Oxytocin and PGF2alpha release in mares resistant and susceptible to persistent mating-induced endometritis

Mares resistant (n=7) and susceptible (n=9) to persistent mating-induced endometritis were artificially inseminated (AI) during oestrus with chilled extended semen. Blood samples were collected from 30 min before AI, for 2 h after AI and again between 16 and 18 h after AI. Samples were assayed for oxytocin and the PGF2alpha metabolite 13,14-dihydro-15-keto PGF2alpha (PGFM). AI caused a significant increase in oxytocin concentrations in both the resistant and susceptible mares and there was no significant difference in oxytocin release between the two groups of mares. Mean PGFM concentrations were significantly higher in the resistant group for the first 30 min after AI. In a second experiment, an i.v. injection of oxytocin (1 iu per 20 kg body weight) was administered to both resistant and susceptible oestrous mares. Blood samples were collected at 5min intervals from 10 min before until 60 min after oxytocin administration. Administration of exogenous oxytocin caused significantly greater PGF2alpha release in resistant than in susceptible mares. The observed pattern of decreased PGFM concentrations in the susceptible group of mares after both AI and oxytocin administration indicates a possible defect in PGF2alpha release at the oxytocin receptor or post-receptor level.