The effect of dantrolene on the enhancement and diminution of tension evoked by staircase and by tetanus in rat muscle

Development of Ryanodine Receptor (RyR) Inhibitors for Skeletal Muscle and Heart Diseases

Ryanodine receptors (RyR) are intracellular calcium (Ca2+) release channels on the sarcoplasmic reticulum of skeletal and cardiac muscles that play a central role in excitation-contraction coupling. Genetic mutations or posttranslational modifications of RyR causes hyperactivation of the channel, leading to various skeletal muscle and heart diseases. Currently, no specific treatments exist for most RyR-associated diseases. Recently, high-throughput screening (HTS) assays have been developed to identify potential candidates for treating RyR-related muscle diseases. These assays have successfully identified several compounds as novel RyR inhibitors, which are effective in animal models. In this review, we will focus on recent progress in HTS assays and discuss future perspectives of these promising approaches.

Drug development for the treatment of RyR1-related skeletal muscle diseases

Type 1 ryanodine receptor (RyR1) is an intracellular Ca²⁺ release channel on the sarcoplasmic reticulum of skeletal muscle, and it plays a central role in excitation-contraction (E-C) coupling. Mutations in RyR1 are implicated in various muscle diseases including malignant hyperthermia, central core disease, and myopathies. Currently, no specific treatment exists for most of these diseases. Recently, high-throughput screening (HTS) assays have been developed for identifying potential candidates for treating RyR-related muscle diseases. Currently, two different methods, namely a FRET-based assay and an endoplasmic reticulum Ca²⁺-based assay, are available. These assays identified several compounds as novel RyR1 inhibitors. In addition, the development of a reconstituted platform permitted HTS assays for E-C coupling modulators. In this review, we will focus on recent progress in HTS assays and discuss future perspectives of these promising approaches.

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

A novel RyR1-selective inhibitor prevents and rescues sudden death in mouse models of malignant hyperthermia and heat stroke

Mutations in the type 1 ryanodine receptor (RyR1), a Ca²⁺ release channel in skeletal muscle, hyperactivate the channel to cause malignant hyperthermia (MH) and are implicated in severe heat stroke. Dantrolene, the only approved drug for MH, has the disadvantages of having very poor water solubility and long plasma half-life. We show here that an oxolinic acid-derivative RyR1-selective inhibitor, 6,7-(methylenedioxy)-1-octyl-4-quinolone-3-carboxylic acid (Compound 1, Cpd1), effectively prevents and treats MH and heat stroke in several mouse models relevant to MH. Cpd1 reduces resting intracellular Ca²⁺, inhibits halothane- and isoflurane-induced Ca²⁺ release, suppresses caffeine-induced contracture in skeletal muscle, reduces sarcolemmal cation influx, and prevents or reverses the fulminant MH crisis induced by isoflurane anesthesia and rescues animals from heat stroke caused by environmental heat stress. Notably, Cpd1 has great advantages of better water solubility and rapid clearance in vivo over dantrolene. Cpd1 has the potential to be a promising candidate for effective treatment of patients carrying RyR1 mutations.

Mutations in ryanodine receptor 1 (RyR1), a Ca2+ release channel in skeletal muscle, cause malignant hyperthermia (MH) and are involved in heat stroke. Here, the authors show that an oxolinic acid-derivative RyR1 inhibitor effectively prevents and treats MH and heat stroke in various MH mouse models.

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.

The force dependence of isometric and concentric potentiation in mouse muscle with and without skeletal myosin light chain kinase

The isometric potentiation associated with myosin phosphorylation is force dependent. The purpose of this study was to assess the influence of a pre-existing period of isometric force on the concentric force potentiation displayed by mouse muscles with and without the ability to phosphorylate myosin. We tested isometric (ISO) and concentric (CON) potentiation, as well as concentric potentiation after isometric force (ISO-CON), in muscles from wild-type (WT) and skeletal myosin light chain kinase-deficient (skMLCK−/−) mice. A conditioning stimulus increased (i.e., potentiated) mean concentric force in the ISO-CON and CON conditions to 1.31 ± 0.02 and 1.35 ± 0.02 (WT) and to 1.19 ± 0.02 and 1.21 ± 0.01 (skMLCK−/−) of prestimulus levels, respectively (data n = 6–8, p < 0.05). No potentiation of mean isometric force was observed in either genotype. The potentiation of mean concentric force was inversely related to relative tetanic force level (P/Po) in both genotypes. Moreover, concentric potentiation varied greatly within each contraction type and was negatively correlated with unpotentiated force in both genotypes. Thus, although no effect of pre-existing force was observed, strong and inverse relationships between concentric force potentiation and unpotentiated concentric force may suggest an influence of attached and force-generating crossbridges on potentiation magnitude in both WT and skMLCK−/− muscles.

