RENAL CLEARANCE OF EIGHTEEN INDIVIDUAL AMINO ACIDS IN HUMAN SUBJECTS 1

Abstract

Four studies were made on the renal clearance of amino acids in 3 normal male subjects. After suitable control periods, Merck''s 10% solution of free amino acids, supplemented with additional DL-methionine, phenylalanine, threonine, and tryptophan, was infused at a constant rate and 3 collection periods were run. Standard clearance techniques were followed, with inulin employed to measure of glomerular filtration rate. Seventeen natural amino acids and 1 unnatural (D-methionine) were assayed microbiologically. The endogenous clearance of each was extremely low, indicating that reabsorption was virtually complete in the fasting state. During the infusion of the amino acid solution there was an increase in the amounts reabsorbed as well as in those excreted. However, even under these conditions of loading, less than 4% of the administered amino acids was excreted. The excretion of isoleucine, leucine, valine, proline, alanine, methionine, arginine, phenylalanine, and lysine remained small, despite significant and, in some instances, striking elevations in plasma concentrations. Hence the occurrence of these amino acids in abnormally large amounts in the urine is more likely to be due to impaired tubular reab-sorption than to a saturation phenomenon. Histidine, serine, glycine, and threonine were excreted in larger amounts when the plasma levels were increased, thus suggesting that maximal reabsorptive capacities might more likely be demonstrated for these individual amino acids. The excretion of cystine increased during the infusion in the absence of any increase in filtered load, thus indicating that one or more amino acids were being reabsorbed at the expense of cystine. The clearance of the unnatural D-isomer of methionine was much higher than that of the natural L-form. In the presence of a diuresis induced by a water load, a significant increase in the clearance of the D-form occurred while that of the L-form decreased. This is taken as evidence of passive reabsorption of the D-form.