Variations in cross-bridge attachment rate and tension with phosphorylation of myosin in mammalian skinned skeletal muscle fibers. Implications for twitch potentiation in intact muscle

Getting the skinny on thick filament regulation in cardiac muscle biology and disease

Thin (actin) filament accessory proteins are thought to be the regulatory force for muscle contraction in cardiac muscle; however, compelling new evidence suggests that thick (myosin) filament regulatory proteins are emerging as having independent and important roles in regulating cardiac muscle contraction. Key to these new findings is a growing body of evidence that point to an influential and, more recently, direct role for ventricular myosin light chain-2 (MLC2v) phosphorylation in regulating cardiac muscle contraction, function, and disease. This includes the discovery and characterization of a cardiac-specific myosin light chain kinase capable of phosphorylating MLC2v as well as a myosin phosphatase that dephosphorylates MLC2v in the heart, which provides added mechanistic insights on MLC2v regulation within cardiac muscle. Here, we review evidence for an emerging and critical role for MLC2v phosphorylation in regulating cardiac myosin cycling kinetics, function, and disease, based on recent studies performed in genetic mouse models and humans. We further provide new perspectives on future avenues for targeting these pathways as therapies in alleviating cardiac disease.

Hill's equation of muscle performance and its hidden insight on molecular mechanisms

Muscles shorten faster against light loads than they do against heavy loads. The hyperbolic equation first used by A.V. Hill over seven decades ago to illustrate the relationship between shortening velocity and load is still the predominant method used to characterize muscle performance, even though it has been regarded as purely empirical and lacking precision in predicting velocities at high and low loads. Popularity of the Hill equation has been sustained perhaps because of historical reasons, but its simplicity is certainly attractive. The descriptive nature of the equation does not diminish its role as a useful tool in our quest to understand animal locomotion and optimal design of muscle-powered devices like bicycles. In this Review, an analysis is presented to illustrate the connection between the historic Hill equation and the kinetics of myosin cross-bridge cycle based on the latest findings on myosin motor interaction with actin filaments within the structural confines of a sarcomere. In light of the new data and perspective, some previous studies of force-velocity relations of muscle are revisited to further our understanding of muscle mechanics and the underlying biochemical events, specifically how extracellular and intracellular environment, protein isoform expression, and posttranslational modification of contractile and regulatory proteins change the interaction between myosin and actin that in turn alter muscle force, shortening velocity, and the relationship between them.

Phosphorylation of myosin regulatory light chain has minimal effect on kinetics and distribution of orientations of cross bridges of rabbit skeletal muscle

Force production in muscle results from ATP-driven cyclic interactions of myosin with actin. A myosin cross bridge consists of a globular head domain, containing actin and ATP-binding sites, and a neck domain with the associated light chain 1 (LC1) and the regulatory light chain (RLC). The actin polymer serves as a "rail" over which myosin translates. Phosphorylation of the RLC is thought to play a significant role in the regulation of muscle relaxation by increasing the degree of skeletal cross-bridge disorder and increasing muscle ATPase activity. The effect of phosphorylation on skeletal cross-bridge kinetics and the distribution of orientations during steady-state contraction of rabbit muscle is investigated here. Because the kinetics and orientation of an assembly of cross bridges (XBs) can only be studied when an individual XB makes a significant contribution to the overall signal, the number of observed XBs was minimized to ∼20 by limiting the detection volume and concentration of fluorescent XBs. The autofluorescence and photobleaching from an ex vivo sample was reduced by choosing a dye that was excited in the red and observed in the far red. The interference from scattering was eliminated by gating the signal. These techniques decrease large uncertainties associated with determination of the effect of phosphorylation on a few molecules ex vivo with millisecond time resolution. In spite of the remaining uncertainties, we conclude that the state of phosphorylation of RLC had no effect on the rate of dissociation of cross bridges from thin filaments, on the rate of myosin head binding to thin filaments, and on the rate of power stroke. On the other hand, phosphorylation slightly increased the degree of disorder of active cross bridges.

A re-interpretation of the rate of tension redevelopment (k(TR)) in active muscle

A slackening to zero tension by large length release (~20%) and a restretch of active muscle fibres cause a fall and a redevelopment in tension. According to the model of Brenner (Proc Natl Acad Sci USA 85(9):3265-3269, 1988), the rate constant of tension redevelopment (k TR) is the sum of attachment and detachment rate constants, hence is limited by the fast reaction. Here we propose a model in which, after restretch, cross-bridges cycle many times by stretching series elastic elements, hence k(TR) is limited by a slow reaction. To set up this model, we made an assumption that the stepping rate (v) decreases linearly with tension (F), which is consistent with the Fenn effect. The distance traveled by a cross-bridge stretches series elastic elements with stiffness σ. With these assumptions, we set up a first order differential equation, which results in an exponential time course with the rate constant k(TR) = ση(0)ν(0)(1 - λ)/F(1), where λ = ν(1)/ν(0), η = step size, the subscript 0 indicates unloaded condition, and the subscript 1 indicate isometric condition. We demonstrate that the ATP hydrolysis rate (=[myosin head]/ν(0)) is proportionate to k(TR) as the ambient temperature is changed, and that the published data fit to this relationship well if λ = 0.28. We conclude that k(TR) is limited by the cross-bridge turnover rate; hence it represents the rate constant of the slowest reaction of the cross-bridge cycle, i.e. the ADP isomerization step before ADP is released.

Myosin phosphorylation and force potentiation in skeletal muscle: evidence from animal models

The contractile performance of mammalian fast twitch skeletal muscle is history dependent. The effect of previous or ongoing contractile activity to potentiate force, i.e. increase isometric twitch force, is a fundamental property of fast skeletal muscle. The precise manifestation of force potentiation is dependent upon a variety of factors with two general types being identified; staircase potentiation referring to the progressive increase in isometric twitch force observed during low frequency stimulation while posttetanic potentiation refers to the step-like increase in isometric twitch force observed following a brief higher frequency (i.e. tetanic) stimulation. Classic studies established that the magnitude and duration of potentiation depends on a number of factors including muscle fiber type, species, temperature, sarcomere length and stimulation paradigm. In addition to isometric twitch force, more recent work has shown that potentiation also influences dynamic (i.e. concentric and/or isotonic) force, work and power at a range of stimulus frequencies in situ or in vitro, an effect that may translate to enhanced physiological function in vivo. Early studies performed on both intact and permeabilized models established that the primary mechanism for this modulation of performance was phosphorylation of myosin, a modification that increased the Ca(2+) sensitivity of contraction. More recent work from a variety of muscle models indicates, however, the presence of a secondary mechanism for potentiation that may involve altered Ca(2+) handling. The primary purpose of this review is to highlight these recent findings relative to the physiological utility of force potentiation in vivo.

Skeletal and cardiac α-actin isoforms differently modulate myosin cross-bridge formation and myofibre force production

Multiple congenital myopathies, including nemaline myopathy, can arise due to mutations in the ACTA1 gene encoding skeletal muscle α-actin. The main characteristics of ACTA1 null mutations (absence of skeletal muscle α-actin) are generalized skeletal muscle weakness and premature death. A mouse model (ACTC(Co)/KO) mimicking these conditions has successfully been rescued by transgenic over-expression of cardiac α-actin in skeletal muscles using the ACTC gene. Nevertheless, myofibres from ACTC(Co)/KO animals generate less force than normal myofibres (-20 to 25%). To understand the underlying mechanisms, here we have undertaken a detailed functional study of myofibres from ACTC(Co)/KO rodents. Mechanical and X-ray diffraction pattern analyses of single membrane-permeabilized myofibres showed, upon maximal Ca(2+) activation and under rigor conditions, lower stiffness and disrupted actin-layer line reflections in ACTC(Co)/KO when compared with age-matched wild-types. These results demonstrate that in ACTC(Co)/KO myofibres, the presence of cardiac α-actin instead of skeletal muscle α-actin alters actin conformational changes upon activation. This later finely modulates the strain of individual actomyosin interactions and overall lowers myofibre force production. Taken together, the present findings provide novel primordial information about actin isoforms, their functional differences and have to be considered when designing gene therapies for ACTA1-based congenital myopathies.

