Two distinct populations of [3H]prazosin and [3H]yohimbine binding sites in the plasma membranes of rat mesenteric artery.

Postsynaptic alpha adrenoceptor subtypes have been investigated by radioligand binding studies in plasma membrane vesicles prepared from rat mesenteric arteries using [3H]prazosin and [3H]yohimbine. Both the radioligands displayed monophasic saturation in binding with a single component on Scatchard analysis. In the estrogenized female rat mesenteric artery, the specific binding of [3H]prazosin was rapid, saturable, reversible and of high affinity (0.65 +/- 0.05 nM) with a maximum binding capacity (Bmax) of 177 +/- 14 fmol/mg of protein. The maximum number of [3H]yohimbine binding sites were 427 +/- 31 fmol/mg of protein with the Kd equal to 34.5 +/- 3.8 nM. There was no evidence of cooperativity in the binding of both the ligands. The Kd values of [3H]prazosin and [3H]yohimbine, calculated from their respective kinetic analyses of binding, were in good agreement with the Kd values estimated from Scatchard plots. Prazosin was 15,000 times more potent in competing at the [3H]prazosin binding sites than at the [3H]yohimbine sites. In contrast, unlabeled yohimbine was 100-fold more potent in competing at the [3H]yohimbine binding sites than at the [3H]prazosin sites. The affinity of BE 2254 was 10,500 times higher for the [3H]prazosin binding sites than its affinity for the [3H]yohimbine binding sites. Non-alpha adrenoceptor antagonists competed poorly for both the radioligand binding sites. The Kd and Bmax of [3H]prazosin and [3H]yohimbine binding in the membranes of male rat mesenteric arteries were not significantly different from the corresponding values in the membranes of estrogenized female rat mesenteric artery.(ABSTRACT TRUNCATED AT 250 WORDS)