Water-permeable and -impermeable barriers of snake distal tubules.

Isolated perfused snake distal tubules transport Na from lumen bath via an amiloride-sensitive transport pathway. Elevation of the luminal Na concentration from 16 to 150 mM leads to non-steady-state electrical behavior and cell swelling. To elucidate the mechanism of cell swelling, the water permeabilities of the epithelium and its apical and basolateral membrane were assessed. Distal tubules were found to be virtually impermeable to transepithelial water flow. Hydraulic conductivity measured 1.2 X 10(-7) cm3 X s-1 X cm-2 X atm-1 in the absence or presence of vasopressin. Effects of transepithelial osmotic pressure gradients on epithelial cell volume revealed the luminal membrane as the water-impermeable cellular barrier and the basolateral membrane as a barrier that is freely permeable to water. Epithelial cell swelling was blocked during perfusion with 150 mM Na when the perfusate also contained amiloride (10(-5) M). These results support the hypothesis that, in the case of transepithelial transport in the presence of high luminal Na concentrations, Na entry across the apical membrane exceeds Na extrusion across the basolateral membrane. Hence, the cells accumulate solute: Na from the apical side and some anion from the apical and/or serosal side. Concomitantly, the epithelial cells swell as water enters across the highly permeable basolateral membrane.