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(The FASEB Journal. 2007;21:938.21)
© 2007 FASEB
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938.21

pH sensitivity of ammonium transport by Rhbg

Nazih L Nakhoul, Solange Abdulnour-Nakhoul, Eric Schmidt, Rienk Doetjes and L Lee Hamm

Medicine, Tulane University, 1430 Tulane Av, SL-45, New Orleans, LA, 70112

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 –27x10–4 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:

  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.
  4. Electroneutral transport of MA by Rhbg is not affected by pH changes.





This Article
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Right arrow Articles by Nakhoul, N. L
Right arrow Articles by Hamm, L L.