Transfer of Radioactive Sodium Across the Placenta of the Gat. ∗

Abstract

Radioactive sodium, Na₂₄, was prepared by use of the electrostatic pressure generator of the Department of Terrestrial Magnetism, Carnegie Institute of Washington. Samples emitting about 105 beta-rays per second were injected intravenously, as the chloride, into pregnant cats. At various intervals of time after injection, fetuses were removed by Caesarian section and a sample of blood taken from the mother. The radioactivity of the samples, in terms of beta-particles per second, was measured by a pressure ionization chamber and string electrometer, using the method previously described. 1

Typical data are presented in Tables I and IL Analysis of these data reveal the following relationships:

1. Table I. The fetus near term comes to within 10% of a limiting equilibrium value with respect to sodium ion in the maternal plasma only after 12 to 18 hours. This is in striking contrast to the extracellular fluid of the mother which comes to the same value in about 4 minutes. 2

2. Table II, column 6. The rate of transfer across the placenta per unit weight of placenta is very low in early stages of pregnancy (gestation age, 15 to 20 days) but increases in linear manner to a value 60 times that of the early stage at a gestation age of 57 days. After this stage, the rate of transfer per unit weight of placenta decreases somewhat until term (62 days).

3. Table II, column 5. The rate of transfer to a unit weight of fetus, however, is high in early stages and falls with the duration of pregnancy. For example, the ratios of rates of transfer for gestation ages of 15-20 days, 40 days and 60 days are 5.5:2.5:1. The relatively high rate of transfer per unit weight of fetus in the youngest fetuses is accounted for by the large size of the placenta compared to that of the fetus (Table II, columns 2 and 3).