The mechanism of suppression of proximal tubular reabsorption by saline infusions

"I don't get no respect": the role of chloride in acute kidney injury

Acute kidney injury (AKI) is a major public health problem that complicates 10-40% of hospital admissions. Importantly, AKI is independently associated with increased risk of progression to chronic kidney disease, end-stage renal disease, cardiovascular events, and increased risk of in-hospital and long-term mortality. The chloride content of intravenous fluid has garnered much attention over the last decade, as well as its association with excess use and adverse outcomes, including AKI. Numerous studies show that changes in serum chloride concentration, independent of serum sodium and bicarbonate, are associated with increased risk of AKI, morbidity, and mortality. This comprehensive review details the complex renal physiology regarding the role of chloride in regulating renal blood flow, glomerular filtration rate, tubuloglomerular feedback, and tubular injury, as well as the findings of clinical research related to the chloride content of intravenous fluids, changes in serum chloride concentration, and AKI. Chloride is underappreciated in both physiology and pathophysiology. Although the exact mechanism is debated, avoidance of excessive chloride administration is a reasonable treatment option for all patients and especially in those at risk for AKI. Therefore, high-risk patients and those with "incipient" AKI should receive balanced solutions rather than normal saline to minimize the risk of AKI.

One physiology does not fit all: a path from data variability to "physiogenetics"?

Data variability is a costly complication of biomedical experimentation because the same experiment must be repeated a sufficient number of times so that the sample mean becomes a credible representation of the entire population. Since sampling is ideally done randomly in populations normalized for environmental and genetic backgrounds, data variability is viewed as a purely statistical issue reflecting the distribution in the population and captured as the standard deviation of the sampled data. The factors contributing to data variability are not analyzed by statistical methods; for want of a better explanation, data scatter is simply attributed to random noise and/or methodological limitations. In this commentary, evidence is discussed that documents an important role of interindividual biological diversity as a cause for data variability based on studies in which repeated sampling in the same individual permitted statistical comparisons between individuals in the same sample. Significant differences were found for proximal fluid reabsorption and plasma renin concentration between sample means of individuals of the same population. Furthermore, arterial blood pressure varied significantly between individual mice independently of strain and sex. Recognition of the extent of interindividual variability has important implications for data reproducibility, data collection, and data presentation in physiological research. Such nonrandom data variability may have different causes, but DNA modifications by genetic or epigenetic mechanisms could generate phenotype variants without being associated with disease symptoms. Exploration of the heritability of phenotypical diversity in physiology may be defined as "physiogenetics," and it would thus be the physiological corollary of pharmacogenetics and pharmacogenomics.

Micropuncture study of nephron function in the rhesus monkey

The function of the proximal and distal tubule was studied in the rhesus monkey during antidiuresis and during the diuresis after furosemide administration (during which extracellular fluid volume was maintained).In the proximal tubule, fluid to plasma ratios for sodium, potassium, and osmolality approximated unity. During antidiuresis, about 30% of the filtered water remained at the end of the accessible portion of this segment (92% of length). Fluid was hypotonic to plasma throughout the distal tubule. 25% of the filtered water was present in the early distal tubule. Small but significant water reabsorption (about 8% of filtered) occurred in remainder of this segment. Tubule fluid to plasma potassium concentration ratios tended to increase along the distal tubule, and the amount of potassium, relative to the amount filtered, increased from 13% in the early portion of this segment to 26% in the late portion.After furosemide was administered animals excreted about one-third of the filtered sodium and water. Despite this diuresis, electrolyte and water reabsorption along the proximal tubule did not differ from values obtained in control animals. Osmolality and sodium concentration of fluid from the distal tubule approached those of plasma. 22% of the filtered sodium (twice the control values) reached the distal tubule, whereas the fraction of filtered water remaining was only slightly increased. These findings indicate that, after the administration of this drug, inhibition of sodium reabsorption occurred in the water-impermeable segment of the nephron, rather than in the proximal tubule. After furosemide administration, all tubule fluid to plasma potassium concentration ratios in the distal tubule were equal to or greater than one, suggesting inhibition of active potassium reabsorption at or prior to this site.Fluid to plasma bicarbonate concentration ratios from the midportion of the proximal tubule were consistently less than one in normal monkeys. After acetazolamide was administered, the bicarbonate concentration of samples of tubule fluid recollected from these same sites was the same as, or higher than in plasma. This fact demonstrates the inhibition of bicarbonate reabsorption in this portion of the tubule.

