Effect of Testosterone and Progesterone on the Estradiol Receptor in the Immature Rat Ovary*

The effect of testosterone and progesterone on the estrogen-stimulated ovarian weight increase and es tradiol receptor (ER) levels was examined in immature, hypophysectomized rats. Nuclear translocation and cytosol replenishment of the ER in the ovary exhibited similarities to those events in the uterus. The ovarian weight increase observed on day 4 in response to three daily injections of 2.5 microgram 17beta-estradiol administered sc in sesame oil was effectively suppressed by one injection of 1 mg testosterone on day 3. In contrast, 1 mg progesterone given on day 3 had no effect on the ovarian weight of the estrogen-primed rat. Nor did progesterone alter the ovarian ER levels in the cytosol and nuclei of similarly treated rats. Further, progesterone receptors were undetectable in the ovarian tissue of immature, hypophysectomized rats given two daily injections of 2.5 microgram estradiol-17beta. In contrast, testosterone significantly reduced the ovarian ER levels both in the cytosol as well as in the nuclei. These results suggest that one of the mechanisms by which testosterone acts in the ovarian tissue is by reducing the available ER for estrogen action.