The renal handling of amylase in normal man.

The renal glomerular and tubular transport rate of amylase was studied by measuring the urinary excretion of this protein before and during inhibition of tubular protein reabsorption by lysine. The excretion of amylase was compared with the excretion of albumin, beta-2 microglobulin and free light chains of immunoglobulins. This investigation showed that amylase is reabsorbed by the tubular cells, but only to a very modest degree compared with the reabsorption of the other three proteins. In the case of amylase only about 45% of the filtered molecules are reabsorbed, whereas more than 90% of the filtered amount of the other molecules is reabsorbed by the tubular cells. The excretion of amylase rose after lysine injection only by a factor 1.8, whereas excretion rose by a factor 28 for albumin, 1,500 for beta-2 microglobulin, 16 for kappa chains and 8 for lambda chains. Minimal values for tubular reabsorption were found to be 5.5 +/- (SD) 4.3 U/min for amylase, 174.0 +/- 35.7 micrograms/min for albumin, 90.5 +/- 14.4 micrograms/min for beta-2-microglobulin, 70.4 +/- 17.4 micrograms/min for kappa chains and 24.2 +/- 9.2 micrograms/min for lambda chains.     

The renal uptake of proteins: a nonselective process in conscious rats

The selectivity of the renal reabsorption of proteins has been investigated by competition experiments in conscious rats. The animals were intravenously injected with increasing doses of proteins over a wide range of net charge and size, including lysozyme, cytochrome C, metallothionein, beta 2-microglobulin, retinol-binding protein, albumin and IgG. The urinary excretion of exogenous proteins injected concomitantly (human beta 2-microglobulin, retinol-binding protein, albumin and/or egg white lysozyme depending on the experiment) and of rat beta 2-microglobulin, albumin and IgG was determined with specific immunoassays. The results show that low molecular weight cationic proteins and low or high molecular weight anionic proteins can increase each other's urinary excretion. Several observations strongly suggest that these effects result from a competitive inhibition of renal uptake. The phenomenon is dose-related in most cases and, as evidenced by cytochrome C injection, transient, reproducible and saturable. In addition, the injected proteins induce a tubular type proteinuria irrespective of their net charge and size. In the case of cationic proteins, this finding excludes the possibility of an enhanced glomerular permeability due to a partial neutralization of the glomerular polyanion which, as demonstrated with protamine sulfate, entails a glomerular type proteinuria. These quantitative data on the mutual inhibition of renal uptake of a wide spectrum of specific proteins lead us to challenge the concept of charge- and size-selective tubular reabsorption of proteins, and to postulate that proteins filtered through the glomeruli are taken up by common tubular endocytotic sites irrespectively of their physicochemical features. As demonstrated by the ability of beta 2-microglobulin and IgG to inhibit the uptake of lysozyme, the affinity of a protein for reabsorption sites is not simply related to its size and net positive charge. Evidence is also presented that proteins, when administered intravenously at high doses, induce a lysosomal enzymuria most likely reflecting a stimulated exocytosis.     

Influence of albumin infusion on the urinary excretion of beta2-microglobulin in patients with proteinuria

Most filtered proteins are reabsorbed by the renal proximal tubule by a mechanism that involves binding to the brush border membrane and endocytosis. Under normal conditions the low-molecular-weight protein beta2-microglobulin (beta2M), which is used to detect tubular injury, is reabsorbed almost completely. However, in proteinuric patients an increased urinary excretion of beta2M may not simply reflect tubular damage but might also result from a decreased tubular reabsorption due to competitive mechanisms. To examine the magnitude of such an effect we have studied the renal effects of albumin infusion (40 g in 2 h of a 20% solution) in 10 patients with a glomerular disease and proteinuria >3.5 g/24 h. Before, during and after albumin infusion the GFR (inulin clearance), RPF (PAH clearance), blood pressure and the urinary excretion of albumin, IgG, transferrin and beta2M were measured. Albumin infusion resulted in a slight decrease of the GFR (72 +/- 11 ml/min before and 67 +/- 10 ml/min after infusion), an increase of the RPF (379 +/- 66 ml/min before and 445 +/- 83 ml/min after), a decrease of the filtration fraction (0.20 before and 0.17 after), and hemodilution. After infusion the urinary excretion of albumin increased from 4.5 +/- 0.7 to 8.4 +/- 1.6 mg/min (p < 0.05). The urinary excretion of IgG and transferrin increased, probably reflecting a change in glomerular size-selectivity. In contrast, the urinary excretion of beta2M did not change significantly (baseline 12 +/- 5 microg/min, end 13 +/- 6 microg/min, percentage change 16.8 +/- 11%). To correct for changes in tubular load we calculated the fractional reabsorption of beta2M. The initial rise in albuminuria during infusion did not affect fractional tubular reabsorption (Delta%: 0. 72 +/- 0.52%, median 0.005%). In the period after infusion a slight decrease was noted (median -0.33%, p < 0.01). A decrease in the fractional reabsorption was particularly observed in patients with pre-existing tubular damage. In conclusion: infusion of albumin in proteinuric patients has no clinically relevant effect on the tubular reabsorption of beta2M. Therefore, beta2M is useful as a parameter to detect tubular injury and alterations in tubular handling of proteins in patients with proteinuria and glomerular diseases.     

