Fetal diuretic responses to maternal hyponatremia: contribution of placental sodium gradient

Maternal hyponatremia induces fetal hyponatremia and increased fetal urine flow. We sought to examine the relative contributions of the placental Nagradient vs. the absolute decrease in fetal plasma Na+ in the fetal diureti c response to hyponatremia. Seven ewes with singleton fetuses (130 +/- 2 da ys) were prepared. Ewes received intravenous 1-desamino-8-D-arginine vasopr essin (20 mu g) and warm tap water (2 liters). Maternal plasma Na+ was decr eased to achieve two levels of maternal hyponatremia. Maternal and fetal bl ood volume were measured with radiolabeled red blood cells. In response to the first decrease in maternal plasma Na+, fetal plasma Na+ did not change initially. Subsequently, fetal plasma Na+ decreased, normalizing the gradie nt. The second decrease in maternal plasma Na+ similarly induced a reduced and normalized placental gradient at lower fetal plasma Na+ values. Fetal u rine flow increased in direct proportion to the degree of fetal hyponatremi a (13, 38, 63, 100%, respectively). Maternal, although not fetal, blood vol ume significantly increased in response to hyponatremia. These results sugg est that chronic fetal hyponatremia will result in a persistent diuresis, d espite placental equilibration.