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.

Length-dependent twitch contractile characteristics of skeletal muscle

The length dependence of force development of mammalian skeletal muscles was evaluated during twitch, double-pulse, and tetanic contractions, and the relation between muscle length and the time-dependent characteristics of twitch and double-pulse contractions were determined. In situ isometric contractions of the rat gastrocnemius muscle were analyzed at seven different lengths, based on a reference length at which the maximal response to double-pulse contractions occurred (Lopt-2P). Twitch and double-pulse contractions were analyzed for developed tension (DT), contraction time (tC), average rate of force development (DT-tC(-1)), half-relaxation time (t50%R), peak rate of relaxation (DT x dtmin(-1)), and 90%-relaxation time (t90%R). Considering the length at which maximal tetanic DT occurred to be the optimal length (Lo-TET), the peak DT for twitch contractions and double-pulse contractions was observed at Lo-TET + 0.75 mm (p < 0.05) and Lo-TET + 0.1 mm (p > 0.05), respectively. When measured at the length for which maximal twitch and double-pulse contractions were obtained, tetanic DT was 95.2 +/- 3 and 99.0 +/- 2% of the maximal value, respectively. These observations suggest that double-pulse contractions are more suitable for setting length for experimental studies than twitch contractions. Twitch and double-pulse contraction tC were 15.53 +/- 1.14 and 25.0 +/- 0.6 ms, respectively, at Lopt-2P, and increased above Lopt-2P and decreased below Lopt-2P. Twitch t50%R was 12.18 +/- 0.90 ms at Lopt-2P, and increased above Lopt-2P and below Lopt-2P. Corresponding changes for double-pulse contractions were greater. Stretching the muscle leads to slower twitch contractions and double-pulse contractions, but the mechanisms of this change in time course remain unclear.

Length dependence of staircase potentiation: interactions with caffeine and dantrolene sodium

In skeletal muscle, there is a length dependence of staircase potentiation for which the mechanism is unclear. In this study we tested the hypothesis that abolition of this length dependence by caffeine is effected by a mechanism independent of enhanced Ca2+ release. To test this hypothesis we have used caffeine, which abolishes length dependence of potentiation, and dantrolene sodium, which inhibits Ca2+ release. In situ isometric twitch contractions of rat gastrocnemius muscle before and after 20 s of repetitive stimulation at 5 Hz were analyzed at optimal length (Lo), Lo - 10%, and Lo + 10%. Potentiation was observed to be length dependent, with an increase in developed tension (DT) of 78 ± 12, 51 ± 5, and 34 ± 9% (mean ± SEM), at Lo - 10%, Lo, and Lo + 10%, respectively. Caffeine diminished the length dependence of activation and suppressed the length dependence of staircase potentiation, giving increases in DT of 65±13, 53 ± 11, and 45 ± 12% for Lo - 10%, Lo, and Lo + 10%, respectively. Dantrolene administered after caffeine did not reverse this effect. Dantrolene alone depressed the potentiation response, but did not affect the length dependence of staircase potentiation, with increases in DT of 58 ± 17, 26 ± 8, and 18 ± 7%, respectively. This study confirms that there is a length dependence of staircase potentiation in mammalian skeletal muscle which is suppressed by caffeine. Since dantrolene did not alter this suppression of the length dependence of potentiation by caffeine, it is apparently not directly modulated by Ca2+ availability in the myoplasm.