The effect of including a series of isometric conditioning contractions to the rowing warm-up on 1,000-m rowing ergometer time trial performance

Rowing requires strength, power, and strength-endurance for optimal performance. A rowing-based warm-up could be enhanced by exploiting the postactivation potentiation (PAP) phenomenon, acutely enhancing power output at the beginning of a race where it is needed most. Minimal research has investigated the effects of PAP on events of longer duration (i.e. 1,000-m rowing). The purpose of this research was to investigate the effects of PAP on 1,000-m rowing ergometer performance through the use of 2 different warm-up procedures: (a) a rowing warm-up combined with a series of isometric conditioning contractions, known as the potentiated warm-up (PW), and (b) a rowing warm-up only (NW). The isometric conditioning contractions in the PW were performed by "pulling" an immovable handle on the rowing ergometer, consisting of 5 sets of 5 seconds (2 seconds at submaximal intensity, and 3 seconds at maximal intensity), with a 15-second recovery between sets. The 1,000-m rowing ergometer time trial was performed after each warm-up condition, whereby mean power output, mean stroke rate, and split time were assessed every 100 m. Ten Australian national level rowers served as the subjects and performed both conditions in a counterbalanced order on separate days. The PW reduced 1,000-m time by 0.8% (p > 0.05). The PW improved mean power output by 6.6% (p < 0.01) and mean stroke rate by 5.2% (p < 0.01) over the first 500 m; resulting in a reduction of 500-m time by 1.9% (p < 0.01), compared with the NW. It appears that the inclusion of isometric conditioning contractions to the rowing warm-up enhance short-term rowing ergometer performance (especially at the start of a race) to a greater extent than a rowing warm-up alone.

Myosin regulatory light chain (RLC) phosphorylation change as a modulator of cardiac muscle contraction in disease

Understanding how cardiac myosin regulatory light chain (RLC) phosphorylation alters cardiac muscle mechanics is important because it is often altered in cardiac disease. The effect this protein phosphorylation has on muscle mechanics during a physiological range of shortening velocities, during which the heart generates power and performs work, has not been addressed. We have expressed and phosphorylated recombinant Rattus norvegicus left ventricular RLC. In vitro we have phosphorylated these recombinant species with cardiac myosin light chain kinase and zipper-interacting protein kinase. We compare rat permeabilized cardiac trabeculae, which have undergone exchange with differently phosphorylated RLC species. We were able to enrich trabecular RLC phosphorylation by 40% compared with controls and, in a separate series, lower RLC phosphorylation to 60% of control values. Compared with the trabeculae with a low level of RLC phosphorylation, RLC phosphorylation enrichment increased isometric force by more than 3-fold and peak power output by more than 7-fold and approximately doubled both maximum shortening speed and the shortening velocity that generated peak power. We augmented these measurements by observing increased RLC phosphorylation of human and rat HF samples from endocardial left ventricular homogenate. These results demonstrate the importance of increased RLC phosphorylation in the up-regulation of myocardial performance and suggest that reduced RLC phosphorylation is a key aspect of impaired contractile function in the diseased myocardium.

Validation of jump squats as a practical measure of post-activation potentiation

To determine if post-activation potentiation (PAP) can augment sports performance, it is pertinent that researchers be confident that any enhancement in performance is attributable to the PAP phenomenon. However, obtaining mechanistic measures of PAP in the daily training environment of highly trained athletes is impractical. We sought to validate jump squats as a practical measure with ecological validity to sports performance against a mechanistic measure of PAP. We assessed the evoked muscle twitch properties of the knee extensors and jump squat kinetics of 8 physically trained males in response to a 5-repetition-maximum back squat conditioning stimulus (CS). Evoked muscle twitch, followed by 3 jump squats, was assessed before and at 4, 8, and 12 min post CS. Time intervals were assessed on separate occasions using a Latin square design. Linear regression was used to determine the relationship between post–pre changes in kinetic variables and muscle twitch peak force (Ft) and twitch rate of force development (RFDt). Large correlations were observed for both concentric relative and absolute mean power and Ft (r = 0.50 ± 0.30) and RFDt (r = 0.56 ± 0.27 and r = 0.58 ± 0.26). Concentric rate of force development (RFD) showed moderate correlations with Ft (r = 0.45 ± 0.33) and RFDt (r = 0.49 ± 0.32). Small-to-moderate correlations were observed for a number of kinetic variables (r = −0.42–0.43 ± 0.32–0.38). Jump squat concentric mean power and RFD are valid ecological measures of muscle potentiation, capable of detecting changes in athletic performance in response to the PAP phenomenon.

Fast skeletal muscle troponin activation increases force of mouse fast skeletal muscle and ameliorates weakness due to nebulin-deficiency

The effect of the fast skeletal muscle troponin activator, CK-2066260, on calcium-induced force development was studied in skinned fast skeletal muscle fibers from wildtype (WT) and nebulin deficient (NEB KO) mice. Nebulin is a sarcomeric protein that when absent (NEB KO mouse) or present at low levels (nemaline myopathy (NM) patients with NEB mutations) causes muscle weakness. We studied the effect of fast skeletal troponin activation on WT muscle and tested whether it might be a therapeutic mechanism to increase muscle strength in nebulin deficient muscle. We measured tension-pCa relations with and without added CK-2066260. Maximal active tension in NEB KO tibialis cranialis fibers in the absence of CK-2066260 was ∼60% less than in WT fibers, consistent with earlier work. CK-2066260 shifted the tension-calcium relationship leftwards, with the largest relative increase (up to 8-fold) at low to intermediate calcium levels. This was a general effect that was present in both WT and NEB KO fiber bundles. At pCa levels above ∼6.0 (i.e., calcium concentrations <1 µM), CK-2066260 increased tension of NEB KO fibers to beyond that of WT fibers. Crossbridge cycling kinetics were studied by measuring k(tr) (rate constant of force redevelopment following a rapid shortening/restretch). CK-2066260 greatly increased k(tr) at submaximal activation levels in both WT and NEB KO fiber bundles. We also studied the sarcomere length (SL) dependence of the CK-2066260 effect (SL 2.1 µm and 2.6 µm) and found that in the NEB KO fibers, CK-2066260 had a larger effect on calcium sensitivity at the long SL. We conclude that fast skeletal muscle troponin activation increases force at submaximal activation in both wildtype and NEB KO fiber bundles and, importantly, that this troponin activation is a potential therapeutic mechanism for increasing force in NM and other skeletal muscle diseases with loss of muscle strength.

