Effects of estradiol-17 beta, naloxone and gonadotropin releasing hormone on postpartum secretion of luteinizing hormone in fall-lambing ewes.

The interaction among exogenous estradiol-17 beta, naloxone and gonadotropin releasing hormone (GnRH) in the control of luteinizing hormone (LH) secretion was studied in intact postpartum ewes nursing their offspring. One-half of 30 fall-lambing ewes were implanted subcutaneously with an estradiol-17 beta containing Silastic capsule between postpartum d 1 and 12 which doubled their serum concentrations of estradiol (16.0 +/- .1 vs 8.4 +/- .1 pg/ml). Blood samples were collected from implanted and non-implanted ewes at 15-min intervals for 5 h on d 3, 8, 13, 20 and 28 postpartum. Pre-injection samples were collected for 1 h, and ewes were injected with saline, naloxone (NAL;1 mg/kg) or GnRH (100 micrograms/ewe). When averaged across all days and implant groups, serum LH in the three post-NAL samples was higher (P less than .05) than in the three pre-NAL samples (3.6 +/- 1.2 vs .6 +/- .2 ng/ml). Post-GnRH concentrations of serum LH were lower (P less than .05) in estradiol-implanted ewes than in non-implanted ewes on d 8 and 13, but there were no differences in any LH characteristics on d 20 and 28 after implant removal on d 12. In non-implanted ewes, serum LH responses to GnRH increased (P less than .05) eightfold from d 3 (3.8 +/- 1.4 ng/ml) to d 8 (31.6 +/- 1.4 ng/ml), remained elevated through d 20, but declined by d 28 (10.8 +/- 1.4 ng/ml).(ABSTRACT TRUNCATED AT 250 WORDS)