On the influence of a leaky tight junction on water and solute transport in epithelia

The role of a leaky tight junction in epithelia is examined by considering the flow of water and solute through a channel consisting of two sections representing the intercellular space and tight junction. Two cases are considered, flow through a channel with a circular cross-section and flow between parallel planes. Analytical solutions are obtained using the isotonic convection approximation. The flow is driven by active transport of solute and imposed concentration and pressure differences. Particular attention is paid to the flux of solute through the tight junction. It is shown that the shape of the channel cross-section is important.The theory is applied to the rat proximal tube epithelium. It is deduced that the emergent osmolarity is close to that predicted for a closed tight junction, but that transepithelial hydrostatic pressure differences are potentially important. The influence of transepithelial concentration differences appears to be unimportant in this model.     

Chloride reabsorption by renal proximal tubules of necturus

Summary Movement of Cl from the lumen ofNecturus proximal tubule into the cells is mediated and dependent on the presence of luminal Na. Intracellular Cl activity was monitored with ion selective microelectrodes. In Cl Ringer's perfused kidneys, cell Cl activity was 24.5±1.1mm, 2 to 3 times higher than that predicted for passive distribution. When luminal NaCl was partially replaced by mannitol (capillaries perfused with Cl Ringer's) cell Cl decreased showing a sigmoidal dependence on luminal NaCl. Peritubular membrane potential was unaltered. Sulfate Ringer's perfusion of the kidneys washed out all cell Cl but did not alter peritubular membrane potential. Chloride did not enter the cell when the tubule lumen was perfused with 100mm KCl, LiCl, or tetramethylammonium Cl. Luminal perfusion of NaCl caused cell Cl to rise rapidly to the same value as the controls in the Cl Ringer's experiments. Perfusion of the tubule lumen with mixtures of NaCl and Na₂SO₄, while the capillaries contained sulfate Ringer's yielded a sigmoidal dependence of cell Cl on luminal NaCl activity. Chloride movement from the lumen into the proximal tubule cells required approximately equal concentrations of Na and Cl. Current clamp experiments indicated that intracellular chloride activity was insensitive to alterations in liminal membrane potential, suggesting that chloride entry was electrically neutral. The transcellular chloride flux was calculated to constitute about one half of the normal chloride reabsorption rate. We conclude that the cell Cl activity is primarily determined by the NaCl concentration in the tubule lumen and that Cl entry across the luminal membrane is mediated.     

A mathematical model of proximal tubule absorption

A previous model of the mechanisms of flow through epithelia was modified and extended to include hydrostatic and osmotic pressures in the cells and in the peritubular capillaries. The differential equations for flow and concentration in each region of the proximal tubule were derived. The equations were solved numerically by a finite difference method. The principal conclusions are: (i) Cell NaCl concentration remains essentially isotonic over the pressure variations considered; (ii) channel NaCl concentration varies only a few mosmol from isotonicity, and the hydrostatic and osmotic pressure differences across the cell wall are of the same order of magnitude; (iii) both reabsorbate osmolality and pressure-induced flow are relatively insensitive to the geometry of the system; (iv) a strong equilibrating mechanism exists in the sensitivity of the reabsorbate osmolality to luminal osmolality; this mechanism is far more significant than any other parameter change.     

Molecular regulation of NKCC2 in the thick ascending limb

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.     

Ion and water fluxes in the ileum of rats

Studies have been carried out on the movement of salt and water across the small intestine of the rat. Segments of the ileum of anesthetized rats have been perfused in vivo with unbuffered NaCl solutions or isotonic solutions of NaCl and mannitol. Kinetic analysis of movements of Na²⁴ and Cl³⁶ has permitted determination of the efflux and influx of Na and Cl. Net water absorption has been measured using hemoglobin as a reference substance. Water was found to move freely in response to gradients of osmotic pressure. Net water flux from isotonic solutions with varying NaCl concentration was directly dependent on net solute flux. The amount of water absorbed was equivalent to the amount required to maintain the absorbed solute at isotonic concentration. These results have been interpreted as indicating that water movement is a passive process depending on gradients of water activity and on the rate of absorption of solute. The effluxes of Na and Cl are linear functions of concentration in the lumen, but both ions are actively transported by the ileum according to the criterion of Ussing (Acta Physiol. Scand., 1949, 19, 43). The electrical potential difference between the lumen and plasma has been interpreted as a diffusion potential slightly modified by the excess of active Cl flux over active Na flux. The physical properties of the epithelial membrane indicate that it is equivalent to a membrane having negatively charged uniform right circular pores of 36 Å radius occupying 0.001 per cent of the surface area.     

