INFLUENCE OF ESTRADIOL ON THE SECRETION OF OXYTOCIN AND PROSTAGLANDIN-F2-ALPHA DURING LUTEOLYSIS IN THE EWE

Twenty ewes of mixed breeds were randomly assigned in equal numbers to one of four groups in a 2 x 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 Blood samples were withdrawn from jugular catheters at 1.5-h intervals from Day 10 to Day 17.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 F2-alpha (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 F2-alpha (PGF2-alpha) 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 PGF2-alpha secretion.