The relationship between peritubular capillary protein concentration and fluid reabsorption by the renal proximal tubule

Renal responses of the fetal lamb to fetal or maternal volume expansion

Fetal and maternal glomerular filtration rate (GFR), renal plasma flow (RPF), urine volume, sodium excretion, and fractional sodium reabsorption were measured in a chronically instrumented sheep preparation. Fetal GFR was essentially stable between 110 and 135 days of gestation (term = 147 days). There was a significant increase in GFR after 135 days. After the infusion of 50 ml of normal saline over a 30-minute period, fetal GFR and sodium excretion increased significantly. Fractional sodium reabsorption was significantly decreased. Thus, the fetus is capable of responding to volume expansion with saline with an increase in GFR and a decrease in fractional sodium reabsorption. After the infusion of 1000 ml of normal saline into the ewe in 1 hour, maternal GFR and RPF rose significantly. Sodium excretion rose 6-fold and fractional sodium reabsorption fell significantly. After the infusion of saline into the ewe, there was no change in fetal GFR, RPF, sodium excretion, urine volume, or fractional sodium reabsorption. Since there were no changes in fetal renal function after maternal volume expansion with saline there was no evidence for the transplacental passage of a natriuretic factor from ewe to fetus.

Efferent arterioles in the cortex of the rat kidney

Although a number of morphological studies have investigated the vascular system of the rat kidney, minimal data are available on the detailed anatomy of the efferent arterioles located throughout the cortex of the kidney. In the present study, the renal vascular system was filled with Microfil and the various efferent arteriole patterns were examined extensively. The efferent vessels of the entire cortex appear to form three major patterns which in turn divided the cortex into three separate regions: the outer, middle and inner cortex. The efferent arterioles of the outer cortex leave the glomerulus and run perpendicular to the kidney capsule. However, as the efferent arterioles ascend, they may show three variations in the way they branch: (1a) the efferent arteriole does not branch until directly beneath the capsule, (1b) the efferent vessel begins to divide into its major branches 100-200 mum below the surface of the kidney and (1c) the efferent vessel has only a short course before giving off many side branches. In the middle cortical area, the branches of the efferent arteriole run lateral to the glomerulus. However, the efferent arterioles of the inner cortex have a few branches which run lateral to the glomerulus while most of them descend into the medulla as vasa rectae. The unique morphological features of the efferent arterioles of the outer cortex are of particular interest in light of the functional data which suggests that the reabsorption of fluid by peritubular capillaries may indeed regulate the rate of net tubular sodium reabsorption.

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.

Filtration of protein in the anti-glomerular basement membrane nephritic rat: a micropuncture study

Production on an anti-glomerular basement membrane (anti-GBM) nephritis in the rat results in a 30-fold increase in glomerular membrane permeability to albumin. The concentration of albumin in glomerular filtrate, estimated from proximal tubular fluid samples, is ten times the normal value. Tubular reabsorption of albumin is not enhanced so that essentially the filtered load is excreted. A nephrotic syndrome develops rapidly. Total kidney glomerular filtration rate (GFR) is reduced to 40% of normal with a proportional reduction in filtration fraction. Glomerulo-tubular balance is maintained since proximal fractional reabsorption remains constant near control levels. Calculated efferent arteriolar plasma colloid osmotic pressure (COP) is about one-third normal. Sodium excretion, sharply curtailed in the first days of anti-GBM nephritis, returns to control values after the fourth postinjection day. Restoration of sodium balance despite reduced filtered load and constant proximal fractional reabsorption must be accomplished by adjustments at a distal site in the nephron.

Effects of nephrectomy on renal salt and water transport in the remaining kidney

Fluid, sodium, and potassium transport was studied in proximal and distal tubules in rats in which one kidney had been removed two weeks after a suprarenal aortic clamp had been placed to prevent adaptive changes in glomerular filtration rate (GFR) in the experimental kidney. Free-flow micropuncture techniques were used and tubular fluid (TF) samples analyzed for inulin, sodium and potassium. In addition, peritubular total protein concentrations and luminal and peritubular hydrostatic pressures were measured. The following changes were observed 15 hr after unilateral nephrectomy: (1) a significant increase in single nephron GFR; (2) unchanged absolute proximal tubular reabsorption rates of fluid and sodium; (3) increased delivery of fluid into distal tubules; (4) increased distal tubular reabsorption of sodium, but of insufficient magnitude to prevent natriuresis; and (5) an augmentation of distal tubular potassium secretion. Reduction of single nephron GRF to control levels by aortic clamping abolished the natriuresis following nephrectomy.

Direct measurement of papillary collecting duct sodium transport in the rat. Evidence for heterogeneity of nephron function during Ringer loading

It has been suggested that collecting duct sodium transport was inhibited by extracellular volume expansion. To directly evaluate this possibility, micropuncture of the papillary collecting duct of young rats was performed during hydropenia and Ringer loading. The possibility of heterogeneity of nephron function was evaluated during Ringer and hyperoncotic albumin loading by comparing the delivery of sodium to the end of the distal tubule of superficial nephrons with papillary base delivery. During hydropenia (n = 14), sodium delivery to the base averaged 0.95% of the filtered sodium load and reabsorption along the collecting duct was noted from base to tip in each collection pair averaging 0.80% of the filtered load. During Ringer loading, sodium delivery to the base was markedly greater than in hydropenia, 11.8 vs. 0.95% of the filtered load (P less than 0.001). Yet, sodium reabsorption was also much greater, 6 vs. 0.8% (P less than 0.001). In 13 paired collections, during Ringer loading, sodium delivery to the papillary base, 12.2% of the filtered load, was consistently greater than late distal tubular delivery from superficial nephrons. 8% (P less than 0.005). In contrast, reabsorption of sodium from late distal tubule to papillary base was found during albumin infusion, 6.2 vs. 3.1% (P less than 0.001). Therefore, these studies demonstrate that: (a) the delivery of sodium to and reabsorption along the papillary collecting duct were markedly greater during Ringer loading than in hydropenia; (b) the amount of sodium delivered to the papillary base was greater than the delivery to the end of the distal tubule of superficial nephrons during Ringer loading, suggesting that deeper nephrons deliver more sodium to the collecting duct in this setting; and (c) the difference in sodium excretion between Ringer loading and hyperoncotic albumin infusion is due to events occurring between the late distal tubule of superficial nephrons and the base of the papillary collecting duct.