Na+ Recirculation and Isosmotic Transport

The Na(+) recirculation theory for solute-coupled fluid absorption is an expansion of the local osmosis concept introduced by Curran and analyzed by Diamond & Bossert. Based on studies on small intestine the theory assumes that the observed recirculation of Na(+) serves regulation of the osmolarity of the absorbate. Mathematical modeling reproducing bioelectric and hydrosmotic properties of small intestine and proximal tubule, respectively, predicts a significant range of observations such as isosmotic transport, hyposmotic transport, solvent drag, anomalous solvent drag, the residual hydraulic permeability in proximal tubule of AQP1 (-/-) mice, and the inverse relationship between hydraulic permeability and the concentration difference needed to reverse transepithelial water flow. The model reproduces the volume responses of cells and lateral intercellular space (lis) following replacement of luminal NaCl by sucrose as well as the linear dependence of volume absorption on luminal NaCl concentration. Analysis of solvent drag on Na(+) in tight junctions provides explanation for the surprisingly high metabolic efficiency of Na(+) reabsorption. The model predicts and explains low metabolic efficiency in diluted external baths. Hyperosmolarity of lis is governed by the hydraulic permeability of the apical plasma membrane and tight junction with 6-7 mOsm in small intestine and < or = 1 mOsm in proximal tubule. Truly isosmotic transport demands a Na(+) recirculation of 50-70% in small intestine but might be barely measurable in proximal tubule. The model fails to reproduce a certain type of observations: The reduced volume absorption at transepithelial osmotic equilibrium in AQP1 knockout mice, and the stimulated water absorption by gallbladder in diluted external solutions. Thus, it indicates cellular regulation of apical Na(+) uptake, which is not included in the mathematical treatment.     

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.     

Ultrastructure of the tubular nephron of the lizard Podarcis (= Lacerta) Taurica

The proximal, intermediate, and distal convoluted tubules of the neprhon of Podarcis (= Lacerta) taurica were examined by electron microscopy. Proximal tubule cells have large, apical cytoplasmic protrusions and microvilli interpreted to function in urate secretion. Adjacent cells are bound apically by tight junctions and desmosomes but interdigitate in their basal region. This situation is repeated in the other tubules with significant differences in intercellular space width. The basal surfaces bear numerous cytoplasmic processes. The intermediate tubule has proximal and distal segments each with dark, ciliated, and light cells, the cuboidal dark cells with dense cytoplasm constituting the main bulk of the wall. As the cells of the proximal and distal segments resemble those of the proximal and distal convoluted tubules, respectively, the intermediate tubule is considered as a transition region. The ciliated cell body has two broad processes extending from the lumen, one to the basement membrane and one to a foot process of a light cell. The light cell is surrounded by dark and ciliated cells. It does not reach the lumen, but contacts the basement membrane through a process running below a ciliated cell to form a mushroom-shaped structure in tubule cross-section, the light cell process forming the stalk and a ciliated cell the cap. The cilia probably propel the glomerular filtrate towards the distal convoluted tubule. This latter tubule has initial, middle, and terminal zones, all nonciliated but with different lumen widths and cell shapes.     

KCl reabsorption by the lower malpighian tubule of rhodnius prolixus: inhibition by Cl(-) channel blockers and acetazolamide

Iono- and osmoregulation by the blood-feeding hemipteran Rhodnius prolixus involves co-ordinated actions of the upper and lower Malpighian tubules. The upper tubule secretes ions (Na(+), K(+), Cl(-)) and water, whereas the lower tubule reabsorbs K(+) and Cl(-) but not water. The extent of KCl reabsorption by the lower tubule in vitro was monitored by ion-selective microelectrode measurement of Cl(-) and/or K(+) concentration in droplets of fluid secreted by Malpighian tubules isolated under oil. An earlier study proposed that K(+) reabsorption involves an omeprazole-sensitive apical K(+)/H(+) ATPase and Ba(2+)-sensitive basolateral K(+) channels. This paper examines the effects acetazolamide and of compounds that inhibit chloride channels, Cl(-)/HCO(3)(-) exchangers and Na(+)/K(+)/2Cl(-) or K(+)/Cl(-) co-transporters. The results suggest that Cl(-) reabsorption is inhibited by acetazolamide and by Cl(-) channel blockers, including diphenylamine-2-carboxylate(DPC) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), but not by compounds that block Na(+)/K(+)/Cl(-) and K(+)/Cl(-) co-transporters. Measurements of transepithelial potential and basolateral membrane potential during changes in bathing saline chloride concentration indicate the presence of DPC- and NPPB-sensitive chloride channels in the basolateral membrane. A working hypothesis of ion movements during KCl reabsorption proposes that Cl(-) moves from lumen to cell through a stilbene-insensitive Cl(-)/HCO(3)(-) exchanger and then exits the cell through basolateral Cl(-) channels.     

