Characterization of α 2-adrenergic receptors on human platelets using [ 3H]yohimbine

Abstract

Summary In order to investigate properties of alpha-adrenergic receptors of human platelets, we have examined the binding of [ 3 H]yohimbine, a potent α 2 -adrenergic 3 antagonist, to intact platelets and platelet membranes. There were 207±41 [ 3 H]yohimbine sites per platelet. These had an equilibrium dissociation constant (K D ) of 2.7 ± 0.7nM, a Hill coefficient of 1.02±0.11, and were competed for by adrenergic compounds stereoselectively and with a rank order expected at α 2 -adrenergic receptors. Addition of Na + and GTP synergistically decreased the affinity of epinephrine in competing for [ 3 H]yohimbine sites in platelet membranes and the resulting affinity of epinephrine in membranes was similar to that obtained with intact platelets. Substitution of sucrose for NaCl increased the affinity of epinephrine for intact platelets. [ 3 H]yohimbine thus appears to be a useful ligand for characterization of platelet α 2 -adrenergic receptors. Our results suggest that extracellular Na + and intracellular GTP and Mg ++ may be physiological determinants of epinephrine binding to these receptors.