Acute effects of prostaglandin F2α on systemic oxytocin and progesterone concentrations during the mid- or late-luteal phase in mares

The acute effects of prostaglandin F2α (PGF) on circulating oxytocin and progesterone concentrations were characterized in mares during the mid- or late-luteal phase. Pony mares were randomly assigned to the following experimental groups based on treatment with PGF (2.5 mg) or saline on Day 8 or Day 13 (Day 0 = ovulation): PGF-8, PGF-13, saline-8, or saline-13 (n = 7/group). Mares were fitted with indwelling, jugular vein catheters and two blood samples (−5 and 0 min) were collected prior to treatment. Treatments were administered into the jugular vein (0 min) and blood collection continued thereafter at 1 min intervals until 5 min and then at 5 min intervals until 60 min. Based on the combined data of −5 and 0 min samples, mares on Day 8 had greater (P < 0.05) oxytocin concentrations than mares on Day 13. On Day 8, PGF treatment resulted in a biphasic pattern of oxytocin release. Oxytocin concentrations increased (P < 0.05) 1 min after PGF treatment, decreased (P < 0.05) from 1 to 10 min, and increased (P < 0.05) from 10 to 30 min. Oxytocin concentrations were greater (P < 0.05) from 1 to 3 min in PGF-treated than saline-treated mares and at most sample times from 15 to 60 min. On Day 13, oxytocin concentrations were greater (P < 0.05) in PGF-treated than in saline-treated mares for most sample times. Mares treated with PGF on Day 8 had greater (P < 0.05) oxytocin concentrations at 25, 30, and 40 min than mares on Day 13. Progesterone concentrations on Day 8 also increased by 1 min after PGF, decreased toward basal concentrations by 2-3 min, and then increased to a maximum 10 min after treatment. Subsequently, circulating progesterone decreased (P < 0.05) below pretreatment concentrations by 40-50 min after PGF. In conclusion, treatment with PGF resulted in an immediate and biphasic increase in progesterone concentrations prior to the expected decrease. Treatment of mares with PGF on Day 8 resulted in an overall greater increase in systemic oxytocin concentrations compared to treatment on Day 13, and the increase on Day 8 was biphasic.