Adrenergic mechanisms in the hindlimb circulation of baboons

The effects of the adrenergic agonists norepinephrine, epinephrine, isoproterenol, and phenylephrine upon femoral arterial blood flow were measured in baboons before and after alpha (phenoxybenzamine) and beta (propranolol) adrenergic receptor blockade. Flow was measured with an electromagnetic flowmeter. Arterial and venous pressures were recorded simultaneously. Femoral vascular resistance was calculated from these data. Catecholamines were injected intra-arterially (10(-3)--10(0) mug, base, kg.(-1) and intravenously (1.0 mug kg.(-1) in a randomized sequence. All four adrenergic amines were vasodilators at low dose (10(-3) mug kg.(-1), intra-arterially) and this effect was abolished during beta adrenergic receptor blockade. Intra-arterial isoproterenol elicited dose-dependent increases in femoral flow; the other amines were vasoconstrictors at high doses. Alpha adrenergic blockade "reversed" the vasoconstrictor effects of these three amines. At the same dose isoproterenol increased flow more through the intra-arterial than the intravenous route. Conversely, norepinephrine and epinephrine were potent femoral vasodilators when injected intravenously. The findings indicate that the classical adrenergic amines are all vasodilators of the subhuman primate hindlimb at low doses due to their interaction with beta receptor sites. The fact that epinephrine and norepinephrine exert a greater increase in flow when given intravenously than when given intra-arterially is presumably secondary to increased arterial pressure, in turn due to the vasoconstrictor effects of these agents on other regional circulations.