Characterization of the Human Platelet α-Adrenergic Receptor

Abstract

Human platelets aggregate and undergo a release reaction when incubated with catecholamines. Indirect evidence indicates that these events are mediated through α-adrenergic receptors. We used [³H]dihydroergocryptine, an α-adrenergic antagonist, to identify binding sites on platelets that have the characteristics of α-adrenergic receptors. Catecholamines compete for the binding sites in a stereo-specific manner with the potency series of (−) epinephrine > (−) norepinephrine > (±) phenylephrine > (−) isoproterenol. The dissociation constant (K_d) of (−) epinephrine is 0.34 μM. Binding is saturable using a platelet particulate fraction but not with intact platelets. There are 0.130 pmol binding sites per milligram protein in fresh platelet membranes. This number represents approximately 100 binding sites per platelet. The K_d for [³H]-dihydroergocryptine was 0.003−0.01 μM. The α-adrenergic antagonist phentolamine (K_d = 0.0069 μM) was much more potent than the β-adrenergic antagonist (±) propranolol (K_d = 27 μM) in competing for the binding sites. The binding data were correlated with catecholamine-induced platelet aggregation and inhibition of basal and prostaglandin E₁-stimulated adenylate cyclase. (−) Epinephrine was more potent than (−) norepinephrine in producing aggregation whereas (−) isoproterenol was ineffective. The threshold dose for inducing aggregation by (−) epinephrine was 0.46 μM. Phentolamine and dihydroergocyrptine blocked this response, whereas (±) propranolol had no effect. (−) Epinephrine and (−) norepinephrine inhibited basal and prostaglandin E₁-stimulated adenylate cyclase in a dose-related manner. The concentration of (−) epinephrine inhibiting adenylate cyclase 50% was 0.7 μM. This inhibition was also blocked by phentolamine and dihydroergocryptine but not by (±) propranolol. The specificity of binding and the close correlation with α-adrenergic receptor-mediated biochemical and physiological responses suggest that the [³H]dihydroergocryptine binding site represents, or is closely related to, the human platelet α-adrenergic receptor. The ability to assay this receptor directly and to correlate these data with independently measured sequelae of receptor activation should facilitate increased understanding of the physiology and pathophysiology of the human platelet α-adrenergic receptor.