Effect of plasma albumin on sodium reabsorption in patients with nephrotic syndrome

Serum creatinine is a poor marker of GFR in nephrotic syndrome

BACKGROUND

In daily clinical practice creatinine clearance is used as marker of glomerular filtration rate (GFR). As a result of the tubular secretion process endogenous creatinine clearance (ECC) overestimates glomerular filtration rate, particularly in patients with impaired renal function. It has been suggested that the tubular handling of creatinine is altered in patients with a nephrotic syndrome.

METHODS

Inulin clearance (GFR) and creatinine clearance (ECC) have been simultaneously measured in a cohort of 42 patients with proteinuria and 45 healthy controls. The clearance of creatinine by tubular secretion (TScreat) can be estimated by ECC-GFR. TScreat was calculated in both groups. Regression analysis was performed to identify factors that independently influence tubular creatinine secretion.

RESULTS

The mean age (+/-SD) of the patients was 41+/-13 years, serum albumin 26+/-9 g/l, median (IQR) proteinuria 4.5 (3.6-8.2) g/10 mmol creatinine, serum creatinine 103 (84-143) micromol/l, ECC 85 (69-118) ml/min/1.73 m2, and GFR 54 (36-83) ml/min/1.73 m2. Median TScreat amounted to 29 (21-36) ml/min/1.73 m2. In the healthy controls serum creatinine was 75 (70-81) micromol/l, ECC 118 (109-125) ml/min/1.73 m2, GFR 106 (102-115) ml/min/1.73 m2, and TScreat 11 (3.5-19) ml/min/1.73 m2. By regression analysis serum albumin was identified as an independent predictor of tubular creatinine secretion. We divided the patients in two subgroups based on serum albumin levels. TScreat was 24 (14-29) ml/min/1.73 m2 in patients with serum albumin levels >25.8 g/l, and 36 (28-54) ml/min/1.73 m2 in patients with serum albumin levels <25.8 g/l (P<0.01).

CONCLUSION

Serum albumin levels influence tubular creatinine secretion. As a result, the endogenous creatinine clearance as well as estimated GFR using a modified MDRD equation more pronouncedly overestimate glomerular filtration rate in nephrotic syndrome.

Pathobiochemistry of nephrotic syndrome

Nephrotic syndrome is a clinical and laboratory syndrome caused by the increased permeability of the glomerular capillary wall for macromolecules. Nephrotic syndrome is a potentially life-threatening state and persistent nephrotic syndrome has a poor prognosis with a high risk of progression to end-stage renal failure and a high risk of cardiovascular complications due to severe hyperlipidemia. Pathogenesis of increased glomerular permeability in different glomerular diseases has not been fully elucidated. Recently, identification of the mutated genes for some podocyte proteins (nephrin, podocin, alpha-actinin-4) in rare familial forms of nephrotic syndrome shed has new light on the molecular mechanisms of glomerular permselectivity. Gradually it becomes apparent that sporadic mutations of podocyte proteins (e.g., podocin) may be present even in some patients with acquired nephrotic syndrome. Expression of other podocyte proteins may change during the course of experimental nephrotic syndrome, possibly as a response to podocyte damage resulting either in apoptosis or stimulation of proliferation and some form of repair, including glomerular sclerosis. Better understanding of these mechanisms could clearly also have therapeutic implications. Glomerular permeability factors are believed to play a role in some noninflammatory glomerular diseases, mainly minimal change disease and focal segmental glomerulosclerosis, but their molecular identification remains elusive, possibly due to the nonhomogeneous nature of the underlying diseases. As an example, focal segmental glomerulosclerosis possibly can be caused by the sporadic mutation of some genes for podocyte proteins, increased production of glomerular permeability factor (possibly by T lymphocytes), or the loss of inhibitors of glomerular permeability factors in nephrotic urine. Clearly the factors causing increased glomerular permeability and factors perpetuating glomerular sclerosis are not necessarily the same. Proteinuria does not seem to be only the consequence of glomerular damage, but it may possibly cause tubular damage and initiate interstitial fibrosis and thus contribute to the progression of chronic renal failure in proteinuric renal diseases. Recent insights into the mechanisms of tubular protein reabsorption may give new tools for preventing the progression of chronic renal disease. Cubilin inhibitors could potentially ameliorate tubular and interstitial damage in patients with heavy proteinuria refractory to treatment. Nephrotic hyperlipidemia is accompanied with increased risk of cardiovascular complications and should be treated in all patients with persistent nephrotic syndrome. The putative positive effect of hypolipidemic drugs (namely statins) on the cardiovascular risk and potentially also on the rate of progression of chronic renal failure remains to be demonstrated in prospective controlled studies. Recent progress in understanding podocyte biology in rare inherited glomerular diseases gives the chance to understand in the near future the molecular pathogenesis of increased glomerular permeability in the much more common acquired forms of nephrotic syndrome.

