Distinction between alpha-adrenergic receptors in eight vascular areas of the dog

The study concerns the heterogeneity of the vascular alpha-adrenergic receptor population studied in anesthetized dogs. Two groups of animals were used: a control group that received only the agonists noradrenaline (1-1024 ng/kg i.v.) and phenylephrine (64-16384 ng/kg i.v.) and a group that additionally received the antagonist phenoxybenzamine (67-2250 micrograms/kg i.v.). Arterial pressure was measured and the vascular responses were simultaneously monitored in liver, stomach, duodenum, jejunum, ileum, colon, kidney and hindleg, simultaneously with electromagnetic blood flow sensors. Phenoxybenzamine with regard to noradrenaline only blocked the renal vasoconstriction. Effects of some dosages of noradrenaline were inhibited only by the highest dose of phenoxybenzamine. In contrast, phenoxybenzamine inhibited phenylephrine-induced vasoconstriction in all the investigated vascular beds, but not completely in the hindleg. The response in the stomach was even reversed to a vasodilation. It is concluded that (1) all vascular beds studied contain alpha 1-adrenergic receptors; (2) noradrenaline exerts its vasoconstrictive activity by interaction with alpha 1-adrenergic receptors but also with phenoxybenzamine-insensitive alpha-adrenergic receptors. This conclusion indicates a heterogeneity of alpha-adrenergic receptors in the vascular beds studied.