Abstract
Renal ammonium (NH3 + NH4+) transport is a key process for body acid-base balance. It is well known that several ionic transport systems allow NH4+ transmembrane translocation without high specificity NH4+, but it is still debated whether NH3, and more generally, gas, may be transported by transmembrane proteins. The human Rh glycoproteins have been proposed to mediate ammonium transport. Transport of NH4+ and/or NH3 by the epithelial Rh C glycoprotein (RhCG) may be of physiological importance in renal ammonium excretion because RhCG is mainly expressed in the distal nephron. However, RhCG function is not yet established. In the present study, we search for ammonium transport by RhCG. RhCG function was investigated by electrophysiological approaches in RhCG-expressing Xenopus laevis oocytes. In the submillimolar concentration range, NH4Cl exposure induced inward currents (IAM) in voltage-clamped RhCG-expressing cells, but not in control cells. At physiological extracellular pH (pHo) = 7.5, the amplitude of IAM increased with NH4Cl concentration and membrane hyperpolarization. The amplitude of IAM was independent of external Na+ or K+ concentrations but was enhanced by alkaline pHo and decreased by acid pHo. The apparent affinity of RhCG for NH4+ was affected by NH3 concentration and by changing pHo, whereas the apparent affinity for NH3 was unchanged by pHo, consistent with direct NH3 involvement in RhCG function. The enhancement of methylammonium-induced current by NH3 further supported this conclusion. Exposure to 500 microm NH4Cl induced a biphasic intracellular pH change in RhCG-expressing oocytes, consistent with both NH3 and NH4+ enhanced influx. Our results support the hypothesis of a specific role for RhCG in NH3 and NH4+ transport.
Highlights
The human Rh family is composed of five known proteins: RhD, RhCE, RhAG, RhBG, and Rh C glycoprotein (RhCG)
The apparent affinity of RhCG for NH4؉ was affected by NH3 concentration and by changing pHo, whereas the apparent affinity for NH3 was unchanged by pHo, consistent with direct NH3 involvement in RhCG function
The aim of our study was to functionally express RhCG in X. laevis oocytes and investigate whether RhCG is involved in ammonium1 (NH4ϩ and/or NH3) transport
Summary
The human Rh family is composed of five known proteins: RhD, RhCE, RhAG, RhBG, and RhCG. Consistent with the involvement of Rh proteins in transmembrane ammonium transport, yeasts deficient in endogenous ammonium transport system (⌬Mep Saccharomyces cerevisiae) are enabled to grow in a low ammonium-containing medium when transformed by RhAG or by RhCG [3]. Based on their finding that Mep and Amt are NH3 channels, Soupene et al [10, 11] raised the hypothesis that Rh proteins are involved in gas transport rather than in ionic transport. Exposure to 500 M NH4Cl induced a biphasic intracellular pH change in RhCG-expressing oocytes, consistent with both NH3 and NH4ϩ influx into the cell. These results are consistent with RhCG-induced NH3 and NH4ϩ transport
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