Collecting duct (Na+/K+)-ATPase activity is correlated with urinary sodium excretion in rat nephrotic syndromes

In puromycin aminonucleoside (PAN)-treated nephrotic rats, sodium retention is associated with increased (Na+/K+)-ATPase activity in the cortical collecting ducts (CCD). This study was undertaken to determine whether stimulation of (Na+/K+)-ATPase in the CCD is a feature of other experimental nephrotic syndromes, whether it might be responsible for renal sodium retention, and whether it is mediated by increased plasma vasopressin levels or activation of calcineurin. For this purpose, the time courses of urinary excretion of sodium and protein, sodium balance, ascites, and (Na+/K+)-ATPase activities in microdissected CCD were studied in rats with PAN or adriamycin nephrosis or HgCl2 nephropathy. The roles of vasopressin and calcineurin in PAN nephrosis were evaluated by measuring these parameters in Brattleboro rats and in rats treated with cyclosporin or tacrolimus. Despite different patterns of changes in urinary sodium and protein excretion in the three nephrotic syndrome models, there was a linear relationship between CCD (Na+/K+)-ATPase activities and sodium excretion in all three cases. The results also indicated that there was no correlation between proteinuria and sodium retention, but ascites was present only when proteinuria was associated with marked reduction of sodium excretion. Finally, the lack of vasopressin in Brattleboro rats or the inhibition of calcineurin by administration of either cyclosporin or tacrolimus did not prevent development of the nephrotic syndrome in PAN-treated rats or stimulation of CCD (Na+/K+)-ATPase. It is concluded that stimulation of Na(+/K+)-ATPase in the CCD of nephrotic rats might be responsible for sodium retention and that this phenomenon is independent of proteinuria and vasopressin and calcineurin activities.

Treatment of edematous disorders with diuretics

Generalized edema results from alterations in renal sodium homeostasis that ultimately result in an expansion of extracellular fluid volume and accumulation of interstitial fluid. The common edematous disorders include congestive heart failure, cirrhosis, nephrotic syndrome, and renal insufficiency. The abnormalities of sodium homeostasis contributing to edema formation in each condition are discussed. Management of volume homeostasis, with an emphasis on the role of diuretic therapy, is reviewed.

Cellular basis for blunted volume expansion natriuresis in experimental nephrotic syndrome

Experimental nephrotic syndrome results in sodium retention, reflecting, at least in part, an intrinsic defect in renal sodium handling in the distal nephron. We studied the relationships among plasma atrial natriuretic peptide (ANP) concentration, sodium excretion (UNaV), and urinary cyclic GMP excretion (UcGMPV) in vivo, and the responsiveness of isolated glomeruli and inner medullary collecting duct (IMCD) cells to ANP in vitro, in rats with adriamycin nephrosis (6-7 mg/kg body weight, intravenously). 3-5 wk after injection, rats were proteinuric and had a blunted natriuretic response to intravenous infusion of isotonic saline, 2% body weight given over 5 min. 30 min after onset of the infusion, plasma ANP concentrations were elevated in normals and were even higher in nephrotics. Despite this, nephrotic animals had a reduced rate of UcGMPV after the saline infusion, and accumulation of cGMP by isolated glomeruli and IMCD cells from nephrotic rats after incubation with ANP was significantly reduced compared to normals. This difference was not related to differences in binding of 125I-ANP to IMCD cells, but was abolished when cGMP accumulation was measured in the presence of 10(-3) M isobutylmethylxanthine or zaprinast (M&B 22,948), two different inhibitors of cyclic nucleotide phosphodiesterases (PDEs). Infusion of zaprinast (10 micrograms/min) into one renal artery of nephrotic rats normalized both the natriuretic response to volume expansion and the increase in UcGMPV from the infused, but not the contralateral, kidney. These results show that, in adriamycin nephrosis, blunted volume expansion natriuresis is associated with renal resistance to ANP, demonstrated both in vivo and in target tissues in vitro. The resistance does not appear related to a defect in binding of the peptide, but is blocked by PDE inhibitors, suggesting that enhanced cGMP-PDE activity may account for resistance to the natriuretic actions of ANP observed in vivo. This defect may represent the intrinsic sodium transport abnormality linked to sodium retention in nephrotic syndrome.

