Concentration-dependent effect of trapidil on slow action potentials in cardiac muscle

Trapidil, a coronary vasodilator and positive inotropic agent, was tested for its ability to affect the normal "fast" action potentials and the "slow" action potentials and contractions of isolated perfused chick hearts, and to affect the tissue cyclic AMP level. At 5 X 10(-3) M, trapidil completely blocked the fast Na+ channels in hearts perfused with normal Tyrode solution, since this dose abolished the action potential when verapamil (2 X 10(-6) M) was present to eliminate the inward slow current. To study effects on the slow channels, the fast Na+ channels were voltage-inactivated by partial depolarization to about -40 mV with an elevated (25 mM) K+-Tyrode solution, resulting in loss of excitability. At low concentrations (1 X 10(-4) - 1 X 10(-3) M), trapidil induced slow action potentials accompanied by contractions, even in the presence of a beta-adrenergic blocker. In contrast, at high concentrations (3 X 10(-3) - 1 X 10(-2) M), trapidil markedly depressed or blocked the isoproterenol-induced slow action potentials. Consistent with this dual action, in hearts perfused with normal Tyrode solution, trapidil exerted a small positive inotropic action at low doses and a considerable negative inotropic action at high doses, even though the intracellular cyclic AMP level was substantially elevated. That is, trapidil has actions similar to those of papaverine. It is concluded that trapidil blocks both fast Na+ channels and slow channels in cardiac muscle, the fast Na+ channels being more sensitive, and that low concentrations of trapidil induce slow channels by elevating the cyclic AMP level because of phosphodiesterase inhibition.