The effect of saline-induced extracellular volume expansion on the kidney function

The functional parameters of renal function of non-hydrated and hydrated dogs (saline-infused to an extent of 1-2% of the body weight) have been compared. The directly measured renal blood flow and the total renal vascular resistance were the same in the two groups. No difference has been found in glomerular filtration rate, the Cinulin was the same in the two groups. There was no important difference in the PAH clearance and PAH extraction. In the hypervolaemic group, the sodium and water excretion was about threefold that of the non-hydrated animals. The plasma protein concentration was significantly lower in the hydrated group. In our experiments we did not find glomerular factors responsible for the increase of sodium and water excretion. The decrease of tubular reabsorption is attributed partly to the decreased plasma protein concentration, partly to unknown (perhaps natriuretic) humoral factors.

Prostaglandins and renal function in acute extracellular volume expansion

Mechanisms determining the natriuresis in ECV-expansion are not yet completely understood. The present study was therefore undertaken to investigate if prostaglandins (PG) are involved in the natriuresis of acute ECV-expansion and by which mechanisms PG may affect renal Na-absorption. In non-expanded rats the PG synthetase inhibitor indomethacin (INDO) had no effect on renal function. In 37 Sprague-Dawley rats ECV-expansion with isotonic saline corresponding to an increase in b.wt. of 10% was induced. Twenty-one animals received an oral dose of 10 mg/kg b.wt. of INDO prior to ECV-expansion. Sixteen animals served as ECV-expanded controls (C). GFR (INDO: 12.5 +/- 1.0; C: 10.5 +/- 0.9 ml/min/kg b.wt.) did not significantly differ in both groups. However, total renal plasma flow (RPF) (INDO: 22.9 +/- 1.8; C: 30.1 +/- 2.7 ml/min/kg b.wt.), urinary flow rate (INDO: 1.11 +/- 0.20; C: 1.93 +/- 0.21 ml/min/kg b.wt.) and urinary excretion of sodium (INDO: 141 +/- 26; C: 267 +/- 46 muEq/min/kg b.wt.) and potassium (INDO: 13.0 +/- 0.9; C: 19.8 +/- 1.7 muEq/min/kg b.wt.) markedly decreased in animals pretreated with INDO. The results indicate that PG are involved in the natriuresis of acute ECV-expansion and suggest, that PG may inhibit the intrinsic capacity for Na-absorption in more proximal parts of the nephron possibly via intrarenal physical factors.

Permselectivity of of the glomerular capillary wall. Studies of experimental glomerulonephritis in the rat using neutral dextran

Polydisperse [3h] dextran was infused into eight Munich-Wistar rats in the early autologous phase of nephrotoxic serum nephritis (NSN), thereby permitting direct measurements of pressures and flows in surface glomeruli and fractional clearances for dextrans [(U/P) dextran/(U/P) inulin] ranging in radius from 18 to 42 A. Despite glomerular injury, evidenced morphologically and by a marked reduction in the glomerular capillary ultrafiltration coefficient, the glomerular filtration rate remained normal because of a compensating increase in the mean net ultrafiltration pressure. In NSN rats, as in normal controls, inulin was found to permeate the glomerular capillary wall without measurable restriction, and dextrans were shown to be neither secreted nor reabsorbed. For dextran radii of 18, 22, 26, 30, 34, 38, and 42 A, (U/P) dextran/(U/P) inulin in NSN and control rats, respectively, averaged 0.90 vs. 0.99, 0.81 vs. 0.97, 0.63 vs. 0.83, 0.38 vs 0.55, 0.20 vs. 0.30, 0.08 vs. 0.11, and 0.02 vs. 0.03. Using a theory based on macromolecular transport through pores, the results indicate that in NSN rats, effective pore radius is the same as in controls, approximately 50 A. In NSN, however, the ratio of total pore surface area to pore length, a measure of the number of pores, is reduced to approximately 1/3 that of control, probably due to a reduction in capillary surface area. These results suggest that proteinuria in glomerular disease is not due simply to increases in effective pore radius or number of pores, as previously believed. Using a second theoretical approach, based on the Kedem-Katchalsky flux equations, dextran permeability across glomerular capillaries was found to be slightly lower, and reflection coefficient slightly higher in NSN than in control rats.

Intrarenal mechanisms of salt retention after bile duct ligation in rats

In order to study renal salt-retaining mechanisms during the early stages of ascites formation, rats were subjected to bile duct ligation. After this procedure, plasma volumes were found to be reduced and hematocrits slightly increased. The whole-kidney glomberular filtration rate and plasma flows were reduced to 59 and 57% of control values, but the filtration fraction was unchanged. Absolute sodium excretion, as well as the fraction of the filtered sodium load excreted, was also significantly reduced. When micropuncture techniques were used to examine the function of single superficial nephrons, the glomerular filtration rate in these nephrons was found to be reduced to 70% of controlled values, and fractional reabsorption was found to be increased at all accessible sites along the nephron. Filtration by intermediate and juxtamedullary nephrons, determined by Hanssen's technique, was reduced to 55 and 48% of control values. By the use of radioactive microspheres, it was demonstrated that blood flow to superficial, intermediate, and juxtamedullary nephrons was reduced to 49, 59, and 73% of control values. Filtration by superficial nephrons decreased much more than plasma flow--a finding which suggests that the measured increase in fractional reabsorption was associated with an increase in the superficial nephron filtration fraction. From this study, it appears that two factors play an important part in the sodium retention observed in the initial stages of ascites formation following bile duct ligation in rats: (a) a decrease in the filtered sodium load and (b) increased fractional reabsorption by the superficial nephrons--the nephrons which show the least decrease in filtration.

