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The object of this study was that of getting additional insight into the mechanism of proteinuria through analysis of the permeability of the glomerular filter for higher molecular weight proteins in patients with proteinuria resulting from diabetic nephropathy. Fifty hospitalized patients with insulin-requiring diabetes mellitus were selected according to the presence of proteinuria and divided into two groups: group I, with or without only minor symptoms of diabetic angiopathy; group II, with moderate to severe diabetic nephropathy and retinopathy grades 1 to 3. In all patients the urinary excretion of haptoglobins (Hp) and of isoagglutins (IA) was measured, as well as the Hp level in the serum and the overall profile of serum and urine proteins. The results indicate that the urinary excretion of Hp and IA could be correlated to some extent with the severity of diabetic nephropathy, whereas the serum level of Hp failed to exhibit any clear-cut anomalies in the presence of diabetic angiopathy. In patients with severe nephropathy, the albumin/globulin ratio in the urine was increased approximately twofold over that in serum, despite a grossly unaltered profile of serum proteins. These results suggest that the "functional pore size" of the renal filter is increased in diabetic nephropathy, but that the mechanism of proteinuria also involves degradation of serum proteins during passage through the kidney.