Regulation of electrolyte composition of fetal rat plasma

Abstract

Is fetal plasma a transudate of maternal plasma? Cl, Na, and K concentrations were measured in plasmas obtained by cardiac puncture of exposed rat fetuses while the maternal rat was under pentobarbital anesthesia. At rest, Cl had 0.95 x the maternal concentration, Na 1.00, and K 1.6x. When hypertonic NaCl was infused into a maternal vein, the Cl in the fetal plasma increased only half as much as in the maternal plasma, a steady differential being maintained at 1–3 hr. In contrast, Na increased alike in both plasmas. When electrolytes were depleted from the mother by intraperitoneal dialysis, both Na and Cl were lost proportionately. Sucrose solution added to the mother's blood diluted the electrolytes of her plasma but increased them in the fetal plasma by withdrawal of water. Analysis of the fetuses showed that water moved out from the fetuses while Na and Cl moved inward after NaCl was given to the mother; depletion of the mother led only to movement of Na and Cl from the fetus. Thus Cl and K are maintained in fetal plasma, when sampled in the above manner, at concentrations differing from those of maternal plasma; and circumstances were found in which these differences are exaggerated. Therefore fetal plasma is not a transudate.