Vol. 282, Issue 3, 1623-1631, 1997

Biphasic Alterations in Cardiac Beta-Adrenoceptor Signal Transduction Mechanism Due to Oxyradicals¹

Sujata Persad, Vijayan Elimban, Jasvinder Kaila and Naranjan S. Dhalla

Institute of Cardiovascular Sciences, St. Boniface General Hospital Research Centre and Department of Physiology, Faculty of Medicine, University of Manitoba, Winnipeg, Canada R2H 2A6

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.