Na, Cl, and Water Transport by Rat Colon

Segments of the colon of anesthetized rats have been perfused in vivo with isotonic NaCl solutions and isotonic mixtures of NaCl and mannitol. Unidirectional and net fluxes of Na and Cl and the net fluxes of water and mannitol have been measured. Net water transport was found to depend directly on the rate of net Na transport. There was no water absorption from these isotonic solutions in the absence of net solute transport, indicating that water transport in the colon is entirely a passive process. At all NaCl concentrations studied, the lumen was found to be electrically negative to the surface of the colon by 5 to 15 mv. Na fluxes both into and out of the lumen were linear functions of NaCl concentration in the lumen. Net Na absorption from lumen to plasma has been observed to take place against an electrochemical potential gradient indicating that Na is actively transported. This active Na transport has been interpreted in terms of a carrier model system. Cl transport has been found to be due almost entirely to passive diffusion.     

Real-time three-dimensional imaging of lipid signal transduction: apical membrane insertion of epithelial Na(+) channels

In the distal tubule, Na⁺ resorption is mediated by epithelial Na⁺ channels (ENaC). Hormones such as aldosterone, vasopressin, and insulin modulate ENaC membrane targeting, assembly, and/or kinetic activity, thereby regulating salt and water homeostasis. Insulin binds to a receptor on the basal membrane to initiate a signal transduction cascade that rapidly results in an increase in apical membrane ENaC. Current models of this signaling pathway envision diffusion of signaling intermediates from the basal to the apical membrane. This necessitates diffusion of several high-molecular-weight signaling elements across a three-dimensional space. Transduction of the insulin signal involves the phosphoinositide pathway, but how and where this lipid-based signaling pathway controls ENaC activity is not known. We used tagged channels, biosensor lipid probes, and intravital imaging to investigate the role of lipids in insulin-stimulated Na⁺ flux. Insulin-stimulated delivery of intracellular ENaC to apical membranes was concurrent with plasma membrane-limited changes in lipid composition. Notably, in response to insulin, phosphatidylinositol 3,4,5-trisphosphate (PIP₃) formed in the basolateral membrane, rapidly diffused within the bilayer, and crossed the tight junction to enter the apical membrane. This novel signaling pathway takes advantage of the fact that the lipids of the plasma membrane's inner leaflet are not constrained by the tight junction. Therefore, diffusion of PIP₃ as a signal transduction intermediate occurs within a planar surface, thus facilitating swift responses and confining and controlling the signaling pathway. phosphatidylinositol 3,4,5-trisphosphate; insulin-stimulated Na⁺ transport; metabolic syndrome; real-time confocal imaging     

Control and Modulation of Fluid Flow in the Rat Kidney

We have developed a mathematical model of the rat’s renal hemodynamics in the nephron level, and used that model to study flow control and signal transduction in the rat kidney. The model represents an afferent arteriole, glomerular filtration, and a segment of a short-loop nephron. The model afferent arteriole is myogenically active and represents smooth muscle membrane potential and electrical coupling. The myogenic mechanism is based on the assumption that the activity of nonselective cation channels is shifted by changes in transmural pressure, such that elevation in pressure induces vasoconstriction, which increases resistance to blood flow. From the afferent arteriole’s fluid delivery output, glomerular filtration rate is computed, based on conservation of plasma and plasma protein. Chloride concentration is then computed along the renal tubule based on solute conservation that represents water reabsorption along the proximal tubule and the water-permeable segment of the descending limb, and chloride fluxes driven by passive diffusion and active transport. The model’s autoregulatory response is predicted to maintain stable renal blood flow within a physiologic range of blood pressure values. Power spectra associated with time series predicted by the model reveal a prominent fundamental peak at ～165 mHz arising from the afferent arteriole’s spontaneous vasomotion. Periodic external forcings interact with vasomotion to introduce heterodynes into the power spectra, significantly increasing their complexity.     

