CATECHOLAMINE TRANSPORT BY THE ORGANIC CATION TRANSPORTER TYPE-1 (OCT1)

1 Liver and kidney extract adrenaline and noradrenaline from the circu lation by a mechanism which does not seem to be one of the classical c atecholamine transporters. The hypothesis that OCT1 is involved-the or ganic cation transporter type 1 which exists in rat kidney and liver-w as tested. 2 Based on human embryonic kidney cells (293), we construct ed a cell line which stably expresses OCT1r (293(OCT1r) cells). Transf ection with OCT1 resulted in a transport activity not only for prototy pical known substrates of OCT1 such as H-3-1-methyl-4-phenylpyridinium and C-14-tetraethylammonium but also for the catecholamines H-3-adren aline. H-3-noradrenaline (H-3-NA) and H-3-dopamine (H-3-DA), the indol eamine H-3-5-hydroxytryptamine (H-3-5HT) as well as the indirect sympa thomimetic C-14-tyramine. 3 For H-3-DA, H-3-5HT and H-3-NA, at non-sat urating concentrations, the rate constants for inwardly directed subst rate 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)) whic h rules out the involvement of classical transporters for biogenic ami nes. 5 The findings demonstrate that OCT1 efficiently transports catec holamines and other biogenic amines and support the hypothesis that OC T1 is responsible for hepatic and renal inactivation of circulating ca techolamines.