Intracellular β-nicotinamide adenine dinucleotide inhibits the skeletal muscle ClC-1 chloride channel

ClC-1 is the dominant sarcolemmal chloride channel and plays an important role in regulating membrane excitability that is underscored by ClC-1 mutations in congenital myotonia. Here we show that the coenzyme β-nicotinamide adenine dinucleotide (NAD), an important metabolic regulator, robustly inhibits ClC-1 when included in the pipette solution in whole cell patch clamp experiments and when transiently applied to inside-out patches. The oxidized (NAD(+)) form of the coenzyme was more efficacious than the reduced (NADH) form, and inhibition by both was greatly enhanced by acidification. Molecular modeling, based on the structural coordinates of the homologous ClC-5 and CmClC proteins and in silico docking, suggest that NAD(+) binds with the adenine base deep in a cleft formed by ClC-1 intracellular cystathionine β-synthase domains, and the nicotinamide base interacts with the membrane-embedded channel domain. Consistent with predictions from the models, mutation of residues in cystathionine β-synthase and channel domains either attenuated (G200R, T636A, H847A) or abrogated (L848A) the effect of NAD(+). In addition, the myotonic mutations G200R and Y261C abolished potentiation of NAD(+) inhibition at low pH. Our results identify a new biological role for NAD and suggest that the main physiological relevance may be the exquisite sensitivity to intracellular pH that NAD(+) inhibition imparts to ClC-1 gating. These findings are consistent with the reduction of sarcolemmal chloride conductance that occurs upon acidification of skeletal muscle and suggest a previously unexplored mechanism in the pathophysiology of myotonia.

Mouse and computational models link Mlc2v dephosphorylation to altered myosin kinetics in early cardiac disease

Actin-myosin interactions provide the driving force underlying each heartbeat. The current view is that actin-bound regulatory proteins play a dominant role in the activation of calcium-dependent cardiac muscle contraction. In contrast, the relevance and nature of regulation by myosin regulatory proteins (for example, myosin light chain-2 [MLC2]) in cardiac muscle remain poorly understood. By integrating gene-targeted mouse and computational models, we have identified an indispensable role for ventricular Mlc2 (Mlc2v) phosphorylation in regulating cardiac muscle contraction. Cardiac myosin cycling kinetics, which directly control actin-myosin interactions, were directly affected, but surprisingly, Mlc2v phosphorylation also fed back to cooperatively influence calcium-dependent activation of the thin filament. Loss of these mechanisms produced early defects in the rate of cardiac muscle twitch relaxation and ventricular torsion. Strikingly, these defects preceded the left ventricular dysfunction of heart disease and failure in a mouse model with nonphosphorylatable Mlc2v. Thus, there is a direct and early role for Mlc2 phosphorylation in regulating actin-myosin interactions in striated muscle contraction, and dephosphorylation of Mlc2 or loss of these mechanisms can play a critical role in heart failure.

Measurement of calcium dissociation rates from troponin C in rigor skeletal myofibrils

Ca2+ dissociation from the regulatory domain of troponin C may influence the rate of striated muscle relaxation. However, Ca²⁺ dissociation from troponin C has not been measured within the geometric and stoichiometric constraints of the muscle fiber. Here we report the rates of Ca²⁺ dissociation from the N-terminal regulatory and C-terminal structural domains of fluorescent troponin C constructs reconstituted into rabbit rigor psoas myofibrils using stopped-flow technology. Chicken skeletal troponin C fluorescently labeled at Cys 101, troponin C^IAEDANS, reported Ca²⁺ dissociation exclusively from the structural domain of troponin C at ∼0.37, 0.06, and 0.07/s in isolation, in the presence of troponin I and in myofibrils at 15°C, respectively. Ca²⁺ dissociation from the regulatory domain was observed utilizing fluorescently labeled troponin C containing the T54C and C101S mutations. Troponin CMIANST54C,C101S reported Ca²⁺ dissociation exclusively from the regulatory domain of troponin C at >1000, 8.8, and 15/s in isolation, in the presence of troponin I and in myofibrils at 15°C, respectively. Interestingly, troponin CIAANST54C,C101S reported a biphasic fluorescence change upon Ca²⁺ dissociation from the N- and C-terminal domains of troponin C with rates that were similar to those reported by troponin CMIANST54C,C101S and troponin C^IAEDANS at all levels of the troponin C systems. Furthermore, the rate of Ca²⁺ dissociation from troponin C in the myofibrils was similar to the rate of Ca²⁺ dissociation measured from the troponin C-troponin I complexes. Since the rate of Ca²⁺ dissociation from the regulatory domain of TnC in myofibrils is similar to the rate of skeletal muscle relaxation, Ca²⁺ dissociation from troponin C may influence relaxation.

H-reflex, muscle voluntary activation level, and fatigue index of flexor carpi radialis in individuals with incomplete cervical cord injury

BACKGROUND

Individuals with incomplete spinal cord injury (SCI) are predisposed to muscle fatigue during voluntary exercise. However, the origin of fatigue is unclear.

OBJECTIVE

The authors examined the motoneuron excitability, muscle activation level, and fatigue properties of the flexor carpi radialis muscle, just below the level of injury.

METHODS

Nine individuals with chronic, incomplete cervical cord injury and 9 age-matched healthy individuals were recruited. The authors tested maximum voluntary contraction (MVC), motoneuron excitability by the maximum amplitude of the H-reflex (Hmax at C-7), and muscle voluntary activation level measured by the interpolated twitch technique. Subjects were fatigued by repetitive maximal voluntary isometric wrist flexion. General fatigue index (GFI), central fatigue index (CFI), and peripheral fatigue index (PFI) of flexor carpi radialis were examined before, during, and immediately after exercise.

RESULTS

The Hmax in the SCI group was significantly higher (P = .0028) than in controls. The MVC (P < .001) and voluntary activation level (P = .016) in the SCI group were significantly lower. The GFI and CFI decreased in both the SCI and the non-SCI groups. The PFI in the SCI group was significantly higher (ie, less fatigue) than that in controls at 30 repetitive contractions.

CONCLUSIONS

In individuals with incomplete SCI, the deficit in central drive is an important source of muscle weakness and fatigue in the muscle below the level of injury.

Regulatory light chain phosphorylation and N-terminal extension increase cross-bridge binding and power output in Drosophila at in vivo myofilament lattice spacing

The N-terminal extension and phosphorylation of the myosin regulatory light chain (RLC) independently improve Drosophila melanogaster flight performance. Here we examine the functional and structural role of the RLC in chemically skinned fibers at various thick and thin filament lattice spacings from four transgenic Drosophila lines: rescued null or control (Dmlc2(+)), truncated N-terminal extension (Dmlc2(Δ2-46)), disrupted myosin light chain kinase phosphorylation sites (Dmlc2(S66A,S67A)), and dual mutant (Dmlc2(Δ2-46; S66A,S67A)). The N-terminal extension truncation and phosphorylation sites disruption mutations decreased oscillatory power output and the frequency of maximum power output in maximally Ca(2+)-activated fibers compressed to near in vivo inter-thick filament spacing, with the phosphorylation sites disruption mutation having a larger affect. The diminished power output parameters with the N-terminal extension truncation and phosphorylation sites disruption mutations were due to the reduction of the number of strongly-bound cross-bridges and rate of myosin force production, with the larger parameter reductions in the phosphorylation sites disruption mutation additionally related to reduced myosin attachment time. The phosphorylation and N-terminal extension-dependent boost in cross-bridge kinetics corroborates previous structural data, which indicate these RLC attributes play a complementary role in moving and orienting myosin heads toward actin target sites, thereby increasing fiber and whole fly power generation.

