Renin-angiotensin-aldosterone system in nephrotic syndrome

Proteasuria-The impact of active urinary proteases on sodium retention in nephrotic syndrome

Sodium retention and extracellular volume expansion are typical features of patients with nephrotic syndrome. In recent years, from in vitro data, endoluminal activation of the epithelial sodium channel (ENaC) by aberrantly filtered serine proteases has been proposed as an underlying mechanism. Recently, this concept was supported in vivo in nephrotic mice that were protected from proteolytic ENaC activation and sodium retention by the use of aprotinin for the pharmacological inhibition of urinary serine protease activity. These and other findings from studies in both rodents and humans highlight the impact of active proteases in the urine, or proteasuria, on ENaC-mediated sodium retention and edema formation in nephrotic syndrome. Targeting proteasuria could become a therapeutic approach to treat patients with nephrotic syndrome. However, pathophysiologically relevant proteases remain to be identified. In this review, we introduce the concept of proteasuria to explain tubular sodium avidity and conclude that proteasuria can be considered as a key mechanism of sodium retention in patients with nephrotic syndrome.

Acute kidney injury complicating nephrotic syndrome of minimal change disease

Minimal change disease accounts for 70% to 90% of cases of nephrotic syndrome in children. It also causes nephrotic syndrome in adults, including patients older than age 60. Renal function is altered moderately in approximately 20% to 30% of patients because foot-process fusion impairs filtration of water and solutes. The glomerular filtration rate is reduced by approximately 20% to 30% and returns to baseline with remission of proteinuria. Over the past 50 years, a number of publications have reported cases of acute kidney injury occurring in approximately one-fifth to one-third of adult cases in the absence of prior or concomitant renal disease. Clinical attributes point to a male predominance, age >50, massive proteinuria, severe hypoalbuminemia, a background of hypertension and vascular lesions on kidney biopsy, along with ischemic tubular necrosis. Acute kidney injury may require dialysis for weeks or months until remission of proteinuria allows resolution of oliguria. In some cases, renal function does not recover. An effect of endothelin-1-induced vasoconstriction at the onset of proteinuria has been proposed to explain tubular cell ischemic necrosis. The main factors causing acute kidney injury in patients with minimal change disease are diuretic-induced hypovolemia and nephrotoxic agents. Acute kidney injury is uncommon in children in the absence of intercurrent complications. Infection, nephrotoxic medication, and steroid resistance represent the main risk factors. In all patients, the goal of supportive therapy is essentially to buy time until glucocorticoids obtain remission of proteinuria, which allows resolution of renal failure.

The pathophysiology of edema formation in the nephrotic syndrome

The mechanism of edema formation in the nephrotic syndrome has long been a source of controversy. In this review, through the construct of Starling's forces, we examine the roles of albumin, intravascular volume, and neurohormones on edema formation and highlight the evolving literature on the role of primary sodium absorption in edema formation. We propose that a unifying mechanism of sodium retention is present in the nephrotic syndrome regardless of intravascular volume status and is due to the activation of epithelial sodium channel by serine proteases in the glomerular filtrate of nephrotic patients. Finally, we assert that mechanisms in addition to sodium retention are likely operant in the formation of nephrotic edema.

Interstitial inflammation, sodium retention, and the pathogenesis of nephrotic edema: a unifying hypothesis

BACKGROUND

The pathophysiology of edema in the nephrotic syndrome is controversial. Some investigators believe that sodium retention may result from a primary renal defect that causes an "overfilled" blood volume. In contrast, other authors believe that fluid escapes the vascular compartment due a low oncotic pressure, and sodium retention is a compensatory physiological response to an "underfilled" blood volume. The patients that best fit the "underfilled" hypothesis are children with minimal-change nephrotic syndrome (MCNS).

METHODS

We analyzed critically the available evidence for and against each proposed pathogenic mechanism in the light of recent evidence indicating that the inflammatory infiltrate may play a role in primary renal sodium retention.

RESULTS

Inflammatory infiltrate in the kidney is a constant characteristic in nephrotic syndrome associated with primary sodium retention and it is absent in most cases of MCNS in children

CONCLUSIONS

We propose that primary sodium retention in the nephrotic syndrome depends on the existence and the intensity of renal inflammatory infiltrate, conspicuously absent in most cases of MCNS in children and present in other conditions associated with massive proteinuria. The tubulointerstitial inflammatory infiltrate is associated with increased vasoconstrictive mediators that result in increased tubular sodium reabsorption and with glomerular hemodynamic changes that reduce filtered sodium load.

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.

