Characterization of two distinct alpha-adrenoceptor binding sites in smooth muscle cell membranes from rat and bovine aorta

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Abstract
In order to characterize postjunctional alpha adrenoceptor binding sites of aortic smooth muscle, the specific binding of (3H)prazosin and (3H)yohimbine to membranes prepared from the medial layers of rat and bovine thoracic aorta was investigated. Binding of (125I)-BE 2254 (2-[B-(4-hydroxyphenyl)-ethylaminomethyl] tetralone) and (3H)RX 781094 (idazoxan) was also examined in bovine membranes. Each of the ligands displayed saturable, specific binding to a single population of sites; the K D values of the respective ligands were similar in the two animal species. The number of (3H)prazison and (125I)BE 2254 binding sites (160–190 fmol · mg protein−1 in the two species) was higher than the number of (3H)yohimbine and (3H)RX 781094 binding sites (110–120 fmol · mg protein−1 in the bovine and 50 fmol · mg protein−1 in the rat). Alpha-adrenoceptor ligands inhibited binding of the ligands with the following orders of potency: prazosin > BE 2254 > yohimbine > RX 781094 > clonidine in the case of (3H)-prazosin, and yohimbine > RX 781094 > clonidine > prazosin in the case of (3H)yohimbine. Methoxamine, in concentrations up to 10 μM, was without effect on the binding of either ligand. The absence or presence of Na+, K+ or Ca2+ added at physiological concentrations did not change the order of potency of displacing ligands whereas Ca2+ reduced by 50% the numbers of (3H)prazosin and (3H)-yohimbine sites and Na+ increased by 3-fold the affinity of (3H)yohimbine. It is concluded that post-junctional membranes from rat and bovine aortic smooth muscle contain two distinct α1- and α2-adrenoceptor binding sites, the number of the latter being less than the number of the former.