Nuclear and Cytoplasmic Estrogen Receptors in Ovine Endometrium During the Estrous Cycle1

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Abstract
Binding of estradiol to a specific endometrial cytoplasmic receptor and translocation of the estrogen-receptor complex to the nucleus was studied in ewes on days 0 (estrus), 3, 6, and 10 of the estrous cycle. Total endometrial cytoplasmic and nuclear receptors were determined by use of the estradiol exchange assay. The equilibrium dissociation constant for the estradiol-receptor complex and the maximum concentration of binding sites were determined to be 8.30 nM and 13,700 molecules per cell, respectively. As determined by exchange assay, total estradiol bound to the cytoplasmic receptor was greater (P < 0.01) on days 0 and 3 (6.84 and 9.56 fmol/.mu.g DNA, respectively) than on days 6 and 10 (1.59 and 2.80 fmol/.mu.g DNA, respectively). Total nuclear-bound estradiol averaged 1.21 .+-. 0.18 (SE) fmol/.mu.g DNA on days 0, 3 and 6; however, it declined to 0.64 fmol/.mu.g DNA on day 10. For each stage of the cycle studied, incubation of endometrium with 2 .times. 10-8 M [3H]estradiol resulted in translocation of 77 .+-. 1% of the available cytoplasmic receptor to the nucleus. The concentration of specific nuclear-bound estradiol in endometrium from both uterine horns following translocation was greater (P < 0.01) on days 0 and 3 (3.03 and 3.75 fmol/.mu.g DNA, respectively) than on days 6 and 10 (1.69 and 1.64 fmol/.mu.g DNA, respectively). Nuclear-bound estradiol following incubation was significantly greater in endometrium from the uterine horn ipsilateral to the ovary bearing the corpus luteum than in endometrium from the contralateral horn on day 10. The observed reduction in total endometrial cytoplasmic and nuclear estrogen receptors during the luteal phase of the estrous cycle of the ewe may be due to exposure of the uterus to increasing concentrations of progesterone which interfere with the replenishment of the cytoplasmic receptor.