Sarcomere length-dependence of activity-dependent twitch potentiation in mouse skeletal muscle

Levosimendan's Effects on Length-Dependent Activation in Murine Fast-Twitch Skeletal Muscle

Levosimendan's calcium sensitizing effects in heart muscle cells are well established; yet, its potential impact on skeletal muscle cells has not been evidently determined. Despite controversial results, levosimendan is still expected to interact with skeletal muscle through off-target sites (further than troponin C). Adding to this debate, we investigated levosimendan's acute impact on fast-twitch skeletal muscle biomechanics in a length-dependent activation study by submersing single muscle fibres in a levosimendan-supplemented solution. We employed our MyoRobot technology to investigate the calcium sensitivity of skinned single muscle fibres alongside their stress-strain response in the presence or absence of levosimendan (100 µM). While control data are in agreement with the theory of length-dependent activation, levosimendan appears to shift the onset of the 'descending limb' of active force generation to longer sarcomere lengths without notably improving myofibrillar calcium sensitivity. Passive stretches in the presence of levosimendan yielded over twice the amount of enlarged restoration stress and Young's modulus in comparison to control single fibres. Both effects have not been described before and may point towards potential off-target sites of levosimendan.

Potentiation of force by extracellular potassium is not dependent on muscle length in mouse EDL muscle

Increases in myofiber extracellular potassium with prolonged contractile activity can potentiate twitch force. Activity-dependent potentiation, another mechanism of force increase in skeletal muscle, has a strong dependence on muscle or sarcomere length. Thus, potassium-mediated twitch potentiation could also be length-dependent. However, this has not been previously investigated. To this end, we used isolated C57BL/6 mouse extensor digitorum longus (EDL) muscles and elicited twitches at 0.9 Lo, Lo, and 1.1 Lo (Lo refers to optimal length) in normal (5 mM) and high (10 mM) potassium solutions. Potentiation magnitude was similar to previous observations and was not significantly different between lengths (0.9 Lo: 12.3 ± 4.4%, Lo: 12.2 ± 3.6%, 1.1 Lo: 11.8 ± 4.8%, values are means ± SD). Exposure to dantrolene sodium, a compound that attenuates calcium release, reduced twitch force across lengths by ∼70%. When dantrolene-affected muscles were subsequently exposed to high potassium, potentiation was similar to that observed in the absence of the former. In total, these findings provide novel information on potassium-mediated twitch potentiation.NEW & NOTEWORTHY Here, we investigated the length-dependence of twitch force potentiation by extracellular potassium in mouse extensor digitorum longus (EDL) in vitro, at 25°C. Potentiation magnitude did not display a statistically significant difference between the examined muscle lengths. These results describe, for the first time, the relationship of this form of potentiation with muscle length, thus furthering the understanding of how it is integrated in in vivo muscle function.

Force potentiation during eccentric contractions in rat skeletal muscle

Postactivation potentiation refers to an acute enhancement of contractile properties following muscle activity. Previously, the effects of prior muscle activation on eccentric force at tetanic activation frequencies have only been sparsely reported. This paper aimed to study acute activity-induced effects on eccentric force of slow and fast-twitch muscles and characterize them in relation to postactivation potentiation. We elicited eccentric contractions in isolated rat extensor digitorum longus and soleus muscles by actively lengthening muscles at a constant velocity. We assessed contractile properties by measuring force over shortly interspaced, identical eccentric, and isometric contractions. We then analyzed stretch force, isometric peak force, rate of force development, and relaxation times. Finally, we compared the time courses for the development and cessation of changes in stretch force to known features of postactivation potentiation. In extensor digitorum longus, muscles stretch force consistently increased in a contraction-to-contraction manner by up to 49% [95% confidence interval (CI): 35-64%] whereas isometric peak force simultaneously showed minor declines (8%, 95% CI: 5-10%). The development and cessation of eccentric force potentiation coincided with the development of twitch potentiation and increases in rate of force development. In soleus muscles we found no consistent eccentric potentiation. Characterization of the increase in eccentric force revealed that force only increased in the very beginning of an active stretch. Eccentric force at tetanic activation frequencies potentiates substantially in extensor digitorum longus muscles over consecutive contractions with a time course coinciding with postactivation potentiation. Such eccentric potentiation may be important in sport performance.NEW & NOTEWORTHY Force during eccentric contractions can increase to a magnitude that may have profound consequences for our understanding of skeletal muscle locomotion. This increase in eccentric force occurs over consecutive, shortly interspaced, tetanic contractions in rat extensor digitorum longus muscles-not in rat soleus muscles-and coincides with well-known traits of postactivation potentiation. Eccentric force potentiation may significantly enhance muscle performance in activities involving stretch-shortening cycles.

