BIPHASIC ALTERATIONS IN CARDIAC BETA-ADRENOCEPTOR SIGNAL-TRANSDUCTIONMECHANISM 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 iso proterenol-stimulated adenylyl cyclase activities showed an increase a t 10 min and a decrease at 30 min of incubation with X plus XO. Treatm ent of membranes with H2O2 also produced biphasic changes in adenylyl cyclase activities. The density of beta(1)-adrenoceptors was decreased when cardiac membranes were treated with X plus XO for 10 and 30 min whereas the affinity of beta(1)-adrenoceptors was increased after 10 m in and reduced after 30 min of incubation. The beta(2)-adrenoceptors w ere not modified at 10 min whereas incubation of cardiac membranes wit h X plus XO for 30 min increased the affinity and decreased the densit y. Cholera toxin-stimulated adenylyl cyclase activity, cholera toxin-c atalyzed ADP-ribosylation and stimulatory guanine nucleotide binding p rotein immunoreactivity in cardiac membranes were increased at 10 min and decreased at 30 min of incubation with X plus XO, However, the per tussis toxin-stimulated adenylyl cyclase activity, pertussis toxin-cat alyzed ADP ribosylation and inhibitory guanine nucleotide binding prot ein immunoreactivity were not affected on treatment of membranes with X plus XO. Addition of superoxide dismutase plus catalase in the incub ation medium prevented the X plus XO-induced alterations in adenylyl c yclase activities, stimulatory guanine nucleotide binding protein-rela ted ADP-ribosylation and changes in the characteristics of beta-adreno ceptors except the increased affinity of beta(1)-adrenoceptors at 10 m in of incubation. These data suggest that alterations in the beta(1)-a drenoceptor-linked stimulatory guanine nucleotide binding protein-aden ylyl cyclase pathway due to X plus XO are biphasic in nature and these changes may likely be due to the formation of H2O2.