Cytoplasm ic Estrogen Receptor, Estrad iol and Progesterone Concentrations in Endometrium of Nonpregnant and Pregnant Pig

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Abstract
Specific cytoplasmic binding of [3H]-estradiol-17.beta. ([3H]-E2.beta.) by cytoplasmic estrogen receptor (CER) in the low-speed supernatant (800 .times. g) of uterine endometrial homogenates was quantified by saturation analysis and compared with cytoplasmic E2.beta. and progesterone in 23 nonpregnant and 19 pregnant pigs. Endometrial samples were recovered from nonpregnant pigs on days 4, 8, 10, 12, 14, 16 and 20 after the onset of estrus and from pregnant pigs on days 10, 12, 14, 16 and 20. Scatchard plots of the ratio of [3H]-E2.beta. bound to free [3H]-E2.beta. vs. [3H]-E2.beta. bound were linear, and Kd (7.3 .+-. 0.6 .times. 10-10 M, mean .+-. standard error of the mean) was not significantly affected by day or status (nonpregnant vs. pregnant). The relative affinity of CER for various estrogens when compared with that for E2.beta. was estrone, 16%; estriol, 6%; estradiol-17.alpha., 4%; and estrone-3-sulfate, 0.3%. Endometrial CER in nonpregnant pigs increased after the onset of estrus to a maximum on day 10 and then declined to day 16 (decline from 1.4 to 0.22 pmol/mg DNA, P .ltoreq. 0.05). Pregnancy did not significantly affect the magnitude or the pattern of CER from day 10 to 20. Cytoplasmic E2.beta. was greater on day 20 in both nonpregnant and pregnant pigs (P .ltoreq. 0.05) than on the earlier days sampled. Cytoplasmic progesterone declined significantly from day 14 to 16 in nonpregnant pigs, at the end of the luteal phase of the estrous cycle, and was significantly greater on days 16 and 20 in pregnant than in nonpregnant pigs (P .ltoreq. 0.05). The high concentration of CER during the midluteal phase of the cycle relative to the follicular phase in the pig, which is the opposite of that in other species, may be of particular physiological significance. Estrogen secretion by the blastocysts in the pig may block the luteolytic effect of the uterus and allow maintenance of pregnancy.

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