Viability of ex-vivo myography as a diagnostic tool for rectus abdominis muscle electrical activity collected at Cesarean section within a diamater cohort study

Background

Ex-vivo myography enables the assessment of muscle electrical activity response. This study explored the viability of determining the physiological responses in muscles without tendon, as rectus abdominis muscle (RAM), through ex-vivo myography to assess its potential as a diagnostic tool.

Results

All tested RAM samples (five different samples) show patterns of electrical activity. A positive response was observed in 100% of the programmed stimulation. RAM 3 showed greater weight (0.47 g), length (1.66 cm), and width (0.77 cm) compared to RAM 1, RAM 2, RAM 4 and RAM 5 with more sustained electrical activity over time, a higher percentage of fatigue was analyzed at half the time of the electrical activity. The order of electrical activity (Mn) was RAM 3 > RAM 5 > RAM 1 > RAM 4 > RAM 2. No electrical activity was recorded in the Sham group.

Conclusions

This study shows that it is feasible to assess the physiological responses of striated muscle without tendon as RAM, obtained at C-section, under ex vivo myography. These results could be recorded, properly analyzed, and demonstrated its potential as a diagnostic tool for rectus abdominis muscle electrical activity.

Does postactivation potentiation (PAP) increase voluntary performance?

The transient increase in torque of an electrically evoked twitch following a voluntary contraction is called postactivation potentiation (PAP). Phosphorylation of myosin regulatory light chains is the most accepted mechanism explaining the enhanced electrically evoked twitch torque. While many authors attribute voluntary postactivation performance enhancement (PAPE) to the positive effects of PAP, few actually confirmed that contraction was indeed potentiated using electrical stimulation (twitch response) at the time that PAPE was measured. Thus, this review aims to investigate if increases in voluntary performance after a conditioning contraction (CC) are related to the PAP phenomenon. For this, studies that confirmed the presence of PAP through an evoked response after a voluntary CC and concurrently evaluated PAPE were reviewed. Some studies reported increases in PAPE when PAP reaches extremely high values. However, PAPE has also been reported when PAP was not present, and unchanged/diminished performance has been identified when PAP was present. This range of observations demonstrates that mechanisms of PAPE are different from mechanisms of PAP. These mechanisms of PAPE still need to be understood and those studying PAPE should not assume that regulatory light chain phosphorylation is the mechanism for such enhanced voluntary performance. Novelty The occurrence of PAP does not necessarily mean that the voluntary performance will be improved. Improvement in voluntary performance is sometimes observed when the PAP level reaches extremely high values. Other mechanisms may be more relevant than that for PAP in the manifestation of acute increases in performance following a conditioning contraction.

