Influence of diet, cortisol and insulin on the activity of pyruvate carboxylase and phosphoenolpyruvate carboxykinase in the rat liver.

The influence of nutritional factors, such as fasting (24, 48 and 72 hours), protein free and high protein diets, and endocrine factors (cortisol and insulin) on the activity of hepatic phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC) in adult rats was studied. The circadian rhythm in the activity of these enzymes was studied at two protein levels: 15% and 50%. Under all conditions which favored gluconeogenesis the activity of PEPCK increased. Cortisol did not further increase the activity of PEPCK in rats fed a high protein diet. Regardless of the protein content of the diet, insulin did not reduce PEPCK activity. Dietary protein level had no influence on the usual rhythm of activity of PEPCK. On the other hand, the high protein diet increased it threefold. Under all experimental conditions we observed no changes in PC activity. This is probably due to the fact that PC is a constitutive enzyme and therefore is not adaptative.