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Whether or not significant amounts of water pass the tight junction (TJ) of leaky epithelia is still unresolved, because it is difficult to separate transcellular water flux from TJ-controlled paracellular water flux. Using an approach without differentiating technically between the transcellular and paracellular route, we measured transepithelial water flux with and without selective molecular perturbation of the TJ to unequivocally attribute changes to the paracellular pathway. To this end, MDCK C7 cells were stably transfected with either claudin-2 or claudin-10b, two paracellular cation-channel-forming TJ proteins that are not endogenously expressed in this cell line. Claudin-2 is typical of leaky, water-transporting epithelia, such as the kidney proximal tubule, whereas claudin-10b is present in numerous epithelia, including water-impermeable segments of the loop of Henle. Neither transfection altered the expression of …