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 1 m m cytosolic Mg²⁺, MHS fibres had a higher sensitivity to caffeine (2-40 m m ) than MHN fibres. The inhibitory effect of Mg²⁺ on caffeine-induced Ca²⁺ release was studied by increasing [Mg²⁺] of the solution containing 40 m m caffeine. Increasing [Mg²⁺] from 1 to 3 m m 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 m m Mg²⁺) decreased the fraction of the total sarcoplasmic reticulum (SR) Ca²⁺ content released in response to 40 m m 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.