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Elevation of Serum Testosterone during Chronic LHRH Agonist Treatment in the Bull^1,2,

Washington State University⁵, Pullman 99164-6332

Abstract

The objective of this study was to try to depress serum testosterone (T) in bulls by prolonged treatment with a potent luteinizing hormone-releasing hormone (LHRH) agonist. Eight sexually mature bulls (325 to 475 kg) were assigned to treatment or control groups. Treatment consisted of 150 ug nafarelin acetate 6-D-2-napthyl-alanine-LHRH (LHRH-A) injected im every 6 h for 15 d. Bovine serum albumin (BSA, .01%) in a carrier solution was injected at the same times in control bulls. Serial 15-min blood samples were collected via jugular cannula during the initial 36 h of treatment and during 6-h windows on d 4, 8 and 14. Bulls were slaughtered and pituitaries and testes collected on d 15. Serum luteinizing hormone (LH), follicle stimulating hormone (FSH) and T were elevated after initial injection of LHRH-A, but returned to basal concentrations by 12, 5 and 17 h, respectively. Prolonged LHRH-A treatment prevented pulsatile LH and T secretion compared with control bulls. Mean serum LH did not differ from that of controls on d 4, 8 and 14 of LHRH-A treatment, while serum T was elevated (P<.01) during the same time periods. Oscillating patterns and mean concentrations of serum FSH were not different between control and LHRH-A-treated bulls. Fifteen days of LHRH-A treatment depressed pituitary LHRH receptor numbers (P<.05) and pituitary LH (P<.01) and FSH (P<.05) concentrations. Testicular LH receptor numbers were elevated (P<.01), but testicular FSH receptor numbers were not altered. In conclusion, treatment of sexually mature bulls with LHRH-A for 15 d depressed pituitary function. Although mean serum LH and FSH were not altered, mean serum T was elevated, probably as a result of elevated testicular LH receptor numbers. Thus, the results are not consistent with the hypothesis that chronic LHRH agonist treatment depresses serum T in the bull.

¹ Sci. Paper No. 6655, College of Agr. Res. Center, Washington State Univ., Pullman. Project 0137.

² This project was supported by the Natl. Inst. of Child Health and Human Devel. Grant #HD 14740.

³ Reprint requests: Dr. J. J. Reeves, Dept. Anim. Sci., Washington State Univ., Pullman, WA 99164-6332.

⁴ The authors thank Dr. Alain Belanger, Laval Univ., City of Quebec for testosterone antiserum and testosterone conjugate used for iodination; Dr. Brian Vickery, Syntex Research, Palo Alto, CA for the nafarelin acetate used to treat bulls; Dr. Jurgen Sandow, Hoeschst Pharmaceutical, West Germany for busserelin used for LHRH receptor assays; NIADD for the LH, hCG and FSH used for hormone assays; and Michael T. O'Connor for assistance in handling the bulls.

⁵ Dept. of Anim. Sci.