The increased proximal tubular reabsorption of sodium and water is maintained in long-term insulin-dependent diabetics with early nephropathy

Proximal tubular reabsorption of sodium and water was investigated in long-term insulin-dependent diabetic patients with normoalbuminuria (group I, n = 19), microalbuminuria (group II, n = 39), diabetic nephropathy (group III, n = 12) and in 13 healthy age-matched subjects. Glomerular filtration rate was measured with the single injection, 51Cr-EDTA technique. The fluid flow rate out of the proximal tubules was assessed by the renal lithium clearance. Although glomerular filtration rate was significantly elevated in the diabetic patients (Group I: 122 +/- 16, Group II: 121 +/- 18, Group III: 110 +/- 17, CONTROLS: 105 +/- 13 ml/min X 1.73 m2), lithium clearance was similar in the four groups (Group I: 19 +/- 6, Group II: 22 +/- 7, Group III: 19 +/- 5, CONTROLS: 23 +/- 4 ml/min X 1.73 m2). Both absolute and fractional proximal reabsorption of sodium and water was enhanced in diabetes. Indices of distal tubular function did not differ between controls and patients with insulin-dependent diabetes. Sodium clearance was about the same in the four groups. Our study suggests that the enhanced proximal reabsorption of sodium and water in insulin-dependent diabetic patients is still observed despite the presence of incipient or overt diabetic nephropathy.