Ionic membrane conductance during the time course of the cardiac action potential.

Abstract

Membrane ionic current-voltage (I-V) relations of the frog [Rana pipiens] ventricular myocardium were measured during the action potential with a new single sucrose gap voltage clamp technique. The I-V relation is linear during the plateau and rapid repolarization phases of the action potential and during the development of the regenerative threshold of repolarization. Time dependent I-V relations during a series of voltage clamp pulses shift continuously into an N-shaped characteristic within 50-100 ms of clamp initiation. The membrane conductance is remarkably constant during the plateau and is about 85 .mu.mhos/.mu.F of membrane capacitance. Chloride conductance is about 18% of the total ionic conductance during the plateau and is not time dependent. The inward Cl- movement during a normal action potential is sufficient to approximately halve the action potential duration. Membrane conductance did not change significantly when Ca2+ was omitted from the bathing medium. Epinephrine increased the duration of the action potential and the total ionic conductance during the plateau in normal and Ca-free media. Separation of Na+ and K+ currents in muscles bathed in zero Ca2+, zero Cl- solution indicates that the inward and outward currents are balanced to within 2% during the slow repolarization. A fine balance between conductance changes and inward and outward going ions mediates the repolarization of the cardiac action potential. The possibility of a cross ionic interaction in the heart cell membrane is suggested.