Urinary excretion of retinol-binding protein in type 1 (insulin-dependent) diabetic patients with microalbuminuria and clinical diabetic nephropathy.

The urinary excretion of retinol-binding protein (RBP) was studied in 101 insulin-dependent diabetic patients allocated to three groups according to 24-h urinary albumin excretion rate (UAE) (median of three urine collections): group 1 (n = 45), normal UAE less than 30 mg/24 h; group 2 (n = 27), microalbuminuria (UAE 30-300 mg/24 h); and group 3 (n = 29), clinical diabetic nephropathy (UAE greater than 300 mg/24 h). We used 23 healthy subjects as controls. Fractional clearance of RBP (FC-RBP) and its 24-h urinary excretion rate (URBP) were higher in each diabetic group than in healthy subjects, the highest values being found in group 3. Groups 1 and 2 did not differ in URBP and FC-RBP. There was a correlation between FC-RBP and haemoglobin A1c in both the total diabetic cohort (P less than 0.001) and in diabetic patients in groups 1 and 2 with a glomerular filtration rate of more than 90 ml/min (P less than 0.05). No correlation was found between FC-RBP and UAE and/or duration of diabetes in any of the diabetic groups. We conclude that the increased urinary excretion of RBP, indicating proximal tubular dysfunction, is already present in normoalbuminuric insulin-dependent diabetic patients and correlates with metabolic control. Further deterioration in proximal tubular function was not observed in microalbuminuric patients, but is a late event in clinical diabetic nephropathy.