Influence of luminal diameter and flow velocity on the isotonic fluid absorption and 36Cl permeability of the proximal convolution of the rat kidney

Reflection coefficient and permeability of urea in the proximal convolution of the rat kidney : An application of non-equilibrium thermodynamics for a multicomponent system with active transport

The transport theory of Kedem and Katchalsky which was derived for passive transport in a two-compartment system is generalized for a multicomponent system with active transport, so that it can be applied to more complicated biological membranes.Equations have been derived to describe the transport of urea through the proximal convolution of the rat kidney and the permeability and the reflection coefficient have been determined. The permeability coefficient[Formula: see text] measured with the microperfusion and stop flow microperfusion methods, was found to be 6.0 and 5.2×10(-5) mm(2)/sec, respectively.The reflection coefficient (σ) was determined in a stationary state situation and found to be 0.68. Earlier free flow micropuncture results together with theP u andσ u of this study indicate that 50% of the filtered urea is reabsorbed proximally and that approximately half of this amount is reabsorbed by solvent drag and the rest by diffusion.In the Appendix, a theoretical treatment of nonelectrolyte transport in a multicomponent system with active transport is given.

Axial flow modulates proximal tubule NHE3 and H-ATPase activities by changing microvillus bending moments

We have previously demonstrated that mouse proximal tubules in vitro respond to changes in luminal flow with proportional changes in Na+ absorption (Du Z, Duan Y, Yan Q, Weinstein AM, Weinbaum S, and Wang T. Proc Natl Acad Sci USA 101: 13068–13073, 2004). It was hypothesized that brush-border microvilli function as a sensor to detect and amplify luminal hydrodynamic forces and transmit them to the actin cytoskeleton. In the present study we examine whether 1) flow-dependent HCO3− transport is proportional to flow-dependent variations in microvillous torque (bending moment); 2) both luminal membrane Na+/H+ exchange (NHE3) and H+-ATPase activity are modulated by axial flow; and 3) paracellular permeabilities contribute to the flux perturbations. HCO3− absorption is examined by microperfusion of mouse S2 proximal tubules in vitro, with varying perfusion rates, and in the presence of the Na/H-exchange inhibitor EIPA, the H+-ATPase inhibitor bafilomycin, and the actin cytoskeleton inhibitor cytochalasin D. Paracellular permeability changes are assessed with measurements of epithelial HCO3− permeability and transepithelial potential difference (PD). It is found that 1) an increase in perfusion rate enhances HCO3− absorption and microvillous torque, and the fractional changes of each are nearly identical; 2) inhibition of NHE3 by EIPA, or H+-ATPase by bafilomycin, produced only partial inhibition of flow-stimulated bicarbonate transport; 3) disruption of the actin cytoskeleton by cytochalasin D blocked the increment of HCO3− absorption by high flow; and 4) HCO3− permeability and transepithelial PD are not modulated by flow. We conclude that flow-dependent modulation of proximal tubule HCO3− reabsorption is due to changes in both NHE3 and H+-ATPase activity within the luminal cell membrane and this requires an intact actin cytoskeleton. Paracellular permeability changes do not contribute to this flow dependence. Perfusion-absorption balance in the proximal tubule is a direct effect of flow-induced torque on brush-border microvilli to regulate luminal cell membrane transporter activity.

Digital image capture and analysis for split-drop micropuncture

A method for automated image capture and analysis of shrinking drop sequences is described. The procedure can be managed by a single operator and allows estimates of proximal tubule volume flux to be calculated during the experiment. Speed of analysis is increased considerably (mean time 1.5 min per sequence) compared with previous methods. The potential exists for measurement of droplets in tubules that follow a tortuous path in the horizontal plane and further increases in capture and analysis rate would allow estimation of volume flux under non-steady state conditions. Film processing is eliminated and the removal of operator errors, bias and fatigue associated with manual measurement, coupled with the greatly reduced time required for analysis of sequences makes the shrinking drop method much more reliable and attractive.

Active and passive components of chloride transport in the rat proximal convoluted tubule

Rat proximal convoluted tubules were perfused in vivo to examine the active and passive components of chloride absorption. Chloride flux was a linear function of the transepithelial electrochemical driving force, yielding a permeability coefficient of 20.6 X 10(-5) cm/s. In the absence of an electrochemical driving force, chloride absorption persisted at the rate of 131 peq/mm X min, thus demonstrating active absorption of chloride. Addition of luminal cyanide to tubules absorbing chloride inhibited net chloride absorption. In tubules perfused with a low luminal chloride concentration in which there was net chloride secretion, addition of luminal cyanide increased the magnitude of net chloride secretion. These studies demonstrate that transepithelial chloride transport involves two components: a passive paracellular flux and an active transcellular flux. Cyanide affects net chloride flux by inhibiting active transcellular chloride absorption.

