The inhibitory effect of Mg²⁺ on halothane induced Ca²⁺ release from the SR was examined in mechanically skinned malignant hyperthermia susceptible (MHS) and non-susceptible (MHN) fibres from human vastus medialis.

Muscle was obtained by open biopsy from patients attending for investigation of MH susceptibility at St James’s Hospital, Leeds, UK. This was done with institutional Research Ethics Committee approval and informed patient consent. All procedures were carried out according to the Declaration of Helsinki. Approximately 1 g of the muscle used in the in vitro contracture test (IVCT), which was performed according to the criteria for malignant hyperthermia (MH) research of the European MH Group (The European Malignant Hyperpyrexia Group, 1984). The IVCT was used as the primary method of categorising MHN and MHS tissue.

Single mechanically skinned fibres were prepared from the remaining 0.2 g of tissue. Preparations were perfused with solutions mimicking the intracellular milieu and changes in [Ca²⁺] were detected using fura-2 fluorescence. Caffeine (40 mM) was briefly applied at 4 min intervals, at a free [Mg²⁺] of 1 mM. Each application of caffeine resulted in a transient increase in [Ca²⁺] within the preparation due to Ca²⁺ release from the sarcoplasmic reticulum (SR). After four to five reproducible caffeine responses were obtained, the preparation was perfused for a further 4 min to allow the SR to re-accumulate Ca²⁺. The solution within the bath was then exchanged for one containing 1, 0.4, 0.2 or 0.1 mM Mg²⁺ and perfusion stopped to restrict the volume of solution surrounding the preparation to approximately 6 µl. After a further minute at the selected [Mg²⁺], 1 mM halothane was rapidly introduced. In MHN muscle, 1 mM halothane never caused a detectable Ca²⁺ release at 1 mM Mg²⁺. However, halothane-induced Ca²⁺ release did occur when the [Mg²⁺] was lowered to a threshold level of 0.4 mM in two patients, 0.2 mM in 10 patients and 0.1 mM in one patient. In MHS muscle, halothane-induced Ca²⁺ release was generally apparent following a smaller reduction in Mg²⁺. Of eight MHS individuals tested, 1 mM halothane induced Ca²⁺ release at 1 mM Mg²⁺ in four patients, 0.4 mM Mg²⁺ in three patients and 0.2 mM Mg²⁺ in one patient.

These results suggest that (i) in MHN fibres, halothane induced Ca²⁺ release is markedly inhibited by physiological levels of cytosolic Mg²⁺ and (ii) the inhibitory effect of cytosolic Mg²⁺ is typically less pronounced in MHS muscle. Reduced inhibition of halothane-induced Ca²⁺ release by cytosolic Mg²⁺ might contribute to the increased susceptibility of MHS muscle to contracture induced by volatile anaesthetics.

This work was supported by the The Wellcome Trust