Functional characterization of human NBC4 as an electrogenic Na+-HCO cotransporter (NBCe2).
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 elicited a large hyperpolarization, indicating that the transporter is electrogenic. The initial CO2-induced decrease in intracellular pH (pH(i)) was followed by a slow recovery that was reversed by removing external Na+. Two-electrode voltage clamp of NBC4c-expressing oocytes revealed large HCO- and Na+-dependent currents. When we voltage clamped V(m) far from NBC4c's estimated reversal potential (E(rev)), the pH(i) recovery rate increased substantially. Both the currents and pH(i) recovery were blocked by 200 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). We estimated the transporter's HCO:Na+ stoichiometry by measuring E(rev) at different extracellular Na+ concentration ([Na+]o) values. A plot of E(rev) against log[Na+]o was linear, with a slope of 54.8 mV/log[Na+]o. This observation, as well as the absolute E(rev) 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.
Virkki
Leila V
LV
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520-8026, USA. leila.virkki@yale.edu
Wilson
Darren A
DA
Vaughan-Jones
Richard D
RD
Boron
Walter F
WF
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000143:Acids / Q000378:metabolism; D000468:Alkalies / Q000378:metabolism; D020816:Amino Acid Motifs; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism; D003001:Cloning, Molecular; D015870:Gene Expression; D006801:Humans; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D017136:Ion Transport / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008969:Molecular Sequence Data; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D016133:Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D012964:Sodium / Q000378:metabolism*; D027982:Sodium-Bicarbonate Symporters / Q000037:antagonists & inhibitors / Q000235:genetics / Q000502:physiology*; D014162:Transfection; D014982:Xenopus laevis
D000143:Acids; D000468:Alkalies; D001639:Bicarbonates; D002712:Chlorides; C422023:SLC4A5 protein, human; D027982:Sodium-Bicarbonate Symporters; D012964:Sodium; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid