Intracellular microelectrodes were used to study the electrophysiologi
cal effects of U-50,488H on guinea-pig papillary muscle. U-50,488H (10
(-5) M) caused a significant decrease in the maximum rate of depolariz
ation and a decrease in the action potential duration. At high concent
ration (5 x 10(-5) M) U-50,488H induced a significant decrease of rest
ing potential, maximum rate of depolarization, action potential amplit
ude and the action potential duration. The administration of MR-2266 (
5 x 10(-7) M) partially antagonized the cardiac electrophysiological e
ffects of U-50,488H. These results suggest that the electrophysiologic
al effects of U-50,488H may be partially due to an interaction with op
ioid receptors.