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

Background. In normal resting muscle, cytosolic Mg2+ exerts a potent inhibitory influence on the sarcoplasmic reticulum (SR) Ca2+ release channel (ryanodine receptor, RyR1). Impaired Mg2+-regulation of RyR1 has been proposed as a causal factor in malignant hyperthermia (MH). The aim of this study was to compare the effects of cytosolic Mg2+ on SR Ca2+ release induced by halothane or sevoflurane in normal (MHN) and MH susceptible (MHS) human skeletal muscle fibres. Methods. Samples of vastus medialis muscle were obtained from patients under investigation for MH susceptibility. Single fibres were mechanically skinned and perfused with solutions mimicking the intracellular milieu. Changes in [Ca2+](i) were detected using fura-2 fluorescence after application of equimolar halothane or sevoflurane. Results. In MHN fibres, concentrations of sevoflurane or halothane as high as 10 mM typically failed to induce SR Ca2+ release at physiological free [Mg2+] (1 mM). However, when [Mg2+] was decreased to 0.4 mM, SR Ca2+ release occurred in 51% (16/33) and 6% (2/33) of MHN fibres after the addition of 1 mM halothane or 1 mM sevoflurane, respectively. Further decreases in [Mg2+] increased the proportion of responsive fibres. In the presence of 0.1 mM [Mg2+], Ca2+ release occurred in all fibres (33/33) after the introduction of 1 mM halothane or 1 mM sevoflurane. In MHS fibres, 1 mM halothane or 1 mM sevoflurane-induced Ca2+ release in 54% (7/13) or 15% (2/13) of fibres, respectively, at 1 mM Mg2+. A decrease in [Mg2+] to 0.2 mM Mg2+ was sufficient to render 100% of MHS fibres (13/13) responsive to 1 mM halothane or 1 mM sevoflurane. Conclusions. In both MHS and MHN fibres (i) halothane is a more potent activator of SR Ca2+ release than sevoflurane and (ii) as with halothane, the efficacy of sevoflurane-induced SR Ca2+ release exhibits a marked dependence on cytosolic [Mg2+]. The marked potentiation of SR Ca2+ release after a moderate reduction in cytosolic [Mg2+] suggests that conditions which cause hypomagnesaemia will increase the probability and possibly severity of an MH event. Conversely, maintenance of a normal or slightly increased cytosolic [Mg2+] may reduce the probability of MH.