PH sensitivity of ammonium transport by Rhbg

Abstract

Rhbg, a non-erythroid glycoprotein, is localized to the basolateral membrane of renal intercalated cells and is involved in NH4+ transport. In this study we characterized the pH sensitivity of Rhbg expressed in Xenopus oocytes. We used two-electrode voltage clamp & ion-selective microelectrodes to measure NH4+currents (INH4+) & changes in pHi. In oocytes expressing Rhbg, 5mM NH4+ induced an inward INH4+ of −79 nA, decreased pHi by 0.13 at a rate of −27×10−4 pH/sec & depolarized the cell by 45 mV. These changes were significantly larger than in H2O-injected oocytes. Methyl ammonium (MA, 5 mM), often used as an NH4+ substrate, induced a current (IMA) 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 & IMA. Lowering bath pH to 6.5 inhibited the MA-induced changes completely. The NH4+-induced changes in pHi & current were similarly sensitive to pH. Exposing the oocyte to MA at low pHi (decreased by butyrate) abolished the MA-induced current however pHi still increased. These data indicate that: Rhbg transport of NH4+ is electrogenic. MA transport by Rhbg is both electroneutral and electrogenic. Electrogenic NH4+ and MA transport is pH sensitive. Electroneutral transport of MA by Rhbg is not affected by pH changes.