Glomerular Charge Selectivity Research Articles

Renal Effects of Sulodexide in Type 2 Diabetic Patients without Nephrotic Range Proteinuria.

Background Glycosaminoglycan plays an important role in the maintenance of glomerular charge selectivity of diabetic nephropathy. Sulodexide, a mixture of naturally occurring glycosaminoglycan polysaccharide components, has shown a nephroprotective effect in an experimental model of diabetic nephropathy. Although sulodexide reduced albuminuria in patients with type 1 and type 2 diabetes, long-term effects in patients with type 2 diabetes with significant proteinuria have not been established. Objectives The study was aimed at investigating the effects of sulodexide on proteinuria and renal function in patients with type 2 diabetes and nephropathy. Methods Fifty-two patients with proteinuria between 500 and 3000 mg/day received sulodexide 200 mg/day for 12 months, while 56 matched patients with type 2 diabetes constituted the control group. All patients received standard metabolic and blood pressure controls. Primary outcome was evaluated as percentage of reduced proteinuria compared with the control group. Renal function was assessed using estimated glomerular filtration rate (GFR). Results Proteinuria significantly increased in the control group [0.9 (IQR 0.3 to 1.78) to 1.16 (IQR 0.44 to 2.23) g/gCr, P = 0.001], whereas it remained stable in the sulodexide group [0.66 (IQR 0.23 to 0.67) to 0.67 (IQR 0.17 to 1.51) g/gCr, P = 0.108]. At 12 months, proteinuria was higher by 19.4% (IQR 10.3 to 37.6) in the control group while proteinuria was lower by -17.7% (IQR -53.1 to 3.2) in the sulodexide group with a significant difference between groups (P = 0.001). Renal function was noted as a change of estimated GFR, and serum creatinine decreased significantly during the study in both groups but did not significantly differ between groups. No significant changes in the blood pressure, fasting plasma glucose, and hemoglobin A1C were reported. Conclusion In addition to standard treatment, sulodexide is efficient in maintaining proteinuria in patients with type 2 diabetes with nonnephrotic range proteinuria, but it did not provide an additional benefit concerning renal disease progression.

Dynamic, size-selective effects of protamine sulfate and hyaluronidase on the rat glomerular filtration barrier in vivo.

The proteinuric actions of protamine sulfate (PS) have classically been, at least partly, attributed to alterations of the negatively charged glomerular endothelial glycocalyx. To investigate whether the charge-selective properties of the glomerular filtration barrier (GFB) would be altered by PS, we assessed the glomerular sieving of conventional, uncharged, polydispersed Ficoll (n-Ficoll) compared with charge modified, conformationally intact, anionic (carboxymethylated) Ficoll (a-Ficoll) before and after systemic infusions of PS in rats. For comparison, we also investigated the impact of hyaluronidase (hyase), which partially degrades the glycocalyx, on GFB permeability. In anaesthetized Wistar rats, blood access was achieved, and the left ureter was cannulated for urine collection. Rats were infused with either n-Ficoll or a-Ficoll before and during systemic infusions with either PS or hyase. Plasma and urine samples were taken repeatedly and analyzed by high-performance size exclusion chromatography to assess glomerular sieving coefficients (θ) for Ficoll (radius 10-80 Å). The GFB showed a significant glomerular charge selectivity for Ficoll molecules of radius 20-35 Å. PS and hyase infusions reversibly increased θ for large Ficoll molecules (Ficoll molecules of radius 50-80 Å). Thus, for PS, θ for a-Ficoll molecules of radius 70 Å increased from 2.47 × 10(-5) ± 1.1(-5) to 7.25 × 10(-5) ± 1.1(-5) (P < 0.05) at 15 min. For hyase, changes in a-Ficoll molecules of radius 50-80 Å were, however, not statistically significant. Neither PS nor hyase had any effect on θ for n-Ficoll molecules of radius 20-45 Å or a-Ficoll molecules of radius 20-45 Å. It is concluded that systemically administered PS and hyase in moderate doses dynamically decreased the size selectivity of the rat GFB without affecting its charge selective properties.

