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

Abstract

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. Based on human embryonic kidney cells (293), we constructed a cell line which stably expresses OCT1r (293OCT1r 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. For 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−1 mg 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−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). Specific 3H‐DA uptake in 293OCT1r 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. 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. British Journal of Pharmacology (1998) 125, 218–224; doi:10.1038/sj.bjp.0702065

Down-regulation of the organic cation transporter 1 of rat liver in obstructive cholestasis

“…The long-term effects of impaired hepatic uptake of Oct1 substrates that largely consist of small organic cationic drugs and important physiological substrates like choline, 4,5 thiamine, 5 tyramine, 6 N-1-methylnicotinamide, 4,5 dopamine, 5,6,8 serotonin, 6,8 adrenaline, 6 noradrenaline, 5,6,8 acetylcholine, 8 and histamine 5,8 are not known but could be important in clinical cholestatic disorders, should the hepatic uptake of these and other cationic solutes and drugs be impaired. It is attractive to speculate whether reductions in the hepatic uptake of neurotransmitters such as serotonin might account for the chronic fatigue seen in many patients with chronic cholestasis.…”

“…Functional studies in Xenopus laevis oocytes showed electrogenic, pHindependent, and Na ϩ -independent uptake of the model organic cation tetraethylammonium (TEA). 2 Further studies in different expression systems identified a wide range of xenobiotics as substrates of Oct1, including tributylmethylammonium (TBuMA), 3 1-methyl-4-phenylpyridinium, 4 -6 azidothymidine, 7 cytosine arabinoside, 7 and physiological substances such as choline 4,5 thiamine, 5 tyramine, 6 N-1-methylnicotinamide, 4,5 dopamine, 5,6,8 serotonin, 6,8 adrenaline, 6 noradrenaline, 5,6,8 acetylcholine, 8 and histamine. 5,8 Studies in Xenopus laevis oocytes demonstrated that rat Oct1 was functionally similar to the hepatic uptake system type I for small organic cations 3 described some years ago.…”

Individual PKC-Phosphorylation Sites in Organic Cation Transporter 1 Determine Substrate Selectivity and Transport Regulation

“…The transport mediated by rOCT1 has been characterized as electrogenic, independent of Na ϩ and pH, and bidirectional by radioactive tracer flux measurements with oocytes from Xenopus laevis expressing rOCT1 (17). rOCT1 can translocate organic cations like TEA ϩ and choline (18), cathecolamines (19), and nucleosides like 2Ј-deoxytubercidin (20), whereas cations like tetrapentylammonium (TPA ϩ ) and cyanine 863 are nontransported inhibitors of the transporter (21). For this transport protein 12 transmembrane domains, a large hydrophilic extracellular loop between the first and the second predicted transmembrane domain and an intracellular localization of the Nand C-termini have been described (22).…”

Interaction of Organic Cations with a Newly Identified Plasma Membrane Monoamine Transporter

“…The OCTs are considered "polyspecifc" or "multispecific" because a wide array of compounds with diverse chemical structures interact with these transporters, either as substrates or as inhibitors (Dresser et al, 2001;Wright and Dantzler, 2004). Classic substrates of the OCTs include MPP ϩ , TEA, guanidine, choline (Busch et al, 1996;Gorboulev et al, 1997;Grundemann et al, 1999;Kekuda et al, 1998;Sweet et al, 2001), the biogenic amines, such as histamine, and the monoamine neurotransmitters (Breidert et al, 1998;Busch et al, 1998;Grundemann et al, 1998;Jonker and Schinkel, 2004). Bulkier and more hydrophobic cations (i.e., the type II cations) are often potent inhibitors, but not substrates, for the OCTs (Nagel et al, 1997).…”