Unsaturated fatty acids activate glycogen phosphorylase in cultured rat hepatocytes

Oleate, linoleate, linolenate, arachidonate and eicosapentaenoate, but not myristate, palmitate and stearate, stimulated glycogen phosphorylase activity by 2-8-fold when added to cultured rat hepatocytes. Addition of BSA or Ca2- to the incubation medium decreased the stimulating effects of the unsaturated fatty acids. The combination of oleate or linolenate, with corticosterone, testosterone or estradiol produced synergistic stimulations of phosphorylase activity. The stimulation of glycogen phosphorylase activity by linolenate was inhibited by staurosporine or sphingosine. Staurosporine (80 nM) alone also decreased basal phosphorylase activities by about 60%. The results show that unsaturated fatty acids can be used as model agonists to stimulate phosphorylase activity by a mechanism that probably involves protein kinase C. On the basis of the fatty acid: BSA ratios used, this stimulation should only occur in vivo at high fatty acid concentrations when accompanied by hypoalbuminaemia.

The long-term outcome of post-ischaemic acute renal failure in the rat. II. A histopathological study of the untreated kidney

Histopathological changes in kidneys subjected to 45 min of isothermic unilateral ischaemia in the acute phase and 1 week and 1 month after primary damage were studied at the electron microscopic level. During the first week after recirculation long homogeneous cylinders, probably consisting of Tamm-Horsfall protein, developed in the medullary parts of the nephron, and after 1 month of recirculation there were two types of nephrons: (1) nephrons with a normal histological appearance and (2) degenerated nephrons. The latter group gave rise to crypts in the outer cortical area. It is hypothesized that the generation of the long Tamm-Horsfall cylinders in the thick ascending limb of the loop of Henle plays an important role in the long-term outcome of the kidney after the primary damage. The persistent blockade caused by these cylinders will lead to precipitation of the ultrafiltrate, resulting in long cell-protein cylinders in the proximal parts of the nephron. This precipitation will proceed in the retrograde direction, reaching the mother glomeruli, eventually leading to total degeneration of the nephron.

Contribution of superficial nephron segments to sodium excretion in experimental glomerulonephritis

The present study examined the contribution of individual superficial nephron segments to sodium excretion in antiglomerular basement membrane nephritis in the rat by sampling the same nephron successively from the end and beginning of the distal tubule and end of the proximal tubule. Whole kidney GFR in glomerulonephritic rats was reduced by approximately 40% from controls; absolute sodium excretion was about 25% of normal. Metabolic balance studies in the awake state had suggested that the animals were in sodium balance. Plasma renin levels before and during micropuncture were similar to controls. These findings suggest that the defect in sodium handling is intrinsic to the kidney. Glomerulotubular balance was maintained along the proximal tubule. Sodium reabsorption in the loop of Henle was reduced in absolute terms but was proportional to the load delivered. Due to the decreased absolute sodium reabsorption in the preceding segments, sodium delivery to the beginning of the distal tubule was comparable in the two groups of animals. Along the distal tubule sodium reabsorption was comparable to control animals. Therefore, the avid urinary sodium retention seen during micropuncture was due to increased sodium reabsorption by segments past the superficial proximal tubule and/or by deep nephrons.

Intra- and extrarenal factors of oedema formation in the nephrotic syndrome

The role of intra- and extrarenal factors in oedema formation in children with nephrotic syndrome is reviewed. Oedema reflects an abnormal accumulation of fluid within the interstitial tissue. At the capillary level oedema develops when increased lymph flow is no longer effective for the removal of interstitial fluid and the maintenance of intravascular volume. Alterations of intrarenal haemodynamics and tubular sodium reabsorption contribute to sodium retention. Recent studies suggest that during oedema formation reduced effective circulatory volume triggers changes in various hormonal systems, such as renin-angiotensin-aldosterone, noradrenaline, dopamine, vasopressin, prostaglandins and natriuretic factors, which contribute to sodium and water retention. It appears that the release of atrial natriuretic peptide following central volume expansion is responsible for the increased urine flow and natriuresis after intravenous administration of albumin.