A stochastic simulation of skeletal muscle calcium transients in a structurally realistic sarcomere model using MCell

Skeletal muscle contraction is initiated when an action potential triggers the release of Ca2+ into the sarcomere in a process referred to as excitation-contraction coupling. The speed and scale of this process makes direct observation very challenging and invasive. To determine how the concentration of Ca2+ changes within the myofibril during a single activation, several simulation models have been developed. These models follow a common pattern; divide the half sarcomere into a series of compartments, then use ordinary differential equations to solve reactions occurring within and between the compartments. To further develop this type of simulation, we have created a realistic structural model of a skeletal muscle myofibrillar half-sarcomere using MCell software that incorporates the myofilament lattice structure. Using this simulation model, we were successful in reproducing the averaged calcium transient during a single activation consistent with both the experimental and previous simulation results. In addition, our simulation demonstrated that the inclusion of the myofilament lattice within our model produced an asymmetric distribution of Ca2+, with more Ca2+ accumulating near the Z-disk and less Ca2+ reaching the m-line. This asymmetric distribution of Ca2+ is also apparent when we examine how the Ca2+ are bound to the troponin-C proteins along the actin filaments. Our simulation model also allowed us to produce advanced visualizations of this process, including two simulation animations, allowing us to view Ca2+ release, diffusion, binding and uptake within the myofibrillar half-sarcomere.

Myosin phosphorylation improves contractile economy of mouse fast skeletal muscle during staircase potentiation

Phosphorylation of the myosin regulatory light chain (RLC) by skeletal myosin light chain kinase (skMLCK) potentiates rodent fast twitch muscle but is an ATP-requiring process. Our objective was to investigate the effect of skMLCK-catalyzed RLC phosphorylation on the energetic cost of contraction and the contractile economy (ratio of mechanical output to metabolic input) of mouse fast twitch muscle in vitro (25°C). To this end, extensor digitorum longus (EDL) muscles from wild-type (WT) and from skMLCK-devoid (skMLCK^-/-) mice were subjected to repetitive low-frequency stimulation (10 Hz for 15 s) to produce staircase potentiation of isometric twitch force, after which muscles were quick frozen for determination of high-energy phosphate consumption (HEPC). During stimulation, WT muscles displayed significant potentiation of isometric twitch force while skMLCK^-/- muscles did not (i.e. 23% versus 5% change, respectively). Consistent with this, RLC phosphorylation was increased ∼3.5-fold from the unstimulated control value in WT but not in skMLCK^-/- muscles. Despite these differences, the HEPC of WT muscles was not greater than that of skMLCK^-/- muscles. As a result of the increased contractile output relative to HEPC, the calculated contractile economy of WT muscles was greater than that of skMLCK^-/- muscles. Thus, our results suggest that skMLCK-catalyzed phosphorylation of the myosin RLC increases the contractile economy of WT mouse EDL muscle compared with skMLCK^-/- muscles without RLC phosphorylation.

Sarcomere mechanics in striated muscles: from molecules to sarcomeres to cells

Muscle contraction is commonly associated with the cross-bridge and sliding filament theories, which have received strong support from experiments conducted over the years in different laboratories. However, there are studies that cannot be readily explained by the theories, showing 1) a plateau of the force-length relation extended beyond optimal filament overlap, and forces produced at long sarcomere lengths that are higher than those predicted by the sliding filament theory; 2) passive forces at long sarcomere lengths that can be modulated by activation and Ca²⁺, which changes the force-length relation; and 3) an unexplained high force produced during and after stretch of activated muscle fibers. Some of these studies even propose "new theories of contraction." While some of these observations deserve evaluation, many of these studies present data that lack a rigorous control and experiments that cannot be repeated in other laboratories. This article reviews these issues, looking into studies that have used intact and permeabilized fibers, myofibrils, isolated sarcomeres, and half-sarcomeres. A common mechanism associated with sarcomere and half-sarcomere length nonuniformities and a Ca²⁺-induced increase in the stiffness of titin is proposed to explain observations that derive from these studies.