Controversies in nephrology: Response to ‘renal albumin handling, facts, and artifacts’

For 40 years indirect measurements of the glomerular sieving coefficient of albumin yielded very low values. The first direct measurement by 2-photon microscopy by Russo et al (Kidney Int (2007) 71, 504-513) gives values 50-times higher. This demonstrated that relatively large quantities of albumin are normally filtered based on size selectivity alone. Most of this albumin is retrieved and returned to the blood supply. These new discoveries represent a paradigm shift in our understanding of albumin processing by the kidney. They also serve to explain several anomalous aspects of previous studies on glomerular filtration and mechanism of albuminuria and support the fact that glomerular charge selectivity is not a major factor controlling glomerular permselectivity.

Disruption of Glomerular Basement Membrane Charge through Podocyte-Specific Mutation of Agrin Does Not Alter Glomerular Permselectivity

Glomerular charge selectivity has been attributed to anionic heparan sulfate proteoglycans (HSPGs) in the glomerular basement membrane (GBM). Agrin is the predominant GBM-HSPG, but evidence that it contributes to the charge barrier is lacking, because newborn agrin-deficient mice die from neuromuscular defects. To study agrin in adult kidney, a new conditional allele was used to generate podocyte-specific knockouts. Mutants were viable and displayed no renal histopathology up to 9 months of age. Perlecan, a HSPG normally confined to the mesangium in mature glomeruli, did not appear in the mutant GBM, which lacked heparan sulfate. Moreover, GBM agrin was found to be derived primarily from podocytes. Polyethyleneimine labeling of fetal kidneys revealed anionic sites along both laminae rarae of the GBM that became most prominent along the subepithelial aspect at maturity; labeling was greatly reduced along the subepithelial aspect in agrin-deficient and conditional knockout mice. Despite this severe charge disruption, the glomerular filtration barrier was not compromised, even when challenged with bovine serum albumin overload. We conclude that agrin is not required for establishment or maintenance of GBM architecture. Although agrin contributes significantly to the anionic charge to the GBM, both it and its charge are not needed for glomerular permselectivity. This calls into question whether charge selectivity is a feature of the GBM.

Effects of ACE Inhibition and Angiotensin II Receptor Blockade on Glomerular Basement Membrane Protein Excretion and Charge Selectivity in Type 2 Diabetic Patients

Angiotensin-converting enzyme (ACE) inhibitors may reduce urinary albumin excretion (UAE) by decreasing glomerular pressure and increasing glomerular charge selectivity through preservation of glycosaminoglycans. The effect of Angiotensin II antagonism on glomerular charge selectivity remains to be determined. The aim of this study was to compare the effects of an AT1 blocker losartan and an ACE inhibitor (ACE-I) enalapril on UAE, extracellular matrix proteins, glycosaminoglycan excretion (UGAG) and red blood cell anionic charge (RBCCh) which are the indirect markers of glomerular basement membrane anionic content in hypertensive Type 2 diabetic patients. Twenty-four patients were randomised into two groups and received either enalapril (520 mg/d) or losartan (50100 mg/d). All parameters were measured at baseline and after six months of treatment. At the end of six months, systolic and diastolic blood pressures (BP), UAE rates, UGAG excretion and RBCCh were significantly and equally reduced in both treatment groups compared with baseline. RBCCh was negatively correlated with UAE (r=-0.57, p<0.0001) and UGAG excretion (r=-0.57, p<0.0001); UAE was correlated with UGAG excretion (r=0.58, p<0.0001). In conclusion, enalapril and losartan treatment were equally effective in reducing BP, UAE as well as UGAG excretion and preserving RBCCh in hypertensive Type 2 diabetic patients. ACE inhibition and AT1-receptor blockade may have favourable effects on preserving glomerular anionic content in hypertensive diabetic patients.