Site and magnitude of the tubular inhibitory effect of expanding the extracellular volume in dogs

Ethacrynic acid inhibits energy-requiring transcellular NaCl reabsorption without affecting NaHCO3 reabsorption. Acetazolamide inhibits NaHCO3 and most of the remaining NaCl reabsorption in the proximal tubules (bicarbonate-dependent reabsorption) but raises distal transcellular NaCl reabsorption. After administration of both diuretics, the remaining bicarbonate-dependent and transcellular reabsorptions become constant until glomerular filtration rate (GFR) is almost halved. The inhibitory effect of expanding the extracellular volume (ECV) until plasma volume and GFR increased 30-40% was examined in anesthetized dogs. Examinations at comparable GFR obtained by altering arterial perfusion pressure showed that the inhibitory effect of ECV expansion was attenuated by administering acetazolamide. Ethacrynic acid amplified the inhibitory effect which for sodium and chloride reabsorption amounted to 6-7% of the filtered load at comparable GFR. An inhibitory effect of ECV expansion of bicarbonate reabsorption was disclosed only after raising plasma bicarbonate concentration. Thus, the small inhibitory effect of massive ECV expansion is confined to proximal tubular bicarbonate-dependent reabsorption and is of the same magnitude as previously demonstrated in experiments of similar design by raising plasma pH by only 0.07 unit. Since ouabain inhibits transcellular NaCl reabsorption, a natriuretic hormone is more likely to be an inhibitor of carbonic anhydrase than of Na,K-ATPase.

Sodium transport inhibitor in proximal tubular urine during acute volume expansion

Harvested proximal tubular fluid from mannitolsaline expanded rats caused a 50% inhibition of transepithelial sodium concentration difference when compared to an artificially prepared test solution used in the same and nonexpanded animals. Because of the methodology employed, none of the usual factors known to affect sodium reabsorption by the kidney could have been responsible for these changes. The factor responsible acts from the luminal side, is an inhibitor and has a short duration of action.

Resistance to diuretics: emphasis on a pharmacological perspective

Resistance to diuretics occurs frequently in clinical settings. Most attention to this phenomenon has been directed toward the pathophysiology of the disease state, with little focus on the pharmacology of the diuretics themselves. This review summarises the pathogenesis and emphasises the pharmacological determinants of response to diuretics, including absorption, delivery to the kidney, and response to amounts of diuretic reaching the site of action. In normal subjects, overall response to organic acid diuretics such as frusemide (furosemide) is determined by the total amount of drug delivered into the urine (reflecting amounts of drug reaching the intraluminal site of action), the time course of delivery, and the relationship between amounts of drug reaching the urine and response (the dynamics of response). The conditions of azotaemia, inhibition of synthesis of prostaglandins, and the oedematous disorders of congestive heart failure, cirrhotic liver disease and nephrotic syndrome are examined in the above context. In azotaemic subjects, delivery of organic acid diuretics to their intraluminal site of action is inhibited by accumulated endogenous organic acids which compete for transport into the nephron at the organic acid secretory site of the proximal tubule. Whether the dynamics of response are changed has not been investigated. During inhibition of synthesis of prostaglandins, and in the oedematous disorders, there appear to be no changes in handling of frusemide; i.e. bioavailability, total drug delivered into the urine and the time course of delivery are comparable with that in normal subjects unless concomitant renal dysfunction exists. Resistance in these conditions is therefore due to a change in the dynamics of response.

