β1- and β1/β2-adrenergic receptor antagonists block 6-nitrodopamine-induced contractions of the rat isolated epididymal vas deferens.

Abstract

6-Nitrodopamine (6-ND) is an endogenous modulator of the contractility in the rat isolated epididymal vas deferens (RIEVD) and considered to be the main peripheral mediator of the emission process. Use of selective and unselective β-adrenergic receptor antagonists has been associated with ejaculatory failure. Here, the effects of selective β1- and β1/β2-adrenergic receptor antagonists on RIEVD contractions induced by 6-ND, dopamine, noradrenaline, adrenaline, and electric-field stimulation (EFS) were investigated. The selective β1-adrenergic receptor antagonists atenolol (0.1 and 1µM), betaxolol (1µM), and metoprolol (1µM) and the unselective β1/β2-adrenergic receptor antagonists propranolol (1 and 10µM) and pindolol (10µM) caused significant rightward shifts of the concentration-response curve to 6-ND (pA2 6.41, 6.91, 6.75, 6.47, and 5.74; for atenolol, betaxolol, metoprolol, propranolol, and pindolol), but had no effect on dopamine-, noradrenaline-, and adrenaline-induced contractions. The effects of selective β1- and β1/β2-adrenergic receptor antagonists at a higher concentration (atenolol 1µM, betaxolol 1µM, metoprolol 1µM, propranolol 10µM, and pindolol 10µM) also reduced the EFS-induced RIEVD contractions in control, but not in RIEVD obtained from L-NAME-treated animals. The selective β1-adrenoceptor agonist RO-363, the selective β2-adrenoceptor agonist salbutamol, and the selective β3-adrenoceptor agonist mirabegron, up to 300µM, had no effect on the RIEVD tone. The results demonstrate that β1- and β1-/β2-adrenoceptor receptor antagonists act as 6-ND receptor antagonists in RIEVD, further confirming the main role of 6-ND in the RIEVD contractility.