Identification of α-Adrenergic Receptors in Human Platelets by [³H]Dihydroergocryptine Binding

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Abstract

Binding of [3H]dihydroergocryptine to platelet lysates appears to have all the characteristics of binding to α-adrenergic receptors. At 25°C binding reaches equilibrium within 20 min and is reversible upon addition of excess phentolamine. Binding is saturable with 183±22 fmol of [3H]dihydroergocryptine bound per mg of protein at saturation, corresponding to 220±26 sites per platelet. Kinetic and equilibrium studies indicate the dissociation constant of [3H]dihydroergocryptine for the receptors is 1-3 nM. The specificity of the binding sites is typical of an α-adrenergic receptor. Catecholamine agonists compete for occupancy of the [3H]dihydroergocryptine binding sites with an order of potency (−)epinephrine> (−)norepinephrine≫ (−)isoproterenol. Stereospecificity was demonstrated inasmuch as the (+)isomers of epinephrine and norepinephrine were 10-20-fold less potent than the (−)isomers. The potent α-adrenergic antagonists phentolamine, phenoxybenzamine, and yohimbine competed potently for the sites, whereas β-antagonists such as propranolol and dichlorisoproterenol were quite weak. Dopamine and serotonin competed only at high concentrations (0.1 mM).

Authors

Kurt D. Newman, Lewis T. Williams, N. Hahr Bishopric, Robert J. Lefkowitz