Glomerular charge selectivity is impaired in hypertensive nephropathy.

In hypertensive nephropathy the morphological changes and albuminuria seem to start in the inner cortex but the mechanism of proteinuria is unknown. We tested the hypothesis of reduced glomerular charge selectivity in the juxtamedullary cortex of old spontaneously hypertensive rats (SHR) as a cause of proteinuria in rats with hypertensive nephropathy. The glomerular charge barrier was evaluated in 80-week-old SHR and age-matched normotensive Wistar-Kyoto rats (WKY) by measuring the glomerular clearance of radiolabelled cationic and anionic chymotrypsinogen (Chym and aChym, MW 25,000) accumulated by the proximal tubular cells in the outer, middle and inner cortex following intravenous injection. The glomerular filtration rates (GFR) in the cortical zones were obtained by aprotinin (MW 6500) and the sieving coefficient (theta) of the protein tracers calculated as their glomerular clearance/GFR. The theta aChym was similar in SHR and WKY except in the inner cortex where it was 35% higher in SHR (0.65+/-0.05) than in WKY (0.48+/-0.03) (P = 0.01). The ratio of theta aChym to theta Chym was increased from 0.55+/-0.06 to 0.77+/-0.05 (P<0.02) in the inner cortex of SHR, whereas theta Chym remained the same in all cortical zones. Finally, the percentage of glomeruli with adsorption droplets in podocytes quantified by light microscopy was higher in the inner than the outer cortex of SHR (P<0.05). The study supports the theory of a functioning glomerular charge barrier. An increased relative clearance of aChym in the inner cortex of SHR indicates impairment of the charge barrier, which, at least in part, could explain the increased protein excretion in SHR with hypertensive nephropathy.

Structural determinants of glomerular permeability.

Recent progress in relating the functional properties of the glomerular capillary wall to its unique structure is reviewed. The fenestrated endothelium, glomerular basement membrane (GBM), and epithelial filtration slits form a series arrangement in which the flow diverges as it enters the GBM from the fenestrae and converges again at the filtration slits. A hydrodynamic model that combines morphometric findings with water flow data in isolated GBM has predicted overall hydraulic permeabilities that are consistent with measurements in vivo. The resistance of the GBM to water flow, which accounts for roughly half that of the capillary wall, is strongly dependent on the extent to which the GBM surfaces are blocked by cells. The spatial frequency of filtration slits is predicted to be a very important determinant of the overall hydraulic permeability, in keeping with observations in several glomerular diseases in humans. Whereas the hydraulic resistances of the cell layers and GBM are additive, the overall sieving coefficient for a macromolecule (its concentration in Bowman's space divided by that in plasma) is the product of the sieving coefficients for the individual layers. Models for macromolecule filtration reveal that the individual sieving coefficients are influenced by one another and by the filtrate velocity, requiring great care in extrapolating in vitro observations to the living animal. The size selectivity of the glomerular capillary has been shown to be determined largely by the cellular layers, rather than the GBM. Controversial findings concerning glomerular charge selectivity are reviewed, and it is concluded that there is good evidence for a role of charge in restricting the transmural movement of albumin. Also discussed is an effect of albumin that has received little attention, namely, its tendency to increase the sieving coefficients of test macromolecules via steric interactions. Among the unresolved issues are the specific contributions of the endothelial glycocalyx and epithelial slit diaphragm to the overall hydraulic resistance and macromolecule selectivity and the nanostructural basis for the observed permeability properties of the GBM.

Puromycin aminonucleoside damages the glomerular size barrier with minimal effects on charge density.

