Replenishment of uterine estrogen receptor in chronically estrogenized rats

Abstract Replenishment of uterine estrogen receptor (ER) following a single injection of estradiol-17β (E 2 ) was examined in chronically estrogenized rats. Subcutaneous implantation of E 2 -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 5μg E 2 in the E 2 -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 μg 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 μg cycloheximide, that effectively blocked protein synthesis in the uterus of the E 2 -implanted rats, completely inhibited the replenishment of soluble ER induced by 5 μg E 2 . These findings, combined with our previous findings that replenishment of ER following a single E 2 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.