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Stimulation of ovarian oxytocin secretion and uterine prostaglandin release by exogenous progesterone early in the cycle of the ovarian auto-transplanted ewe

The present study was undertaken to determine whether the administration of progesterone, early in the oestrous cycle, had an influence on ovarian oxytocin secretion and on peripheral concentrations of the prostaglandin F₂ metabolite 13,14-dihydro-15-keto PGF₂ (PGFM) in the ovarian auto-transplanted ewe. Twelve ewes with ovarian auto-transplants (n = 6 per group) were randomly assigned to receive an i.m. injection of progesterone (12.5 mg) or vehicle, twice a day, on days 1, 2 and 3 of the oestrous cycle. Beginning on day 7, blood samples were collected at intervals of 1 h from the ovarian and contralateral jugular veins for up to 70 h. Ovarian oxytocin secretion rate and jugular concentrations of PGFM and progesterone were determined by radioimmunoassay. The number of ewes that showed pulses of both ovarian oxytocin and PGFM was significantly (P < 0.05) greater in progesterone-treated ewes than in control ewes. In progesterone-treated ewes, the average number of ovarian oxytocin pulses per ewe was 9.66 ±5.5 (mean ± SD) and the interval between pulses was 7.18 ± 5.8 h. The mean amplitude and amount of oxytocin released, as calculated by the area under the curve of ovarian oxytocin pulses, were 6.27 ± 1.98 ng min^–1 and (10.05 ± 8.91 ng min^–1 ), respectively (where is the number of hours between the last time point before and the first time point after a significant increase in hormone concentration was detected by the Pulsar program). The mean amplitude and area under the curve of PGFM pulses were 317.22 ± 5.65 pg ml^–1 and (383.36 ± 1.77 pg ml^–1), respectively. The average number of pulses of plasma PGFM observed per ewe was 5.8 ± 1.9 and interpulse interval for plasma PGFM pulses was 10.32 ± 8.7 h between day 7 and day 9 after oestrus. These data indicate that administration of progesterone during the first 3 days of the oestrous cycle results in the premature release of ovarian oxytocin and uterine prostaglandin F₂.