Nebivolol: Endothelium-Mediated Vasodilating Effect

Nebivolol, a racemic mixture of (S,R,R,R) and (R,S,S,S) enantiomers, is the most beta1-selective adrenoceptor antagonist currently available for clinical use. However, its haemodynamic effects differ from those of classical beta-adrenoceptor antagonists as a result of a vasodilating action. Nebivolol is devoid of alpha-adrenergic antagonist actions, and the detailed mechanism of its vasodilator action is unknown. Nebivolol relaxes precontracted strips of canine coronary and carotid artery only if the endothelium is intact, and such relaxation is antagonized by inhibition of nitric oxide (NO) synthase, implicating the endothelial L-arginine/NO mechanism. Nebivolol and atenolol have been compared in phenylephrine preconstricted dorsal hand veins of 11 healthy men. Nebivolol caused venodilation, which was antagonized by N(G)-monomethyl-L-arginine (LNMMA), whereas atenolol did not, suggesting that such a mechanism could also operate in human veins. Venodilation could be functionally important in reducing cardiac pre-load. Beta2-adrenoceptor stimulation increases forearm blood flow (FBF) by activating the L-arginine/NO pathway but nebivolol lacks direct beta2-adrenoceptor agonist activity. Resistance vessel function has been studied by measuring FBF by venous occlusion plethysmography in healthy men during brachial artery infusions of racemic nebivolol and its enantiomers, atenolol, carbachol (a stable analogue of acetylcholine that vasodilates this vascular bed, in part by activating the L-arginine/NO pathway), nitroprusside (a NO donor that causes non-endothelium-dependent vasodilation through the same effector mechanism as endothelium-dependent agonists) and LNMMA. Nebivolol (354 microg/min) increased FBF by 91 +/- 18% (mean +/- SE, n = 8; p < 0.01), whereas an equimolar dose of atenolol had no significant effect. LNMMA inhibited responses to nebivolol and carbachol to a significantly greater extent than it reduced responses to nitroprusside. Antagonism of nebivolol by LNMMA was abolished by L-arginine. The (S,R,R,R) and (R,S,S,S) enantiomers caused similar increases of FBF. To determine whether brachial artery infusion of nebivolol causes vasodilation in the forearm resistance vasculature of patients with essential hypertension and to investigate the possible involvement of the L-arginine/NO pathway, we studied otherwise healthy volunteers with uncomplicated essential hypertension. Nebivolol (88.5, 177 and 354 microg/min, each dose for 6 min) caused similar vasodilatation as in normotensive subjects, and these responses were sensitive to inhibition by LNMMA. If acute effects of nebivolol on the L-arginine/NO pathway persist during chronic treatment of patients with hypertension or heart failure, this could reduce cardiac after-load as well as pre-load, improve organ perfusion and reduce atherogenesis and thrombosis.