The effect of ICI 182,780, oestrogen antagonist, on the concentrations of oxytocin and uterine PGF(2α) was investigated in intact Border Leicester Merino cross ewes during the late oestrous cycle. Twelve cyclic ewes (n = 6 per group) were randomly assigned to receive, at 6 h intervals, intra-muscular injection of either peanut oil or ICI 182,780 (1.5 mg kg-1 day-1) in oil for 2 days, starting at 1900 h on day 13 until 1300 h on day 15 post-oestrus. Hourly blood samples were collected via a jugular catheter from 0800 h on day 14 for 37 h and then daily over days 16, 17 and 18 post-oestrus. Peripheral plasma concentrations of oxytocin, the metabolite of prostaglandin F(2α), 15-keto-13,14-dihydro-prostaglandin F(2α), (PGFM) and progesterone were measured by radioimmunoassay. All ewes treated with ICI 182,780 exhibited functional luteal regression as indicated by a marked reduction in plasma progesterone concentrations to less than 1000 pg/ml over the period of 18-36 h during sampling period on days 14 and 15 of the oestrous cycle. In five of six vehicle-treated ewes, progesterone concentrations declined between day 16 and clay 18 post-oestrus. In the remaining control ewe, progesterone concentrations reach less than 1000 pg/ml within 36 h of the commencement of the sampling period. During the frequent sampling period, the number of oxytocin pulses in the ICI 182,780 treated ewes was significantly higher compared to control ewes (2.7 ± 0.3 vs. 0.8 ± 0.3). The mean amplitude of oxytocin pulses observed was also greater (70.4 ± 19.5 pg/ml) in ewes treated with ICI 182,780, but was not significantly different from control ewes (33.5 ± 12.9 pg/ml). Oxytocin pulses may however have occurred following the initial two ICI 182,780 injections but before commencing blood sampling. The oxytocin pulses were detected at a mean of 3.2 ± 0.2 h following each injection with ICI 182,780 during blood sampling. In the ICI 182,780-treated ewes, the pulsatile pattern of plasma PGFM in jugular blood samples over the 37 h sampling period on days 14 and 15 post-oestrus had a higher amplitude (512.9 ±158.9 vs. 121.7 ± 78.7 pg/ml) and pulse area (618.1 ± 183.3 vs. 151.5± 102.9 (pg/ml)τ) compared to the vehicle-treated ewes (P 0.05) respectively. The average number of PGFM pulses observed per ewe was 3.0 ± 0.7 in the ICI 182,780-treated group and was significantly (P < 0.02) higher than the number of pulses (0.5 ± 0.3) observed in ewes treated with vehicle alone. The PGFM pulses were detected at 4.2 ± 0.6 h following each injection with ICI 182,780 during blood sampling. The percentage of PGFM pulses that occurred coincidently with a significant elevation of oxytocin concentrations was 44.4% in ICI 182,780-treated compared to 66.6% in control ewes. We conclude that administration of oestrogen antagonist ICI 182,780 accelerated development of the luteolytic mechanism by enhancing pulsatile secretion of oxytocin and PGFM which suggests that ICI 182,780 acts as an agonist for oxytocin and prostaglandin F(2α) release in intact ewes when administered at 1.5 mg/kg/day over Day 13 to 15 post-oestrus.
- Oestrogen antagonist
- Sheep endocrinology