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 chann els in muscle. The properties of individual sparks provide information rega rding the opening of sarcoplasmic reticulum Ca2+ channels within functionin g cells. Here we use high-speed confocal microscopy to show that individual Ca2+ sparks activated by membrane depolarization in single frog skeletal m uscle fibers can be terminated prematurely by repolarization. Thus, either voltage sensor deactivation on repolarization or release channel inactivati on during continued depolarization can terminate the Ca2+ release channel a ctivity underlying voltage-activated Ca2+ sparks in skeletal muscle.