Comparative evaluation of fractional excretion of sodium following saline infusion in transplanted kidneys and in isolated perfused kidneys in conditions of previous high or low dietary sodium intake

The increase of fractional excretion of sodium following the infusion of isotonic saline solution is quantitatively the same in dog kidneys perfused by a whole animal or by a pump-oxygenator machine at identical arterial and venous pressures. The equal response found in both experimental conditions demonstrates that the dilution of the blood, and primarily the decrease of post-glomerular plasma oncotic pressure, represents the only extrarenal humoral factor of significant importance in the reduction of fractional reabsorption of sodium following large saline infusion. The renal response to hemodilution is reduced similarly in both types of experiments when the dogs have been previously submitted to a dietary deprivation of sodium. The decreased sensitivity of the kidney to the stimulus of hemodilution appears as a major determinant of the poor natriuretic response to saline loading of sodium-deprived animals.

Glomerular filtration response to elevated ureteral pressure in both the hydropenic and the plasma-expanded rat

The factors affecting glomerular ultrafiltration with elevated ureteral pressure were examined in both plasma-expanded (2.5% body weight) and hydropenic Munich-Wistar rats. Elevated ureteral pressure (20 mm Hg) alternated as the initial condition in both groups. Glomerular capillary hydrostatic pressure (PG) and Bowman's space pressure (Pt) were measured directly in surface glomeruli with a servonulling device (deltaP = PG - PT), systemic (piA) and efferent (piE) peritubular capillary oncotic pressures were estimated by microprotein methods, and single-nephron glomerular filtration rates (sngfr) were determined by micropuncture techniques under control ureteral pressure and after increased ureteral pressure in both experimental groups. All data were then applied to equations describing the process of glomerular ultrafiltration to define the profile of effective filtration pressure (EFP = deltaP - pi) and the glomerular permeability coefficient (LpA), where sngfr = LpA-EFP. In plasma-expanded rats, sngfr fell from 44.8 +/- 2.2 to 38.5 +/- 1.5 nilters/min g-1 kidney weight (P less than 0.025) with elevated ureteral pressure entirely as a result of a decrease in the hydrostatic pressure gradient (deltaP), since PG did not rise and nephron plasma flow remained constant. In hydropenic rats, sngfr fell from 34.7 +/- 1.6 to 27.3 +/- 1.6 nliters/min g-1 kidney weight with increased ureteral pressure. PG rose 8.7 mm Hg (P less than 0.001) due to an increase in vascular resistance between the peritubular capillaries and the renal vein which prevented the reduction in deltaP. The reduction in sngfr appeared to result from a reduction in LpA resulting in disequilibration of EFP. Nephron plasma flow was not changed. The filtration response to elevated ureteral pressure was modified by the prior state of volume expansion and was not associated with changes in either nephron blood flow or afferent arteriolar resistance.

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

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.

Pathogenesis of acute renal failure following temporary renal ischemia in the rat

In this study, we characterized the sequence of several intrarenal events and evaluated their relative importance in the pathogenesis of unilateral oliguric acute renal failure induced experimentally in rats by complete occlusion of a renal artery for 1 hour. Kidneys were studied prior to occlusion and 1-3 hours and 22-26 hours after release of the temporary occlusion. Renal blood flow measured by an electromagnetic flow transducer was reduced to 40-50% of control during both postocclusion periods. Flow of tubular fluid was markedly reduced, and the damaged kidneys were oliguric. Proximal and distal convolutions were filled with fluid and dilated 1-3 hours after occlusion; their pressures were greatly heterogeneous and were elevated, on the average, to 31 and 16 mm Hg, respectively. Glomerular capillary pressure at this time was normal or slightly increased. Histological sections showed extensive tubular obstruction. We conclude that initially the oliguria is primarily due to intraluminal obstruction in the absence of predominant increases in preglomerular vascular resistance. Observations at 22-26 hours after occlusion indicated acute tubular necrosis. Moreover, the combined involvement of preglomerular vasoconstriction, presisting tubular obstruction, and passive backflow of tubular fluid appeared to be important in the maintenance of the oliguria. Glomerular capillary, proximal intratubular, and peritubular capillary hydrostatic pressures were reduced below control values. After acute volume expansion, the reduced pressures and renal blood flow were reversed, yet the experimental kidneys remained oliguric. Thus, it is clear that tubular obstruction is a significant factor responsible for both the genesis and the maintenance of oliguria in this experimental model of ischemia-induced acute renal failure.

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.

Permselectivity of the glomerular capillary wall to macromolecules. II. Experimental studies in rats using neutral dextran

To determine the permselectivity characteristics of the glomerular capillary wall, known molecular size fractions of [3H]dextran, prepared by gel chromatography, were infused into normally hydrated Wistar rats, thus permitting simultaneous measurement of Bowman's space/plasma water (BS/P) and urine/plasma water (U/P) concentration ratios, along with glomerular pressures and flows. Since (BS/P)inulin = 1.01 +/- 0.01 SE(n = 34, radius = approximately 14 A) and since (BS/P)dextran/(BS/P)inulin equaled (U/P)dextran/(U/P)inulin for dextrans ranging in molecular radius from 21 to 35 A, these findings validate that dextrans are neither secreted nor reabsorbed. For dextran radii of 20, 24, 28, 32, 36, 40, and 44 A, (U/P)dextran/(U/P)inulin averaged 0.99, 0.92, 0.69, 0.42, 0.19, 0.06, and 0.01, respectively. In accord with theoretical predictions that these fractional dextran clearances should vary appreciably with changes in glomerular transcapillary pressures and flows, an increase in glomerular plasma flow rate, induced in these same rats by plasma volume expansion, resulted in a highly significant lowering of fractional clearance of all but the smallest and largest dextrans studied. These findings emphasize that fractional solute clearances alone are inadequate to describe the permselective properties of the glomerular capillary wall unless glomerular pressures and flows are also known. This sensitivity of fractional dextran clearance to changes in plasma flow indicates that dextrans are transported across the capillary not only by bulk flow but also to an important extent by diffusion.

