title: Transepithelial Na+ transport and the intracellular fluids: a computer study.
abstract: Computer simulations of tight epithelia under three experimental conditions have been carried out, using the rheogenic nonlinear model of Lew, Ferreira and Moura (Proc. Roy. Soc. London. B 206:53-83, 1979) based largely on the formulation of Koefoed-Johnsen and Ussing (Acta Physiol. Scand. 42: 298-308. 1958). First, analysis of the transition between the short-circuited and open-circuited states has indicated that (i) apical Cl- permeability is a critical parameter requiring experimental definition in order to analyze cell volume regulation, and (ii) contrary to certain experimental reports, intracellular Na+ concentration (ccNa) is expected to be a strong function of transepithelial clamping voltage. Second, analysis of the effects of lowering serosal K+ concentration (csK) indicates that the basic model cannot simulate several well-documented observations; these defects can be overcome, at least qualitatively, by modifying the model to take account of the negative feedback interaction likely to exist between the apical Na+ permeability and ccNa. Third, analysis of the strongly supports the concept that osmotically induced permeability changes in the apical intercellular junctions play a physiological role in conserving the body's stores of NaCl. The analyses also demonstrate that the importance of Na+ entry across the basolateral membrane is strongly dependent upon transepithelial potential, cmNa and csK; under certain conditions, net Na+ entry could be appreciably greater across the basolateral than across the apical membrane.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D001826:Body Fluids / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D003201:Computers; D004848:Epithelium / Q000378:metabolism; D005246:Feedback; D007424:Intracellular Fluid / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*
chemical_list: D012964:Sodium; D011188:Potassium