Study: Oxytocin Does Not Cause Cardiac Changes in Mares

Study: Oxytocin Does Not Cause Cardiac Changes in Mares

“Parturition in the horse (and in all other mammals) is a very complex condition, and we still have a lot to learn,” Aurich said. “It is absolutely amazing what happens in the dam and what she can cope with!”

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The hormone oxytocin is associated with foaling—it causes the mare’s uterus to contract, open the cervix, and push out the foal. However, unlike in humans, it doesn’t seem to cause the cardiac changes that occur during birth. And that, researchers say, could be reassuring news when a retained placenta requires oxytocin injections.

“Oxytocin infusion is the best treatment of retained placenta in the horse, and the present study shows that this method is safe with regard to cardiac function in horses,” said Christine Aurich, DVM, PhD, head of the Graf Lehndorff Institute in Neustadt, Germany.

In their study, Aurich and her fellow researchers injected 27 study mares with either oxytocin or saline, intravenously, in three stages: an hour after foaling, 12 hours after foaling, and seven days after the first post-foaling ovulation. They studied cardiac changes after the injections in these otherwise healthy mares to see what effects, if any, the drug (or saline control) had on the mares’ hearts and circulatory systems.

Blood parameters and electrocardiogram readings did not reveal any cause-effect relationship between oxytocin injections and cardiac changes, Aurich said. This is contrary to popular scientific belief that oxytocin causes these changes—likely due to the fact that cardiac changes do occur during labor and foaling, at the same time that oxytocin is released naturally. However, the current study suggests it’s not the oxytocin that’s responsible for those cardiac effects, but other phenomena relating to the birthing process.

“Interestingly, in a separate study we found that oxytocin treatment of mares with retained placenta induced a transient cortisol (stress hormone) release, but this was the only effect,” Aurich said.

However, oxytocin does have a pronounced effect on uterine contractions, she added. And those contractions will have their own effects.

“Oxytocin given as bolus or infusion will induce uterine contractions that will induce some abdominal pain and distress in mares,” she said. “This can certainly be the reason for sweating and colic signs that we often see. So this is related to oxytocin’s effect on the uterus, but not on the heart or—as has also been suspected—on the sweat glands in the skin.”

In mares with retained placentas, the effects could be even more obvious, Aurich added.

“We assume that the effect of oxytocin on the uterus is probably more pronounced in mares where the placenta is still attached to parts of the uterine wall,” she said. “The fact that especially the uterus and, in most cases, also the cervix is stretched due to the presence of the placenta may enhance the oxytocin effect.”

But for all that, the oxytocin does not induce heart rate changes, atrioventricular blocks, or other cardiac problems, Aurich said. And that’s a good thing, because those results could cause serious health problems in the mare. “If an overdose of oxytocin were given, and oxytocin were associated with cardiac changes, the worst case would be a cardiac arrest with subsequent death of the horse,” she said. “Fortunately, this does not seem to occur.”

Aurich and her team continue to attempt to unravel the many mysteries still remaining in understanding all that happens during foaling, she said. “Parturition in the horse (and in all other mammals) is a very complex condition, and we still have a lot to learn,” Aurich said. “It is absolutely amazing what happens in … the dam and what she can cope with!”

Their research is ongoing.

The study, “Oxytocin treatment does not change cardiovascular parameters, hematology and plasma electrolytes in parturient horse mares,” was published in Theriogenology

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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