A model of NaCl and water flow through paracellular pathways of renal proximal tubules

Abstract

To explain how hydrostatic pressure differences between tubule lumen and interstitium modulate isotonic reabsorption rates, we developed a model of NaCl and water flow through paracellular pathways of the proximal tubule. Structural elements of the model are a tight junction membrane, an intercellular channel whose walls transport NaCl actively at a constant rate, and a basement membrane. Equations of change were derived for the channel, boundary conditions were formulated from irreversible thermodynamics, and a pressure-area relationship typical of thin-walled tubing was assumed. The boundary value problem was solved numerically. The principal conclusions are: 1) channel NaCl concentration must remain within a few mOsm of isotonic values for reabsorption rates to be modulated by transtubular pressure differences known to affect this system: 2) basement membrane and channel wall parameters determine reabsorbate tonicity; tight junction parameters affect the sensitivity of reabsorption to transmural pressure; 3) channel NaCl concentration varies inversely with transmural pressure difference; this concentration variation controls NaCl diffusion through the tight junction; 4) modulation of NaCl diffusion through the tight junction controls the rate of isotonic reabsorption; modulation of water flow can increase sensitivity to transmural pressure; 5) no pressure-induced change in permeability of the tight junction or basement membrane is needed for pressure to modulate reabsorption; and 6) system performance is indifferent to the distribution of active transport sites, to the numerical value of the compliance function, and to the relationship between lumen and cell pressures.

Maturation of Jejunum and Ileum in Rats

“…In "leaky" epithelia, such as that of the small intestine, the proximal kidney tubule, and the gall bladder, where the electric conductance across the intercellular spaces forms around 80-96% of the total transepithelial conductance (10)(11)(12)(13), most of the passive ion transport across the epithelium occurs through the intercellular spaces (10,(14)(15)(16)(17)(18)(19). Water flows into these Response of the Intestine to Hypertonic Solutions during Maturation spaces in response to a local osmotic gradient produced by active or passive solute transport into these spaces (10,(14)(15)(16)(17)(18)(19).…”

“…Water flows into these Response of the Intestine to Hypertonic Solutions during Maturation spaces in response to a local osmotic gradient produced by active or passive solute transport into these spaces (10,(14)(15)(16)(17)(18)(19). From the mucosal medium, flow of water into the intercellular spaces can be directly through the tight junctions (18) and/or through the cell membranes, and is associated with dilatation of the spaces (20)(21)(22).…”

Models of coupled salt and water transport across leaky epithelia

“…(A ~ and pO are area and pressure at level flow.) An alternative empirical compliance relation has been utilized by Huss and Marsh (1975) and Huss and Stephenson (1979) in their models of proximal tubule in which channel area is related to channel pressure through an exponential function. Nevertheless, over a wide range of serosal pressures the linear compliance law has been shown to give an adequate simulation of the changes in channel dimensions (Weinstein & Stephenson, 1979).…”

Microscopical determination of the filtration permeability of the mucosal surface of the goldfish intestinal epithelium

“…If in the control situation the fold is distended, the force necessary for this distention is produced by a pressure built up in the epithelial layer during transport (cf. Ogilvie, McIntosh & Curran, 1963;Huss & Marsh, 1975). This pressure pushes the cells apart till elas-tic forces counteract these dilating forces.…”