L-type amino acid transporters in two intestinal epithelial cell lines function as exchangers with neutral amino acids

The present study examined the functional characteristics of the inward [[sup.14]C]-L-leucine transporter in two intestinal epithelial cell lines (human Caco-2 and rat IEC-6). The uptake of [[sup.14]C]-L-leucine was largely promoted through an energy-dependent and sodium-insensitive transporter, although a minor component of [[sup.14]C]-L-leucine uptake (~15%) required extracellular sodium. [[sup.14]C]-L-leucine uptake was insensitive to N-(methylamino)-isobutyric acid, but competitively inhibited by 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BCH). Both L- and D-neutral amino acids, but not acidic and basic amino acids, markedly inhibited [[sup.14]C]-L-leucine accumulation. The efflux of [[sup.14]C]-L-leucine was markedly increased (P < 0.05) by L-leucine and BCH, but not by L-arginine. In IEC-6 cells, but not in Caco-2 cells, the uptake of [[sup.14]C]-L-leucine at acidic pH (5.0 and 5.4) was greater (P < 0.05) than at pH 7.4. In conclusion, it is likely that system BO might be responsible for the sodium-dependent uptake of L-leucine in Caco-2 and IEC-6 cells, whereas sodium-independent uptake of L-leucine may include system LAT1, whose activation results in transstimulation of L-leucine outward transfer. J. Nutr. 132: 733-738, 2002. KEY WORDS: * L-system * leucine * Caco-2 cells * IEC-6 cells * amino acid exchanger