Influence of Estradiol on the Secretion of Oxytocin and Prostaglandin F2α during Luteolysis in the Ewe1

Twenty ewes of mixed breeds were randomly assigned in equal numbers to one of four groups in a 2 × 2 factorial design. The factors were x-irradiation to destroy ovarian follicles or sham irradiation and the administration of estradiol-containing or empty (placebo) implants. Surgery for irradiation was performed on Day 8 of the cycle. Blood samples were withdrawn from jugular catheters at 1.5-h intervals from Day 10 to Day 17. Luteolysis was not observed by Day 17 in 4 of 5 placebo-treated ewes after destruction of ovarian follicles. Luteolysis was observed in 4 of 5 ewes of the sham-irradiated, placebo-treated group and in all ewes that received estradiol whether or not ovarian follicles had been destroyed. The longest (p < 0.07) interval between peaks of 13,14-dihydro-15-keto-prostaglandin F2α (PGFM) was observed in the x-irradiated, placebo-treated group, whereas the administration of estradiol reduced (p < 0.01) the interval between PGFM peaks. These findings indicate that a short interpulse interval in the secretion of prostaglandin F2α (PGF2α) is associated with luteolysis. It is possible that the reduced interpulse interval was either an effect of estradiol that caused luteolysis or a secondary event resulting from luteolysis. The administration of estradiol decreased (p < 0.05) the number of episodes of oxytocin secretion during luteolysis and increased (p < 0.01) the interval between episodes. These findings suggest a more sluggish pattern in the secretion of oxytocin in response to estradiol or luteolysis, although similar results could be obtained if the sampling frequency failed to detect high-frequency pulses of short duration. Failure to detect high-frequency bursts in the secretion of oxytocin may be one explanation for the decrease (p < 0.05) in the percentage of coincident episodes in the secretion of oxytocin and PGFM in the estradiol-treated groups. An alternative explanation is that an estrogen-sensitive pulse generator other than oxytocin is responsible for PGF2α secretion.