Effects of hormone and glucose administration on hepatic glucose and glycogen metabolism in vivo. A 13C NMR study

Effects of peripheral venous injection of glucagon and insulin on 1-13C]glucose incorporation into hepatic glycogen of rats were studied by 13C NMR in vivo. Each animal was given a continuous somatostatin infusion and a 100-mg intravenous injection of 1-13C] glucose in NMR experiments or unlabeled glucose in parallel experiments for determination of serum glucose. Insulin administration caused serum glucose to fall below basal levels and accelerated the loss of hepatic 1-13C]glucose; these effects were counteracted by the addition of glucagon. Glucagon administration alone did not affect serum glucose or hepatic 1-13C] glucose but caused the loss of 1-13C]glucose from glycogen and inhibited 1-13C]glucose incorporation into glycogen. Insulin did not alter 1-13C]glucose incorporation into glycogen when given alone or in combination with glucagon. The data are consistent with a model in which liver glycogen synthesis increases linearly with hepatic glucose concentration above a threshold glucose concentration. Insulin did not alter the rate constant or the threshold for synthesis.