The nephrotic syndrome and its complications

Modern views of the pathogenesis and natural history of nephrotic syndrome have changed substantially since the early studies by Cotugno and Bright. Contrary to beliefs held 20 years ago, we do not possess a unique satisfying explanation for the induction, maintenance, and resolution of nephrotic edema, and many concepts firmly established as "classic" are now being revised or reconsidered. These include the relationship between urinary protein losses and hypoalbuminemia, which is complicated by several factors such as daily protein intake, albumin catabolism, and the possible role of albumin loss at extrarenal sites. The influence of lowered plasma albumin on the decrease in plasma volume is also quite complex, due to technical difficulties in measuring plasma volume and turnover of radio-labeled albumin. The most contentious areas are how sodium and water retention are initiated and maintained and the relationship between hypoalbuminemia, plasma oncotic pressure, and edema. While aldosterone excretion and plasma concentrations are elevated in nephrotic patients, data on the renin-angiotensin system are controversial and the renal handling of sodium is related to a host of factors including glomerular filtration rate, altered proximal tubular reabsorption, and the role of vasodilators or vasoconstrictors. The complications of nephrotic syndrome are protean and relatively common. Among those are acute renal failure, thrombosis, infections, and hyperlipidemia. Since the introduction of percutaneous biopsy, the spectrum of lesions underlying nephrotic syndrome has widened considerably, the most common being minimal change, especially in children. There are very few prognostic indicators by which response to treatment may be predicted and these include persistent microscopic hematuria.

Atrial natriuretic peptide and other vasoactive hormones in nephrotic syndrome

Plasma levels of atrial natriuretic peptide (ANP), arginine vasopressin (AVP), renin activity (PRA), aldosterone (PA), catecholamines and urinary prostaglandins (PG), as well as renal function were measured in children in the edematous state of the nephrotic syndrome before and after infusion of human serum albumin. Before albumin infusion, plasma levels of AVP, PRA, PA and noradrenaline (NA) and urinary excretion of PGE2, PGE-Met, PGF2 alpha were elevated. The mean value of plasma ANP was in the normal range. Albumin infusion produced a 36% increase in the calculated plasma volume. It was associated with a fivefold rise in the plasma level of ANP (31.6 +/- 22.6 vs. 151.4 +/- 52 fmol/ml mean, SD), and a significant fall in the levels of PRA, AVP, PA, and NA. Similarly, urinary concentration of PGE2, PGE-Met and PGF2 alpha fell. Urine flow, GFR, UNaV, FENa, and COsm increased significantly, while CH2O remained unchanged. The diuresis, natriuresis and GFR correlated with the level of plasma ANP, while urinary sodium excretion did not correlate with PA or NA levels. These findings suggest that ANP plays an important role in albumin induced natriuresis in children with nephrotic syndrome.

Distal blockade in experimental glomerulonephritis: the role of the diluting segment in sodium retention

In the light of accumulating evidence implicating the diluting segment as the site of final regulation of sodium excretion by the nephron, we produced in this experiment distal blockade in anti-glomerular basement membrane (GBM) glomerulonephritic (GN) rats by the administration of furosemide and polythiazide. This allowed to dissociate the sodium reabsorption that occurs in the proximal tubule from the one that occurs more distally and permitted an appreciation of the rôle played by the diluting segment in the sodium retention of anti GBM GN. In a previous experiment we showed that GN conscious or anaesthetized rats presented an increase in Na tubular reabsorption and failed to raise their fractional and absolute excretion of sodium as normal one did after rapid volume expansion. In this study distal blockade corrected almost completely the difference in sodium excretion that existed between GN and normal groups before the administration of diuretics, pointing to the important rôle played by the diluting segment in the sodium retention of experimental GN.

