Two nonerythrocyte Rh glycoproteins (Rhbg and Rhcg) were recently cloned and localized to the intercalated cells of the renal collecting duct. The apical Rhcg and the basolateral Rhbg are likely involved in NH3 and/or NH4+ transport, yet the characteristics of this transport are not clear. In this study, we characterized NH4+ transport by Rhbg expressed in Xenopus oocytes. We used two-electrodes voltage-clamp and ion-selective microelectrodes to measure NH4+-induced currents (INH4) and changes in pHi, respectively. In oocytes expressing Rhbg, 5 mM NH4+ induced an inward INH4 of −79 nA, decreased pHi (ΔpHi) by 0.13 at a rate (dpHi/dt) of −27 × 10−4 pH/s and depolarized the cell by 45 mV. These changes were significantly lager than in control oocytes. I-V plots in the presence of NH4+ showed substantial increase in conductance. Amiloride (1 mM) inhibited INH4, ΔpHi and dpHi/dt in oocytes expressing Rhbg but not in control oocytes. Methyl ammonium (MA at 5 mM), often used as an NH4+ substrate, induced a current of −63 nA in Rhbg oocytes but did not cause any change in control oocytes. Unlike NH4+, MA elicited a pHi increase but also induced a depolarization of the cell. Exposing the oocyte to MA at alkaline bath pH (8.2) enhanced both the pHi increase and the MA-induced current. Lowering bath pH to 6.5 inhibited the MA-induced changes completely. The NH4+-induced changes were similarly sensitive to pH. Exposing the oocyte to MA at low pHi (decreased by butyrate) abolished the MA-induced current and depolarization; however, pHi still increased. These data indicate that (1) Rhbg transport of NH4+ is electrogenic, (2) MA transport by Rhbg is both electroneutral and electrogenic, (3) electrogenic NH4+ and MA transport is pH sensitive, and (4) electroneutral transport of MA by Rhbg is apparently not affected by pH changes.
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