Analysis of adrenergic responses in the mesenteric vascular bed of the cat; evidence that vascular beta 2-adrenoceptors are innervated.

Responses to sympathetic nerve stimulation and pressor hormones were investigated in the feline mesenteric vascular bed under conditions of controlled blood flow. Sympathetic nerve stimulation and norepinephrine produced frequency- and dose-dependent increases in mesenteric vascular resistance. However, when alpha-receptors were blocked with the non-equilibrium alpha-receptor antagonist, phenoxybenzamine, nerve stimulation and norepinephrine produced frequency- and dose-dependent decreases in mesenteric vascular resistance. These reductions in mesenteric vascular resistance were unchanged after indomethacin or atropine, whereas propranolol converted the mesenteric vasodilator responses to small vasoconstrictor responses. In these studies, responses to a variety of vasoconstrictor agents were enhanced after administration of propranolol. Sotalol, a nonselective beta blocker with little membrane stabilizing activity, also enhanced vasoconstrictor responses. The present data suggest that both alpha- and beta-adrenergic receptors are innervated in the feline mesenteric vascular bed, and that vasodilator responses to norepinephrine and sympathetic nerve stimulation are independent of activation of muscarinic receptors or formation of products in the cyclooxygenase pathway. These data also demonstrate that there is a nonspecific potentiation of intestinal vasoconstrictor responses after beta-adrenergic receptor blockade that is independent of a membrane-stabilizing or receptor-mediated mechanism.