RISC (REPOLARIZATION-INDUCED STOP OF CAFFEINE-CONTRACTURE) IS NOT DUETO STORE DEPLETION IN CULTURED MURINE SKELETAL-MUSCLE

We have combined the patch-clamp technique with Fura-2 measurements to investigate whether RISC (repolarization-induced stop of caffeine-con tracture) is a consequence of store depletion in cultured skeletal mus cles of rats and mice. Weak depolarizations (-45 to -40 mV) of long du ration induced a barely detectable Ca2+ transient. Even under these co nditions, caffeine-activated Ca2+ transients (CafTs) were terminated u pon membrane repolarization (-70 mV) at all stages of CafT. Following the peak of the CafT, massive Ca2+ release was elicited by either flas h-photolysis of caged Ca2+ or further depolarization to 0 mV, demonstr ating the lack of store depletion. Thus, RISC is not due primarily to store depletion but to closure of the Ca2+ release channels possibly t hrough a mechanical interaction with voltage sensors. RISC was not pre sent in rat heart muscle, further supporting a role of direct interact ion in skeletal muscle.