BACKGROUND Recent work suggests that impaired Mg(2+) regulation of the ryanodine receptor is a common feature of both pig and human malignant hyperthermia. Therefore, the influence of [Mg(2+)] on halothane-induced Ca(2+) 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 [Mg(2+)]. Sarcoplasmic reticulum Ca(2+) release was detected using fura-2 or fluo-3. RESULTS In MHN fibers, 1 mm halothane consistently did not induce sarcoplasmic reticulum Ca(2+) release in the presence of 1 mm Mg(2+). It was necessary to increase the halothane concentration to 20 mm or greater before Ca release occurred. However, when [Mg(2+)] was reduced below 1 mm, halothane became an increasingly effective stimulus for Ca(2+) release; e.g., at 0.4 mm Mg(2+), 58% of MHN fibers responded to halothane. In MHS fibers, 1 mm halothane induced Ca(2+) release in 57% of MHS fibers at 1 mm Mg(2+). Reducing [Mg(2+)] increased the proportion of MHS fibers that responded to 1 mm halothane. Further experiments revealed differences in the characteristics of halothane-induced Ca(2+) release in MHS and MHN fibers: In MHN fibers, at 1 mm Mg(2+), 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 C(2+)a release, which then propagated along the fiber. However, propagated Ca(2+) release was observed in MHN fibers when halothane was applied at an Mg(2+) concentration of 0.4 mm or less. CONCLUSIONS When Mg(2+) inhibition of the ryanodine receptor is reduced, the halothane sensitivity of MHN fibers and the characteristics of the Ca release process approach that of the MHS phenotype. In MHS fibers, reduced Mg(2+) inhibition of the ryanodine receptor would be expected to have a major influence on halothane sensitivity. The Mg dependence of the halothane response in MHN and MHS may have important clinical implications in circumstances where intracellular [Mg(2+)] deviates from normal physiologic concentrations.