The role of carbonic anhydrase II on HCO 3 − ‐initiated transport through the SLC4A4 transporter NBCe1

According to some reports, carbonic anhydrase II (CAII) binds the electrogenic Na/HCO 3 co‐transporter NBCe1, enhancing transport of HCO 3 − ‐related species. Solid‐phase binding assay data shows that CAII does not interact directly with C‐termini of 3 SLC4s. In 2006, the Boron Lab (BL) showed that neither pure CAII injected into Xenopus oocytes expressing NBCe1 nor CAII genetically fused to the NBCe1 C‐terminus influenced NBCe1's slope conductance (G). Yet in 2007, Becker & Deitmer (BD) reached the opposite conclusion. One difference between the studies was that BL's I‐V protocol involved 60 ms steps in 20 mV increments from the reversal potential (E rev ). BD made long‐duration voltage clamps tens of mV from E rev , producing currents that markedly altered intracellular ionic composition. We first hypothesized that CAII distributed throughout the cytosol (rather than that attached to the membrane) enhanced transport in the BD protocol by dissipating long‐distance HCO 3 − and pH i gradients—an effect that should be accentuated by prolonged V h steps far from E rev . However, we found that even increasing V h steps to 30 s did not reveal an effect of CAII on G in our hands. Another difference between the studies was that pH i was substantially lower in the absence than in the presence of a CAII inhibitor with BD—an effect that could have led to an overestimation of CAII inhibition. We are now evaluating that hypothesis.