Human organic cation transporter 3 mediates the transport of antiarrhythmic drugs
Introduction
The transepithelial transport of organic cations plays an important role in the elimination of xenobiotics from the body by the liver and kidney (Pritchard and Miller, 1993). In the liver, numerous organic cations, including endogenous metabolites, drugs and xenobiotics, are removed from the circulation. This clearance process is achieved via the basolateral transport system, which mediates the hepatocellular uptake of organic cations. In the kidney, the secretion of organic cations is an important physiological function of the renal proximal tubule. The process of secreting organic cations through the proximal tubule cells is achieved via unidirectional transcellular transport involving the uptake of organic cations from blood across the basolateral membrane into the cells, followed by extrusion across the brush-border membrane into the proximal tubule fluid.
Functional studies led to the identification of two distinct classes of organic cation transport systems, one driven by the transmembrane potential difference and another driven by the transmembrane H+ gradient (Kekuda et al., 1998). Recently, cDNAs encoding the human organic cation transporters (human-OCTs) have been successively cloned including human-OCT1 (Gorboulev et al., 1997), human-OCT2 (Gorboulev et al., 1997), human-OCT2-A (Urakami et al., 2002) and human-OCT3 (Wu et al., 2000). Human-OCT1, human-OCT2A and human-OCT3 mRNAs were shown to be expressed in the liver, whereas human-OCT2, human-OCT2A and human-OCT3 mRNA were expressed in the kidney (Gorboulev et al., 1997, Urakami et al., 2002, Wu et al., 2000).
Pharmacokinetically, as indicated by the urinary excretion rates of unchanged drugs shown in Table 1, antiarrhythmic drugs are eliminated by the liver as well as the kidney. Almost all of the antiarrhythmic drugs possess cationic moieties, and the organic cation transport systems have been considered to play an important role in the pharmacokinetic handling of antiarrhythmic drugs. However, it remains unclarified whether OCTs including human-OCT3 mediate the transport of antiarrhythmic drugs.
The purpose of this study was to elucidate the molecular mechanism underlying the transport of antiarrhythmic drugs using cells from the second segment of the proximal tubule (S2) cells of mice expressing human-OCT3 (S2 human-OCT3). The antiarrhythmic drugs tested in the current study are listed in Table 1.
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Materials
[3H]histamine (458.8 GBq/mmol), [14C]lidocaine (20.0 GBq/mmol) and [14C]phenytoin (1.827 GBq/mmol) were purchased from Perkin Elmer Life Sciences (Boston, MA). [3H]quinidine (740 GBq/mmol) was purchased from Muromachi Chemicals (Tokyo, Japan). Antiarrhythmic drugs, namely, disopyramide, lidocaine, mexiletine, phenytoin, procainamide and quinidine were, obtained from Sigma (St. Louis, MO). Cibenzoline and pilsicanide were kind gifts from Fujisawa (Osaka, Japan) and Daiichi (Tokyo, Japan),
Quinidine and lidocaine uptake by human-OCT3
We have elucidated whether human-OCT3 mediates the uptake of quinidine. Human-OCT3 exhibited a time-dependent uptake of quinidine up to 15 min. In order to further elucidate the property of quinidine transport mediated by human-OCT3, we performed kinetic analysis of quinidine uptake. Human-OCT3 mediated a dose-dependent uptake of quinidine (Fig. 1A), and the Eadie–Hofstee analysis revealed that the Km value for human-OCT3-mediated quinidine uptake was 216±23.4 μM.
We have also elucidated whether
Discussion
In 1998, rat-OCT3 cDNA was isolated from placenta (Kekuda et al., 1998). Subsequent studies revealed that rat-OCT3 is identical to the extraneuronal monoamine transporter that has been described functionally as a transporter specific for monoamines (Wu et al., 1998). Human-OCT3 cDNA has been cloned from human placenta and revealed to be a potential-sensitive organic cation transporter (Grundemann et al., 1999, Wu et al., 2000). Human-OCT3 was shown to mediate the transport of various cationic
Acknowledgements
This study was supported in part by grants-in-aid from the Ministry of Education, Culture, Sports, Science and Technology (Nos. 11671048, 11694310, 13671128 and 15590858), the Science Research Promotion Fund of the Japan Private School Promotion Foundation and Research on Health Sciences Focusing on Drug Innovation from Japan Health Sciences Foundation.
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