The role of sub-optimal preovulatory oestradiol secretion in the aetiology of premature luteolysis during the short oestrous cycle in the cow

Premature regression of the corpus luteum, following the first post partum ovulation, is often preceded by sub-optimal preovulatory oestradiol secretion and accompanied by elevated levels of oxytocin receptors early in the luteal phase. We have investigated the role of preovulatory oestradiol in the control of subsequent oxytocin receptor concentration and activity by treating ovariectomised cows, over a simulated 48 h follicular phase, with high (600 μg per day) medium (300 μg per day) or low (150 μg per day) levels of oestradiol. These doses of oestradiol generated mean±S.E.M. plasma oestradiol concentrations of 12.1±1.0, 4.9±0.5 and 2.9±0.4 pg ml−1, respectively. In Study 1 (n=4 per group), we found that by day 4 following oestrus there was a significant (P<0.05) effect of the level of oestradiol on the inhibition of oxytocin binding activity measured in endometrial biopsy samples. This had fallen to mean±S.E.M. concentrations of 25±2 fmol per mg protein in the high group, 47±8 fmol per mg protein in the medium group and 65±12 fmol per mg protein in the low group. In Study 2, cows (n=3 per group) were treated with the same three levels of oestradiol followed by treatment with increasing levels of progesterone from days 3 to 6 following oestrus, generating mean±S.E.M. plasma concentrations of 2.17±0.18 ng ml−1 by day 6. On day 6, there was a significant (P<0.01) effect of the level of oestradiol on PGF2α release in response to oxytocin challenge. High, medium and low oestradiol groups exhibiting mean±S.E.M., increase plasma PGF2α metabolite concentrations of 10.0±2.2, 21.3±4.3 and 41.3±1.2 pg ml−1, respectively, during the hour after oxytocin administration. From these results, we postulate that at the first post partum ovulation a low level of preovulatory oestradiol can result in the early generation of a luteolytic mechanism during the subsequent luteal phase due to impaired inhibition of oxytocin receptors allowing increased PGF2α release.