Glomerular size and charge selectivity in insulin-dependent diabetes mellitus

The pathogenesis of clinical nephropathy in Type 1 (insulin-dependent) diabetes was investigated by measuring renal fractional clearances of albumin, total IgG, IgG4 and beta 2-microglobulin, four plasma proteins which differ in size and charge. Seventy patients and eleven control subjects were studied. In diabetic patients with normal urinary albumin excretion (less than 30 mg/24 hr), fractional IgG clearance was two to three times higher than in control subjects, whereas fractional clearance of the anionic plasma proteins IgG4 and albumin was similar to that of control subjects. These alterations indicate an increase in anionic pore charge within the glomerular basement membrane concomitant with an increase in either pore size or impairment of tubular reabsorption. Diabetic patients, whose urinary albumin excretion has started to rise (30 to 100 mg/24 hr), had unchanged fractional IgG compared to patients with normal albumin excretion, while fractional IgG4 and albumin clearances were increased three- to fourfold; indicating unchanged glomerular pore size, but a decrease in anionic pore charge. In patients demonstrating urinary albumin excretion of greater than 100 mg/24 hr fractional IgG clearance increased to the same extent as fractional albumin clearance, indicating an increase in large pore area. Fractional beta 2-microglobulin clearances were similar to that of control subjects in the different patient groups indicating unchanged tubular reabsorption of proteins. Thus, the increase in large pore area seen in patients with clinical nephropathy is preceded by loss of anionic charge in the glomerular basement membrane. It is likely that this loss of anionic charge is due to loss of heparan sulphate-proteoglycan.     

The effect of acute and chronic protein loading on urinary pepsinogen A excretion

In 8 healthy male volunteers, urinary excretion (UE) and fractional clearance (FC) of pepsinogen A (PGA), beta 2-microglobulin (beta 2-m) and albumin were measured after 6 days high protein diet (HPD; 2.0 g/kg/day) and compared to values obtained after 6 days low protein diet (LPD; 0.5 g/kg/day). In addition, the effect of an acute protein load (APL; 500 g beef) on these variables were measured. Both chronic and acute protein loading induced a rise in glomerular filtration rate (GFR) of about 10% together with a parallel rise in effective renal plasma flow. UE PGA and FC PGA increased both after HPD (UE PGA 1,707 +/- 1,106 ng/min; FC PGA 23 +/- 12%) as compared to LPD (UE PGA 1,200 +/- 987 ng/min, p less than 0.01; FC PGA 18 +/- 12%, p less than 0.05), and after APL (UE PGA 2,276 +/- 1,389 ng/min; FC PGA 26 +/- 16%) as compared to baseline (UE PGA 1,418 +/- 965 ng/min, p less than 0.02; FC PGA 21 +/- 12%, p less than 0.05). UE and FC of beta 2-m and albumin were not affected by protein loading. As PGA is nearly freely filtered, it is concluded that the increase in fractional PGA clearance reflects a decrease in fractional tubular PGA reabsorption. Our results suggest that an increase in fractional protein clearance after protein loading is not necessarily due to an impaired glomerular permselectivity but represents a decreased fractional tubular reabsorption as a result of a GFR-mediated increase in filtered load without a concomitant increase in tubular reabsorption.     

