Regulation of the ryanodine receptor (RYR) by Mg²⁺ and SR luminal Ca²⁺ was studied 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. MHS fibres had a higher sensitivity to caffeine (2-40 mM) than MHN fibres. For example, in MHS fibres, the amplitude of the caffeine-induced Ca²⁺ transient resulting from rapid application of 10 mM caffeine was 86.7 ± 6.7 % (n = 9, mean ± S.E.M.) of the maximum response (to 40 mM/zero Mg²⁺), compared with 37 ± 5.9 % (n = 10, mean ± S.E.M.) in MHN fibres. The inhibitory effect of Mg²⁺ on caffeine-induced Ca²⁺ release was studied by increasing the [Mg²⁺] of the solution containing 40 mM caffeine. Increasing [Mg²⁺] from 1 to 3 mM reduced the amplitude of the caffeine-induced Ca²⁺ transient by 77 ± 7.4 % (n = 8, mean ± S.E.M.) in MHN fibres. However, the caffeine-induced Ca²⁺ transient decreased by only 24 ± 8.1 % (n = 9, mean ± S.E.M.) in MHS fibres. In MHN fibres, reducing the Ca²⁺ loading period from 4 to 1 min (at 1 mM Mg²⁺) decreased the fraction of the total SR Ca²⁺ content released in response to 40 mM caffeine by 90.4 ± 6.2 % (n = 6). However, in MHS fibres the response was reduced by only 31.2 ± 17.4 % (n = 6, mean ± S.E.M.) under similar conditions. These results suggest that human MH is associated with reduced inhibition of the RYR by (i) cytosolic Mg²⁺ and (ii) SR Ca²⁺ depletion. Both of these effects may contribute to increased sensitivity of the RYR to caffeine and volatile anaesthetics.

This work was supported by the The Wellcome Trust.

All procedures accord with current local guidelines and the Declaration of Helsinki.