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

Abstract

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 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.

3. For ³H-DA, ³H-5HT and ³H-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 μl min⁻¹ mg protein⁻¹. In wild type cells (293_WT) the corresponding k_in's were considerably lower, being 0.94±0.40, 0.47±0.08 and 0.23±0.05 μl 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 ³H-DA uptake in 293_OCT1r cells 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.

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.

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