The effect of intravenous aldosterone on the glomerular filtration rate and urinary excretion of sodium and potassium was evaluated in 12 newborns (aged 7 to 21 days) and 12 infants (aged 2 to 12 months). A decrease in sodium excretion was demonstrated in the latter group only. There was a decreased excretion of potassium in both groups, so that the ratio of sodium to potassium excreted was not significantly altered. The filtered loads and tubular reabsorption of sodium, measured as tubular rejection fraction, were not influenced by aldosterone. The effect of intravenous spirolactene was studied in 9 newborns (aged 4 to 20 days) and 13 infants (aged 2 to 8 months). Increased sodium excretion and decreased potassium excretion were demonstrated in both groups. A significantly increased sodium : potassium ratio and a significant increase of the tubular rejection fraction were observed in response to spirolactone administration in both age groups.
Spirolactone is a competitive antagonist of aldosterone, effective only under conditions in which the hormone is exerting its physiological action on the kidney. As it was fully effective even in the newborns, the results thus suggest that aldosterone is of physiological importance in the control of urinary sodium and potassium excretion even at this age. The lack of response to intravenous aldosterone can be interpreted as a reflection of the full prior stimulation of the newborn kidney by endogenous aldosterone.