Figure 1.

¹³C NMR glutamate turnover study. (A) Intramuscular label flow schematic resulting from 2-¹³C acetate administration (V_tca, TCA cycle flux). (B) A spectrum acquired from the rat hindlimb at 135–150 min is shown below. The 2-¹³C glutamate peak appears at 55.5 ppm on the shoulder of the creatine/phosphocreatine peak (54.4 ppm), and the 4-¹³C glutamate peak obscured by a coresonating aliphatic lipid peak appears at 34.4 ppm. Therefore, all spectra were baseline subtracted before peak integration. 3-¹³C glutamate (27.9 ppm) and 2-¹³C acetate (24.2 ppm) were not observed, because they reside in the frequency bandwidth that was partially suppressed because of the aliphatic lipid suppression pulse sequence used. Turnover of the 2- and 4-¹³C glutamate peaks in the hindlimb muscles of control and T₃-treated rats is illustrated in the baseline subtracted spectra shown above. 2-¹³C acetate label incorporates rapidly into 4-¹³C glutamate and more slowly into 2-¹³C glutamate (second turn of the TCA cycle). (C) The label turnover in 2- and 4-¹³C glutamate was slower in the control vs. T₃-treated group as reflected by the representative turnover curves. R² ≥ 0.96 (n = 14) for the best fit curves to the mathematical model.