A COMPARISON OF α2‐ADRENORECEPTOR BINDING CHARACTERISTICS OF INTACT HUMAN PLATELETS IDENTIFIED BY [3H]‐YOHIMBINE AND [3H]‐DIHYDROERGOCRYPTINE

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Abstract
The binding characteristics of [3H]-yohimbine on intact human platelets were investigated and compared to those of [3H]-dihydroergocryptine. A significant correlation (r = 0.92, P < 0.01, n = 7) was observed between the inhibition of [3H]-yohimbine binding and of noradrenaline[norepinephrine]-induced platelet aggregation, indicating that [3H]-hohimbine binds to the .alpha.2-adrenoreceptor responsible for noradrenaline-induced platelet aggregation. The binding capacity of [3H]-yohimbine in 20 young male volunteers was 39 .+-. 2.2 fmol/108 platelets (mean). This is considerably lower than the binding capacity of [3H]-dihydroergocryptine previously reported. Simultaneous analysis of [3H]-yohimbine and [3H]-dihydroergocryptine binding to 10 subjects showed a significantly lower binding capacity for [3H]-yohimbine (P < 0.001, paired t-test). This difference was not present in every subject, and was consistent within individuals on repeated testing. Possible causes of this difference in radioligand binding are discussed.