Variation of cytoplasmic rat uterine androgen receptors.

The number of testosterone binding sites present in rat uterine cytosol varied regularly during the estrous cycle, reaching a trough at metestrus and a peak at proestrus. Treating ovariectomized and adrenalectomized rats for 2 days with estradiol resulted in a 3-4-fold increase in the number of binding sites per uterus. Estradiol withdrawal induced a decrease in uterine androgen receptors. Testosterone or progesterone treatment also increased the number of these sites, but to a lesser degree. When administered together with estradiol, testosterone did not enhance the stimulatory effect of the latter, whereas progesterone even reduced it. Testosterone or progesterone did not prevent the number of receptors from declining after estradiol withdrawal. Thus the changes in the number of cytoplasmic androgen receptors in the uterus during the rat estrous cycle is mainly controlled by the rise and fall of the serum levels of estradiol.