Hyperoncotic albumin infusion in experimental glomerulonephritis in rats: A micropuncture study

The effect of infusion of hyperonciotic (25 g% salt-poor bovine plasma albumin) (0.06 g% of body weight) has been studied using clearance and micropuncture techniques in animals with two forms of immunologically induced experimental glomerulonephritis and in age-matched control rats. In control rats the significant natriuresis and diuresis after hyperoncotic albumin were associated with decreased fractional and absolute proximal reabsorption, decreased calculated efferent oncotic pressure, and a transitory rise in efferent arteriolar hydrostatic pressure. These findings, however, do not necessarily indicate a predominant role for peritubular "physical factors" in the control of proximal reabsorption. In glomerulonephritic rats a smaller diuresis and an insignificant natriuresis were found after hyperoncotic albumin, especially in anti-GBM nephritis, when no change in SNGFR or proximal absolute reabsorption occurred. In AICN there was a rise in SNGFR and a fall in absolute reabsorptive rate, especially in those nephrons with high filtration rates. There was no evidence that any alteration in efferent arteriolar hydrostatic pressure of calculated efferent oncotic pressure had occurred.

Renin secretion at the individual nephron level

Blood was collected from the descending aorta, from a renal efferent arteriole and from the renal vein of a rat. The renin concentrations of the blood samples were measured. The renal vein renin concentration was 673 +/- 81 (SE) ng ml-1h-1 which was significantly higher than the concentration in the aorta of 456 +/- 50 (SE) ng ml-1h-1. The concentration of renin in the renal efferent arteriole was significantly lower than that in the aorta. These observations imply that net renin secretion is a combination of two processes; removal between artery and efferent arteriole and entry between efferent arteriole and renal vein. It appears that renin is released into the interstitium and enters the circulation at the capillary level rather than being released into the afferent or efferent arterioles. This mode of secretion supports the suggestion that the renin angiotensin system may primarily work intra-renally rather than through the systemic circulation.

The effect on total renal and tubular function and plasma renin of a moderate isotonic saline load in rats anesthetized with amytal and inactin

The renal effects of i.v. saline loading equal to 1% body weight (b.wt.) were studied in 2 groups of rats: group I was anesthetized with Amytal (15 mg/100 g b.wt. plus supplementary doses), group II with Inactin (12.5-15.0 mg/100 g b.wt.). In group I the saline load caused an increase in urine flow (+92%), solute excretion (+67%), inulin clearance (CIN) (+24%), PAH clearance (+31%) and absolute proximal reabsorption rate (+27%). Proximal fractional reabsorption and filtration fraction (FF) remained unchanged, while plasma oncotic pressure (COP) decreased by 10%. Plasma renin fell and there was an inverse relationship between renin concentration and proximal reabsorption rate. In group II urine flow and solute excretion increased after saline (+85% and 110%, respectively); CIN and absolute proximal reabsorption rate was lower than in group I and failed to increase after saline. Proximal fractional reabsorption was also lower and decreased after saline. COP as well as renin decreased as in group I, but no relationship between renin concentration and proximal reabsorption rate could be demonstrated. The data indicate that Inactin depresses both resting proximal tubular reabsorptive capacity and the tubular response to a physiological volume expansion. The results are compatible with the hypothesis that the renin-angiotensin system is significantly involved in regulation of proximal tubular function, while they are incompatible with the idea that peritubular COP plays any major role in this adjustment.

Effects of norepinephrine and angiotensin II on the determinants of glomerular ultrafiltration and proximal tubule fluid reabsorption in the rat

In 26 Wistar rats with surface glomeruli, the determinants of glomerular ultrafiltration and peritubular capillary uptake of proximal reabsorbate were studied before and during intravenous infusions of norepinephrine or angiotensin II. Regardless of whether renal perfusion pressure (AP) was permitted to increase, both hormones produced elevations in single nephron filtration fraction due to declines in glomerular plasma flow with little change in nephron glomerular filtration rate. The resulting large increases in the efferent arteriolar oncotic pressure, piE, were accompanied by equivalent increases in the mean glomerular transcapillary hydraulic pressure difference, deltaP. Equality of piE and deltaP, denoting filtration pressure equilibrium, obtained before and during infusion of either hormone. Por both hormones, when elevations in AP were allowed, marked and roughly proportional increases in the resistance to blood flow through single afferent and efferent arterioles occurred, whereas when increases in AP were prevented by partial aortic constriction increases in resistance were confined primarily to the efferent arteriole. Tespite the marked increases in piE, absolute rates of proximal tubule fluid reabsorption, on the average, were unchanged by these hormones due to the opposing effects of marked decreases in efferent arteriolar plasma flow rate and, to a lesser extent, increases in peritubular capillary hydraulic pressure.

Effects of acute unilateral renal denervation in the rat

Studies were undertaken to characterize the renal responses to acute unilateral renal denervation and the mechanisms involved in these responses. Denervation was produced in anesthetized nondiuretic rats by application of phenol to the left renal artery. Studies were also performed in sham-denervated nondiuretic rats. Whole kidney and individual nephron studies were performed before and after denervation or sham denervation. Denervation increased urine volume from the left kidney to about twice its control value (P less than 0.001) and increased urinary sodium excretion from 332 neq min minus -1 to 1,887 neq min minus -1 (P less than 0.001). Glomerular filtration rate (GFR) and renal plasma flow (RPF) remained unchanged in both kidneys after the procedure. The innervated right kidney showed no changes in urine volume or in sodium excretion. After denervation, late proximal ratio of tubular fluid inulin concentration to that of plasma [(F/P)In] decreased from 2.23 to 1.50 (P less than 0.001) while single nephron GFR remained unchanged. Absolute reabsorption decreased from 16.5 to 9.9 n. min minus -1 (P less than 0.001). (F/P)In ratios were also decreased in early distal (from 6.21 to 3.18, P less 0.001) and late distal convolutions (from 16.41 to 8.33, P less than 0.001) during the experimental period. (F/P)Na ratios remained unchanged in the early distal convolutions, but increased from 0.18 to 0.38 (P less than 0.01) in late distal convolutions after denervation. Absolute Na reabsorption after denervation increased in the loop of Henle, distal convolution, and collecting ducts. Any changes in intrarenal hydrostatic pressures after denervation were always small. There were no changes in GFR, RPF, urine volume, urinary sodium excretion, or late proximal (F/P)In after sham denervation. We conclude that the diuresis and natriuresis seen after acute renal denervation were caused by a marked depression of sodium and water reabsorption in the proximal tubule with partial compensation in more distal nephron segments. These responses appeared to be unrelated to systemic or intrarenal hemodynamic changes. The results demonstrate an effect of the renal nerves on proximal tubular function.

