Cellular permeation pathways in a leaky epithelium: the human amniochorion

The presence of cellular permeation pathways in human fetal membranes at term was evaluated. Electrical parameters (transepithelial potential [TEP], and conductance [Gt], and intracellular potentials [cell PD]), and water and urea diffusional coefficients (Pdw, Pdu), were determined in Ussing-like chambers. In amniochorion, the TEP was practically 0 (0.1 +/- 0.03 mV), and the Gt very high (144 +/- 14 mS/cm2). The Cell PD of amnion cells was -37 +/- 3 mV. Increasing the [K+] of the amniotic perfusate between 5.8 and 125.8 mM depolarized the cells with a slope of 23 mV. The deletion of Na+ hyperpolarized the cells, whereas amiloride and ouabain depolarized them. The Pdw and Pdu were determined in intact amnion and chorion and in their epithelial cell layers. The Pdw/Pdu ratio in amnion was 4.0, and 7.0 in its cell layer; the ratio in chorion was 2.5, and 3.3 in its cell layer. The amniochorion is a leaky structure, but its cellular layers possess definite transcellular permeation pathways. The ionic conductances in amnion cells are complex, with the Cell PD being determined by at least K+ and Na+ conductances, and ouabain- and amiloride-sensitive pathways. The amnion is a more effective diffusional barrier to water and urea than chorion is; its diffusional characteristics are comparable to those of nystatin-treated lecithin: cholesterol bilayers and the membranes of human erythrocytes.