Magnesium Sulfate Is a Poor Inhibitor of Oxytocin-Induced Contractility in Pregnant Sheep

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Abstract
The aim of this study was to determine if magnesium sulfate has a direct inhihitory effect in vivo on oxytocin-induced myometnal contractility. We used 13 preterm non-laboring sheep to evaluate the ability of magnesium sulfate to inhibit oxytocin-induced contractions. In each of seven animals, we achieved three sequential steady state concentrations of magnesium (3.5, 5.5, and 8.3 mEq/L). A bolus of oxytocin was given before magnesium infusion and after 140 min at each steady state concentration. The myometrial response to each oxytocin injection was quantified. In six other animals, we infused magnesium sulfate over 24 h and achieved an average concentration of 7.7 mEq/L. Oxytocin boluses were given after 4 and 24 h of magnesium infusion and the myometrial response was quantified. Magnesium sulfate did not significantly reduce oxytocin-induced contractility with either short (2 h) or prolonged (24 h) infusion. There was no dose related contraction inhibition by magnesium sulfate over concentrations ranging from 1.8 to 8.3 mEq/L. Magnesium sulfate is a poor inhibitor of oxytocin-induced contractility in this animal model. The purported mechanism of action of magnesium sulfate in vitro may not be applicable to the intact animal.

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