BETA-ADRENOCEPTOR-LINKED SIGNAL-TRANSDUCTION IN ISCHEMIC-REPERFUSED HEART AND SCAVENGING OF OXYRADICALS

In order to examine the mechanisms of ischemia-reperfusion induced cha nges in beta-adrenoceptor-linked signal transduction pathway, isolated rat hearts perfused in the absence or presence of superoxide dismutas e (SOD) plus catalase (CAT) were made ischemic for 30 min and then rep erfused for 60 min. The left ventricular developed pressure as well as the rate of contraction and rate of relaxation were markedly decrease d, whereas the left ventricular end-diastolic pressure increased in th e 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. T he affinities and densities of beta(1)- and beta(2)-adrenoceptors were decreased in the ischemic-reperfused hearts; the magnitude of changes in beta(1)-adrenoceptors was greater than that in beta(2)-adrenocepto rs. The isoproterenol-stimulated adenylyl cyclase activity was depress ed in both ischemic hearts and ischemic-reperfused hearts. The basal a nd forskolin-stimulated adenylyl cyclase activities were unaltered due to ischemia but were increased upon reperfusion. The NaF- and 5'-Guan ylyl-imidodiphosphate[Gpp(NH)p]-stimulated adenylyl cyclase activities were depressed in the ischemic hearts and increased in the ischemic-r eperfused hearts. Cholera toxin (CT)-stimulated adenylyl cyclase as we ll as the CT-catalysed ADP-ribosylation activity and stimulatory G pro tein (G(s) protein) immunoreactivity were decreased in the ischemic he arts and increased in the reperfused hearts. Pertussis toxin (PT)-stim ulated adenylyl cyclase activiyy was unaltered in both ischemic and is chemic-reperfused hearts, whereas the PT-catalysed ribosylation and in hibitory G protein (G(i) protein) immunoactivity were slightly increas ed in the reperfused myocardium. Thus the inability Bf isoproterenol t o stimulate adenylyl cyclase in the ischemic-reperfused hearts may be due to alterations mainly in the characteristics of beta(1)-adrenocept ors including density, affinity and coupling with the adenylyl cyclase . Scavenging of oxyradicals by the addition of SOD plus CAT in the per fusion medium prevented the reperfusion-induced changes in contractile function, inotropic responses of the heart to isoproterenol, activati on of adenylyl cyclase by isoproterenol, as well as densities and affi nities of beta-adrenoceptors in cardiac membranes. These results sugge st that the depressed contractile activity and the attenuated inotropi c 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. (C) 1997 Academic Press Limited.