Abstract
The Na+ recirculation theory for solute-coupled fluid absorption is an expansion of the local osmosis concept introduced by Curran and analyzed by Diamond & Bossert. Based on studies on small intestine the theory assumes that the observed recirculation of Na+ serves regulation of the osmolarity of the absorbate. Mathematical modeling reproducing bioelectric and hydrosmotic properties of small intestine and proximal tubule, respectively, predicts a significant range of observations such as isosmotic transport, hyposmotic transport, solvent drag, anomalous solvent drag, the residual hydraulic permeability in proximal tubule of AQP1 (−/−) mice, and the inverse relationship between hydraulic permeability and the concentration difference needed to reverse transepithelial water flow. The model reproduces the volume responses of cells and lateral intercellular space (lis) following replacement of luminal NaCl by sucrose as well as the linear dependence of volume absorption on luminal NaCl concentration. Analysis of solvent drag on Na+ in tight junctions provides explanation for the surprisingly high metabolic efficiency of Na+ reabsorption. The model predicts and explains low metabolic efficiency in diluted external baths. Hyperosmolarity of lis is governed by the hydraulic permeability of the apical plasma membrane and tight junction with 6–7 mOsm in small intestine and ≤ 1 mOsm in proximal tubule. Truly isosmotic transport demands a Na+ recirculation of 50–70% in small intestine but might be barely measurable in proximal tubule. The model fails to reproduce a certain type of observations: The reduced volume absorption at transepithelial osmotic equilibrium in AQP1 knockout mice, and the stimulated water absorption by gallbladder in diluted external solutions. Thus, it indicates cellular regulation of apical Na+ uptake, which is not included in the mathematical treatment.
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Notes
Considering bidirectional fluxes of isotopes through two membranes in series we arrived at the same general conclusion (Larsen et al., 2006).
In the AQP1(−/−) computations the relationship between J V and Δπ is not strictly linear but upward concave. In the original paper P f was estimated near equilibrium, which resulted in a somewhat larger P f (Larsen et al., 2006)).
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The study is supported by a frame grant from the Danish Natural Science Research Council, 272-05-0417.26
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Larsen, E., Møbjerg, N. Na+ Recirculation and Isosmotic Transport. J Membrane Biol 212, 1–15 (2006). https://doi.org/10.1007/s00232-006-0864-x
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DOI: https://doi.org/10.1007/s00232-006-0864-x