Modulation of caffeine contractures in mammalian skeletal muscles by variation of extracellular potassium

Voltage modulates halothane-triggered Ca2+ release in malignant hyperthermia-susceptible muscle

Malignant hyperthermia can result from mutations in the ryanodine receptor that favor anesthetic-induced Ca²⁺ release. Zullo et al. find that membrane potential modulates the effect of the volatile anesthetic halothane on skeletal muscle ryanodine receptors possessing the Y524S mutation.

Malignant hyperthermia (MH) is a fatal hypermetabolic state that may occur during general anesthesia in susceptible individuals. It is often caused by mutations in the ryanodine receptor RyR1 that favor drug-induced release of Ca²⁺ from the sarcoplasmic reticulum. Here, knowing that membrane depolarization triggers Ca²⁺ release in normal muscle function, we study the cross-influence of membrane potential and anesthetic drugs on Ca²⁺ release. We used short single muscle fibers of knock-in mice heterozygous for the RyR1 mutation Y524S combined with microfluorimetry to measure intracellular Ca²⁺ signals. Halothane, a volatile anesthetic used in contracture testing for MH susceptibility, was equilibrated with the solution superfusing the cells by means of a vaporizer system. In the range 0.2 to 3%, the drug causes significantly larger elevations of free myoplasmic [Ca²⁺] in mutant (YS) compared with wild-type (WT) fibers. Action potential–induced Ca²⁺ signals exhibit a slowing of their time course of relaxation that can be attributed to a component of delayed Ca²⁺ release turnoff. In further experiments, we applied halothane to single fibers that were voltage-clamped using two intracellular microelectrodes and studied the effect of small (10-mV) deviations from the holding potential (−80 mV). Untreated WT fibers show essentially no changes in [Ca²⁺], whereas the Ca²⁺ level of YS fibers increases and decreases on depolarization and hyperpolarization, respectively. The drug causes a significant enhancement of this response. Depolarizing pulses reveal a substantial negative shift in the voltage dependence of activation of Ca²⁺ release. This behavior likely results from the allosteric coupling between RyR1 and its transverse tubular voltage sensor. We conclude that the binding of halothane to RyR1 alters the voltage dependence of Ca²⁺ release in MH-susceptible muscle fibers such that the resting membrane potential becomes a decisive factor for the efficiency of the drug to trigger Ca²⁺ release.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

Dihydropyridine (DHPR) and ryanodine receptors (RyRs) are central to transduction of transverse (T) tubular membrane depolarisation initiated by surface action potentials into release of sarcoplasmic reticular (SR) Ca2+ in skeletal muscle excitation-contraction coupling. Electronmicroscopic methods demonstrate an orderly positioning of such tubular DHPRs relative to RyRs in the SR at triad junctions where their membranes come into close proximity. Biochemical and genetic studies associated expression of specific, DHPR and RyR, isoforms with the particular excitation-contraction coupling processes and related elementary Ca2+ release events found respectively in skeletal and cardiac muscle. Physiological studies of intramembrane charge movements potentially related to voltage triggering of Ca2+ release demonstrated a particular qγ charging species identifiable with DHPRs through its T-tubular localization, pharmacological properties, and steep voltage-dependence paralleling Ca2+ release. Its nonlinear kinetics implicated highly co-operative conformational events in its transitions in response to voltage change. The effects of DHPR and RyR agonists and antagonists upon this intramembrane charge in turn implicated reciprocal rather than merely unidirectional DHPR-RyR interactions in these complex reactions. Thus, following membrane potential depolarization, an orthograde qγ-DHPR-RyR signaling likely initiates conformational alterations in the RyR with which it makes contact. The latter changes could then retrogradely promote further qγ-DHPR transitions through reciprocal co-operative allosteric interactions between receptors. These would relieve the resting constraints on both further, delayed, nonlinear qγ-DHPR charge transfers and on RyR-mediated Ca2+ release. They would also explain the more rapid charging and recovery qγ transients following larger depolarizations and membrane potential repolarization to the resting level.