Ionic requirements of proximal tubular fluid reabsorption flow dependence of fluid transport

The effects of changes in luminal flow rate on fluid absorption in rat renal proximal convoluted tubules were studied by continuous luminal and peritubular microperfusion methods. Luminal flow rate was varied over a range from 5 to 45 nl . min-1, and the effects of transepithelial chloride and bicarbonate gradients were tested. Fluid absorption across the proximal convoluted tubule increased with luminal flow rate in the absence of luminal bicarbonate and organic solutes but in the presence of transepithelial chloride and bicarbonate gradients and active sodium transport. Augmenting perfusion rate from 5 to 45 nl . min-1 resulted in an increase of volume absorption from 0.49 to 3.37 nl . min-1 per millimeter length of tubule-1. The chloride concentration change in the collected perfusate decreased from 5.9 to 2.6 mEq . liter-1 . mm length tubule-1 over the same perfusion range. Thus, tubular chloride concentration rises with perfusion rate such that the steepest transepithelial chloride gradients are maintained at the highest flow rates. Flow dependence continued, albeit at reduced rate, in the absence of active sodium transport (cyanide perfusion) but in the presence of chloride and bicarbonate gradients. Flow dependence disappeared in the absence of both active sodium transport and transepithelial anion gradients. Luminal and peritubular perfusion experiments with symmetrical bicarbonate-free solutions that contained only phosphate buffer showed that even under those conditions fluid movement driven by cyanide-sensitive active transport increased with flow rate.

A mathematical model of fluid transport in the kidney

A mathematical model of the rat kidney is developed from glomerular and tubular submodels. It is assumed that all nephrons are identical, that the hydraulic pressure in the tubules obeys Hagen-Poiseuille's law, that the rate of fluid reabsorption depends on the flow rate of tubular fluid, and that the tubules are distensible. The independent variables of the model are selected to comply with experimental measurements in the hydropenic rat. The model is used to evaluate the mechanism of glomerulotubular balance: changing the mean ultrafiltration pressure to the glomerular capillaries has a substantial influence on glomerular filtration rate (GFR). A change in the rate of fluid reabsorption in the proximal tubules has a strong influence on GFR notwithstanding that the change in GFR is smaller than that in the rate of fluid reabsorption. The calculated values for the hydraulic pressure profile in the tubular system and the interstitial pressure during ureteral obstruction are in close agreement with experimental measurements. Increasing the arterial haematocrit above normal causes a substantial decrease in GFR, whilst reducing it below normal has only a small effect on GRF.

Dependency of proximal tubular fluid transport on the load of glomerular filtrate

In hydropenic rats, the reabsorption of glomerular filtrate by the proximal convoluted tubules was measured before and after reduction of its intratubular flow rate. Three different protocols were used. (1) In 26 tubules (14 rats), nephron glomerular filtration rate (SNGFR) was varied from 37.2 +/- 7.3 to 20.4 +/- 7.1 nl/min by microperfusing their loops of Henle at 0 to 5 nl/min and 40 nl/min, respectively. This 43% reduction of SNGFR was followed by a 36.0 +/- 23.3% reduction of volume reabsorption rate (P less than 0.001). Between both parameters a linear regression line can be calculated, which is given by y = 0.92 chi + 0.0017. (2) In 17 tubules (14 rats), SNGFR was altered again by feedback from 46.0 +/- 9.7 to 28.8 +/- 9.3 nl/min. The volume resorption from the first half of the proximal convoluted tubule was compared with the reabsorption in its late proximal segments, which were microperfused with proximal tubular fluid at a rate of 20 nl/min. The 36.8% reduction of SNGFR was followed by only a 28.2% reduction of volume reabsorption rate in the first half of the tubule. In the microperfused segments, however, reabsorption remained unaltered. (3) In 29 tubules (21 rats), at the midpoint of proximal convolutions, some of the tubule fluid was removed by a suction pump, and volume reabsorption rate in the late segments was compared with that in the early parts of this tubule, when SNGFR remained stable. The reduction of intratubular flow from 27.7 +/- 8.5 to 14.7 +/- 5.8 nl/min, which is 53% of control, were followed by a reduction of volume reabsorption rate in the late segment to 60.6% control. Between both parameters a regression line was calculated, which is given by y = 0.76 chi +/- 0.01. We conclude that the rate of volume reabsorption by the proximal tubule depends on its intratubular load of glomerular filtrate and, further, that this dependency accounts predominantly for the maintenance of glomerular tubular balance under conditions of hydropenia.

