Inhibition of cross-bridge formation has no effect on contraction-associated phosphorylation of p38 MAPK in mouse skeletal muscle

Creatine Kinase Equilibration and ΔGATP over an Extended Range of Physiological Conditions: Implications for Cellular Energetics, Signaling, and Muscle Performance

In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK Keq″) over wide but physiologically and experimentally relevant ranges of pH, Mg2+ and temperature. Our empirical formula for CK Keq″ is based on experimental measurements. It can be used to estimate [ADP] when [ADP] is below the resolution of experimental measurements, a typical situation because [ADP] is on the order of micromolar concentrations in living cells and may be much lower in many in vitro experiments. Accurate prediction of [ADP] is essential for in vivo studies of cellular energetics and metabolism and for in vitro studies of ATP-dependent enzyme function under near-physiological conditions. With [ADP], we were able to obtain improved estimates of ΔGATP, necessitating the reinvestigation of previously reported ADP- and ΔGATP-dependent processes. Application to actomyosin force generation in muscle provides support for the hypothesis that, when [Pi] varies and pH is not altered, the maximum Ca2+-activated isometric force depends on ΔGATP in both living and permeabilized muscle preparations. Further analysis of the pH studies introduces a novel hypothesis around the role of submicromolar ADP in force generation.

Strength Training: In Search of Optimal Strategies to Maximize Neuromuscular Performance

Training with low-load exercise performed under blood flow restriction can augment muscle hypertrophy and maximal strength to a similar extent as the classical high-load strength training method. However, the blood flow restriction method elicits only minor neural adaptations. In an attempt to maximize training-related gains, we propose using other protocols that combine high voluntary activation, mechanical tension, and metabolic stress.

Do metabolites that are produced during resistance exercise enhance muscle hypertrophy?

Many reviews conclude that metabolites play an important role with respect to muscle hypertrophy during resistance exercise, but their actual physiologic contribution remains unknown. Some have suggested that metabolites may work independently of muscle contraction, while others have suggested that metabolites may play a secondary role in their ability to augment muscle activation via inducing fatigue. Interestingly, the studies used as support for an anabolic role of metabolites use protocols that are not actually designed to test the importance of metabolites independent of muscle contraction. While there is some evidence in vitro that metabolites may induce muscle hypertrophy, the only study attempting to answer this question in humans found no added benefit of pooling metabolites within the muscle post-exercise. As load-induced muscle hypertrophy is thought to work via mechanotransduction (as opposed to being metabolically driven), it seems likely that metabolites simply augment muscle activation and cause the mechanotransduction cascade in a larger proportion of muscle fibers, thereby producing greater muscle growth. A sufficient time under tension also appears necessary, as measurable muscle growth is not observed after repeated maximal testing. Based on current evidence, it is our opinion that metabolites produced during resistance exercise do not have anabolic properties per se, but may be anabolic in their ability to augment muscle activation. Future studies are needed to compare protocols which produce similar levels of muscle activation, but differ in the magnitude of metabolites produced, or duration in which the exercised muscles are exposed to metabolites.

Selective inhibition of ATPase activity during contraction alters the activation of p38 MAP kinase isoforms in skeletal muscle

Muscle contractions strongly activate p38 MAP kinases, but the precise contraction-associated sarcoplasmic event(s) (e.g., force production, energetic demands, and/or calcium cycling) that activate these kinases are still unclear. We tested the hypothesis that during contraction the phosphorylation of p38 isoforms is sensitive to the increase in ATP demand relative to ATP supply. Energetic demands were inhibited using N-benzyl-p-toluene sulphonamide (BTS, type II actomyosin) and cyclopiazonic acid (CPA, SERCA). Extensor digitorum longus muscles from Swiss Webster mice were incubated in Ringer's solution (37°C) with or without inhibitors and then stimulated at 10 Hz for 15 min. Muscles were immediately freeze-clamped for metabolite and Western blot analysis. BTS and BTS + CPA treatment decreased force production by 85%, as measured by the tension time integral, while CPA alone potentiated force by 310%. In control muscles, contractions resulted in a 73% loss of ATP content and a concomitant sevenfold increase in IMP content, a measure of sustained energetic imbalance. BTS or CPA treatment lessened the loss of ATP, but BTS + CPA treatment completely eliminated the energetic imbalance since ATP and IMP levels were nearly equal to those of non-stimulated muscles. The independent inhibition of cytosolic ATPase activities had no effect on contraction-induced p38 MAPK phosphorylation, but combined treatment prevented the increase in phosphorylation of the γ isoform while the α/β isoforms unaffected. These observations suggest that an energetic signal may trigger phosphorylation of the p38γ isoform and also may explain how contractions differentially activate signaling pathways.

