Analysis of responses to dopamine in the mesenteric vascular bed of the cat.

Responses to dopamine were investigated in the feline mesenteric vascular bed under conditions of controlled blood flow. Intra-arterial injections of dopamine in doses of 3, 10 and 30 microgram caused biphasic changes in mesenteric arterial perfusion pressure. The pressor component of the biphasic response was blocked whereas the depressor component was enhanced after administration of phenoxybenzamine in a dose that reversed vasoconstrictor responses to norepinephrine. Depressor responses to dopamine in phenoxybenzamine treated animals were not blocked by propranolol or atropine in doses that inhibited responses to isoproterenol or acetylcholine or by indomethacin in a dose that inhibited responses to the prostaglandin precursor arachidonic acid. Depressor responses to dopamine were inhibited by bulbocapnine in a dose that was without significant effect on depressor responses to isoproterenol or nitroglycerin. The inhibitory effects of bulbocapnine were reversible and vasodepressor response to dopamine returned to control levels 15-30 min after the bulbocapnine infusion was discontinued. These data suggest that the pressor effects of dopamine in the feline mesenteric vascular bed are due to activation of alpha adrenergic receptors whereas the vasodilator activity results from activation of vascular dopamine receptors that are distinct from beta adrenergic or cholinergic receptors and does not involve the release of dilator prostaglandin.