Cytoplasmic Estrogen Receptors and Estrogen Concentrations in Bovine Uterine Endometrium*

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Abstract
Specific cytoplasmic binding of 17.beta.-[3H]estradiol ([3H]E2.beta.) by unoccupied receptors in bovine uterine endometrium was determined by saturation analysis and correlated with endometrium levels of E2.beta. in 21 cows. The concentrations of these estrogen receptors (ER) during the estrous cycle were significantly greater during proestrus, estrus and postestrus (days 18-20 and 0-4 of the cycle) than during the midluteal period (days 10-12; P < 0.05). These increases in ER concentration paralleled increases in endometrial and plasma E2.beta. concentrations. In a preliminary experiment involving 6 heifers killed during the estrous cycle, a comparison of the ER concentrations of the horns ipsilateral and contralateral to the corpus luteum showed no significant differences, as did a study of the Kd of the steroid-receptor interaction during the estrous cycle and early pregnancy. The order of inhibition of cytoplasmic binding of [3H]E2.beta. by estrogens was as follows: E2.beta. > E1 > E2.alpha. > E3 at 4.degree. C. The concentration of ER in 6 pregnant animals was higher on days 2-3 than on days 13-14 after insemination, which was similar to that found in cycling animals; however, the difference between days was not significant. Ovariectomy of 2 heifers resulted in slightly higher ER concentrations than in intact heifers; whereas immunization of 2 heifers with an E2.beta. conjugate resulted in levels 2-fold higher. In these altered animals, plasma progesterone (P4) was nearly nondetectable. ER concentration was inversely related to the level of plasma P4 in cycling heifers (r = -0.58, P < 0.01). Endometrial estrogen and plasma estrogen levels were also inversely correlated with plasma P4 levels (r = -0.65, P < 0.01 and r = -0.48, P < 0.05, respectively). Thus, if P4 level influences ER levels, the hormone may make the tissue less responsive to ovarian estrogens at such times as the luteal phase of the cycle and during pregnancy.