Convective paracellular solute flux. A source of ion-ion interaction in the epithelial transport equations.

Abstract

An electrolyte model of an epithelium (a cell and a tightjunction in parallel, both in series with a lateral interspace basement membrane) is analyzed using the formalism of nonequilibrium thermodynamics . It is shown that if the parallel structures are heteroporous (i .e ., reflection coefficients for two ion species differ between the components), then a cross-term will appear in the overall transport equations of the epithelium . Formally, this cross-term represents an ion-ion interaction . With respect to the rat proximal tubule, data indicating epithelial ionic reflection coefficients less than unity, together with the assumption of no transcellular solvent drag, imply the presence of convective paracellular solute flux . This means that a model applicable to a heteroporous structure must be used to represent the tubule, and, in particular, the crossterms for ion-ion interaction must also be evaluated in permeability determinations. A series of calculations is presented that permits the estimation of the Na-Cl interaction for rat proximal tubule from available experimental data . One consequence of tubule heteroporosity is that an electrical potential may be substantially less effective than an equivalent concentration gradient in driving reabsorptive ion fluxes .

Modeling oxygen consumption in the proximal tubule: effects of NHE and SGLT2 inhibition

“…In the rat, the reflection coefficient of Cl Ϫ (taken to be ϭ 0.30 in the present model) is much smaller than that of other major solutes ( ϭ 0.75-0.90) (49). Thus the model predicts that paracellular convective solute flux contains a disproportionately large amount of Cl Ϫ , which, taken in isolation, raises the transepithelial voltage, enhances paracellular Na ϩ diffusion and thereby increases the metabolic efficiency of Na ϩ transport.…”

“…In the rat a significant fraction of water reabsorption is thought to occur paracellularly, as suggested by several studies showing reflection coefficients Ͻ1 for the main solutes (9,12,49,54). The role of paracellular water transport is less clear in the mouse.…”

Flow-dependent transport in a mathematical model of rat proximal tubule

“…These models have indicated that paracellular fluxes of Na ϩ and Cl Ϫ constitute an important fraction of their overall reabsorption; that these fluxes assume greater significance later along the tubule (where transepithelial Cl Ϫ and HCO 3 Ϫ gradients are well developed); and that both diffusive and convective components of paracellular Cl Ϫ flux are substantial (45). Nevertheless, these models have supported the primacy of luminal membrane Na ϩ /H ϩ exchange in determining the rate of overall proximal Na ϩ reabsorption (47).…”

Efficiency of Primary Saliva Secretion: An Analysis of Parameter Dependence in Dynamic Single-Cell and Acinus Models, with Application to Aquaporin Knockout Studies

“…the paracellular pathway and the cell) through which water also travels, then cross-terms in overall (or “composite”) epithelium system equations can still occur. These represent formal ion-ion interactions and can have important consequences for the interpretation of tracer flux experiments and determination of permeabilities (Weinstein, 1987). Since in our model we assume a fully transcellular pathway for water (consistent with the assumption of an osmotic mechanism as discussed in the introduction), these formal composite-system interactions are not present (Weinstein, 1987; Friedman, 2008).…”