Myosin light chain kinase and the role of myosin light chain phosphorylation in skeletal muscle

Skeletal muscle myosin light chain kinase (skMLCK) is a dedicated Ca(2+)/calmodulin-dependent serine-threonine protein kinase that phosphorylates the regulatory light chain (RLC) of sarcomeric myosin. It is expressed from the MYLK2 gene specifically in skeletal muscle fibers with most abundance in fast contracting muscles. Biochemically, activation occurs with Ca(2+) binding to calmodulin forming a (Ca(2+))(4)•calmodulin complex sufficient for activation with a diffusion limited, stoichiometric binding and displacement of a regulatory segment from skMLCK catalytic core. The N-terminal sequence of RLC then extends through the exposed catalytic cleft for Ser15 phosphorylation. Removal of Ca(2+) results in the slow dissociation of calmodulin and inactivation of skMLCK. Combined biochemical properties provide unique features for the physiological responsiveness of RLC phosphorylation, including (1) rapid activation of MLCK by Ca(2+)/calmodulin, (2) limiting kinase activity so phosphorylation is slower than contraction, (3) slow MLCK inactivation after relaxation and (4) much greater kinase activity relative to myosin light chain phosphatase (MLCP). SkMLCK phosphorylation of myosin RLC modulates mechanical aspects of vertebrate skeletal muscle function. In permeabilized skeletal muscle fibers, phosphorylation-mediated alterations in myosin structure increase the rate of force-generation by myosin cross bridges to increase Ca(2+)-sensitivity of the contractile apparatus. Stimulation-induced increases in RLC phosphorylation in intact muscle produces isometric and concentric force potentiation to enhance dynamic aspects of muscle work and power in unfatigued or fatigued muscle. Moreover, RLC phosphorylation-mediated enhancements may interact with neural strategies for human skeletal muscle activation to ameliorate either central or peripheral aspects of fatigue.

The significance of regulatory light chain phosphorylation in cardiac physiology

It has been over 35 years since the first identification of phosphorylation of myosin light chains in skeletal and cardiac muscle. Yet only in the past few years has the role of these phosphorylations in cardiac dynamics been more fully understood. Advances in this understanding have come about with further evidence on the control mechanisms regulating the level of phosphorylation by kinases and phosphatases. Moreover, studies clarifiying the role of light chain phosphorylation in short and long term control of cardiac contractility and as a factor in cardiac remodeling have improved our knowledge. Especially important in these advances has been the use of gain and loss of function approaches, which have not only testedthe role of kinases and phosphatases, but also the effects of loss of RLC phosphorylation sites. Major conclusions from these studies indicate that (i) two negatively-charged post-translational modifications occupy the ventricular RLC N-terminus, with mouse RLC being doubly phosphorylated (Ser 14/15), and human RLC being singly phosphorylated (Ser 15) and singly deamidated(Asn14/16 to Asp); (ii)a distinct cardiac myosin light kinase (cMLCK) and a unique myosin phosphatase targeting peptide (MYPT2) control phosphoryl group transfer;and (iii) ablation of RLC phosphorylationdecreases ventricular power, lengthens the duration of ventricular ejection, and may also modify other sarcomeric proteins (e.g., troponin I) as substrates for kinases and/or phosphatases. A long term effect of low levels of RLC phosphorylation in mouse models also involves remodeling of the heart with hypertrophy, depressed contractility, and sarcomeric disarray. Data demonstrating altered levels of RLC phosphorylation in comparisons of samples from normal and stressed human hearts indicate the significance of these findings in translational medicine.

Myosin light-chain phosphorylation and potentiation of dynamic function in mouse fast muscle

The intent of this study was to determine if the stimulation-induced increase or "potentiation" of dynamic function of mouse extensor digitorum longus muscle (in vitro 25°C) during work cycles is graded to myosin regulatory light-chain (RLC) phosphorylation. To do this, concentric force and muscle work output during sinusoidal length changes were determined before (unpotentiated) and after (potentiated) the application of conditioning stimuli (CS) producing incremental elevations in RLC phosphorylation from rest. Sine wave excursion was from 1.09 to 0.91 of L (o) with a period of 142 ms; stimulating muscles to twitch and generate force during these cycles produced plots of force × displacement termed work loops. Stimulation at 2.5-, 5.0-, and 100-Hz elevated RLC phosphorylation from 0.16±0.02 (rest) to 0.29±0.03, 0.45±0.02 and 0.56±0.02 mol phos per mole RLC, respectively (n= 6-7, P<0.05). These CS potentiated mean concentric force (at all lengths) to 1.14±0.02, 1.26±0.04 and 1.41±0.06 of pre-stimulus, control levels (all n= 5-7, P<0.05) while work was increased to 1.07±0.02, 1.17±0.02 and 1.34±0.03 of controls, respectively. In a No CS condition that did not elevate RLC phosphorylation, neither mean concentric force nor work was altered. Thus, strong correlations between RLC phosphorylation and mean concentric force and work support the hypothesis that this molecular mechanism modulates muscle power output. No length-dependence for concentric force potentiation was observed in any condition, an outcome suggesting that interactions between instantaneous variations in muscle length and shortening velocity during work cycles modulates the potentiation response.

Potentiation of the triceps brachii during voluntary submaximal contractions

INTRODUCTION

The effects of postactivation potentiation (PAP) on evoked contractions are well understood, but less is known about the effect of PAP on voluntary submaximal contractions. Using a measure of neuromuscular efficiency (NME) [NME = (mV EMG / Nm torque)] we explored the effects of PAP in the triceps brachii at two muscle lengths.

METHODS

Evoked twitch and NME were compared at short (40° elbow flexion) and long (120°) muscle lengths. At each length, 12 subjects performed a contraction of 25% maximum voluntary contraction (MVC) torque before and after a potentiating MVC.

RESULTS

Twitch torque potentiated more at short length (216.9 ± 169.3%) than at long length (77.3 ± 32.6%), but PAP moderately improved NME only at short length (12.2 ± 8.7%).

CONCLUSIONS

The greater capacity for PAP at the short length is reflected by greater NME. Compared with evoked responses, the relatively small change in NME suggests a different and more modest role of PAP during voluntary submaximal contractions.

Doublet electrical stimulation enhances torque production in people with spinal cord injury

BACKGROUND

Muscle fatigue prevents repetitive use of paralyzed muscle after spinal cord injury (SCI).

OBJECTIVE

This study compared the effects of hybrid patterns of muscle stimulation in individuals with acute and chronic SCI.

METHODS

Individuals with chronic (n = 11) or acute paralysis (n = 3) underwent soleus muscle activation with a constant (CT) or doublet (DT) stimulation train before and at various times after a fatigue protocol.

RESULTS

The chronically paralyzed soleus was highly fatigable with a fatigue index (FI) of 19% ± 6%, whereas the acutely paralyzed soleus was fatigue resistant (FI = 89% ± 8%). For the chronically paralyzed group, the DT protocol caused less than 5% improvement in peak and mean force relative to the CT protocol before fatigue; however, after fatigue the DT protocol caused an increase in peak and mean force (>10%), compared with the CT protocol (P < .05). As the chronically paralyzed muscle developed low-frequency fatigue, the DT protocol became more effective than the CT protocol (P < .05). The DT protocol increased the rate of torque development before fatigue (42% ± 78%), after fatigue (62% ± 52%), and during recovery (87% ± 54% to 101% ± 56%; P < .05). The acutely paralyzed group showed minimal change in peak and mean torque with the DT protocol.

CONCLUSIONS

Chronic reduced activity is associated with muscle adaptations (slow to fast) that render the muscle more amenable to force enhancement through doublet train activation after fatigue. These findings are applicable to patients using neuromuscular stimulation.

