Voltage‐controlled Ca2+ release in normal and ryanodine receptor type 3 (RyR3)‐deficient mouse myotubes

Abstract

1 Primary cultured myotubes were derived from satellite cells of the diaphragm obtained from both normal mice (RyR3+/+) and mice with a targeted mutation eliminating expression of the type 3 isoform of the ryanodine receptor (RyR3−/−). Using the whole‐cell patch clamp technique, L‐type Ca²⁺ currents were measured during step depolarizations. Simultaneously, intracellular Ca²⁺ transients were recorded with the fluorescent indicator dye fura‐2. 2 After correction for non‐instantaneous binding of Ca²⁺ to the indicator dye and taking into account the dynamics of Ca²⁺ binding to intracellular constituents, an estimate of the time course of the Ca²⁺ release rate from the sarcoplasmic reticulum (SR) was obtained. 3 The calculated SR Ca²⁺ release flux exhibited a marked peak within less than 12 ms after the onset of the voltage‐clamp depolarization and fell rapidly thereafter to a five times lower, almost steady level. It declined rapidly after termination of the depolarization. 4 Signals in normal and RyR3‐deficient myotubes showed no significant difference in the activation of Ca²⁺ conductance and in amplitude, time course and voltage dependence of the Ca²⁺ efflux from the SR. 5 In conclusion, the characteristics of voltage‐controlled Ca²⁺ release reported here are similar to those of mature mammalian muscle fibres. In contrast to differences observed in the contractile properties of RyR3‐deficient muscle fibres, a contribution of RyR3 to excitation‐contraction coupling could not be detected in myotubes.