α2-Adrenergic receptor binding in human platelets: Alterations during the menstrual cycle

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Abstract
The effect of several clinically important variables on the characteristics of .alpha.2-adrenergic receptors in human platelet membranes was studied. The number and affinity of the receptor binding sites were determined from radioligand binding experiments, with [3H]yohimbine being the radioligand of choice. Platelets from female subjects had a cyclic variation in the number of .alpha.2-adrenergic receptors that coincided with their menstrual cycles. The number of .alpha.2-receptors was highest at the onset of menses and dropped to 74-79% of that value during the middle of the cycle. In concurrent experiments, comparable cyclic changes in receptor binding sites in platelets from male subjects were not observed. There was no age-dependent alteration in receptor number in a sample of 39 subjects ranging in age from 8-80 yr, but the number of .alpha.2-receptors in platelets from male and female subjects differed. The possibility of a circadian rhythm in .alpha.2-receptor number was tested but no cyclic changes as a function of time of day were found. There was no alteration in .alpha.2-adrenergic receptor binding in the platelets from 5 subjects with Parkinson''s disease. There was no change in receptor affinity as a function of any of the variables tested. These data should apply to the design of further studies on the clinical importance of platelet .alpha.2-adrenergic receptors.

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