Mechanism of cardiovascular action of trapidil

In anaesthetized, open-chest dogs N,N-diethyl-5-methyl[1,2,4]triazolo[1,5-alpha]pyrimidine-7-amine (trapidil) in doses of 0.3--3 mg/kg i.v. produced increases in coronary sinus outflow and heart rate and decreases in systemic blood pressure and coronary resistance in a dose-dependent manner. Trapidil produced an increase in myocardial oxygen consumption but virtually no change in coronary arteriovenous oxygen difference. At 1.8 mg/kg i.v. of the drug coronary resistance fell to half of the pre-drug value and coronary sinus outflow almost doubled, and so did myocardial oxygen consumption. In isolated, blood-perfused dog heart preparations, trapidil produced coronary vasodilator and positive inotropic and chronotropic effects. Theophylline produced similar effects. Trapidil was a more positive inotropic than positive chronotropic agent, and so was theophylline but to a lesser degree than trapidil. In producing vasodilator and positive inotropic effects trapidil was about 3 times more effective than theophylline. Trapidil and theophylline inhibited the cyclic AMP phosphodiesterase (PDE) activity in crude extracts prepared from the dog ventricular muscle. In this respect trapidil was nearly 3 times more potent than theophylline. It is suggested that PDE inhibition would be a fundamental mechanism of action of trapidil.