3H-Yohimbine, a potent and selective pharmacological antagonist of α2-adrenergic receptors, labeled human platelet membrane α2-receptors with high affinity. Binding was rapid and reversible at 25°C. Both saturation and kinetic experiments indicated a single order of binding sites, with an equilibrium KD value of 1.0-1.5 nM. Low Mg2+ concentrations increased the KD for 3H-yohimbine without altering the Bmax. The 3H-yohimbine site exhibited α2-receptor specificity: (-)-norepinephrine and (-)-isoproterenol were 4.8 and 330 times less potent than (-)-epinephrine; (-)-catecholamines were 17-35 times more potent than corresponding (+)-catecholamines; the selective α1-antagonist prazosin was 340 times less potent than yohimbine. Catecholamine agonists exhibited shallow curves in inhibiting 3H-yohimbine binding, with pseudo-Hill coefficients (nH) of less than 1.0, whereas the nH of antagonists was 1.0. No specific binding of 3H-prazosin to platelet membranes was observed, indicating the absence of α1-receptors. 3H-Yohimbine labeled fewer platelet sites than did 3H-dihydroergocryptine under identical conditions (80 vs 130 receptors/ cell), and may be a more specific and useful antagonist probe of platelet α2-receptors than 3H-dihydroergocryptine.
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)
- Pharmacology, Toxicology and Pharmaceutics(all)