Facultad de Farmacia, Departamento de Fisiología y Zoología (Biomembranes Group), Universidad de Sevilla, 41012Sevilla, Spain.
J Cell Physiol. 2010 May;223(2):451-9. doi: 10.1002/jcp.22054.
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.
免疫测定和顶端膜小泡转运测量表明，大鼠肾皮质和髓质的顶端膜存在 OCTN2 和 OCTN3 蛋白，并以钠离子依赖和非依赖的方式转运 L-[(3)H]-肉碱。OCTN2 介导了在髓质中测量到的钠离子/ L-肉碱转运活性，因为：(i) 转运表现出与皮质钠离子/ L-肉碱转运体相同的特征，(ii) 髓质表达 OCTN2 mRNA 和蛋白。钠离子非依赖的 L-肉碱转运活性似乎由 OCTN2 和 OCTN3 介导，因为：(i) 钠离子非依赖的 L-肉碱摄取被抗 OCTN2 和抗 OCTN3 抗体抑制，(ii) 动力学研究表明涉及高亲和性和低亲和性转运系统，以及 (iii) Western 和免疫组织化学研究表明，OCTN3 蛋白位于肾脏上皮细胞的顶端膜上。钠离子非依赖的 L-肉碱摄取表现出腔内 L-肉碱或甜菜碱的跨刺激作用。这种跨刺激作用被抗 OCTN3 抗体抑制，但不受抗 OCTN2 抗体抑制，表明 OCTN3 可以作为 L-肉碱/有机化合物交换体发挥作用。这是首次报道在肾髓质中存在功能性顶端 OCTN2，以及在肾皮质和髓质中均存在功能性顶端 OCTN3。