Androgen receptors in muscle fibers induce rapid loss of force but not mass: implications for spinal bulbar muscular atrophy

INTRODUCTION

Testosterone (T) induces motor dysfunction in transgenic (Tg) mice that overexpress wild-type androgen receptor (AR) in skeletal muscles. Because many genes implicated in motor neuron disease are expressed in skeletal muscle, mutant proteins may act in muscle to cause dysfunction in motor neuron disease.

METHODS

We examined contractile properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles in vitro after 5 and 3 days of T treatment in motor-impaired Tg female mice.

RESULTS

Both muscles showed deficits in tetanic force after 5 days of T treatment, without losses in muscle mass, protein content, or fiber number. After 3 days of T treatment, only SOL showed a deficit in tetanic force comparable to that of 5 days of treatment. In both treatments, EDL showed slowed twitch kinetics, whereas SOL showed deficits in the twitch/tetanus ratio.

CONCLUSIONS

These results suggest calcium-handling mechanisms in muscle fibers are defective in motor-impaired mice.

Molecular cloning and in silico analysis of the duck (Anas platyrhynchos) MEF2A gene cDNA and its expression profile in muscle tissues during fetal development

The role of myogenic enhancer transcription factor 2a (MEF2A) in avian muscle during fetal development is unknown. In this work, we cloned the duck MEF2A cDNA sequence (GenBank accession no. HM460752) and examined its developmental expression profiles in cardiac muscle, non-vascular smooth muscle and skeletal muscle. Duck MEF2A cDNA comprised 1479 bp encoding 492 amino acid residues. In silico analysis showed that MEF2A contained MADS (MCM1, AGAMOUS, DEFICIENS and SRF - serum response factor), MEF2 and mitogen-activated protein kinase (MAPK) transcription domains with high homology to related proteins in other species. Modified sites in these domains were conserved among species and several variants were found. Quantitative PCR showed that MEF2A was expressed in all three muscles at each developmental stage examined, with the expression in smooth muscle being higher than in the other muscles. These results indicate that the conserved domains of duck MEF2A, including the MADS and MEF2 domains, are important for MEF2A transcription factor function. The expression of MEF2A in duck smooth muscle and cardiac muscle suggests that MEF2A plays a role in these two tissues.

The effects of active and passive stretching on muscle length

Active stretch is necessary for regulating muscle fiber length (ie, the number of series sarcomeres). Elevated cytoplasmic calcium is the proposed component of contractile activity required to activate signaling pathways for sarcomere number regulation. Passive stretch reduces muscle tissue stiffness, most likely by signaling connective tissue remodeling via fibroblasts. Passive stretch may induce sarcomere addition if the muscle fibers are lengthened sufficiently to raise cytoplasmic calcium through stretch-activated calcium channels. The magnitude of stretch in vivo is limited by the physiologic range of movement and stretch pain tolerance. The greatest effect of stretching muscle fibers is expected when the lengthening exceeds the optimum fiber length (Lo).

A myosin II ATPase inhibitor reduces force production, glucose transport, and phosphorylation of AMPK and TBC1D1 in electrically stimulated rat skeletal muscle

