DIFFERENTIAL RESPONSES IN ANTERIOR-PITUITARY LUTEINIZING-HORMONE (LH)CONTENT AND LH-BETA-SUBUNIT AND ALPHA-SUBUNIT MESSENGER-RNA, AND PLASMA-CONCENTRATIONS OF LH AND TESTOSTERONE, IN BULLS TREATED WITH THE LH-RELEASING HORMONE AGONIST DESLORELIN

Anterior pituitary gland contents of LH and LH beta- and alpha-subunit mRNAs, and circulating concentrations of LH and testosterone, were de termined in bulls treated with the LH-releasing hormone (LHRH) agonist deslorelin. Brahman (Bos indicus) bulls (14-month-old) were allocated to two groups and received the following: Control (n = 5), no treatme nt; Deslorelin (n = 4), four deslorelin implants (approximately 200 mu g total deslorelin/day) for 36 d. Plasma concentrations of LH were hi gher in bulls treated with deslorelin on Day 1, had returned to typica l levels by Day 8, and did not differ for control bulls and bulls trea ted with deslorelin from Day 8 to Day 29. Pituitary content of LH on D ay 36 was reduced (P < 0.001) in bulls treated with deslorelin (33 +/- 4 ng/mg) compared with control bulls (553 +/- 142 ng/mg). Relative pi tuitary content of LH beta-subunit mRNA was also reduced on Day 36 in bulls treated with deslorelin (Control, 0.65 +/- 0.10; Deslorelin, 0.2 2 +/- 0.04; P = 0.003). However, alpha-subunit mRNA relative content d id not differ (Control, 0.73 +/- 0.15; Deslorelin, 1.06 +/- 0.12; P > 0.05). Plasma concentrations of testosterone were increased over the p eriod of the experiment in the bulls treated with deslorelin compared with control bulls. This is the first demonstration of reduced pituita ry content of LH beta-subunit mRNA and LH, and unaltered content of al pha-subunit mRNA, in bulls treated with LHRH agonist. This was associa ted with apparently typical plasma concentrations of LH and elevated p lasma testosterone. The anterior pituitary in bulls treated with LHRH agonist, therefore, undergoes classical desensitization and downregula tion, but plasma LH and testosterone are not suppressed. (C) Elsevier Science Inc. 1997.