α 2 ‐Adrenergic receptor binding in human platelets: Alterations during the menstrual cycle

We studied the effect of several clinically important variables on the characteristics of α 2 ‐adrenergic receptors in human platelet membranes. The number and affinity of the receptor binding sites were determined from radioligand binding experiments, with [ 3 H]yohimbine being the radioligand of choice. Platelets from female subjects had a cyclic variation in the number of α 2 adrenergic receptors that coincided with their menstrual cycles. The number of α 2 receptors was highest at the onset of menses and dropped to 74% to 79% ofthat value during the middle of the cycle. In concurrent experiments we did not observe comparable cyclic changes in receptor binding sites in platelets from male subjects. There was no age‐dependent alteration in receptor number in a sample of 39 subjects ranging in age from 8 to 80 yr, but the number of α 2 ‐receptors in platelets from male and female subjects differed. We also tested the possibility of a circadian rhythm in α 2 ‐receptor number but found no cyclic changes as a function of time of day. There was no alteration in α 2 ‐adrenergic receptor binding in the platelets from five subjects with Parkinson's disease. Finally, 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 α 2 ‐adrenergic receptors. Clinical Pharmacology and Therapeutics (1983) 34, 90–96; doi: 10.1038/clpt.1983.135