A comparison of 13C nuclear magnetic resonance and 14C tracer studies of hepatic metabolism.

The gluconeogenic pathway from 13C-labeled substrates, each of which contained the 14C-labeled counterpart at a tracer level, has been followed in isolated rat liver cells and in isolated perfused mouse liver. The gluconeogenic flux from glycerol, the synthesis of glycogen, the stimulation of glycogenolysis by glucagon, the recycling of triacylglycerol, and an increase in pentose cycle activity under the influence of phenazine methosulfate were all observed directly in the 13C NMR spectra of perfused liver or isolated hepatocytes. The relative concentrations of 13C label at specific carbons measured by the NMR spectra under these conditions agreed closely with 14C isotopic distributions measured in extracts of the same doubly labeled samples for specific activities of greater than or equal to 3%. The label distributions measured by both methods were the same to within the experimental errors, which ranged from +/- 2% to +/- 7% in these experiments.