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

X-ray diffraction analysis of the effects of myosin regulatory light chain phosphorylation and butanedione monoxime on skinned skeletal muscle fibers

The phosphorylation of the myosin regulatory light chain (RLC) is an important modulator of skeletal muscle performance and plays a key role in posttetanic potentiation and staircase potentiation of twitch contractions. The structural basis for these phenomena within the filament lattice has not been thoroughly investigated. Using a synchrotron radiation source at SPring8, we obtained X-ray diffraction patterns from skinned rabbit psoas muscle fibers before and after phosphorylation of myosin RLC in the presence of myosin light chain kinase, calmodulin, and calcium at a concentration below the threshold for tension development ([Ca(2+)] = 10(-6.8)M). After phosphorylation, the first myosin layer line slightly decreased in intensity at ∼0.05 nm(-1)along the equatorial axis, indicating a partial loss of the helical order of myosin heads along the thick filament. Concomitantly, the (1,1/1,0) intensity ratio of the equatorial reflections increased. These results provide a firm structural basis for the hypothesis that phosphorylation of myosin RLC caused the myosin heads to move away from the thick filaments towards the thin filaments, thereby enhancing the probability of interaction with actin. In contrast, 2,3-butanedione monoxime (BDM), known to inhibit contraction by impeding phosphate release from myosin, had exactly the opposite effects on meridional and equatorial reflections to those of phosphorylation. We hypothesize that these antagonistic effects are due to the acceleration of phosphate release from myosin by phosphorylation and its inhibition by BDM, the consequent shifts in crossbridge equilibria leading to opposite changes in abundance of the myosin-ADP-inorganic phosphate complex state associated with helical order of thick filaments.

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.

The activation time-course of contractile elements estimated from in vivo fascicle behaviours during twitch contractions

To better understand the cascade from neural activation up to force production within in vivo contracting muscle-tendon units, we estimated activation of contractile elements from experimentally measured human fascicle length change and force using a Hill-type muscle model. The experiment was conducted with respect to twitch contractions of the tibialis anterior muscle at three joint angles. As muscle contractile element force is a function of its length and velocity, the activation of contractile elements was calculated using a Hill-type muscle model and measured data. The results were able to reproduce the continuous rising activation of contractile elements after termination of electromyographic activity, the earlier shift of peak activation in time compared to twitch force, and the differences in time-course activation at three different joint angles. These findings are consistent with the predicted change in the activation of contractile elements from previous reports. Also, the results suggest that the time-course of the activation of contractile elements was greatly influenced by the change in force generating capacities related to both length and velocity, even in fixed end contractions, which could result from muscle-tendon interaction.

Extra forces evoked during electrical stimulation of the muscle or its nerve are generated and modulated by a length-dependent intrinsic property of muscle in humans and cats

Extra forces or torques are defined as forces or torques that are larger than would be expected from the input or stimuli, which can be mediated by properties intrinsic to motoneurons and/or to the muscle. The purpose of this study was to determine whether extra forces/torques evoked during electrical stimulation of the muscle or its nerve with variable frequency stimulation are modulated by muscle length/joint angle. A secondary aim was to determine whether extra forces/torques are generated by an intrinsic neuronal or muscle property. Experiments were conducted in 14 able-bodied human subjects and in eight adult decerebrate cats. Torque and force were measured in human and cat experiments, respectively. Extra forces/torques were evoked by stimulating muscles with surface electrodes (human experiments) or by stimulating the nerve with cuff electrodes (cat experiments). In humans and cats, extra forces/torques were larger at short muscle lengths, indicating that a similar regulatory mechanism is involved. In decerebrate cats, extra forces and length-dependent modulation were unaffected by intrathecal methoxamine injections, despite evidence of increased spinal excitability, and by transecting the sciatic nerve proximal to the nerve stimulations. Anesthetic nerve block experiments in two human subjects also failed to abolish extra torques and the length-dependent modulation. Therefore, these data indicate that extra forces/torques evoked during electrical stimulation of the muscle or nerve are muscle length-dependent and primarily mediated by an intrinsic muscle property.

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.

