Typical NMR spectra of perfused and isolated liver of rat fed ad libitum. 3A: 13C NMR spectrum. An external silicone reference gives a resonance at 0 ppm. Peak assignments: (a and h) fatty acids chains; (b) C-1 glycogen; (c) C-1α and C-1β glucose (mainly exogenous glucose of the perfusate); (d) glucose and glycogen (C-3β, C-5β glucose, glycogen; C-2 glucose; C-3α glucose; C-2, C-5α glucose, C-5 glycogen; C-4αβ glucose, glycogen); (e) C-6 glucose, glycogen; (f) choline; (g) ethanolamine. The chemical shift scale δ is given in parts per million (ppm) according to: chemical shift (Hz) = δ(ppm) × A(MHz), A being the frequency of the spectrometer. The unit ppm is used owing to the order value (10-6) of a constant characterizing the chemical nature of the nucleus. This scale allows easy comparison between spectra obtained in spectrometers operating at different magnetic fields. 3B. 31P NMR spectrum. Major resonances are assigned to (a) phosphomonoesters, (b) phosphocholine, (c) intracellular inorganic phosphate, (d) glycerol-3- phosphorylcholine and glycerol-3-phosphorylethanolamine, (e) nucleoside-'-triphosphates (γNTP) and diphosphates (βNDP), (f) α-NTP and β-NDP, (g, h) nicotinamide adenine dinucleotide and uridine-'-diphosphoglucose, (i) β-NTP.