Abstract
Immunological assays and transport measurements in apical membrane vesicles revealed that the apical membrane of rat kidney cortex and medulla presents OCTN2 and OCTN3 proteins and transports L‐[^3^H]‐carnitine in a Na^+^‐dependent and ‐independent manner. OCTN2 mediates the Na^+^/L‐carnitine transport activity measured in medulla because (i) the transport showed the same characteristics as the cortical Na^+^/L‐carnitine transporter and (ii) the medulla expressed OCTN2 mRNA and protein. The Na^+^‐independent L‐carnitine transport activity appears to be mediated by both OCTN2 and OCTN3 since: (i) Na^+^‐independent L‐carnitine uptake was inhibited by both, anti‐OCTN2 and anti‐OCTN3 antibodies, (ii) kinetics studies revealed the involvement of a high‐ and a low‐affinity transport systems, and (iii) Western and immunohistochemistry studies revealed that OCTN3 protein is located at the apical membrane of the kidney epithelia. The Na^+^‐independent L‐carnitine uptake exhibited trans‐stimulation by intravesicular L‐carnitine or betaine. This trans‐stimulation was inhibited by anti‐OCTN3 antibody, but not by anti‐OCTN2 antibody, indicating that OCTN3 can function as an L‐carnitine/organic compound exchanger. This is the first report showing a functional apical OCTN2 in the renal medulla and a functional apical OCTN3 in both renal cortex and medulla. J. Cell. Physiol. 223: 451–459, 2010. © 2010 Wiley‐Liss, Inc.