ESSENTIAL ROLE OF EXTRACELLULAR CA2+ FOR THE MAINTENANCE OF THE CA2+ ACTIVATED K+-CHANNEL ACTIVITY IN GUINEA-PIG VAS-DEFERENS

The characteristics of Ca2+ activated K+ channels of enzymatically dissociated single smooth muscle cells of vas deferens of the adult guinea pig were studied by the whole cell clamp technique. A stepwise depolarization to -30 mV induced a fast transient cuff ent followed by the spontaneous transient outward currents (STOCs). STOCs were eliminated by application of K+ channel blocker TEA. STOCs appeared only when Ca2+ was present in extracellular medium, and disappeared immediately once Ca2+ was removed from the medium. Therefore, STOCs in the present preparation can be ascribed to the activity of Ca2+ activated K+ channels. Furthermore, even in the absence of Ca2+, STOCs were evoked transiently by application of caffeine, indicating the involvement of the Ca2+ induced Ca2+ release (CICR) mechanism. Thus, it is likely that Ca2+ activated K+ channels are operated via CICR mechanism and extracellular Ca2+ is essential for keeping their constant activity.