Bidirectional sodium fluxes across the placenta of conscious sheep

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Abstract
Conscious pregnant sheep in the last 3 wk of gestation were studied 1-3 days after surgery. Fetal plasma Na concentration was significantly lower than maternal. A mean electrical potential difference (PD) of 34 .+-. 4 (SE) mV (n = 24) was recorded between maternal and fetal intravascular catheters, the mother being positive with reference to fetus. Unidirectional fetomaternal (Jf .fwdarw. m) and maternofetal (Jm .fwdarw. f) Na fluxes were determined by application of Fick''s principle to uterine and umbilical circulations following injection of 22NaCl or 24NaCl to fetus or mother, respectively. Blood flows were measured by an antipyrine technique. Jm .fwdarw. f = 0.142 .+-. 0.029 mmol/min (n = 10); Jf .fwdarw. m = 0.137 .+-. 0.015 mmol/min (n = 21). Jm .fwdarw. f increased as a linear function of calculated fetal wt. In 7 sheep both Jm .fwdarw. f and Jf .fwdarw. m were measured in a single experiment. The measured ratio Jm .fwdarw. f/Jf .fwdarw. m was significantly different from the ratio predicted using Ussing''s flux ratio equation. There is probably a transplacental Na pump active in the direction fetus to mother.

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