Renal sodium handling in minimal change nephrotic syndrome

Renal sodium handling was studied in 23 children at three different stages of the minimal change nephrotic syndrome--the oedema forming state, proteinuric steady state, and remission. Clearances of inulin and para-aminohippuric acid and urinary sodium excretion were determined basally, after intravenous infusion of isotonic saline and hyperoncotic albumin, and after furosemide injection. Absolute and fractional basal sodium excretion were significantly lower in oedema forming patients than in proteinuric patients in steady state, and non-proteinuric patients. In contrast to proteinuric patients in steady state and non-proteinuric patients, the oedema forming patients failed to respond to isotonic saline infusion with increased sodium excretion. After diuretic blockade with furosemide, the fractional sodium excretion of the oedema forming patients increased to values no different from those of the non-proteinuric patients, whereas the fractional sodium excretion of the steady state patients increased to significantly higher values. The plasma aldosterone concentration was within normal limits in 11 of 14 proteinuric patients, and did not correlate with the basal sodium excretion. Thus, sodium retention in the minimal change nephrotic syndrome was found only in oedema forming patients, and since this is not related to the plasma aldosterone concentration it may be caused by an intrarenal mechanism, probably sited in distal parts of the nephron.

Role of plasma vasopressin in the impairment of water excretion in nephrotic syndrome

To verify whether or not an increased secretion of ADH may cause the water retention commonly observed in nephrotic syndrome, 16 nephrotic patients and 13 normal control subjects were studied in basal conditions and following a water load or an iso-osmotic blood volume expansion by 20% albumin infusion. In the basal condition there were no differences in plasma ADH, urine output, urinary osmolality (UOsm), and plasma renin activity between nephrotic patients and control subjects; POsm, PNa+, UNaV, and blood volume (BV) instead, were significantly lower in nephrotic patients than in control subjects. Following the water load control subjects reached a minimal UOsm of 82 +/- 12 mOsm/kg at 60 min and excreted completely the ingested water in 150 min; nephrotic patients reached a minimal UOsm of 160 +/- 111 mOsm/kg at 120 min, and the water was eliminated completely in 240 min. Plasma ADH decreased significantly in the first hour following water load only in control subjects. A significant direct correlation was observed between plasma ADH and POsm in control subjects (ADH = -85 + 0.30 POsm, P less than 0.001) but not in nephrotic patients. Plasma ADH was inversely correlated with BV in nephrotic patients (ADH = 15.47 -0.17 BV, P less than 0.001) but not in normal control subjects. In nephrotic patients with reduced BV the expansion of BV with 20% albumin was effective in reducing the plasma levels of ADH and promoting a water diuresis. Our results demonstrate a sustained volume mediated secretion of ADH in the nephrotic syndrome, which is responsible for the impairment in water excretion.

Relationship of kidney function to immunopathology in chronic serum sickness of rats

This study sought to clarify the relationship between kidney function and immunopathology in chronic serum sickness (CSS) of rats. CSS was induced by chronic intravenous immunization with bovine serum albumin. Whole kidney function was studied during the course of CSS by assays of serum and urine. Single nephron function was evaluated by micropuncture techniques. Three categories (mild, moderate, severe) of kidney disease were identified from the analysis of kidney function in rats with CSS. Those categories represented distinct stages in the natural history of CSS nephritis. The three stages identified by measurements of function corresponded to distinct categories of kidney immunopathology. In rats with milk CSS, immune deposits were limited to the mesangium; histopathology was slight or absent. The only detectable change in protein handling was a small elevation of albumin concentration in tubule fluid. Abnormal proteinuria was a feature of moderate CSS; whole kidney glomerular filtration was not decreased despite evidence of significant immunopathology of glomeruli. Compromise of whole kidney function including decreased sodium excretion was only detected in the severe stage of CSS in association with diffuse proliferative glomerulonephritis. The transitions from mild to moderate and moderate to severe CSS were not gradual but occurred as discrete, sudden events.

Role for intrarenal mechanisms in the impaired salt excretion of experimental nephrotic syndrome

