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Serum Luteinizing Hormone, Testosterone and Androstenedione in Pubertal and Prepubertal Bulls after Gonadotropin Releasing Hormone^{1, 2,}

Michigan State University⁴, East Lansing 48824

Abstract

Nine bulls, three each at 2, 4 and 6 months of age, were each given three doses (200, 400 or 800 µg; im) of GnRH, one on each of 3 consecutive days. Jugular blood was sampled at frequent intervals through indwelling cannulae. Serum LH increased about 24-fold within 45 min after GnRH injection, and declined to pretreatment concentrations within 6 hr after GnRH. Duration and magnitude of the increase in serum LH after GnRH was similar at all ages of bulls and at all doses of GnRH, except that the duration of the LH response was prolonged with the highest dose (800 µg) of GnRH. In addition, the magnitude of LH release decreased with repeated administrations of GnRH. Serum testosterone increased about threefold in response to the increased LH release after GnRH in 6-month-old bulls but not in 2- or 4-month-old bulls. Unlike testosterone, androstenedione increased significantly at all ages and the pattern of the androstenedione response to GnRH generally was similar to that for LH.

¹ Published with the approval of the Director of Michigan Agriculture Experiment Station as Paper No. 6843.

² Gonadotropin releasing hormone (GnRH) was generously supplied by Dr. R. Rippel, Abbott Laboratories, North Chicago, Illinois.

³ Present address: Kasetsart University, Bangkok 9, Thailand.

⁴ Animal Reproduction Laboratory, Department of Dairy Science.