Involvement of dihydropyridine receptors in terminating Ca2+ release in rat skeletal myotubes

1. Combined patch-clamp and fura-2 measurements were performed in order to investigate the effect of dihydropyridine (DHP) antagonists on termination of sarcoplasmic reticulum (SR) Ca2+ release in cultured rat skeletal myoballs. 2. Ca2+ transients induced by 10 mM caffeine were curtailed by depolarization (e.g. +20 mV for 1 s) and subsequent repolarization (-70 mV). This phenomenon is termed RISC (repolarization-induced stop of caffeine-induced Ca2+ release). 3. At 0.5 to 1 microM, DHP antagonists (nifedipine or (+)PN200-110) strongly inhibited RISC and also slowed the decay of intracellular Ca2+ concentration ([Ca2+]i) following repolarization after depolarization-induced Ca2+ release (-20 or -10 mV for 5 s). 4. The activation time course of the Ca2+ channel associated with the DHP receptor (DHPR) was investigated by measuring DHP-sensitive Ca2+ channel tail currents, while varying the duration of depolarizing pulses. The tail currents increased with pulse duration and peaked around 0.7, 0.9 and 1.1 s for depolarizations to +70, +40 and +20 mV, respectively. These values are compatible with the activation time course of RISC (0.5-1 s to maximally activate RISC at +20 to +60 mV). 5. These results suggest that the DHPR in T-tubular membranes regulates closing of the ryanodine receptor (RyR)-Ca2+ release channel complex through membrane potential change.