Joint angle dependence of intermuscle difference in postactivation potentiation

The purpose of this study was to examine the effect of ankle joint angle on the intermuscle difference in postactivation potentiation (PAP) between the medial gastrocnemius (MG) and soleus (SOL) muscles. At the neutral position of joint angle, dorsiflexion of 20 degrees , and plantarflexion of 20 degrees , twitch responses were evoked by stimulating the posterior tibial nerve with supramaximal intensity before and after a 10-s maximal voluntary plantarflexion at each joint angle. Mechanical properties of the MG and SOL muscles were assessed simultaneously and separately by using mechanomyography (MMG), and the extent of potentiation of each muscle was evaluated by peak-to-peak amplitude of the MMG signal. The MG showed greater potentiation than the soleus after the conditioning MVC in the neutral and dorsiflexion position, while in the plantarflexion position no significant difference was found in PAP between MG and SOL. These results suggest that the difference in the magnitude of PAP between synergistic muscles is determined by a combination of the joint angle- and fiber composition-dependence of PAP.

Dynamics of muscle fibre growth during postnatal mouse development

Background

Postnatal growth in mouse is rapid, with total skeletal muscle mass increasing several-fold in the first few weeks. Muscle growth can be achieved by either an increase in muscle fibre number or an increase in the size of individual myofibres, or a combination of both. Where myofibre hypertrophy during growth requires the addition of new myonuclei, these are supplied by muscle satellite cells, the resident stem cells of skeletal muscle.

Results

Here, we report on the dynamics of postnatal myofibre growth in the mouse extensor digitorum longus (EDL) muscle, which is essentially composed of fast type II fibres in adult. We found that there was no net gain in myofibre number in the EDL between P7 and P56 (adulthood). However, myofibre cross-sectional area increased by 7.6-fold, and length by 1.9-fold between these ages, resulting in an increase in total myofibre volume of 14.1-fold: showing the extent of myofibre hypertrophy during the postnatal period. To determine how the number of myonuclei changes during this period of intense muscle fibre hypertrophy, we used two complementary mouse models: 3F-nlacZ-E mice express nlacZ only in myonuclei, while Myf5^nlacZ/+ mice have β-galactosidase activity in satellite cells. There was a ~5-fold increase in myonuclear number per myofibre between P3 and P21. Thus myofibre hypertrophy is initially accompanied by a significant addition of myonuclei. Despite this, the estimated myonuclear domain still doubled between P7 and P21 to 9.2 × 10³ μm³. There was no further addition of myonuclei from P21, but myofibre volume continued to increase, resulting in an estimated ~3-fold expansion of the myonuclear domain to 26.5 × 10³ μm³ by P56. We also used our two mouse models to determine the number of satellite cells per myofibre during postnatal growth. Satellite cell number in EDL was initially ~14 satellite cells per myofibre at P7, but then fell to reach the adult level of ~5 by P21.

Conclusions

Postnatal fast muscle fibre type growth is divided into distinct phases in mouse EDL: myofibre hypertrophy is initially supported by a rapid increase in the number of myonuclei, but nuclear addition stops around P21. Since the significant myofibre hypertrophy from P21 to adulthood occurs without the net addition of new myonuclei, a considerable expansion of the myonuclear domain results. Satellite cell numbers are initially stable, but then decrease to reach the adult level by P21. Thus the adult number of both myonuclei and satellite cells is already established by three weeks of postnatal growth in mouse.

