Figure 2.

Localized ¹³C MR spectra acquired at 4 Tesla from the midline occipito-parietal lobe of a volunteer infused with ¹³C-labeled glucose, lactate or acetate. Upper spectrum: acquired during the last 18 min of a 2-hour [1-¹³C] glucose infusion. Middle spectrum: acquired during the last 18 min of a 2-hour [3-¹³C] lactate infusion ([Lac]_Plasma ~1.5 mmol/L and ¹³C fractional enrichment, ~30%). Lower spectrum: acquired during the last 18 min of a 2-hour [2-¹³C] acetate infusion. Spectra are scaled to NAA C3 to exhibit the differences in ¹³C fractional enrichment reached for glutamate (Glu) and glutamine (Gln) and aspartate. The highest fractional enrichment is attained with glucose as label precursor. For glucose or lactate as precursor the majority of labeling appears in glutamate C4, consistent with the majority of brain metabolism of these substrates occurring in the neurons which contain the majority of glutamate under normal conditions (25). In contrast label from [2-¹³C] acetate is highly enriched in glutamine C4, consistent with the localization of acetate metabolism in the astrocyte TCA cycle as shown in figure 1 resulting in preferential labeling of glutamine C4.

Abbreviations Glu: Glutamate, Gln: Glutamine, Asp: Aspartate, NAA: N-Acetyl aspartate, GABA: Gamma-Aminobutyric Acid.