Urinary excretion of plasma proteins in diabetic subjects. Increased excretion of kappa light chains in diabetic patients with and without proliferative retinopathy

The urinary excretion of beta2-microglobulin, albumin, kappa light chains, transferrin, and IgG as well as their concentration ratios were assessed in 27 nondiabetic patients with proteinuria and in 72 IDDM patients, 41 with proliferative retinopathy (PR) and 31 without retinopathy, matched for age, duration of diabetes, and treatment. The mean excretions of albumin, transferrin, and IgG were similar in patients with nondiabetic proteinuria and in IDDM patients with PR and were significantly higher than in IDDM patients without retinopathy. Despite similar albumin excretion, the amount of excreted kappa light chains was significantly higher in IDDM patients than in patients with nondiabetic proteinuria, resulting in an elevated kappa chain/albumin ratio. Furthermore, diabetic subjects without microalbuminuria showed increased kappa chain/albumin ratio, indicating that increased urinary excretion of kappa chains may be an early sign of diabetic nephropathy. Determination of kappa light chain excretion may have clinical implications in the differentiation between proteinuria of diabetic and nondiabetic origin. The ratio kappa chain/albumin was independent of the excretion of beta2-microglobulin in patients with PR, suggesting that the reduced ability to reabsorb immunoglobulin light chains may occur earlier than that of beta2-microglobulin in the development of tubular dysfunction in insulin-dependent diabetes mellitus.     

Enzymuria and tubular proteinuria in diabetic rats: a 12-week follow-up study.

Various biochemical parameters of renal tubular function were examined for a period of up to 12 weeks in rats rendered diabetic by an i.v. injection of streptozotocin. Except for a statistically significant decrease in the urinary excretion of gamma-glutamyl-transpeptidase to 64% of control values, the urinary excretion of beta-N-acetyl-D-glucosaminidase, beta-galactosidase, alanine aminopeptidase, and lactate dehydrogenase significantly increases in diabetic rats to between 154% and 712% of control values. This increased enzymuria is not correlated to the marked polyuria induced by diabetes (r between 0.14 and 0.35, not significant). Enzymuria is also accompanied by a 10-fold increase in the urinary excretion of the low molecular weight protein beta 2-microglobulin while the excretion of albumin is not significantly modified, indicating impairment of tubular reabsorption in diabetic animals. Clearance studies reveal that the clearance of both beta 2-microglobulin and infused egg-white lysozyme are also increased. Finally the histopathologic examination of paraffin sections of the kidney show hydropic degenerescence and pycnosis of the tubular cells. It is concluded that early-stage diabetes results in tubular impairment and that the streptozotocin-rat model appears well suited to the study of these early signs of renal dysfunction.     

Albumin transport and processing by the proximal tubule: physiology and pathophysiology

PURPOSE OF REVIEW: Significant epidemiological and clinical trial evidence supports the association between increased urinary albumin excretion, cardiovascular events and renal failure. An increase in albumin excretion has traditionally been considered to reflect a 'glomerular' leak of protein; however, it is now recognized that significant tubular reabsorption of albumin occurs under physiological conditions that may be modified by genetic determinants, systemic disease and drug therapies. RECENT FINDINGS: The endocytosis of albumin by the proximal tubule is a highly regulated process depending on protein-protein interactions between several membrane proteins and scaffolding and regulatory molecules. The elucidation of these interactions is an ongoing research focus. There is also mounting evidence for a transcytotic pathway for retrieval of albumin from the tubular filtrate. The molecular basis for the role of albuminuria in both interstitial renal disease and cardiovascular pathology continues to be defined. The clinical implications of albuminuria due to a glomerular leak vs. reduced tubular reabsorption of albumin are, however, now under consideration. In particular, the prognostic implication of microalbuminuria induced by the more potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors is under study. SUMMARY: The currently defined mechanisms underpinning the tubular reabsorption of albumin, how these are modified by pathology and pharmacology, and the clinical implications are the subject of this review.     

