Membrane mechanisms for transepithelial amino acid absorption and secretion.

Although amino acids are normally almost completely absorbed by the proximal nephron, specific amino acids may also exhibit active secretion under the appropriate circumstances. This brief review examines details of the influx and efflux transport processes across both lumina and basolateral membranes of the proximal tubule and through the paracellular pathway as determinants of the direction and rate of transepithelial amino acid transport. Net absorption depends upon active uptake from the lumen into tubular cells by cotransport with Na+. At least some proximal tubule cells also actively accumulate amino acids from their peritubular environment by active transport across basolateral membranes. The net direction of transepithelial movement depends on the relative rates of passive amino acid exit from the cell across luminal and basolateral membranes. Since exit across the basolateral membrane appears to be facilitated whereas luminal exit is not, normally net absorption occurs. The minimum luminal amino acid concentration and excretion rate are also dependent on a relatively low paracellular amino acid permeability, especially in the latest segments of the proximal nephron where the transepithelial concentration gradient is the greatest.