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Blood Serum Testosterone after Sexual Preparation or Ejaculation, or after Injections of LH or Prolactin in Bulls¹

Michigan State University², East Lansing 48823

Abstract

Two experiments were conducted to determine testosterone responses to ejaculation and to injections of LH or prolactin. In the first experiment, 12 bulls were ejaculated weekly for 4 weeks; blood was sampled 30 min. before and 5 and 30 min. after ejaculation. Six mature bulls had higher (P < .01) blood serum testosterone (9.3 ng/ml) than six young bulls (4.3 ng/ml). Relative to 30 min. before ejaculation, testosterone increased at 5 and 30 min. after ejaculation in all of the mature bulls but in only two of the young bulls (80% vs. 25%, respectively; P < .01). Since we reported earlier that prolactin increased six-fold but LH did not increase significantly after ejaculation in the same bulls, a second experiment was designed to determine testosterone response to injected LH or prolactin. Blood was sampled (jugular cannulae) hourly during two 24-hr. periods from five bulls to determine diurnal changes in hormones. An average of six elevations (12 ng/ml) in serum prolactin occured at irregular intervals during each 24-hr. period; about 30% coincided with feeding and bedding (0800 to 1000 hr.), but the others were not related obviously to environmental stimuli. These increases in prolactin had no apparent relationship with serum testosterone. An average of four spikes (1 ng/ml) in serum LH occurred at irregular intervals during each 24-hr. period in each bull. Of 39 LH spikes, 25 (64%) were accompanied or followed within 1 hr. by elevations (>2 ng/ml) in serum testosterone. After infusion of LH (NIH-LH-B5), blood serum LH disappeared with T = 26 min. for the first hour and T = 65 min. during the next 3 hours. The comparable half-times after infusion of proclactin (NIH-P-B2 or 3) were 21 and 190 min., respectively. After injections of 2 or 4 mg LH, blood testosterone doubled within 1 hr., peaked at 1.5 to 2 hr. and remained high until at least 4 hr. after LH. In contrast, prolactin did not increase serum testosterone, and injection of prolactin with LH resulted in no higher testosterone than LH alone.

¹ Published with the approval of the Director of the Agricultural Experiment Station as Paper No. 6215. Supported by the National Association of Animal Breeders. Select Sires, Inc. provided the bulls and part of the facilities for this research. Dr. L. E. Reichert provided highly purified LH (LER-1072-2) for iodination in the radioimmunoassay, and Dr. G. D. Niswender furnished rabbit antiserum to testosterone-3-oxime bovine serum albumen.

² Department of Dairy Science.

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