Contraction-stimulated glucose transport by skeletal muscle appears to be caused by the cumulative effects of multiple inputs [potentially including AMP-activated protein kinase (AMPK), Ca(2+) flux, and force production], making it challenging to isolate the roles of these putative regulatory factors. To distinguish the effects of force production from the direct consequences of Ca(2+) flux, the predominantly type II rat epitrochlearis muscle was incubated without (vehicle) or with N-benzyl-p-toluenesulfonamide (BTS), a highly specific myosin II ATPase inhibitor that prevents force production by electrically stimulated (ES) type II fibers without altering cytosolic Ca(2+). In ES muscles, BTS vs. vehicle had an 84% reduction in force production and a 57% decrement in contraction-stimulated 3-O-methylglucose transport (3MGT). BTS did not alter the ES increase in phosphorylation of CaMKII (indicative of cytosolic Ca(2+)) or the amount of glycogen depletion. ES caused significant reductions in ATP (48%) and phosphocreatine (67%) concentrations for vehicle-treated muscles. For BTS-treated muscles, ES did not reduce ATP and caused only a 42% decrease in phosphocreatine. There was an ES increase in phosphorylation of AMPK, acetyl-CoA carboxylase (an AMPK substrate), and TBC1D1 for vehicle-treated muscles but not for BTS-treated muscles. These results point toward an essential role for tension-related events, including AMPK activation, in the 57% contraction-stimulated increase in 3MGT that was inhibited by BTS and further suggest a possible role for TBC1D1 phosphorylation. Non-tension-related events (e.g., increased cytosolic Ca(2+) rather than increased AMPK and TBC1D1 phosphorylation) are implicated in the contraction-stimulated increase in 3MGT that persisted in the presence of BTS.

Kinase-specific responsiveness to incremental contractile activity in skeletal muscle with low and high mitochondrial content

Muscle contractions activate protein kinases, leading to signal transduction. We hypothesized that kinase activation would be influenced by mitochondrial content, as well as by contractile activity-induced increases in muscle O(2) consumption (Vo(2)). Kinase phosphorylation in high-oxidative red and low-oxidative white tibialis anterior (TA) muscle (RTA and WTA, respectively) with 2.5-fold differences in mitochondrial content were compared. Stimulation of the TA muscle elicited large increases in Vo(2) (3- to 6-fold and 4- to 60-fold above resting levels in WTA and RTA, respectively). At rest, AMP-activated protein kinase (AMPK), p38, p42, and p44 activation were nearly twofold greater in WTA than in RTA, suggesting an inverse relationship between mitochondrial content and kinase activation in resting muscle. During contractions, similar degrees of phosphorylation in RTA and WTA were evident as a function of Vo(2) for p38 and p42. During increases in Vo(2) up to sixfold above rest, greater responses were observed in RTA than in WTA for AMPK and p44, whereas Akt activation was greater in WTA. In RTA, elevations in Vo(2) elicited increases in AMPK and p44 activation, whereas Akt, p38, and p42 were less sensitive to increments in Vo(2). Reactive oxygen species (ROS) production was greater in mitochondria from white muscle, but when it was calculated in the context of the whole muscle, ROS production was twofold greater in red than in white myofibers. Thus mitochondrial content influences ROS production and is inversely related to kinase activation in resting muscle. During contractions, kinases are differentially sensitive to contraction-induced increments in Vo(2), suggesting that muscle mitochondrial content is important, but it is not the sole determinant of kinase activation during exercise of different intensities.

Causes of excitation-induced muscle cell damage in isometric contractions: mechanical stress or calcium overload?

Prolonged or unaccustomed exercise leads to muscle cell membrane damage, detectable as release of the intracellular enzyme lactic acid dehydrogenase (LDH). This is correlated to excitation-induced influx of Ca2+, but it cannot be excluded that mechanical stress contributes to the damage. We here explore this question using N-benzyl-p-toluene sulfonamide (BTS), which specifically blocks muscle contraction. Extensor digitorum longus muscles were prepared from 4-wk-old rats and mounted on holders for isometric contractions. Muscles were stimulated intermittently at 40 Hz for 15-60 min or exposed to the Ca2+ ionophore A23187. Electrical stimulation increased 45Ca influx 3-5 fold. This was followed by a progressive release of LDH, which was correlated to the influx of Ca2+. BTS (50 microM) caused a 90% inhibition of contractile force but had no effect on the excitation-induced 45Ca influx. After stimulation, ATP and creatine phosphate levels were higher in BTS-treated muscles, most likely due to the cessation of ATP-utilization for cross-bridge cycling, indicating a better energy status of these muscles. No release of LDH was observed in BTS-treated muscles. However, when exposed to anoxia, electrical stimulation caused a marked increase in LDH release that was not suppressed by BTS but associated with a decrease in the content of ATP. Dynamic passive stretching caused no increase in muscle Ca2+ content and only a minor release of LDH, whereas treatment with A23187 markedly increased LDH release both in control and BTS-treated muscles. In conclusion, after isometric contractions, muscle cell membrane damage depends on Ca2+ influx and energy status and not on mechanical stress.