A functional model of the rat kidney

A mathematical model of the nephron was developed by writing a set of material balance equations for the flow of urea, salt and water along the foregoing study and are taken here as a basis, in particular the model configuration of the collecting duct system. The stimulation of the model equatentration profiles which at the ends of the several tubular sections were consistent with the values observed in experimental investigations.e medullary interstitial solute concentration profiles are taken to increase linearly in outer and inner zone. The several transeptithelial fluxes are driven by diffusion, osmosis, solvent drag and active transport. The development of osmotic gradient in the inner medulla is taken here to be caused by active secretion of salt into the descending LImb of Henle's loop. The parameters in the flux equations for all parts of the nephron and the concentration values at the end of each tubular section are determined by collecting and averaging the values given in literature and by extrapolating the measurement data. The simulation of the model equations with these averaged parameters resulted in concentration profiles which at the ends of the several tubular sections were consistent with the values observed in experimental investigations.

Electrochemical forces for chloride transport in the proximal tubules of the rat kidney

The electrochemical forces for chloride transport in the proximal tubule of the rat kidney were studied using micropuncture techniques. Electrical transmembrane potentials were recorded in randomly punctured tubules with Ling-Gerhard electrodes. Chloride activities in the luminal, cellular and interstitial compartments were measured with ion selective micro-electrodes. Electrical potential measurements between cell to interstitium and lumen to interstitium were -72.1 +/- 2.6 mV and +0.5 +/- 1.4 mV (mean +/- S.D.) respectively. The calculated chloride concentrations for lumen, cell and interstitium were 133.0 +/- 10.3 mM, 8.5 +/- 1.0 mM and 99.1 +/- 3.2 mM (mean +/- S.D.) respectively. The net electrochemical forces, qualitatively, offer a passive chloride ion pathway through the tubular wall and a chloride equilibrium over the luminal membrane seems to exist.

Single nephron filtration, luminal flow and tubular fluid reabsorption along the proximal convolution and the pars recta of the rat kidney as influenced by luminal pressure changes

Intratubular pressures were measured in free flow and after blockade of tubular flow at different distances from the glomerulum in the kidney of Wistar rats. Free flow pressure was ffp = 13.3 +/- 2.5 Torr and stop flow pressure sfp = 41.7 +/- 3.8 Torr. With increasing distance of the blockade from the glomerulum the intratubular pressure decreased being 22.4 +/- 2.1 Torr, when the tubule was blocked at the end of the pars recta. In a second series single nephron filtration rate (gfr) and late proximal flow rates (V) were measured at different intratubular pressures. Free flow gfrf was 26.5 +/- 5.9 nl/min and Vf = 14.7 +/- 4.0 nl/min. The difference of these flow rates divided by tubular length results in a local reabsorption rate of C = 2.9 +/- 0.9 nl/min-mm in the proximal convolution. In the pars recta local reabsorbtion rate was 1.0 +/- 0.3 nl/min-mm. In the proximal convolution C increased with increasing intratubular pressure: deltaC/deltaitp = (2.7 +/- 1.2)-10(-2) nl/min-mm-Torr. Filtration was in disequilibrium in these animals under all conditions examined, hydraulic filtration conductance was K = 1.2 +/- 0.4 nl/min-Torr. Modified methods have been used for intratubular pressure and for flow rate measurements in order to reduce experimental procedure. It is shown, that fractional reabsorption, calculated on the basis of pressure measurements, is a good approximation to results usually obtained by inulin measurements.

