Existence of Two Types of Postjunctional Alpha Adrenoceptors in the Isolated Canine Intermediate Auricular Artery

Four alpha adrenergic agonists (phenylephrine, xylazine, clonidine and norepinephrine) and two antagonists (prazosin and yohimbine) were used to investigate the characteristics of the postjunctional alpha adrenoceptors involved in contraction of the isolated and perfused canine intermediate auricular artery. All agonists showed almost equal potencies for inducing vasoconstriction in the perfused arterial preparations (i.e., all agonists caused strong vasoconstrictor responses in a dose-related manner; the threshold dose of each agonist was within a dose range of 0.003 to 0.01 micrograms and 0.3 to 1.0 micrograms of each agent caused approximately 200 mm Hg increase in perfusion pressure). Prazosin inhibited phenylephrine-induced vasoconstriction in a competitive manner, but yohimbine did not significantly influence phenylephrine-induced responses. Xylazine-induced responses were inhibited by both prazosin and yohimbine, but the former was less potent than the latter at a low dose and the antagonistic property of prazosin against xylazine was not competitive. The pA2 values of prazosin against phenylephrine, xylazine and clonidine were 7.10, 6.82 and 6.99, respectively, and that of yohimbine against xylazine was 7.16. These results support the theory that not only alpha-1 but also alpha-2 adrenoceptors are involved in the contractile responses of the isolated and perfused canine intermediate auricular artery.