2 DISTINCT POPULATIONS OF [H-3]PRAZOSIN AND [H-3] YOHIMBINE BINDING-SITES IN THE PLASMA-MEMBRANES OF RAT MESENTERIC-ARTERY

  • 1 January 1985
    • journal article
    • research article
    • Vol. 233  (1) , 195-203
Abstract
Postsynaptic .alpha. adrenoceptor subtypes were investigated by radioligand binding studies in plasma membrane vesicles prepared from rat mesenteric arteries using [3H]prazosin and [3H]yohimbine. Both 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 completed 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. There was a decrease in the Bmax for yohimbine in arteries from chemically sympathectomized estrogenized female rats which resulted in values not significantly different from that for prazosin. The binding sites for .alpha. adrenoceptor antagonists are similar in Bmax and Kd 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. The existence of 2 populations of .alpha. adrenoceptor antagonist binding sites in the purified plasma of rat mesenteric artery is indicated.

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