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

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Abstract
1. Membrane ionic current‐voltage (I‐V) relations of the frog ventricular myocardium were measured during the action potential with a new single sucrose gap voltage clamp technique. 2. 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. 3. Time dependent I‐V relation during a series of voltage clamp pulses of clamp initiation. 4. The membrane conductance is remarkably constant during the plateau and is about 85 mumhos/muF of membrane capacitance. 5. Chloride conductance is about 18% of the total ionic conductance during the plateau and is not time dependent. Two inward Cl‐movement during a normal action potential is sufficient to approximately halve the action potential duration. 6. Membrane conductance did not change significantly when Ca2+ was omitted from the bathing medium. 7. Epinephrine increased the duration of the action potential and the total ionic conductance during the platiau in normal and Ca‐free media. 8. 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. 9. The results indicates that a fine balance between conductance changes cardiac action potential. The possibility of a cross ionic interaction in the heart cell membrane is suggested.