Renal handling of sodium in premature and full-term neonates: a study using clearance methods during water diuresis.

A study using fractional clearances during orally induced water diuresis was designed to delineate the mechanism underlying defective tubular reabsorption of sodium in very low-birth-weight neonates. The use of clearance methodology during maximal water diuresis may give an indirect estimate of distal sodium delivery [urine volume (V), CH2O + CNa + K], sodium reabsorption at the diluting segments (CH2O), and proportion of the distal load reabsorbed distally (CH2O/CH2O + CNa + K), when all values are corrected to 100 ml glomerular filtration rate. The study was carried out in 28 healthy newborn infants who were grouped according to conceptual age (CA): 13 infants with mean birth weight of 1370 +/- 330 g and mean CA of 31.8 wk (range, 28-34 wk), and 15 infants with mean birth weight of 2330 +/- 550 g and mean CA of 37.9 wk (range, 35-41 wk). All studies were performed at 6-7 days of age. It was demonstrated that higher urinary osmolality (67.5 +/- 23.2 versus 52.9 +/- 9.4 mOsm/kg, P less than 0.0025) and higher fractional sodium excretion (2.3 +/- 1.8 versus 0.9 +/- 0.5 ml/dl glomerular filtration, P less than 0.01) observed in the group of very preterm infants resulted from significantly decreased proximal (V: 18.7 +/- 6.0 versus 13.3 +/- 3.6 ml/dl glomerular filtration, P less than 0.005; CH2O + CNa + K: 17.1 +/- 5.2 versus 11.9 +/- 3.3 ml/dl glomerular filtration, P less than 0.005) and distal (CH2O/CH2O + CNa + K X 100: 81.9 +/- 8.2 versus 88.2 +/- 4.5%, P less than 0.01) tubular sodium reabsorption.(ABSTRACT TRUNCATED AT 250 WORDS)