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

Abstract

The transport of three neutral amino acid analogs has been studied in an epithelial cell line from a pig kidney. 2-Aminoisobutyric acid is accumulated by LLC-PK1 cells against a concentration gradient through a mechanism with several features of a carrier-mediated process. The influx is accounted for by a saturable Na+-dependent and nonsaturable Na+-independent process. The total influx of 2-(methylamino)isobutyric acid is also mediated through Na+-dependent and Na+-independent systems. Part of the Na+-dependent influx of 2-aminoisobutyric acid was competitively inhibited by 2-(methylamino)isobutyric acid (Ki = 4.2 mM).l Selectivity studies indicate that the 2-(methylamino)isobutyric acid-sensitive part of 2-aminoisobutyric acid influx is mediated through the A system, whereas the insensitive part occurs through the ASC system. Cycloleucine transport also involves a Na+-dependent and Na+-independent process. The Na+-dependent influx is completely inhibited by 2-aminoisobutyric acid but not perceptibly by 2-(methylamino)isobutyric acid. This influx probably represents entry through the ASC system. The Na+-independent component is completely inhibited by 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid, a specific substrate for the L system. Uptake studies from either side of the monolayers indicate that these transport systems are exclusively located in basolateral membranes of the cultured renal cells.