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

Area of Science:

• Neuroscience
• Biochemistry
• Cell Biology

Background:

• Hepatic encephalopathy involves brain ammonium accumulation, impacting glutamatergic neurotransmission.
• Glutamate uptake in glial cells shows conflicting responses to acute ammonium exposure.
• The underlying mechanisms for these opposing effects on glutamate uptake remain unclear.

Purpose of the Study:

• To investigate the direct effects of ammonium on individual excitatory amino acid transporter (EAAT) isoforms.
• To elucidate the mechanisms behind ammonium's influence on glutamate transport.
• To explain the disparate findings regarding ammonium's impact on glial glutamate uptake.

Main Methods:

• Expressed EAAT1, EAAT2, and EAAT3 in Xenopus oocytes.
• Measured ammonium's effects on [(3)H]glutamate uptake and glutamate transport currents.
• Investigated the role of extracellular pH and intracellular pH changes.

Main Results:

• Ammonium significantly increased EAAT1- and EAAT3-mediated glutamate uptake and currents, but not EAAT2.
• Ammonium enhanced EAAT3 transport capacity without altering glutamate or Na+ affinity.
• The stimulatory effect of ammonium on EAAT3 was pH-dependent, suggesting mediation by ammonia (NH(3)).
• Intracellular acidification, not alkalinization, occurred, and ammonium also stimulated pH-independent transport.

Conclusions:

• Ammonium directly stimulates EAAT1 and EAAT3 function, likely via ammonia (NH(3)).
• The observed effects are not due to intracellular alkalinization or altered ion concentrations.
• Isoform-specific effects of ammonium on EAATs, coupled with cell-specific EAAT expression, likely explain conflicting reports on ammonium's impact on brain glutamate uptake.