Staircase in mammalian muscle without light chain phosphorylation

In disuse atrophied skeletal muscle, the staircase response is virtually absent and light chain phosphorylation does not occur. The purpose of the present study was to determine if staircase could be restored in atrophied muscle with continued absence of myosin light chain phosphorylation, by reducing what appears to be an otherwise enhanced calcium release. Control (untreated) and sham-operated female Sprague-Dawley rats were compared with animals after 2 weeks of complete inactivity induced by tetrodotoxin (TTX) application to the left sciatic nerve. In situ isometric contractile responses of rat gastrocnemius muscle were analyzed before and after administration of dantrolene sodium (DS), a drug which is known to inhibit Ca2+ release in skeletal muscle. Twitch active force (AF) was attenuated by DS from 2.2 +/- 0.2 N, 2.7 +/- 0.1 N and 2.4 +/- 0.2 N to 0.77 +/- 0.2 N, 1.05 +/- 0.1 N and 1.01 +/- 0.2 N in TTX (N = 5), sham (N = 11) and control (N = 7) muscles, respectively. Following dantrolene treatment, 10 s of 10-Hz stimulation increased AF to 1.32 +/- 0.2 N, 1.52 +/- 0.1 N and 1.45 +/- 0.2 N for the TTX, sham and control groups, respectively, demonstrating a positive staircase response. Regulatory light chain (R-LC) phosphorylation was lower for TTX-treated (5.5 +/- 5.5%) than for control (26.1 +/- 5.3%) and sham (20.0 +/- 5%) groups. There was no significant change from resting levels for any of the groups after DS treatment (P = 0.88). This study shows that treatment with dantrolene permits staircase in atrophied muscle as well as control muscle, by a mechanism which appears to be independent of R-LC phosphorylation.

In vivo 31P NMR assessed effects of dantrolene on mechanics and energy metabolism in tetanic stimulated rat gastrocnemius

Dantrolene does not affect fatigue from submaximal effort and MVC while it decreases twitch tension. We hypothesize that dantrolene could modify the relation between energy metabolism and fatigue by inhibiting calcium release from the sarcoplasmic reticulum. The effects of dantrolene (10 mg) on mechanical and metabolic parameters of gastrocnemius muscle were examined by 31P NMR during an in vivo fatigue test. The fatigue test constituted of three successive 20 min periods of increased stimulation rhythms and followed by a 20 min recovery period. 31P NMR was used to determine phosphocreatine (PCr), ATP and intracellular pH changes, while tension was recorded. We showed that dantrolene increased mechanical fatigue while PCr levels were similar to those from control animals. Acidosis was most prominent in dantrolene treated rats. These results suggest that dantrolene firstly affects calcium cycling with additive effects to fatigue and, secondly, modifies the activation of oxidative metabolism and the energy cost of the generated tension.

Dantrolene sodium and fatigue of long duration

A long-lasting impairment of muscular force generation follows fatiguing exercise (fatigue of long duration), the physiological basis of which is not well understood. To investigate the role of reduced calcium release in long-lasting fatigue, we examined the effects of dantrolene sodium, which selectively decreases calcium release from the sarcoplasmic reticulum. The drug impaired muscle function in a pattern identical to that of long-lasting fatigue. The results are consistent with either independent effects of dantrolene and exercise at the same site in the excitation-contraction coupling chain, or independent actions at separate serial sites.

Myosin light chain phosphorylation and tension potentiation in mouse skeletal muscle

In intact mammalian fast-twitch skeletal muscle, a quantitative relationship exists between the phosphate content of myosin P-light chain (PLC) and the extent of isometric twitch tension potentiation. It has been proposed that PLC phosphorylation causes twitch potentiation in intact muscle by rendering the contractile element more sensitive to activation by Ca2+. If this hypothesis is correct, then an obligatory experimental outcome is that the slope of the "PLC phosphate vs. isometric tension potentiation (ITP)" relationship should increase when the amount of Ca2+ available to activate the contractile element is decreased. Intact mouse extensor digitorum longus muscles were studied in the absence and presence of sodium dantrolene, an agent that partially inhibits Ca2+ release from the sarcoplasmic reticulum (SR). Treatment of muscles with dantrolene produced a 73% reduction in isometric twitch tension and an approximately threefold increase in the slope of the PLC phosphate vs. ITP relationship. Under experimental conditions that produced fused, tetanic contractions equal to 0.52, 0.72, 0.94, and 1.0 force production, contraction-induced changes in PLC phosphate content were directly proportional to the extent of contractile element activation, whereas the extent of ITP was inversely proportional to the extent of contractile element activation. These data indicate that the slope of the PLC phosphate vs. ITP relationship varies inversely as a function of the amount of Ca2+ that is released from the SR to activate the contractile element during both twitch and fused, submaximal tetanic contractions. Furthermore, these findings support the hypothesis that ITP in intact skeletal muscle is due to a PLC phosphorylation-induced sensitization of the contractile element to activation by Ca2+.

