Independence of urea and water transport in rat inner medullary collecting duct.

Several published models of the renal concentration mechanism have assumed a reflection coefficient for urea in the inner medullary collecting duct (IMCD) that is less than unity, implying direct coupling between water and urea transport. In the present study, we used isolated perfused terminal IMCD segments and mathematical modeling of IMCD transport to determine the validity of this assumption. Mathematical simulations of IMCD transport, using recently published data on urea and water permeability, revealed that the method previously used to measure the reflection coefficient for urea underestimates the true value. The modeling results allowed us to design two new experimental protocols to determine the reflection coefficient for urea. In the first protocol, we measured the ability of a transepithelial urea gradient to induce a water flux, correcting for the dissipation of the urea gradient by rapid passive urea permeation. In the second protocol, we directly measured the solvent drag of urea resulting from an osmotically induced water flux. Both protocols yielded values for the urea reflection coefficient that were not significantly different from unity (0.92 +/- 0.04 and 1.07 +/- 0.05, respectively). Thus we find no evidence for direct coupling between urea and water transport in the rat terminal IMCD.