Postsynaptic alpha-adrenoceptor subtypes in rabbit blood vessels and rat anococcygeus muscle studied in vitro.

Postsynaptic alpha-adrenoceptor subtypes were investigated in vitro, employing rabbit aorta, pulmonary artery, and portal vein, and rat anococcygeus. Phenylephrine (alpha 1-selective), alpha-methylnoradrenaline (mixed agonist), and xylazine (alpha 2-selective) were used as agonists, and prazosin (alpha 1-selective) and rauwolscine (alpha 2-selective) as antagonists. In all tissues, agonist concentration-response curves were monophasic and their shape was unaltered by either antagonist. In rabbit blood vessels, prazosin was as potent against alpha-methylnoradrenaline as against phenylephrine and was 1,000 times more potent than rauwolscine; xylazine was a partial agonist of low potency. Hence, the postsynaptic receptors of these tissues are alpha 1. In the rat anococcygeus, there was some evidence for an alpha 2-receptor: xylazine was a full and potent agonist, rauwolscine was more potent against xylazine than against alpha-methylnoradrenaline or phenylephrine, and in experiments protecting against irreversible blockade by phenoxybenzamine, xylazine afforded significantly greater protection to alpha-methylnoradrenaline than to phenylephrine. However, prazosin was more potent than rauwolscine against xylazine and was equipotent against all agonists. Hence, although the smooth muscle cells of the rat anococcygeus may contain alpha 2-adrenoceptors, the predominant population is alpha 1.