Teleological significance of beta adrenergic receptors in muscle blood vessels.

Teleological significance of the pressence of beta adrenergic receptors in blood vessels of skeletal muscle seems to be in redistributing blood flow from viscera to skeletal muscle when muscular activity is required or anticipated. Adrenaline secreted from the adrenal medulla in such occasions constricts blood vessels in viscera as its alpha effect and dilates blood vessels in muscle as its beta effect. The latter effect is considered to be especially important in the rat which lacks cholinergic vasodilator fibers. Beta receptor activity detected as vasodilation on administration of adrenaline is most marked in skeletal muscle, but its teleological significance is obscure5>. At least in the rat the presence of such a vasodilator mechanism seems to be advantageous in increasing muscle blood flow when muscular activity is required or anticipated. In a previous study9>, we observed a conspicuous beta adrenergic vasodilatation in transposition response of the rat, induced by transposing the animal from its home cage to a new cage. In this responce in which the sympathoadrenal system is excited presumably triggered by the stress of change of habitat, cardiac output is increased and arterial pressure remains almost unchanged: total peripheral resistance is decreased. Beta adrenoceptor blockade with propranolol converts this isopressor response to a pressor one. The arterial pressure and total peripheral resistance are then both elevated on transposition of the rat. In search for the vascular bed responsible for the decrease in total peripheral resistance in transposition response, changes in peripheral blood flow were observed using an electromagnetic flow probe chronically implanted at the terminal aorta, renal artery, superior mesenteric artery, and common carotid artery8>. Of these four arteries, blood flow was increased in transposition response only in the terminal aorta supplying the hindquarters. Blood flow was markedly decreased in the superior mesenteric and renal arteries and was almost unchanged in the common carotid artery. The increase in hindquarter flow in transposition response was greatly diminished after