The initiation of phasic and tonic contraction by potassium and the effect of calcium, multivalent cations and Ca-antagonist on potassium contracture in guinea-pig vas deferens.

Abstract

The role of Ca in phasic and tonic contractions of high-K-induced contractures of guinea-pig vas deferens was studied. It was shown that phasic contraction was initiated by a burst of action potentials, and tonic contraction by sustained depolarization. Phasic contraction seemed to be related to the rate and degree of depolarization, whereas tonic contraction seemed to be related only to the degree of depolarization. Tonic contraction was more sensitive to a reduction of extracellular Ca, though both types of contraction were abolished by Ca removal. The tonic contraction observed with isotonic recording was more sensitive to extracellular Ca than that observed with isometric recording. Maximum contractions were achieved with lower Ca than that in isometric measurement. Verapamil blocked tonic contraction at a low concentration, and both contractions at a high concentration. Mn or La blocked the phasic and tonic contractions first. After prolonged exposure to either ion, however, tonic contraction could be induced. The results suggest that both phasic and tonic contractions were induced by Ca-influx through the cell membrane, and that tonic contraction was maintained by a constantly elevated intracellular Ca concentration which is the consequence of an altered equilibration of Ca influx and Ca extrusion through the cell membrane.