Effects of NH4Cl metabolic acidosis on proximal and distal tubular sodium reabsorption in newborn infants.

The present study was undertaken to assess the influence of acute metabolic acidosis on proximal and distal tubular sodium reabsorption in a group of 7 one-week-old neonates with a mean birth weight of 2,164 g (range: 1,300-3,750 g) and a mean gestational age of 34 weeks (range: 28-40 weeks) undergoing an NH4Cl loading test. Free water clearance during water diuresis was used to estimate separately the proximal and distal sodium reabsorption. In response to NH4Cl metabolic acidosis urine flow rate (p less than 0.01) and free water clearance (p less than 0.025) increased significantly, and there was a general tendency for creatinine, osmolar and sodium clearance to increase without achieving statistical significance. The absolute amount of sodium delivered to (p less than 0.01) and reabsorbed in the distal nephron (p less than 0.01) increased significantly in spite of the constant fractional distal reabsorption. The enhanced distal sodium reabsorption during acidosis is assumed to be related to the acidosis-induced increase in aldosterone production.