Glutamate decreases pyruvate carboxylase activity and spares glucose as energy substrate in cultured cerebellar astrocytes

The effects of glutamate on [U-C-13]glucose metabolism were studied in cerebellar astrocytes using C-13 magnetic resonance spectroscopy. Labeled glutamate, glutamine, aspartate, lactate, and alanine were observed both in the cell extracts and in media, and, additionally, labeled glycogen was detected in the cell extracts. However, only labeled lactate and alanine were quantifiable in the medium in addition to [U-C-13]glucose. In the presence of unlabeled glutamate, the amount of [U-C-13]glucose removed from the medium was decreased, indicating that glutamate might spare glucose as an energy substrate and thus decrease the uptake of glucose. Labeled glycogen, [4,5-C-13]glutamate, [3,4,5-C-13]glutamate, [3,4-C-13]aspartate, and [U-C-13]alanine were increased in the presence of glutamate. However, the increase in the amount of [3,4,5-C-13]glutamate from the second turn in the tricarboxylic acid (TCA) cycle was less pronounced than that of [4,5-C-13]glutamate from the first turn in the TCA cycle. This indicates the dilution of label, probably resulting from the synthesis of unlabeled oxaloacetate from glutamate in the TCA cycle. Furthermore, exogenous glutamate had an inhibiting effect on pyruvate carboxylation, presumably by formation of oxaloacetate from 2-oxoglutarate derived from glutamate. It could be shown that glucose is a better substrate for energy production than glutamate; it is, however, less efficient in labeling amino acids than glutamate in cerebellar astrocytes. (C) 2001 Wiley-Liss, Inc.