Abnormal circadian rhythm of diuresis or nocturnal polyuria in a subgroup of children with enuresis and hypercalciuria is related to increased sodium retention during daytime.

PURPOSE: In a subgroup of children with enuresis an increase in nighttime water and solute excretion has been documented. To investigate if modifications in renal function are involved in nocturnal enuresis, we assessed circadian variation in natriuresis and tubular sodium handling in polyuric hypercalciuric children.

MATERIALS AND METHODS: A total of 10 children with proved hypercalciuria and nocturnal polyuria and 10 age matched controls were included in the study. A 24-hour urine collection was performed in 8 sampling periods for measurement of urinary sodium excretion. Segmental tubular sodium transport was investigated during a daytime oral water load test and calculated according to standardized clearance methodology.

RESULTS: The children with enuresis showed a marked increase in the fractional excretion of sodium during the night (0.93% +/- 0.36%), while daytime sodium excretion was decreased (0.84% +/- 0.23%). Analysis of segmental tubular sodium transport revealed decreased delivery of sodium to distal tubule (C(H2O) + C(Na) = 10.7 ml/100 ml glomerular filtration rate), indicating increased proximal tubular sodium reabsorption but also stimulation of distal sodium reabsorption as demonstrated by increased fractional distal sodium reabsorption (92.9% +/- 2.2%, controls 90.5% +/- 2.9%). Increased distal reabsorption was associated with increased fractional potassium excretion (17.5% +/- 2.7%, controls 13.6% +/- 6.4%), indicating increased distal tubular sodium/potassium exchange.

CONCLUSIONS: No intrinsic defect in renal tubular sodium transport was found, but during the day increased sodium reabsorption in proximal and distal tubules was observed, suggesting extrarenal factors to be involved in altered circadian variation in solute and water excretion by the kidney.