Myotubes expressing wild type RyR1 (WT) or RyR1 with one of three malignant hyperthermia mutations R615C, R2163C, and T4826I (MH) were exposed sequentially to 60 mM KCl in Ca2+-replete and Ca2+-free external buffers (Ca+ and Ca-, respectively) with 3 min of rest between exposures. Although the maximal peak amplitude of the Ca2+ transients during K+ depolarization was similar for WT and MH in both external buffers, the rate of decay of the sustained phase of the transient during K+ depolarization (decay rate) in Ca+ was 50% slower for MH. This difference was eliminated in Ca-, and the relative decay rates were faster for both genotypes than in Ca+. The integrated Ca2+ transient in Ca- compared with Ca+ was reduced by 50-60% for MH and 20% for WT. The decay rate was not affected by [K+] x [Cl-] product or NiCl2 (2 mM) supplementation of Ca-. The addition of La2+ (0.1mM), or SKF 96365 (20 μM) to Ca+ significantly accelerated decay rates for both WT and MH, but their effect was significantly greater in MH. Nifedipine (1 μM) had no effect, suggesting that the mechanism for this difference was not a reduction in L-type Ca2+ channel Ca2+ current. These data strongly suggest: 1) the decay rate in skeletal myotubes is related in part to Ca 2+ entry through the ECCE channel; 2) the MH mutations enhance ECCE compared with wild type; and 3) the increased Ca2+ entry might play a significant role in the pathophysiology of MH.
ASJC Scopus subject areas