Signaling to myosin regulatory light chain in sarcomeres

Myosin regulatory light chain (RLC) phosphorylation in skeletal and cardiac muscles modulates Ca(2+)-dependent troponin regulation of contraction. RLC is phosphorylated by a dedicated Ca(2+)-dependent myosin light chain kinase in fast skeletal muscle, where biochemical properties of RLC kinase and phosphatase converge to provide a biochemical memory for RLC phosphorylation and post-activation potentiation of force development. The recent identification of cardiac-specific myosin light chain kinase necessary for basal RLC phosphorylation and another potential RLC kinase (zipper-interacting protein kinase) provides opportunities for new approaches to study signaling pathways related to the physiological function of RLC phosphorylation and its importance in cardiac muscle disease.

The effect of sprinting after each set of heavy resistance training on the running speed and jumping performance of young basketball players

The purpose of this study was to investigate the effect of a 10-week heavy resistance combined with a running training program on the strength, running speed (RS), and vertical jump performance of young basketball players. Twenty-six junior basketball players were equally divided in 2 groups. The control (CON) group performed only technical preparation and the group that followed the combined training program (CTP) performed additionally 5 sets of 8-5 repetition maximum (RM) half squat with 1 30-m sprint after each set. The evaluation took place before training and after the 5th and 10th weeks of training. Apart from the 1RM half squat test, the 10- and 30-m running time was measured using photocells and the jump height (squat, countermovement jump, and drop jump) was estimated taking into account the flight time. The 1RM increased by 30.3 +/- 1.5% at the 10th week of training for the CTP group (p < 0.05), whereas the CON group showed no significant increase (1.1 +/- 1.6%, p > 0.05). In general, all measured parameters showed a statistically significant increase after the 5th and 10th weeks (p < 0.05), in contrast to the CON group (p > 0.05). This suggests that the applied CTP is beneficial for the strength, RS, and jump height of young basketball players. The observed adaptations in the CTP group could be attributed to learning factors and to a more optimal transfer of the strength gain to running and jumping performance.

Length dependence of force generation exhibit similarities between rat cardiac myocytes and skeletal muscle fibres

According to the Frank-Starling relationship, increased ventricular volume increases cardiac output, which helps match cardiac output to peripheral circulatory demand. The cellular basis for this relationship is in large part the myofilament length-tension relationship. Length-tension relationships in maximally calcium activated preparations are relatively shallow and similar between cardiac myocytes and skeletal muscle fibres. During twitch activations length-tension relationships become steeper in both cardiac and skeletal muscle; however, it remains unclear whether length dependence of tension differs between striated muscle cell types during submaximal activations. The purpose of this study was to compare sarcomere length-tension relationships and the sarcomere length dependence of force development between rat skinned left ventricular cardiac myocytes and fast-twitch and slow-twitch skeletal muscle fibres. Muscle cell preparations were calcium activated to yield 50% maximal force, after which isometric force and rate constants (k(tr)) of force development were measured over a range of sarcomere lengths. Myofilament length-tension relationships were considerably steeper in fast-twitch fibres compared to slow-twitch fibres. Interestingly, cardiac myocyte preparations exhibited two populations of length-tension relationships, one steeper than fast-twitch fibres and the other similar to slow-twitch fibres. Moreover, myocytes with shallow length-tension relationships were converted to steeper length-tension relationships by protein kinase A (PKA)-induced myofilament phosphorylation. Sarcomere length-k(tr) relationships were distinct between all three cell types and exhibited patterns markedly different from Ca(2+) activation-dependent k(tr) relationships. Overall, these findings indicate cardiac myocytes exhibit varied length-tension relationships and sarcomere length appears a dominant modulator of force development rates. Importantly, cardiac myocyte length-tension relationships appear able to switch between slow-twitch-like and fast-twitch-like by PKA-mediated myofibrillar phosphorylation, which implicates a novel means for controlling Frank-Starling relationships.

Differential roles of regulatory light chain and myosin binding protein-C phosphorylations in the modulation of cardiac force development

Phosphorylation of myosin regulatory light chain (RLC) by myosin light chain kinase (MLCK) and myosin binding protein-C (cMyBP-C) by protein kinase A (PKA) independently accelerate the kinetics of force development in ventricular myocardium. However, while MLCK treatment has been shown to increase the Ca(2+) sensitivity of force (pCa(50)), PKA treatment has been shown to decrease pCa(50), presumably due to cardiac troponin I phosphorylation. Further, MLCK treatment increases Ca(2+)-independent force and maximum Ca(2+)-activated force, whereas PKA treatment has no effect on either force. To investigate the structural basis underlying the kinase-specific differential effects on steady-state force, we used synchrotron low-angle X-ray diffraction to compare equatorial intensity ratios (I(1,1)/I(1,0)) to assess the proximity of myosin cross-bridge mass relative to actin and to compare lattice spacings (d(1,0)) to assess the inter-thick filament spacing in skinned myocardium following treatment with either MLCK or PKA. As we showed previously, PKA phosphorylation of cMyBP-C increases I(1,1)/I(1,0) and, as hypothesized, treatment with MLCK also increased I(1,1)/I(1,0), which can explain the accelerated rates of force development during activation. Importantly, interfilament spacing was reduced by 2 nm (3.5%) with MLCK treatment, but did not change with PKA treatment. Thus, RLC or cMyBP-C phosphorylation increases the proximity of cross-bridges to actin, but only RLC phosphorylation affects lattice spacing, which suggests that RLC and cMyBP-C modulate the kinetics of force development by similar structural mechanisms; however, the effect of RLC phosphorylation to increase the Ca(2+) sensitivity of force is mediated by a distinct mechanism, most probably involving changes in interfilament spacing.

Dihydrotestosterone activates the MAPK pathway and modulates maximum isometric force through the EGF receptor in isolated intact mouse skeletal muscle fibres

It is generally believed that steroid hormones have both genomic and non-genomic (rapid) actions. Although the latter form an important component of the physiological response of these hormones, little is known about the cellular signalling pathway(s) mediating these effects and their physiological functions in adult mammalian skeletal muscle fibres. Therefore, the primary aim of this study was to investigate the non-genomic actions of dihydrotestosterone (DHT) and their physiological role in isolated intact mammalian skeletal muscle fibre bundles. Our results show that treating the fibre bundles with physiological concentrations of DHT increases both twitch and tetanic contractions in fast twitch fibres. However, it decreases them in slow twitch fibres. These changes in force are accompanied by an increase in the phosphorylation of MAPK/ERK1/2 in both fibre types and that of regulatory myosin light chains in fast twitch fibres. Both effects were insensitive to inhibitors of Src kinase, androgen receptor, insulin-like growth factor 1 receptor and platelet-derived growth factor receptor. However, they were abolished by the MAPK/ERK1/2 kinase inhibitor PD98059 and the epidermal growth factor (EGF) receptor inhibitor tyrphostin AG 1478. In contrast, testosterone had no effect on force and increased the phosphorylation of ERK1/2 in slow twitch fibres only. From these results we conclude that sex steroids have non-genomic actions in isolated intact mammalian skeletal muscle fibres. These are mediated through the EGF receptor and one of their main physiological functions is the enhancement of force production in fast twitch skeletal muscle fibres.

