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1Liver and kidney extract adrenaline and noradrenaline from the circulation by a mechanism which does not seem to be one of the classical catecholamine transporters. The hypothesis that OCT1 is involved–the organic cation transporter type 1 which exists in rat kidney and liver–was tested.2Based on human embryonic kidney cells (293), we constructed a cell line which stably expresses OCT1r (293OCT1rcells). Transfection with OCT1 resulted in a transport activity not only for prototypical known substrates of OCT1 such as 3H‐1‐methyl‐4‐phenylpyridinium and 14C‐tetraethylammonium but also for the catecholamines 3H‐adrenaline, 3H‐noradrenaline (3H‐NA) and 3H‐dopamine (3H‐DA), the indoleamine 3H‐5‐hydroxytryptamine (3H‐5HT) as well as the indirect sympathomimetic 14C‐tyramine.3For 3H‐DA, 3H‐5HT and 3H‐NA, at non‐saturating concentrations, the rate constants for inwardly directed substrate flux (kin) were 6.9±0.8, 3.1±0.2, and 1.2±0.1 μl min−1mg protein−1. In wild type cells (293WT) the corresponding kin's were considerably lower, being 0.94±0.40, 0.47±0.08 and 0.23±0.05 μl min−1mg protein−1(n=12). The indirectly determined half‐saturating concentrations of DA, 5HT, and NA were 1.1 (95% c.i.: 0.8, 1.4), 0.65 (0.49, 0.86), and 2.8 (2.1, 3.7) mmol l−1(n=3).4Specific 3H‐DA uptake in 293OCT1rcells was resistant to cocaine (1 μmol l−1), 3H‐5HT uptake was resistant to citalopram (300 nmol l−1) and 3H‐NA uptake was resistant to desipramine (100 nmoll−1), corticosterone (1 μmol l−1), and reserpine (10 nmol l−1) which rules out the involvement of classical transporters for biogenic amines.5The findings demonstrate that OCT1 efficiently transports catecholamines and other biogenic amines and support the hypothesis that OCT1 is responsible for hepatic and renal inactivation of circulating catecholamines.

The recently cloned apical renal transport system for organic cations (OCT2) exists in dopamine-rich tissues such as kidney and some brain areas (Gründemann, D., Babin-Ebell, J., Martel, F., Örding, N., Schmidt, A., and Schömig, 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 293OCT2rcell line. Expression of OCT2r in 293 cells induces specific transport of tritiated dopamine, noradrenaline, adrenaline, and 5-hydroxytryptamine (5-HT). Initial rates of specific3H-dopamine, 3H-noradrenaline,3H-adrenaline, and 3H-5-HT transport were saturable, the Kmvalues being 2.1, 4.4, 1.9, and 3.6 mmol/liter. The corresponding Vmaxvalues 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 3H-dopamine uptake into 293OCT2rcells with an Kiof 5.1 (2.6, 9.9) nmol/liter. In situhybridization 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.