Amino acid reabsorption in the proximal tubule of rat kidney: Stereospecificity and passive diffusion studied by continuous microperfusion

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Abstract
Renal tubular reabsorption of glycine and of thel- andd-isomers of histidine, serine, phenylalamine, methionine, proline and cystine was investigated in vivo et situ by continuous microperfusion of single proximal convolutions of the rat kidney. In the case of glycine and thel-isomers, tubular reabsorption is saturable to a great extent. Thed-amino acids are reabsorbed much more slowly than the respectivel-forms. Furthermore in the case of methionine and perhaps also of proline, serine and phenylalanine, the fractional reabsorption decreases in the presence of high concentrations of thel-form. This indicates that thed-isomers also have a measurable affinity for the reabsorption mechanisms of the renal tubule. The very poor reabsorption ofd-amino acids in the presence of theirl-isomers indicates that simple passive diffusion plays only a relatively small role in tubular amino acid reabsorption. Permeability coefficients estimated from these findings are in the range from 1–5×10−7 cm2·s−1. These values are very similar to those found for other organic molecules of comparable molecular weights.