Pathways for hydraulically and osmotically-induced water flows across epithelia.

EVIDENCE linking an extracellular shunt pathway1 for ions with intercellular junctions2, with reports that those junctions are permeable to large molecules such as sucrose3, inulin3, lanthanum hydroxide4 and horseradish peroxidase5 in various fluid-transporting epithelia suggests that water could also traverse this pathway. There is evidence, moreover, for a high hydraulic-conductance pathway across gastric mucosa6 and rumen7 if hydrostatic pressure is the driving force. Yet, except for the kidney proximal tubule8, measurements of osmotically-induced water flows across several ‘leaky’ epithelia have not in general yielded values9 consistent with a high-conductance pathway. It has been argued, also, that the extracellular shunt is not a significant route for osmotic flow in the gallbladder10. To investigate these apparent discrepancies, we have measured the hydraulic (Lp) and osmotic (PO S) conductances of rabbit corneal endothelium. We report here that a large conductance for both hydraulic and osmotic flows either is present or can be demonstrated in appropriate conditions, and that its value is of the order expected for bulk water flow across the intercellular junctions.