Mechanisms of ammonia and ammonium transport by rhesus-associated glycoproteins

Abstract

In this study we characterized ammonia and ammonium (NH3/NH4+) transport by the rhesus-associated (Rh) glycoproteins RhAG, Rhbg, and Rhcg expressed inXenopusoocytes. We used ion-selective microelectrodes and two-electrode voltage clamp to measure changes in intracellular pH, surface pH, and whole cell currents induced by NH3/NH4+and methyl amine/ammonium (MA/MA+). These measurements allowed us to define signal-specific signatures to distinguish NH3from NH4+transport and to determine how transport of NH3and NH4+differs among RhAG, Rhbg, and Rhcg. Our data indicate that expression of Rh glycoproteins in oocytes generally enhanced NH3/NH4+transport and that cellular changes induced by transport of MA/MA+by Rh proteins were different from those induced by transport of NH3/NH4+. Our results support the following conclusions:1) RhAG and Rhbg transport both the ionic NH4+and neutral NH3species;2) transport of NH4+is electrogenic;3) like Rhbg, RhAG transport of NH4+masks NH3transport; and4) Rhcg is likely to be a predominantly NH3transporter, with no evidence of enhanced NH4+transport by this transporter. The dual role of Rh proteins as NH3and NH4+transporters is a unique property and may be critical in understanding how transepithelial secretion of NH3/NH4+occurs in the renal collecting duct.