Malignant hyperthermia mutation Arg615Cys in the porcine ryanodine receptor alters voltage dependence of Ca2+ release

1. Ca2+ inward current and fura-2 Ca2+ transients were simultaneously recorded in porcine myotubes, Myotubes from normal pigs and cells from specimens homozygous for the Arg615Cys (malignant hyperthermia) mutation of the skeletal muscle ryanodine receptor RyR1 were investigated. We addressed the question whether this mutation alters the voltage dependence of Ca2+ release from the sarcoplasmic reticulum. 2. Thr time course of the total flux of Ca2+ into the myoplasm was estimated. Analysis showed that the largest input Ca2+ flux occurred immediately after depolarization. Amplitude and time course of the Ca2+ flux at large depolarizations were not significantly different in the Arg615Cys myotubes. 3. Ca2+ release from the sarcoplasmic reticulum was activated at more negative potentials than the L-type Ca2+ conductance. In the controls, the potentials for half-maximal activation (V-1/2) were -9.0 mV and 16.5 mV, respectively. 4. In myotubes expressing the Arg615Cys mutation, Ca2+ release was activated at significantly lower depolarizing potentials (V-1/2 = -23.5 mV) than in control myotubes. In contrast, V-1/2 conductance activation (13.5 mV) was not significantly different from controls. 5. The specific shift in the voltage dependence of Ca2+ release caused by this mutation can be well described by altering a voltage-independent reaction of the ryanodine receptor that coupled to the voltage-dependent transitions of the L-type Ca2+ channel.