Characterization of postsynaptic alpha-adrenergic receptors by [3H]-dihydroergocryptine binding in muscular arteries from the rat mesentery.

Abstract

Alpha-adrenergic receptors are likely to be important determinants of the effects of catecholamines on vascular resistance. To study the alpha-adrenergic receptor in muscular arteries of the type that determine vascular resistance, we characterized and quantitated alpha-adrenergic receptors in a particulate fraction of the highly reactive, richly innervated arteries of the rat mesentery. With the ligand [3H]-dihydroergocryptine ([3H]-DHEC), specific binding (displaceable by 5 microM phentolamine) is saturable. There is a single class of binding sites with a dissociation constant (Kd) of 2.9 nM and a maximal binding capacity of 68 fmoles of [3H]-DHEC per mg of particulate fraction protein. Catecholamines compete for [3H]-DHEC binding stereospecifically and with the alpha-adrenergic potency series of (-)epinephrine greater than (-)norepinephrine greater than (-)isoproterenol. Binding is rapid (t 1/2 less than or equal to 2 mins) and rapidly reversible (t 1/2 less than or equal to 2 mins). Inhibition of [3H]-DHEC binding by the alpha-adrenergic antagonist phentolamine (Kd = 3.0 nM) is much greater than by the beta-adrenergic antagonist propranolol (Kd = 8200 nM). The alpha 1-selective antagonist prazosin (Kd = 63 nM) is 20 times more potent in competing for [3H]-DHEC binding than is the alpha 2-selective antagonist yohimbine (Kd = 1250 nM), thus suggesting that the alpha-adrenergic receptor identified is predominantly of the alpha 1 subtype that is responsible for vascular smooth muscle contraction. This extension of radioligand binding techniques to highly innervated muscular arteries of the type contributing to vascular resistance will allow the study of the role of the vascular alpha-adrenergic receptor in various physiologic states and models of hypertension.