Energy conservation attenuates the loss of skeletal muscle excitability during intense contractions

High-frequency stimulation of skeletal muscle has long been associated with ionic perturbations, resulting in the loss of membrane excitability, which may prevent action potential propagation and result in skeletal muscle fatigue. Associated with intense skeletal muscle contractions are large changes in muscle metabolites. However, the role of metabolites in the loss of muscle excitability is not clear. The metabolic state of isolated rat extensor digitorum longus muscles at 30 degrees C was manipulated by decreasing energy expenditure and thereby allowed investigation of the effects of energy conservation on skeletal muscle excitability. Muscle ATP utilization was reduced using a combination of the cross-bridge cycling blocker N-benzyl-p-toluene sulfonamide (BTS) and the SR Ca2+ release channel blocker Na-dantrolene, which reduce activity of the myosin ATPase and SR Ca2+-ATPase. Compared with control muscles, the resting metabolites ATP, phosphocreatine, creatine, and lactate, as well as the resting muscle excitability as measured by M-waves, were unaffected by treatment with BTS plus dantrolene. Following 20 or 30 s of continuous 60-Hz stimulation, BTS-plus-dantrolene-treated muscles showed a 25% lower ATP utilization compared with control muscles. Furthermore, the ability of muscles to maintain excitability during high-frequency stimulation was significantly improved in BTS-plus-dantrolene-treated muscles, indicating a strong link between metabolites, energetic state, and the excitability of the muscle.

Mechanical load plays little role in contraction-mediated glucose transport in mouse skeletal muscle

The factors responsible for control of glucose transport during exercise are not fully understood. We investigated the role of mechanical load in contraction-mediated glucose transport in an isolated muscle preparation. Mouse extensor digitorum longus muscles were stimulated with repeated contractions for 10 min with or without N-benzyl-p-toluene sulphonamide (BTS, an inhibitor of myosin II ATPase) to block crossbridge activity. BTS inhibited force production during repeated contraction to approximately 5% of control. In contrast, BTS had little effect on glucose transport in the basal state (control = 0.55 +/- 0.04; BTS = 0.47 +/- 0.09 micromol (20 min)(-1) ml(-1)) or after contraction (control = 2.27 +/- 0.15; BTS = 2.10 +/- 0.16 micromol (20 min)(-1) ml(-1)). BTS did not significantly alter the contraction-mediated changes in high-energy phosphates, glutathione status (a measure of oxidant status) or AMP-activated protein kinase activity. In conclusion, these data show that mechanical load plays little role in contraction-mediated glucose transport. Instead, it is likely that the increased glucose transport during contraction is a consequence of the increase in myoplasmic Ca(2+) and the subsequent alterations in metabolism, e.g. increased energy turnover and production of reactive oxygen species.

Influence of activation frequency on cellular signalling pathways during fatiguing contractions in rat skeletal muscle

Activation frequency as a regulator of physiological responses in skeletal muscle, independent of contractile force, has received little attention. Here, the length-tension and force-frequency relationships were employed to keep active contractile force equal, despite a twofold difference in stimulation frequency (15 versus 30 Hz). Rat tibialis anterior muscles were tested in situ using 15 Hz stimulation at optimal length (15 Hz) and 30 Hz stimulation at shortened and lengthened positions (30 Hz(sub) and 30 Hz(supra)). Muscles were subjected to 1, 15, 30 and 80 Hz stimulation trains before and after 2 min of fatiguing stimulation. The principal findings were that the two 30 Hz protocols produced greater 38 kDa MAPK (p38) phosphorylation than the 15 Hz protocol (1.4- to 1.5-fold versus 1.1-fold), as well as greater fatigue (65-78 versus 43% decline in contraction force). In contrast, c-jun amino terminal kinase (JNK) phosphorylation appeared most responsive to total (active plus passive) tension such that the changes followed the pattern: 30 Hz(supra) > 15 Hz > 30 Hz(sub), while 44 and 42 kDa extracellular regulated kinase (ERK1/2) phosphorylation was not significantly increased in response to any of the protocols studied. Neither glycogen depletion nor myofibre damage accounted for any of the findings, but a decline in muscle excitation (m-wave) may have contributed to the greater fatigue seen at higher frequencies. These data suggest that neuromuscular activation frequency can influence certain signalling pathways in skeletal muscle, independent of force production.

