Relationship between Plasma Oxytocin and Placental Retention Time Immediately after Foaling in Heavy Draft Mares

抄録

The aims of this study were to determine the relationship (1) between oxytocin release during parturition especially during the third stage of labor and placental retention time, and (2) between hand-milking or suckling and oxytocin release after foaling. Blood was sampled every 5 min from foaling to expulsion of the placenta in 9 heavy draft mares to evaluate circulating oxytocin. The relationship between the oxytocin concentration and recorded placental retention, hand milking and suckling times were investigated. Results: (1) High peak of oxytocin concentration observed around foaling in 7 mares. (2) Peaks of oxytocin were observed after expulsion of the placenta and suckling or after hand-milking about 2 hr after foaling in 2 mares. (3) The mean oxytocin concentration in mares which, had a shorter placental retention time (i.e.<1 hr) was significantly lower than that of mares which had a longer placental time (i.e.>1 hr). (4) A significant negative correlation was observed between oxytocin concentrations immediately after foaling and placental retention times. (5) Oxytocin increased distinctly after hand-milking and suckling in 2 mares. The response was most extensive in the mare, which had the lowest oxytocin concentration immediately after foaling and the longest placental retention time. In conclusion, the oxytocin concentration immediately after foaling is negatively related to the placental retention time. Low circulating oxytocin immediately after foaling may result in placental retention.