus fibres loaded with the fast Ca(2+) dye Magfluo-4 AM, and adhered to Laminin, to test whether repetitive stimulation induces progressive changes in the kinetics of Ca(2+) release and reuptake in a fibre-type-dependent fashion. We applied a protocol of tetani of 350 ms, 100 Hz, every 4 s to reach a mean amplitude reduction of 25% of the first peak. Morphology type I (MT-I) and morphology type II (MT-II) fibres underwent a total of 96 and 52.8 tetani (P < 0.01 between groups), respectively. The MT-II fibres (n = 18) showed significant reductions of the amplitude (19%), an increase in rise time (8.5%) and a further reduction of the amplitude/rise time ratio (25.5%) of the first peak of the tetanic transient after 40 tetani, while MT-I fibres (n = 5) did not show any of these changes. However, both fibre types showed significant reductions in the maximum rate of rise of the first peak after 40 tetani. Two subpopulations among the MT-II fibres could be distinguished according to Ca(2+) reuptake changes. Fast-fatigable MT-II fibres (fMT-II) showed an increase of 32.2% in the half-width value of the first peak, while for fatigue-resistant MT-II fibres (rMT-II), the increase amounted to 6.9%, both after 40 tetani. Significant and non-significant increases of 36.4% and 11.9% in the first time constant of decay (t(1)) values were seen after 40 tetani in fMT-II and rMT-II fibres, respectively. MT-I fibres did not show kinetic changes in any of the Ca(2+) reuptake variables. All changes were reversed after an average recovery of 7.5 and 15.4 min for MT-I and MT-II fibres, respectively. Further experiments ruled out the possibility that the differences in the kinetic changes of the first peak of the Ca(2+) transients between fibres MT-I and MT-II could be related to the inactivation of Ca(2+) release mechanism. In conclusion, we established a model of enzymatically dissociated fibres, loaded with Magfluo-4 and adhered to Laminin, to study muscle fatigue and demonstrated fibre-type-dependent, fatigue-induced kinetic changes in both Ca(2+) release and reuptake.

Sodium bicarbonate ingestion and repeated swim sprint performance

The purpose of the present investigation was to observe the ergogenic potential of 0.3 g·kg-1 of sodium bicarbonate (NaHCO3) in competitive, nonelite swimmers using a repeated swim sprint design that eliminated the technical component of turning. Six male (181.2 ± 7.2 cm; 80.3 ± 11.9 kg; 50.8 ± 5.5 ml·kg-1·min-1 VO2max) and 8 female (168.8 ± 5.6 cm; 75.3 ± 10.1 kg; 38.8 ± 2.6 ml·kg-1·min-1 VO2max) swimmers completed 2 trial conditions (NaHCO3 [BICARB] and NaCl placebo [PLAC]) implemented in a randomized (counterbalanced), single blind manner, each separated by 1 week. Swimmers were paired according to ability and completed 8, 25-m front crawl maximal effort sprints each separated by 5 seconds. Blood acid-base status was assessed preingestion, pre, and postswim via capillary finger sticks, and total swim time was calculated as a performance measure. Total swim time was significantly decreased in the BICARB compared to PLAC condition (p = 0.04), with the BICARB condition resulting in a 2% decrease in total swim time compared to the PLAC condition (159.4 ± 25.4 vs. 163.2 ± 25.6 seconds; mean difference = 4.4 seconds; 95% confidence interval = 8.7-0.1). Blood analysis revealed significantly elevated blood buffering potential preswim (pH: BICARB = 7.48 ± 0.01, PLAC = 7.41 ± 0.01) along with a significant decrease in extracellular K+ (BICARB = 4.0 ± 0.1 mmol·L-1, PLAC = 4.6 ± 0.1 mmol·L-1). The findings suggest that 0.3 g·kg-1 NaHCO3 ingested 2.5 hours before exercise enhances the blood buffering potential and may positively influence swim performance.

Effects of various sodium bicarbonate loading protocols on the time-dependent extracellular buffering profile

