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It has been reported that patients with type I insulin dependent diabetes mellitus (IDDM) are characterized by reduced Na+ excretion during water immersion and saline infusion and abnormal glomerulo-tubular balance. Aims of the present study were therefore to investigate firstly the fractional tubular Na+ reabsorption during saline infusion to clarify the altered tubular site and secondly the glomerulo-tubular balance during acute increase of glomerular filtration rate induced by sodium acetoacetate infusion in IDDM. During saline and euglycaemic glucose clamp, after an overnight fast, glomerular filtration rate, renal plasma flow, filtration fraction and plasma sodium were 99 +/- 15 ml min-1 1.73 m-2, 452 +/- 109 ml min-1 1.73 m-2, 0.23 +/- 0.04 and 142 +/- 8 mmol l-1 (Mean +/- SD) in 10 type I insulin dependent diabetic patients and 96 +/- 18, 452 +/- 87, 0.21 +/- 0.02, 143 +/- 2 in five matched normal subjects, respectively. The lithium and sodium clearances were significantly lower in diabetic patients than in normal subjects (23 +/- 5 ml min-1 1.73 m-2 vs 28 +/- 6, p less than 0.05 and 1.1 +/- 0.4 vs 1.6 +/- 0.3, p less than 0.01 respectively). The fractional lithium reabsorption was greater (0.77 +/- 0.04 vs 0.72 +/- 0.03, p less than 0.05) and the distal fractional sodium reabsorption smaller (0.22 +/- 0.04 vs 0.27 +/- 0.03, p less than 0.01) in the diabetic patients compared to the controls.(ABSTRACT TRUNCATED AT 250 WORDS)