Oxytocin induction of pulses of a prostaglandin metabolite and luteolysis in mares

A procedure for oxytocin (OT) administration on Day 13 postovulation was developed in mares for stimulation of a pulse of PGFM (a PGF2α metabolite) that mimics a natural PGFM pulse during luteolysis. Bolus treatment with each of five OT doses (1-10 IU/mare, n = 3) stimulated a burst of PGFM that was maximum in 4 minutes and was unlike a natural pulse. A 2-hour OT infusion of 1.25, 2.5, or 5 IU/100 kg (n = 4) induced a PGFM pulse similar to reported pulses; lower doses did not. The peak of an induced pulse (approximately 260-380 pg/mL) seemed similar to reported natural peaks (approximately 200-300 pg/mL), using the same assay system. The interval from nadir to nadir was 6.6 ± 0.2 hours. Percentage decrease in progesterone (P4) within 8 hours was greater (P < 0.05) for doses of 1.25, 2.5, or 5 IU/100 kg (43%-50%) than that for a vehicle group (11%). Treatment with flunixin meglumine (1.0 mg/kg), a cyclooxygenase inhibitor, decreased (P < 0.008) P4 concentration, but treatment 2 hours before the beginning of OT infusion (2.5 IU/100 kg) did not prevent the OT-induced PGFM pulses and the decrease in P4. In conclusion, a PGFM pulse was simulated by infusion of OT during 2 hours but not by a single OT bolus, and an OT-simulated PGFM pulse stimulated a decrease in P4 that was not prevented by a cyclooxygenase inhibitor. These are the first firm demonstrations that OT in mares as in other species has a role in luteolysis.