Protruding masticatory (superfast) myosin heads from staggered thick filaments of dog jaw muscle revealed by X-ray diffraction

To characterize the structure of jaw muscle fibres expressing masticatory (superfast) myosin, X-ray diffraction patterns of glycerinated fibres of dog masseter were compared with those of dog tibialis anterior in the relaxed state. Meridional reflections of masseter fibres were laterally broad, indicating that myosin filaments are staggered along the filament axis. Compared with tibialis anterior fibres, the peak of the first myosin layer line of masseter fibres was lower in intensity and shifted towards the meridian, while lattice spacings were larger at a similar sarcomere length. These suggest that the myosin heads of masticatory fibres are mobile, and tend to protrude from the filament shaft towards actin filaments. Lowering temperature or treating with N-phenylmaleimide shifted the peak of the first myosin layer line of tibialis anterior fibres towards the meridian and the resulting profile resembled that of masseter fibres. This suggests that the protruding mobile heads in the non-treated masticatory fibres are in the ATP-bound state. The increased population of weakly binding cross-bridges may contribute towards the high specific force of masticatory fibres during contraction. Electron micrographs confirmed the staggered alignment of thick filaments along the filament axis within sarcomeres of masticatory fibres, a feature that may confer efficient force development over a wide range of the sarcomere lengths.

The molecular effects of skeletal muscle myosin regulatory light chain phosphorylation

Phosphorylation of the myosin regulatory light chain (RLC) in skeletal muscle has been proposed to act as a molecular memory of recent activation by increasing the rate of force development, ATPase activity, and isometric force at submaximal activation in fibers. It has been proposed that these effects stem from phosphorylation-induced movement of myosin heads away from the thick filament backbone. In this study, we examined the molecular effects of skeletal muscle myosin RLC phosphorylation using in vitro motility assays. We showed that, independently of the thick filament backbone, the velocity of skeletal muscle myosin is decreased upon phosphorylation due to an increase in the myosin duty cycle. Furthermore, we did not observe a phosphorylation-dependent shift in calcium sensitivity in the absence of the myosin thick filament. These data suggest that phosphorylation-induced movement of myosin heads away from the thick filament backbone explains only part of the observed phosphorylation-induced changes in myosin mechanics. Last, we showed that the duty cycle of skeletal muscle myosin is strain dependent, consistent with the notion that strain slows the rate of ADP release in striated muscle.

Malignant familial hypertrophic cardiomyopathy D166V mutation in the ventricular myosin regulatory light chain causes profound effects in skinned and intact papillary muscle fibers from transgenic mice

Transgenic (Tg) mice expressing approximately 95% of the D166V (aspartic acid to valine) mutation in the ventricular myosin regulatory light chain (RLC) shown to cause a malignant familial hypertrophic cardiomyopathy (FHC) phenotype were generated, and the skinned and intact papillary muscle fibers from the Tg-D166V mice were examined using a Guth muscle research system. A large increase in the Ca(2+) sensitivity of force and ATPase (Delta pCa(50)>0.25) and a significant decrease in maximal force and ATPase were observed in skinned muscle fibers from Tg-D166V mice compared with control mice. The cross-bridge dissociation rate g was dramatically decreased, whereas the energy cost (ATPase/force) was slightly increased in Tg-D166V fibers compared with controls. The calculated average force per D166V cross-bridge was also reduced. Intact papillary muscle data demonstrated prolonged force transients with no change in calcium transients in Tg-D166V fibers compared with control fibers. Histopathological examination revealed fibrotic lesions in the hearts of the older D166V mice. Our results suggest that a charge effect of the D166V mutation and/or a mutation-dependent decrease in RLC phosphorylation could initiate the slower kinetics of the D166V cross-bridges and ultimately affect the regulation of cardiac muscle contraction. Profound cellular changes observed in Tg-D166V myocardium when placed in vivo could trigger a series of pathological responses and result in poor prognosis for D166V-positive patients.

Ablation of ventricular myosin regulatory light chain phosphorylation in mice causes cardiac dysfunction in situ and affects neighboring myofilament protein phosphorylation

There is little direct evidence on the role of myosin regulatory light chain phosphorylation in ejecting hearts. In studies reported here we determined the effects of regulatory light chain (RLC) phosphorylation on in situ cardiac systolic mechanics and in vitro myofibrillar mechanics. We compared data obtained from control nontransgenic mice (NTG) with a transgenic mouse model expressing a cardiac specific nonphosphorylatable RLC (TG-RLC(P-). We also determined whether the depression in RLC phosphorylation affected phosphorylation of other sarcomeric proteins. TG-RLC(P-) demonstrated decreases in base-line load-independent measures of contractility and power and an increase in ejection duration together with a depression in phosphorylation of myosin-binding protein-C (MyBP-C) and troponin I (TnI). Although TG-RLC(P-) displayed a significantly reduced response to beta(1)-adrenergic stimulation, MyBP-C and TnI were phosphorylated to a similar level in TG-RLC(P-) and NTG, suggesting cAMP-dependent protein kinase signaling to these proteins was not disrupted. A major finding was that NTG controls were significantly phosphorylated at RLC serine 15 following beta(1)-adrenergic stimulation, a mechanism prevented in TG-RLC(P-), thus providing a biochemical difference in beta(1)-adrenergic responsiveness at the level of the sarcomere. Our measurements of Ca(2+) tension and Ca(2+)-ATPase rate relations in detergent-extracted fiber bundles from LV trabeculae demonstrated a relative decrease in maximum Ca(2+)-activated tension and tension cost in TG-RLC(P-) fibers, with no change in Ca(2+) sensitivity. Our data indicate that RLC phosphorylation is critical for normal ejection and response to beta(1)-adrenergic stimulation. Our data also indicate that the lack of RLC phosphorylation promotes compensatory changes in MyBP-C and TnI phosphorylation, which when normalized do not restore function.

Three-dimensional reconstruction of tarantula myosin filaments suggests how phosphorylation may regulate myosin activity

Muscle contraction involves the interaction of the myosin heads of the thick filaments with actin subunits of the thin filaments. Relaxation occurs when this interaction is blocked by molecular switches on these filaments. In many muscles, myosin-linked regulation involves phosphorylation of the myosin regulatory light chains (RLCs). Electron microscopy of vertebrate smooth muscle myosin molecules (regulated by phosphorylation) has provided insight into the relaxed structure, revealing that myosin is switched off by intramolecular interactions between its two heads, the free head and the blocked head. Three-dimensional reconstruction of frozen-hydrated specimens revealed that this asymmetric head interaction is also present in native thick filaments of tarantula striated muscle. Our goal in this study was to elucidate the structural features of the tarantula filament involved in phosphorylation-based regulation. A new reconstruction revealed intra- and intermolecular myosin interactions in addition to those seen previously. To help interpret the interactions, we sequenced the tarantula RLC and fitted an atomic model of the myosin head that included the predicted RLC atomic structure and an S2 (subfragment 2) crystal structure to the reconstruction. The fitting suggests one intramolecular interaction, between the cardiomyopathy loop of the free head and its own S2, and two intermolecular interactions, between the cardiac loop of the free head and the essential light chain of the blocked head and between the Leu305-Gln327 interaction loop of the free head and the N-terminal fragment of the RLC of the blocked head. These interactions, added to those previously described, would help switch off the thick filament. Molecular dynamics simulations suggest how phosphorylation could increase the helical content of the RLC N-terminus, weakening these interactions, thus releasing both heads and activating the thick filament.

