Effect of NaCl load administered to the fetus on the bidirectional movement of 51Cr-EDTA across rat placenta

A hypothesis that, in the rat, fluid circulates across the placenta, with circulation being maintained by active transport of Na+ from mother to fetus, has been tested. Transfer of 51Cr-EDTA from mother to fetus and from fetus to mother has been measured and the respective unidirectional transfer constants, Kmf and Kfm, have been calculated. Immediately before the transfer measurement, the fetuses were injected intravenously with 10 microliters of isotonic glucose (controls); with 30 or 300 microliters of isotonic saline; or with 10, 30, or 60 microliters of 9% NaCl. In controls, Kmf of 51Cr-EDTA was 2.0 +/- 0.6 microliters/min, and Kfm was 4.3 +/- 1.0 microliters/min. Injecting the fetus with NaCl had no effect on Kmf, whereas the Kfm was increased significantly in a dose-dependent way. In other experiments, 51Cr-EDTA was injected into nephrectomized maternal animals, and the radioactivity of maternal and fetal plasma was followed for 30 h. The time course of fetal plasma radioactivity supported the thesis that the transfer of 51Cr-EDTA across the rat placenta is highly asymmetrical.