A mathematical model of compartmentalized neurotransmitter metabolism in the human brain

After administration of enriched [1-13C]glucose, the rate of 13C label incorporation into glutamate C4, C3, and C2, glutamine C4, C3, and C2, and aspartate C2 and C3 was simultaneously measured in six normal subjects by 13C NMR at 4 Tesla in 45-ml volumes encompassing the visual cortex. The resulting eight time courses were simultaneously fitted to a mathematical model. The rate of (neuronal) tricarboxylic acid cycle flux (VPDH), 0.57 ± 0.06 μmol·g-1·min-1, was comparable to the exchange rate between (mitochondrial) 2-oxoglutarate and (cytosolic) glutamate (Vx, 0.57 ± 0.19 μmol·g-1·min-1), which may reflect to a large extent malate-aspartate shuttle activity. At rest, oxidative glucose consumption [CMRGlc(ox)] was 0.41 ± 0.03 μmol·g-1·min-1, and (glial) pyruvate carboxylation (VPC) was 0.09 ± 0.02 μmol·g-1·min-1. The flux through glutamine synthetase (Vsyn) was 0.26 ± 0.06 μmol·g-1·min-1. A fraction of Vsyn was attributed to be from (neuronal) glutamate, and the corresponding rate of apparent glutamatergic neurotransmission (VNT) was 0.17 ± 0.05 μmol·g-1·min-1. The ratio [VNT/CMRGlc(ox)] was 0.41 ± 0.14 and thus clearly different from a 1:1 stoichiometry, consistent with a significant fraction (∼90%) of ATP generated in astrocytes being oxidative. The study underlines the importance of assumptions made in modeling 13C labeling data in brain.