The L-3,4-dihydroxyphenylalanine transporter in human and rat epithelial intestinal cells is a type 2 hetero amino acid exchanger

Information on the intestinal transport of L-3,4-dihydroxyphenylalanine (L-DOPA) is scarce. We present here the functional characteristics and regulation of the apical inward L-DOPA transport in two intestinal epithelial cell lines (human Caco-2 and rat IEC-6). The inward transfer of L-DOPA and L-leucine was promoted through an energy-driven system but with different sensitivity to extracellular Na(+) concentration: a minor component of L-leucine uptake (approximately 25%) was found to require extracellular Na(+) in comparison with L-DOPA transport which was Na(+)-independent. L-DOPA and L-leucine uptake was insensitive to N-(methylamino)-isobutyric acid, but competitively inhibited by 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BCH). L- and D-neutral amino acids, but not acidic and basic amino acids, markedly inhibited L-DOPA and [(14)C]L-leucine accumulation in both cell lines. The [(14)C]L-DOPA and [14C]L-leucine outward were markedly increased by L-leucine and BCH present in extracellular medium, but not by L-arginine. In both cell lines, L-DOPA transport was stimulated by acidic pH in comparison with [(14)C]L-leucine inward which was pH-independent. In conclusion, it is likely that system B(0) might be responsible for the Na(+)-dependent uptake of L-leucine in Caco-2 and IEC-6 cells, whereas sodium-independent uptake of L-leucine and L-DOPA may include system type 1 and type 2 L-amino acid transporter (LAT1 and LAT2), the activation of which results in trans-stimulation of substrates outward transfer.