In vivo Carbon-13 Nuclear Magnetic Resonance Studies of Heart Metabolism

Guinea pig heart metabolism was studied in vivo by13C NMR at 20.18 MHz. High-quality proton-decoupled13C NMR spectra with excellent signal-to-noise ratios and resolution could be obtained in 6 min. Natural-abundance spectra showed resonances that could be assigned to fatty acids, but glycogen was not seen. During intravenous infusion of D-[1-13C]glucose and insulin, the time course of myocardial glycogen synthesis was followed serially for up to 4 hr. Anoxia resulted in degradation of the labeled glycogen within 6 min and appearance of13C label in lactic acid. Infusion of sodium [2-13C]acetate resulted in incorporation of label into the C-4, C-2, and C-3 positions of glutamate and glutamine, reflecting ``scrambling'' of the label expected from tricarboxylic acid cycle activity. Examination of the31P NMR spectrum of the guinea pig heart in vivo demonstrated no change in the high-energy phosphates during the time periods of the13C NMR experiments. Our studies indicate that13C NMR is a unique nondestructive tool for the study of heart metabolism in vivo.