FUNCTIONAL-CHARACTERISTICS AND MEMBRANE LOCALIZATION OF RAT MULTISPECIFIC ORGANIC CATION TRANSPORTERS, OCT1 AND OCT2, MEDIATING TUBULAR SECRETION OF CATIONIC DRUGS

We have isolated a kidney-specific organic cation transporter, rat OCT 2, which is distinct from rat OCT1 (Okuda M, Saito H, Urakami Y, Takan o M and Inui K (1996) Biochem Biophys Res Commun 224:500-507). In our study, the functional characteristics and membrane localization of OCT 1 and OCT2 were investigated by uptake studies using MDCK cells transf ected with rat OCT1 or OCT2 cDNA (MDCK-OCT1 or MDCK-OCT2) and immunolo gical studies. Tetraethylammonium (TEA) uptake by both MDCK-OCT1 and M DCK-OCT2 cells was markedly elevated when TEA was added to the basolat eral medium, but not to the apical medium. Efflux of TEA from MDCK-OCT 1 and MDCK-OCT2 cells was not changed by extracellular pH from 5.4 to 8.4, whereas TEA uptake by both transfectants was decreased by acidifi cation of extracellular medium. Apparent K-m values for TEA uptake by MDCK-OCT1 and MDCK-OCT2 cells were 38 and 45 mu M, respectively. Altho ugh various hydrophilic organic cations such as 1-methyl-4-phenylpyrid inium, cimetidine, quinidine, nicotine, N-1-methylnicotinamide and gua nidine markedly inhibited TEA uptake by both MDCK-OCT1 and MDCK-OCT2 c ells, there were no significant differences in the apparent inhibition constants [K-i] against these organic cations between both transfecta nts. Furthermore, immunological studies using a polyclonal antibody ag ainst OCT1 revealed that OCT1 was expressed in the basolateral membran es but not in the brush-border membranes of the rat kidney. These resu lts suggested that both OCT1 and OCT2 are basolateral-type organic cat ion transporters with broad substrate specificities, mediating tubular secretion of cationic drugs.