Influence of indomethacin on cation excretion after acute unilateral nephrectomy in dogs

Acute unilateral nephrectomy (AUN) causes functional changes in the remaining kidney. Since renal prostaglandins (PGs) may participate in this response, we investigated the effect of the PG synthetase inhibitor, indomethacin (INDO), on the function of the remaining kidney after AUN. Glomerular filtration rate (GFR), effective renal plasma flow (ERPF), filtration fraction (FF), sodium excretion, and potassium excretion were measured for 1 hr prior to and 3 hr after AUN in dogs anesthetized with sodium pentobarbital. Group I and II animals underwent AUN. A third group was sham-operated (Group III). Group I received INDO (2 mg/kg) before and 2 hr after AUN, while Groups II and III received a buffered saline vehicle. AUN alone (Group II) had no effect on GFR, ERPF, FF, or sodium excretion, while potassium excretion was increased. In animals administered INDO prior to AUN (Group I), the increase in potassium excretion was abolished, FF rose and sodium excretion, while potassium excretion was increased. In animals administered INDO prior to AUN (Group I), the increase in potassium excretion was abolished, FF rose and sodifore and 2 hr after AUN, while Groups II and III received a buffered saline vehicle. AUN alone (Group II) had no effect on GFR, ERPF, FF, or sodium excretion, while potassium excretion was increased. In animals administered INDO prior to AUN (Group I), the increase in potassium excretion was abolished, FF rose and sodium excretion was decreased. Results suggest that renal PGs participate in the compensatory response of the remaining kidney to AUN and may specifically play a role in increased cation excretion following AUN.

Effects of a new diuretic (piretanide) compared with furosemide on renal diluting and concentrating mechanisms in patients with the nephrotic syndrome

The mode and site of action of the new diuretic agent piretanide were investigated in man. The effect of a single oral dose of 12 mg was compared with that of furosemide 80 mg p.o. iin 6 patients with the nephrotic syndrome. Piretanide showed a greater diuretic and natriuretic response than furosemide during maximal water diuresis, but the effect was similar in the hydropenic state. Like furosemide, piretanide decreased free water clearance relative to delivery during water diuresis and the reabsorption of solute-free water during hydropenia. This inhibitory action on both the concentrating and diluting urinary mechanisms demonstrates an effect of piretanide localized in the ascending limb of Henle's loop. Consistent alterations in glomerulur filtration rate, renal blood flow or filtration fraction were not observed.

The clinical physiology of water metabolism. Part I: The physiologic regulation of arginine vasopressin secretion and thirst

Water balance is tightly regulated within a tolerance of less than 1 percent by a physiologic control system located in the hypothalamus. Body water homeostasis is achieved by balancing renal and nonrenal water losses with appropriate water intake. The major stimulus to thirst is increased osmolality of body fluids as perceived by osmoreceptors in the anteroventral hypothalamus. Hypovolemia also has an important effect on thirst which is mediated by arterial baroreceptors and by the renin-angiotensin system. Renal water loss is determined by the circulating level of the antidiuretic hormone, arginine vasopressin (AVP). AVP is synthesized in specialized neurosecretory cells located in the supraoptic and paraventricular nuclei in the hypothalamus and is transported in neurosecretory granules down elongated axons to the posterior pituitary. Depolarization of the neurosecretory neurons results in the exocytosis of the granules and the release of AVP and its carrier protein (neurophysin) into the circulation. AVP is secreted in response to a wide variety of stimuli. Change in body fluid osmolality is the most potent factor affecting AVP secretion, but hypovolemia, the renin-angiotensin system, hypoxia, hypercapnia, hyperthermia and pain also have important effects. Many drugs have been shown to stimulate the release of AVP as well. Small changes in plasma AVP concentration of from 0.5 to 4 muU per ml have major effects on urine osmolality and renal water handling.

Effects of dopamine on whole kidney function and proximal transtubular volume fluxes in the rat

The renal effects of dopamine were studied using clearance and micropuncture techniques in rats. Intravenous infusion of dopamine (4.7 - 10(-6) mol - kg-1 - h-1) increased glomerular filtration rate and renal blood flow. Renal blood flow was measured by an electromagnetic flowmeter. The increase in filtered fluid and sodium was nearly completely matched by increased tubular reabsorption. Thus, only a small rise in urine flow and in urinary sodium excretion was observed. The micropuncture experiments using the split oil droplet method of Gertz demonstrated a stimulation of the transepithelial fluid transfer after injection of dopamine (10(-4) M) into the proximal tubular lumen. This effect was abolished by simultaneous injection of propranolol (10(-3) M) which, by its own, did not affect transtubular volume fluxes. It is concluded that dopamine, by stimulation of beta-adrenoceptors, may increase reabsorptive capacity of the proximal tubular epithelium independent of changes in renal hemodynamics.

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.

