Postnatal changes in proximal and distal tubular sodium reabsorption in healthy very-low-birth-weight infants.

We studied 7 healthy very-low-birth-weight male infants with a mean birth weight of 1,195 g and a gestational age of 29.0 weeks, from birth to 6 weeks. The filtered Na load increased fivefold but urine Na excretion decreased from 3.4 to 0.1% of the filtered Na load. The efficiency of Na reabsorption increased 8% in the proximal tubule compared with 66% in the distal tubule. Because greater than 90% of Na reabsorption always occurred proximally, the smaller percentage increase in efficiency of the proximal tubule contributed as much as the larger percentage increase in efficiency of the distal tubule to the postnatal improvement in renal Na conservation.