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.

Dynamics of glomerular ultrafiltration in the rat. VIII. Effects of hematocrit

This study was undertaken in an effort to examine the effects of selective variations in systemic hematocrit on the preglomerular, glomerular, and postglomerular micocirculation in the rat. By isovolemic exchange transfusions, systemic hematocrit (control 51 ml/100 ml) was either reduced (21 ml/100 ml, N equal 7 rats) or elevated (62 ml/100 ml, N equal 7). Single nephron glomerular filtration rate varied inversely and filtration fraction varied directly with the changes in hematocrit. The fall in filtration fraction with decreased hematocrit was due to a decline in the measured glomerular transcapillary hydraulic pressure difference and to a marked increased in the initial glomerular plasma flow rate. Afferent (RA)and efferent (RE) arteriolar resistance declined with the fall in hematocrit; RA fell proportionately more than did RE. The rise in filtration fraction with the elevation in hematocrit was due to a marked increase in in part due to a relatively greater rise in RE than in RA. These findings provide an attractive explanation for the general tendency for filtration fraction to vary directly with hematocrit in anemic and polycythemic states in man.

The mechanism of acute renal failure after uranyl nitrate

Administration of 25 mg/kg uranyl nitrate (UN) to rats leads to a brief period of polyuria followed by progressive oliguria with death at 5 days. Factors that determine glomerular filtration rate (GFR) were examined in control Munich-Wistar rats (n equals 16) and 2 h after either 15 mg/kg (n equals 8) or 25 mg/kg (n equals 7) of UN (i.v.) utilizing direct measurements of hydrostatic and oncotic pressures and plasma flow. Total kidney GFR was reduced to 47% of control in the low dose group and to 21% in the high dose group. The simultaneous nephron filtration rate (sngfr) was 28.6 plus or minus 0.8 nl/min/g kidney wt in control, 29.1 plus or minus 1.0 in the low dose group, and 18.1 plus or minus 1.2 (P less than 0.001) in the higher dose group. This disparity in UN effect upon GFR and sngfr was due to tubular back-diffusion of solute through damaged epithelia beyond the early proximal tubule as demonstrated by microinjection of inulin and mannitol in the proximal tubule. Inulin "leak" persisted at 6 h after UN when tubular pressure had returned to normal. Comparison of sngfr measured in early vs. late proximal tubule revealed no difference after high dose UN, suggesting no significant leak of inulin from the early proximal tubule, and that the decreased sngfr was due to primary reductions in ultrafiltration. Nephron plasma flow was equal to control at both doses of UN. Also directly measured hydrostatic pressure gradient across the glomerular capillary was not changed. The effective filtration pressure achieved equilibrium in control of animals but became significantly positive at the efferent end of the capillary at both doses of UN and increased. Total glomerular permeability (LpA) was progressively reduced from control (0.089 plus or minus 0.005 nl/s/g kidney wt/mm Hg) at low dose UN (0.047 plus or minus 0.013) and high dose 0.024 plus or minus 0.003 nl/s/g kidney wt/mm Hg). Therefore UN decreases GFR by two mechanisms: (1) tubular damage leading to back-diffusion of solutes and (b) a primary reduction in sngfr due to reduced LpA.

Effect of increased peritubule protein concentration on proximal tubule reabsorption in the presence and absence of extracellular volume expansion

The effect of increased peritubule capillary oncotic pressure on sodium reabsorption by the proximal tubule of the dog was investistigated after extracellular volume expansion (ECVE) with Ringer's solution or during continued hydropenia. Control measurements were made after ECVE or during hydropenia and again during renal arterial infusion with hyperoncotic albumin solution. Absolute reabsorption by the proximal tubule was calculated from fractional reabsorption and single nephron filtration rates as determined by micropuncture. Direct measurements of efferent arteriole protein were used to determine efferent arteriolar oncotic pressure. Albumin infused into the renal artery after ECVE significantly increased efferent oncotic pressure by 17.6 plus or minus 5.3 mm Hg. Fractional and absolute reabsorption by the proximal tubule increased from 20 plus or minus 6 to 37 plus or minus 5% and from 22 plus or minus 6 to 36 plus or minus 7 nl/min, respectively. During hydropenia, the albumin infusion significantly increased efferent oncotic pressure by 15.0 plus or minus 4.4 mm Hg. However, in contrast to the effect seen during ECVE, neither fractional nor absolute reabsorption was changed, delta equals 0.3 plus or minus 1.5% and 3 plus or minus 5 nl/min, respectively. Single nephron filtration rates were not significantly different between the groups and were unchanged by the albumin infusion. Peritubule capillary hydrostatic pressures, measured with a null-servo device, were not changed by the albumin infusion in either group. Renal interstitial hydrostatic pressure, measured from chronically implanted polyethylene capsules, was decreased significantly from 7.2 plus or minus 0.9 to 3.4 plus or minus 0.6 mm Hg in the hydropenic group and from 0.6 plus or minus 0.6 to 4.8 plus or minus 0.7 mm Hg in the Ringer's expanded group. In the hydropenic group, the increase in efferent oncotic pressure was nearly compensated for by changes in interstitial forces so that the calculated net force for capillary uptake was almost unchanged, 17.8 mm Hg before vs. 21.4 mm Hg during the albumin infusion. The increased efferent oncotic pressure in the Ringer's expanded group was not compensated, so that the calculated net force for uptake was increased, 11.9 mm Hg before to 22.2 mm Hg during the albumin infusion. Thus, while the increase in efferent oncotic pressure during albumin infusion was not significantly different between the groups, absolute and fractional reabsorptions were increased only in the animals in which the extracellular volume was expanded. The results suggest that ECVE alters the effect of increased peritubule oncotic pressure on sodium reabsorption by the proximal tubule.