A unilateral model of puromycin aminonucleoside (PAN)-induced albuminuria was produced in Munich-Wistar rats to examine the mechanisms responsible for renal salt retention. 2 wk after selective perfusion of left kidneys with PAN (n = 8 rats) or isotonic saline (control, n = 7 rats), increases in albumin excretion and decreases in sodium excretion were demonstrated in PAN-perfused but not in nonperfused kidneys of PAN-treated rats although systemic plasma protein concentration remained at control level. Total kidney glomerular filtration rate (GFR) and superficial single nephron (SN) GFR were also reduced selectively in PAN-perfused kidneys, on average by approximately 30%, due primarily to a marked decline in the glomerular capillary ultrafiltration coefficient (Kf), which was also confined to PAN-perfused kidneys. Values for absolute proximal reabsorption (APR) were also selectively depressed in PAN-perfused kidneys, in keeping with a similarly selective decline in peritubular capillary oncotic pressure measured in these kidneys, the latter also a consequence of the fall in Kf. In a separate group of seven PAN-treated rats, however, no differences were detected between PAN-perfused and nonperfused kidneys in the absolute amount of sodium reaching the early (0.77 +/- 0.09 neq/min vs. 0.74 +/- 0.08, P greater than 0.40) and late portions of superficial distal tubules (0.31 +/- 0.02) neq/min vs. 0.32 +/- 0.05, P greater than 0.50), despite the lesser filtered load of sodium in PAN-perfused kidneys. Suppressed sodium reabsorption in both proximal convoluted tubules and short loops of Henle of PAN-perfused kidneys contributed to this equalization of sodium delivery rates to the late distal tubule, as did comparable reabsorption along distal convolutions. In two additional groups of PAN-treated rats, infusion of saralasin (0.3 mg/kg per h, i.v.) led to substantial increases in total kidney GFR and SNGFR in PAN-perfused but not in nonperfused kidneys. Despite these increases in total and SNGFR, urinary sodium excretion by PAN-perfused kidneys remained at a level far below that for nonperfused kidneys, again indicating that the antinatriuresis characterizing the PAN-perfused kidney is due to alterations in sodium handling by the tubules rather than changes in GFR. These results therefore indicate (a) that reductions in Kf and depressed sodium reabsorption by proximal tubules and Henle's loop segments in this model are brought about by intrarenal rather than circulating or systemic factors, and (b) assuming that superficial nephrons are representative of the entire nephron population, renal salt retention in this model is due primarily to intrarenal factor(s) acting beyond the distal convolution.

Pathogenesis of sodium and water retention in edematous disorders

Edema is a collection of fluid within the body's interstitial space which occurs when there is an alteration of the Starling forces which control transfer of fluid from the vascular compartment to surrounding tissue spaces. Generalized edema results when altered Starling forces affect all capillary beds, such as occurs in cardiac failure, cirrhosis, and nephrotic syndrome. Common to these conditions is the development of increased total body sodium and water content. The kidneys play an essential role in the retention of this sodium and water. In this article we shall discuss the signals the kidneys receive for sodium and water retention in these edematous disorders (afferent mechanisms). We shall also examine the means by which the kidney responds to these signals and retains sodium and water (efferent mechanisms). As shall become apparent these edematous states may share many of the same afferent and efferent mechanisms for sodium and water retention.

Formation and structure of human Hageman factor fragments

Autodigestion of activated Hageman factor (HFa) yields a 40,000-mol wt activated enzyme as well as Hageman factor fragment (HFf); HFf consists of two molecular weight species of 28,500 and 30,000. We have investigated the structure of these active fragments and demonstrate that upon reduction, each possesses a heavy chain of 28,000. The associated light chains were identified by subjecting iodinated proteins to two-dimensional slab gel electrophoresis in which the second dimension is run reduced. The 40,000-dalton enzyme has a light chain of 15,000, the 30,000-dalton form of HFf has a light chain of 2,000 and we have suggestive evidence of a light chain associated with the 28,500-dalton form of HFf (putative mol wt approximately 500). We also demonstrate that the 30,000-dalton form of HFf precedes the 28,500 form. These data indicate that digestion of native HF to form HFa precedes cleavages that fragment the molecule and diminish its molecular weight. The 28,500-dalton light chain of HFa becomes the heavy chain of each of the fragmentation products while cleavage at different points along the heavy chain of HFa determines which fragments will be produced. In contrast to autoactivation, kallikrein digestion of HFa yields primarily HFf; however, the 40,000-dalton enzyme may be seen when prekallikrein-deficient (Fletcher trait) plasma is activated.

