Epithelial Impermeability to Water: A Second Look

Impermeability of the ascending limb of the Henle loop for water is traditionally regarded as essential for countercurrent multiplication in the kidney. Similar claims have been made about permeability properties of the collecting duct and some other epithelia. It is not clear, however, how a structure based on phospholipid bilayers can be water-impermeant if phospholipid bilayers themselves have measurable permeability. The presence of two membranes separated by the cytoplasm may only account for a several-fold reduction in permeability compared to a single bilayer. By analyzing published data, we conclude that these tubules do have a finite water permeability, especially the collecting duct. Although the results on isolated ascending limbs vary among authors, osmotic shock experiments clearly indicate that both the collecting duct and the ascending Henle loop are sufficiently water-permeable to observe volume regulation effects. We conclude that these epithelia by themselves do not display unusual resistance to water flow; it can be estimated that 20-50% of the fluid entering the tubules can be reabsorbed into a strongly hypertonic medulla. It is possible, however, that unstirred layers in the intact kidney may contribute to the apparent low permeability of the tubules.