Application of a fiber-matrix model to transport in renal tubules.
The effects of tight junction structure on water and solute fluxes across proximal tubular epithelium were examined with fiber-matrix equations previously derived by Curry and Michel (1980. Microvascular Research. 20:96-99). Using plausible estimates of tight junction fiber length and width the model predicts solute (Ps) and water permeability (Lp) coefficients that agree with the measured values. When fiber-matrix and pore models were compared for physiologically relevant ranges of matrix void fraction (80-98%) and pore radii (0-20 A), the fiber-matrix model predicted a 10-fold higher Lp/Ps ratio. Lp/Ps was most sensitive to small changes in tight junction structure when void fractions exceeded 90%. Void fractions of 96.5% and 97.1% predicted previously measured values for Lp and solute permeabilities in rat and rabbit proximal tubules. These values are consistent with void fractions and permeabilities of artificial membranes. The fiber-matrix tight junction model was incorporated into a model of reabsorption from the rat proximal tubule developed by Weinstein (1984). American Journal of Physiology. 247:F848-F862.) A void fraction of 98% predicted the experimental results for isosmotic reabsorption driven by active transport. Changing void fraction over the range of 97-99% produced a 50-75% change in predicted volume reabsorption with active transport. According to the fiber-matrix model: (a) solute permeabilities alone cannot be used to predict Lp, (b) previously measured solute permeabilities in the proximal tubule are compatible with significant water reabsorption through a water-permeable tight junction, and (c) hydraulic and solute permeabilities may be sensitive to small changes in tight junction fiber length and diameter or ionic strength within the tight junction.
Fraser
W D
WD
Toronto General Hospital, Ontario, Canada.
Baines
A D
AD
D000042:Absorption; D000818:Animals; D001693:Biological Transport, Active; D002463:Cell Membrane Permeability; D004848:Epithelium / Q000378:metabolism; D007365:Intercellular Junctions / Q000378:metabolism; D007687:Kidney Tubules, Proximal / Q000378:metabolism* / Q000648:ultrastructure; D008353:Mannitol / Q000378:metabolism; D008433:Mathematics; D008567:Membranes, Artificial; D008954:Models, Biological; D011817:Rabbits; D051381:Rats; D014867:Water / Q000378:metabolism
D008567:Membranes, Artificial; D014867:Water; D008353:Mannitol