Mild renal injury in Behçet's disease

AIM: The aim of this study is to investigate the frequency of microalbuminuria and abnormal urinary beta2-microglobulin excretion in patients with Beh?et's disease (BD). MATERIALS AND METHODS: Twenty-eight patients and 27 healthy controls were included in this study. Urine albumin/creatinine and beta2-microglobulin/creatinine ratios were calculated. RESULTS: The frequency of microalbuminuria and abnormal urinary beta2-microglobulin excretion was higher among patients with BD than in control group, but this was not statistically significant (p > 0.05). CONCLUSION: Microalbuminuria and abnormal beta2-microglobulin excretion are markers of renal injury, which have not been investigated in BD previously. Renal injury in BD is more frequent than has been recognized and it is most often in mild nature.     

Prednisolone can increase glomerular permeability to proteins in nephrotic syndrome

In patients with a nephrotic syndrome administration of prednisolone causes an increase of proteinuria. To elucidate the mechanism of this effect we have studied the acute proteinuric effect of prednisolone, 125 to 150 mg intravenously, in nine patients (7 M, 2F) with a nephrotic syndrome. Mean age (+/- SD) of the patients was 53 +/- 6 years, mean endogenous creatinine clearance 104 +/- 30 ml/min, and mean proteinuria 7.7 +/- 3.0 g/24 hr. After administration of prednisolone, urinary total protein excretion rose in all patients from a mean (+/- SEM) of 4.89 +/- 0.59 mg/min before to 9.09 +/- 0.99 mg/min at five hours after administration (P less than 0.01). Glomerular filtration rate (inulin clearance), effective renal plasma flow (PAH clearance), and filtration fraction did not change significantly. The increases of urinary excretion of albumin (median %: +92%), IgG (median %: +88%), and transferrin (median %: +76%) were comparable and correlated significantly. Urinary excretion of beta 2-microglobulin did not change significantly however. We conclude that intravenous administration of prednisolone to patients with a nephrotic syndrome causes an increase in urinary protein excretion rate which cannot be explained by changes in renal hemodynamics or tubular protein reabsorption, and which therefore must be the result of a change in glomerular permselectivity characteristics.     

Factors causing variation in urinary N-nitrosamine levels in enterocystoplasties

OBJECTIVE: To evaluate changes in protein leakage in the glomerular filtration barrier, and in the ability of the tubule to reabsorb proteins during and after acute urinary retention (AUR). PATIENTS AND METHODS: Glomerular and tubular function was investigated in 24 men during AUR (mean age 68 years, mean retention time 31 h and mean retention volume 1140 mL) who were then followed for 6 months by measuring the urinary excretion of glomerular and tubular proteins, and the glomerular filtration rate (GFR). Retention was relieved by inserting a suprapubic catheter and the cause of retention treated one month later. No patient had a previous renal disease or diabetes. RESULTS: During AUR, and after 1 and 6 months, albuminuria was detected in 100%, 92% and 54% of patients, and increased excretion of alpha1-microglobulin in 52%, 36% and 58%, of IgG in 79%, 58% and 40%, and of IgG4 in 67%, 42% and 20%, respectively. The mean GFR was normal during retention and during the follow-up. CONCLUSION: AUR causes disturbances in both the glomerular filtration and tubular reabsorption of proteins. Albuminuria and increased excretion of IgG, IgG4 and alpha1-microglobulin occurred in most patients during AUR. After relieving retention, the albuminuria and elevated alpha1-microglobulin excretion persisted, indicating slight glomerular dysfunction and a permanent defect in the proximal tubule to reabsorb proteins. This could be caused partly by previous chronic obstruction. AUR should be relieved immediately and the basic cause treated effectively to prevent further deterioration of renal function.     

Treatment-related changes in urinary excretion of high and low molecular weight proteins in patients with idiopathic membranous nephropathy and renal insufficiency.

