Effect of insulin on incorporation of 14C-labeled pyruvates and bicarbonate into blood glucose of fasted mice.

Abstract

The incorporations of [1-14C]- or [2-14C]pyruvate, [1-14C]aspartate and of NaH14CO3 into blood glucose and liver proteins were studied in fasted or fed mice with or without insulin. Fasting increased the incorporation of NaH14Co3 into blood glucose and liver proteins. The stimulatory effect of fasting on pyruvate carboxylase activity was not modified by insulin under these experimental conditions. In fasted mice, insulin strongly decreased the incorporation of [14C]pyruvate into glucose but was without effect on that of NaH14CO3 when the animals were killed 10 min after the administration of radioactive precursors. When the animals were killed 3 min after injection of [14C]pyruvate, insulin also increased the incorporation of this radioactive precursor into glucose. The disappearance of the injected [14C]glucose from the blood is much more rapid in fasted insulin-treated mice than in fasted untreated mice. This probably is also the case for the neosynthesized glucose, which explains the apparent inhibitory effect of insulin on gluconeogenesis from pyruvate in contrast to its effect on gluconeogenesis from [14C]bicarbonate. The incorporation of [14C]pyruvate into glucose is much more rapid than that of bicarbonate. Apparently this does not indicate an inhibitory action of insulin on gluconeogenesis in 24-h-fasted mice, at least at the pyruvate carboxylation step.