Modulation of tamoxifen-specific binding sites and estrogen receptors by estradiol and progesterone in the neonatal uterus of guinea pig.

Estradiol administration to newborn guinea pigs (30 ng/g body wt) provokes an 84 +/- 10% and 145 +/- 10% (means +/- SE) increase of tamoxifen-specific binding site (site B) in the uterine cytosol and in the 0.6 M KCl nuclear extract, respectively, 48 h after hormone administration. The simultaneous injection of progesterone (15 micrograms/g body wt) with estradiol antagonizes this estrogen-induced increase of site B. Parallel modifications of estrogen receptor are observed following the same estradiol treatment: after translocation to the nuclear fraction, cytoplasmic estrogen receptor is replenished 24 h after hormone administration with a net increase of 60 +/- 10% (mean +/- SE) after 48 h. The increase of estrogen receptor is also antagonized by the simultaneous administration of progesterone. It is concluded that the levels of both tamoxifen-specific binding site and estrogen receptor are modulated by estradiol and progesterone in the neonatal uterus of guinea pig.