The Flux from Glucose to Glutamate in the Rat Brain in vivo as Determined by 1 -Observed,13 C-Edited NMR Spectroscopy

The rate of incorporation of carbon from [1-¹³C]glucose into the [4-CH₂] and [3-CH₂] of cerebral glutamate was measured in the rat brain in vivo by ¹H-observed, ¹³C-edited (POCE) nuclear magnetic resonance (NMR) spectroscopy. Spectra were acquired every 98 s during a 60-min infusion of [1-¹³C]glucose. Complete time courses were obtained from six animals. The measured intensity of the unresolved [4-¹³CH₂] resonances of glutamate and glutamine increased exponentially during the infusion and attained a steady state in ∼20 min with a first-order rate constant of 0.130 ± 0.010 min⁻¹ (t_1/2 = 5.3 ± 0.5 min). The appearance of the [3-¹³CH₂] resonance in the POCE difference spectrum lagged behind that of the [4-¹³CH₂] resonance and had not reached steady state at the end of the 60-min infusion (t_1/2 = 26.6 ± 4.1 min). The increase observed in ¹³C-labeled glutamate represented isotopic enrichment and was not due to a change in the total glutamate concentration. The glucose infusion did not affect the levels of high-energy phosphates or intracellular pH as determined by ³¹P NMR spectroscopy. Since glucose carbon is incorporated into glutamate by rapid exchange with the tricarboxylic acid (TCA) cycle intermediate α-ketoglutarate, the rate of glutamate labeling provided an estimate of TCA cycle flux. We have determined the flux of carbon through the TCA cycle to be ≈1.4 μmol g⁻¹ min⁻¹. These experiments demonstrate the feasibility of measuring metabolic fluxes in vivo using ¹³C-labeled glucose and the technique of ¹H-observed, ¹³C-decoupled NMR spectroscopy.