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      Size and Charge Selectivity of the Glomerular Filter in Patients with Insulin-Dependent Diabetes mellitus: Urinary Immunoglobulins and Glycosaminoglycans

      a , b
      S. Karger AG
      Dimethylmethylene blue, Immunoglobulins, Selectivity index

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          Background: In diabetic nephropathy, a reduction in negative membrane charge in the glomerular filter, i.e., the number of sulphated groups of glycosaminoglycans, has been argued to lead to increases in excretion of negatively charged molecules, such as albumin and IgG4. However, albuminuria and an increased excretion rate of IgG may also be caused by an increase in radius or number of glomerular large pores. Methods: Timed urinary excretion rates of sulphated glycosaminoglycans, albumin, and IgG2, IgG4, and IgM were analyzed in 94 patients with insulin-dependent diabetes mellitus with different degrees of nephropathy and compared with the excretion rates in 26 control subjects. Sulphated glycosaminoglycans were measured spectrophotometrically after addition of 1,9-dimethylmethylene blue. Albumin and immunoglobulins were measured by immunoassays. Results: With increases in the albumin excretion rate the excretion of IgG2, IgG4, and IgM also increased, in contrast to a decrease in glycosaminoglycans and the ratio between IgG2 and IgG4 (selectivity index). This index decreased from 6.2 to 0.7 (median; p < 0.01). However, with linear regression analysis the excretion rates of albumin and immunoglobulins were not associated with those of glycosaminoglycans. Conclusion: In diabetic nephropathy changes in both large-pore number and in charge selectivity may be pathogenic mechanisms for albuminuria.

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          Size-selectivity of the glomerular barrier to high molecular weight proteins: upper size limitations of shunt pathways.

          To evaluate the large pore radius of the glomerular capillary filter, plasma-to-urine fractional clearances of a number of endogenous proteins were assessed in normal and in nephrotic Wistar rats in which proximal tubular reabsorption had been inhibited using lysine. The proteins studied varied in radius from 16.2 A (Beta 2-microglobulin) to 90 A (alpha 2-macroglobulin). The nephrotic syndrome was induced by puromycin aminonucleoside (PAN). A marked restriction of the transport of large proteins across the glomerular capillary wall was found, indicating that there are no non-discriminatory 'shunt pathways' in the glomerular barrier. Rather, there seems to be large pores of radius 110 to 115 A accounting for the clearance of large proteins into the primary urine. This protein excretion pattern was almost the same for control and nephrotic rats, except that in the latter, the number of large pores was increased 170 times. The ratio between the number of large and small pores was calculated to be approximately equal to 7 x 10(-7) in normal rats and to 1.2 x 10(-4) in PAN nephrotic rats, assuming no classic shunt pathways. If classic shunt pathways had still existed, they would normally contribute to no more than approximately equal to 10(-5) of the total glomerular filtration rate. We postulate that very large macromolecules like IgM will not pass the glomerular filter at all under normal conditions, whereas the urine concentration of alpha2-macroglobulin will normally be extremely low.
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            Proteinuria selectivity index based upon alpha 2-macroglobulin or IgM is superior to the IgG based index in differentiating glomerular diseases. Technical note.

            The proteinuria selectivity index (SI) may be used to describe changes of the glomerular permeability for macromolecules in glomerular diseases. Proteins the size of alpha 2-macroglobulin (alpha 2 M) or IgM cannot normally pass the glomerular barrier, whereas IgG can pass through the large pores of glomerular basement membrane. Comparison of the clearance of the three high-molecular-weight proteins to that of albumin may be useful in characterization and diagnosis of different glomerular diseases as well as in understanding of the permeability characteristics of the glomerular filter. Three types of SI, each calculated as a ratio of clearance of either IgG, alpha 2M or IgM to that of albumin, were measured in 199 proteinuric patients. The patients were subdivided into eight different biopsy-verified glomerular diseases. Two diagnoses could be clearly distinguished using SI based on alpha 2M (alpha 2 M SI) or IgM (IgM SI). Both alpha 2M SI and IgM SI were significantly lower in minimal change nephropathy and higher in crescentic necrotizing glomerulonephritis than in all the other diagnoses. The SI based on IgG (IgG SI) was less useful in determining specific diagnoses, since patients with minimal change nephropathy could not be distinguished from those with other types of primary glomerulonephritis and patients with crescentic necrotizing glomerulonephritis did not differ from those with diabetic nephropathy. The findings of this study indicate that alpha 2M SI and IgM SI are superior to IgG SI in characterization of glomerular disorders and might replace the IgG SI for this purpose.
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              Urinary glycosaminoglycan excretion in NIDDM subjects: its relationship to albuminura.

              Nephropathy is a serious microvascular complication of diabetes mellitus which is preceded by a period of microalbuminura. Increased loss of proteoglycan (PG) from glomerular basement (GBM) has been postulated to alter glomerular charge selectivity which contributes to urinary loss of albumin. In this study we measured the excretion of urinary glycosaminoglycans (GAG), the degradation products of PG, in 82 non-insulin-dependent (NIDDM) (Type 2) diabetic and 34 non-diabetic subjects. We found that diabetic subjects had a significantly higher GAG urinary excretion rate compared to non-diabetic subjects (12.54 +/- 5.67 vs 8.80 +/- 3.99 micrograms glucuronic acid min-1, p = 0.0001). Categorizing for albuminuric status shows that the diabetic normo-, micro- and macroalbuminuric groups have a higher GAG excretion rate than non-diabetic subjects. Heparan sulphate (HS) GAG urinary excretion was measured in 25 samples from diabetic subjects and 18 non-diabetic subjects. Diabetic subjects excreted more HS GAG than controls both as a rate or as a percentage of total GAG (3.70 +/- 1.94 vs 2.38 +/- 1.48 micrograms glucosamine min-1, p = 0.02; 31.6% +/- 12.5 vs 23.1% +/- 10.4, p = 0.02). Categorizing for albuminuric status shows that micro- and macro-albuminuric groups have a significantly higher HS GAG excretion rate than non-diabetic subjects. We conclude that, as in IDDM, excretion of GAG and HS GAG is higher in NIDDM and may precede the development of microalbuminuria.

                Author and article information

                S. Karger AG
                December 1999
                30 November 1999
                : 83
                : 4
                : 301-307
                Divisions of aDiabetology and Endocrinology and bNephrology, Medical Department, University Hospital, Lund, Sweden
                45421 Nephron 1999;83:301–307
                © 1999 S. Karger AG, Basel

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                Page count
                Tables: 3, References: 40, Pages: 7
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                Self URI (journal page): https://www.karger.com/SubjectArea/Nephrology
                Original Paper

                Cardiovascular Medicine,Nephrology
                Selectivity index,Immunoglobulins,Dimethylmethylene blue
                Cardiovascular Medicine, Nephrology
                Selectivity index, Immunoglobulins, Dimethylmethylene blue


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