Pulsatile luteinizing hormone secretion in the castrate male bovine: effects of testosterone or estradiol replacement therapy.

Luteinizing hormone (LH) secretory profiles have been determined for the male bovine following castration and steroid replacement therapy. Serum LH concentrations increased approximately threefold during the first week following castration and thereafter remained elevated (6.6 +/- .7 ng/ml). Castrates not receiving steroid replacement showed a rhythmic pattern of LH release that was of high frequency (mean pulse interval; 85 +/- 5 min) and high amplitude (mean peak concentration, 11.2 +/- 1.4 ng/ml). Chronic administration of estradiol-17 beta via subdermal Silastic implants reduced mean serum LH concentrations (2.1 +/- .3 ng/ml) and blocked the pulsatile pattern of LH release in all steers. Similar administration of testosterone suppressed mean serum LH and blocked pulsatile LH release in two of four animals. The number of implants used in this study provided physiological concentrations of estradiol (9.8 +/- 1.5 pg/ml) and testosterone (4.1 +/- .2 ng/ml) in systemic blood for the two respective treatment groups. Differences in the LH secretory profiles among testosterone-implanted steers may have been related, in part, to differences in the amounts of steroid not bound to serum proteins. These findings demonstrate that estradiol is a particularly potent inhibitor of pulsatile LH secretion in the male bovine and suggest that gonadal steroid feedback on LH secretion may, in part, be imposed at the level of the hypothalamus. The mechanism for pulsatile LH release is discussed relative to a centrally-located luteinizing hormone releasing hormone pulse generator.