1. The renal response to volume expansion produced by water immersion to the neck at 35°C was examined in eight young normotensive uncomplicated insulin-dependent diabetic subjects and in eight matched normal control subjects.

2. Both the diabetic and normal subjects manifested a renal response of natriuresis and kaliuresis on immersion, but the natriuretic response was reduced in the diabetic group. Thus the induced excretion of sodium over the 4 h of immersion was 40 ± 5 mmol (mean ± sem) in the normal group compared with 22 ± 4 mmol in the diabetic group (P < 0.02).

3. In the normal subjects creatinine clearance did not change during immersion compared with pre-immersion control values while in the diabetic group it rose from pre-immersion control values of 112 ± 11 ml/min to a mean value of 127 ± 11 ml/min during immersion (P < 0.01).

4. The diabetic subjects thus excreted less sodium despite an increased filtered load during water immersion. Fractional excretion of sodium was significantly reduced in the diabetic subjects compared with the normal control subjects (P < 0.05).

5. The suppression of plasma renin and aldosterone was similar in normal and diabetic groups.

6. Tubular sodium retention could be an early functional change in the diabetic kidney, and be implicated in the development of diabetic nephropathy.

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