Acute effects of prostaglandin F 2α on systemic oxytocin and progesterone concentrations during the mid- or late-luteal phase in mares

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The acute effects of prostaglandin F 2α (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.