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Vol. 299, Issue 1, 392-398, October 2001
Department of Biopharmaceutical Sciences (Y.S., C.L.B., L.M.M.,
B.F., K.M.G.) and Division of Clinical Pharmacology and Experimental
Therapeutics (Y.S., K.M.G.), University of California San Francisco,
San Francisco, California
Organic cation transporters (OCT1-3) play an important role in renal
elimination of many drugs. The goals of this study were to 1) identify
a cell culture model which constitutively expressed OCT2 that could be
used to study the characteristics and regulation of this transporter,
and 2) to study the mechanisms by which xenobiotics and hormones
regulate the activity of OCT2. We characterized the endogenous organic
cation transporter (OCT) activity in Madin-Darby canine kidney
(MDCK) cells. The activity was localized to the basolateral membrane
and was pH and membrane potential-dependent. The uptake of the model
organic cation, tetraethylammonium, was saturable
(Km, 19.5 ± 4.6 µM;
Vmax, 350 ± 19.4 pmol/mg of protein/10 min) and was inhibited by known OCT inhibitors (e.g., cimetidine and
quinidine). A cDNA fragment (711 base pairs) isolated by reverse transcriptase-polymerase chain reaction (RT-PCR) was greater than 83%
identical to OCT2 cDNAs from mammalian species; no OCT1 or OCT3 was
detected by RT-PCR, suggesting that OCT2 may be the primary basolateral
OCT in MDCK. OCT2 mRNA levels were increased significantly following
exposure of MDCK to the steroid hormones, dexamethasone (2.0-fold),
hydrocortisone (2.4-fold), and testosterone (1.8-fold) with comparable
increases in activity. Other compounds tested, including the cytochrome
P450 inducers, rifampicin, phenobarbital, and phenytoin, and the OCT
substrates, verapamil and metformin, had no inducing effects.
Collectively, these data indicate that MDCK can serve as a useful and
convenient tool in screening candidate drugs for interaction with OCT2
and for studying the regulation of this transporter. Furthermore, our
data demonstrate that steroid hormones induce the transcription of OCT2
in the kidney.
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