Sodium-bicarbonate cotransporter NBCn1/Slc4a7 inhibits NH4Cl-mediated inward current in Xenopus oocytes

The electroneutral Na+-HCO3- cotransporter NBCn1 (SLC4A7) contributes to intracellular pH maintenance and transepithelial HCO3- movement. In this study, we expressed NBCn1 in Xenopus oocytes and examined the effect of NBCn1 on oocyte NH4+ transport by analysing changes in membrane potential, current and intracellular pH mediated by NH4Cl. In the presence of HCO3-/CO2, applying NH4Cl (20mm) produced intracellular acidification of oocytes. The acidification was faster in oocytes expressing NBCn1 than in control oocytes injected with water; however, NH4Cl-mediated membrane depolarization was smaller in oocytes expressing NBCn1. In HCO3-/CO2-free solution, NH4Cl produced a smaller inward current in NBCn1-expressing oocytes (56% inhibition by 20mm NH4Cl, measured at -60 mV), while minimally affecting intracellular acidification. The inhibition of the current by NBCn1 was unaffected when BaCl2 replaced KCl. Current-voltage relationships showed a positive and nearly linear relationship between NH4Cl-mediated current and voltage, which was markedly reduced by NBCn1. Large basal currents (before NH4Cl exposure) were produced in NBCn1-expressing oocytes owing to the previously characterized channel-like activity of NBCn1. Inhibiting this channel-like activity by Na+ removal abolished the inhibitory effect of NBCn1 on NH4Cl-mediated currents. The currents were progressively reduced over 72-120 h after NBCn1 cRNA injection, during which the channel-like activity was high. These results indicate that NBCn1 stimulates NH4+ transport by its Na+-HCO3- cotransport activity, while reducing NH4+ conductance by its channel-like activity.