Biology of Reproduction, Vol 23, 72-77, Copyright © 1980 by Society for the Study of Reproduction

Cytoplasm ic Estrogen Receptor, Estradiol and Progesterone Concentrations in Endometrium of Nonpregnant and Pregnant Pigs

¹ U. S. Department of Agriculture, Science and Education Administration, Agricultural Research, Animal Science Institute, Reproduction Laboratory, Beltsville, Maryland 20705

Specific cytoplasmic binding of [³H]-estradiol-17 ([³H]-E₂) by cytoplasmic estrogen receptor (CER) in the low-speed supernatant (800 x g) of uterine endometrial homogenates was quantified by saturation analysis and compared with cytoplasmic E₂ 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 bound to free [³H]-E₂ vs [³H]-E₂ bound were linear, and the dissociation constant (7.3 ± 0.6 x 10^-l0 M, mean ± SEM) 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 was as follows: estrone, 16%; estriol, 6%; estradiol-17, 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 pmole/ mg of DNA, P0.05). Pregnancy did not significantly affect the magnitude or the pattern of CER from Day 10 to 20. Cytoplasmic E₂ was greater on Day 20 in both nonpregnant and pregnant pigs (P0.05) 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 (P0.05). 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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ACKNOWLEDGMENTS We thank Ms. Kathy Ogle for technical assistance and Ms. Linda Neuenhahn for typing this manuscript.