Glutamate and glutamine metabolism and compartmentation in astrocytes

Metabolism of glutamate and glutamine in cultured mouse cerebral cortical astrocytes has been investigated using either radioactively labelled (14C) amino acids or 13C-labelled amino acids combined with NMR spectroscopy of cell extracts and lyophilyzed incubation media. Using [U-13C]glutamate it has been shown that in astrocytes exogenously supplied glutamate is primarily (70%) metabolized oxidatively through the tricarboxylic acid (TCA) cycle and to a lesser extent (30%) directly to glutamine. Glutamate metabolized in the TCA cycle is to a large extent recovered as lactate showing that the astrocyte-specific enzyme, malic enzyme is functionally active. Incubation with [U-14C]glutamine led to a higher specific radioactivity in glutamate than in glutamine. It could also be shown that glutamate and glutamine were metabolized differently to aspartate and alanine. These results taken together strongly suggest that glutamate/glutamine metabolism in astrocytes is compartmentalized and a model with multiple cytoplasmic and mitochondrial compartments of these amino acids is proposed.