Postnatal Changes in Proximal and Distal Tubular Sodium Reabsorption in Healthy Very-Low-Birth-Weight Infants

Abstract

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 > 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.

Sodium and potassium clearances by the maturing kidney: clinical-molecular correlates

“…In human infants, Rodriguez-Soriano et al [45] and Leslie et al [46], using clearance techniques, estimated that the percentage of filtered sodium reabsorbed by the proximal tubule increases by about 5% between 28 and 34 weeks GA, whereas the percentage of distal sodium reabsorption increases by >15% during this same period. Because the proximal tubule reabsorbs >70% of the filtered load of sodium, the small percentage increase in fractional reabsorption by this segment contributes as much as the larger percentage increase in efficiency of the distal tubule to the postnatal increase in renal sodium retention.…”

Effect of a 24-hour infusion of an isotonic electrolyte replacement fluid on the renal clearance of electrolytes in healthy neonatal foals

“…[3][4][5][6][7][8][9] Hypernatremia and volume overload (resulting from the limited ability of the kidneys to increase urinary sodium excretion) are thought to be the mechanism for these conditions. [3][4][5][6][7][10][11][12][13][14] These phenomena have been identified in neonatal humans, dogs, and sheep and are the basis for the recommendation to use fluids containing low sodium concentrations for maintenance fluid therapy in neonatal foals. 1,2,4,[10][11][12][13]15,16 Results of previous studies 17,18 in nursing neonatal foals have been used to establish reference values for urinary clearance of creatinine and electrolytes, percentage creatinine clearance (Cr cl ) of electrolytes (fractional excretion), urine osmolality (Osm u ) and specific gravity, glomerular filtration rate, and effective renal plasma flow.…”

“…[3][4][5][6][7][10][11][12][13][14] These phenomena have been identified in neonatal humans, dogs, and sheep and are the basis for the recommendation to use fluids containing low sodium concentrations for maintenance fluid therapy in neonatal foals. 1,2,4,[10][11][12][13]15,16 Results of previous studies 17,18 in nursing neonatal foals have been used to establish reference values for urinary clearance of creatinine and electrolytes, percentage creatinine clearance (Cr cl ) of electrolytes (fractional excretion), urine osmolality (Osm u ) and specific gravity, glomerular filtration rate, and effective renal plasma flow. In adult horses, IV administration of saline (0.9% NaCl) solution or 5% dextrose in water increases urinary excretion of sodium and chloride; however, information regarding the effects of administration of any type of fluid in neonatal foals is lacking.…”

Fetal Endocrine Maturation and Perinatal Renal Function

“…The result is an increase in filtration fraction, GFR, 2,15 and urineconcentrating ability. 16 Although sodium excretion may increase transiently in the immediate perinatal period, 23 within 4 hours after birth 2 sodium reabsorption increases markedly, 17 reducing both total and fractional sodium excretions. Thus, the renal capacity to reabsorb sodium increases during a period when GFR and the filtered sodium load also are increasing.…”