Endogenous proteins as markers of glomerular function and dysfunction
Abstract: Plasma and urine concentrations of endogenous proteins are frequently used in the diagnosis of kidney diseases and in studies of glomerular filter function. The main issues addressed in these studies were: storage of urine samples for subsequent protein analysis, use of protein concentrations in urine and in plasma in health and as markers of glomerular diseases, and the application of renal plasma-to-urine clearance of endogenous proteins in estimating the size-selectivity properties of the glomerular capillary wall. Studies of the stability of endogenous proteins in urine stored under different conditions showed certain proteins (immunoglobulin G, protein HC, and a1-antitrypsin) to deteriorate in native urine stored at -20°C, and a1-antitrypsin to be also unstable in native urine kept at room temperature or at 4°C. The addition of the preservative solution employed in these studies was shown to allow reliable measurements to be made of all the proteins investigated, and under all conditions tested, with the exception of a1-antitrypsin in frozen urine. Measurements of urine concentrations of endogenous proteins in healthy adults, using rapid, generally available methods, showed that the same upper reference limits for urinary protein excretion may be used for both genders and regardless of age or the type of urine collection. Moreover, the protein content in normal urine does not correlate to the presence of haematuria or granular casts in urinary sediment. Increased plasma concentrations of acute phase proteins, a1-antitrypsin, haptoglobin and orosomucoid, but not C-reactive protein, were detected in patients with primary chronic glomerulonephritides. The findings imply that, despite the indolent clinical picture, persistent inflammatory processes occur in chronic glomerulonephritides and that, the three first-mentioned acute phase proteins may be used as markers of these diseases. Moreover, the C-reactive protein level may be used to diagnose infections or other inflammatory conditions affecting patients with chronic glomerulonephritides. Urine excretion of glycosaminoglycans was decreased in patients with primary glomerulonephritis or renal amyloidosis. Significant differences in this variable were also observed between various kinds of glomerular diseases, urine concentrations being lower in acute glomerulonephritis compared to the chronic forms of the disease, and in amyloidosis compared to other glomerular diseases. These findings indicate that urinary glycosaminoglycans excretion can not only be used as a marker of glomerulonephritis or renal amyloidosis, but it can also be used in the differential diagnosis of the acute and chronic forms of the former disease, and in screening for amyloidosis in patients with glomerular diseases or with chronic inflammatory diseases, with or without clinical signs of renal involvement. Finally, based on findings in the study of fractional protein clearance in rats with inhibited tubular protein reabsorption, the large pore radius of the glomerular basement mem-brane could be estimated to be 110-115 Å. Thus, the existence of ‘shunt pathways’ in the glomerular basal membrane is open to question.
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