Determinants of glomerular filtration in experimental glomerulonephritis in the rat

Pressures and flows were measured in surface glomerular capillaries, efferent arterioles, and proximal tubules of 22 Wistar rats in the early autologous phase of nephrotoxic serum nephritis (NSN). Linear deposits of rabbit and rat IgG and C3 component of complement were demonstrated in glomerular capillary walls by immunofluorescence microscopy. Light microscopy revealed diffuse proliferative glomerulonephritis, and proteinuria was present. Although whole kidney and single nephron glomerular filtration rate (GFR) in NSN (0.8 plus or minus 0.04 SE2 ml/min and 2 plus or minus 2 nl/min, respectively) remained unchanged from values in 16 weight-matched NORMAL HYDROPENIC control rats (0.8 plus or minus 0.08 and 28 plus or minus 2), important alterations in glomerular dynamics were noted. Mean transcapillary hydraulic pressure difference (deltaP) averaged 41 plus or minus 1 mm Hg in NSN versus 32 plus or minus 1 in controls (P LESS THAN 0.005). Oncotic pressures at the afferent (piA) end of the glomerular capillary were similar in both groups ( 16 mm /g) but increased much less by the efferent end (piE) in NSN (to 29 plus or minus 1 mm Hg) than in controls (33 plus or minus 1, P less than 0.025). Hence, equality between deltaP and piE, denoting filtration pressure equilibrium, obtained in control but not in NSN rats. While glomerular plasma flow rate was slightly higher in NSN (88 plus or minus 8 nl/min) than in controls (76 plus or minus 6, P greater than 0.2), the failure to achieve filtration equilibrium in NSN rats was primarily the consequence of a marked fall in the glomerular capillary ultrafiltration coefficient, Kf, to a mean value of 0.03 nl/(s times mm Hg), considerably lower than that found recently for the normal rat, 0.08 nl/(s times mm Hg). Thus, despite extensive glomerular injury, evidenced morphologically and by the low Kf, GFR remained normal. This maintenance of GFR resulted primarily from increases in deltaP, which tended to increase the net driving force for filtration, and thereby compensate for the reduction in Kf.

Diuretics: mechanism of action and clinical application

Despite the bewildering number of diuretics available to the physician, these drugs can be divided into 4 main groups, characterised by their site of action on sodium reabsorption in the kidney. Drugs acting on the ascending limb of the loop of Henle have a powerful but short acting diuretic effect; they include frusemide, ethacrynic acid and bumetanide. The benzothiadiazines and related compounds have a moderate diuretic action spread over a longer period, whilst the potassium-sparing diuretics, triamterene, amiloride and spironolactone, have only a weak diuretic effect but a marked ability to diminish urinary potassium excretion. The fourth group is made up of miscellaneous substances which function as vasodilator or osmotic agents. The pathogenesis of oedema formation in heart failure is outlined and a logical approach to treatment suggested. Duiretics are being increasingly used in the treatment of non-oedematous states, in particular hypertension, diabetes insipidus and hypercalciuria; their exact role in pregnancy and acute renal failure remains controversial. Side-effects can be related to their effect on electrolyte excretion and include hypokalaemia, hyponatraemia, hyperkalaemia and hyperuricaemia. The incidence of disturbed carbohydrate tolerance in previously normal individuals is low. Other less common side-effects are also discussed.

The ontogenetic development of concentration differences for protein and ions between plasma and cerebrospinal fluid in rabbits and rats

1. The purpose of this study was to study in rats and rabbits the ontogenetic development of the blood-brain barrier to macromolecules and the ontogenetic development of concentration differences between plasma and cerebrospinal fluid for ions which are known to be transported actively across the choroid plexus and the blood-brain barrier.2. By comparing the development of concentration differences for ions with the development of the blood-brain barrier to macromolecules we wanted to evaluate an eventual relationship between the development of these two functions of the blood-brain barrier.3. The concentration of protein in cerebrospinal fluid and plasma was measured in foetal, juvenile and adult rabbits and in new-born, juvenile and adult rats. The concentration of protein was similar in rabbit foetuses at 23 days of gestational age (term at 31 days) and in new-born rats, and the ratio decreases at approximately the same rate in the two species.4. The high concentration of proteins in cerebrospinal fluid might reflect either a high rate of entry of protein into the brain or a low production rate of cerebrospinal fluid. Injection of Diamox(R) (100 mg/kg) 2 hr before sampling of cerebrospinal fluid did not change the concentration of protein in cerebrospinal fluid in new-born rats whereas it increased the concentration in older rats. This finding suggests that new-born rat produces little (if any) cerebrospinal fluid suggesting that the high concentration of protein in cerebrospinal fluid in new-born rats reflect a low rate of turnover of cerebrospinal fluid.5. The concentration of sodium, potassium, chloride and magnesium in plasma and cisternal cerebrospinal fluid was measured in rabbits of different age, from 23 days of gestation until adulthood, and in rats of different ages from birth until adulthood.6. Concentration differences between plasma and cerebrospinal fluid for these were established in the youngest animals examined, indicating that the active transport mechanisms for these ions were functioning at an age where the concentration of protein in cerebrospinal fluid was very high.7. The maintenance of concentration differences for ions at a time where the concentration of proteins in cerebrospinal fluid is high, is difficult to explain if the high concentration of proteins in cerebrospinal fluid is due to a leakiness of the intercellular junctions between cerebral endothelial cells. However, the findings might be explained either by a low rate of production of cerebrospinal fluid in the youngest animals and/or by a pinocytotic transfer of proteins across the blood-brain barrier in these animals.8. In rats concentration gradients for ions are established at an age (new-borns) with a low or absent bulk formation of cerebrospinal fluid and at an age where the capillaries are still not enveloped by astrocytic foot processes. These facts suggest that the active transport mechanisms for the ions must be located in the cerebral endothelial cells.

