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).

The [³H]dihydroergocryptine binding sites could also be demonstrated in intact platelets where they displayed comparable specificity, stereospecificity, and saturability. Saturation binding studies with the intact platelets indicated 220±45 receptors per platelet, in good agreement with the value derived from studies with platelet lysates. Ability of α-adrenergic agonists to inhibit adenylate cyclase and of α-adrenergic antagonists to antagonize this inhibitory effect directly paralleled ability to interact with the [³H]dihydroergocryptine binding sites. These data demonstrate the feasibility of directly studying α-adrenergic receptor binding sites in human platelets with [³H]dihydroergocryptine.