On the mechanisms of renal sodium handling in newborn infants.

In an attempt to delineate the specific tubular defect in sodium reabsorption in low-birth-weight neonates, fractional sodium excretion (CNa/CCr), distal tubular sodium delivery (CNa+CH2O), and tubular sodium reabsorption [(CH2O/CH2O+CNa) X 100] were determined in 8 healthy premature and 10 full-term newborn infants. The mean birth weight was 1,701 g (range: 1,240--2,120 g) and the mean gestational age was 32.6 weeks (range: 28--35 weeks) for premature; and 3,199 g (range: 2,670--3,670 g) and 38.9 weeks (range: 38--41 weeks) for full-term neonates. It was demonstrated that the significantly higher fractional sodium excretion in premature infants (1.44 +/- 0.33 SE versus 0.36 +/- 0.09%), p less than 0.01) resulted from significantly decreased proximal (CNa+CH2O : 0.674 +/- 0.105 versus 0.360 +/- 0.069 ml/min/1.73m2, p less than 0.05) and distal [(CH2O/CH2O+CNa) X 100:69.9 +/- 3.3 versus 85.8 +/- 3.4%, p less than 0.01] tubular sodium reabsorption.