How descending limb of Henle's loop permeability affects hypertonic urine formation

An isomorphic, dynamic model of the antidiuretic medulla and distal tubule was used to determine how vasa recta (VR) plasma flow and the nature of descending limb (DLH) equilibration influences inner medullary urine concentrating ability in the steady state. Four DLH modes were examined: water recycling with limited solute permeability, ideal water recycling, mixed water-solute recycling, and strict solute recycling. Each DLH mode was evaluated with and without thin ascending limb (tALH) NaCl active transport. Results indicate that VR plasma flow strongly affects medullary solute accumulation, that NaCl reabsorption from passive tALH is insufficient to establish a positive corticomedullary NaCl gradient, that even limited DLH solute entry compromises the medulla's ability to establish an axial osmotic gradient without active transport, and that with tALH active NaCl transport, positive inner medullary NaCl and osmotic gradients could be established with all four DLH modes. We conclude that tALH NaCl active transport or some other form of osmotic work must be invoked to account quantitatively for inner medullary concentrating effects in rodents.