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Plasma Estrogens in Pregnant and Postpartum Mares^{1, 2,}

Washington State University, Pullman 99163

Abstract

Reliable and sensitive radioimmunoassays for estrone and estradiol-17 ß3 were developed. The minimum amount of estrogen detectable in an assay tube was 4 pg in both assay systems. The cross-reactions of other estrogens with respect to estradiol-17ß (100%) were: estrone, 62%; estradiol-17 33%; estriol, 27%; equilin, 17%; and equilenin, 19%. Non-estrogenic steroids did not cross-react with the antiserum used. When 25 to 200 pg of each estrogen was added to plasma from an ovariectomized animal and subsequently processed as an unknown, recovery was quantitative. Sephadex LH-20 column chromatography was used to separate fraction E₁ (estrone, equilin and equilenin) and fraction E₂ (estradiol) after ether extraction. Residues of the chromatography fractions produced inhibition curves parallel to those obtained with standard estrone and estradiol-17ß. Inter and intra assay precision were 7.8% and 4.9%, respectively. Blood samples were collected via jugular puncture from five Thoroughbred mares and one Arabian mare at 4-day intervals from breeding to day 72 of gestation and from day 300 of gestation until parturition. In addition, samples were collected at day 80 of gestation and at monthly intervals from day 90 to day 300 of gestation. During the postpartum period blood samples were collected every other day during estrus and every fourth day when the mare was not in heat. Blood samples were collected from two additional mares during the postpartum period. The concentration of E₁ remained below 20 pg/ml from breeding to day 80 of gestation, increased to 828 ±151 pg/ml (mean ± SE) at 210 days of gestation, then declined until parturition; basal levels were found by 1 day postpartum. The concentration of E₂ remained below 15 pg/ml until day 90 of gestation, increased to 71 ±18 pg/ml at 240 days of gestation, then declined until 300 days of gestation and remained relatively unchanged until parturition; basal levels were found by 1 day postpartum. One mare aborted 203 days post insemination; the concentrations of E₁ and E₂ were greatest at the 180 day sampling and had returned to basal levels by the 210 day sampling. Another mare did not foal nor was observed to abort; however, a single estrogen peak was noted 270 days postinsemination in this mare.

The concentration of total estrogens, as estrone equivalent, in the plasma of mares during the postpartum period was determined. The concentration of estrogens increased during estrus in those animals which demonstrated foal heat. In some cases an estrogen peak was found at the time foal heat was expected when behavioral estrus was not exhibited by the mares. Another mare did not demonstrate foal heat nor have a detectable estrus during the early postpartum period.

¹ Scientific Paper 3916 College of Agriculture Research Center, Washington State University. Project 0116.

² The authors wish to thank the National Cancer Institute, NIH, for the generous gift of estradiol-17ß 17-hydrogen succinate-BSA preparation NSC-88522E.

³ NDEA Fellow.

⁴ Present address: Department of Physiology, Colorado State University, Fort Collins 80521.

⁵ Present address: Department of Veterinary Science, University of Wisconsin, Madison 53706.