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 oxytoci n in the isolated perfused rat heart model and to see whether pregnanc y 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 wer e measured. Hearts were exposed serially to 2, 4, and 6 mU/ml concentr ations of oxytocin. The experiment was repeated in 15 pregnant animals at a magnesium level of 5 mEq/L. RESULTS: Hearts from pregnant rats h ad lower heart rates than did hearts from nonpregnant animals. Oxytoci n exposure diminished heart rate but increased left ventricular systol ic pressure and contractility in a dose-dependent manner. Pregnancy di d not alter this response. Magnesium levels of 5 mEq/L reversed the ef fects of oxytocin on contractility and left ventricular systolic press ure.CONCLUSION: Oxytocin is a negative chronotropic and positive inotr opic agent in hearts from pregnant and nonpregnant rats. Magnesium res erves the positive inotropic effects of oxytocin.