α7β1-Integrin regulates mechanotransduction and prevents skeletal muscle injury

α7β1-Integrin links laminin in the extracellular matrix with the cell cytoskeleton and therein mediates transduction of mechanical forces into chemical signals. Muscle contraction and stretching ex vivo result in activation of intracellular signaling molecules that are integral to postexercise injury responses. Because α7β1-integrin stabilizes muscle and provides communication between the matrix and cytoskeleton, the role of this integrin in exercise-induced cell signaling and skeletal muscle damage was assessed in wild-type and transgenic mice overexpressing the α7BX2 chain. We report here that increasing α7β1-integrin inhibits phosphorylation of molecules associated with muscle damage, including the mitogen-activated protein kinases (JNK, p38, and ERK), following downhill running. Likewise, activation of molecules associated with hypertrophy (AKT, mTOR, and p70S6k) was diminished in mice overexpressing integrin. While exercise resulted in Evans blue dye-positive fibers, an index of muscle damage, increased integrin protected mice from injury. Moreover, exercise leads to an increase in α7β1protein. These experiments provide the first evidence that α7β1-integrin is a negative regulator of mechanotransduction in vivo and provides resistance to exercise-induced muscle damage.

Cross bridges account for only 20% of total ATP consumption during submaximal isometric contraction in mouse fast-twitch skeletal muscle

It is generally believed that cross bridges account for >50% of the total ATP consumed by skeletal muscle during contraction. We investigated the effect of N-benzyl-p-toluene sulfonamide (BTS), an inhibitor of myosin ATPase, on muscle force production and energy metabolism under near-physiological conditions (50-Hz stimulation frequency at 30 degrees C results in 35% of maximal force). Extensor digitorum longus muscles from mice were isolated and stimulated to perform continuous isometric tetanic contractions. Metabolites of energy metabolism were analyzed with fluorometric techniques. ATP turnover was estimated from the changes in phosphocreatine (PCr), ATP, and lactate (-2DeltaATP - DeltaPCr + [1.5Deltalactate]). During contractions (2-10 s), BTS decreased force production to approximately 5% of control. Under these conditions, BTS inhibited ATP turnover by only 18-25%. ATP turnover decreased markedly and similarly with and without BTS as the duration of contraction progressed. In conclusion, cross bridges (i.e., actomyosin ATPase) account for only a small fraction (approximately 20%) of the ATP consumption during contraction in mouse fast-twitch skeletal muscle under near-physiological conditions, suggesting that ion pumping is the major energy-consuming process.

Measurement of activation energy and oxidative phosphorylation onset kinetics in isolated muscle fibers in the absence of cross-bridge cycling

This study utilized N-benzyl-p-toluene sulfonamide (BTS), a potent inhibitor of cross-bridge cycling, to measure 1) the relative metabolic costs of cross-bridge cycling and activation energy during contraction, and 2) oxygen uptake kinetics in the presence and absence of myosin ATPase activity, in isolated Xenopus laevis muscle fibers. Isometric tension development and either cytosolic Ca2+ concentration ([Ca2+]c) or intracellular Po2 (PiO2) were measured during contractions at 20 degrees C in control conditions (Con) and after exposure to 12.5 microM BTS. BTS attenuated tension development to 5+/-0.4% of Con but did not affect either resting or peak [Ca2+]c during repeated isometric contractions. To determine the relative metabolic cost of cross-bridge cycling, we measured the fall in PiO2) (DeltaPiO2; a proxy for Vo2) during contractions in Con and BTS groups. BTS attenuated DeltaP(iO2) by 55+/-6%, reflecting the relative ATP cost of cross-bridge cycling. Thus, extrapolating DeltaPiO2 to a value that would occur at 0% tension suggests that actomyosin ATP requirement is approximately 58% of overall ATP consumption during isometric contractions in mixed fiber types. BTS also slowed the fall in PiO2) (time to 63% of overall DeltaPiO2) from 75+/-9 s (Con) to 101+/-9 s (BTS) (P<0.05), suggesting an important role of the products of ATP hydrolysis in determining the Vo2 onset kinetics. These results demonstrate in isolated skeletal muscle fibers that 1) activation energy accounts for a substantial proportion (approximately 42%) of total ATP cost during isometric contractions, and 2) despite unchanged [Ca2+]c transients, a reduced rate of ATP consumption results in slower Vo2 onset kinetics.

