Cell osmotic water permeability of isolated rabbit proximal straight tubules

'What controls aqueous humour outflow resistance?'

The bulk of aqueous humour outflow resistance is generated in or near the inner wall endothelium of Schlemm's canal in normal eyes, and probably also in glaucomatous eyes. Fluid flow through this region is controlled by the location of the giant vacuoles and pores found in cells of the endothelium of Schlemm's canal, but the flow resistance itself is more likely generated either in the extracellular matrix of the juxtacanalicular connective tissue or the basement membrane of Schlemm's canal. Future studies utilizing in vitro perfusion studies of inner wall endothelial cells may give insights into the process by which vacuoles and pores form in this unique endothelium and why inner wall pore density is greatly reduced in glaucoma.

Mathematical models of renal fluid and electrolyte transport: acknowledging our uncertainty

Mathematical models of renal tubular function, with detail at the cellular level, have been developed for most nephron segments, and these have generally been successful at capturing the overall bookkeeping of solute and water transport. Nevertheless, considerable uncertainty remains about important transport events along the nephron. The examples presented include the role of proximal tubule tight junctions in water transport and in regulation of Na(+) transport, the mechanism by which axial flow in proximal tubule modulates solute reabsorption, the effect of formate on proximal Cl(-) transport, the assessment of potassium transport along collecting duct segments inaccessible to micropuncture, the assignment of pathways for peritubular Cl(-) exit in outer medullary collecting duct, and the interaction of carbonic anhydrase-sensitive and -insensitive pathways for base exit from inner medullary collecting duct. Some of these uncertainties have had intense experimental interest well before they were cast as modeling problems. Indeed, many of the renal tubular models have been developed based on data acquired over two or three decades. Nevertheless, some uncertainties have been delineated as the result of model exploration and represent communications from the modelers back to the experimental community that certain issues should not be considered closed. With respect to model refinement, incorporating more biophysical detail about individual transporters will certainly enhance model reliability, but ultimate confidence in tubular models will still be contingent on experimental development of critical information at the tubular level.

NaCl reflection coefficients in proximal tubule apical and basolateral membrane vesicles. Measurement by induced osmosis and solvent drag

Two independent methods, induced osmosis and solvent drag, were used to determine the reflection coefficients for NaCl (sigma NaCl) in brush border and basolateral membrane vesicles isolated from rabbit proximal tubule. In the induced osmosis method, vesicles loaded with sucrose were subjected to varying inward NaCl gradients in a stopped-flow apparatus. sigma NaCl was determined from the osmolality of the NaCl solution required to cause no initial osmotic water flux as measured by light scattering (null point). By this method sigma NaCl was greater than 0.92 for both apical and basolateral membranes with best estimates of 1.0. sigma NaCl was determined by the solvent drag method using the Cl-sensitive fluorescent indicator, 6-methoxy-N-[3-sulfopropyl]quinolinium (SPQ), to detect the drag of Cl into vesicles by inward osmotic water movement caused by an outward osmotic gradient. sigma NaCl was determined by comparing experimental data with theoretical curves generated using the coupled flux equations of Kedem and Katchalsky. By this method we found that sigma NaCl was greater than 0.96 for apical and greater than 0.98 for basolateral membrane vesicles, with best estimates of 1.0 for both membranes. These results demonstrate that sigma NaCl for proximal tubule apical and basolateral membranes are near unity. Taken together with previous results, these data suggest that proximal tubule water channels are long narrow pores that exclude NaCl.

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.

The continuous measurement of tubular volume changes in response to step changes in contraluminal osmolality

A new method to measure time dependent (t) volume (V) changes in proximal straight tubules (PST) is described. V is calculated from diameter (d) measurements for which a video camera and an integrating circuit are used. A tubular image of high optical contrast is recorded with the TV camera such that the scan lines run crosswise to the tubule. The video signal is analyzed by a special processor which adds 225 tubular diameters of each TV frame and feeds this analog signal to a pen recorder. The fractional error in d measurements is 10(-3). Diameter changes of less than 0.05 micron can be detected, as compared to the usual error of a single measurement of about 0.4 micron. Pcbos, the osmotic water permeability of the contraluminal cell membrane was measured by setting up osmotic steps across it in less than 0.1 s and following the ensuing delta d/delta t. The time delay between solution change and the linear part of the osmotic response was 0.51 +/- 0.05 s. Pcbos was found to be 50.4 (+/- 8.7) X 10(-4) cm3.cm-2 of basement membrane area .s-1.osmolar-1.