Nephrotic edema--pathogenesis and treatment

The cardinal features of the nephrotic syndrome are albuminuria, hypoalbuminemia, and edema. Traditionally, albuminuria was thought to be responsible primarily for the development of hypoalbuminemia. A decreased plasma-albumin concentration accompanied by a decreased plasma-oncotic pressure was thought responsible for the development of edema and secondary salt retention by the kidney. However, new findings have prompted a reevaluation of these relationships. For example, increased renal catabolism and blunted hepatic synthesis appear to play major roles in the development of hypoalbuminemia. Evidence suggests that primary, rather than secondary, salt retention by the kidney and activation of mechanisms that limit fluid movement across the capillary wall participate in the pathogenesis of the nephrotic syndrome and related edema. The treatment of patients with the nephrotic syndrome should limit proteinuria. This can be accomplished by administering angiotensin-converting enzyme inhibitors, lowering the protein content of the diet, and cautiously using non-steroidal antiinflammatory agents.

Reversible renal failure in the nephrotic syndrome

Acute, usually reversible, renal failure has been observed in patients with normal or minimally altered glomeruli on renal biopsy. This review aims to examine the clinical features of acute renal failure in these patients and to evaluate factors that may contribute to the reduction in glomerular filtration rate (GFR). In an analysis of 79 cases affecting 75 patients reported in the English literature since 1966, with acute renal failure associated with minimal change disease or mild histopathological changes in glomeruli, the average age was 58 +/- 2 years (mean +/- 5 SEM), urine protein excretion 11.6 +/- 0.6 g/d, and serum albumin level 19 +/- 1 g/L (1.9 +/- 0.1 g/dL). Acute renal failure was documented an average of 29 +/- 5 days after onset of nephrotic syndrome, and persisted for 7 weeks in 62 episodes in the 58 patients in whom recovery of renal function occurred. Fourteen patients died of uremia or required chronic dialysis, and 3 were lost to follow-up. Although plasma volume depletion was sometimes cited as the cause of renal failure, objective signs of hypovolemia were not documented and most patients did not improve after treatment designed to correct volume deficits. In contrast, histopathological changes consistent with acute tubular necrosis (ATN) were observed in at least 60% of cases. Since the pathogenesis of acute renal failure in minimal change nephrotic syndrome is unknown, we evaluated hemodynamic determinants of GFR in patients with minimal change disease with normal or near-normal renal function, and in relevant animal models, to obtain insights into the effect of nephrotic syndrome on GFR. Although acute renal failure is uncommon, GFR is reduced concurrently with nephrotic syndrome in approximately 30% of children and adults. Absolute and effective blood volume and renal plasma flow are relatively well preserved. However, clinical and experimental observations suggest that the glomerular ultrafiltration coefficient may be reduced by as much as 50%. These findings, together with renal biopsy changes in cases with acute renal failure, suggest that severe reductions in GFR in some patients with minimal change nephrotic syndrome may result from an interaction between acute ischemic tissue injury and preexisting intrinsic renal abnormalities.

Renal sodium retention in the nephrotic syndrome

Where do the experiments on rat kidneys exposed to puromycin leave us in attempting to evaluate further the pathophysiology of oedema in human nephrotic syndrome? They cannot be considered conclusive. The arguments favouring an intrinsic abnormality in the kidney as a major factor in sodium retention, rather than this being a secondary response to humoral and neural influences stimulated by changes in actual or perceived plasma volume, would be strengthened by experiments using another animal model of the nephrotic syndrome. But taken together with the longstanding clinical observations demonstrating the absence of any correlation between plasma oncotic pressure and diuresis or sodium retention, the work particularly of Dorhout-Mees and his colleagues on plasma volume, the frequent failure of infusions of salt-free albumin to induce natriuresis, and the profound resistance to intensive diuretic therapy in many nephrotics, the evidence for an important, probably predominant, role of intrinsic renal retention of sodium is strong. This is not to negate the importance of Starling forces in determining the distribution of the retained salt and water, nor to suggest that, on occasion at least, hypovolaemia, relative or absolute, may contribute to sodium retention. Certainly hypotensive shock due to hypovolemia in nephrotic patients untreated by diuretics has been observed often enough, particularly in children, and the risk of over-enthusiastic diuretic treatment resulting in tubular necrosis in the course of management of minimal change disease, is well recognised. An important role for leakage of plasma from the vascular compartment in the initiation of the oedema of the nephrotic state is certainly likely, but to consider this the major continuing mechanism is really not tenable.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute post-streptococcal glomerulonephritis without proteinuria

Two cases of histologically proven acute post-streptococcal glomerulonephritis presenting as congestive cardiac failure with normal blood pressure are reported. Proteinuria was not a finding. These features are discussed.