Identification of adrenoceptors and dopamine receptors mediating vascular responses in the superior mesenteric arterial bed of the rat.

The nature of the adrenoceptors and dopamine receptors mediating vascular responses in the in-situ blood perfused superior mesenteric arterial bed of the rat have been studied. alpha 1-Adrenoceptor agonists produced vasoconstriction but alpha 2-agonists had no significant effect on vascular resistance. The vasoconstrictor effects of noradrenaline were antagonized by low doses of prazosin (26 nmol kg-1 i.v.). Isoprenaline and salbutamol produced vasodilation when the vasculature was preconstricted with arginine vasopressin. The responses to isoprenaline were potently antagonized by propranolol (1.69 mumol kg-1 i.v.) and weakly but significantly reduced by practolol (3.75 mumol kg-1 i.v.) whereas the responses to salbutamol were unaffected by the same dose of practolol. After preconstriction of the vasculature and alpha-adrenoceptor blockade, dopamine and apomorphine produced dilator responses with both compounds producing the same maximal response and apomorphine being 1.8 times more potent than dopamine. The dopamine responses were present after the animals had been pithed and were resistant to spiperone (506 nmol kg-1 i.v.) but were antagonized by cis-alpha-flupenthixol (460 nmol kg-1 i.v.). These results suggest that this vascular bed possesses vasoconstrictor alpha 1- but not alpha 2-adrenoceptors, vasodilator beta 1- and beta 2-adrenoceptors and vasodilator dopamine receptors which appear similar to the D1-type found centrally.