Catecholamine transport by the organic cation transporter type 1 (OCT1)

1 Liver 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. 2 Based on human embryonic kidney cells (293), we constructed a cell line which stably expresses OCT1r (293(OCT1r) cells). Transfection with OCT1 resulted in a transport activity not only for prototypical known substrates of OCT1 such as H-3-1-methyl-4-phenylpyridinium and C-14-tetraethylammonium but also for the catecholamines H-3-adrenaline. H-3-noradrenaline (H-3-NA) and H-3-dopamine (H-3-DA), the indoleamine H-3-5-hydroxytryptamine (H-3-5HT) as well as the indirect sympathomimetic C-14-tyramine. 3 For H-3-DA, H-3-5HT and H-3-NA, at non-saturating concentrations, the rate constants for inwardly directed substrate flux (k(in)) were 6.9+/-0.8, 3.1+/-0.2, and 1.2+/-0.1 mu l min(-1) mg protein(-1). In wild type cells (293(WT)) the corresponding k(in)'s mere considerably lower, being 0.94+/-0.40, 0.47+/-0.08 and 0.23 +/- 0.05 mu l min(-1) mg 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). 4 Specific H-3-DA uptake in 293(OCT1r) cells was resistant to cocaine (1 mu mol l(-1)), H-3-5HT uptake was resistant to citalopram (300 nmol l(-1)) and H-3-NA. uptake was resistant to desipramine (100 nmoll(-1)), corticosterone (1 mu mol l(-1)), and reserpine (10 nmol l(-1)) which rules out the involvement of classical transporters for biogenic amines. 5 The 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.