ABSTRACT
Our purpose was to determine the dynamic changes in plasma and cardiac oxytocin during gestation, to study the cardiac effects of oxytocin in the isolated perfused rat heart model and whether pregnancy modifies the hormone's action. Cardiac and plasma oxytocin were evaluated at three stages of gestation [6,13 and 20 days] and at 2 days postpartum. In addition, at the same stages, hearts were excised and attached to a Langendorf's apparatus. Heart rate [HR] and left ventricular developed pressure [LVDP] were measured while being exposed serially to same plasma concentrations of oxytocin observed at each studied stage. Compared with non-pregnant controls, plasma and cardiac oxytocin were significantly and respectively decreased by 41.9%; 38% in early, 42.5%; 36.6% in mid and 8,9%; 47.9% in late gestation, to increase again by 178.1% and 81% postpartum. Hearts from pregnant rats had higher HR, LVDP and delta P/delta t[max] than did hearts from non-pregnant animals. Postpartum, hearts showed lower HR by 9.2%, LVDP by 6.9% and delta P/delta t [max] by 13.9% compared to non-pregnant rats. During pregnancy, reproductive hormones may regulate cardiac oxytocin release and consequently its negative chronotropic and inotropic effects. After parturition, an activated oxytocin system may help the body to get rid of the excess volume