Changes in estrogen receptor levels during deciduomata development in the pseudopregnant rat

The presence of a cytoplasmic estradiol receptor with an affinity constant of 10(10)M-1 has been demonstrated in the differentiated fibroblasts comprising the deciduomata in pseudopregnant rats. The receptor had a sedimentation coefficient of 6S in sucrose density gradients containing a physiological concentration of salt, and estradiol binding was completely abolished by a hundred-fold excess of unlabeled estradiol. During the early stages of decidualization (days 2 and 3), the decidualized uterine horn or isolated deciduomal tissue contained a concentration of receptor comparable to that in non-gravid uteri from rats ovariectomized at estrus. By day 5 of decidualization, the concentration of estradiol binding in deciduomal tissue decreased to about one-half the concentration measurable at days 2 and 3 of decidualization despite continued tissue growth until day 7. By day 7 of decidualization, estradiol binding had decreased to about 20% of the concentration on days 2 and 3 of decidualization. Cytosol from untreated uterine horns of rats bearing deciduomata bound the same amount of estradiol through day 7 as that in uteri of rats ovariectomized at estrus. These observations are discussed in terms of steroid hormone involved in deciduomal growth and regression.