Solvent drag measurement of transcellular and basolateral membrane NaCl reflection coefficient in kidney proximal tubule

The NaCl reflection coefficient in proximal tubule has important implications for the mechanisms of near isosmotic volume reabsorption. A new fluorescence method was developed and applied to measure the transepithelial (sigma NaClTE) and basolateral membrane (sigma NaClcl) NaCl reflection coefficients in the isolated proximal straight tubule from rabbit kidney. For sigma NaClTE measurement, tubules were perfused with buffers containing 0 Cl, the Cl-sensitive fluorescent indicator 6-methoxy-N-[3-sulfopropyl] quinolinium and a Cl-insensitive indicator fluorescein sulfonate, and bathed in buffers of differing cryoscopic osmolalities containing NaCl. The transepithelial Cl gradient along the length of the tubule was measured in the steady state by a quantitative ratio imaging technique. A mathematical model based on the Kedem-Katchalsky equations was developed to calculate the axial profile of [Cl] from tubule geometry, lumen flow, water (Pf) and NaCl (PNaCl) permeabilities, and sigma NaClTE. A fit of experimental results to the model gave PNaCl = (2.25 +/- 0.2) x 10(-5) cm/s and sigma NaClTE = 0.98 +/- 0.03 at 23 degrees C. For measurement of sigma NaClbl, tubule cells were loaded with SPQ in the absence of Cl. NaCl solvent drag was measured from the time course of NaCl influx in response to rapid (less than 1 s) Cl addition to the bath solution. With bath-to-cell cryoscopic osmotic gradients of 0, -60, and +30 mosmol, initial Cl influx was 1.23, 1.10, and 1.25 mM/s; a fit to a mathematical model gave sigma NaClbl = 0.97 +/- 0.04. These results indicate absence of NaCl solvent drag in rabbit proximal tubule. The implications of these findings for water and NaCl movement in proximal tubule are evaluated.     

Isosmotic volume reabsorption in rat proximal tubule

A theoretical model incorporation both active and passive forces has been developed for fluid reabsorption from split oil droplets in rat intermediate and late proximal tubule. Of necessity, simplifying assumptions have been introduced; we have assumed that the epithelium can be treated as a single membrane and that the membrane "effective" HCO3 permeability is near zero. Based on this model with its underlying assumptions, the following conclusions are drawn. Regardless of the presence or absence of active NaCl transport, fluid reabsorption from the split oil droplet is isosmotic. The reabsorbate osmolarity can be affected by changes in tubular permeability parameters and applied forces but is not readily altered from an osmolarity essentially equal to that of plasma. In a split droplet, isosmotic flow need not be a special consequence of active Na transport, is not the result of a particular set of permeability properties, and is not merely a trivial consequence of a very high hydraulic conductivity; isosmotic flow can be obtained with hydraulic conductivity nearly an order of magnitude lower than that previously measured in the rat proximal convoluted tubule. Isosmotic reabsorption is, in part, the result of the interdependence of salt and water flows, their changing in parallel, and thus their ratio, the reabsorbate concentration being relatively invariant. Active NaCl transport can cause osmotic water flow by reducing the luminal fluid osmolarity. In the presence of passive forces the luminal fluid can be hypertonic to plasma, and active NaCl transport can still exert its osmotic effect on volume flow. There are two passive forces for volume flow: the Cl gradient and the difference in effective osmotic pressure; they have an approximately equivalent effect on volume flow. Experimentally, we have measured volume changes in a droplet made hyperosmotic by the addition of 50 mM NaCl; the experimental results are predicted reasonably well by our theoretical model.     

Cobalt-dependent stimulation of sodium transport in the amphibian skin and nephron

One to ten millimolar CoCl2, when applied to the outer surface of the apical membrane of the frog skin (Rana temporaria), reversibly increased the potential differences and short-circuit current. These observations suggest that Co2+ may control the gating system of the sodium channel. In the kidney of the newt (Triturus vulgaris), proximal reabsorption is increased under the influence of 0.5 mM CoCl2 injected into the lumen. When CoCl2 (0.5 mM) was injected into the lumen of the distal tubule of the newt kidney, Na+ and Cl- reabsorption was stimulated simultaneously Ca2+ transport was inhibited. The data obtained suggested that Co2+ may affect the state of the sodium and calcium channels of nonexcitable membranes of amphibia, and thus may be involved in the regulation of the function of the renal tubules.     

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

An electrolyte model of an epithelium (a cell and a tight junction 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 cross-terms 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.     

Axial heterogeneity and filtered-load dependence of proximal bicarbonate reabsorption.

