Down-regulation of beta-adrenoceptors and loss of Gs alpha subunit levels in ventricular myocardium of rats treated with isoproterenol.
We investigated the influence of chronic beta-adrenergic stimulation on the beta-adrenoceptor-G protein-adenylyl cyclase system in rat ventricular myocardium. The rats received twice-daily injections of 4 mg/kg isoproterenol (ISO) alone or with 8 mg/kg propranolol (PROP) for 4 days. The ISO (10 microM)-induced increase in tissue cAMP production was lower (50%) after chronic ISO treatment than in control myocardium. The beta-adrenoceptor density decreased by 43% in ventricular membranes from ISO-treated rats. Immunoblotting techniques using specific antibodies against G proteins revealed that ventricular myocardium contains three Gs alpha subunit isoforms of 45, 47 and 52kDa. ISO treatment decreased levels of the three Gs alpha subunits by a total of 40%, while no change in Gi alpha (40/41kDa) and Gcommon beta (35/36kDa) levels were found in the same membrane preparations. The antagonist PROP almost totally blocked the effects of ISO treatment on cAMP, beta-adrenoceptors and Gs alpha subunits. These results suggest that chronic beta-adrenergic stimulation causes not only down-regulation of beta-adrenoceptors, but also loss of Gs alpha subunit levels in rat ventricular myocardium.