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Pulsatile Luteinizing Hormone Secretion in the Castrate Male Bovine: Effects of Testosterone or Estradiol Replacement Therapy¹

US Department of Agriculture^,4,5,, Clay Center, NE 68933

Abstract

Luteinizing hormone (LH) secretory profiles have been determined for the male bovine following castration and steroid replacement therapy. Serum LH concentrations increased approximately threefold during the first week following castration and thereafter remained elevated (6.6 ± .7 ng/ml). Castrates not receiving steroid replacement showed a rhythmic pattern of LH release that was of high frequency (mean pulse interval; 85 ± 5 min) and high amplitude (mean peak concentration, 11.2 ± 1.4 ng/ml). Chronic administration of estradiol-17β via subdermal Silastic implants reduced mean serum LH concentrations (2.1 ± .3 ng/ml) and blocked the pulsatile pattern of LH release in all steers. Similar administration of testosterone suppressed mean serum LH and blocked pulsatile LH release in two of four animals. The number of implants used in this study provided physiological concentrations of estradiol (9.8 ± 1.5 pg/ml) and testosterone (4.1 ± .2 ng/ml) in systemic blood for the two respective treatment groups. Differences in the LH secretory profiles among testosterone-implanted steers may have been related, in part, to differences in the amounts of steroid not bound to serum proteins. These findings demonstrate that estradiol is a particularly potent inhibitor of pulsatile LH secretion in the male bovine and suggest that gonadal steroid feedback on LH secretion may, in part, be imposed at the level of the hypothalamus. The mechanism for pulsatile LH release is discussed relative to a centrally-located luteinizing hormone releasing hormone pulse generator.

¹ Published as Paper No. 6770, Journal Series, Nebraska Agr. Exp. Sta. The authors acknowledge the technical assistance of Jim Green, Cindy Rainbolt, Kathy Sorenson and Kathy Leising and the cooperation of the Univ. of Nebraska and the Nebraska Agr. Exp. Sta. In addition, we gratefully acknowledge Dr. D. J. Bolt, USDA, ARS, Beltsville, MD, for LH antiserum (DJB 3-12/11), Dr. L. E. Reichert, Emory Univ., Atlanta, GA, for purified ovine LH (LER 1065-C2) and the Endocrine Study Section, National Institutes of Health, Bethesda, MD, for the bovine reference standard (NIH-LH-B10).

² Michael J. D'Occhio is a Univ. of Nebraska Research Fellow located at the Roman L. Hruska U.S. Meat Animal Research Center, Clay Center, NE.

³ Tom W. Gettys is a Clemson Univ. Graduate Fellow located at the Roman L. Hruska U.S. Meat Animal Research Center, Clay Center, NE.

⁴ Roman L. Hruska U.S. Meat Animal Research Center, ARS.

⁵ Mention of trade names or companies does not constitute an implied warranty or endorsement by the authors or USDA.