A theoretical model was developed to examine the role of physical and chemical factors in the control of bicarbonate reabsorption in the renal proximal tubule. Included in the model were axial and radial variations in the concentrations of HCO3-, CO2 and related chemical species in the tubule lumen and epithelial cells. Relations between these concentrations and the solute fluxes across the brush border and basolateral membranes were also included, as were reaction rate and equilibrium expressions to describe the various buffering processes in the lumen and cells. The two most critical membrane parameters, the rate constant for H+ secretion at the brush border and the effective permeability of HCO3- at the basolateral membrane, were evaluated by comparing model predictions with available free-flow micropuncture data in the rat. It was found that the experimental observations could be explained only by decreasing one or both of these membrane parameters with axial position, suggesting a progressive decrease in HCO3- reabsorptive capacity along the tubule. For single nephron filtered loads of HCO3- up to about 1,400 pmol/min, absolute bicarbonate reabsorption was predicted to increase nearly in proportion to filtered load, whereas it was calculated to be relatively constant at higher filtered loads, irrespective of how filtered load was assumed to be varied. These predictions are in excellent agreement with most of the available micropuncture data in rats, as is the prediction that HCO3- reabsorption should change in parallel with CO2 partial pressure in the filtrate, at a given filtered load of HCO3-. Certain discrepancies between the model predictions and experimental observations are evident at very high filtered loads, and the implications of these are discussed in terms of possible adaptive responses of the tubule.     

A Model of Peritubular Capillary Control of Isotonic Fluid Reabsorption by the Renal Proximal Tubule

A mathematical model of peritubular transcapillary fluid exchange has been developed to investigate the role of the peritubular environment in the regulation of net isotonic fluid transport across the mammalian renal proximal tubule. The model, derived from conservation of mass and the Starling transcapillary driving forces, has been used to examine the quantitative effects on proximal reabsorption of changes in efferent arteriolar protein concentration and plasma flow rate. Under normal physiological conditions, relatively small perturbations in protein concentration are predicted to influence reabsorption more than even large variations in plasma flow, a prediction in close accord with recent experimental observations in the rat and dog. Changes either in protein concentration or plasma flow have their most pronounced effects when the opposing transcapillary hydrostatic and osmotic pressure differences are closest to equilibrium. Comparison of these theoretical results with variations in reabsorption observed in micropuncture studies makes it possible to place upper and lower bounds on the difference between interstitial oncotic and hydrostatic pressures in the renal cortex of the rat.     

A parallel path model forNecturus proximal tubule

Summary A parallel path model based on the principles of nonequilibrium thermodynamics was developed for theNecturus proximal tubule. The cellular path was represented as a luminal membrane followed by an irreversible active NaCl transport system in the peritubular barrier. The shunt pathway was described as three coarse barriers in series: tight junction, lateral intercellular spaces, and basement membrane with connective tissue. Volume and solute flows were predicted by the model equations as a function of applied electric current. Variations of the model parameters revealed the quantitative importance of the shunt path properties and the relative insensitivity of epithelial transport to changes in most cell parameters. Circulation of electric current and solute within the epithelium were shown to significantly influence the bahavior of the tubule in the presence of an electric field. Values for all transport parameters of the shunt path and epithelium were calculated and compared with available experimental evidence. Volume flow and electric currents predicted by the model compared favorably with experimental observations.     

Chemical kinetic and diffusional limitations on bicarbonate reabsorption by the proximal tubule.

It is accepted that bicarbonate reabsorption in the proximal tubule is mediated by H+ secretion, but several aspects of this process have remained controversial. To examine some of these issues, we have developed a model that allows for spatial variations in the concentrations of CO2, HCO3-, and H2CO3 within the tubule lumen and cell cytoplasm, passive transport of these substances across cell membranes, carbonic anhydrase-catalyzed interconversion of HCO3- and CO2 within the cell and at the luminal membrane surface, and the corresponding uncatalyzed reactions in lumen and cell. Most of the required kinetic and transport parameters were estimated from physicochemical data in the literature, whereas intracellular pH and HCO3- permeability at the basal cell membrane, found to be the most significant parameters under normal conditions, were adjusted to yield reabsorption rates of "total CO2" (tCO2, the sum of CO2, HCO3- and H2CO3) comparable to measured values in the rat. Our results suggest that for normal carbonic anhydrase activity, almost all tCO2 leaves the lumen as CO2, yet the transepithelial differences in CO2 partial pressure does not exceed approximately 2 mm Hg. Electrochemical potential gradients favor substantial passive backleak of HCO3- from cell to lumen. Gradients in CO2 partial pressure remain small during simulated inhibition of carbonic anhydrase, with approximately 70% of tCO2 leaving the lumen as H2CO3 in this case, and the remainder as CO2. Predicted tCO2 reabsorption rates for carbonic anhydrase inhibition are approximately of normal, in good agreement with recent measurements in the rat, indicating that the concept of "carbonic acid recycling" is viable.