Although much research has investigated the types of exercise that are enhanced with sodium bicarbonate (NaHCO3) ingestion, to date, there has been limited research on the dosage and timing of ingestion that optimizes the associated ergogenic effects. This study investigated the effects of various NaHCO3 loading protocols on the time-dependent blood-buffering profile. Eight male volunteers (age, 22.4 +/- 5.7 yr; height, 179.8 +/- 9.6 cm, body mass, 76.3 +/- 14.1 kg) completed Part A, measures of alkalosis throughout 120 minutes after ingestion of various single NaHCO3 dosages (0.3 gxkg-1, 0.2 gxkg-1, 0.1 gxkg-1, and placebo); and Part B, similar profiles after alternative NaHCO3 loading protocols (single morning dosage [SMD], single evening dosage [SED], and dosages ingested on 3 consecutive evenings [CED]). Results from Part A are as follows. Blood buffering in the 0.1 gxkg-1 condition was significantly lower than the 0.2 g.kg-1 and 0.3 gxkg-1 conditions (p < 0.002), but there was no significant differences between the 0.2 gxkg -1and 0.3 g.kg-1 conditions (p = 0.34). Although the blood buffering was relatively constant in the 0.1 and 0.2 conditions, it was significantly higher at 60 minutes than at 100 minutes and 120 minutes in the 0.3 gxkg-1 condition (p < 0.05). Results from Part B are as follows. Blood buffering for SMD was significantly higher than for SED and CED (p < 0.05). Blood buffering in the SMD condition was significantly lower at 17:00 hours than at 11:00 hours (p = 0.007). The single 0.2 and 0.3 gxkg-1 NaHCO3 dosages appeared to be the most effective for increasing blood-buffering capacity. The 0.2 gxkg-1 dosage is best ingested 40 to 50 minutes before exercise and the 0.3 gxkg-1 dosage 60 minutes before exercise.

Myosin heavy chain isoform composition and Ca(2+) transients in fibres from enzymatically dissociated murine soleus and extensor digitorum longus muscles

Electrically elicited Ca(2+) transients reported with the fast Ca(2+) dye MagFluo-4 AM and myosin heavy chain (MHC) electrophoretic patterns were obtained in intact, enzymatically dissociated fibres from adult mice extensor digitorum longus (EDL) and soleus muscles. Thirty nine fibres (23 from soleus and 16 from EDL) were analysed by both fluorescence microscopy and electrophoresis. These fibres were grouped as follows: group 1 included 13 type I and 4 type IC fibres; group 2 included 2 type IIC, 3 IIA and 1 I/IIA/IIX fibres; group 3 included 4 type IIX and 1 type IIX/IIB fibres; group 4 included 2 type IIB/IIX and 9 type IIB fibres. Ca(2+) transients obtained in groups 1, 2, 3 and 4 had the following kinetic parameters (mean +/- s.e.m.): amplitude (F/F): 0.61 +/- 0.05, 0.53 +/- 0.08, 0.61 +/- 0.06 and 0.61 +/- 0.03; rise time (ms): 1.64 +/- 0.05, 1.35 +/- 0.05, 1.18 +/- 0.06 and 1.14 +/- 0.04; half-amplitude width (ms): 19.12 +/- 1.85, 11.86 +/- 3.03, 4.62 +/- 0.31 and 4.23 +/- 0.37; and time constants of decay (tau(1) and tau(2), ms): 3.33 +/- 0.13 and 52.48 +/- 3.93, 2.69 +/- 0.22 and 41.06 +/- 9.13, 1.74 +/- 0.06 and 12.88 +/- 1.93, and 1.56 +/- 0.11 and 9.45 +/- 1.03, respectively. The statistical differences between the four groups and the analysis of the distribution of the parameters of Ca(2+) release and clearance show that there is a continuum from slow to fast, that parallels the MHC continuum from pure type I to pure IIB. However, type IIA fibres behave more like IIX and IIB fibres regarding Ca(2+) release but closer to type I fibres regarding Ca(2+) clearance. In conclusion, we show for the first time the diversity of Ca(2+) transients for the whole continuum of fibre types and correlate this functional diversity with the structural and biochemical diversity of the skeletal muscle fibres.

Role of TRPC1 channel in skeletal muscle function

Skeletal muscle contraction is reputed not to depend on extracellular Ca2+. Indeed, stricto sensu, excitation-contraction coupling does not necessitate entry of Ca2+. However, we previously observed that, during sustained activity (repeated contractions), entry of Ca2+ is needed to maintain force production. In the present study, we evaluated the possible involvement of the canonical transient receptor potential (TRPC)1 ion channel in this entry of Ca2+ and investigated its possible role in muscle function. Patch-clamp experiments reveal the presence of a small-conductance channel (13 pS) that is completely lost in adult fibers from TRPC1(-/-) mice. The influx of Ca2+ through TRPC1 channels represents a minor part of the entry of Ca(2+) into muscle fibers at rest, and the activity of the channel is not store dependent. The lack of TRPC1 does not affect intracellular Ca2+ concentration ([Ca2+](i)) transients reached during a single isometric contraction. However, the involvement of TRPC1-related Ca2+ entry is clearly emphasized in muscle fatigue. Indeed, muscles from TRPC1(-/-) mice stimulated repeatedly progressively display lower [Ca2+](i) transients than those observed in TRPC1(+/+) fibers, and they also present an accentuated progressive loss of force. Interestingly, muscles from TRPC1(-/-) mice display a smaller fiber cross-sectional area, generate less force per cross-sectional area, and contain less myofibrillar proteins than their controls. They do not present other signs of myopathy. In agreement with in vitro experiments, TRPC1(-/-) mice present an important decrease of endurance of physical activity. We conclude that TRPC1 ion channels modulate the entry of Ca(2+) during repeated contractions and help muscles to maintain their force during sustained repeated contractions.

