The effect of castration, estradiol and LHRH [luteinizing hormone releasing hormone] on LH secretion of lambs fed different levels of dietary energy

To examine the effect of diet on luteinizing hormone (LH) secretion, basal and luteinizing hormone releasing hormone (LHRH)-induced LH release was compared in intact or castrated-estradiol-17 beta implanted Finn-Dorset lambs. Ten to 12 wk old ram (n = 20) and ewe lambs (n = 20) were maintained under a 8L:16D photoperiod and fed for high (HG, 163 to 168 g/d) or low (LG, 76 to 103 g/d) rates of gain. Eight to 10 wk later, baseline LH concentrations were determined in blood samples collected at 20 min intervals for 7 h. The following day, lambs were given an iv injection of 5 micrograms of estradiol-17 beta followed within 4 h by LHRH (.5 or 2.5 micrograms). Baseline concentrations of LH for HG ewes were threefold greater than for LG ewes (4.2 vs 1.4 ng/ml), respectively. Time to peak response was inversely related to dietary energy level (P less than .025). Basal LH levels were similar across diets in rams. Total LH release following LHRH was dose-dependent (P less than .005). Effects of gonadal feedback were tested in a second group (n = 24) of castrated lambs. Changes in LH secretion were not different between diets within 3 to 4 wk after castration. A subcutaneous silastic implant (22 mm) of estradiol-17 beta inhibited (P less than .01) LH concentrations across diets in both ewes and rams. No differences in estradiol feedback on LH secretion (at the dose of steroid tested) were detected between HG and LG lambs. Within 8 d, however, basal LH concentrations were 60% lower (P less than .01) in HG vs LG ewes. Furthermore, peak LHRH-induced LH release was greater (P less than .025) in LG vs HG lambs of both sexes. Estradiol inhibited basal LH secretion in ewes and rams but facilitated LH release in lambs with a reduced rate of gain.