Cellular and whole muscle studies of activity dependent potentiation

With a single activation, a skeletal muscle fiber, motor unit or whole muscle will yield a twitch contraction. The twitch is not an "all-or-none" response, but a submaximal response that can vary from one time to another. Prior activation causes myosin regulatory light chain (RLC) phosphorylation, by an enzyme called myosin light chain kinase. This phosphorylation dissipates slowly over the next several minutes due to a slow activity of light chain phosphatase. Phosphorylation of the RLC increases the mobility of the S1 head of myosin, bringing the S1 head in closer proximity to the myosin binding sites on actin. This increased mobility increases the rate of engagement of cross-bridges and increases the rate of force development and contraction magnitude on subsequent twitch or other submaximal contractions. We call this increased contractile response "activity dependent potentiation". With sequential twitches or incompletely fused tetanic contractions, the term staircase is used to describe the progressive increase in amplitude of contraction. If a twitch is elicited after a tetanic contraction, we call the enhanced response posttetanic potentiation. Stretching a muscle fiber to a longer length will also bring the actin filaments close to the myosin heads. This increases the Ca²(+) sensitivity, independent of RLC phosphorylation. At long sarcomere lengths, the impact of RLC phosphorylation is diminished, because Ca²(+) sensitivity is already increased. Similarly, lowering the temperature at which the muscle is tested increases Ca²(+) sensitivity. At low temperatures, staircase and posttetanic potentiation are diminished, but RLC phosphorylation still occurs. Activity dependent potentiation is an important functional modulator of contractile response.

Potentiation of isometric and isotonic contractions during high-frequency stimulation

Activity dependent potentiation is an enhanced contractile response resulting from previous contractile activity. It has been proposed that even a maximal effort contraction may be enhanced by prior activity if there is an increase in the peak rate of force development. This should increase the peak active force during a very brief maximal effort contraction. The purpose of these experiments was to evaluate potentiation during brief sequential contractions with high-frequency stimulation. For this experiment, the rat medial gastrocnemius muscle was isolated in situ. Sequential stimulation (two contractions per second for 4 s) with 200, 300, or 400 Hz doublets, triplets, and quadruplets was applied. A small degree of force potentiation was observed in isometric contractions at the reference length (RL), but the activity dependent potentiation of isometric contractions was greater at short muscle length. For example, peak rate of force development for 200 Hz doublets increased significantly from the first to the eighth contraction (from 0.30+/-0.02 to 0.34+/-0.02 N.s(-1) at RL and from 0.18+/-0.02 to 0.28+/-0.01 N.s(-1) at RL-3 mm). During isotonic contractions, there were significant increases in peak shortening from the first to the eighth contraction. With 200 Hz doublet stimulation, shortening increased from 0.85+/-0.14 to 1.14+/-0.17 mm, and this corresponded with an increase in peak velocity (from 116+/-18 to 136+/-19 mm.s(-1)). Remarkably, even 400 Hz quadruplets showed a significant increase in shortening during repeated contractions (2 s(-1)). These observations indicate the possibility that activity dependent potentiation can result in significant improvement in both isometric and dynamic contractions, even when activated at very high frequency.

Dantrolene, like fatigue, has a length-dependent effect on submaximal force-length relationships of rat gastrocnemius muscle

AIM

Fatigue is length-dependent; relative active force depression is greater when measured at short lengths than at long lengths. Several unsatisfactory mechanisms have been proposed to explain this length dependence of fatigue, including: damaged myofilaments, stretch of 'in-series' structures, impaired t-tubule conduction and reduced intensity of activation. Dantrolene targets the ryanodine receptors, inhibiting stimulation-induced release of Ca2+. The purpose of this study was to determine if the force depression caused by dantrolene treatment also has a length dependence.

METHODS

Submaximal (single-pulse, double-pulse and 50 Hz stimulation) active force-length relationships were obtained from the medial gastrocnemius muscle of anaesthetized rats, before and after intravenous injection with dantrolene dissolved in propylene glycol.

RESULTS

Dantrolene treatment was sufficient to reduce twitch amplitude by 25%. Similar to the consequences of repetitive stimulation, dantrolene treatment caused the same decrease in absolute active force across a broad range of test lengths, for twitch, double-pulse and 50 Hz contractions. Considering that active force is smaller at short lengths than at long lengths, this similar absolute force decrease represents a greater relative decrease at short lengths. Clearly, there is a length-dependent impact of attenuated Ca2+ release by dantrolene on relative active force.

