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 m...Continue Reading
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