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Am J Physiol Cell Physiol 282: C1278-C1289, 2002. First published January 16, 2002; doi:10.1152/ajpcell.00589.2001
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Vol. 282, Issue 6, C1278-C1289, June 2002

Functional characterization of human NBC4 as an electrogenic Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter (NBCe2)

Leila V. Virkki1, Darren A. Wilson2, Richard D. Vaughan-Jones2, and Walter F. Boron1

1 Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520; and 2 University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom

We have functionally characterized Na+-driven bicarbonate transporter (NBC)4, originally cloned from human heart by Pushkin et al. (Pushkin A, Abuladze N, Newman D, Lee I, Xu G, and Kurtz I. Biochem Biophys Acta 1493: 215-218, 2000). Of the four NBC4 variants currently present in GenBank, our own cloning efforts yielded only variant c. We expressed NBC4c (GenBank accession no. AF293337) in Xenopus laevis oocytes and assayed membrane potential (Vm) and pH regulatory function with microelectrodes. Exposing an NBC4c-expressing oocyte to a solution containing 5% CO2 and 33 mM HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> elicited a large hyperpolarization, indicating that the transporter is electrogenic. The initial CO2-induced decrease in intracellular pH (pHi) was followed by a slow recovery that was reversed by removing external Na+. Two-electrode voltage clamp of NBC4c-expressing oocytes revealed large HCO<UP><SUB>3</SUB><SUP>−</SUP></UP>- and Na+-dependent currents. When we voltage clamped Vm far from NBC4c's estimated reversal potential (Erev), the pHi recovery rate increased substantially. Both the currents and pHi recovery were blocked by 200 µM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). We estimated the transporter's HCO<UP><SUB>3</SUB><SUP>−</SUP></UP>:Na+ stoichiometry by measuring Erev at different extracellular Na+ concentration ([Na+]o) values. A plot of Erev against log[Na+]o was linear, with a slope of 54.8 mV/log[Na+]o. This observation, as well as the absolute Erev values, are consistent with a 2:1 stoichiometry. In conclusion, the behavior of NBC4c, which we propose to call NBCe2-c, is similar to that of NBCe1, the first electrogenic NBC.

intracellular pH; 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid; microelectrodes; stoichiometry; Xenopus laevis oocytes


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