Response of ewes to a single injection of an analogue of PGF-2 given at different stages of the oestrous cycle

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Abstract
Six groups of 4 ewes were given a single i.m. injection of 100 .mu.g ICI 80,996 [16-aryloxyprostaglandin, cloprostenol] on days 3, 5, 8, 11, 14 or 16 of the estrous cycle. On day 3 treatment was only partly effective in causing luteal regression, with only 2 of the 4 treated ewes returning to estrus. Injection had no apparent effect when given on day 16, at a time when luteal regression had already started naturally. Treatment from day 5-14 inclusive of the estrous cycle evoked a rapid fall in progesterone concentrations to basal values of < 0.5 ng/ml within 24 h with a high degree of synchrony in the return of estrus (37.7 .+-. 1.6 h) and the timing of the lutropin peak (45.3 .+-. 3.2 h) after injection. The corpus luteum of the ewe is evidently responsive to ICI 80,996 only from day 5-14 of the estrous cycle.

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