Chloride concentration gradient in newborn dogs in the presence of distal nephron blockade

Renal tubular Na+, Cl-, and H2O reabsorption was determined in 14 newborn dogs, 3--29 days of age, and in three adult dogs. In all animals NaCl reabsorption beyond the proximal tubule was blocked with ethacrynic acid (2 mg/kg) and amiloride (2.4 mg/kg). During distal blockade, fractional reabsorption of NaCl and water in both newborns and adults was approximately 70%, and there was a urine-to-plasma chloride gradient equal to 1.34 +/- 0.01, indicating that the proximal tubules of the newborn as well as those of the adult can generate a transtubular Cl- gradient. Upon administration of acetazolamide (50 mg/kg), there was a dramatic increase in excretion of Na+, Cl-, HCO3-, and water, and a decrease in the transtubular chloride gradient. After acetazolamide, the degree of inhibition of HCO3 reabsorption was well correlated with that of Na+, (r = 0.77) or Cl- (r = 0.74), and Na+ or Cl- inhibition exceeded that of HCO3-. In the newborn animal, the ratio of inner-to outer cortical nephron function is high at birth and declines rapidly during the first few weeks of life. However, there was no correlation between age and changes in either electrolyte excretion or in the transtubular chloride gradient. Therefore, the newborn dog possesses Cl- permselective tubules in the inner cortex that, in the presence of intact HCO3- reabsorption, are capable of establishing a functional transtubular Cl- gradient contributing to NaCl reabsorption.