Organic anion transporting polypeptide mediates organic anion/HCO3/- exchange

Lisa M. Satlin, Vipul Amin, Allan W. Wolkoff

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Abstract

Organic anion transporting polypeptide (oatp) is an integral membrane protein cloned from rat liver that mediates Na+-independent transport of organic anions such as sulfobromophthalein and taurocholic acid. Previous studies in rat hepatocytes suggested that organic anion uptake is associated with base exchange. To better characterize the mechanism of oatp-mediated organic anion uptake, we examined transport of taurocholate in a HeLa cell line stably transfected with oatp under the regulation of a zinc-inducible promoter (Shi, X., Bai, S., Ford, A. C, Burk, R. D., Jacquemin, E., Hagenbuch, B., Meier, P. J., and Wolkoff, A. W. (1995) J. Biol. Chem. 270, 25591-25595). Whereas noninduced transfected cells showed virtually no uptake of [3H]taurocholate, taurocholate uptake by induced cells was Na+- independent and saturable (K(m) = 19.4 ± 3.3 μM; V(max) = 62.2 ± 1.4 pmol/min/mg protein; n = 3). To test whether organic anion transport is coupled to HCO3/- extrusion, we compared the rates of taurocholate- dependent HCO3/efflux from alkali-loaded noninduced and induced cells. Monolayers grown on glass coverslips were loaded with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; intracellular pH (pH(i)) was measured by excitation ratio fluorometry. Noninduced and induced cells were alkalinized to an equivalent pH(i) (~7.7) by transient exposure to a 50 mM HCO3/-, Cl+-free solution. In the absence of extracellular Cl- and taurocholate, isohydric reduction of superfusate HCO3/- concentration from 50 to 25 mM resulted in an insignificant change in pH(i) over time (dpH(i)/dt) in both groups. Addition of 25 μM taurocholate to the superfusate led to a rapid fall in pHi in induced (-0.037 ± 0.011 pH units/min to pHi of 7.41 ± 0.14) but not in noninduced (0.003 ± 0.006 pH units/min to pH(i) of 7.61 ± 0.08) cells (p < 0.03). These data indicate that oatp-mediated taurocholate transport is Na+-independent, saturable, and accompanied by HCO3/- exchange. We conclude that organic anion/base exchange is an important, potentially regulatable component of oatp function.

Original languageEnglish (US)
Pages (from-to)26340-26345
Number of pages6
JournalJournal of Biological Chemistry
Volume272
Issue number42
DOIs
StatePublished - Oct 17 1997

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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