Exogenous glutamate concentration regulates the metabolic fate of glutamate in astrocytes

The metabolic fate of glutamate in astrocytes has been controversial since several studies reported > 80% of glutamate was metabolized to glutamine; however, other studies have shown that half of the glutamate was metabolized via the tricarboxylic acid (TCA) cycle and half converted to glutamine. Studies were initiated to determine the metabolic fate of increasing concentrations of [U-13C] glutamate in primary cultures of cerebral cortical astrocytes from rat brain. When astrocytes from rat brain were incubated with 0.1 mM [U-13C] glutamate 85% of the 13C metabolized was converted to glutamine. The formation of [1,2,3-13C3] glutamate demonstrated metabolism of the labeled glutamate via the TCA cycle. When astrocytes were incubated with 0.2-0.5 mM glutamate, 13C from glutamate was also incorporated into intracellular aspartate and into lactate that was released into the media. The amount of [13C] lactate was essentially unchanged within the range of 0.2-0.5 mM glutamate, whereas the amount of [13C] aspartate continued to increase in parallel with the increase in glutamate concentration. The amount of glutamate metabolized via the TCA cycle progressively increased from 15.3 to 42.7% as the extracellular glutamate concentration increased from 0.1 to 0.5 mM, suggesting that the concentration of glutamate is a major factor determining the metabolic fate of glutamate in astrocytes. Previous studies using glutamate concentrations from 0.01 to 0.5 mM and astrocytes from both rat and mouse brain are consistent with these findings.