Increased proximal tubular reabsorption of sodium in childhood diabetes mellitus.

Type 1 diabetes mellitus is associated with an increase in total exchangeable body sodium. To delineate a site of possible altered sodium handling, proximal tubular sodium reabsorption (PTRNa) was measured in 30 diabetic children, age 12.0 (range 7-16) yr, duration of diabetes 4.5 (range 0.2-12) yr, and compared with 10 non-diabetic children, age 10.0 (range 8.6-12.5) yr. PTRNa was calculated from the fractional clearance of lithium, which was determined from a single blood sample and a random untimed urine sample, taken between 0700 and 0830 h at home, fasting, before insulin therapy. PTRNa was significantly increased in the diabetic children compared with the non-diabetic children (81.6(SE 1.0) vs 74.2(2.6)%, p = 0.014). There was no relationship of PTRNa with age, duration of diabetes, metabolic control (glycosylated haemoglobin, plasma and urinary glucose, plasma lactate), or urinary protein excretion (albumin, N-acetyl-beta-D-glucosaminidase). Elevated sodium reabsorption in the proximal renal tubule may account for the high total exchangeable body sodium found in Type 1 diabetic patients.