The purpose of this investigation was to determine if alteration in the function of the dihydropyridine receptor may in turn modify halothane-induced contractures in muscle bundles from patients susceptible to malignant hyperthermia (MH). The effects of Ca(2+)-free Krebs Ringer (KR) solution, 5 microM verapamil, 5 microM nifedipine, and 10 microM of the Ca2+ agonist BAY K 8644 on halothane-induced contracture were therefore investigated. The halothane-induced contracture was prevented in the absence of extracellular Ca2+ and significantly reduced in the presence of verapamil or nifedipine. BAY K 8644 significantly enhanced the 0.5-, 1.0-, and 1.5-vol % halothane-induced contracture in MH-susceptible muscle bundles. When BAY K 8644 was dissolved in Ca(2+)-free KR solution, no contracture was observed in MH-susceptible muscle bundles. These results on cut MH-susceptible human muscle bundles support the hypothesis that halothane-induced contracture in MH can be modified by the binding of Ca2+ agonists or antagonists to the dihydropyridine receptor. The role of Ca2+ entry phenomena remains unclear, but the results suggest that extracellular Ca2+ is required to reprime or to bind to some sites of the dihydropyridine receptors.
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