Depiction of 13C labeling of cerebral metabolites from labeled glucose and acetate.
(A) Metabolism of [U-13C6]glucose via glutamatergic and GABAergic TCA cycles labels Glu4,5 which is de-carboxylated to GABA1,2 in GABAergic neurons by GAD. Labeling of Gln4,5 occurs from Glu4,5 and GABA1,2 via glutamate-glutamine and GABA-glutamine cycle. (B) As transporters of acetate are present on astroglia only, [2-13C]acetate is selectively transported and metabolized in these cells and labels Gln4 by the combined action of astroglial TCA cycle and glutamine synthetase. Neurotransmitters Glu4 and GABA2 are labeled from Gln4 via glutamate-glutamine and GABA-glutamine substrates cycling between astroglia and neurons. Isotopomers of Glu4, GABA2 and Gln4 derived from [U-13C6]glucose and [2-13C]acetate are different. Open and filled circles represent 12C and 13C atoms, respectively.