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Physiological Levels of Ammonia Regulate Glutamine Synthesis from Extracellular Glutamate in Astrocyte Cultures
Author(s) -
Waniewski Robert A.
Publication year - 1992
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1992.tb09292.x
Subject(s) - glutamine , glutamate receptor , ammonia , extracellular , astrocyte , transamination , biochemistry , metabolism , glutamine synthetase , ammonium chloride , deamination , amino acid , chemistry , ammonium , intracellular , biology , endocrinology , enzyme , central nervous system , receptor , organic chemistry
The effect of ammonia on glutamate accumulation and metabolism was examined in astrocyte cultures prepared from neonatal rat cortices. Intact astrocytes were incubated with 70 μ M l ‐[ 14 C(U)]glutamate and varying amounts of ammonium chloride. The media and cells were analyzed separately by HPLC for amino acids and labelled metabolites. Extracellular glutamate was reduced to 8 μ M by 60 min. Removal of glutamate from the extracellular space was not altered by addition of ammonia. The rate of glutamine synthesis was increased from 3.6 to 9.3 nmol/mg of protein/min by addition of 100 μ M ammonia, and intracellular glutamate was reduced from 262 to 86 nmol/mg of protein after 30 min. The metabolism of accumulated glutamate was matched nearly perfectly by the synthesis of glutamine, and both processes were proportional to the amount of added ammonia. The transamination and deamination products of glutamate were minor metabolites that either decreased or remained unchanged with increasing ammonia. Thus, ammonia addition stimulates the conversion of glutamate to glutamine in intact astrocyte cultures. At physiological concentrations of ammonia, glutamine synthesis appears to be limited by the rate of glutamate accumulation and the activity of competing reactions and not by the activity of glutamine synthetase.