Effect of mannitol on glomerular ultrafiltration in the hydropenic rat

The effect of mannitol upon glomerular ultrafiltration was examined in hydropenic Munich-Wistar rats. Superficial nephron filtration rate (sngfr) rose from 32.0+/-0.9 nl/min/g kidney wt to 42.0+/-1.6 (P < 0.001) in eight rats. Hydrostatic pressure gradients acting across the glomerular capillary (DeltaP) were measured in glomerular capillaries and Bowman's space with a servo-nulling device, systemic (piA) and efferent arteriolar oncotic pressures (piE) were determined by microprotein analysis. These data were applied to a computer-based mathematical model of glomerular ultrafiltration to determine the profile of effective filtration pressure (EFP = DeltaP - pi) and total glomerular permeability (L(p)A) in both states. Filtration equilibrium obtained in hydropenia (L(p)A >/= 0.099+/-0.006 nl/s/g kidney wt/mm Hg) and sngfr rose because EFP increased from a maximum value of 4.2+/-1.1 to 12.8+/-0.5 mm Hg after mannitol (P <0.01). This increase was due to both increased nephron plasma flow and decreased piA. Computer analysis of these data revealed that more than half (>58%) of this increase was due to decreased piA, consequent to dilution of protein. Since EFP was disequilibrated after mannitol, L(p)A could be calculated accurately (0.065 +/- 0.003 nl/s/g kidney wt/mm Hg) and was significantly lower than the minimum estimate in hydropenia.Therefore, sngfr does increase with mannitol and this increase is not wholly dependent upon an increase in nephron plasma flow since the major factor increasing EFP was decreased piA.

Effects of dietary sodium and of acute saline infusion on the interrelationship between dopamine excretion and adrenergic activity in man

The effects of dietary sodium and of saline infusion on urinary dopamine and norepinephrine and on the relationship of these catecholamines to adrenergic activity were determined. In seven normal subjects on a 9-meq sodium intake, urinary dopamine and norepinephrine were 136+/-18 (SE) and 37.4+/-5.3 mug/day, respectively. When sodium intake was increased to 209 or 259 meq/day, urinary dopamine increased to 195+/-20 mug/day (P<0.01) whereas urinary norepinephrine decreased to 21.1+/-3.0 mug/day (P<0.01). Infusion of saline in seven subjects increased sodium excretion and urinary dopamine (from 2.18+/-0.22 to 2.79+/-0.19 mug/20 min, P<0.01), but decreased plasma dopamine-beta-hydroxylase by 33% and urinary norepinephrine insignificantly. The clearance of inulin and p-aminohippurate did not change significantly and filtration fraction was the same. The data indicate that an increase in dietary sodium or infusion of saline results in an apparent decrease in adrenergic activity and an increase in urinary dopamine. Dopamine excretion would thus appear to relate inversely to adrenergic activity and to parallel sodium excretion. These findings suggest a possible role for dopamine and norepinephrine in the regulation of renal sodium excretion.

Pressure control of sodium reabsorption and intercellular backflux across proximal kidney tubule

The magnitude of changes in luminal hydrostatic pressure (DeltaP(L)), peritubular capillary hydrostatic pressure (DeltaP(PT)), and peritubular capillary colloid osmotic pressure (Deltapi) was determined in the Necturus kidney during volume expansion (VE). The specific effects of separate changes of each pressure parameter on proximal net sodium transport (J(Na)) were studied in isolated perfused kidneys. The combined effect of DeltaP(L), DeltaP(PT), and Deltapi, of a magnitude similar to that induced by volume expansion, decreases J(Na) by 26% in the perfused kidney. A major portion of the natriuresis in VE is due to changes in intrarenal pressures. The effect of Deltapi on the permeability characteristics of Necturus proximal tubule was studied. With increasing Deltapi, the ionic conductance of the paracellular shunt pathway decreased, since transepithelial input and specific resistance rose significantly, whereas cellular membrane resistance remained unchanged. Transepithelial permeability coefficients for sodium chloride and raffinose changed inversely proportional to transepithelial resistance, indicating an alteration of a paracellular permeation route. Net passive sodium backflux and active transport flux components were calculated. Increased net sodium transport with rising Deltapi is accompanied by a significant drop in passive back diffusion, without an increment in the active flux component. Change in passive sodium ion back diffusion thus appears to be a key physiological factor in the control of transepithelial sodium transport.

Nephron stop-flow pressure response to obstruction for 24 hours in the rat kidney

Complete ureteral ligation of 24-h duration significantly reduced stop-flow and estimated glomerular capillary pressures in nephrons accessible to micropuncture in obstructed kidneys. In kidneys without ureteral obstruction, a similar response occurred in single tubules blocked for 24 h without affecting nearby unblocked tubules. Thus, the response to tubular obstruction occurs on an individual nephron basis and results from constriction of individual afferent arterioles. The mechanism leading to the response is unknown, but a feedback mechanism operating through the juxtaglomerular apparatus of individual nephrons is an attractive possibility.

Salt-poor human albumin in management of nephrotic syndrome

Thirteen patients with the nephrotic syndrome were treated with a high-protein diet, a 0.5 g sodium intake (equivalent to 1.3 g sodium chloride), and frusemide in increasing dosage. One became oedema-free with frusemide 240 mg daily, three became oedema-free with frusemide 500 mg daily, and two required a combination of high-dose frusemide and spironolactone. In three there was an appreciable increase in the blood urea, one patient developed hyponatraemia, and in two there was no weight loss. In these six patients infusions of human salt-poor albumin produced a prompt diuresis, loss of weight, and correction of the abnormal biochemical findings. In the seventh severely oedematous patient combined albumin and diuretic therapy led to a loss of 27 kg in 14 days.

Current concepts of sodium chloride and water transport by the mammalian nephron

The decision of the editors to solicit a review for the Medical Progress series of this journal devoted to current concepts of the renal handling of salt and water is sound in that this important topic in kidney physiology has recently been the object of a number of new, exciting and, in some instances, quite unexpected insights into the mechanisms governing sodium excretion. These developments have come about largely as a consequence of the fact that segments of nephrons previously inaccessible to direct study are now readily accessible. Many of the findings to be discussed argue for extensive revision of a number of our current widely held views concerning the renal handling of sodium chloride and water. In the opinion of the authors, the strength of this argument rests in the fact that many of these new findings were obtained under circumstances that enabled workers to gain more direct access to the nephron than has been possible heretofore. This is not to say that areas of controversy and disagreement no longer exist. Wherever possible, these have been identified. In attempting to provide a comprehensive review of this topic, it has been necessary at times to overgeneralize and to disregard minor deficiencies in some of the studies cited. Finally, we wish to emphasize that a considerable portion of the information contained herein derives from work still under active investigation. Much of this contemporary work will undoubtedly withstand the rigors of future experimental scrutiny. It is inevitable, however, as William James so aptly noted in the quotation cited below, that some of our present ideas will need to be abandoned or revised in favor of newer, more convincing evidence. Seen in this light, the present effort is intended as nothing more than a timely survey of this active and fertile topic in renal physiology.

