Differentiating among HCO3−, CO3=, and H+ movements across membranes has long seemed impossible. We now seek to discriminate unambiguously among three alternate mechanisms: the inward flux of 2 HCO3− (mechanism 1), the inward flux of 1 CO3= (mechanism 2), and the CO2/HCO3−-stimulated outward flux of 2 H+ (mechanism 3).
As a test case, we use electrophysiology and heterologous expression in Xenopus oocytes to examine SLC4 family members that appear to transport “bicarbonate” (“HCO3−”).
First, we note that cell-surface carbonic anhydrase should catalyze the forward reaction CO2+OH–→HCO3− if HCO3− is the substrate; if it is not, the reverse reaction should occur. Monitoring changes in cell-surface pH (ΔpHS) with or without cell-surface carbonic anhydrase, we find that the presumed Cl-“HCO3” exchanger AE1 (SLC4A1) does indeed transport HCO3− (mechanism 1) as long supposed, whereas the electrogenic Na/“HCO3” cotransporter NBCe1 (SLC4A4) and the electroneutral Na+-driven Cl-“HCO3” exchanger NDCBE (SLC4A8) do not. Second, we use mathematical simulations to show that each of the three mechanisms generates unique quantities of H+ at the cell surface (measured as ΔpHS) per charge transported (measured as change in membrane current, ΔIm). Calibrating ΔpHS/ΔIm in oocytes expressing the H+ channel HV1, we find that our NBCe1 data align closely with predictions of CO3= transport (mechanism 2), while ruling out HCO3− (mechanism 1) and CO2/HCO3−-stimulated H+ transport (mechanism 3).
Our surface chemistry approach makes it possible for the first time to distinguish among HCO3−, CO3=, and H+ fluxes, thereby providing insight into molecular actions of clinically relevant acid-base transporters and carbonic-anhydrase inhibitors.