Evidence for a change in oestradiol negative feedback and LH pulse frequency in post-partum ewes

Plasma LH levels and the effects of ovariectomy and of chronic and acute administration of oestradiol-17 beta were compared in post-partum (anoestrous) and cyclic Merino ewes during the breeding season. Plasma LH and LH pulses were detected in more post-partum (anoestrous) than cyclic (dioestrous) ewes (10/10 and 4/10 respectively). The post-partum ewes had a greater pulse frequency (1.8 +/- 0.42 compared with 0.4 +/- 0.16 pulses/6 h) and higher plasma LH levels (1.16 +/- 0.32 compared with 0.17 +/- 0.05 ng/ml). Ovariectomized post-partum ewes demonstrated fewer LH pulses (3.8 +/- 0.36 compared with 5.0 +/- 0.15 pulses/6 h) than did ovariectomized cyclic ewes. The depression of plasma LH following acute i.v. injection of 3 micrograms oestradiol-17 beta was similar (0.05 < P < 0.1) for the two groups (80.5 +/- 3.6 and 66.3 +/- 4.1%). At 7 days after ovariectomy and an oestradiol implant more cyclic ewes than post-partum ewes showed detectable plasma LH and LH pulses (6/10 compared with 1/9 ewes). By 66 h after implant removal pulse frequency was less in the post-partum ewes (3.2 +/- 0.55 compared with 5.5 +/- 0.56 pulses/6 h). These findings indicate that in post-partum ewes there is an increased inhibitory (negative-feedback) effect of oestradiol on LH release and a lower intrinsic frequency of pulsatile release of LH. These factors may be casually related to ovarian acyclicity.