We have investigated the effects of systemic administration of the oxytocin antagonist (OTA) L-368,899 on luteolytic PGF(2alpha) release in ewes. In the first study, carried out in four ovariectomized ewes primed with progesterone to induce responsiveness to oxytocin, 3-h i.v. infusions of 3, 10 and 30 microg/kg/min OTA, carried out on days 12, 14, 16 and 18 in a Latin Square design, resulted in a significant attenuation of the oxytocin induced increase in PGFM concentration at all doses (OTA 139+/-8.3% of pre-oxytocin baseline; control 206.8+/-18.7%; P<0.005). In a further study, continuous infusion of cyclic ewes (n=6) with 10 microg/kg/min OTA from day 13 to day 17 of the cycle resulted in a reduction in both the frequency (OTA 1.0+/-0.4/ewe; control 2.2+/-0.2/ewe; P<0.05) and amplitude (OTA 31.8+/-11.0 pg/ml; control 68.8+/-10.4 pg/ml; P<0.05) of endogenous PGFM episodes compared to control ewes (n=5) measured during daily 8-h sampling windows on days 14-17. This reduction in PGFM concentrations was accompanied by a modest extension in the day of luteolysis (progesterone <0.5 ng/ml) to day 17.5+/-0.4 in the OTA treated group compared with day 16.4+/-0.5 in the control group (P=0.07). The results demonstrate that treatment with OTA caused a significant reduction in episodes of increased PGFM concentration during the period of luteolysis and may provide an approach by which to reduce early pregnancy failure.
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