Premium
Is the electrogenic Na/HCO 3 cotransporter a CO 2 channel?
Author(s) -
Occhipinti Rossana,
Lu Jing,
Boron Walter F
Publication year - 2016
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.30.1_supplement.971.2
Subject(s) - chemistry , cotransporter , intracellular ph , membrane , biophysics , extracellular , membrane potential , dissolution , intracellular , inorganic chemistry , permeability (electromagnetism) , analytical chemistry (journal) , biochemistry , sodium , chromatography , biology , organic chemistry
It has now become clear that not all gases cross all biological membranes simply by dissolving into the lipid phase of the membrane, but that in some cases dissolved gases cross membranes via channels formed by integral membrane proteins. Currently, we know of three families of gas channels—the aquaporins (AQPs), rhesus (Rh) proteins, and urea transporters (UTs)—that can augment CO 2 and/or NH 3 permeability. In this study, we investigate the possibility that a fourth family exists, exemplified by the electrogenic Na/HCO 3 cotransporter (NBCe1, SLC4A4), which may be a CO 2 channel. We investigate the CO 2 permeability of NBCe1 by combining electrophysiology experiments with mathematical modeling. In the electrophysiology work, we use microelectrodes to monitor intracellular pH (pH i ) of oocytes injected with water (controls) or cRNA encoding human NBCe1‐A, which transports one Na + and one CO 3 = (carbonate) ion into the cell. We determine the maximal rate of pH i change, (dpH i /dt) max , as we switch the extracellular solution from CO 2 /HCO 3 − ‐free to (1) equilibrated (EQ) CO 2 /HCO 3 − at pH 7.50, or (2) an out‐of‐equilibrium (OOE) solution at the same pH, but containing only CO 2 (“pure CO 2 ”), or (3) an OOE solution containing only HCO 3 − (“pure HCO 3 − ”). We find that exposing control oocytes to “pure HCO 3 − ” elicits virtually no change in pH i , whereas exposing them to “pure CO 2 ” or EQ CO 2 /HCO 3 − causes pH i to fall rapidly and at nearly the same rate. Exposing NBCe1‐A oocytes to “pure HCO 3 − ” causes pH i to rise moderately fast (CO 3 = uptake), whereas exposing them to “pure CO 2 ” causes pH i to fall rapidly (CO 2 uptake). Intuition suggests that exposing an NBCe1‐A oocyte to EQ CO 2 /HCO 3 − will produce a pH i trajectory halfway between those of “pure HCO 3 − ” and “pure CO 2 ”. Surprisingly, we find that exposing NBCe1‐A oocytes to EQ causes pH i to fall about as rapidly as with “pure CO 2 ”. Thus, NBCe1‐A, in the presence of HCO 3 − , appears to increase CO 2 permeability. To assist us in interpreting our pH i data, we extended our 3D reaction‐diffusion mathematical model of CO 2 fluxes across the oocyte membrane to include NBCe1‐mediated transmembrane fluxes of CO 3 = (informed by two‐electrode voltage clamp data). The model can reproduce the pH i data over the first 15 sec of a physiological experiment, for both “pure HCO 3 − ” and “pure CO 2 ”. However, reproducing the fast pH i decrease of EQ CO 2 /HCO 3 − requires that we increase CO 2 permeability, thereby supporting the hypothesis that NBCe1 may conduct CO 2 while transporting CO 3 = . Support or Funding Information Supported by grants from the Office of Naval Research (N00014‐11‐1‐0889, N00014‐14‐1‐0716) and NIH (U01‐GM111251, K01‐DK107787)