Puromycin aminonucleoside (PAN) has been suggested to reduce glomerular charge density, to create large glomerular "leaks," or not to affect the glomerular barrier. Therefore, we analyzed glomerular charge and size selectivity in vivo and in isolated kidneys perfused at 8 degrees C (cIPK) in control and PAN-treated rats. The fractional clearances (theta) for albumin and Ficoll of similar hydrodynamic size were 0.0017 +/- 0.0004 and 0.15 +/- 0.02, respectively, in control cIPKs. Two-pore analysis gave similar results in vivo and in vitro, with small- and large-pore radii of 47-52 and 85-105 A, respectively, in controls. Puromycin increased the number of large pores 40-50 times, the total pore area over diffusion distance decreased by a factor of 25-30, and the small-pore radius increased by 33% (P < 0.001 for all comparisons of size selectivity and theta). The effect of PAN was less dramatic on the estimated wall charge density, which was 73% of that of controls. We conclude that puromycin effectively destroys the glomerular size barrier with minimal effects on charge density.

A gel-membrane model of glomerular charge and size selectivity in series.

We have analyzed glomerular sieving data from humans, rats in vivo, and from isolated perfused rat kidneys (IPK) and present a unifying hypothesis that seems to resolve most of the conflicting results that exist in the literature. Particularly important are the data obtained in the cooled IPK, because they allow a variety of experimental conditions for careful analysis of the glomerular barrier; conditions that never can be obtained in vivo. The data strongly support the classic concept of a negative charge barrier, but separate components seem to be responsible for charge and size selectivity. The new model is composed of a dynamic gel and a more static membrane layer. First, the charged gel structure close to the blood compartment has a charge density of 35-45 meq/l, reducing the concentration of albumin to 5-10% of that in plasma, due to ion-ion interactions. Second, the size-selective structure has numerous functional small pores (radius 45-50 A) and far less frequent large pores (radius 75-115 A), the latter accounting for 1% of the total hydraulic conductance. Both structures are required for the maintenance of an intact glomerular barrier.

Reference intervals for the markers of proteinuria with a standardised bed-rest collection of urine.

Reference intervals for markers of proteinuria or glomerular charge selectivity were measured in 61 healthy female and 61 healthy male individuals. Timed bed-rest and daytime collections were used to assess significance of preanalytical variability of results. Bed-rest collections are advisable for research on renal damage, whereas in routine care, robust protein/creatinine ratios work as practical estimates of protein excretion rates, the correlations to excretion rates improving with increasing proteinuria. For glomerular charge selectivity, pancreatic/salivary isoamylase clearance ratio showed lower within-subject biological variation than IgG/IgG4 clearance ratio, allowing more accurate classification into normal and reduced charge selectivity. With our method, the lower 2.5% reference intervals for isoamylase clearance ratio were 1.1 in men and 1.9 in women.

Glomerular permselectivity at the onset of nephropathy in type 2 diabetes mellitus.

The development of microalbuminuria in individuals with type 2 diabetes mellitus is associated with a 10-fold increase in the risk of progression to overt nephropathy and eventual end-stage renal failure. The present study reports long-term (up to 8 yr) follow-up of 43 Pima Indians with type 2 diabetes detected on screening to have microalbuminuria. The natural history of albuminuria in these individuals included progression to overt proteinuria (urinary albumin excretion > or = 300 mg/d) in half of the participants by 7 yr of follow-up. The size selectivity of the glomerular barrier was also investigated using dextran sieving and pore theory. Whereas a comparison group of macroalbuminuric Pima Indians had an excess of large pores that served as a macromolecular "shunt," individuals with microalbuminuria had a shunt size no different from long-term diabetic, normoalbuminuric control subjects. An abrupt transition from little or no relationship to a highly significant and positive relationship between increasing albuminuria and shunt size occurred at an albumin-to-creatinine ratio of approximately 3000 mg/g. Shunt size in macroalbuminuric individuals correlated with the extent of foot process broadening. Podocyte foot processes in microalbuminuric participants were not different from those in control subjects. In conclusion, although microalbuminuria in type 2 diabetic Pima Indians often heralds progressive glomerular injury, it is not the result of defects in the size permselectivity of the glomerular barrier but rather of changes in either glomerular charge selectivity or tubular handling of filtered proteins or of a combination of these two factors.

