Acute Effect of Ammonia on Branched‐Chain Amino Acid Oxidation and Incorporation into Proteins in Astrocytes and in Neurons in Primary Cultures

14 CO 2 production and incorporation of label into proteins from the labeled branched‐chain amino acids, leucine, valine, and isoleucine, were determined in primary cultures of neurons and of undifferentiated and differentiated astrocytes from mouse cerebral cortex in the absence and presence of 3 mM ammonium chloride. Production of 14 CO 2 from [I‐ 14 C]leucine and [I‐ 14 C]valine was larger than 14 CO 3 production from [U‐ 14 C]leucine and [U‐ 14 C]valine in both astrocytes and neurons. In most cases more 14 CO 2 was produced in astrocytes than in neurons. Incorporation of labeled branched‐chain amino acids into proteins varied with the cell type and with the amino acid. Addition of 3 mM ammonium chloride greatly suppressed 14 CO 2 production from [I‐ 14 C]‐labeIed branched chain amino acids but had little effect on 14 CO 2 production from [U‐ 14 C]‐labeled branched‐chain amino acids in astrocytes. Ammoniumion, at this concentration, suppressed the incorporation of label from all three branched‐chain amino acids into proteins of astrocytes. In contrast, ammonium ion had very little effect on the metabolism (oxidation and incorporation into proteins) of these amino acids in neurons. The possible implications of these findings are discussed, especially regarding whether they signify variations in metabolic fluxes and/or in magnitudes of precursor pools.