Gluconeogenesis in the kidney in vivo in fed rats. Circadian change and substrate specificity

The gluconeogenesis in the kidney in vivo in fed rats was studied by measuring the rate of synthesis of 14C-labeled blood glucose derived from 14C-labeled substrates injected into functionally hepatectomized rats. Among the substrates for the gluconeogenesis in the kidney examined, the most effective substrates were lactate, glycerol, pyruvate and glutamine, whose contributions to the renal gluconeogenesis were 54.0%, 26.5%, 4.1%, and 5.5% at 2:00 a.m., when the activity of phosphoenolpyruvate carboxykinase [EC 4.1.1.32] was highest, and 50.0%, 31.0%, 5.3%, and 3.2% at 2:00 pm., when it was lowest. These findings showed that there was no significant daily change in the total renal gluconeogenesis. Based on these data, we propose as a working hypothesis that a major function of the renal gluconeogenesis, utilizing lactate, glycerol, pyruvate, and glutamine as preferred substrates, is to meet the constant minimum requirement for blood glucose in the brain.