Urinary Glycosaminoglycan Excretion in Newly Diagnosed Essential Hypertensive Patients

Glycosaminoglycans (GAGs) are major components of the basement membranes and play a key role in their molecular organization and function (1)(2)(3). Some authors have proposed that increased loss of proteoglycan from glomerular basement membrane (GBM) alters glomerular charge selectivity, which contributes to urinary loss of albumin (4)(5)(6)(7)(8). Several clinical studies have shown that the GAG content of human GBM is significantly decreased and that urinary loss of GAG is markedly increased in diabetic patients (7)(8)(9). Among the possible mechanisms of diabetic microalbuminuria are decreased synthesis (10) and/or increased loss of GAGs (9)(11) from the GBM. Although the importance of microalbuminuria is unclear in hypertensive patients, it may be an early marker of glomerular functional and structural changes (12)(13). Because hypertension is considered a systemic disease, basement membrane changes should be widespread. Whether basement membranes from other tissues are also affected and whether these changes are the result of hypertension remains to be determined. Therefore, in our study, we examined the effects of hypertension on basement membrane anionic charges in newly diagnosed, untreated essential hypertensive patients. Twelve nonsmoking, essential hypertensive patients, stage I or II according to the sixth report of Joint National Committee (14), and 12 age- and sex-matched healthy normotensive controls were included in the study. All patients gave informed consent, and the institutional local ethics committee approved the …

Glomerular charge selectivity for horseradish peroxidase and albumin at low and normal ionic strengths.

The classical concept of a negative glomerular charge barrier has recently been questioned, mainly based on the somewhat high clearance for anionic horseradish peroxidase (HRP). The validity of using anionic HRP can be tested by changing the properties of the charge barrier. A rather unequivocal approach is to reduce the ionic composition of the perfusate and hence increase the Debye length. We determined the glomerular clearance for horseradish peroxidase and serum albumin, using isolated rat kidneys perfused at 8 degrees C to reduce the tubular modification of the primary urine. The perfusate contained trace amounts of the neutral 125I-nHRP and the anionic 131I-aHRP and were otherwise identical except for different ionic strengths, 152 mM and 34 mM, respectively. During control, the fractional clearance (theta) was 0.11 +/- 0.015 for nHRP and 0.045 +/- 0.010 for aHRP, with an average clearance ratio (n/a) of 2.8 +/- 0.24. Low ionic strength reduced theta for aHRP to 0.027 +/- 0.006, giving an increased clearance ratio for HRP of 4.2 +/- 0.44. The existence of a negative charge barrier is supported by the experiments. The result obtained during normal perfusion is compatible with a charge density (omega) of 34 mEq L-1, using a model of homogeneously charged membrane. Low ionic strength perfusion reversibly reduced the concentration of fixed charges to 12 mEq L-1, suggesting an almost threefold increase of the glomerular membrane volume. Thus, the glomerular charge barrier should be regarded to have a dynamic gel structure rather than being a rigid membrane.

Glomerular Charge and Size Selectivity Assessed by Changes in Salt Intake in Type 2 Diabetic Patients