Renal consequences of valvular heart disease

Three aspects of potential renal involvement in patients with valvular heart disease are discussed. These include (1) disturbances in renal salt and water handling and their implications with respect to diuretic management; (2) the hemodynamic effects of angiographic contrast agents with identification of potentially dangerous effects on the kidneys; and (3) the histologic patterns of bacterial endocarditis in the kidney and their similarity to those of immunologically mediated disease.

Effects of acute volume expansion and altered hemodynamics on renal tubular function in chronic caval dogs

It is well established that dogs with chronic partial constriction of the thoracic inferior vena cava develop sodium retention, ascites, and respond poorly to acute saline loading. A group of such chronic caval dogs, and a group of normal controls were studied during hydropenia, and again after acute saline loading by clearance and recollection micropuncture techniques. After volume expansion, the caval dogs excreted 52 muEq/min per kidney of sodium compared with 370 muEq/min per kidney for the normal controls. During hydropenia and after the saline infusions, single nephron filtration rates, fractional reabsorption of sodium within the proximal tubule, and proximal delivery of filtrate to the distal nephron were comparable in both groups of dogs. Micropuncture of distal tubular segments confirmed that the loop of Henle was the major site for salt and water retention in the expanded caval dogs. Alteration of intrarenal hemodynamics by vasodilating one kidney and elevating systemic arterial blood pressure induced a normal natriuretic response in the saline-loaded caval dogs. Proximal tubular function remained unchanged and the loop of Henle appeared to be the major site responsive to these hemodynamic maneuvers. These same experiments in saline-loaded control dogs had no effect on function of the proximal or distal nephron and did not increase urinary excretion of sodium or water. These experiments provide evidence that the loop of Henle is the major site for sodium retention in volume-expanded chronic caval dogs excreting minimal amounts of sodium.

The relative contributions of reabsorptive rate and redistributed nephron filtration rate to changes in proximal tubular fractional reabsorption during acute saline infusion and aortic constriction in the rat

The absolute rate of reabsorption by superficial rat proximal tubules was measured by the in situ microperfusion technique under conditions of hydropenia, infusion of saline, and infusion of saline plus aortic constriction sufficient to decrease whole kidney filtration rate below hydropenic levels. Fractional reabsorption was measured in adjacent filtering nephrons by collecting and recollecting tubular fluid from late proximal convolutions during each experimental condition. During hydropenia, the absolute rate of proximal tubular reabsorption averaged 3.56 +/-0.60 nl/min per mm and late proximal tubular fractional reabsorption averaged 0.56 +/-0.10. From these two measurements and measurements of tubule length to the site of micropuncture, a value for filtration rate was calculated for filtering nephrons. During hydropenia this value averaged 32.9 +/-7.1 nl/min. Saline infusion increased sodium excretion to 5.5% of the filtered load as the absolute rate of proximal tubular reabsorption decreased 38% and fractional reabsorption decreased 45%. Calculated superficial nephron filtration rate increased 21% which on the average was identical with the simultaneously measured increase in whole kidney filtration rate. Similar results were obtained in a separate group of animals by the technique of total collection of late proximal tubular fluid. Aortic constriction during saline infusion decreased whole kidney and calculated nephron filtration rate to the same degree and to values lower than those during hydropenia. Fractional reabsorption increased but not to hydropenic values. The persistent natriuresis during aortic constriction was associated with a continued depression of the absolute rate of proximal tubular reabsorption which was sufficient to maintain an increased delivery of filtrate out of the proximal tubule despite the fall in nephron filtration rate. These results indicate that depressed fractional reabsorption in the proximal tubule during acute saline infusion is due predominantly to a decrease in absolute reabsorptive rate and to a lesser extent to an increase in superficial nephron filtration rate which is proportional to the increase in whole kidney filtration. Continued natriuresis when filtration rate is decreased during saline infusion can be accounted for entirely by the persistent large reduction in the absolute rate of proximal tubular reabsorption.