Pathophysiology of Tamm-Horsfall protein

Tamm-Horsfall protein, a renal glycoprotein present in normal urine, is the primary constituent of urinary casts. Immunoelectron microscopy has shown that this protein is localized selectively along surface membranes of the thick ascending loop of Henle. In this surface membrane site, the unique aggregation and gel formation of Tamm-Horsfall protein in response to increasing concentrations of electrolytes within physiologic ranges may influence the permeability characteristics of this nephron segment. These aggregation characteristics also play a role in pathologic conditions and lead to the prolonged persistence of interstitial Tamm-Horsfall protein deposits in several tubulointerstitial diseases. Recent studies have demonstrated immunologic responses to this protein, including an immune complex tubulointerstitial nephritis in rats mediated by autoantibodies to Tamm-Horsfall protein.

Renal sodium retention during volume expansion in experimental nephrotic syndrome

We have studied sodium retention during volume expansion in rats with autologous immune complex nephropathy (AICN), a model of nephrotic syndrome (NS) in which GFR after volume expansion was not different from that in adjuvant-injected controls (C). AICN rats developed heavy proteinuria (298 +/- 27 vs. less than 10 mg/day), hypoalbuminemia (2.14 +/- 0.15 vs. 3.08 +/- 0.12 g/100 ml) and hypercholesterolemia (181 +/- 22 vs. 58 +/- 4 mg/100 ml). After saline, there were no significant differences in blood pressure (119 +/- 2 vs. 114 +/- 2 mm Hg), renal plasma flow (4.9 +/- 0.41 vs. 4.1 +/- 0.28 ml/min), inulin clearance (1.37 +/- 0.06 vs. 1.55 +/- 0.10 ml/min), or SNGFR (47 +/- 2 vs. 53 +/- 4 nl/min). Sodium excretion, however, was significantly lower in NS rats (4.7 +/- 1.1 vs. 9.2 +/- 1.2 muEq/min). Proximal sodium reabsorption was decreased in NS rats (35 +/- 2 vs. 41 +/- 2%, 2.5 +/- 0.2 vs. 3.3 +/- 0.2 nEq/min). Sodium delivery into the loop, however, was equal in NS and C, since the slightly lower filtered load in NS rats offset the depression in proximal reabsorption. Sodium reabsorption by the loop and by the distal convoluted tubules were equal in NS and C. Thus, sodium delivered into the cortical collecting ducts was the same in both groups (0.33 +/- 0.17 vs. 0.34 +/- 0.07 nEq/min; 4.5 +/- 0.6% of filtered sodium vs. 4.4 +/- 0.3%). The percent of filtered sodium excreted in the urine, however, was significantly lower in the NS rats, 2.18 +/- 0.48% vs. 4.0 +/- 0.58%. We conclude that antinatriuresis in this model of NS is determined beyond the superficial late distal convoluted tubule. The inability to excrete the sodium load during volume expansion is due to either enhanced reabsorption by the collecting duct or to abnormal function in deep nephrons.

Sodium wasting, acidosis and hyperkalemia induced by methicillin interstitial nephritis. Evidence for selective distal tubular dysfunction

A 61 year old male patient was studied who manifested dehydration, azotemia, acidosis and hyperkalemia six weeks after exposure to methicillin. Thyroid and adrenal glucocorticoid and mineralocorticoid function were normal. The dehydration was found to be caused by a profound sodium-losing nephropathy; urinary sodium ranged from 78 to 101 meq/day during a salt restricted diet. A distal renal tubular acidosis and a quantitively impaired ability to excrete potassium were also found. These defects were relatively unresponsive to mineralocorticoid or prednisone therapy. A renal biopsy specimen showed an interstitial nephritis which selectively affected distal tubules and was thought to be secondary to methicillin. The data suggest functional impairment specific for the distal tubule, but with only a modest decrease in the glomerular filtration rate.

Reversibility of the "salt-losing" tendency of chronic renal failure

We examined the salt-losing tendency of chronic renal failure in five patients with glomerular filtration rates varying from 5.2 to 16.0 ml per minute. When the studies began, patients were in sodium balance on metabolic diets containing 58 to 342 meq of sodium daily. The sodium content of the diet was then reduced in stages at intervals of at least one week over four to 14 weeks with careful clinical and laboratory monitoring. In four of the patients a salt-losing tendency developed, temporarily requiring intravenous salt replacement in two. At the completion of the studies all the patients remained in sodium balance while ingesting a mean (+/- S.D.) of 5.0 +/- 2.9 meq of sodium per day. There was no change in renal function. Thus, the salt-losing tendency of chronic renal failure is reversible and appears to be a manifestation of the long-term adaptation for sodium excretion.

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.