To evaluate glomerular charge selectivity in patients with type 2 diabetes, we studied changes in fractional clearance of proteins with different sizes and charges when patients were placed on two diets with different salt contents. Nineteen patients with type 2 diabetes and normoalbuminuria (< 20 micrograms/min, n = 8), microalbuminuria (20-100 micrograms/min, n = 7), or advanced albuminuria (> 100 micrograms/min, n = 4) were placed on a low-salt diet (85 mEq of sodium daily) or a high-salt diet (255 mEq of sodium daily) for 1 week, and then on the other diet, in random order. Fractional clearances of albumin and immunoglobulin G (IgG) were calculated on the last 3 days of each diet. In patients with normoalbuminuria, the high-salt diet increased the fractional clearance of IgG, which is electrically neutral, but the fractional clearance of albumin, which is anionic, was unaltered, suggesting that the pore charge of the glomerular barrier was unaffected. However, in patients with microalbuminuria, the high-salt diet increased the fractional clearances of IgG and albumin equally, indicating some neutralization of the pore charge. Fractional clearance of IgG in these first two groups was similar when salt intake was low, so pore size was the same in these groups. In patients with advanced albuminuria, fractional clearance of IgG was higher than in the other groups, indicating that size selectivity had worsened. In type 2 diabetic patients, charge selectivity is lost before size selectivity as diabetic nephropathy progresses.

Glomerular charge selectivity for proteins larger than serum albumin as revealed by lactate dehydrogenase isoforms.

It is well known that macromolecules like albumin are markedly restricted in their passage across the glomerular capillary wall. However, the relative importance of solute size, charge and shape is currently debated since much of the previous work is based on dextran in neutral or charge-modified forms. These polymers have certain drawbacks that make them less suitable for analysis of capillary permeability and the notion of a glomerular charge barrier has therefore been questioned. Moreover, macromolecules larger than albumin (mol. wt. 69,000) have been suggested to pass through nonselective 'shunt' pathways. In order to study glomerular permeability, isolated rat kidneys were perfused with albumin solutions containing trace amounts of two differently radiolabelled isoenzymes of lactate dehydrogenase (LDH) at low temperature to inhibit tubular function. The isoenzymes have similar size (mol. wt. 140,000) and shape but differ in charge, one carrying a negative net surface charge (LDH1, -19) and the other being slightly cationic (LDH5, +2). The urine and perfusate samples were subjected to high pressure liquid chromatography (HPLC) gel-filtration to allow for measurements of intact LDH. The fractional clearance was 0.11% +/- 0.04% for the anionic LDH1 and 0.56% +/- 0.07% for LDH5, whereas that for albumin was 0.21% +/- 0.03% at a glomerular filtration rate of 0.11 +/- 0.01 mL min-1 g-1 kidney wet weight. The results were analysed using a homogenously charged membrane model and are compatible with a charge density of 35 mEq L-1, with 95% confidence interval of 26-41 mEq L-1. These findings suggest a significant glomerular charge selectivity for proteins substantially larger than albumin. The charge density is, however, far less than estimated from dextran studies.

Effect of interleukin-8 on glomerular sulfated compounds and albuminuria.

To evaluate the effect of interleukin-8 (IL8) on glomerular basement membrane (GBM) sulfated compounds and albuminuria, we infused IL8 in 1% bovine serum albumin (BSA) for 5 days into the left renal artery of Holtzman male rats at the rate of 10 microliters/h using an osmotic pump. Control rats received 1% BSA. A significant increase in urinary albumin/creatinine ratio was seen on the last day of IL8 infusion (0.38 +/- 0.11, mean +/- SEM) when compared with albumin/creatinine ratio prior to infusion (0.19 +/- 0.04, P = 0.04). No significant differences in urinary albumin excretion prior to and after infusion of 1% BSA were observed. On the last day of infusion, rats were injected with 35sulfate (1.0 mCi/200 g body weight) intraperitoneally and killed after 8 h. Glomeruli were isolated and GBM obtained. After 5 days of IL8 administration, there was a significant increase in 35sulfate uptake by GBM of the infused kidney (76 +/- 10 cpm/dry glomerular weight, mean +/- SEM) compared with the uptake seen in the contralateral kidney (53 +/- 9, P = 0.05). The in vivo infusion of IL8 increased the 35sulfate uptake by GBM and augmented the urinary albumin/creatinine ratio, suggesting that IL8 may induce albuminuria by altering the metabolism of the GBM sulfated compounds. This hypothesis needs to be confirmed by studies on glomerular charge selectivity and GBM anionic sites during the course of the infusion. Moreover, the persistence of the effect needs to be evaluated by prolonging the infusion for more than 5 days.

