Evidence of active sodium transport in the visceral yolk sac of the rat in vitro.

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Abstract
Na transport was studied in the isolated rat visceral yolk sac from day 17.5 of gestation to term. The transepithelial potential difference (pd) and the short circuit current (scc) in the isolated yolk sac varied with gestational age, with peak values at day 19.5. The maximal pd and scc were 3.85 .+-. 0.32 mV (the fetal side positive) and 19.5 .+-. 5.3 .mu.A cm-2, respectively. Simultaneous determination of the 2-way Na+ flux and the scc revealed a preferential active movement of Na in the maternal to fetal direction. The net flux was 50% higher than the scc. Both the pd and the scc were reduced by cooling and by the uncoupling agent 2,4-dinitrophenol. The scc altered in a curvilinear fashion with the Na+ concentration in the bathing solution, with an apparent Km of about 20 mM-Na+. Removing Cl- from the bathing solutions had no effect on the pd and scc. Addition of amiloride (10-4 M) to either side of the visceral yolk sac had no effect on the scc but application of ouabain (10-5 M) to the fetal side caused a profound fall in the scc. The possible physiological role of this active Na transport by the visceral yolk sac in the formation of amniotic fluid is discussed.

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