BACKGROUND: In patients with idiopathic membranous nephropathy, an increased urinary excretion of high (IgG) and low [beta(2)-microglobulin (beta(2)M), alpha(1)-microglobulin (alpha(1)M)] molecular weight proteins predicts prognosis and precedes renal insufficiency. We have studied the changes in the urinary excretion of these proteins in patients with idiopathic membranous nephropathy and renal insufficiency during and after treatment with cyclophosphamide and steroids, and investigated their value in predicting long-term outcome. METHODS: Standardized measurements of urinary IgG, albumin, beta(2)M and alpha(1)M were performed at 0, 2, 6 and 12 months in 11 patients, at 12 months in 25 patients and in 17 of these last patients after 2-5 years. RESULTS: We observed a rapid improvement of glomerular permselectivity and tubular protein reabsorption within 2 months after the start of therapy. Despite a partial remission of proteinuria within 12 months in most patients, evidence of tubulo-interstitial injury remained apparent. Neither absolute levels of urinary IgG, beta(2)M or alpha(1)M at baseline or at 12 months nor the percentage reduction between baseline and 12 months clearly predicted the occurrence of a remission or a relapse to nephrotic range proteinuria. In the case of a persistent stable remission, we observed a gradual decrease of urinary beta(2)M towards normal values. CONCLUSIONS: In patients with idiopathic membranous nephropathy and renal insufficiency, treatment with cyclophosphamide and steroids resulted in an improvement of glomerular permeability and tubular proteinuria. Tubular proteinuria remained present for many years, even in patients with stable remission of proteinuria. Measurements of urinary proteins at 12 months after treatment start lacked predictive accuracy.     

The mechanisms of exercise-induced proteinuria--relationship between urinary excretion of proteins and lactate after exhaustive exercise

Exercise-induced proteinuria is of the mixed glomerular-tubular type and has been thought to be due to increased glomerular permeability to macromolecular proteins in response to activation of the renin-angiotensin system and catecholamines and a reduction in renal hemodynamics. The excretion of the tubular type of proteins, however, can't be explained by the above mechanism. Two groups, consisting of subjects with the highest (H-alb, n = 20) and the lowest (L-alb, n = 20) excretion of albumin 30 min after exhaustive exercise were selected from a total of 69 normal male participants. No differences in VO2max, maxHR, changes in plasma angiotensin II, catecholamines, urine volume, or Ccr following exercise were observed between the two groups. The increase in blood lactate concentration immediately after exercise, increases in urinary excretion of lactate, pyruvate, alpha 1 M, beta 2 M, albumin and decrease in Cl- excretion 30 min after exercise in the H-alb group were significantly greater than in the L-alb group. The greater the urinary excretion of lactate and pyruvate, the greater the excretion of albumin, alpha 1M and beta 2M, and the less the urinary excretion of Cl-. The coefficient of correlation between the urinary excretion of lactate and beta 2M was 0.757, and between urinary excretion of albumin and beta 2M, 0.756 (p less than 0.001). These results suggest that exercise-induced organic acids and/or decrease in renal circulatory pH caused by organic acids may alter renal glomerular permeability and inhibit renal tubular reabsorption of low molecular weight proteins (alpha 1M, beta 2M). The permeability of the glomerular basement membrane (GBM) to macromolecular proteins may be altered by a reduction in the charge barrier of the GBM, which may be caused by the over-production of organic acids and a lower pH.     

Effects of pentoxifylline on the urinary protein excretion profile of type 2 diabetic patients with microproteinuria: a double-blind, placebo-controlled randomized trial

AIMS: The purpose of this study was to identify the effect of pentoxifylline on the urinary protein excretion profile in type 2 diabetic patients. METHODS: 40 type 2 nonhypertensive diabetic patients were randomly allocated to receive either pentoxifylline 400 mg t.i.d. or placebo daily for 16 weeks. Eligible subjects were those with urinary albumin excretion between 20 and 200 microg/min. Subjects receiving angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), calcium antagonists, and diuretics as well as those with reduced renal function, pregnancy, urinary tract infection, and smoking were not included. A 6-month pretreatment stabilization phase aimed to reduce and stabilize fasting serum glucose levels was carried out. Urinary proteins were identified by electrophoresis, and immunodetection was identified by Western blot. Electrophoretic analysis was performed using molecular weight markers of 150, 132, 77, and 66 kDa to identify high-weight proteins, and 54, 41, 36, 27, 21, 14.3, and 12 kDa to identify low-weight proteins. RESULTS: At baseline, subjects in both groups who showed a glomerular tubular pattern did not differ in their urinary excretion profile. The urinary proteins identified were immunoglobulin G, ceruloplasmin, transferrin, and albumin (glomerular pattern) as well as alpha1-antitrypsin, alpha1-acid glycoprotein, collagenase inhibitor, alpha1-microglobulin, trypsin inhibitor, lysozyme, and beta2-microglobulin (tubular pattern). Subjects who received pentoxifylline had reduced urinary excretion of high- and low-molecular weight proteins. CONCLUSIONS: Urinary protein excretion in type 2 diabetic subjects shows a mixed, glomerular and tubular, pattern. Pentoxifylline reduces the excretion of both high and low molecular-weight urinary proteins.     