Effect of perfusion rate on the fluxes of water, sodium, chloride and urea across the proximal convoluted tubule

Studies were undertaken to examine the mechanism whereby changes in intraluminal flow rates after reabsorption in the isolated perfused proximal tubule of the rabbit. All protocols employed the technique of in vitro perfusion of isolated segments of the proximal convoluted tubule. Stepwise elimination of d-glucose and l-alanine from an artifical perfusate stimulating ultrafiltrate decreased the unidirectional flux of sodium, transtubular potential difference, and net water absorption. Using isosmolal ultrafiltrate as the perfusate, net fluid reabsorption and the unidirectional lumen-to-bath flux of sodium and chloride decreased with a decrease in flow rate below 11 nl/min, but neither net fluid reabsorption nor the unidirectional fluxes of sodium and chloride increased further as the perfusion rate was increased above 11 nl/min. The unidirectional flux of 14C-urea was not affected by changes of perfusion rate from 1.6 to 44 nl/min. The dependence of net fluid reabsorption and unidirectional fluxes of sodium and chloride on flow rate per se, and not on intraluminal hydrostatic pressure or geometry, was established by demonstrating their decrease despite a rise in intraluminal pressure and inside diameter produced by counterpressure at the collecting end of the tubule, while flow was decreased. Ouabain decreased net fluid reabsorption to near zero at all flow rates, but ouabain had no effect on the flow-dependency of unidirectional sodium anf sodium was eliminated with a decrease in bicarbonate concentration and removal of d-glucose and l-alanine from the perfusate. Thus, the present studies demonstrate that net water and unidirectional sodium and chloride fluxes are flow-dependent. At flow rates somewhere below 11 nl/min, unidirectional fluxes decreased with decreasing perfusion rates; however, at perfusion rates greater than 11 nl/min, there was no further effect of perfusion rate on either net water absorption or the unidirectional fluxes of sodium or chloride. These effects may be partly mediated through the flow-dependent changes in the intraluminal concentration of bicarbonate, d-glucose, and 1-alanine.

Effect of intraluminal bicarbonate and chloride on fluid absorption by the rat renal proximal tubule

In order to study mechanisms of fluid transport in the rat renal proximal convoluted tubule, the effects of large variations in intraluminal HCO3- and Cl- concentrations were measured by microperfusion techniques. No differences in rates of fluid transport were found when intraluminal HCO3- was varied from 4 to 30 mEq/liter and Cl- from 146 to 120 mEq/liter. Inhibition of H+ secretion with benzolamide had no effect on fluid absorption when little or no HCO3- was present in the lumen, but did reduce fluid transport when 25 mEq of HCO3- was present. If several different mechanisms are responsible for proximal fluid transport, such as nonelectrogenic active NaHCO3 transport, passive chloride diffusion and active sodium transport linked to H+ secretion, the above observations imply that they all operate at approximately the same rate, since the dominant driving force would have been different with each perfusion solution. The data seem more compatible with the view that active sodium transport is the major driving force for fluid absorption in the proximal tubule, that this is not linked to H+ secretion and that anions modify the rate of absorption only to the degree that they are able to accompany sodium across the epithelium. An additional observation was that absorption of isotonic NaCl was very slow in short segments of tubule, as compared to HCO3--containing perfusion solutions. Although the mechanism is uncertain, these data suggest that a finite amount of intraluminal HCO3- is necessary for optimal proximal fluid transport.

The hydraulic conductivity of the rat proximal tubular wall determined with colloidal solutions

The hydraulic conductivity of the rat proximal tubular wall was determined using colloidal solutions perfused in short (50--200 mum) (SMP) or long (90--200 mum) (LMP) proximal tubular segments. In SMP human serum albumin (HSA) or polyvinylpyrrolidone (PVP) was added to raffinose solutions. A Lp of 0.019 nl-min-1-mm-1-mm Hg-1 was found when high colloid concentrations were used while values of 0.055--0.092 were found when low colloid concentrations were used. In other experiments, the Lp was determined by perfusing short tubular segments with pure raffinose solutions. A value of 0.015 nl-min-1-mm-1-mm Hg-1 was found. This is twice the value found when raffinose solutions were perfused through long tubular segments and it is concluded that the short microperfusion technique overestimates Lp with a factor of two. When microperfusions of long tubular segments were conducted, PVP was added to an equilibrium solution consisting of NaCl (110 mM) and raffinose (80 mM). Lp was found to be 0.018--0.021 when high colloid concentrations were used, while a value of 0.029 was found when a low colloid concentration was used. As found in both SMP and LMP a decrease in Lp's with increasing colloid concentrations indicates that a significant influence of radial concentration differences is highly probable. It is therefore suggested that the highest Lp derived when using the lowest colloid concentrations represents the best estimate. With this Lp value (0.03--0.05 nl-min-1-mm-1-mm Hg-1) and the existing transtubular hydrostatic and oncotic pressure difference it can be calculated that these passive forces might constitute the driving force for 1/3 of the fluid reabsorbed in the proximal tubule.