Time‐of‐Day Effects on Myoelectric and Mechanical Properties of Muscle During Maximal and Prolonged Isokinetic Exercise

The aim of this study was to examine the time-of-day (TOD) effects in myoelectric and mechanical properties of muscle during a maximal and prolonged isokinetic exercise. Twelve male subjects were asked to perform 50 maximal voluntary contractions (MVC) of the knee extensor muscles at a constant angular velocity of 2.09 rad . sec(-1), at 06 : 00 and 18 : 00 h. Torque and electromyographic (EMG) parameters were recorded for each contraction, and the ratio between these values was calculated to evaluate variations of the neuromuscular efficiency (NME) with fatigue and with TOD. The results indicated that maximal torque values (T(45)Max) was significantly higher (7.73%) in the evening than in the morning (p<0.003). The diurnal variation in torque decrease was used to define two phases. During the first phase (1st to the 26th repetition), torque values decreased fast and values were higher in the evening than in the morning, and during the second phase (27th to the 50th repetition), torque decreased slightly and reached a floor value that appeared constant with TOD. The EMG parameters (Root Mean Square; RMS) were modified with fatigue, but were not TOD dependent. The NME decrease-significantly with fatigue, showing that peripheral factors were mainly involved in the torque decrease. Furthermore, NME decrease was greater at 18 : 00 than at 06 : 00 h for the vastus medialis (p<0.05) and the vastus lateralis muscles (p<0.002), and this occurred during the first fatigue phase of the exercise. In conclusion, the diurnal variation of the muscle fatigue observed during a maximal and prolonged isokinetic exercise seems to reflect on the muscle, with a greater contractile capacity but a higher fatigability in the evening compared to the morning.

Mechanisms of excitation-contraction uncoupling relevant to activity-induced muscle fatigueThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

If the free [Ca2+] in the cytoplasm of a skeletal muscle fiber is raised substantially for a period of seconds to minutes or to high levels just briefly, it leads to disruption of the normal excitation-contraction (E-C) coupling process and a consequent long-lasting decrease in force production. It appears that the disruption to the coupling occurs at the triad junction, where the voltage-sensor molecules (dihydropyridine receptors) normally interact with and open the Ca2+ release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum (SR). This disruption results in inadequate release of SR Ca2+ upon stimulation. Such E-C uncoupling may underlie the long-duration low-frequency fatigue that can occur after various types of exercise, as well as possibly being a contributing factor to the muscle weakness in certain muscle diseases. The process or processes causing the disruption of the coupling between the voltage sensors and the release channels is not known with certainty, but might be associated with structural changes at the triad junction, possibly caused by activation of the Ca2+-dependent protease, µ-calpain.

Sequential effects of GSNO and H2O2 on the Ca2+ sensitivity of the contractile apparatus of fast- and slow-twitch skeletal muscle fibers from the rat

Reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) and nitric oxide (NO), have been shown to differentially alter the Ca2+ sensitivity of the contractile apparatus of fast-twitch skeletal muscle, leading to the proposal that normal muscle function is controlled by perturbations in the amounts of these two groups of molecules ( 28 ). However, no previous studies have examined whether these opposing actions are retained when the contractile apparatus is subjected to both molecule types. Using mechanically skinned fast- and slow-twitch skeletal muscle fibers of the rat, we compared the effects of sequential addition of nitrosoglutathione (GSNO), a NO donor, and H2O2 on the Ca2+ sensitivity of the contractile apparatus. As expected from previous reports in fast-twitch fibers, when added separately, GSNO (1 mM) reduced the Ca2+ sensitivity of the contractile apparatus, whereas H2O2 (10 mM; added during contractions) increased the Ca2+ sensitivity of the contractile apparatus. When added sequentially to the same fiber, such that the oxidation by one molecule (e.g., GSNO) preceded the oxidation by the other (e.g., H2O2), and vice versa, the individual effects of both molecules on the Ca2+ sensitivity were retained. Interestingly, neither molecule had any effect on the Ca2+ sensitivity of slow-twitch skeletal muscle. The data show that H2O2 and GSNO retain the capacity to independently affect the contractile apparatus to modulate force. Furthermore, the absence of effects in slow-twitch muscle may further explain why this fiber type is relatively insensitive to fatigue.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