Microheterogeneity of urinary albumin and tubular proteinuria in juvenile diabetes mellitus

We studied differential urinary albumin excretion by a double one-dimensional gel electrophoresis with decyl sodium sulphate-polyacrylamide gel electrophoresis in the first, and isoelectric focusing in the second dimension in 37 diabetic children and 20 healthy subjects. In addition, total proteins, albumin, ₂-microglobulin and molecular size distribution of urinary proteins were measured. the latter using sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Whilst albuminuria was not significantly different from controls we found an increased microheterogeneity of urinary albumin in 38% of patients. In addition, low molecular weight protein (P<0.05) and ₂-microglobulin excretion (P<0.01) were elevated. It is suggested that the appearance of highly heterogenous albumin in the pI range of 5.3–5.9 is the result of a decreased tubular reabsorption.     

Effects of low-dose nifedipine on urinary protein excretion rate in patients with renal disease

Background: The observation that proteinuria is an important determinant of the progression of renal disease has prompted numerous studies on the effects of antihypertensive agents on protein excretion. Reports on the proteinuria effects of calcium-channel blockers are quite controversial. It has been suggested that the short-acting dihydropyridine calcium-channel blocker nifedipine increases protein excretion by interference with tubular protein reabsorption. Methods: In a randomized controlled trial 10 patients with renal disease and proteinuria were treated with a dose of 10 mg nifedipine o.d. (slow release formulation) for 1 week. The acute effects on renal and systemic haemodynamics and on urinary albumin, IgG, and {beta}2-microglobulin excretion were investigated during a clearance study in the supine position after the first dose. After 1 week of treatment urinary protein excretion rates were measured in 24-h urine samples collected in the ambulatory patient in consecutive fractions of 4-8 h during normal daily activities. Results: After the first dose nifedipine lowered mean arterial blood pressure in the supine position by 7±1 mmHg (<0.01), attenuated proximal tubular sodium reabsorption (fractional excretion of sodium 3.48±0.49 vs 2.62±0.35% during control, P<0.02), but did not affect proximal tubular protein reabsorption (fractional urinary excretion of {beta}2-microglobulin (0.97±0.30 vs 0.98±0.32% during control, NS). The decrease in blood pressure was not accompanied by decreases in urinary albumin or IgG excretion rates. The selectivity index as well as GFR, RPF, and FF did not change. Continued treatment for 1 week with nifedipine did not influence 24-h protein excretion. However, we observed a rise of proteinuria during daily activities in the first 4 h after drug intake compared to the start of the study with the patients in supine position. During nifedipine the increase in proteinuria was more marked and correlated with the selectivity index. Conclusions: (1) Nifedipine 10 mg orally did not impair tubular protein reabsorption. (2) Nifedipine had no immediate antiproteinuric effect despite the observed blood pressure reduction. (3) Nifedipine increased proteinuria in ambulatory urine collections. This latter observation might explain the seemingly different effects of dihydropyridine calcium-channel blockers as reported in previous studies.     

Characterization of proteinuria and tubular protein uptake in a new model of oral L-lysine administration in rats

