Variation in plasma concentrations of oestradiol-17β and their relationship to those of progesterone, 13,14-dihydro-15-keto-prostaglandin F-2α and oxytocin across pregnancy and at parturition in pony mares

Abstract

Concentrations of plasma progesterone were similar to values reported in the literature except that a significant decrease in progesterone during the last day, but before parturition, was detected by systematic, high-intensity blood sampling. Mean concentrations of oestradiol-17β increased sharply and significantly, plateaued for 132.8 ± 1.5 days (mean ± s.e.m., N = 9), then declined sharply in each mare. There was obvious variation between mares in when these increases and decreases in oestradiol-17β occurred, with the events being related closely to ambient photoperiod conditions rather than to the stage of pregnancy. Concentration of 13,14-dihydro-15-keto-prostaglandin F-2α (PGFM) remained at low levels (< 400 pg/ml) until Day 200 then increased to peak pregnancy levels (> 2000 pg/ml) by Day 300 and remained at this value until parturition. The concentrations of oxytocin remained basal (< 15μU/ml) throughout pregnancy and increased only at the beginning of the expulsive stage of labour. There was an increase, although not statistically significant, in the relative concentrations of oestradiol-17β to progesterone beginning 3 days before parturition, with the highest value of the ratio occurring at fetal delivery. Far more striking were acute changes in PGFM and oxytocin during parturition. Maximal concentrations of PGFM (~ 30 ng/ml) and oxytocin (> 200 μU/ml) were measured between rupture of the chorioallantois and the completion of delivery. Closely time samples from one animal showed that oxytocin increased (more than 10 standard deviations of the mean levels during late pregnancy for this animal) before any change in PGFM. In another distocic mare, both oxytocin and PGFM peaked in the initial stages of delivery but only oxytocin remained elevated until the distocia was remedied. The results suggest that an abrupt increase in oxytocin secretion precipitates the expulsive phase of parturition in mares.