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Nutrient Metabolism

L-Type Amino Acid Transporters in Two Intestinal Epithelial Cell Lines Function as Exchangers with Neutral Amino Acids¹

Institute of Pharmacology & Therapeutics, Faculty of Medicine, 4200–319 Porto, Portugal

²To whom correspondence should be addressed. E-mail: patricio.soares{at}mail.telepac.pt.

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.

KEY WORDS: • L-system • leucine • Caco-2 cells • IEC-6 cells • amino acid exchanger

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