Molecular Pharmacology, Vol 16, 77-90, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Characterization of Alpha-Adrenergic Receptors in Guinea-pig Vas Deferens by [³H]Dihydroergocryptine Binding

¹ Department of Pharmacology, Wayne State University School of Medicine, Detroit, Michigan 48201

[³H]Dihydroergocryptine binds to membrane sites on guinea-pig vas deferens, which have the characteristics expected of -adrenoreceptors. Specific binding is rapid, reversible and saturable, with a maximal occupancy of 190 fmol [³H]dihydroergocryptine bound/mg protein. Saturable binding occurs to a single population of high affinity sites (K_D = 1.55 nM) with no evidence for cooperative interactions. [³H]Dihydroergocryptine binding is markedly pH and temperature sensitive. While Na⁺ and K⁺ do not affect binding, maximal [³H]dihydroergocryptine binding observed in the presence of Mg²⁺ is antagonized by increasing concentrations of Ca²⁺. -Adrenoreceptor agonists and antagonists of several classes inhibit binding to vas deferens membranes. Catecholamines show marked stereo-specificity in relation to inhibition of binding of [³H]dihydroergocryptine. The potency for displacement of [³H]dihydroergocryptine by phenylethylamine agonists is identical to their agonistic potency in eliciting contractile responses of the isolated guinea-pig vas deferens. Some discrepancies between binding and physiological affinities of imidazoline -adrenoreceptor ligands were observed and discussed. There is close agreement between dissociation constants of azapetine and dihydroergocryptine, as estimated from radioligand binding and antagonism of physiological responses. As no other biogenic amine receptors binds [³H]dihydroergocryptine, there is no interference in using this radioligand to study -adrenoreceptor mechanisms in guinea-pig vas deferens.