Alterations in β-Adrenoceptor Mechanisms in Hearts Perfused with Xanthine Plus Xanthine Oxidase

Abstract

Background: Although β-adrenoceptors and adenylyl cyclase are known to be affected upon exposing cardiac membranes to some oxyradical generating systems, the results are conflict ing. Furthermore, functional significance of alterations in the β-adrenoceptor-adenylyl cyclase system in terms of changes in the inotropic responses to catecholamines is not clear. Methods and Results: The positive inotropic effect of isoproterenol was augmented on per fusing the isolated rat hearts with xanthine (X) plus xanthine oxidase (XO) for 5 minutes but was attenuated by perfusion for 15 minutes. The isoproterenol-stimulated adenylyl cyclase activity in cardiac membranes showed an increase at 10 minutes and a decrease at 30 minutes perfusion of hearts with X plus XO. The density of β-adrenoceptors in cardiac membranes was reduced after 10 minutes and 30 minutes of perfusion with X plus XO, whereas the affin ity of β-adrenoceptors was increased after 10 minutes and reduced after 30 minutes. Although β-adrenoceptors were unaltered by 10 minutes of perfusion with X plus XO, their affinity was increased and density was decreased by 30 minutes of perfusion. The agonist competition curves using isoproterenol indicated an increase in the number of coupled recep tors in the high affinity state on 10 minutes of perfusion and an increase in the low affinity state of coupled receptor due to 30 minutes of perfusion with X plus XO. The basal as well as forskolin-, NaF- and Gpp(NH)p-stimulated adenylyl cyclase activities in cardiac membranes exhibited an increase after 10 minutes and decrease after 30 minutes of perfusion with X plus XO. Although the presence of superoxide dismutase plus catalase in the perfusion medium prevented most of the alterations due to X plus XO, it did not alter the increased affinity of the β-adrenoceptor upon perfusing hearts for 10 minutes with X plus XO. Conclusions: The results in this study suggest the biphasic nature of the oxyradical-induced alterations in both the inotropic responses to catecholamines and the β-adrenoceptor-medi ated signal transduction mechanism in the heart.