Segmental Nephron Sodium and Potassium Reabsorption in Newborn and Adult Dogs during Saline Expansion

Abstract

Studies were carried out in 23 anesthetized neonatal dogs aged 2 to 20 days and in 16 adult dogs to compare the effects of saline volume expansion on renal tubular Na and K reabsorption between newborn and adult animals. Proximal- and distal-tubule function was estimated by the distal-nephron-blockade technique using ethacrynic acid and amiloride. During saline infusion, which increased extracellular volume by approximately 30% for both age groups, total nephron fractional Na reabsorption was 0.91 for the adult but 0.98 for the puppy (P < 0.01). However, proximal tubule fractional Na reabsorption was greater in the adult (0.64) than in the puppy (0.48, P < 0.01) whereas distal nephron fractional Na reabsorption was much greater in the newborn (0.51) than in the adult (0.26, P < 0.01). Sodium reabsorption normalized to kidney weight was lower in all segments of the neonatal kidney than in the adult kidney. The filtered sodium load was lower in the newborn (27.0 μeq min^–1g^–1) than in the adult (105.0, P < 0.01), and the Na load to the distal nephron was also lower in the newborn (14.0 μeq min^–1g^–1) than in the adult (37.2, P < 0.01). Fractional K excretion was similar in both age groups even though the fraction of filtered K escaping proximal-tubule reabsorption was greater in the newborn than in the adult, indicating greater net K fractional reabsorption in the distal nephron of the newborn than in the adult kidney. These results indicate that in response to saline expansion there is a greater proximal tubule natriuresis in the neonate than in the adult but overall renal Na excretion is less in the newborn animal due to enhanced fractional Na reabsorption in the neonatal distal nephron, particularly in Henle's Loop.