Accelerated reabsorption in the proximal tubule produced by volume depletion

The renal response to chronic depletion of extracellular volume was examined using the techniques of micropuncture. Depletion of salt and water was produced by administration of furosemide to rats maintained on a sodium-free diet. There was a marked fall in body weight, plasma volume, and glomerular filtration rate. The intrinsic reabsorptive capacity of the proximal tubule, measured by the split-droplet technique, was greatly enhanced. The acceleration of proximal fluid reabsorption could not be accounted for by changes in filtration rate, tubular geometry, or aldosterone secretion. The half-time of droplet reabsorption in the distal tubule was not altered by sodium depletion. An increase in the reabsorption of fluid in the proximal tubule, as demonstrated directly in the present experiments, provides an explanation for a variety of clinical phenomena associated with volume depletion.

Effect of saline infusions on intrarenal distribution of glomerular filtrate and proximal reabsorption in the dog

The effect of acute extracellular volume expansion with saline on the intrarenal distribution of glomerular filtrate, was studied in dogs utilizing micropuncture techniques in which samples were obtained by both recollection and from new tubules. Recollection was examined in seven dogs during continuous hydropenia and in five dogs during continuous saline diuresis. Recollection was associated with an increase in nephron flow rate of 8% during hydropenia and 27% during saline diuresis. In addition, during continuous saline diuresis, shortened transit times and lowered intratubular pressures were recorded in previously punctured tubules. Despite increased tubular flow, fractional reabsorption was unchanged. Nephron glomerular filtration rates (gfr) were measured during hydropenia and then after acute volume expansion in 10 dogs. In the repunctured tubules gfr rose 38% more than total glomerular filtration rate (GFR). In contrast, when new tubules were punctured during volume expansion, nephron gfr and total GFR changed proportionately. The disproportionate rise in nephron gfr after volume expansion noted with the recollection technique appears to be artifactual when contrasted to micropuncture of new tubules. With acute volume expansion, fractional reabsorption decreased 15% in recollected samples and 16% in newly sampled tubules. Increased nephron gfr cannot account for the fall in fractional reabsorption. It is concluded that in dogs, saline diuresis is not associated with redistribution of filtrate from deep to superficial nephrons, and that the fall in proximal fractional reabsorption is caused by diminished absolute reabsorption.

Demonstration of independent roles of proximal tubular reabsorption and intratubular load in the phenomenon of glomerulotubular balance during aortic constriction in the rat

The mechanism of glomerulotubular balance was investigated by microperfusion of the rat proximal tubule at two different rates before and after contriction of the aorta sufficient to produce a 50% reduction in whole kidney filtration rate and plasma flow. At a perfusion rate of 28 nl/min the absolute rate of proximal tubular reabsorption averaged 4.80+/-0.28 nl/mm.min in the absence of aortic constriction. Reducing the perfusion rate by one-half resulted in only a 22% decrease in the absolute rate of reabsorption, and imbalance between load and reabsorption resulted as fractional reabsorption of the perfused volume increased from 0.56 to 0.83 at 3 mm length of perfused tubule. These observations support other studies indicating that changing the load presented to the individual proximal tubule does not change reabsorptive rate sufficiently to result in glomerulotubular balance. Aortic constriction decreased the absolute rate of proximal tubular reabsorption approximately 50%, resulting in imbalance between load and reabsorption at the higher perfusion rate (fractional reabsorption of the perfused volume fell to 0.23 at 3 mm). Thus, the decrease in proximal tubular reabsorption necessary for glomerulotubular balance will occur independent of a change in the load presented for reabsorption. Balance between load and reabsorption was produced artificially by combining aortic constriction and a reduction in perfusion rate proportional to the reduction in whole kidney filtration rate. Mathematical analysis of the data suggests that the absolute rate of reabsorption along the accessible length of the proximal tubule is constant and is not proportional to the volume of fluid reaching a given site. Thus, there appears to be no contribution to glomerulotubular balance of any intra- or extratubular mechanism directly coupling load and the rate of proximal tubular reabsorption. It is concluded that glomerulotubular balance during aortic constriction is a consequence of hemodynamic effects of the maneuver to decrease filtration rate and the rate of proximal tubular reabsorption independently but in an approximately proportional manner.