The effect of chelerythrine on depolarization-induced force responses in skinned fast skeletal muscle fibres of the rat

1 We examined the effect of the protein kinase C (PKC) inhibitor chelerythrine on depolarization-induced force responses (DIFRs) and sarcoplasmic reticulum (SR) function in single, mechanically skinned skeletal muscle fibres of the rat. 2 In this study, the DIFRs in the skinned fibres normally underwent an irreversible loss of excitation-contraction coupling (ECC) after 10-15 responses. Chelerythrine (12 micro M) was shown to restore ECC in these fibres. Restored force responses were similar in peak (control 50.8+/-6.4%, chelerythrine 56.9+/-12.4% of maximum force, P=0.42, n=21), but significantly broadened compared to initial control responses (full-width at half maximum, control; 3.7+/-0.3 s, chelerythrine; 13.3+/-1.1 s, P<0.001). Early exposure to chelerythrine prevented run-down of DIFRs. Chelerythrine also induced spontaneous force responses in some fibres. 3 The PKC inhibitors calphostin C and staurosporine did not restore ECC, and the PKC activator phorbol 12-myristate 13-acetate did not promote loss of ECC in the skinned fibres. 4 Chelerythrine significantly increased SR Ca(2+) loading by 8.4+/-1.7% (P=0.02, n=9) and SR Ca(2+) release by at least 14.1+/-2.7% (P=0.004, n=11) in the skinned fibres. 5 Chelerythrine had no significant effect on maximum force production or the [Ca(2+)] producing half maximal activation of the myofilaments. However, chelerythrine did have a small effect on the slope of the force-Ca(2+) relationship (P=0.02, n=10). 6 Chelerythrine reverses the use-dependent loss of excitation-contraction coupling in skinend skeletal muscle fibres by a PKC independent pathway. Chelerythrine may be an important pharmacological probe for examining the mechanisms of contraction-induced muscle injury.

Different Ca2+ releasing action of caffeine and depolarisation in skeletal muscle fibres of the rat

1. The relative abilities of caffeine and transverse tubular (T-) system depolarisation to induce Ca2+ release in mammalian skeletal muscle were compared in mechanically skinned fibres of the rat, in order to determine whether normal excitation-contraction (E-C) coupling is achieved by up-regulating the Ca2+-induced Ca2+ release process, as caffeine is known to do. 2. Caffeine triggered Ca2+ release in soleus (slow-twitch) fibres at much lower concentrations than in extensor digitorum longus (EDL) (fast-twitch) fibres when the sarcoplasmic reticulum (SR) of each type was loaded with Ca2+ at close to endogenous levels. The difference in caffeine sensitivity resulted at least in part from the SR being loaded endogenously at near maximal capacity in soleus fibres but at less than half of maximal capacity in EDL fibres. The caffeine sensitivity could be reversed by reversing the relative level of SR loading. 3. The ability of caffeine to induce Ca2+ release was markedly reduced by lowering the level of SR loading or by raising the free [Mg2+] from 1 to 3 mM. Caffeine, even at 30 mM, triggered little or no Ca2+ release in EDL fibres (a) at 1 mM (physiological) Mg2+ when the SR was loaded at two-thirds or less of the endogenous level, and (b) at 3 mM Mg2+ when the SR was loaded at close to the endogenous level. In contrast, depolarisation potently elicited Ca2+ release under these conditions in the same fibres. 4. The inability of 30 mM caffeine to induce Ca2+ release under certain conditions was not attributable to desensitisation or inactivation of the release channels, because there was no response even upon initial exposure to caffeine and depolarisation always remained able to trigger Ca2+ release. It instead appeared that caffeine was a relatively ineffectual stimulus in EDL fibres except under conditions where (a) the SR was heavily loaded, (b) the free [Mg2+] was low, or (c) a high [Cl-] was present. 5. These results show that the normal E-C coupling mechanism in mammalian skeletal muscle does not involve just enhancing Ca2+-induced Ca2+ release, and evidently requires the removal or bypassing of the inhibitory effect of Mg2+ on the Ca2+ release channels.

