The effects of nebivolol, the racemic mixture of the SRRR and RSSS enantiomers, on beta-adrenoceptor-mediated cAMP accumulation in living cardiac cells were compared to those of beta-adrenoceptor antagonists. Serum-free cultivation of cardiac cells from ventricles of 2 to 3-day-old Wistar rats resulted in a population of contractile cardiac cells almost free of mesenchymal non-myocardial cells. Isoproterenol stimulated beta 1- as well as beta 2-adrenoceptor sites. Selective beta 1- and beta 2-receptor site occlusion, in the presence of an appropriate concentration of the selective beta 2-adrenoceptor antagonist, ICI 118-551, or the selective beta 1-adrenoceptor antagonist, CGP 20712-A, showed that the receptor population consisted of mostly the beta 1-adrenergic subtype. The latter could be specifically stimulated by noradrenaline. Nebivolol and d-nebivolol (SRRR) inhibited noradrenaline-induced cAMP accumulation with IC50 values of 22 and 15 nM, respectively. CGP 20712-A was 10 times more active and atenolol was 7 times less active than nebivolol. Both assays, beta-adrenoceptor binding and cAMP accumulation, evidenced beta-adrenoceptor antagonistic properties only for the d-enantiomer of nebivolol (SRRR). 1-Nebivolol (RSSS) showed no beta-adrenergic activity.
Adrenergic beta-Antagonists Animals