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Analysis of the coupling of receptors and the adenylate cyclase is facilitated by radioligand analysis of receptor occupancy. The platelet α-receptor has been characterized by [3H]dihydroergocryptine binding to membranes, but poorly reproducible results are obtained with intact platelets. We incubated washed platelets with [3H]dihydroergocryptine and measured bound counts after 6-fold dilution with plasma and centrifugation. After 10 minutes, phentolamine (5μM) suppressible counts equivalent to 400 molecules per platelet were found with 1/2 saturation at 1.2nM. There was considerable intra-experimental variation, and non-specific binding “space” was greater than 10μl/108 platelets. Time course of displacement with cold dihydroergocryptine, phentolamine or yohimbine was biphasic with a rapid (2 min) component amounting to 20-30% followed by a further 20% over the next 60 minutes. These results, which confirm those of others, suggest that equilibrium is not reached in a reasonable period of time and that more than one binding mechanism may be involved. In contrast, [3H]yohimbine bound with simple first order kinetics (37°) giving k1 = 5.25 × 105M-1 and k-1 = 3.33 × 10-3sec-1. Scatchard analysis revealed kd = 4.8 nM, Bmax = 180 molecules per platelet. Non-specific binding “space” was less than 1μl/108 platelets. Binding was completely reversed or prevented by epinephrine, clonidine, ρ-aminoclonidine, phentolamine, dihydroergotamine and dihydroergocryptine in that order of potency. Prazosin, an α1-antagonist, was less potent than epinephrine. Yohimbine blocks the ability of epinephrine to potentiate and induce aggregation, and to inhibit the adenylate cyclase. It is concluded that this novel radioligand has clear advantages over dihydroergocryptine for analysis of receptor occupancy in intact platelets.

Selective Binding Of [3H]Yohimbine To The α2-Receptor Of Intact Platelets -- Comparison With [3H] Dihydroergocryptine Binding