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Serum LH Concentration following Castration, Steroid Hormone and Gonadotropin Releasing Hormone Treatment in the Bovine^{1 ,2,}

Oregon State University^,4, Corvallis, 97331

³ Address reprint requests to Dr. Swanson.

Abstract

Serum LH concentrations were less than 1 ng/ml immediately prior to castration of four 5-month-old Holstein bulls. A significant increase occurred 7 hr post-castration and serum LH remained elevated thereafter, interspersed with non-rhythmic episodic fluctuations of this gonadotropin. Single injections of testosterone or dihydrotestosterone, given iv or im, did not depress post-castration serum LH, whereas a single injection of estradiol (.5 or 2.0 µg/kg) caused a significant reduction in serum LH within 3 hr of administration. Repeated administration of testosterone propionate (100 mg TP at 12-hr intervals for 3 days) resulted in significantly reduced serum LH concentrations and episodic increases were eliminated. However, these concentrations of LH were still significantly higher than serum LH prior to castration. Administration of 5 µg gonadotropin releasing hormone (GnRH) at 4 hr and at 28 hr following a 4-day treatment with TP (119 mg at 12-hr intervals) resulted in a slight increase (P= .08) in total LH released as compared to the release of LH induced by injection of GnRH prior to TP treatment. We conclude that testosterone does not exert a negative feedback effect at the level of the pituitary. In fact, under the conditions of these experiments, testosterone slightly enhanced the ability of the anterior pituitary to respond to GnRH.

¹ Technical Paper No. 3967, Oregon Agricultural Experiment Station.

² The authors express their appreciation to Dr. Russ Mauer, Abbott Labs, for the generous supply of GnRH. The expert technical assistance of Susan Bigelow is acknowledged. These data are taken from a thesis submitted by M. S. McCarthy to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science. A preliminary report of part of this work has appeared in abstract form (J. Anim. Sci. 39:217, 1974).

⁴ Department of Animal Science