Dynamics of glomerular ultrafiltration in the rat. V. Response to ischemic injury

An experimental model of postischemic, acute renal failure has been developed in Wistar rats with surface glomeruli, thereby making possible a direct assessment of the mechanisms responsible for the fall in glomerular filtration rate that characterizes this disorder. Whole kidney and cortical single nephron filtration rates were reduced proportionately, on average by approximately 40%, after 3 h of nearly complete occlusion of the ipsilateral renal artery. The possibility of a significant transtubular leak of inulin was excluded. This decline in filtration rate occurred in the absence of measured changes in mean arterial pressure, mean glomerular transcapillary hydrostatic pressure, or net ultrafiltration pressure at afferent and efferent ends of the glomerular capillary. Net ultrafiltration pressure at the efferent end of the capillary approached zero both before and after ischemic injury, demonstrating that filtration pressure equilibrium was achieved throughout this study. Single nephron filtration fraction remained unchanged, indicating that the fall in filtration rate was accompanied by a proportional decline in glomerular plasma flow. The results indicate that the fall in filtration rate was solely the consequence of this fall in glomerular plasma flow. Since filtration rate per nephron is equal to the product of the ultrafiltration coefficient and mean ultrafiltration pressure, this product must also have fallen in proportion to the decline in glomerular plasma flow. Evidence is presented to indicate that a change in ultrafiltration coefficient is not required to account for the observed fall in filtration rate. The reduction in glomerular plasma flow, occuring in the absence of a concomitant decline in mean glomerular capillary hydrostatic pressure, resulted from large and proportional increases in afferent and efferent arteriolar resistances. These resistance changes appear to play a fundamental role in the pathogenesis of this form of acute renal failure.

Renal physiology in children

The human kidney may not reach functional maturity until the age of 18 months. In the preceding months there are limitations in response to such stresses as salt and water loading, acidosis and dehydration. It is seen that these limitations must not be taken out of context of the overall situation of health or disease and of actual solute load. Almost all drug dosages for children are calculated on the basis of body size, and it should be recalled that in the first months of life there exists relative renal insufficiency on the basis of body weight or surface area in comparison with older children and adults.

Precise physiologic data are necessary for completely successful management of the critically ill patient. It is seen from this review that not only are there gaps in our knowledge of developmental renal physiology, but also the process of development has introduced another variable into the evaluation of the individual patient.

Mechanism of natriuresis after closure of chronic arteriovenous shunts

Animals subjected to certain cardiovascular manipulations, such as arteriovenous fistulas, diminish their urinary sodium excretion. It has been shown that closure of such fistulas results in a prompt increase in the rate of sodium excretion. However, the nature of the renal mechanisms increasing the excretion of sodium when the initial cardiovascular abnormality is corrected has remained unclear. Since the elucidation of such mechanisms might provide information pertinent to other sodium-retaining states, the effect of closure of chronic Teflon-Silastic arteriovenous shunts was studied in desoxycorticosterone acetate (DOCA)-treated dogs by utilizing micropuncture techniques.Nephron filtration rates were measured first during a control period with open arteriovenous shunts and then again after closure of the shunts in 12 dogs. Nephron filtration rate rose 32% while total glomerular filtration rate (GFR) decreased 8%. After closure of the arteriovenous shunt, fractional reabsorption increased 6%, while total kidney filtration fraction increased from 0.31 to 0.35. Renal plasma flow decreased from a mean of 111 ml/min to 90 ml/min. Closure of the arteriovenous shunts increased sodium excretion from a mean of 21 mueq/min to 45 mueq/min. Concomitantly, a redistribution of filtrate to superficial nephrons occurred. Since pharmacological doses of DOCA were being administered while total GFR was not increased and fractional reabsorption of sodium in the proximal tubule was not inhibited, it was concluded that filtrate distribution to superficial nephrons may have contributed to the observed natriuresis, although alternate explanations were also deemed possible.

A comparison of the segmental analysis of sodium reabsorption during Ringer's and hyperoncotic albumin infusion in the rat

Studies were designed to compare the segmental analysis of sodium reabsorption along the nephron during volume expansion with either 10% body weight Ringer's or 0.6% body weight hyperoncotic albumin. Total kidney and nephron glomerular filtration rate increased similarly with both, but urinary sodium excretion (12.7 vs. 4.0 mueq/min, P < 0.001) and fractional sodium excretion (5.0 vs. 1.6%, P < 0.001) increased to a greater extent with Ringer's. Fractional reabsorption of sodium in the proximal tubule was diminished in both groups but to a significantly greater extent during Ringer's (P < 0.005). Absolute reabsorption was inhibited only in the Ringer's group. Delivery of filtrate out of the proximal tubule was greater in the Ringer's studies, 45 vs. 37 nl/min (P < 0.001). However, both fractional and absolute sodium delivery to the early and late distal tubule were not significantly different in the two groups. Fractional reabsorption in the collecting duct decreased from 96% in hydropenia to 31% during Ringer's but fell only slightly to 80% in the albumin studies. Absolute collecting duct reabsorption was also greater in the albumin studies, 0.55 vs. 0.21 neq/min (P < 0.001), which could totally account for the difference in urinary sodium excretion between the two groups. (22)Na recovery in the final urine after end distal microinjections was 71% during Ringer's infusion and 34% during albumin (P < 0.001). From these data we conclude that: (a) Ringer's solution has a greater inhibitory effect on proximal tubular sodium reabsorption, and (b) in spite of this effect, differences in mucosal to serosal collecting duct sodium transport are primarily responsible for the greater natriuresis during Ringer's infusion.