Enhanced relaxation and reduced positive inotropic effects of amrinone in ventricular muscle from cats with subacute heart failure. Implications for drug therapy.

Previously, we reported that amrinone increases isometric twitch force but relaxes K+-induced contracture in muscles from normal cat right ventricle. This study evaluated its effects on diseased cardiac tissue. Right-ventricular papillary muscles were obtained from cats with subacute right-ventricular failure (3-14 days after partial pulmonary-artery ligation) and studied in vitro during stimulation (0.5 Hz) and exposure to high-K+ Tyrode solution. Active isometric twitch force and rate of force development (dP/dt) were significantly lower in muscles from hearts with right-ventricular failure compared to control muscles. In addition, while time to peak force was not different, duration of the twitch was significantly longer. In contrast to its positive inotropic actions in control muscles, amrinone (5.3 X 10(-4) M) had no significant effects on twitch force and dP/dt in muscles from failed ventricles. Time to peak force was not changed by amrinone in either group, but unlike its action in control muscle, duration of the twitch was reduced in failed muscle. Amrinone reduced K+-contracture force similarly in both control and failed muscles. Isoproterenol (10(-6) M) significantly increased twitch force and dP/dt and reduced K+-contracture force in both muscle groups. Since amrinone appears to be a phosphodiesterase inhibitor, our data indicate that cyclic AMP (cAMP)-related relaxation processes, but not cAMP-related contractile processes, can be enhanced by phosphodiesterase inhibitors in experimental heart failure. Furthermore, amrinone's reduced positive inotropic effect in failed myocardium suggests that its improvement of ventricular function in patients reflects, in part, enhancement of relaxation.