Replenishment of uterine estrogen receptor in chronically estrogenized rats

Replenishment of uterine estrogen receptor (ER) following a single injection of estradiol-17 beta (E2) was examined in chronically estrogenized rats. Subcutaneous implantation of E2-pellet for 7 days in ovariectomized rats resulted in a significant stimulation of uteri with regard to wet tissue weight, DNA content and progesterone receptor content, with a shift of ER distribution. An intraperitoneal injection of 5 micrograms E2 in the E2-implanted rats induced a significant decrease in soluble ER (from 141.1 +/- 12.6 to 69.2 +/- 8.8 fmol/mg protein) with a concomitant increase in nuclear ER (from 58.2 +/- 8.6 to 129.2 +/- 11.6 fmol/100 micrograms DNA) 1 h after the injection. However, soluble ER was rapidly replenished, which was accompanied by nuclear ER reduction, and both values returned to the pre-injection levels at 4 h after the injection. An administration of 150 micrograms cycloheximide, that effectively blocked protein synthesis in the uterus of the E2-implanted rats, completely inhibited the replenishment of soluble ER induced by 5 micrograms E2. These findings, combined with our previous findings that replenishment of ER following a single E2 administration in the pituitary of chronically estrogenized rats was inhibited by cycloheximide, suggest that replenishment of ER is entirely dependent on protein synthesis in chronically estrogenized rats.