α-Adrenergic Receptors in Human Platelets

Abstract

Stimulation of human platelets by adrenaline and noradrenaline causes platelet aggregation and inhibition of platelet adenylate cyclase; both effects involve α-adrenergic receptors, since both are blocked by α-adrenergic blocking agents. The binding of [3H]dihydroergonine (3H-DHE), a potent α-adrenergic antagonist in the platelet system, was used to characterize the α-adrenergic receptors in both particulate preparations from human platelets and in intact platelets. Binding of 3H-DHE to platelet particles was rapid and reversible, with an association rate constant of 2.2·107 liters · mol−1 · min−1 and a dissociation rate constant of 0.027 min−1 at 25°. 3H-DHE binding was of high affinity, with an equilibrium dissociation constant (KD) of 6.3 nM and was saturable with 220 fmol bound/mg of particle protein. No cooperativity was detectable. The α-adrenergic agonist (−)-adrenaline competed for 3H-DHE binding with a KD of about 0.5 μM. The order of potencies for adrenergic agonists was (−)-adrenaline > (−)-noradrenaline (NA) » (±)-methoxamine » (−)-isoprenaline and for adrenergic antagonists was dihydroergonine > dihydroergotamine = yohimbine > phentolamine » tolazoline = azapetine » (−)-propranolol = (−)-pindolol. (+)-NA was about 20 times less potent than (−)-NA. The specificities of the various agonists and antagonists for competition of 3H-DHE binding were in good agreement with their effects on platelet aggregation and adenylate cyclase. In intact platelets, binding of 3H-DHE reached equilibrium within about 10 min at 25° and was saturable with about 2000 binding sites/platelet.