Renal function and glomerular permselectivity late after living related donor transplantation.

Living related kidney transplantation is the preferable procedure for renal replacement therapy. The aim of the current study was to determine systemic hemodynamic and intrarenal adaptions in donors and recipients late after living related kidney transplantation. Furthermore, glomerular permselectivity was assessed in these subjects. We studied mean blood pressure (MAP), glomerular filtration rate (GFR), renal plasma flow (RPF), microalbuminuria (MIA), 24-hr urinary protein excretion, and glomerular permselectivity (fractional clearance of neutral dextrans [thetaD] as a marker for size selectivity and fractional clearance of dextran sulfate [thetaDS] to assess charge selectivity) in 22 donors and 22 recipients. MAP was normal in the donor group (102 +/- 4 mmHg), but five patients had blood pressure above 140/90 mmHg. This 18%, however, is lower than the prevalence of hypertension in the age-adjusted general population in Austria. The recipients also had normal MAP at the time of study (99 +/- 3); however, 13 needed antihypertensive therapy. GFR and RPF were lower in recipients than in donors (53 +/- 8 vs. 72 +/- 11 and 314 +/- 74 vs. 412 +/- 86 ml/min respectively). In the donor group, GFR was 137 +/- 45% of the expected age-adjusted mean value/kidney due to hyperfiltration. Proteinuria and MIA were higher in the recipients than in the donors (0.39 +/- 0.22 vs. 0.07 +/- 0.04 g/day, 137 +/- 136 vs. 26 +/- 15 mg/day). Nonetheless, five donors had an elevated MIA. A higher need for antihypertensive medication could be observed in recipients with previous rejection episodes, as well as a significantly higher urinary protein excretion and MIA (0.7 +/- 0.42 vs. 0.24 +/- 0.14 g/day, 336 +/- 380 vs. 48 +/- 32 mg/day). ThetaDS was significantly higher in the recipients, whereas thetaDS of the donors was identical to the value obtained from 18 healthy controls (0.7 +/- 0.08 vs. 0.6 +/- 0.06). OD was similar in all groups studied. In conclusion, 76 months after uninephrectomy for renal donation, mild changes in glomerular permselectivity occurred in a subset of donors without affecting renal excretory function. In recipients, proteinuria was due to a defect in glomerular charge selectivity.

Evidence of Impaired Glomerular Charge Selectivity in Nondiabetic Subjects With Microalbuminuria

Microalbuminuria is associated with excess cardiovascular morbidity and mortality in diabetic and nondiabetic subjects. Loss of glomerular charge selectivity may explain the development of microalbuminuria in diabetic subjects. The primary population in this cross-sectional study was 124 subjects aged 40 to 75 years without glucose intolerance and with a previous (3 years before the present study) urinary albumin excretion rate (UAE) in the normal (<20 microgram/min) or microalbuminuric (>20 microgram/min) range. The secondary population consisted of 39 offspring aged 15 to 40 years. The main outcome measures included UAE, urinary IgG/IgG4 selectivity index (SI), and the presence of ischemic heart disease as determined by questionnaire or ECG. Among the primary population, a significant inverse correlation was found between SI and UAE (r=-.40, P<.001). Reduction in SI could be demonstrated in subjects with UAE >10 microgram/min. In multiple regression analysis reduction in SI was found with increasing age, independent of UAE. In 20 subjects with clinical cardiovascular disease a reduction in SI was found (1.9+/-0.6 versus 2.6+/-1.3, P=.001) without concomitant elevation in UAE (P=.99). Offspring from parents with microalbuminuria had an SI comparable to offspring from parents with normal UAE (2.7+/-0.7 versus 3.3+/-1.6, P=.17). In nondiabetic subjects the development of microalbuminuria is associated with reduced SI, suggesting impairment of glomerular charge selectivity. SI may offer a more sensitive monitoring of abnormalities in glomerular permselectivity than does measurements of UAE, but the ability of SI to predict development of cardiovascular disease needs to be evaluated prospectively.

