Central hemodynamic effects of oxytocin and interaction with magnesium and pregnancy in the isolated perfused rat heart

OBJECTIVE: Our purpose was to determine the cardiac effects of oxytocin in the isolated perfused rat heart model and to see whether pregnancy or pharmacologic levels of magnesium modifies the hormone's action. STUDY DESIGN: Hearts were excised from 30 female Sprague-Dawley rats (15 pregnant, 15 nonpregnant) and attached to a Langendorf's apparatus. Heart rate, left ventricular systolic pressure, and contractility were measured. Hearts were exposed serially to 2, 4, and 6 mU/ml concentrations of oxytocin. The experiment was repeated in 15 pregnant animals at a magnesium level of 5 mEq/L. RESULTS: Hearts from pregnant rats had lower heart rates than did hearts from nonpregnant animals. Oxytocin exposure diminished heart rate but increased left ventricular systolic pressure and contractility in a dose-dependent manner. Pregnancy did not alter this response. Magnesium levels of 5 mEq/L reversed the effects of oxytocin on contractility and left ventricular systolic pressure. CONCLUSION: Oxytocin is a negative chronotropic and positive inotropic agent in hearts from pregnant and nonpregnant rats. Magnesium reserves the positive inotropic effects of oxytocin.