Recurrence quantification analysis of surface electromyographic signal: sensitivity to potentiation and neuromuscular fatigue

This study aimed to assess the capacity of recurrence quantification analysis (RQA) to detect potentiation and to determine the fatigue components to which RQA is sensitive. Fifteen men were divided in two groups [8 endurance-trained athletes (END) and 7 power-trained athletes (POW)]. They performed a 10-min intermittent (5s contraction, 5s rest) knee extension exercise at 50% of their maximal voluntary isometric contraction. Muscular fatigue and potentiation were evaluated with neurostimulation technique. Mechanical (peak torque, Pt) and electrophysiological (M-wave) responses following electrical stimulation of the femoral nerve were measured at rest and every 10s throughout exercise. Vastus lateralis muscle activity (root mean square, RMS) was recorded during each contraction, and RMS was normalized to M-wave area (RMS/M). During contraction, muscle activity was analyzed with RQA to obtain the percentage of determinism (%Det). At the beginning of exercise, a significant Pt increase (+52%, P<0.001) was observed in both groups, indicating potentiation. At this time, %Det remained constant in both groups, indicating that RQA did not detect potentiation. Thereafter, Pt decreased in POW from 5min 30s of exercise (-30%, P<0.001), reflecting impairment in excitation-contraction coupling, and %Det increased from 3min 30s (P<0.01). In END, Pt remained high and %Det was unchanged. These two results indicated that RQA detected the peripheral component of fatigue. Conversely, RQA did not detect central adaptation to fatigue since %Det remained constant when a significant increase in RMS/M (P<0.01) appeared in END.

Postactivation potentiation of short tetanic contractions is differently influenced by stimulation frequency in young and elderly adults

The purpose of this study was to examine the effects of postactivation potentiation (PAP) on the torque and rate of torque development for contractions evoked by short trains of stimuli at different frequencies, in young and elderly adults. Individual mechanical contributions to each electrical stimulus within trains were also analysed. Single pulse, and two- (PT2) and three-pulse trains (PT3) delivered at 20, 50, 80 and 100 Hz were evoked before and during a 10 min period after a 6-s conditioning MVC. The results show that PAP of the torque for PT2 decreased with the increase of the stimulation frequency for young (from 184.5 to 140.4% of control values) and elderly (from 140.5 to 109.6%). Regardless of the stimulation frequency, the peak of PAP was greater in young than in elderly and occurred immediately after the conditioning MVC but was delayed for the 100 Hz condition in elderly adults. For PT3, the results were similar although the extent of PAP was less. The PAP of the mechanical contributions within the trains also decreased with the augmentation of the stimulation frequency. For most of the frequencies above 20 Hz, the peak of PAP for each mechanical contribution was delayed by 1 min after the conditioning MVC. These results indicate an age- and frequency-related PAP saturation of the successive mechanical contributions within a train of stimuli that decrease with time. The functional implication of the findings is that PAP effect is lower and delayed at high compared with low activation rate.

Sarcomere lattice geometry influences cooperative myosin binding in muscle

In muscle, force emerges from myosin binding with actin (forming a cross-bridge). This actomyosin binding depends upon myofilament geometry, kinetics of thin-filament Ca²⁺ activation, and kinetics of cross-bridge cycling. Binding occurs within a compliant network of protein filaments where there is mechanical coupling between myosins along the thick-filament backbone and between actin monomers along the thin filament. Such mechanical coupling precludes using ordinary differential equation models when examining the effects of lattice geometry, kinetics, or compliance on force production. This study uses two stochastically driven, spatially explicit models to predict levels of cross-bridge binding, force, thin-filament Ca²⁺ activation, and ATP utilization. One model incorporates the 2-to-1 ratio of thin to thick filaments of vertebrate striated muscle (multi-filament model), while the other comprises only one thick and one thin filament (two-filament model). Simulations comparing these models show that the multi-filament predictions of force, fractional cross-bridge binding, and cross-bridge turnover are more consistent with published experimental values. Furthermore, the values predicted by the multi-filament model are greater than those values predicted by the two-filament model. These increases are larger than the relative increase of potential inter-filament interactions in the multi-filament model versus the two-filament model. This amplification of coordinated cross-bridge binding and cycling indicates a mechanism of cooperativity that depends on sarcomere lattice geometry, specifically the ratio and arrangement of myofilaments.

Striated muscle is highly structured, and the molecular organization of muscle filaments varies within individuals (by fiber type) and taxonomically. The consequences of filament arrangement on muscle contraction, however, remain largely unknown. We explore how filament arrangement affects force production in muscle using spatially explicit models of many interacting myofilaments. Our analysis incorporates molecular scale force balance equations with Monte Carlo simulations of both actin–myosin interactions and thin-filament Ca²⁺ activation. Simulations show that a more physiological representation of vertebrate striated muscle amplifies force production, coordinates dynamic actin–myosin cycling, and may optimize energetics of contraction (force generated per ATP consumed). This coordinated myosin behavior indicates a mechanism of cooperativity in muscle that depends on the ratio and arrangement of filaments. We also demonstrate the importance of mechanical coupling between myosin molecules by varying filament stiffness. Our simulations show a tradeoff between the way myosin molecules partition energy from ATP hydrolysis into force transmitted throughout the filaments versus distortions within the filaments. These findings present a possible consequence of organization in muscle, where the ratio and arrangement of muscle filaments affects contractile performance for the given function across different muscle types.

Troponin and titin coordinately regulate length-dependent activation in skinned porcine ventricular muscle

We investigated the molecular mechanism by which troponin (Tn) regulates the Frank-Starling mechanism of the heart. Quasi-complete reconstitution of thin filaments with rabbit fast skeletal Tn (sTn) attenuated length-dependent activation in skinned porcine left ventricular muscle, to a magnitude similar to that observed in rabbit fast skeletal muscle. The rate of force redevelopment increased upon sTn reconstitution at submaximal levels, coupled with an increase in Ca²⁺ sensitivity of force, suggesting the acceleration of cross-bridge formation and, accordingly, a reduction in the fraction of resting cross-bridges that can potentially produce additional active force. An increase in titin-based passive force, induced by manipulating the prehistory of stretch, enhanced length-dependent activation, in both control and sTn-reconstituted muscles. Furthermore, reconstitution of rabbit fast skeletal muscle with porcine left ventricular Tn enhanced length-dependent activation, accompanied by a decrease in Ca²⁺ sensitivity of force. These findings demonstrate that Tn plays an important role in the Frank-Starling mechanism of the heart via on–off switching of the thin filament state, in concert with titin-based regulation.

Myosin regulatory light chain phosphorylation attenuates cardiac hypertrophy

Hyperphosphorylation of myosin regulatory light chain (RLC) in cardiac muscle is proposed to cause compensatory hypertrophy. We therefore investigated potential mechanisms in genetically modified mice. Transgenic (TG) mice were generated to overexpress Ca2+/calmodulin-dependent myosin light chain kinase specifically in cardiomyocytes. Phosphorylation of sarcomeric cardiac RLC and cytoplasmic nonmuscle RLC increased markedly in hearts from TG mice compared with hearts from wild-type (WT) mice. Quantitative measures of RLC phosphorylation revealed no spatial gradients. No significant hypertrophy or structural abnormalities were observed up to 6 months of age in hearts of TG mice compared with WT animals. Hearts and cardiomyocytes from WT animals subjected to voluntary running exercise and isoproterenol treatment showed hypertrophic cardiac responses, but the responses for TG mice were attenuated. Additional biochemical measurements indicated that overexpression of the Ca2+/calmodulin-binding kinase did not perturb other Ca2+/calmodulin-dependent processes involving Ca2+/calmodulin-dependent protein kinase II or the protein phosphatase calcineurin. Thus, increased myosin RLC phosphorylation per se does not cause cardiac hypertrophy and probably inhibits physiological and pathophysiological hypertrophy by contributing to enhanced contractile performance and efficiency.

