Mg2+ dependence of halothane-induced Ca2+ release from the sarcoplasmic reticulum in skeletal muscle from humans susceptible to malignant hyperthermia

Abstract

Background: Recent work suggests that impaired Mg2+ regulation of the ryanodine receptor is a common feature of both pig and human malignant hyperthermia. Therefore, the influence of [Mg2+] on halothane-induced Ca2+ release from the sarcoplasmic reticulum was studied in malignant hyperthermia-susceptible (MHS) or -nonsusceptible (MHN) muscle. Methods: Vastus medialis fibers were mechanically skinned and perfused with solutions containing physiologic (1 mM) or reduced concentrations of free [Mg2+]. Sarcoplasmic reticulum Ca2+ release was detected using fura-2 or fluo-3. Results: In MHN fibers, 1 mM halothane consistently did not induce sarcoplasmic reticulum Ca2+ release in the presence of 1 mM Mg2+. It was necessary to increase the halothane concentration to 20 mM or greater before Ca2+ release occurred. However, when [Mg 2+] was reduced below 1 mM, halothane became an increasingly effective stimulus for Ca2+ release; e.g., at 0.4 mM Mg2+, 58% of MHN fibers responded to halothane. In MHS fibers, 1 mM halothane induced Ca2+ release in 57% of MHS fibers at 1 mM Mg2+. Reducing [Mg2+] increased the proportion of MHS fibers that responded to 1 mM halothane. Further experiments revealed differences in the characteristics of halothane-induced Ca2+ release in MHS and MHN fibers: In MHN fibers, at 1 mM Mg2+, halothane induced a diffuse increase in [Ca 2+], which began at the periphery of the fiber and spread slowly inward. In MHS fibers, halothane induced a localized Ca2+ release, which then propagated along the fiber. However, propagated Ca2+ release was observed in MHN fibers when halothane was applied at an Mg 2+ concentration of 0.4 mM or less. Conclusions: When Mg2+ inhibition of the ryanodine receptor is reduced, the halothane sensitivity of MHN fibers and the characteristics of the Ca2+ release process approach that of the MHS phenotype. In MHS fibers, reduced Mg2+ inhibition of the ryanodine receptor would be expected to have a major influence on halothane sensitivity. The Mg2+ dependence of the halothane response in MHN and MHS may have important clinical implications in circumstances where intracellular [Mg2+] deviates from normal physiologic concentrations.