DEVELOPMENTAL ASPECTS OF RENAL TUBULAR REABSORPTION OF WATER IN THE LAMB FETUS

The effects of intrauterine maturation on the tubular reabsorption of water by the fetal kidney was determined in 44 chronically catheterized fetal lambs divided in three groups. Fourteen fetuses were studied between 101 and 119 days, 13 between 120 and 130 days, and 17 between 131 and 142 days. The urinary flow rate increased (P < 0.01) when fetuses <120 days were compared to fetuses between 120-130 days of gestation. A 35% increase in Uosm was found in fetuses over 130 days and was paralleled by significant increases in (U/P) iothalamate and (U/P) osm ratios. An increase in (U/P) urea ratio in fetuses over 130 days correlated with an increase in (U/P) iothalamate (r = 0.87). The (U/P) ratios for Na+ and Cl− were not correlated with fetal age and did not increase during gestation. The percent of filtered water excreted (V/GFR) was high before 120 days of gestation (26.8 ± 3.04%) and remained elevated in fetuses of 120-130 days (28.7 ± 2.53%). After 130 days of gestation, it decreased significantly to 18.3 ± 2.56% (P < 0.05). Similarly, CH2O/GFR values were, respectively, 12.02 ± 1.53 and 13.18 ± 1.46 ml/min/100 ml GFR in groups of fetuses of less than 120 days and between 120-130 days; after 130 days CH2O/GFR decreased significantly to 5.82 ± 1.97 ml/min/ 100 ml GFR (P < 0.02). Cosm/GFR remained unchanged in all three groups of fetuses at 14.76 ± 2.57, 15.49 ± 1.90, and 12.51 ± 1.34 ml/min/100 ml GFR, respectively. The simultaneous significant decreases in V/GFR and CH2O/GFR without changes in Cosm/GFR in fetuses over 130 days suggest that the increase in Uosm in near-term fetuses represents an increase in free water reabsorption by the fetal kidney. Speculation: The increase in free water reabsorption by the near-term fetal kidney may represent either an increase in end-organ responsiveness to arginine vasopressin or maturation of mechanisms involved in the buildup of the osmotic gradient in the fetal medulla or both. © 1979 International Pediatric Research Foundation, Inc.