Serum LH concentration following castration, steroid hormone and gonadotropin releasing hormone treatment in the male bovine.

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 /lg/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. Administrat ion of 5 rig gonadotropin releasing hormone (GnRH) at 4 hr and at 28 hr following a 4-day t reatment 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 abil i ty of the anterior pi tui tary to respond to GnRH.