Pharmacological analysis of the postjunctional alpha-adrenoceptors of the rat anococcygeus muscle and vas deferens.

The alpha-adrenoceptor sub-type of the rat vas deferens was characterized with noradrenaline (alpha-1, alpha-2-agonist), phenylephrine (alpha-1-agonist), prazosin (alpha-1-antagonist) and yohimbine (alpha-2-antagonist) and compared with the results obtained in the rat anococcygeus muscle. In the vas deferens, both prazosin and phentolamine were competitive antagonists of noradrenaline and phenylephrine whereas yohimbine was a non-competitive antagonist irrespective of the concentrations of these antagonists. On the other hand, in the anococcygeus muscle, at low concentrations, prazosin was non-competitive against noradrenaline whilst it was competitive against phenylephrine. Yohimbine behaved as a competitive antagonist of noradrenaline at both low and high concentrations whilst at low concentrations it was non-competitive against phenylephrine. These results suggest a predominance of alpha-1-adrenoceptors in the vas deferens located postjunctionally with a small population of alpha-2-subtype adrenoceptors, whilst the anococcygeus muscle seems to contain relatively more alpha 2-adrenoceptors postjunctionally than the vas deferens.