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Regulation of Luteinizing Hormone Secretion in Prepubertal Heifers: Increased Threshold to Negative Feedback Action of Estradiol^1,2,

University of Wisconsin⁴, Madison 53706

Abstract

Three experiments were conducted to evaluate negative feedback action of estradiol-17β (E2) on luteinizing hormone (LH) release in ovariectomized heifers and to determine the response at different ages. In the first experiment, iv injections of .2 or 2.0 µg E2/kg body weight (BW) reduced serum concentrations of LH by arresting episodic mode of secretion. Results of a second experiment indicated a positive relationship (P<.05) between dose of E2 and duration of LH suppression. Mean duration of suppression with the high (2.0 µg E2/kg BW), medium (2 µg E2/kg BW) and low (.1 µg E2/kg BW) doses was 6.5, 4.5 and 3.5 h, respectively. Neither magnitude nor rate of LH suppression was related (P>.25) to dose in the first two experiments. Results of third experiment showed that both magnitude and duration of suppression were directly proportional to dose of E2 (0, .1 and 1.0 µg E2/kg BW). The response lasted longer in heifers 4 mo of age than in heifers 8 and 12 mo of age. E2 was also more effective in reducing LH pulses in heifers 4 mo of age than in heifers 8 and 12 mo of age. These results show that E2 inhibits LH release by influencing pulsatile mode of secretion, and that the threshold to negative feedback increases as heifers approach puberty.

¹ Supported by the College of Agricultural and Life Sciences, Meat and Anim. Sci. Paper No. 749, Grant RR-00167 from the National Institutes of Health, and Grant No. 5T32 HD 07007-03 awarded by the National Institutes of Child Health and Human Development, DHEW. Publication No. 21-011 from the Wisconsin Regional Primate Research Center.

² The authors thank Dr. L. Reichert, Dr. G. Niswender and the Endocrine Study Section, National Institutes of Health, for donating luteinizing hormone used for iodination, luteinizing hormone antisera and luteinizing hormone used for standards, and the Upjohn Company for donating estradiol-tyrosine methylester. We also thank Dr. R. H. Stauffacher for assistance with surgery; Mr. E. Hoch, Mr. P. Bringle, Mr. J. Kane and Ms. J. Hillstrom for their excellent technical assistance, and Ms. S. Kading for typing this manuscript.

³ Trainee of the Endocrinology-Reproductive Physiology Program.

⁴ Dept. of Meat and Anim. Sci. and Wisconsin Regional Primate Research Center.