Placental transport of sodium in the guinea-pig.

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Abstract
The mechanism of placental transport of Na was studied in guinea pigs in placentae with intact umbilical blood circulation or in the preparation of the placenta perfused in situ. A constant level of 22Na was maintained in maternal plasma for 60 min, and from the quantity of 22Na recovered from the fetus at the end of this period the influx of Na from mother to fetus was calculated. Ligation of the omphalomesenteric vessels (supplying the everted yolk sac with blood) had no effect on the influx, the corresponding values of influx in the control and treated fetuses being 0.235 .+-. 0.020 and 0.247 .+-. 0.029 .mu.mol/min .cntdot. g fetal weight (n = 6, the limits are standard error of mean). The specific activity of Na in amniotic fluid was below that of the maternal or fetal plasma Na by 2 orders of magnitude. The extraplacental transport of Na into the fetus was negligibly low. The electrical potential difference (pd) and unidirectional fluxes of Na across the placenta perfused in situ were measured by means of 22Na and 24Na administered to the opposite sides of the placental barrier. The fluxes varied with the weight of the fetuses whose placentae were perfused. The flux from the maternal to the fetal side was 0.270 .+-. 0.017 .mu.mol/min .cntdot. g fetal weight, the flux from the fetal to the maternal side was 0.340 .+-. 0.018 .mu.mol/min .cntdot. g fetal weight (n = 38). The corresponding pd was -20.7 .+-. 1.2 mV (fetal side negative). The active component of Na transport across the placenta was calculated from the unidirectional fluxes and the pd. The active transport was directed from the fetal to the maternal side and its rate was 0.211 .+-. 0.015 .mu.mol/min .cntdot. g fetal weight (n = 38). During perfusion of the placenta with KCN (10-3 M) the active transport decreased by approximately 1/3. The flux of Na from the fetal to the maternal side of the perfused placenta was higher than the flux from the maternal to the fetal side. A similar asymmetry of Na fluxes was observed in the non-perfused placenta, the flux from mother to fetus being 0.180 .+-. 0.013 .mu.mol/min .cntdot. g fetal weight and the flux from fetus to mother 0.235 .+-. 0.024 .mu.mol/min .cntdot. g fetal weight (n = 12). The asymmetry of Na fluxes was caused by the anesthesia and/or by the trauma of the operation rather than by the perfusion of the placenta. The permeabilities of the perfused placenta to Na and sucrose measured simultaneously from the maternal to the fetal side were 0.0767 .+-. 0.0183 and 0.0324 .+-. 0.0094 cm3/min (n = 7), respectively. The permeability values bear the same relation to each other as the respective coefficients of free diffusion in water, suggesting that the passive transport of Na across the placenta takes place as simple diffusion through wide aqueous channels. The observations were consistent with 2 different mechanisms of Na transport across the placenta: simple diffusion (presumably bidirectional) and unidirectional active transport from the fetal to the maternal side of the barrier.

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