Androgen negative feedback during the early rise in LH secretion in bull calves.

A transient elevation in mean circulating concentrations of LH and FSH occurs in the young bull calf prior to 24 weeks of age. The functional significance of this is not clear. To see if changes in the ability of androgens to suppress gonadotrophin secretion were involved in the start of this early rise in LH secretion or the cessation of the early rise in LH and FSH secretion, bull calves were treated with flutamide (androgen receptor blocker; n = 5; 9 mg flutamide/kg body weight in propylene glycol (i.m./s.c.) in three equal portions at 12-h intervals) at 8, 16 and 24 weeks of age and bled every 15 min for 12 h beginning after the third flutamide treatment; control bulls received vehicle at these times. Control bulls (n = 5) were bled every 15 min for 12 h at 4, 8, 12, 16 and 24 weeks of age, and all bulls were bled weekly. Serum samples were assayed for concentrations of LH, FSH and testosterone. Based on weekly and intensive bleedings for control and flutamide-treated bulls, an early rise in LH (8-18 weeks of age) and FSH (4-24 weeks of age) secretion was seen in all bull calves (P < 0.05). At 8 weeks of age flutamide treatment resulted in increased mean serum LH concentrations (P < 0.05); at 16 weeks of age it resulted in increased basal and mean LH concentrations and increased LH pulse frequency (P < 0.05); and at 24 weeks of age in increased mean LH concentrations, LH pulse frequency and amplitude (P < 0.05) in comparison with control bulls.(ABSTRACT TRUNCATED AT 250 WORDS)