Sodium/Bicarbonate Cotransporter NBCn1/Slc4a7 Inhibits NH4Cl-mediated Inward Current in Xenopus Oocytes

• Type of Material: Article
• Authors: Soojung Lee, Emory University; Inyeong Choi, Emory University
• Language: English
• Date: 2011-08
• Publisher: Wiley: 12 months
• Publication Version: Author Accepted Manuscript (After Peer Review)
• Copyright Statement: © 2011 The Authors. Journal compilation © 2011 The Physiological Society
• Final Published Version (URL): http://ep.physoc.org/content/96/8/745
• Title of Journal or Parent Work: Experimental Physiology
• ISSN: 0958-0670
• Volume: 96
• Issue: 8
• Start Page: 745
• End Page: 755
• Grant/Funding Information: This work was supported by Emory University Research Committee Grant and National Institutes of Health Grant GM078502 (I. C.).
• Keywords: bicarbonate; Xenopus; voltage-clamp
• Research Categories: Biology, Physiology

Abstract

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 analyzing changes in membrane potential, current, and intracellular pH mediated by NH4Cl. In the presence of HCO3−/CO2, applying NH4Cl (20 mM) 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 20 mM 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 due to the previously characterized channel-like activity of NBCn1. Inhibiting this channel-like activity by Na+ removal abolished NBCn1’s inhibitory effect 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 by its Na/HCO3 cotransport activity stimulates NH4+ transport, while reducing NH4+ conductance by its channel-like activity.