Nonequilibrium thermodynamic model of the rat proximal tubule epithelium

The rat proximal tubule epithelium is represented as well-stirred, compliant cellular and paracellular compartments bounded by mucosal and serosal bathing solutions. With a uniform pCO2 throughout the epithelium, the model variables include the concentrations of Na, K, Cl, HCO3, H2PO4, HPO4, and H, as well as hydrostatic pressure and electrical potential. Except for a metabolically driven Na-K exchanger at the basolateral cell membrane, all membrane transport within the epithelium is passive and is represented by the linear equations of nonequilibrium thermodynamics. In particular, this includes the cotransport of Na-Cl and Na-H2PO4 and countertransport of Na-H at the apical cell membrane. Experimental constraints on the choice of ionic conductivities are satisfied by allowing K-Cl cotransport at the basolateral membrane. The model equations include those for mass balance of the nonreacting species, as well as chemical equilibrium for the acidification reactions. Time-dependent terms are retained to permit the study of transient phenomena. In the steady state the energy dissipation is computed and verified equal to the sum of input from the Na-K exchanger plus the Gibbs free energy of mass addition to the system. The parameter dependence of coupled water transport is studied and shown to be consistent with the predictions of previous analytical models of the lateral intercellular space. Water transport in the presence of an end-proximal (HCO3-depleted) luminal solution is investigated. Here the lower permeability and higher reflection coefficient of HCO3 enhance net sodium and water transport. Due to enhanced flux across the tight junction, this process may permit proximal tubule Na transport to proceed with diminished energy dissipation.

Lack of effect of peritubular protein on passive NaCl transport in the rabbit proximal tubule

The effect of peritubular protein removal on passive NaCl transport was examined in the isolated rabbit proximal convoluted tubule (PCT). Three modes of passive NaCl transport were tested: (a) paracellular backflux of NaCl, (b) convective flow of NaCl through junctional complexes, and (c) anion gradient-dependent NaCl transport. The effect of peritubular protein removal on the paracellular permeability to NaCl was examined using transepithelial specific resistance. Eight PCT were perfused with ultrafiltrate (UF) and bathed in either serum or UF. Transepithelial specific resistance averaged 14.5 +/- 1.9 in the presence and 13.7 +/- 1.7 omega cm2 in the absence of peritubular protein. The effect of peritubular protein removal on the convective flow of a NaCl solution across functional complexes was examined in the absence of active transport by using colloid osmotic pressure (COP) gradients. 12 PCT were perfused with simple salt solutions in Donnan equilibrium with and without protein at 20 degrees C. A COP gradient of 60.1 and -60.1 mmHg drove only 0.06 and -0.23 nl/min, respectively. These values are approximately 10% of the value predicted for an effect of peritubular protein on NaCl solution flow (1.98 nl/min) and are approximately equal to the value predicted for pure water equilibration for the small osmotic pressure difference between solutions in Donnan equilibrium (0.17-0.18 nl/min). The effect of peritubular protein removal on the passive absorption of NaCl driven by anion concentration gradients was examined in seven PCT perfused with a high chloride solution simulating late proximal tubular fluid and bathed in either serum or UF at 20 degrees C. Volume absorption averaged 0.34 +/- 0.20 in the presence and 0.39 +/- 0.20 nl/mm min in the absence of peritubular protein. In conclusion, peritubular protein removal did not significantly affect any of the three distinct modes of passive NaCl transport tested. The lack of effect of peritubular protein removal on passive paracellular NaCl transport suggests that protein modulates an active transcellular NaCl transport process.

Fluid reabsorption by Necturus proximal tubule perfused with solutions of normal and reduced osmolarity

Fluid absorption in Necturus proximal tubule was studied when the kidneys were perfused with solutions of different osmolarities. The rate of fluid absorption was inversely proportional to the perfusion fluid osmolarity, while Na uptake remained constant. No difference was detected between the collected and injected luminal fluid, i.e. reabsorption was isotonic at normal and reduced osmolarities. The transtubular osmotic permeability remained fairly constant under the different perfusion osmolarities. Using our experimental results to test various models based on osmotic equilibration across the tubule wall we show that none of these provides an adequate mechanism for fluid absorption in this epithelium.