Fig. 3.

Carbon-2 and carbon-5 regions of ¹³C NMR spectra of MAG derived from natural abundance glucose (A), medium glucose from myotube culture (B, protocol 3), medium glucose from skeletal muscle tissue incubation (C, protocol 4), and glycogen from skeletal muscle tissue incubation (D, protocol 4). Protocol 3 is myotube culture using D-PBS containing 20 mM [U-¹³C₃]lactate, 5% ²H₂O, 2% FBS, 5 mM glucose, and 10 mM glutamine. Protocol 4 is muscle tissue incubation using 0.1% BSA, 10 mM glucose, 10 mM glutamine, 20 mM lactate (20% [U-¹³C₃]lactate), and 5% ²H₂O. All of the doublets are natural abundance, and all quartets are excess enrichment derived from [U-¹³C₃]lactate. The ¹³C excess enrichment of quartet in skeletal muscle glycogen (D) is much higher compared with the rest (see Table 2). Insets: D12, doublet from coupling of carbon-1 with carbon-2; D23, doublet from coupling of carbon-2 with carbon-3; Q, doublet of doublets, or quartet, arising from coupling of carbon-2 with both carbons-1 and -3, or from coupling of carbon-5 with both carbons-4 and -6; D45, doublet from coupling of carbon-4 with carbon-5; D56, doublet from coupling of carbon-5 with carbon-6; S, singlet.