TY - JOUR
T1 - Elevated ammonium levels
T2 - differential acute effects on three glutamate transporter isoforms
AU - Søgaard, Rikke
AU - Novak, Ivana
AU - MacAulay, Nanna
PY - 2012/3
Y1 - 2012/3
N2 - 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.
AB - 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.
KW - Ammonium Chloride
KW - Animals
KW - Biological Transport
KW - Carbon Radioisotopes
KW - Cloning, Molecular
KW - Excitatory Amino Acid Transporter 1
KW - Excitatory Amino Acid Transporter 2
KW - Excitatory Amino Acid Transporter 3
KW - Female
KW - Glutamic Acid
KW - Humans
KW - Hydrogen-Ion Concentration
KW - Methylamines
KW - Oocytes
KW - Patch-Clamp Techniques
KW - Protein Isoforms
KW - Quaternary Ammonium Compounds
KW - Scintillation Counting
KW - Synaptic Transmission
KW - Tritium
KW - Xenopus laevis
U2 - 10.1152/ajpcell.00238.2011
DO - 10.1152/ajpcell.00238.2011
M3 - Journal article
C2 - 22159086
SN - 0363-6143
VL - 302
SP - C880-C891
JO - American Journal of Physiology: Cell Physiology
JF - American Journal of Physiology: Cell Physiology
IS - 6
ER -