Na+-coupled transport of L-carnitine via high-affinity carnitine transporter OCTN2 and its subcellular localization in kidney

The mechanism of Na+-dependent transport of L-carnitine via the carnitine/o rganic cation transporter OCTN2 and the subcellular localization of OCTN2 i n kidney were studied. Using plasma membrane vesicles prepared from HEK293 cells that were stably transfected with human OCTN2, transport of L-carniti ne via human OCTN2 was characterized. Uptake of L-[H-3]carnitine by the OCT N2-expressing membrane vesicles was significantly increased in the presence of an inwardly directed Na+ gradient, with an overshoot, while such transi ent uphill transport was not observed in membrane vesicles from cells that were mock transfected with expression vector pcDNA3 alone. The uptake of L- [H-3]carnitine was specifically dependent on Na+ and the osmolarity effect showed that Na+ significantly influenced the transport rather than the bind ing. Changes of inorganic anions in the extravesicular medium and of membra ne potential by valinomycin altered the initial uptake activity of L-carnit ine by OCTN2. In addition, the flues of L-carnitine and Na+ were coupled wi th 1:1 stoichiometry. Accordingly, it was clarified that Na+ is coupled wit h flux of L-carnitine and the flux is an electrogenic process. Furthermore, OCTN2 was localized on the apical membrane of renal tubular epithelial cel ls. These results clarified that OCTN2 is important for the concentrative r eabsorption of L-carnitine after glomerular filtration in the kidney. (C) 2 001 Elsevier Science B.V. All rights reserved.