Ankle dorsiflexor twitch properties in malignant hyperthermia

A noninvasive method to diagnose malignant hyperthermia (MH) was sought. To this end, in vivo isometric twitch properties of the ankle dorsiflexor muscles were studied in three groups: (1) MH-susceptible patients (n = 12), (2) relatives (n = 12) of MH-susceptible patients who were judged to be MH resistant, and (3) a group of normal volunteers (n = 42) chosen from the community. Twitch properties were studied under resting state conditions and with 1 or 2 Hz stimulation to produce the negative staircase twitch response. There was a high degree of overlap between the ranges of the measured twitch parameters of all groups. Thus, the techniques presented in this study have no value in diagnosing susceptibility to MH. Several physiological features of human isometric twitch properties were demonstrated: (1) slowing of twitch speed with advancing age, (2) strong positive correlation between body weight and twitch torque, and (3) a negative staircase response typical of that described in other mammalian twitch studies.

Fatigability of rat hindlimb muscle: associations between electromyogram and force during a fatigue test

1. An experimental protocol designed to assess fatigability in motor units (Burke, Levine, Tsairis & Zajac, 1973) has been applied to the whole muscles of anaesthetized adult rats, and the association between the electromyogram (EMG) and force was monitored over the course of the test. 2. Both test muscles (soleus and extensor digitorum longus) exhibited a wide range of fatigability, which was defined as the decline in isometric peak force at 6 min, such that the data could be separated into five levels of fatigability. Fatigue indices for each test muscle were distributed across three levels. 3. The EMG was quantified with four measures of amplitude, four of duration, and one interaction term (area). Correlation analyses indicated that the EMG was adequately represented by one measure of amplitude (absolute amplitude), one of duration (peak-to-peak duration) and area. The best single measure was area. 4. The EMG-force associations for soleus varied markedly among its three fatigability groups. In contrast, over the course of the test, all three extensor digitorum longus groups displayed qualitatively similar EMG-force associations. 5. Multiple regression analyses indicated that the EMG parameters were able to predict peak force better for extensor digitorum longus than for soleus. Furthermore, for both test muscle, the prediction was best for the most fatigable group. 6. The associations between EMG and force exhibited three patterns for the two test muscles and three levels of fatigability. These differences suggested variation in the mechanisms, related to both fibre-type composition and susceptibility to fatigue, that dictate the performance elicited by this particular stimulus regimen. The mechanisms seem to include both intracellular and transmission processes.

Relationship between parvalbumin content and the speed of relaxation in chronically stimulated rabbit fast-twitch muscle

The time courses of changes in parvalbumin (PA) content, isometric twitch tension, and half-relaxation time (1/2 RT) were studied in rabbit tibialis anterior muscle following chronic 10 Hz nerve stimulation of 1-21 days. Up to 5 days stimulation had no effect on PA content, but it induced a slight (10-15%) increase in the 1/2 RT. This change occurred together with the previously observed 50% decrease in Ca2+-uptake by the SR (Leberer et al. 1987). While prolonged stimulation produced no further decrease in the Ca2+-uptake by the SR, PA content declined after 5 days of stimulation. The reduction in PA content was accompanied by a progressive lengthening of the 1/2 RT. However, the increase in 1/2 RT was particularly pronounced after PA had fallen below 50% of its normal value. A 90% reduction in PA coincided with a 60% increase in the 1/2 RT. By this time the staircase phenomenon, normally observed in fast-twitch muscle, was completely abolished. Although the changes in PA content and 1/2 RT were not linearly related, these results suggest that PA plays an important role in the relaxation process of mammalian fast-twitch muscle.

