Replenishment of pituitary estrogen receptor in chronically estrogenized rats

Replenishment of pituitary estrogen receptor (ER) following a single injection of estradiol-17 beta (E2) was examined in chronically estrogenized ovariectomized rats. Subcutaneous implantation of E2-pellet in castrated rats for 7 days caused a shift of ER distribution in the pituitary (39.9 +/- 5.7 fmol/10 mg tissue in the cytosol vs 24.8 +/- 1.9 fmol/10 mg tissue in the nucleus). An intraperitoneal injection of 5 micrograms E2 in the E2-implanted rats resulted in a significant decrease in cytosol ER with a concomitant significant increase in nuclear ER (16.6 +/- 1.2 fmol in the cytosol vs. 36.7 +/- 4.1 fmol in the nucleus at 1 h). However, cytosol ER was rapidly replenished, which was accompanied by nuclear ER reduction, and both values returned to the pre-injection levels at 4 h. An administration of 150 micrograms cycloheximide, that effectively blocked protein synthesis in the E2-implanted and 5 micrograms E2 injected rats, completely inhibited the replenishment of ER. These findings suggest that replenishment of ER following E2 administration in the pituitary of chronically estrogenized rats is entirely dependent on protein synthesis.