Patterns of LH Secretion in Castrated Bulls (Steers) During Intravenous Infusion of Androgenic and Estrogenic Steroids: Pituitary Response to Exogenous Luteinizing Hormone-Releasing Hormone1

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Abstract
Five steers were used in a 5 x 5 Latin square design to investigate negative feedback on luteinizing hormone (LH) secretion by androgenic and estrogenic steroids. Vehicle (200 microliter 30% ethanol/min), testosterone (20 micrograms/min), 5 alpha-dihydrotestosterone (20 micrograms/min), estrone (2 micrograms/min) and estradiol-17 beta (2 micrograms/min) were administered by continuous 12 h infusion at weekly intervals. At the end of each infusion period animals were given an intravenous injection of luteinizing hormone-releasing hormone (LHRH, 1000 ng). LH secretion in steers was characterized by pulses which occurred at intervals of 40 to 60 min. Estradiol-17 beta abolished pulsatile LH release and suppressed mean LH levels. The LH response to LHRH was greater however, in steers infused with estradiol-17 beta than in steers infused with vehicle only. The latter finding indicates a positive feedback action of estradiol-17 beta on the pituitary. In contrast to the results obtained with estradiol-17 beta, infusion with estrone had no effect on either the pattern of LH secretion or the pituitary response to LHRH. Testosterone and 5 alpha-dihydrotestosterone infusion also failed to influence LH secretory patterns in steers. Similar results for testosterone were obtained in a second experiment where this steroid was infused at a rate of 80 micrograms/min. The results of this study suggest that in the male bovine, estradiol-17 beta exerts negative feedback at the level of the hypothalamus to block pulsatile LHRH release. The results also support the contention that chronic castrates are insensitive to acute androgen feedback.

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