Abstract
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 (B(max)) of 177 ± 14 fmol/mg of protein. The maximum number of [3H]yohimbine binding sites was 427 ± 31 fmol/mg of protein with the K(d) equal to 34.5 ± 3.8 nM. There was no evidence of cooperativity in the binding of both the ligands. The K(d) values of [3H]prazosin and [3H]yohimbine, calculated from their respective kinetic analyses of binding, were in good agreement with the K(d) 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 K(d) and B(max) 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. There was a decrease in the B(max) for yohimbine in arteries from chemically sympathectomized estrogenized female rats which resulted in values not significantly different from that for prazosin. These data show that the binding sites for alpha adrenoceptor antagonists are similar in B(max) and K(d) in arteries from male and female rats and that sympathectomy lowers the number of yohimbine binding sites either by removing contaminating neural membrane or by modulating the number of postsynaptic sites. In conclusion, these results indicate the existence of two populations of alpha adrenoceptor antagonist binding sites in the purified plasma membrane of rat mesenteric artery.