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

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.