A mathematical model of fatigue in skeletal muscle force contraction

The ability for muscle to repeatedly generate force is limited by fatigue. The cellular mechanisms behind muscle fatigue are complex and potentially include breakdown at many points along the excitation-contraction pathway. In this paper we construct a mathematical model of the skeletal muscle excitation-contraction pathway based on the cellular biochemical events that link excitation to contraction. The model includes descriptions of membrane voltage, calcium cycling and crossbridge dynamics and was parameterised and validated using the response characteristics of mouse skeletal muscle to a range of electrical stimuli. This model was used to uncover the complexities of skeletal muscle fatigue. We also parameterised our model to describe force kinetics in fast and slow twitch fibre types, which have a number of biochemical and biophysical differences. How these differences interact to generate different force/fatigue responses in fast- and slow- twitch fibres is not well understood and we used our modelling approach to bring new insights to this relationship.

Chloride conductance in the transverse tubular system of rat skeletal muscle fibres: importance in excitation-contraction coupling and fatigue

Contraction in skeletal muscle fibres is governed by excitation of the transverse-tubular (t-) system, but the properties of the t-system and their importance in normal excitability are not well defined. Here we investigate the properties of the t-system chloride conductance using rat skinned muscle fibres in which the sarcolemma has been mechanically removed but the normal excitation-contraction coupling mechanism kept functional. When the t-system chloride conductance was eliminated, either by removal of all Cl(-) or by block of the chloride channels with 9-anthracene carboxylic acid (9-AC) or by treating muscles with phorbol 12,13-dibutyrate, there was a marked reduction in the threshold electric field intensity required to elicit a t-system action potential (AP) and twitch response. Calculations of the t-system chloride conductance indicated that it constitutes a large proportion of the total chloride conductance observed in intact fibres. Blocking the chloride conductance increased the size of the twitch response and was indicative that Cl(-) normally carries part of the repolarizing current across the t-system membrane on each AP. Block of the t-system chloride conductance also reduced tetanic force responses at higher frequency stimulation (100 Hz) and greatly reduced twitch responses in the period shortly after a brief tetanus, owing to rapid loss of t-system excitability during the AP train. Blocking activity of the Na(+)-K(+) pump in the t-system membrane caused loss of excitability owing to K(+) build-up in the sealed t-system, and this occurred approximately 3-4 times faster when the chloride conductance was blocked. These findings show that the t-system chloride conductance plays a vital role during normal activity by countering the effects of K(+) accumulation in the t-system and maintaining muscle excitability.

Blood flow restriction during low-intensity resistance exercise increases S6K1 phosphorylation and muscle protein synthesis

Low-intensity resistance exercise training combined with blood flow restriction (REFR) increases muscle size and strength as much as conventional resistance exercise with high loads. However, the cellular mechanism(s) underlying the hypertrophy and strength gains induced by REFR are unknown. We have recently shown that both the mammalian target of rapamycin (mTOR) signaling pathway and muscle protein synthesis (MPS) were stimulated after an acute bout of high-intensity resistance exercise in humans. Therefore, we hypothesized that an acute bout of REFR would enhance mTOR signaling and stimulate MPS. We measured MPS and phosphorylation status of mTOR-associated signaling proteins in six young male subjects. Subjects were studied once during blood flow restriction (REFR, bilateral leg extension exercise at 20% of 1 repetition maximum while a pressure cuff was placed on the proximal end of both thighs and inflated at 200 mmHg) and a second time using the same exercise protocol but without the pressure cuff [control (Ctrl)]. MPS in the vastus lateralis muscle was measured by using stable isotope techniques, and the phosphorylation status of signaling proteins was determined by immunoblotting. Blood lactate, cortisol, and growth hormone were higher following REFR compared with Ctrl (P < 0.05). Ribosomal S6 kinase 1 (S6K1) phosphorylation, a downstream target of mTOR, increased concurrently with a decreased eukaryotic translation elongation factor 2 (eEF2) phosphorylation and a 46% increase in MPS following REFR (P < 0.05). MPS and S6K1 phosphorylation were unchanged in the Ctrl group postexercise. We conclude that the activation of the mTOR signaling pathway appears to be an important cellular mechanism that may help explain the enhanced muscle protein synthesis during REFR.