Corpus luteum function in the ewe: effect of PGF2alpha and prostaglandin synthetase inhibitors.

A series of experiments were conducted to evaluate the effects of mode and frequency of administration and estrous cycle stage on the response of the cycling ewe to PGF2alpha. The effects of dexamethasone, arachadonic acid and prostaglandin synthetase inhibitors on estrous cycle length and plasma progesterone levels were also determined. Intramuscular administration of 5 or 10 mg of PGF2alpha, on days 8 and 9 after estrus (5 ewes/group), significantly (p less than .01) shortened the mean length of the estrous cycle and the interval from the end of treatment to estrus. Mean plasma progesterone levels, 24 hours after initial injection, were significantly (p less than .01) lowered. When administered on day 8 only, these doses were considerably less effective in shortening estrous cycle length or lowering plasma progesterone levels. Intravaginal administration of PGF2alpha, by polyurethane tampon, was also largely ineffective. Treatment of ewes with 10 mg of PGF2alpha, by polyurethanetampon, was also largely ineffective. Treatment of ewes with 10 mg of PGF2alpha i.m., on days 3 and 4 of the estrous cycle, resulted in a return to estrus in 2 days in 25% of the treated animals. Plasma progesterone levels of PGF2alpha-treated ewes were significantly lower than controls on the second, third and fourth days after the start of dosing. It would appear that PGF2alpha exerts a retarding effect on developing CL functionality. The prostaglandin synthetase inhibitors, aspirin, flufenamic acid and 1-p-chlorobenzylidene-2-methyl-5-methoxy-3-indenylacetic acid, were administered orally or parenterally for 16 days beginning on day 8 of the estrous cycle. These compounds failed to prolong estrous cycle length. Parenteral administration of dexamethasone did not result in PGF2alpha release in the cycling ewe, at least not in quantities sufficient to induce luteolytis. The prostaglandin precursor, arachadonic acid, also was not luteolytic when given parenterally to cycling ewes.