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Negative Effect of Estradiol on Luteinizing Hormone Throughout the Ovulatory Luteinizing Hormone Surge in Mares¹

Eutheria Foundation,³ Cross Plains, Wisconsin 53528 Department of Pathobiological Sciences,⁴ University of Wisconsin, Madison, Wisconsin 53706

ABSTRACT

The negative effect of estradiol-17ß (E2) on LH, based on exogenous E2 treatments, and the reciprocal effect of LH on endogenous E2, based on hCG treatments, were studied throughout the ovulatory follicular wave during a total of 103 equine estrous cycles in seven experiments. An initial study developed E2 treatment protocols that approximated physiologic E2 concentrations during the estrous cycle. On Day 13 (ovulation = Day 0), when basal concentrations of E2 and LH precede the ovulatory surges, exogenous E2 significantly depressed LH concentrations to below basal levels. Ablation of all follicles 10 mm when the largest was 20 mm resulted in an increase in percentage change in LH concentration within 8 h that was greater (P < 0.03) than for controls or E2-treated/follicle-ablated mares. Significant decreases in LH occurred when E2 was given when the largest follicle was either 25 mm, 28 mm, 35 mm, or near ovulation. Treatment with 200 or 2000 IU of hCG did not affect E2 concentrations during the initial portion of the LH surge (largest follicle, 25 mm), but 2000 IU significantly depressed E2 concentrations before ovulation (largest follicle, 35 mm). Results indicated a continuous negative effect of E2 on LH throughout the ovulatory follicular wave and may be related to the long LH surge and the long follicular phase in mares. Results also indicated that a reciprocal negative effect of LH on E2 does not develop until the E2 surge reaches a peak.

estradiol, follicle, human chorionic gonadotropin, luteinizing hormone, mares, ovulation

¹Supported by the Eutheria Foundation (Cross Plains, WI); Project P1-P7-OG-05.

Correspondence: ²O.J. Ginther, Department of Pathobiological Sciences, School of Veterinary Medicine, 1656 Linden Dr., University of Wisconsin, Madison, WI 53706. FAX: 608 262 7420; e-mail: ginther{at}svm.vetmed.wisc.edu