Effects of Mg2+ and SR luminal Ca2+ on caffeine-induced Ca2+ release in skeletal muscle from humans susceptible to malignant hyperthermia

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. Preparations were perfused with solutions mimicking the intracellular milieu and changes in [Ca2+] were detected using fura-2 fluorescence. At I mm cytosolic Mg2+, MHS fibres had a higher sensitivity to caffeine (2-40 mm) than MHN fibres. The inhibitory effect of Mg2+ on caffeine-induced Ca2+ release was studied by increasing [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) in MHN fibres. However, the caffeine-induced Ca2+ transient decreased by only 24 +/- 8.1 % (n = 9) 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 sarcoplasmic reticulum (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) under similar conditions. These results suggest that human malignant hyperthermia (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.