Effects of dantrolene and methylxanthines on the sensory nerve terminal of the frog muscle spindle

The application of 1.5-4 microM dantrolene decreased the threshold and the current sensitivity of the rhythmic hyperpolarizations that occur during depolarization of the sensory nerve terminal in the frog muscle spindle. The higher concentration provoked spontaneous rhythmic changes even without depolarization. Methylxanthines (5 mM caffeine, theophylline or pentylene-tetrazole) increased the threshold and the sensitivity. Electron microscopic observations of the dantrolene-treated spindles revealed numerous electron-dense deposits associated with the cytoplasmic membrane of the sensory terminals and with mitochondrial membranes. The deposits were found to contain K+ and Ca2+ by energy dispersive X-ray microanalysis. Electron-dense deposits containing Ca2+ were usually observed in the inner capsular space and in the mitochondria of the sensory terminals perfused by normal or high Ca2+ Ringer solutions. They were reduced in number following incubation with methylxanthines. The amplitudes of afferent spikes and the spindle potential were increased by methylxanthines in much the same way as by K+ channel blockers, suggesting that GK of the terminal membrane may be reduced by methylxanthines. We suggest that methylxanthines may modulate the terminal responses both as a K+ channel blocker and by enhancing the release of Ca2+ from a storage site, perhaps in the inner capsular space, whereas dantrolene has the opposite effect.

Dantrolene--in vitro studies in malignant hyperthermia susceptible (MHS) and normal skeletal muscle

Dantrolene sodium, a hydantoin analogue, is efficacious in the therapy of malignant hyperthermia (MH). In order to improve our knowledge of the mode of action of dantrolene, we have examined the influence of dantrolene sodium on: (1) twitch and resting tensions, in the absence and the presence of caffeine, of intact skeletal muscle fascicles; and (2) caffeine induced tension rises of single chemically skinned skeletal muscle fascicles. We have found that dantrolene appears to exert its beneficial action on malignant hyperthermia susceptible (MHS) skeletal muscle by an indirect action on the sarcoplasmic reticulum (SR). Thus dantrolene inhibits twitch tensions of skeletal muscle fascicles, probably by indirectly preventing the release of calcium from the SR. To a lesser extent dantrolene inhibits caffeine induced contractures of skeletal muscle fascicles, probably by indirectly accelerating the uptake of calcium into the SR. Because the former effect is greater than the latter in vivo dantrolene sodium is effective only when given prior to total loss of calcium from the SR. Vigilant temperature and EKG monitoring of all patients during anaesthesia is, therefore, essential.

Evoked responses in normal and diseased muscle with particular reference to twitch potentiation

The compound muscle action potential and isometric twitch tension evoked by single and repetitive electrical stimuli are indicators of the number of motor units activated and of the contractile properties of the muscle. The action potentials and mechanical responses were recorded in proximal and distal muscles in patients with myasthenia gravis and myopathy and compared with findings in normal subjects. In normal muscle, at low rates of stimulation (2-3 s-1) the decrement was at most 5% in the action potential and 15-24% in the twitch tension. Tetanic stimuli (50 s-1) were unsuitable for diagnostic purposes because of movement artefact. In patients with myasthenia gravis, the incidence and size of the decrement of evoked responses were greater in the platysma than in the elbow flexors and the adductor pollicis (ADP) muscles. The 2-3 times greater post-tetanic facilitation (PTF) of the action potential in the platysma than in extremity muscles also indicates a more severe functional block in facial muscle. The PTF is an indicator of recruitment of blocked fibres. The maximal decrement was grossly related to the titre of antibodies against the acetylcholine receptor. To reveal failure of neuromuscular transmission in patients with myasthenia gravis without a decrement, a small dose of d-tubocurarine (0.2 mg in 30 ml of saline) was injected i.v. in the upper arm in a regional curare test. The sensitivity was greater in patients with myasthenia gravis than in controls and in patients with myopathy. Potentiation of twitch tension reflects contractile properties. In normal muscle twitch potentiation in the staircase (1-3 s-1, 100 s in duration) and after tetanus (50 s-1, 1.5 s in duration) was 2-3 times greater in the platysma than in the elbow flexors and ADP, presumably related to the greater proportion of fast-twitch fibers in facial muscle. The amplitude of the action potential and the twitch tension varied proportionally with the number of fibers activated and the difference in the decrements of the action potential and the twitch during the staircase in some patients with myasthenia gravis showed that the staircase phenomenon was diminished suggesting abnormalities in the excitation-contraction coupling. The diminution of the staircase and post-tetanic potentiation (PTP) in myopathy also indicates impairment of the excitation-contraction coupling. In rats with severe chronic myasthenia gravis, the staircase and PTP were decreased even when the failing neuromuscular transmission was circumvented by applying direct stimuli to the extensor digitorum longus muscle (EDL).

