Transport of monoamine transmitters by the organic cation transporter type 2, OCT2

The recently cloned apical renal transport system for organic cations (OCT2) exists in dopamine-rich tissues such as kidney and some brain areas (Grundemann, D., Babin-Ebell, J., Martel, F., Ording, N., Schmidt, A., and Schomig, E. (1997) J. Biol. Chem. 272, 10408-10413). The study at hand was performed to answer the question of whether OCT2 accepts dopamine and other monoamine transmitters as substrate. 293 cells were stably transfected with the OCT2r cDNA resulting in the 293(OCT2r) cell line. Expression of OCT2r in 293 cells induces specific transport of tritiated dopamine, noradrenaline, adrenaline, and B-hydroxytryptamine (5-HT). Initial rates of specific H-3-dopamine, H-3-noradrenaline, H-3-adrenaline, and H-3-5-HT transport were saturable, the K-m values being 2.1, 4.4, 1.9, and 3.6 mmol/liter. The corresponding V-max values were 3.9, 1.0, 0.59, and 2.5 nmol min(-1).mg of protein(-1), respectively. 1,1'-diisopropyl-2,4'-cyanine (disprocynium24), a known inhibitor of OCT2 with a potent eukaliuric diuretic activity, inhibited SH-dopamine uptake into 293(OCT2r) cells with an K-i of 5.1 (2.6, 9.9) nmol/liter. In situ hybridization reveals that, within the kidney, the OCT2r mRNA is restricted to the outer medulla and deep portions of the medullary rays indicating selective expression in the S3 segment of the proximal tubule. These findings open the possibility that OCT2r plays a role in renal dopamine handling.