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: 110117, Controls: 105 + 13 ml/minxl.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/ minx 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.