Action of dantrolene sodium on single motor units of cat muscle in vivo

Dantrolene sodium, a skeletal muscle relaxant, was investigated for its action on single motor units of the peroneus tertius muscle in cats anaesthetized with pentobarbitone sodium. Motor axons were isolated in ventral root filaments and their muscle units were identified as either fast-fatiguable (FF), fast-resistant (FR), fast-intermediate (FI) or slow-resistant (S). Dantrolene sodium (2 mg/kg) was administered intravenously in a solution of 1,2-propanediol. Effects were observed on the twitches, unfused tetanic contractions and maximal tetanic tensions of 78 motor units in 5 experiments. Contractile tension was depressed whereas muscle action potentials appeared unaffected. Maximal tetani were less depressed than unfused tetani and twitches. The reduction of tension was more pronounced for fast (FF, FR and FI) than for slow units. After drug injection, the mean tensions developed at the end of a 3 s period of stimulation at 40/s were: 13.1%, 10.6% and 12.7% of pre-drug control for FF, FR and FI units, respectively, and 67.1% for S units. Upon prolonged stimulation at 40/s fast units depressed by Dantrolene sodium were able to potentiate back to their initial pre-drug tension.

Effects of calcium antagonists on contraction of a holothurian muscle

Contractions evoked by acetylcholine or by tetanic stimulation differed from caffeine contractures, in muscle strips isolated from the longitudinal muscle bands of the body wall of Isostichopus badionotus (Selenka), an aspidochirote holothurian. Tetanic contractions and responses to acetylcholine remained reproducible for hours in sea water or artificial sea water. Caffeine contractures declined rapidly during a series of repetitions, carried out in a bath medium which had a calcium content equivalent to that of sea water. Manganese, lathanum, and dantrolene have been used as calcium antagonists, with the objective of differentiating the calcium stores used in reproducible contractions from the calcium stores used in rapidly extinguishing contractures. Muscle strips were pretreated with an ionophore (X-537A) to confer reproducibility in a series of caffeine contractures, before use of calcium antagonists. For reproducible caffeine contractures, the order of effectiveness of calcium antagonists was lanthanum greater than manganese greater than dantrolene. The order of reversibility was manganese greater than dantrolene greater than lanthanum. For acetylcholine contractions, or tetanic contractions, the order of effectiveness of calcium antagonists was lanthanum greater than manganese and the order of reversibility was manganese greater than lanthanum. Dantrolene reversibility enhanced contractions.

Enhancement and diminution of mechanical tension evoked by staircase and by tetanus in rat muscle

