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Vol. 303, Issue 3, 959-968, December 2002
Department of Pharmacology and Therapeutics (K.B.G., G.L., M.T.P.,
and D.S.S.), Department of Internal Medicine (D.S.S.), Department of
Pediatrics and Child Health (D.S.S.), and Centre on Aging (D.S.S.),
University of Manitoba, Winnipeg, Manitoba, Canada; and Institute of
Anatomy and Cell Biology (V.G., C.V., and H.K.), Wurzburg, Germany
In renal proximal tubules, the organic cation transporters rOCT1 and
rOCT2 are supposed to mediate the first step in organic cation
secretion. We investigated whether previously described differences in
amantadine and tetraethylammonium (TEA) uptake into isolated renal
proximal tubules could be explained by differences in their transport
by rOCT1 and rOCT2. By expressing rOCT1 and rOCT2 in
Xenopus oocytes and HEK 293 cells, we demonstrated that both transporters translocated amantadine. In Xenopus
oocytes, the inhibitory potency of several rOCT1/2 inhibitors was
similar for amantadine compared to TEA uptake and supports amantadine transport by rOCT1 and rOCT2. In proximal tubules, procainamide, quinine, cyanine863, choline, and guanidine in
concentrations that inhibit rOCT1/2-mediated TEA or amantadine uptake
in Xenopus oocytes exhibited no effect on amantadine
uptake. At variance, these inhibitors blocked TEA uptake into proximal
tubules. Amantadine and TEA transport were sensitive to modulation by
25 mM bicarbonate. The effect of bicarbonate on organic cation
transport was dependent on substrate (amantadine or TEA), cell system
(oocytes, HEK 293 cells, or proximal tubules), and transporter (rOCT1
or rOCT2). In proximal tubules, only amantadine uptake was stimulated
by bicarbonate. The data suggested that rat renal proximal tubules contain an organic cation transporter in addition to rOCT1 and rOCT2
that mediates amantadine uptake and requires bicarbonate for optimal
function. TEA uptake by the basolateral membrane may be mediated mainly
by rOCT1 and rOCT2, but these transporters may be in a different
functional or regulatory state when expressed in cells or oocytes
compared with expression in vivo.
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