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.