Polarized amino acid transport by an epithelial cell line of renal origin (LLC-PK1). The apical systems.

The transport of D-aspartate has been studied in an epithelial cell line from a pig kidney. The amino acid is accumulated by LLC-PK1 cells without evidence of metabolism. The accumulation against a concentration gradient occurs through a mechanism with several features of a carrier-mediated process. The influx may be accounted for by a saturable Na+-dependent and nonsaturable Na+-independent process. The presence of Na+ in the incubation medium increases Vmax without affecting Km. A number of differences in the apparent affinities and specifities allows one to differentiate between this and the acidic amino acid transport system from other tissues. Polarized uptake from either side of the monolayers indicates that the acidic amino acid transport system is preferentially located in the apical membrane of the cultured renal cells. The apical localization of this system clearly contrasts with the basolateral localization of the other three neutral amino acid transport systems reported previously, indicating a high degree of cell polarization. The present study shows a close similarity between the Na+-dependent acidic amino acid transport system in LLC-PK1 cells and the system present in the apical membrane of the proximal tubular cells.