Is the electrogenic Na/HCO3 cotransporter a CO2 channel?

Abstract

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 CO2 and/or NH3 permeability. In this study, we investigate the possibility that a fourth family exists, exemplified by the electrogenic Na/HCO3 cotransporter (NBCe1, SLC4A4), which may be a CO2 channel.We investigate the CO2 permeability of NBCe1 by combining electrophysiology experiments with mathematical modeling. In the electrophysiology work, we use microelectrodes to monitor intracellular pH (pHi) of oocytes injected with water (controls) or cRNA encoding human NBCe1‐A, which transports one Na+ and one CO3= (carbonate) ion into the cell. We determine the maximal rate of pHi change, (dpHi/dt)max, as we switch the extracellular solution from CO2/HCO3−‐free to (1) equilibrated (EQ) CO2/HCO3− at pH 7.50, or (2) an out‐of‐equilibrium (OOE) solution at the same pH, but containing only CO2 (“pure CO2”), or (3) an OOE solution containing only HCO3− (“pure HCO3−”).We find that exposing control oocytes to “pure HCO3−” elicits virtually no change in pHi, whereas exposing them to “pure CO2” or EQ CO2/HCO3− causes pHi to fall rapidly and at nearly the same rate. Exposing NBCe1‐A oocytes to “pure HCO3−” causes pHi to rise moderately fast (CO3= uptake), whereas exposing them to “pure CO2” causes pHi to fall rapidly (CO2 uptake). Intuition suggests that exposing an NBCe1‐A oocyte to EQ CO2/HCO3− will produce a pHi trajectory halfway between those of “pure HCO3−” and “pure CO2”. Surprisingly, we find that exposing NBCe1‐A oocytes to EQ causes pHi to fall about as rapidly as with “pure CO2”. Thus, NBCe1‐A, in the presence of HCO3−, appears to increase CO2 permeability.To assist us in interpreting our pHi data, we extended our 3D reaction‐diffusion mathematical model of CO2 fluxes across the oocyte membrane to include NBCe1‐mediated transmembrane fluxes of CO3= (informed by two‐electrode voltage clamp data). The model can reproduce the pHi data over the first 15 sec of a physiological experiment, for both “pure HCO3−” and “pure CO2”. However, reproducing the fast pHi decrease of EQ CO2/HCO3− requires that we increase CO2 permeability, thereby supporting the hypothesis that NBCe1 may conduct CO2 while transporting CO3=.Support or Funding InformationSupported by grants from the Office of Naval Research (N00014‐11‐1‐0889, N00014‐14‐1‐0716) and NIH (U01‐GM111251, K01‐DK107787)