8-MONTH LONGITUDINAL-STUDY OF URINARY-EXCRETION OF ALBUMIN AND TUBULAR PROTEINS IN DIABETIC SUBJECTS

To evaluate the importance of tubular proteinuria in diabetic nephropathy, we studied the serial changes of micro-albuminuria, microproteinuria and protein patterns patterns by sodium dodecyl suphate polyacrylamide gel electrophoresis (SDS-PAGE) in 38 diabetic patients over 8 months. There was a significant correlation between the amount of micro-albuminuria measured by radio-immunoassay and the amount of microproteinuria quantitated by the Coomassie brilliant blue dye binding method (r = 0.976; p < 0.0001). Micro-albuminuria of 50 mg/day was equivalent to microproteinuria of 190 mg/day. Among the 38 diabetic patients, 26 had micro-albuminuria above 50 mg/day, while 12 had micro-albuminuria below this level. There was a significant correlation between the amount of microproteinuria and haemoglobin A1, showing that the quantity of microproteinuria was affected by metabolic control. Diabetic patients with micro-albuminuria of above 50 mg/day have a significantly higher diastolic blood pressure than those below this level. Among the diabetic patients with micro-albuminuria of less than 50 mg/day, the amount of micro-albuminuria and microproteinuria remained constant, whereas progressive increases in micro-albuminuria and microproteinuria were observed among the 12 diabetic subjects with micro-albuminuria above 50 mg/day. These support the prognostic importance of this quantity of micro-albuminuria. The protein patterns as revealed by SDS-PAGE with Coomassie blue staining show a significant loss of low-molecular-weight proteins in 7 patients, which may therefore suggest tubular damage. The loss of tubular proteins persisted over a period of 8 months in all 7 subjects, and the amount gradually increased over this period. To conclude, our results indicate that diabetic proteinuria is characterized by urinary loss of albumin as well as low-molecular-weight proteins. The latter is associated with tubular damage, which may be one of the manifestations of the renal complications of diabetes mellitus.