Elevated ammonium levels: differential acute effects on three glutamate transporter isoforms

Research output: Contribution to journalJournal articleResearchpeer-review

Increased ammonium (NH(4)(+)/NH(3)) in the brain is a significant factor in the pathophysiology of hepatic encephalopathy, which involves altered glutamatergic neurotransmission. In glial cell cultures and brain slices, glutamate uptake either decreases or increases following acute ammonium exposure but the factors responsible for the opposing effects are unknown. Excitatory amino acid transporter isoforms EAAT1, EAAT2, and EAAT3 were expressed in Xenopus oocytes to study effects of ammonium exposure on their individual function. Ammonium increased EAAT1- and EAAT3-mediated [(3)H]glutamate uptake and glutamate transport currents but had no effect on EAAT2. The maximal EAAT3-mediated glutamate transport current was increased but the apparent affinities for glutamate and Na(+) were unaltered. Ammonium did not affect EAAT3-mediated transient currents, indicating that EAAT3 surface expression was not enhanced. The ammonium-induced stimulation of EAAT3 increased with increasing extracellular pH, suggesting that the gaseous form NH(3) mediates the effect. An ammonium-induced intracellular alkalinization was excluded as the cause of the enhanced EAAT3 activity because 1) ammonium acidified the oocyte cytoplasm, 2) intracellular pH buffering with MOPS did not reduce the stimulation, and 3) ammonium enhanced pH-independent cysteine transport. Our data suggest that the ammonium-elicited uptake stimulation is not caused by intracellular alkalinization or changes in the concentrations of cotransported ions but may be due to a direct effect on EAAT1/EAAT3. We predict that EAAT isoform-specific effects of ammonium combined with cell-specific differences in EAAT isoform expression may explain the conflicting reports on ammonium-induced changes in glial glutamate uptake.
Original languageEnglish
JournalAmerican Journal of Physiology: Cell Physiology
Issue number6
Pages (from-to)C880-C891
Number of pages12
Publication statusPublished - 2012

    Research areas

  • Ammonium Chloride, Animals, Biological Transport, Carbon Radioisotopes, Cloning, Molecular, Excitatory Amino Acid Transporter 1, Excitatory Amino Acid Transporter 2, Excitatory Amino Acid Transporter 3, Female, Glutamic Acid, Humans, Hydrogen-Ion Concentration, Methylamines, Oocytes, Patch-Clamp Techniques, Protein Isoforms, Quaternary Ammonium Compounds, Scintillation Counting, Synaptic Transmission, Tritium, Xenopus laevis

ID: 38331150