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Vol. 59, Issue 2, 358-366, February 2001
Faculty of Pharmaceutical Sciences (R.O., I.T., Y.S., A.T.) and
Institute for Experimental Animals, Faculty of Medicine (H.N., N.H.),
Kanazawa University, Kanazawa, Japan; Core Research for Evolutional
Science and Technology, Japan Science and Technology
Corporation, Kawaguchi, Japan (I.T., Y.S., A.T.); and Chugai Research
Institute for Molecular Medicine Inc., Ibaraki, Japan (J.N., A.O.,
M.S.)
OCTN2 is an Na+-dependent transporter for carnitine, which
is essential for fatty acid metabolism, and its functional defect leads
to fatal systemic carnitine deficiency (SCD). It also transports the
organic cation tetraethylammonium (TEA) in an
Na+-independent manner. Here, we studied the
multifunctionality of OCTN2, by examining the transport characteristics
in cells transfected with mouse OCTN2 and in juvenile visceral
steatosis (jvs) mice that exhibit a SCD phenotype owing
to mutation of the OCTN2 gene. The physiological significance of OCTN2
as an organic cation transporter was confirmed by using
jvs mice. The embryonic fibroblasts from jvs mice exhibited significantly decreased transport of
[14C]TEA. Pharmacokinetic analysis of
[14C]TEA disposition demonstrated that jvs
mice showed decreased tissue distribution and renal secretory
clearance. In transport experiments using OCTN2-expressing cells, TEA
and carnitine showed mutual trans-stimulation effects in
their transport, implying a carnitine/TEA exchange mechanism. In
addition, Na+ affected the affinity of carnitine for OCTN2,
whereas Na+ is unlikely to be involved in TEA transport.
This is the first molecular and physiological demonstration of the
operation of an organic cation transporter in renal apical membrane.
The results are consistent with the physiological coupling of carnitine
reabsorption with the secretion of organic cations.
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