Transepithelial osmolality differences, hydraulic conductivities, and volume absorption in the proximal tubule

The weight of current evidence indicates that the proximal tubule has a high transepithelial osmotic water permeability in the range of 3500-6000 microns/sec, which is attributable in large part to the high water permeabilities of the cell membranes. Water movement through these membranes may occur through specialized, proteinaceous channels that can be blocked by sulfhydryl reagents. The water channels probably exclude even the smallest solutes and allow only single file movement of the water molecules as do the water channels previously described in the red blood cell and vasopressin-responsive epithelia. If a significant fraction of the water flow also occurs through the junctional complexes, it seems likely that these junctions could be a site for solute solvent coupling which would contribute to solute absorption by solvent drag and which would be responsible for non-unity reflection coefficients for some solutes such as Na+ and Cl-. This possibility is still a matter of vigorous debate. Since the transepithelial water permeability is high, only a very small osmolality difference (1-10 mOsM) is required to drive normally observed rates of volume absorption both in vivo and in vitro. The osmolality difference is produced at least in part by dilution of the luminal fluid and is possibly augmented by the development of interstitial hyperosmolality because of the rapid transport of preferentially absorbed solutes. In the future it is likely that the most important work in this field will relate to the factors that alter transepithelial water permeability and the solute and water permeabilities of the junctional complexes. Investigation in this area is essential in understanding how changes in capillary and interstitial hydrostatic and colloid osmotic pressure may affect volume absorption.