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

Renal tubular reabsorption of glycine and of the L- and D-isomers of histidine, serine, phenyl-alanine, 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 the L-isomers, tubular reabsorption is saturable to a great extent. The D-amino acids are reabsorbed much more slowly than the respective L-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 the L-form. This indicates that the D-isomers also have a measurable affinity for the reabsorption mechanisms of the renal tubule. The very poor reabsorption of D-amino acids in the presence of their L-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 X 10(-7) cm2 - s-1. These values are very similar to those found for other organic molecules of comparable molecular weights.