Calcium Release in FDB Muscle Fibers of the RyR-R163C Mutant Model of Malignant Hyperthermia

Abstract

The R163C is one of the most frequent missense mutations of the human RyR1; it confers malignant hyperthermia susceptibility and results in central cores. Alterations of the excitation contraction (EC) coupling associated with the R163C mutation have been studied in myotubes and, to a lesser extent, in adult FDB fibers of the transgenic R163C mouse. In order to further evaluate the pathophysiology of the EC coupling in these animals, we studied the properties of Ca2+ release microdomains evoked by action potentials (AP), and measured the rate of Ca2+ release in global (spatially averaged) transients evoked by APs and/or voltage clamp pulses. Current- and voltage-clamp experiments were done in adult FDB fibers isolated from R163C and control mice internally equilibrated with a solution containing 250 μM OGB-5N and 30:15 mM EGTA:Ca2+. The properties of AP-evoked global Ca2+ transients were similar in fibers of R163C and control animals. Also, peak Ca2+ release fluxes of transients elicited by depolarizations to +60mV were not significantly different in R163C (236±60 μM/ms, n=3) with respect to those in control fibers (242±58 μM/ms, n=6). However, the voltage-dependence of the peak Ca2+ fluxes was significantly shifted to more negative potential (by 10 mV, n=3) in fibers from R163C. Finally, the amplitude of Ca2+ release microdomains varied significantly along consecutive triads of R163C fibers, which contrasts with the monotonous dependence with position along the fiber's longitudinal axis in control fibers. Altogether our results show that though EC coupling is on-average mildly compromised in R163C mutants, Ca2+ release sites at the triads are conspicuously altered by the RyR1 mutation. Supported by NIH grants AR047664, AR041802, and AR054816. The R163C mice were kindly provided by Dr. I. Pessah, UC Davis.