Two mechanisms for termination of individual Ca2+ sparks in skeletal muscle

Ca2+ sparks are brief, localized elevations of myoplasmic [Ca2+] caused by release of increments of Ca2+ via sarcoplasmic reticulum Ca2+ release channels in muscle. The properties of individual sparks provide information regarding the opening of sarcoplasmic reticulum Ca2+ channels within functioning cells. Here we use high-speed confocal microscopy to show that individual Ca2+ sparks activated by membrane depolarization in single frog skeletal muscle fibers can be terminated prematurely by repolarization. Thus, either voltage sensor deactivation on repolarization or release channel inactivation during continued depolarization can terminate the Ca2+ release channel activity underlying voltage-activated Ca2+ sparks in skeletal muscle.