Effect of Estrogen on Androgen Receptor Dynamics in Female Rat Pituitary*

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Abstract
We examined the role of estrogen in regulating the number of androgen receptors (AR) in the pituitary gland of the female rat. Female Sprague-Dawley rats (240-280 g) that were ovariectomized for 3 days were used in this study. AR numbers were determined in pituitary cytosol and nuclear fractions by binding and exchange assays using the synthetic ligand 17.alpha.-methyl-3H-trienolone. The administration of a single dose of estradiol benzoate [(EB) 10 .mu.g/100 g BW] to ovariectomized female rats resulted in a 60% increase in cytosolic AR which was significantly (P < 0.01) elevated above that of oil-treated controls by 12 h post EB. Cytosolic AR levels remained elevated for as long as 48 h post EB (213% of controls). Saturation analysis of pituitary cytosolic AR revealed a single, high affinity binding site for 17.alpha.-methyl-3H-trienolone exhibiting an apparent dissociation constant (Kd) of 0.5 .times. 10-10 M in both EB- and oil-treated animals the adminstration of cycloheximide (1 mg/kg BW) before EB administration prevented the EB-induced rise in AR when measured 8 h post EB. When dihydrotestosterone (1.5 mg) was injected 24 h after EB or oil, there was a rapid increase in nuclear AR accompanied by a rapid decrease in cytosolic AR. The increase in nuclear AR was significantly greater (P < 0.05) in EB-pretreated animals vs. oil-treated controls. These observations show that a potential synergism exists between androgen and estrogen in the female rat pituitary and suggest that androgens may play an important role in regulating cyclic pituitary hormone release.