Relationships between intracellular amino acid levels and protection against injury to isolated proximal tubules

Metabolism and cellular levels of glycine, alanine, and other relevant amino acids in proximal tubules were studied during models of acute injury and protection by glycine. Freeze-clamped, normal rabbit renal cortex was very rich in glycine (66.8 nmol/mg protein) and glutamate and also had substantial levels of taurine, alanine, glutamine, serine, and aspartate. Isolated proximal tubules were severely depleted of all these amino acids (glycine, 2.1 nmol/mg protein). During 37 degrees C incubation in presence of alanine, tubules recovered only glutamate to a level approximating that in vivo (38.8 nmol/mg protein, 15.2 mM). Glycine added to medium at levels ranging from 0.25 to 2 mM was actively concentrated four- to sixfold by tubule cells. Two millimolar glycine potently protected tubules from lethal cell injury induced by hypoxia, antimycin A, or ouabain. Glycine levels of injured tubules rapidly equilibrated with medium, irrespective of whether glycine was loaded by preincubation or was added concomitantly with the injury maneuver. Metabolism of glycine during protection, assessed by changes in total levels, gas chromatography-mass spectroscopy determination of the fate of [13C]glycine, and redistribution of label from [3H]glycine was minimal. The data suggest that glycine plays an essential, constitutive role in maintenance of tubule cell structural integrity independently of common metabolic pathways. Intracellular amino acid content is sufficiently labile for depletion of structurally essential amino acids to potentially occur in a variety of settings, but, even with severe ATP depletion or Na+ pump inhibition, supplemental glycine is readily available to intracellular sites of action.