1. Potentiation of the isometric twitch tension was compared during and after the staircase and after tetanic stimuli in the fast-twitch extensor digitorum longus muscle of adult Lewis rats at 37-38 degrees C.2. With up to 250 stimuli the potentiation rose with an increase in both the frequency and number of stimuli in the staircase (2-5/sec) and the tetanus (100-167/sec). After a tetanus of 375 stimuli (125/sec) the potentiation was smaller. The potentiation 2 sec after a tetanus of 250 stimuli (167/sec) was + 132 +/- 5% (n = 21, s.e. of mean) which was greater (P < 0.001) than at the 250th stimulus at 5/sec, +92+/-3% (n = 21, s.e. of mean).3. After the staircase the decay of potentiation was initially slow and later more rapid. This was taken to indicate both the recovery of a process that diminished twitch tension and the decay of a process causing potentiation. After 250 stimuli (5/sec) the rate of decay of the processes causing diminution and potentiation had time constants of 34.5 +/- 3.8 sec (n = 18, s.e. of mean) and 102.2 +/- 6.6 sec (n = 20, s.e. of mean) respectively. Compared with the potentiation, the process causing diminution became relatively more pronounced the greater the frequency of stimuli.4. The decay of post-tetanic potentiation showed an initial rapid and a later slower phase of decay. After a tetanus of 250 stimuli (167/sec) the rates of decay had time constants of 5.7 +/- 0.8 sec (n = 16, s.e. of mean) and 113.5 +/- 8.7 sec (n = 19, s.e. of mean) respectively.5. Compared with the unpotentiated response the time course of the twitch was shortened initially in the staircase and when the post-tetanic potentiation was low. The contraction time was then increasingly prolonged the greater the potentiation and the greater the number of stimuli in the staircase and in the tetanus. The half-relaxation time was the more prolonged the greater the number of stimuli.6. Potentiation can be described in terms of a two-compartment model of processes which show saturation. Both compartments were activated in a tetanus whereas only the compartment with a slow rate of decay was activated in the staircase. It is speculated that the two compartments are related to the excitation-contraction coupling. The process that caused diminution of twitch tension during the staircase may be due to fatigue. It is suggested that the energy consumption in 250 twitches is about 10 times greater than in a tetanus of 250 stimuli which may explain the presence of fatigue after the staircase whereas it was absent after the tetanus.

Temperature dependence of enhancement and diminution of tension evoked by staircase and by tetanus in rat muscle

1. The effect of temperature (20-37.5 degrees C) on the potentiation of twitch tension was examined during and after the staircase (250 stimuli, 5/sec) and after the tetanus (188 stimuli, 125/sec) in the extensor digitorum longus muscle of adult Lewis rats.2. During the staircase at 20 degrees C the twitch tension decreased (negative staircase) by 10-20%. At 25-30 degrees C the staircase was initially negative and later positive. At 37.5 degrees C the staircase was positive throughout the train. Both at the end of the staircase and 2 sec after the tetanus the potentiation increased linearly with increasing temperature.3. After the staircase and the tetanus at 20-30 degrees C the twitch tension increased initially rapidly and later after the staircase at a slower rate. Maximal potentiation at 20 degrees C was attained 3 min after the staircase (+ 30 +/- 3%, n = 10, s.e. of mean) and 1 min after the tetanus (+ 16 +/- 1%, n = 10, s.e. of mean). At 37.5 degrees C the potentiation decayed rapidly after the staircase and the tetanus.4. During the staircase the time course of the twitch was shortened twice as much at 20 as at 37.5 degrees C. At the end of the staircase and 2 sec after the tetanus the contraction time was the more prolonged the greater the potentiation. At maximal potentiation the contraction time was prolonged three times as much at 20 degrees C (+ 19 +/- 3%, n = 10, s.e. of mean) as at 37.5 degrees C (P < 0.005). The half-relaxation time at the end of the staircase was prolonged 10 times more at 20 than at 37.5 degrees C (P < 0.02).5. When extrapolated to time zero after the staircase and the tetanus the potentiation at 20 degrees C was still marked (20-50%). The rate of decay of potentiation (time constant, 20 degrees C, 561.2 +/- 37.4 sec, n = 20, s.e. of mean) increased with increasing temperature (Q(10) = 2.6). The event of potentiation with a fast rate of decay, present after the tetanus but not after the staircase at 37.5 degrees C, was abolished below 30 degrees C.6. The increase in twitch tension after the staircase and the tetanus at 20-30 degrees C was taken to indicate the recovery of events that diminished the twitch, occurring simultaneously with potentiation.7. (i) One process of diminution, present after the staircase but not after the tetanus, increased on cooling and was assumed to be due to fatigue. The rate of recovery of the process (time constant, 20 degrees C, 79.6 +/- 7.4 sec, n = 10, s.e. of mean) increased with increasing temperature (Q(10) = 1.9). The half-relaxation time of the last twitch in the staircase was the more prolonged the greater the process. (ii) Another process causing diminution was present after the staircase and the tetanus at 20-30 degrees C. It recovered at 20 degrees C with a time constant of 14.9 +/- 2.2 sec (n = 10, s.e. of mean). This process, possibly responsible for the initially negative staircase, was not thought to be due to fatigue. It may reflect a diminished depolarization of the transverse tubules by repetitive stimuli.