Some metabolic effects of ammonia on astrocytes and neurons in primary cultures

Some metabolic effects on primary cultures of neurons or astrocytes were studied following acute or chronic exposure to pathophysiological concentrations (usually 3 mM) of ammonia. Three parameters were investigated: (1) 14CO2 production from 14C-labeled substrates [glucose, pyruvate, branched-chain amino acids (leucine, valine, isoleucine), and glutamate]; (2) interconversion between glutamate and glutamine; and (3) incorporation of label from labeled branched-chain amino acids into proteins. Neither acute nor chronic exposure to ammonia had any effect on 14CO2 production from [U-14C]glucose in astrocytes and neurons, whereas under certain conditions 14CO2 production from [1-14C]pyruvate in astrocytes was inhibited by ammonia. Production of 14CO2 from [1-14C]branched-chain amino acids was inhibited by acute, but stimulated by chronic, exposure to ammonia (3 mM) in astrocytes, with less effect in neurons. Production of 14CO2 from [1-14C]glutamate in both astrocytes and neurons was inhibited by acute exposure to ammonia. In astrocytes, glutamate levels tended to decrease and glutamine levels tended to increase following acute exposure to ammonia; in neurons, both glutamine and glutamate levels decreased. Protein content (per culture dish) increased in astrocytes but not in neurons, after chronic exposure to ammonia, possibly as a result of enhanced protein synthesis and/or by inhibition of protein degradation.