Interaction between signalling pathways involved in skeletal muscle responses to endurance exercise

The purpose of this review is to summarise the latest literature on the signalling pathways involved in transcriptional modulations of genes that encode contractile and metabolic proteins in response to endurance exercise. A special attention has been paid to the cooperation between signalling pathways and coordinated expression of protein families that establish myofibre phenotype. Calcium acts as a second messenger in skeletal muscle during exercise, conveying neuromuscular activity into changes in the transcription of specific genes. Three main calcium-triggered regulatory pathways acting through calcineurin, Ca(2+)-calmodulin-dependent protein kinases (CaMK) and Ca(2+)-dependent protein kinase C, transduce alterations in cytosolic calcium concentration to target genes. Calcineurin signalling, the most important of these Ca(2+)-dependent pathways, stimulates the activation of many slow-fibre gene expression, including genes encoding proteins involved in contractile process, Ca(2+) uptake and energy metabolism. It involves the interaction between multiple transcription factors and the collaboration of other Ca(2+)-dependent CaMKs. Although members of mitogen-activated protein kinase (MAPK) pathways are activated during exercise, their integration into other signalling pathways remains largely unknown. The peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator-1alpha (PGC-1alpha) constitutes a pivotal factor of the circuitry which coordinates mitochondrial biogenesis and which couples to the expression of contractile and metabolic genes with prolonged exercise.

Role of Ca2+/calmodulin-dependent kinases in skeletal muscle plasticity

In skeletal muscle, the increase in intracellular Ca(2+) resulting from motor activation plays a key role in both contractile activity-dependent and fiber type-specific gene expression. These motor activation-dependent signals are linked to the amplitude and duration of the Ca(2+) transients that are decoded downstream by Ca(2+)-dependent transcriptional pathways. Evidence is mounting that the Ca(2+)/calmodulin-dependent kinases (CaMKs) such as CaMKII play an important role in regulating oxidative enzyme expression, mitochondrial biogenesis, and expression of fiber type-specific myofibrillar proteins. CaMKIV has been shown to promote mitochondrial biogenesis and a mild fast-to-slow fiber type transition but has recently been shown to not be required for activity-dependent changes in muscle phenotype. CaMKII is known to decode frequency-dependent information and is activated during hypertrophic growth and endurance adaptations and also is upregulated during muscle atrophy. CaMKII has also been shown to remain active in a Ca(2+)-independent manner after acute and prolonged exercise, and, therefore, is implicated as a mechanism for muscle memory. This mechanism can sense altered functional demands and trigger activation of an adaptational response that is dose dependently related to the activation level. This class of enzymes may therefore be the ideal decoders of information encoded by the intensity, frequency, and duty cycle of muscle activation and thus translate level of muscle activation into phenotypic adaptations through regulation of important muscle genes.

N-Benzyl-p-toluene sulphonamide allows the recording of trains of intracellular action potentials from nerve-stimulated intact fast-twitch skeletal muscle of the rat

In skeletal muscle, the intracellular recording of trains of action potentials is difficult owing to the movement of the muscle upon stimulation. A potential tool for the removal of muscle movement is the cross-bridge cycle blocker, N-benzyl-p-toluene sulphonamide (BTS), although the effects of BTS on the passive and active membrane properties of intact muscle fibres are not known. Rat extensor digitorum longus (EDL) muscle was used to show that 50 mum BTS reduced tetanic force to approximately 10% of control force, without markedly altering muscle excitability. Incubation with BTS did not alter intracellular K+ content or Na+-K+ pump activity, but produced minor decreases in intracellular Na+ content (7%), resting 22Na+ influx (14%) and excitation-induced 22Na+ influx (29%). Despite these alterations to Na+ fluxes, BTS did not impair muscle excitability, since membrane conductance, resting membrane potential (RMP), rheobase current and the amplitude, overshoot and maximum rate of depolarization of the action potential were all unaltered. However, BTS did induce a small (8%) decrease in the maximum rate of repolarization of the action potential and an increase in the refractory period. The minor effects of BTS on muscle membrane properties did not compromise the ability of the muscle to propagate action potentials, even during tetanic stimulation. In conclusion, BTS can be used successfully to reduce contractility, allowing the intracellular recording of action potentials during both twitch and tetanic contraction of nerve-stimulated muscle, thus making it an excellent tool for the study of electrophysiology in fast-twitch skeletal muscle.