INHIBITION OF BETA-MEDIATED BUT NOT ALPHA(1)-MEDIATED ADRENERGIC RESPONSES IN ISOLATED HEARTS AND CARDIOMYOCYTES BY NITRIC-OXIDE AND 8-BROMO CYCLIC GMP

Objectives: The study was carried out to assess the effect of nitric o xide (NO) generation or inhibition of NO synthase on the cardiac respo nse to beta- and alpha(1)-adrenergic agonists. In addition, we determi ned the effects of the cell-permable analogue of cGMP, 8-bromo-cGMP (8 Br-cGMP). Methods: Experiments were done in electrically-paced isolate d perfused rat hearts as well as in ventricular myocytes. Hearts were exposed to either the beta-adrenoceptor agonist, isoproterenol(0.1 mu M), or the alpha(1)-adrenoceptor agonist, phenylephrine (2 mu M in the presence of equimolar concentrations of propranolol), either with eac h drug alone or in the presence of the NO donors S-nitrosoacetylpenici llamine (SNAP, 10 mu M) and 3-morpholino-sydnonimine (SIN-1, 10 mu M), the NO synthase inhibitor L-NAME (10 mu M) or 8Br-cGMP (50 mu M). The se concentrations of SNAP and 8Br-cGMP increased tissue cGMP levels ap proximately 3-fold after 15 min treatment. Myocardial contractility wa s assessed by determining left ventricular pressure with a fluid-fille d balloon inserted into the left ventricle. Similar experiments were p erformed in myocytes in which cell shortening and intracellular calciu m transients were determined although concentrations of isoproterenol and phenylephrine in myocytes were higher (1 and 5 mu M, respectively) than those used in isolated hearts in order to achieve optimum respon ses. Results: In isolated hearts isoproterenol increased developed pre ssure by about 50%, which was totally prevented by SNAP and SIN-1 and unaffected by L-NAME. 8Br-cGMP, however, did not significantly diminis h the positive inotropic effect of isoproterenol. Phenylephrine increa sed developed pressure of isolated hearts by about 30%, but this was t otally unaffected by either SNAP, SIN-I or 8Br-cGMP. In myocytes, isop roterenol significantly increased the calcium transient by more than 5 0% and cell shortening by about 70%. Both effects were significantly a ttenuated by SNAP, SIN-I and 8Br-cGMP but unaffected by L-NAME. Phenyl ephrine significantly increased cell shortening and the calcium transi ent, but these responses were unaffected either by SNAP or 8Br-cGMP. C onclusions: The present study demonstrates that NO as well as guanylat e cyclase inhibitors and, to a lesser extent, 8Br-cGMP attenuate beta- receptor-mediated cardiac responses and supports the concept that NO s erves as an endogenous regulator of beta-mediated effects of catechola mines in the heart. In addition, our findings suggest that the antiadr energic effects of NO are restricted to these receptors but likely do not involve alpha(1)-mediated effects.