Fetal fluid responses to long-term 5 M NaCl infusion: where does all the salt go?

The fetus must obtain Na and Cl ions in order to grow. However, the regulation of electrolyte acquisition by the fetus is not well understood. To explore fetal electrolyte balance, we intravenously infused 5 M NaCl at a rate equal to 80% of the total fetal body Na+ and Cl- content per day (240 mM/day) for 3 days into late-gestation fetal sheep. We hypothesized that the increase in fetal osmolality resulting from the infusion would cause a transplacental water movement into the fetal compartment, leading to hydrops fetalis and/or polyhydramnios. The fetal-to-maternal osmotic gradient was initially -2.8 +/- 0.9 (SE) mosmol/kgH2O and rose by 4.8 +/- 1.8 mosmol/kgH2O during the infusion. Fetal plasma [Na+] and [Cl-] increased (3.0 +/- 0.4 and 5.5 +/- 0.5 meq/l, respectively), but the normal maternal-to-fetal transplacental concentration gradients for these ions were not reversed. Most of the infused Na+ (92 +/- 14%) and Cl- (82 +/- 12%) was excreted by the fetus in large volumes of hypotonic urine. Amniotic fluid osmolality and [Na+] were unchanged, but amniotic [Cl-] increased 5.7 +/- 2.4 meq/l. The amniotic plus allantoic fluid volume, as estimated by ultrasonography, was increased (43.5 +/- 14.5%) at day 2 and returned to control by day 3 of infusion. There was no fetal edema during the study or at autopsy. In light of these results, we propose a novel and somewhat complex mechanism for transplacental fluid and electrolyte movement in which placental capillary permeability increases along the length of the capillary.(ABSTRACT TRUNCATED AT 250 WORDS)