Intravenous infusion of basic amino acids is used experimentally and pharmacologically to prevent renal proximal tubular uptake of filtered proteins. Intravenously injected L-lysine is rapidly cleared from plasma and the effect on tubular protein reabsorption is transient. To obtain a more sustained effect, we developed a model of oral L-lysine administration and characterized this model by analyzing urinary protein excretion and proximal tubule uptake of filtered proteins. Rats placed in metabolic cages were treated with 20 mmol/kg/6 h of L-lysine, glycine, or water. Urines were analyzed for proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and radioimmunoassay. Proximal tubule uptake of proteins and expression of apical membrane receptors were investigated by immunocytochemistry. In vitro uptake and receptor expression were studied using a yolk sac cell line. L-lysine administration produced increased urinary excretion of a large number of proteins while the effect on tubular accumulation of selected proteins was variable. L-lysine treatment induced changes in the localization of two receptors responsible for tubular endocytosis of filtered proteins. In conclusion, oral L-lysine treatment induced proteinuria, in particular albuminuria, as efficiently as previous reports on intravenous infusion. The effect on tubular protein accumulation was variable suggesting differential effects on tubular reabsorption and degradation of filtered proteins. Changes in tubular protein handling were accompanied by changes in the localization of the endocytic receptors, megalin, and cubilin. In vitro experiments supported the in vivo observations. The findings suggest that L-lysine may affect receptor trafficking in addition to possible effects on the direct binding of ligands to the receptors.     

Urinary excretion of alpha 2 mu-globulin and albumin by adult male rats following treatment with nephrotoxic agents

Nephrotoxic agents such as puromycin aminonucleoside (PAN) and sodium maleate (MAL) have been used to induce experimental glomerular proteinuria and tubular disease, respectively. Current studies show that PAN caused a massive loss of albumin in the urine while not affecting the excretion of the smaller, sex-dependent alpha 2 mu-globulin. On the other hand, MAL which inhibits the reabsorption of proteins, increased the loss of both alpha 2 mu and albumin. Both nephrotoxic agents increased the excretion of albumin when administered to female rats. MAL-induced proteinuria was used as a direct measure of the renal load for alpha 2 mu and albumin. Renal loads and excretion of alpha 2 mu and albumin were measured in male rats maintained on 0, 20, and 50% casein diets. On the protein-free diet, the excretion of both alpha 2 mu and albumin was reduced (less than 1 mg daily); their renal loads were almost totally reabsorbed. On a 50% casein diet the reabsorption of alpha 2 mu was reduced from a normal of 60% to 10% of the renal load. Thus the high protein diet increased the excretion of alpha 2 mu while having little effect on the excretion of albumin. We suggest that dietary protein exerts two levels of control on the excretion of protein in the urine of the adult male rat. Protein-deficient diets stimulate the general reabsorption of proteins thereby minimizing the excretion of alpha 2 mu and albumin. High protein diets appear to reduce selectively the reabsorption of alpha 2 mu-globulin, thereby increasing its excretion in the urine.     

Reversible microproteinuria induced by L-aspartate infusion

Infusion of an acidic amino acid, L-aspartate, to 10 volunteers resulted in transient, significant increases in urinary excretion of the major urinary trypsin inhibitor (p less than 0.002) and beta 2-microglobulin (p less than 0.02). Simultaneously with the proteinuria, urinary pH rose significantly (p less than 0.02). These changes appeared following the infusion and after the excretion of L-aspartate had reached the preinfusion level. Albumin excretion was unchanged indicating that the proteinuria was due to a decreased tubular reabsorption. The mechanism for the reversible tubular proteinuria is unknown. A simple pH effect due to alkalization of the urine was excluded, as NaHCO3 infusion was not followed by an increase in the excretion of the major urinary trypsin inhibitor and beta 2-microglobulin.     

Beta-2-microglobulin in sickle cell anaemia. Evidence of increased tubular reabsorption

Sickle cell nephropathy is characterized by severe defects in renal medullary functions. In contrast, both proximal tubular secretion and reabsorption are found to be elevated. Since proximal tubular cells are involved in reabsorption and catabolism of beta 2-microglobulin (beta 2-m), we studied serum concentration and urinary excretion of this low molecular weight protein in sickle cell nephropathy. Serum beta 2-m concentration was higher in sickle cell anaemia (SCA) patients compared to control persons. beta 2-m excretion, however, was normal and beta 2-m clearance was not significantly decreased. Fractional beta 2-m excretion was significantly lower and therefore tubular reabsorption of beta 2-m was increased in the SCA patients. There was no correlation between serum beta 2-m concentration and the tubular reabsorption of beta 2-m and phosphate. These findings are further evidence of a particular function of the proximal tubule in sickle cell nephropathy.