Differences in Insulin Receptors between Men and Menstruating Women and Influence of Sex Hormones on Insulin Binding during the Menstrual Cycle*

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Abstract
Specific binding of [125I]insulin to circulating monocytes and erythrocytes from 9 normal menstruating women and 9 normal men was determined during a 28 day period (1 sample every 7 days). In women, insulin binding was higher to both monocytes (P < 0.001) and erythrocytes (P < 0.02) in the follicular phase than in the luteal phase. In men, insulin binding to monocytes was similar to the follicular phase values for women; however, insulin binding to erythrocytes from men showed higher values than insulin binding to erythrocytes from women in both the follicular (P < 0.001) and luteal (P < 0.001) phases. These differences were due primarily to changes in receptor concentration rather than receptor affinity. An inverse relationship was found between insulin binding to monocytes and levels of 17.beta.-estradiol, progesterone and 17.alpha.-hydroxyprogesterone; this relationship was not observed in insulin binding to erythrocytes. Sex hormones may play a role in the control of insulin receptors. Other factors may exist during the follicular phase that lower insulin binding to erythrocyte insulin receptors. If insulin receptors on circulating cells reflect the behavior of the main insulin target tissues, the present data might, in part, explain the reduction in glucose tolerance reported by various authors in the 2nd half of the menstrual cycle.