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J. Fischbarg, A. Rubashkin, P. Iserovich, J.A. Hernandez; Epithelial Fluid Transport: Protruding Macromolecules and Space Charges Can Bring About Electro–Osmotic Coupling at the Tight Junctions . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2907.
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We have reported evidence for a central role for electro–osmosis in fluid transport across corneal endothelium (Sanchez JM et al., J. Membr Biol 2002). The present work investigates whether the idea of epithelial fluid transport based on electro–osmotic coupling at the level of the leaky tight junctions (TJ) can be further supported by a plausible theoretical model of the junctions.
We develop a model based on electro–osmotic coupling at leaky tight junctions (TJ). In it, the cell membranes limiting the TJs possess surface electrical charges, but in addition we also consider the fixed electrical charges of macromolecules in strands protruding into the TJ. The model embodies systems of electro–hydrodynamic equations for the intercellular pathway. For the case of the TJ, the expressions utilized are the Brinkman and the Poisson–Boltzmann differential equations.
After linearization, we are able to obtain analytical solutions for a system of the two equations above, and are able to derive expressions for the fluid velocity profile and the electrostatic potential in the TJ. Importantly, electrolytes would appear at a lower concentration in the strand regions of the TJ due to a diminished partition coefficient. We employ geometrical parameters and experimental data from the corneal endothelium, and verify that the model successfully reproduces the experimental data for fluid flow, electrical current, and the coupling between them.
Our results suggest that electro–osmotic coupling at the TJ can account for fluid transport by the corneal endothelium. Efficient electro–osmotic coupling would occur due to unique environmental characteristics found only in the strand regions of leaky tight junctions, surrounded by a solution relatively depopulated of ions, and subject to a very intense transverse electric field. Electro–osmotic coupling at the tight junctions could therefore represent one of the basic mechanisms driving fluid transport by the paracellular route across some leaky epithelia, a process that remains unexplained.
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