Biphasic alterations in cardiac beta-adrenoceptor signal transduction mechanism due to oxyradicals.

To assess the effects of oxyradicals on cardiac beta-adrenoceptors, G-proteins and adenylyl cyclase, rat heart membranes were incubated with xanthine (X) plus xanthine oxidase (XO) for different intervals. The basal as well as forskolin-, NaF-, 5'-guanylylimidodiphosphate and isoproterenol-stimulated adenylyl cyclase activities showed an increase at 10 min and a decrease at 30 min of incubation with X plus XO. Treatment of membranes with H2O2 also produced biphasic changes in adenylyl cyclase activities. The density of beta1-adrenoceptors was decreased when cardiac membranes were treated with X plus XO for 10 and 30 min whereas the affinity of beta1-adrenoceptors was increased after 10 min and reduced after 30 min of incubation. The beta2-adrenoceptors were not modified at 10 min whereas incubation of cardiac membranes with X plus XO for 30 min increased the affinity and decreased the density. Cholera toxin-stimulated adenylyl cyclase activity, cholera toxin-catalyzed ADP-ribosylation and stimulatory guanine nucleotide binding protein immunoreactivity in cardiac membranes were increased at 10 min and decreased at 30 min of incubation with X plus XO. However, the pertussis toxin-stimulated adenylyl cyclase activity, pertussis toxin-catalyzed ADP ribosylation and inhibitory guanine nucleotide binding protein immunoreactivity were not affected on treatment of membranes with X plus XO. Addition of superoxide dismutase plus catalase in the incubation medium prevented the X plus XO-induced alterations in adenylyl cyclase activities, stimulatory guanine nucleotide binding protein-related ADP-ribosylation and changes in the characteristics of beta-adrenoceptors except the increased affinity of beta1-adrenoceptors at 10 min of incubation. These data suggest that alterations in the beta1-adrenoceptor-linked stimulatory guanine nucleotide binding protein-adenylyl cyclase pathway due to X plus XO are biphasic in nature and these changes may likely be due to the formation of H2O2.