Researchers in Poland recently looked into the issue of retained placenta in draft mares, desiring to answer the question of why treatment of a retained placenta with repeated injections of oxytocin so often fails. They put forth the hypothesis that it may be the “expression of oxytocin receptors that differs in mares that retain fetal membranes due to secondary uterine atony” (lack of physiologic tone, muscle weakness). The objective of this study was to determine whether the expression of oxytocin receptors in equine placental tissue differs in these draft mares that normally pass their fetal membranes vs. those mares that retain them.
Placental biopsies were taken from eight draft mares during foaling. Four mares passed their placenta shortly after foaling (control group), and four mares retained their placenta due to secondary uterine atony. The number of oxytocin receptors was estimated, and the presence and location of these receptors were determined.
Results showed that the oxytocin receptor expression was nearly 50 times less intense in the uterus of mares that retained their placenta due to secondary uterine atony, and it was difficult to determine where the receptors were located. In the control group, oxytocin receptors were found throughout the layers of the placenta, with the most intense concentration seen where the uterus contacts the allantochorion (embryonic membrane composed of the fused allantois and chorion). It was also noted that the mares that retained their fetal membranes were, on average, 8.4 years older than the control mares. Whether this lack of oxytocin receptors was due to a lower initial number of the receptors or depletion and slower resynthesis of them, it was thought that age-related changes may have led to the secondary uterine atony. The researchers concluded, “Inadequate expression of oxytocin receptors may be a cause of uterine atony leading to fetal membrane retention”.
When we give oxytocin to a mare to help her expel the placenta, we rely on that oxytocin being able to bind on the appropriate receptors in the endometrium and myometrium of the uterus to cause the muscle contractions that are needed to encourage the placenta to be passed. If there are not enough of these receptors, then the oxytocin that is given cannot bind in a high enough quantity to cause the response by the uterus that we need. It is important to take this into account, especially when dealing with the older broodmare.
Scientific Article: Rapacz-Leonard, A. et al “Expression of oxytocin receptors is greatly reduced in the placenta of heavy mares with retained fetal membranes due to secondary uterine atony”. Equine Vet J 2015, on line.