Effect of Estrogens on Uterine Tone and Life Span of the Corpus Luteum in Mares2

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Abstract
The role of estrogens in development of tense uterine tone and persistence of the corpus luteum which occur in early pregnant and pseudopregnant mares was studied. The compressed thickness of the cranial end of a uterine horn increased (P<.05) between days 10 and 50 post-ovulation in pregnant pony mares. In nonpregnant, ovarian intact mares, daily (days 10 to 60) administration of progesterone (P4) (100 mg/day) did not increase (P>.05) thickness over the day 10 (diestrus) value. Similar P4 treatment in ovariectomized mares increased (P<.05) thickness between days 10 and 40 and at day 50 mean thickness was not different (P>.05) from that of P4 treated ovarian intact mares, although the mean for each group at day 50 was less (P<.05) than the mean for pregnant mares. In another experiment, P4 (100 mg/day), . 25 mg estradiol-17β (E2) + P4, or. 5 mg E2 + P4 was given daily for 50 days to seasonally anovulatory mares. Uterine thickness in P4 and .25 mg E2 + P4 treated mares increased gradually until the end of treatment and then decreased, but values for .25 mg E2+P4 mares were not greater (P>.05) than those for P4 treated mares. Thickness in mares treated with .5 mg E2 + P4 was greater (P<.05) by day 30 than for all other groups. Treatment (days 7 to 18) with E2 (5mg/day) or estrone (100 mg/day) appeared to lengthen and diethylstilbesterol (DES, 5 mg/day) lengthened (P<.05) the interovulatory interval over that of E2 (.5 mg/day) or vehicle. The DES treatment caused significant luteal maintenance on day 18 as indicated by color and diameter and tended (P<.10) to cause increased luteal weight. Results support the hypothesis that estrogens play a role in the tense uterine tone and luteal maintenance associated with pregnancy and pseudopregnancy in mares. Copyright © 1978. American Society of Animal Science . Copyright 1978 by American Society of Animal Science.

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