Electrophysiological effects of opioid receptor selective agonists on guinea-pig papillary muscle

The cardiac electrophysiological effects of DAMGO, DPDPE and U-50,488H which are selective opioid agonists for mu, delta and kappa receptors, respectively, were studied in the isolated guinea-pig right ventricular papillary muscles. Neither DAMGO (5.10(-6)-5.10(-5)M) nor DPDPE (5.10(-6)-5.10(-5)M) produced any significant effect on the action potential characteristics. However, U.-50,488H (10(-5),5.10(-5) M) caused a concentration-dependent reduction in the maximum rate of depolarization of phase 0 (Vmax) and in the action potential duration measured at 50% repolarization, APD50 and 90% repolarization, APD90. At 5.10(-5) M it also produced a decrease of action potential amplitude (APA). These results suggest that the reported electrophysiological effects of U-50,488H on the cardiac muscle action potential, may be, at least in part, explained by a direct cardiac action.