CONCLUSION

This study demonstrates that there is a length dependence of force depression associated with decreased Ca2+ release due to dantrolene treatment; therefore, if fatigue is due to decreased Ca2+ release, then additional length-dependent mechanisms are not required to explain the reported length dependence of force depression.

Low-frequency fatigue, post-tetanic potentiation and their interaction at different muscle lengths following eccentric exercise

Low-frequency fatigue (LFF) and post-tetanic potentiation (PTP) were quantified at different muscle lengths in rat medial gastrocnemius (GM) muscle. In situ experiments were performed on GM muscle-tendon complexes of anaesthetised (urethane, 1.5 g kg(-1) i.p.) Wistar rats (N=8). Force-length characteristics were determined at maximal (200 Hz) and submaximal (60 Hz) stimulation. Data for submaximally stimulated muscle were obtained in a non-potentiated and in a potentiated condition. LFF was induced by a series of 40 eccentric contractions. Post-exercise (40-80 min), data for the force-length relationships were obtained once more. Whereas force loss at 200 Hz-stimulation was least at optimum muscle length, L(0,200 Hz), (17.0+/-1.4%, mean +/-S.E.M.), force loss at 60 Hz-stimulation was maximal near L(0,200 Hz) (55.1+/-4.3% at L(0,200 Hz)-1 mm). When the muscle was potentiated, force loss at 60 Hz-stimulation was maximal at short muscle length: L(0,200 Hz)-4 mm (53.5+/-3.8%). The extent of LFF, quantified by a decrease in the 60:200 Hz force ratio, varied with muscle length: LFF increased with decreasing muscle lengths when muscles were potentiated. However, in the non-potentiated condition, LFF was maximal at a length just below L(0,200 Hz); the 60:200 Hz force ratio had decreased to 54.6+/-5.9% of the pre-exercise ratio at L(0,200 Hz)-1 mm. Compared with the non-potentiated condition, LFF was less pronounced in the potentiated condition. PTP counteracted LFF particularly at long muscle lengths. However, at short muscle lengths, LFF was still observed in potentiated muscles.

The length dependence of muscle active force: considerations for parallel elastic properties

The purpose of this study was to choose between two popular models of skeletal muscle: one with the parallel elastic component in parallel with both the contractile element and the series elastic component (model A), and the other in which it is in parallel with only the contractile element (model B). Passive and total forces were obtained at a variety of muscle lengths for the medial gastrocnemius muscle in anesthetized rats. Passive force was measured before the contraction (passive A) or was estimated for the fascicle length at which peak total force occurred (passive B). Fascicle length was measured with sonomicrometry. Active force was calculated by subtracting passive (A or B) force from peak total force at each fascicle or muscle length. Optimal length, that fascicle length at which active force is maximized, was 13.1 +/- 1.2 mm when passive A was subtracted and 14.0 +/- 1.1 mm with passive B (P < 0.01). Furthermore, the relationship between double-pulse contraction force and length was broader when calculated with passive B than with passive A. When the muscle was held at a long length, passive force decreased due to stress relaxation. This was accompanied by no change in fascicle length at the peak of the contraction and only a small corresponding decrease in peak total force. There is no explanation for the apparent increase in active force that would be obtained when subtracting passive A from the peak total force. Therefore, to calculate active force, it is appropriate to subtract passive force measured at the fascicle length corresponding to the length at which peak total force occurs, rather than passive force measured at the length at which the contraction begins.

Role of Calcium Sensitivity Modulation in Skeletal Muscle Performance

A common mechanism affecting Ca2+ sensitivity in skeletal muscle is the proximity of myosin heads with actin filaments, a function of myofilament lattice spacing and myosin head mobility with respect to the myosin filament. This is an important mechanism of pCa2+50 modulation by length, pH, regulatory light-chain phosphorylation, and temperature.