Electrogenic ammonium transport by renal Rhbg
N L Nakhoul1, E Schmidt, S-M Abdulnour-Nakhoul
1Department of Medicine, Section of Nephrology, SL-45, Tulane University School of Medicine, 1430 Tulane Avenue, New Orleans, LA 70112, USA. nakhoul@tulane.edu
Summary
The Rhbg glycoprotein facilitates electrogenic ammonium (NH4+) transport in kidney cells, exhibiting Michaelis-Menten kinetics and transporting methyl ammonium. This clarifies its role in renal ammonia handling.
Area of Science:
- Nephrology
- Molecular Biology
- Biophysics
Background:
- Non-erythrocyte Rh glycoproteins, Rhbg and Rhcg, are found in renal collecting duct intercalated cells.
- Their precise roles in ammonia (NH3) and ammonium (NH4+) transport remain unclear.
Purpose of the Study:
- To investigate the mechanism of NH4+ transport mediated by Rhbg and Rhcg.
- To characterize the kinetic properties and substrate specificity of Rhbg-mediated NH4+ transport.
Main Methods:
- Expression of Rhbg and Rhcg in Xenopus oocytes.
- Two-electrode voltage-clamp electrophysiology to measure NH4+-induced currents.
- Ion-selective microelectrodes to monitor intracellular pH (pHi) changes.
Main Results:
- Rhcg expression showed minimal NH4+-induced currents compared to controls.
- Rhbg expression significantly increased NH4+-induced inward currents, decreased pHi, and caused depolarization.
- Rhbg-mediated NH4+ transport followed Michaelis-Menten kinetics, with an apparent Km of 2.3 mM, and was inhibited by amiloride.
- Rhbg also transported methyl ammonium, indicating substrate versatility.
Conclusions:
- Rhbg mediates electrogenic NH4+ transport with saturation kinetics.
- Rhbg is capable of transporting methyl ammonium.
- These findings elucidate Rhbg's function in renal ammonium homeostasis.
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