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 o n the beta-adrenoceptor-G protein-adenylyl cyclase system in rat ventr icular myocardium. The rats received twice-daily injections of 4 mg/kg isoproterenol (ISO) alone or with 8 mg/kg propranolol (PROP) for 4 da ys. The ISO (10 muM)-induced increase in tissue cAMP production was lo wer (50%) after chronic ISO treatment than in control myocardium. The beta-adrenoceptor density decreased by 43% in ventricular membranes fr om ISO-treated rats. Immunoblotting techniques using specific antibodi es against G proteins revealed that ventricular myocardium contains th ree Gsalpha subunit isoforms of 45, 47 and 52kDa. ISO treatment decrea sed levels of the three Gsalpha subunits by a total of 40%, while no c hange in Gialpha (40/41kDa) and Gcommonbeta (35/36kDa) levels were fou nd in the same membrane preparations. The antagonist PROP almost total ly blocked the effects of ISO treatment on cAMP, beta-adrenoceptors an d Gsalpha subunits. These results suggest that chronic beta-adrenergic stimulation causes not only down-regulation of beta-adrenoceptors, bu t also loss of Gsalpha subunit levels in rat ventricular myocardium.