The Journal of Membrane Biology

, Volume 38, Issue 4, pp 359–386 | Cite as

Pathways for movement of ions and water across toad urinary bladder

III. Physiologic significance of the paracellular pathway
  • Mortimer M. Civan
  • Donald R. DiBona


Hypertonicity of the mucosal bathing medium increases the electrical conductance of toad urinary bladder by osmotic distension of the epithelial “tight” or limiting junctions. However, toad urine is not normally hypertonic to plasma. In this study, the transmural osmotic gradient was varied strictly within the physiologic range; initially hypotonic mucosal bathing media were made isotonic by addition of a variety of solutes. Mucosal NaCl increased tissue conductance substantially. This phenomenon could not have reflected solely an altered conductance of the transcellular active transport pathway since mucosal KCl also increased tissue conductance, whether or not Na+ was present in the bathing media. The effect of mucosal NaCl could not have been mediated solely by a parallel transepithelial pathway formed by damaged tissue since mucosal addition of certain nonelectrolytes also increased tissue conductance. Finally, the osmotically-induced increase in conductance could not have occurred solely in transcellular transepithelial channels in parallel with the active pathway for Na+, since the permeability to22Na from serosa to mucosa (s tom) was also increased by mucosal addition of NaCl; a number of lines of evidence suggest thats-to-m movement of Na+ proceeds largely through paracellular transepithelial pathways. The results thus establish that the permeability of the limiting junctions is physiologically dependent on the magnitude of the transmural osmotic gradient. A major role is proposed for this mechanism, serving to conserve the body stores of NaCl from excessive urinary excretion.


Electrical Conductance Human Physiology Urinary Bladder Urinary Excretion Active Transport 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Springer-Verlag New York Inc 1978

Authors and Affiliations

  • Mortimer M. Civan
    • 1
    • 2
  • Donald R. DiBona
    • 1
    • 2
  1. 1.Departments of Physiology and MedicineUniversity of Pennsylvania School of MedicinePhiladelphia
  2. 2.Laboratory of Renal BiophysicsMassachusetts General HospitalBoston

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