Microheterogeneity of urinary albumin and tubular proteinuria in juvenile diabetes mellitus

Time to abandon microalbuminuria?

The term microalbuminuria--a urinary albumin excretion (UAE) between 20 and 200 microg/min--has been introduced to identify subjects at increased risk of renal and cardiovascular disease. However, the relationship between albuminuria and risk is not restricted to the microalbuminuric range and extends to as low as 2-5 microg/min. On the contrary, the increase of UAE above 200 microg/min (macroalbuminuria) heralds the onset of proteinuria (urinary protein excretion above 0.5 g/24 h) and progressive renal and cardiovascular disease. Albuminuria is a component of the metabolic syndrome and may represent a marker of the increased risk of renal and cardiovascular disease associated with insulin resistance and endothelial dysfunction. Proteinuria is a sign of established kidney damage and plays a direct pathogenic role in the progression of renal and cardiovascular disease. Albuminuria reflects functional and potentially reversible abnormalities initiated by glomerular hyperfiltration, proteinuria, a size-selective dysfunction of the glomerular barrier normally associated with glomerular filtration rate (GFR) decline that may result in end-stage renal disease. Thus, the limit of 200 microg/min segregates patients with albuminuria or proteinuria who are at quite different risk. Among subjects with albuminuria, however, there is a continuous relationship between albumin excretion and risk and no lower bound between normal albuminuria and microalbuminuria can be identified that segregates subjects at different risk. Thus, the terms microalbuminuria and macroalbuminuria could be replaced by the concepts of albuminuria- and proteinuria-associated diseases. Future studies are needed to identify levels of albuminuria below which therapy is no longer beneficial.

Electrophoretic analysis of urinary proteins in diabetic adolescents

Pathological changes in the urine sodium dodecyl sulphate gel electrophoresis (SDS PAGE) patterns often precede the occurrence of any sign of renal involvement in diabetes. However, data concerning the most frequent SDS PAGE pattern of the urine in early stages of type I diabetes mellitus are controversial. In the present study an SDS PAGE technique has been used that provides an adequate sensitivity for the detection of the abnormal pattern. Urinary proteins have been analyzed by SDS PAGE in twenty two diabetic adolescents and twenty four age matched controls. Albumin concentration, and N acetyl-beta-D-glucosaminidase (NAG) activity were also measured in the same samples. There was no significant difference in urine albumin concentration and NAG activity between diabetic children and controls. However twelve patients showed an electrophoretic pattern characteristic for glomerulopathy, two had a pattern indicating tubular dysfunction and another two patients had a mixed pattern. Among the twenty four controls only three showed abnormal electrophoretic patterns. The results support the view that early stages of diabetic nephropathy may involve both glomerular and tubular dysfunction. However the exact clinical and prognostic significance of the information provided by SDS PAGE analysis remains to be elucidated.

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.

Detection and semiquantitation of albumin forms in fresh human plasma separated on gradient polyacrylamide gel by means of electroblotting on agarose gel matrix

Albumin in vitro contains several molecular forms, while in vivo it exists mainly as a monomer with a small fraction of a dimer. The aim of the present work was to detect and estimate albumin forms in fresh blood samples. The available analytical methods at present are inadequate for this purpose. An improved immunoblotting method was used where plasma was subjected to electrophoretic separation on 4-25% gradient polyacrylamide gels followed by immunoblotting on agarose gel containing anti-human albumin. The interference from the huge amount of the monomer in plasma was overcome by cutting the monomer region from the polyacrylamide gel before immunoblotting. After staining of the agarose gel, it revealed the presence of seven stained bands of albumin in addition to the monomer. These bands represent albumin aggregates and complexes of varying molecular masses (112-428 kDa). These albumin forms accounted for 0.7% of the total plasma albumin and their estimated level was 30.7 mg/dL. This study shows that the native albumin in blood has several molecular forms. It is concluded that albumin in healthy human subjects may form association complexes of varying molecular masses with other macromolecules in blood and these complexes are expected to be of physiological relevance.

Renal tubular handling of potassium in children with insulin-dependent diabetes mellitus

To clarify the mechanism by which renal potassium (K) excretion is reduced in children with insulin-dependent diabetes mellitus, we studied two groups of patients: (A) at diagnosis and (B) after at least 1 year of follow-up. Group A (15 children) was studied twice: on the day of admission and after 1 month of insulin therapy. On admission, urinary K excretion, fractional K excretion, and transtubular K concentration gradient (TTKG) were significantly decreased, but became normal after extended insulin therapy. TTKG was inversely correlated with blood glucose (P < 0.001) and hemoglobin A1c (HbA1c, P < 0.001). Group B (73 children with a mean follow-up of 54 +/- 36 months) was subdivided according to the TTKG: 30 patients had a low TTKG < 4.0 (median 3.2) and 43 patients had a normal TTKG > or = 4.0 (median 5.2). Patients had a low TTKG and those with a normal TTKG had an identical duration of follow-up and similar values for plasma renin activity, aldosterone concentration, calciuria, magnesiuria, albumin excretion rate, and creatinine clearance. However, those with a low TTKG had significantly higher blood HbA1c levels, urine volume, and glucosuria. Logistic regression analysis showed that the only independent variables predicting a low TTKG were blood HbA1c and glucosuria (P < 0.001). These data confirm that a reduced renal K excretion is a characteristic feature of diabetic children; this is reversible with appropriate insulin therapy, largely depends on the metabolic control of the disease, and, specifically, on the degree of hyperglycemia and/or glucosuria.