Involvement of Mg2+ in terminating Ca2+ release in cultured rat skeletal muscle.

Combined patch-clamp and fura-2 measurements were performed to investigate the mechanism that terminates Ca2+ release in rat skeletal myoballs. When cells were intracellularly perfused with solution containing 1 mM free Mg2+, the caffeine (10 mM)-induced Ca2+ transient was abruptly terminated by membrane repolarization (-70 mV). With low intracellular Mg2+ (e.g. 50 microM) perfusion, however, repolarization failed to terminate the caffeine transient. The results show that intracellular Mg2+ is necessary for repolarization-induced closing of the Ca2+ release channel.