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Regulation of Circulating Gonadotropins by the Negative Effects of Ovarian Hormones in Mares¹

Eutheria Foundation, Cross Plains, Wisconsin 53528

The functional and temporal relationships between circulating gonadotropins and ovarian hormones in mares during Days 7–27 (ovulation = Day 0) was studied using control, follicle ablation, and ovariectomy groups (n = 6 mares/group). In the follicle-ablation group, all follicles 6 mm were ablated on Day 7, and every 2 days thereafter, newly emerging follicles were also ablated. Estradiol concentrations decreased (P < 0.01) similarly in the controls and the follicle-ablation group between Days 7 and 11 and by Day 15 began to increase in the controls and continued to decrease in the follicle-ablation group. Concentrations of progesterone were not affected by follicle ablation, but diameter of the corpus luteum was greater (P < 0.05) by Day 21 in the follicle-ablation group; these results indicated that the follicles were involved in morphologic luteolysis, but not in functional luteolysis. Concentrations of LH were higher (P < 0.05) on Days 15 and 16 in the follicle-ablation group than in the controls, indicating an initial negative effect of follicles on LH. Immunoreactive inhibin and estradiol decreased (P < 0.0001) and FSH and LH increased (P < 0.05) within 1 or 2 days after ovariectomy; these changes occurred more slowly in the follicle-ablation group. The maximum value for an FSH surge in each control mare was below the lower 95% confidence limit in the ovariectomy group. Maximum concentration for the periovulatory LH surge in the controls was not different from the mean maximum LH concentrations in the ovariectomy group. Our interpretation is that the gonadotropin surges resulted from changes in the magnitude of the negative effects of ovarian hormones on the positive effects of extraovarian control. There was no indication of a positive ovarian effect on either FSH or LH.

estradiol, follicle, follicle-stimulating hormone, gonadotropins, inhibin, progesterone

² Correspondence: O.J. Ginther, Animal Health and Biomedical Sciences, 1656 Linden Drive, University of Wisconsin, Madison WI 53706. FAX: 608 262 7420; ginther{at}svm.vetmed.wisc.edu

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				O.J Ginther, M.D Utt, M.A Beg, E.L Gastal, and M.O Gastal Negative Effect of Estradiol on Luteinizing Hormone Throughout the Ovulatory Luteinizing Hormone Surge in Mares Biol Reprod, September 1, 2007; 77(3): 543 - 550. [Abstract] [Full Text] [PDF]

				O.J. Ginther, E.L. Gastal, M.O. Gastal, M.A.R. Siddiqui, and M.A. Beg Relationships of Follicle Versus Oocyte Maturity to Ultrasound Morphology, Blood Flow, and Hormone Concentrations of the Preovulatory Follicle in Mares Biol Reprod, August 1, 2007; 77(2): 202 - 208. [Abstract] [Full Text] [PDF]

				E L Gastal, M O Gastal, and O J Ginther Relationships of changes in B-mode echotexture and colour-Doppler signals in the wall of the preovulatory follicle to changes in systemic oestradiol concentrations and the effects of human chorionic gonadotrophin in mares. Reproduction, April 1, 2006; 131(4): 699 - 709. [Abstract] [Full Text] [PDF]