Control of proximal tubule fluid reabsorption in experimental glomerulonephritis

We have recently shown that in the early autologous phase of nephrotoxic serum nephritis (NSN) single nephron glomerular filtration rate is unchanged from values in normal hydropenic control rats, but that single nephron filtration fraction and efferent arteriolar oncotic pressure (piE) are reduced because of a marked reduction in the glomerular capillary ultrafiltration coefficient. The present study was undertaken to examine the influence of this decline in piE as well as the other known determinants of peritubular capillary fluid exchange on absolute proximal fluid reabsorption (APR) in NSN. The findings indicate that APR and proximal fractional reabsorption are reduced significantly in NSN, relative to values in a separate group of age and weight-matched normal hydropenic control rats studied concurrently. In addition to the measured decline in piE, efferent arteriolar plasma flow (Qe) and peritubular capillary hydraulic pressure (Pc) were found to increase significantly, while interstitial oncotic pressure, estimated from hilar lymph, was not significantly different from values in control rats. Using a mathematical model of peritubular capillary fluid uptake we found that, assuming that the capillary permeability-surface area product and interstitial hydraulic pressure are unchanged in NSN, the observed changes in piE and Pc are sufficient to offset the effect of the increase in QE, yielding a calculated reduction in APR of approximately 4 nl/min, in excellent agreement with the observed mean decline of 4.1 nl/min. These findings suggest that control of APR in NSN is mediated by the same factors that regulate APR under normal physiological conditions, namely, the imbalance of forces governing peritubular capillary uptake of isotonic reabsorbate.

Variation of proximal tubular reabsorptive capacity by volume expansion and aortic constriction during constancy of peritubular capillary protein concentration in rat kidney

The present study was undertaken in anesthetized rats to examine the effect of aortic constriction and volume expansion on proximal tubular sodium reabsorption during constancy of flow and fluid composition in the peritubular circulation of the kidney. Efferent arterioles and branch capillaries were perfused at 625 nl/min with an artificial perfusate containing 9 g per oiter of albumin both before and during either aortic constriction or saline infusion. Results of recollection micropuncture studies in those tubules surrounded by artificially perfused capillaries were compared with results in control tubules in which the peritubular capillary flow and fluid composition were allowed to change during aortic constriction or volume expansion following saline infusion. Changes in single nephron filtration rate, fractional and absolute reabsorption induced by both aortic constriction and saline infusion were found to be qualitatively and quantitatively comparable in tubules with constant peritubular capillary microperfusion and in the control tubules with changing peritubular capillary environment due to the experimental maneuvers. Taken together, therefore, the present results indicate that with the use of the peritubular microperfusion technique no evidence is found to support a role of alteration in the peritubular environment in modulating the effect of aortic constriction or saline infusion on tubular sodium reabsorption in the rat nephron. Rather, these results provide indirect evidence in support of intraluminal factors as mediating these responses in tubular reabsorption to volume expansion and aortic constriction.

Quantitative importance of changes in postglomerular colloid osmotic pressure in mediating glomerulotubular balance in the rat

In recent studies in this laboratory employing normal hydropenic rats we have demonstrated that the reduction in absolute proximal reabsorption that attends the experimental reduction of single nephron glomerular filtration rate (SNGFR) (glomerulotubular balance) is mediated, at least in part, by the accompanying decline in postglomerular vascular protein concentration, and therefore, postglomerular colloid osmotic pressure (piEA). The present study was undertaken to define the quantitative contribution of these changes in piEA to the changes in absolute proximal reabsorption measured under these conditions. A protocol was employed which enabled us to examine the effects on absolute proximal reabsorption of reductions in filtered load brought about under conditions in which piEA remained essentially unchanged. Thus, after partial aortic constriction in 16 plasma-loaded rats, near constancy of piEA was observed in 10 (a change in efferent arteriolar protein concentration of 0.4 g/100 ml or less) and in these, uniform reductions in SNGFR averaging 16.7 nl/min were attended by reductions in absolute proximal reabsorption averaging only 1.7 nl/min, or 7% of preconstriction values. These findings, taken together with previous observations from this laboratory, suggest that the proximal reabsorptive adjustment that characterizes glomerulotubular balance in the rat is markedly blunted when changes in piEA are prevented. In the remaining six rats, a mean reduction in filtered load comparable to that observed in the above group was attended by slightly to moderately greater reductions in efferent arteriolar protein concentration, thereby fulfilling less well the stated aim of this study. Nevertheless, in accord with the above conclusion, these relatively greater reductions in piEA were accompanied by correspondingly greater reductions in absolute proximal reabsorption.