The uterus as a source of oxytocin in cyclic mares.

In a recent study, continuous administration of oxytocin by subcutaneous minipump to mares from day 8 to 20 after ovulation prevented luteolysis in most of the treated but none of the control mares, indicating a role for oxytocin in cyclical luteolysis in mares. In the present study, measurement of oxytocin concentrations in uterine flushings recovered from nine mares during days 14-18 after ovulation gave values that were many times higher than those measured concurrently in peripheral plasma. Furthermore, intrauterine administration of oxytocin to four mares on day 14 after ovulation stimulated a large increase in peripheral plasma 13,14-dihydro-15-keto PGF2alpha concentrations without a corresponding increase in plasma oxytocin concentrations. These findings indicate that oxytocin is secreted into the uterine lumen during late dioestrus and early oestrus in mares, where it stimulates PGF2alpha release and, thus, induces luteolysis.