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

Abstract

The present study examined the functional characteristics of the inward [14C]-L-leucine transporter in two intestinal epithelial cell lines (human Caco-2 and rat IEC-6). The uptake of [14C]-L-leucine was largely promoted through an energy-dependent and sodium-insensitive transporter, although a minor component of [14C]-L-leucine uptake (∼15%) required extracellular sodium. [14C]-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 [14C]-L-leucine accumulation. The efflux of [14C]-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 [14C]-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 B0 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.