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Vol. 298, Issue 1, 110-115, July 2001
Division of Clinical Pharmacology and Toxicology, Department of
Medicine, University Hospital, Zürich, Switzerland (J.E.v.M.,
P.J.M.); Department of Pharmacokinetics and Drug Delivery, Groningen
University Institute for Drug Exploration, Groningen, The Netherlands
(J.E.v.M., G.M.M.G., D.K.F.M.); Sub-department of Human Nutrition and
Epidemiology, Nutrition, Metabolism, and Genomics Group, Wageningen
University and Research Center, Wageningen, The Netherlands (M.M.); and
Department of Anatomy, University of Würzburg, Würzburg,
Germany (H.K.)
Previous inhibition studies with taurocholate and cardiac glycosides
suggested the presence of separate uptake systems for small "type
I" (system1) and for bulky "type II" (system2) organic cations in rat hepatocytes. To identify the transport systems involved
in type I and type II organic cation uptake, we compared the organic
cation transport properties of the rat and human organic cation
transporter 1 (rOCT1; hOCT1) and of the organic anion-transporting polypeptides 2 and A (rat Oatp2; human OATP-A) in cRNA-injected Xenopus laevis oocytes. Based on characteristic
cis-inhibition patterns of rOCT1-mediated
tributylmethylammonium and Oatp2-mediated rocuronium uptake, rOCT1 and
Oatp2 could be identified as the organic cation uptake systems1 and 2, respectively, in rat liver. While hOCT1 exhibited similar transport
properties as rOCT1, OATP-A- but not Oatp2-mediated rocuronium uptake
was inhibited by the OATP-A substrate
N-methyl-quinidine. The latter substrate was also
transported by rOCT1 and hOCT1, demonstrating distinct organic cation
transport activities for rOCT1 and Oatp2 and overlapping organic cation
transport activities for hOCT1 and OATP-A. Finally, the data
demonstrate that unmethylated quinidine is transported by rOCT1, hOCT1,
and OATP-A at pH 6.0, but not at pH 7.5, indicating that quinidine
requires a positive charge for carrier-mediated uptake into
hepatocytes. In conclusion, the studies demonstrate that in rat liver
the suggested organic cation uptake systems1 and 2 correspond to rOCT1
and Oatp2, respectively. However, the rat-based type I and II organic
cation transporter classification cannot be extended without
modification from rat to human.
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