Effects of increased sodium delivery on distal tubular sodium reabsorption with and without volume expansion in man

The separate effects of volume expansion and of increased delivery of sodium on sodium reabsorption in the diluting segment of the distal nephron were studied in man. In six normal subjects during a sustained water diuresis, sodium delivery to the distal nephron was increased without volume expansion by the administration of acetazolamide. In these subjects, free water clearance rose linearly as a function of urine flow. In five patients with complete, central diabetes insipidus, distal sodium delivery was increased by the infusion of hypertonic saline during a sustained water diuresis. In four of these five patients, changes in free water clearance were also observed during hypertonic saline diuresis in the presence of distal blockade of sodium reabsorption with chlorothiazide. At high rates of distal delivery the following observations were made: (a) free water clearance was lower and fractional sodium excretion higher during saline diuresis compared to acetazolamide diuresis; (b) although free water clearance was moderately reduced by chlorothiazide at low rates of urine flow, there was no difference in free water clearance between saline loading alone and saline plus chlorothiazide at high rates of urine flow; and (c) during saline loading free water clearance rose without evidence of a limit when increased distal delivery was accompanied by spontaneous increases in glomerular filtration rate, but tended toward a limit when glomerular filtration rate remained constant. The data indicate that during acute volume expansion with saline, there is a decrease in the fraction of delivered sodium reabsorbed in the distal nephron when compared to the response of the distal nephron to comparable increases in distal sodium delivery in the absence of volume expansion.

Effects of 0-9 per cent saline infusion on urinary and renal tissue composition in the hydropaenic, normal and hydrated conscious rat

1. Changes in water and solute outputs of hydropaenic, normal and hydrated conscious rats were determined during intravenous infusion (0.2 ml./min) of isotonic (0.9%) saline for 4 hr; renal tissue composition was determined before, and after 1 or 2 hr, infusion.2. In normal and hydrated rats increased excretion of water and sodium was such that urinary output matched intravenous input from about 2 hr. In hydropaenic rats, the diuretic and natriuretic response was much reduced; a retention of infused saline, equivalent to 15% body weight, occurred over 4 hr.3. A considerable increase in urea output and clearance, and a smaller increase in potassium and ammonium outputs, occurred in all groups.4. The corticomedullary osmolal gradients characteristic of non-diuretic rats were largely dissipated during saline infusion: by 1 hr in normal and hydrated rats, and by 2 hr in the hydropaenic group.5. These changes were ascribable mainly to an increase in tissue water content in all segments, particularly in the hydropaenic group; and to a profound decrease in urea content in all groups.6. Changes in tissue sodium content were smaller, and differed between segments and between the differently hydrated groups. A decrease in papillary content occurred in hydropaenic and normal groups and an increase in cortical and outer medullary content occurred in all groups.7. After 2 hr saline infusion, incomplete papillary-urinary osmotic equilibration was evident in all groups.8. These changes in medullary osmolality and in papillary-urinary osmotic equilibration preceded the maximal diuresis, and must contribute to the diuresis induced by saline infusion, as in water and osmotic diureses.

Influence of sodium balance on the ability of diuretics to inhibit tubular reabsorption. A study of factors that influence renal tubular sodium reabsorption in man

Chlorothiazide and ethacrynic acid were given to normal humans and patients with cirrhosis and ascites, with replacement of urinary losses, to obtain information on (1) the influence of different states of sodium balance and excretion on the effectiveness of the drugs in blocking tubular reabsorption and promoting excretion of sodium, and (2) the influence of different states of sodium balance on reabsorption of sodium by different segments of the nephron of man. The results demonstrated that in normal man the natriuretic response to these agents is markedly influenced by the existing state of sodium balance. The differences in response to the drugs related both to differences in the filtered load of sodium and to differences in the fraction of filtered sodium reabsorbed at the diuretic-sensitive distal tubular sites. Sodium loading in the presence of a mineralocorticoid was associated with a thiazideinduced increment in the excretion of sodium which was 10 times greater than that observed during sodium depletion. This effect is suggestive of decreased proximal tubular reabsorption. In patients with cirrhosis and sodium retention the fraction of filtered sodium reabsorbed at both diuretic-sensitive tubular sites was diminished, suggesting increased proximal tubular reabsorption. In addition to demonstrating a striking influence of sodium balance on the natriuretic response to the diuretics, the results suggest that in man sodium diuresis is associated with decreased fractional reabsorption by the proximal tubule and increased fractional and absolute sodium reabsorption by the distal tubule. In patients accumulating edema and ascites, an increase in the fractional reabsorption of sodium appears to occur in the proximal tubule and could account for limited effectiveness of the diuretic agents.