Vasoconstrictor responses of isolated and perfused canine mesenteric arteries to alpha-adrenoceptor agonists

Using isolated and perfused canine mesenteric artery, vasoconstrictor responses to alpha-adrenoceptor stimulating agents were compared. The order of potencies for inducing vasoconstriction was epinephrine greater than norepinephrine greater than phenylephrine greater than methoxamine greater than dopamine much greater than tyramine greater than clonidine greater than or equal to xylazine. A selective alpha 2-stimulant, xylazine, did not induce any significant vasoconstrictor response. Tyramine produced only a slight vasoconstriction. Responses to norepinephrine, phenylephrine and methoxamine were competitively inhibited by phentolamine treatment. From these results, it is concluded that the adrenoceptor in the canine mesenteric artery may be characterized as an alpha 1-receptor and that release of norepinephrine by tyramine is not marked in this relatively large vessel.