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Cytosolic and nuclear estrogen receptors in the ovary and uterus of immature rats and hamsters were determined to evaluate why exogenous estrogens were ineffective in stimulating follicular maturation in the hamster compared to the rat. Animals were injected sc with oil or single injection of 1 mg estradiol cyclopentylpropionate (ECP) on Day 23 or a daily injection of 2 mg diethylstilbestrol (DES) on Days 23-25 and killed on Day 26. Total binding sites for estrogen in ovarian cytosol of control hamsters were half the number in the rat ovary (28 fmole/mg protein) and about 50% of the receptors were occupied in the hamster. The apparent affinity of the estrogen-cytosol receptor complex was also lower in the hamster (Kd; 1.41 nM) than in the rat (Kd; 0.52 nM). After ECP treatment, there was a tendency for translocation in all 4 tissues examined even though some differences were not statistically significant. However, after DES treatment both cytosol and nuclear estrogen receptors decreased in both species. This discrepancy may be due to the difference in the time course of the nuclear translocation, the difference in metabolism and difference in the binding potencies of ECP and DES. The lack of ovarian responsiveness to estrogen in the hamster thus appears to be due to the reduced number of cytosol receptor sites which have a low affinity for estrogen and are already partially occupied.

Estrogen Receptors in Ovary and Uterus of Immature Hamster and Rat: Effects of Estrogens