The rest-dependent depression of action potential duration in rabbit myocardium and the possible role of the transient outward current. A pharmacological analysis.

A pharmacological approach has been used to investigate the mechanism of the action potential depression after a 1 min rest in rabbit myocardium. This paradoxical depression was not prevented by potentiators of the slow inward current (isoproterenol and 5'-guanylylimidodiphosphate). This depression was completely removed by a specific blocker of the transient outward current, 4-aminopyridine. It appears reasonable to assume that rabbit ventricular fibers have a transient outward current and the activation of this current coupled to the masking of the slow inward current probably accounts for the depression of the rested action potential plateau phase.