Diurnal variation in glomerular charge selectivity, urinary albumin excretion and blood pressure in insulin-dependent diabetic patients

The urinary albumin excretion rate (AER) in a subgroup of patients with insulin-dependent diabetes mellitus (IDDM) steadily increases. In these patients a concomitant reduction of the glomerular charge selectivity index (SI) has been demonstrated. The aim of the present study was to evaluate whether diurnal variation in AER could be related to a diurnal variation in SI and/or a diurnal blood pressure variation. Thirty-three patients with IDDM, 27 with normal albumin excretion (AER < 20 micrograms/min; group D(o)) and six with incipient nephropathy (AER from 20 to 200 micrograms/min; group DA), were studied. AER and SI (renal clearance ratio of total-IgG/IgG4) were measured in three different urine collecting periods: period A (8:00 a.m. to 12:00 a.m.), period B (12:00 a.m. to bedtime) and period C (bedtime to 8:00 a.m.). A significant increase in SI was seen during the nighttime: period A, 1.6 (0.2 to 3.8; mean, range); period B, 1.7 (0.3 to 3.0); and period C, 2.0 (0.2 to 4.0); P = 0.01. Corresponding to this observation, an overall significant decrease in AER was found: period A, 10 (3 to 137) micrograms/min (median, range); period B, 8 (3 to 84) micrograms/min; and period C, 5 (0 to 78) micrograms/min; P < 0.001. In all three sampling periods a negative correlation was found between AER and SI. When group D(o) was analyzed alone, the results were similar. Diurnal variation in blood pressure was significantly positively correlated with AER in group DA, but was not correlated to variation in AER in D(o). We suggest that in normoalbuminuric IDDM patients diurnal variation in AER is related to diurnal variation in SI.

Renal and systemic transvascular albumin leakage in severe atherosclerosis.

Microalbuminuria was recently proposed as a novel atherogenic risk factor. The pathophysiological link between microalbuminuria and atherosclerosis may be mediated through an increased generalized transvascular leakage of albumin. To investigate this hypothesis, urinary albumin excretion and clearance and systemic transvascular albumin leakage (TERalb) were measured in 23 patients with severe clinical atherosclerosis and 25 healthy controls. In addition, renal clearances of three other endogenous plasma proteins (IgG, IgG4, and beta 2-microglobulin) and of creatinine were measured. Measurements of urine and serum proteins were done by enzyme-linked immunosorbent assays. TERalb was measured by the fractional disappearance rate of 125I-albumin from the total intravascular compartment in 1 hour after intravenous injection. Glomerular filtration rate was estimated as creatinine clearance. Urinary albumin excretion (geometric means [95% confidence intervals], 10.5 [6.1 to 18.3] versus 5.7 [4.7 to 6.9] micrograms/min; P < .05), fractional urinary albumin clearance (2.8 [1.6 to 4.8] x 10(-6) versus 1.3 [1.0 to 1.6] x 10(-6); P < .05), and TERalb (6.0 [5.5 to 6.5] versus 5.1 [4.5 to 5.8] %/h; P < .05) were higher in patients than in control subjects. Glomerular charge selectivity (ratio of IgG clearance to IgG4 clearance) was lower in patients than in control subjects (1.5 [1.1 to 2.0] versus 2.3 [2.0 to 2.6]; P < .05). These alterations were independent of blood pressure, glomerular filtration rate, tubular function, and smoking status. It is concluded that atherosclerotic vascular disease is associated with renal and systemic transvascular leakiness for albumin. Theoretically, such leakiness may in addition allow for an increased lipid insudation into the large vessel wall, thereby linking microalbuminuria to atherogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)