In order to examine the mechanisms of ischemia-reperfusion induced changes in beta-adrenoceptor-linked signal transduction pathway, isolated rat hearts perfused in the absence or presence of superoxide dismutase (SOD) plus catalase (CAT) were made ischemic for 30 min and then reperfused for 60 min. The left ventricular developed pressure as well as the rare of contraction and rate of relaxation were markedly decreased, whereas the left ventricular end-diastolic pressure increased in the ischemic hearts. A significant increase in the density and affinity of beta 1-adrenoceptors without any changes in the characteristics of beta 2-adrenoceptors was evident in cardiac membranes obtained from the ischemic hearts. The recovery of contractile abnormalities in the ischemic heart was depressed upon reperfusion; the ischemic-reperfused hearts also showed attenuated inotropic responses to isoproterenol. The affinities and densities of beta- and beta-adrenoceptors were decreased in the ischemic-reperfused hearts; the magnitude of changes in beta 1-adrenoceptors was greater than that in beta 2-adrenoceptors. The isoproterenol-stimulated adenylyl cyclase activity was depressed in both ischemic hearts and ischemic-reperfused hearts. The basal and forskolin-stimulated adenylyl cyclase activities were unaltered due to ischemia but were increased upon reperfusion. The NaF- and 5'-Guanylyl-imidodiphosphate[Gpp(NH)p]-stimulated adenylyl cyclase activities were depressed in the ischemic hearts and increased in the ischemic reperfused hearts. Cholera toxin (CT)-stimulated adenylyl cyclase as well as the CT-catalysed ADP-ribosylation activity and stimulatory G protein (Gs protein) immunoreactivity were decreased in the ischemic hearts and increased in the reperfused hearts. Pertussis toxin (PT)-stimulated adenylyl cyclase activity was unaltered in both ischemic and ischemic-reperfused hearts, whereas the PT-catalysed ribosylation and inhibitory G protein (Gi protein) immunoactivity were slightly increased in the reperfused myocardium. Thus the inability of isoproterenol to stimulate adenylyl cyclase in the ischemic-reperfused hearts may be due to alterations mainly in the characteristics of beta 1-adrenoceptors including density, affinity and coupling with the adenylyl cyclase. Scavenging of oxyradicals by the addition of SOD plus CAT in the perfusion medium prevented the reperfusion-induced changes in contractile function, inotropic responses of the heart to isoproterenol, activation of adenylyl cyclase by isoproterenol, as well as densities and affinities of beta-adrenoceptors in cardiac membranes. These results suggest that the depressed contractile activity and the attenuated inotropic responses of ischemic-reperfused hearts to isoproterenol as well as the defects in beta-adrenoceptor-linked signal transduction may be due to the formation of oxyradicals in the myocardium.