RENAL CONCENTRATING MECHANISMS IN NEWBORN INFANTS. EFFECT OF DIETARY PROTEIN AND WATER CONTENT, ROLE OF UREA, AND RESPONSIVENESS TO ANTI-DIURETIC HORMONE *

Abstract

Observations were made of the effects of various levels of intake of protein, urea, NaCl, and water on the concentrating mechanism of young full-term and prematurely born infants. In terms of current concepts of renal concentrating mechanisms, the failure of the young infant consistently to produce urine as hypertonic to plasma as that seen in the adult appears to be explained best, not by differences in tubular permeability to water in response to antidiuretic hormone, but rather through differences in diet and metabolism of protein, and most likely through differences in other mechanisms involved in the production of an area of interstitial hypertonicity around the collecting tubule. The young infant normally receives a diet high in protein content. However, by virtue of his strong anabolic state, as well as the form utilized for urinary nitrogen, little urea is available for excretion. High protein intake, or supplementation of the diet with urea, provides a significantly increased amount of urea for excretion, and results during hydropenia in a greatly increased urinary osmolality and urine urea concentration. The usual infant diet provides a large intake of water. Ingestion of a diet with low water content results in increased urinary osmolality in response to dehydration. Values for net reabsorption of osmotically free water in infants approach those seen in adults, indicating no difference in the effect of antidiuretic hormone on the permeability of the collecting tubule to water.