Modulation of the actomyosin interaction during fatigue of skeletal muscle

Fatigue of skeletal muscle involves many systems beginning with the central nervous system and ending with the contractile machinery. This review concentrates on those factors that directly affect the actomyosin interaction: the build-up of metabolites; myosin phosphorylation; and oxidation of the myofibrillar proteins by free radicals. The decrease in [ATP] and increase in [ADP] appear to play little role in modulating function. The increase in phosphate inhibits tension. The decrease in pH, long thought to be a major factor, is now known to play a more minor role. Myosin phosphorylation potentiates the force achieved in a twitch, and a further role in inhibiting velocity is proposed. Protein oxidation can both potentiate and inhibit the actomyosin interaction. It is concluded that these factors, taken together, do not fully explain the inhibition of the actomyosin interaction observed in living fibers, and thus additional modulators of this interaction remain to be discovered.

Children are more susceptible to central fatigue than adults

Performance in high-intensity exercise is dependent on the ability to activate motor units. The main aim of this study was to test the hypothesis that adult men and women (age 19-27 years) are able to maintain higher levels of voluntary activation (VA) in knee extensor muscles than boys and girls (age 12-14 years). The volunteers (n = 7 in each group) performed three 5-s maximal voluntary contractions (MVCs) and a continuous 2-min MVC. The VA and fatigue of the muscles was assessed by applying 250-ms 100-HZ test tetani (TT100HZ). During brief MVCs girls showed lower VA than women, but the difference between boys and men was not significant. During the 2-min MVC, VA in boys and girls was more depressed than in adults. The end-exercise values of the relative TT100HZ torque correlated with the average VA during the exercise. Thus, the results of the study support the hypothesis that children are more susceptible to central fatigue than adults. This should be taken into account when evaluating results of fitness tests that require high levels of motor unit activation.

Inhibition of skeletal muscle ClC-1 chloride channels by low intracellular pH and ATP

Skeletal muscle acidosis during exercise has long been thought to be a cause of fatigue, but recent studies have shown that acidosis maintains muscle excitability and opposes fatigue by decreasing the sarcolemmal chloride conductance. ClC-1 is the primary sarcolemmal chloride channel and has a clear role in controlling muscle excitability, but recombinant ClC-1 has been reported to be activated by acidosis. Following our recent finding that intracellular ATP inhibits ClC-1, we investigated here the interaction between pH and ATP regulation of ClC-1. We found that, in the absence of ATP, intracellular acidosis from pH 7.2 to 6.2 inhibited ClC-1 slightly by shifting the voltage dependence of common gating to more positive potentials, similar to the effect of ATP. Importantly, the effects of ATP and acidosis were cooperative, such that ATP greatly potentiated the effect of acidosis. Adenosine had a similar effect to ATP at pH 7.2, but acidosis did not potentiate this effect, indicating that the phosphates of ATP are important for this cooperativity, possibly due to electrostatic interactions with protonatable residues of ClC-1. A protonatable residue identified by molecular modeling, His-847, was found to be critical for both pH and ATP modulation and may be involved in such electrostatic interactions. These findings are now consistent with, and provide a molecular explanation for, acidosis opposing fatigue by decreasing the chloride conductance of skeletal muscle via inhibition of ClC-1. The modulation of ClC-1 by ATP is a key component of this molecular mechanism.

Postactivation potentiation in a human muscle: effect on the load-velocity relation of tetanic and voluntary shortening contractions

Recently it was demonstrated that postactivation potentiation (PAP), which refers to the enhancement of the muscle twitch torque as a result of a prior conditioning contraction, increased the maximal rate of torque development of tetanic and voluntary isometric contractions (3). In this study, we investigated the effects of PAP and its decay over time on the load-velocity relation. To that purpose, angular velocity of thumb adduction in response to a single electrical stimulus (twitch), a high-frequency train of 15 pulses at 250 Hz (HFT(250)), and during ballistic voluntary shortening contractions, performed against loads ranging from 10 to 50% of the maximum torque, were recorded before and after a conditioning 6-s maximal voluntary contraction (MVC). The results showed an increase of the peak angular velocity for the different loads tested after the conditioning MVC (P < 0.001), but the effect was greatest for the twitch ( approximately 182%) compared with the HFT(250) or voluntary contractions ( approximately 14% for both contraction types). The maximal potentiation occurred immediately following the conditioning MVC for the twitch, whereas it was reached 1 min later for the tetanic and ballistic voluntary contractions. At that time, the load-velocity relation was significantly shifted upward, and the maximal power of the muscle was increased ( approximately 13%; P < 0.001). Furthermore, the results also indicated that the effect of PAP on shortening contractions was not related to the modality of muscle activation. In conclusion, the findings suggest a functional significance of PAP in human movements by improving muscle performance of voluntary dynamic contractions.

Double phosphorylation of the myosin regulatory light chain during rigor mortis of bovine Longissimus muscle

To investigate changes in myosin light chains (MyLCs) during postmortem aging of the bovine longissimus muscle, we performed two-dimensional gel electrophoresis followed by identification with matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The results of fluorescent differential gel electrophoresis showed that two spots of the myosin regulatory light chain (MyLC2) at pI values of 4.6 and 4.7 shifted toward those at pI values of 4.5 and 4.6, respectively, by 24 h postmortem when rigor mortis was completed. Meanwhile, the MyLC1 and MyLC3 spots did not change during the 14 days postmortem. Phosphoprotein-specific staining of the gels demonstrated that the MyLC2 proteins at pI values of 4.5 and 4.6 were phosphorylated. Furthermore, possible N-terminal region peptides containing one and two phosphoserine residues were detected in each mass spectrum of the MyLC2 spots at pI values of 4.5 and 4.6, respectively. These results demonstrated that MyLC2 became doubly phosphorylated during rigor formation of the bovine longissimus, suggesting involvement of the MyLC2 phosphorylation in the progress of beef rigor mortis.

KEYWORDS

Bovine; myosin regulatory light chain (RLC, MyLC2); phosphorylation; rigor mortis; skeletal muscle.

Enhanced skeletal muscle contraction with myosin light chain phosphorylation by a calmodulin-sensing kinase

Repetitive low frequency stimulation results in potentiation of twitch force development in fast-twitch skeletal muscle due to myosin regulatory light chain (RLC) phosphorylation by Ca(2+)/calmodulin (CaM)-dependent skeletal muscle myosin light chain kinase (skMLCK). We generated transgenic mice that express an skMLCK CaM biosensor in skeletal muscle to determine whether skMLCK or CaM is limiting to twitch force potentiation. Three transgenic mouse lines exhibited up to 22-fold increases in skMLCK protein expression in fast-twitch extensor digitorum longus muscle containing type IIa and IIb fibers, with comparable expressions in slow-twitch soleus muscle containing type I and IIa fibers. The high expressing lines showed a more rapid RLC phosphorylation and force potentiation in extensor digitorum longus muscle with low frequency electrical stimulation. Surprisingly, overexpression of skMLCK in soleus muscle did not recapitulate the fast-twitch potentiation response despite marked enhancement of both fast-twitch and slow-twitch RLC phosphorylation. Analysis of calmodulin binding to the biosensor showed a frequency-dependent activation to a maximal extent of 60%. Because skMLCK transgene expression is 22-fold greater than the wild-type kinase, skMLCK rather than calmodulin is normally limiting for RLC phosphorylation and twitch force potentiation. The kinase activation rate (10.6 s(-1)) was only 3.6-fold slower than the contraction rate, whereas the inactivation rate (2.8 s(-1)) was 12-fold slower than relaxation. The slower rate of kinase inactivation in vivo with repetitive contractions provides a biochemical memory via RLC phosphorylation. Importantly, RLC phosphorylation plays a prominent role in skeletal muscle force potentiation of fast-twitch type IIb but not type I or IIa fibers.