Excitation-contraction coupling in skeletal muscle: comparisons with cardiac muscle

1. The present review describes the mechanisms involved in controlling Ca2+ release from the sarcoplasmic reticulum (SR) of skeletal muscle, which ultimately regulates contraction. 2. Comparisons are made between cardiac and skeletal muscle with respect to: (i) the role of the dihydropyridine receptors (DHPR) as Ca2+ channels and voltage-sensors; (ii) the regulation of the ryanodine receptor (RyR)/Ca2+-release channels in the SR; and (iii) the importance of Ca2+-induced Ca2+ release. 3. It is shown that the key differences of the skeletal muscle Ca2+-release channel (RyR1), namely the increase in its stimulation by ATP and its inhibition by Mg2+, are critical for its direct regulation by the associated DHPR and, consequently, for the fast, accurate control of skeletal muscle contraction.

Dantrolene inhibition of sarcoplasmic reticulum Ca2+ release by direct and specific action at skeletal muscle ryanodine receptors

The skeletal muscle relaxant dantrolene inhibits the release of Ca2+ from the sarcoplasmic reticulum during excitation-contraction coupling and suppresses the uncontrolled Ca2+ release that underlies the skeletal muscle pharmacogenetic disorder malignant hyperthermia; however, the molecular mechanism by which dantrolene selectively affects skeletal muscle Ca2+ regulation remains to be defined. Here we provide evidence of a high-affinity, monophasic inhibition by dantrolene of ryanodine receptor Ca2+ channel function in isolated sarcoplasmic reticulum vesicles prepared from malignant hyperthermia-susceptible and normal pig skeletal muscle. In media simulating resting myoplasm, dantrolene increased the half-time for 45Ca2+ release from both malignant hyperthermia and normal vesicles approximately 3.5-fold and inhibited sarcoplasmic reticulum vesicle [3H]ryanodine binding (Ki approximately 150 nM for both malignant hyperthermia and normal). Inhibition of vesicle [3H]ryanodine binding by dantrolene was associated with a decrease in the extent of activation by both calmodulin and Ca2+. Dantrolene also inhibited [3H]ryanodine binding to purified skeletal muscle ryanodine receptor protein reconstituted into liposomes. In contrast, cardiac sarcoplasmic reticulum vesicle 45Ca2+ release and [3H]ryanodine binding were unaffected by dantrolene. Together, these results demonstrate selective effects of dantrolene on skeletal muscle ryanodine receptors that are consistent with the actions of dantrolene in vivo and suggest a mechanism of action in which dantrolene may act directly at the skeletal muscle ryanodine receptor complex to limit its activation by calmodulin and Ca2+. The potential implications of these results for understanding how dantrolene and malignant hyperthermia mutations may affect the voltage-dependent activation of Ca2+ release in intact skeletal muscle are discussed.

BAY K 8644 and ClO4- potentiate caffeine contracture without Ca2+ release channel activation

Effects of perchlorate (ClO4-) and BAY K 8644 on caffeine contracture and Ca2+ release channel current were studied in frog skeletal muscle. Single fibers produced a small transient contracture on addition of 2.2 mM caffeine. ClO4 at 10 mM enhanced caffeine contracture 3.7-fold. This effect was inhibited by 10 microM nifedipine pretreatment. An increase in caffeine contracture was also obtained after exposure to 0.1 microM BAY K 8644 for 1 h. At 20 mM, external K+ potentiated caffeine contracture 2.2-fold. ClO4- (< 10 mM) and BAY K 8644 (0.1-1 microM) did not affect open probability (Po), unitary conductance, and open and closed time constants of the Ca2+ release channel current. BAY K 8644 at 0.1 microM did not further enhance the channel that had been activated by 2 mM caffeine. However, 20-30 mM ClO4 increased Po significantly and led the channel to a long open state by increasing the slow open time constant and decreasing the fast closed time constant. These results suggest that binding of ClO4 and BAY K 8644 to dihydropyridine receptors elicits a further increase in Ca2+ release from the sarcoplasmic reticulum.