Relative contribution of glycogenolysis and gluconeogenesis to hepatic glucose production in control and diabetic rats. A re-examination in the presence of euglycaemia

Several studies have suggested that, in non-insulin-dependent diabetes mellitus, augmented gluconeogenesis is responsible for increased endogenous glucose production (EGP) and in the end determines fasting hyperglycaemia. However, human and animal studies have been conducted by comparing euglycaemic control subjects to hyperglycaemic diabetic probands. We measured EGP and hepatic gluconeogenesis comparing control and diabetic rats in the fasting state (with diabetic animals in hyperglycaemia), re-examining them in the presence of identical euglycaemia (with diabetic rats made acutely euglycaemic through i.v. phloridzin) or during a hyperinsulinaemic clamp. All rats were infused with [3-H-3]-glucose and [U-C-14]-lactate; the ratio between C-14-uridine-diphosphoglucose (reflecting C-14-glucose 6-phosphate) and 2 . C-14-phosphoenolpyruvate specific activities (both purified by high performance liquid chromatography from liver) measured hepatic gluconeogenesis. In diabetic animals, although overall ECP (similar to 19.5 . mg . kg(-1) min(-1)) remained unaffected by experimental euglycaemia, the contribution of glycogenolysis largely increased (from 5.4 to 11.7 mg . kg(-1) . min(-1), hyper- vs euglycaemia) while gluconeogenesis decreased (from 14.0 to 8.1 mg . kg(-1) . min(-1)); both were responsible for the augmented EGP (control rats, EGP: 12.7 mg . kg(-1) . min(-1); gluconeogenesis: 5.9 mg . kg(-1) . min(-1) glycogenolysis: 6.7 mg . kg(-1) . min(-1)). Finally, during insulin clamp, gluconeogenesis and glycogenolysis were similarly decreased, and both contributed to the hepatic insulin-resistance of diabetic animals. We conclude that, in this model of non-insulin-dependent diabetes, augmented gluconeogenesis is not primarily responsible for fasting hyperglycaemia and hepatic insulin resistance. Finally, failure to accurately match the experimental conditions in which diabetic and control humans or animals are compared affects gluconeogenesis, overestimating its role in determining hyperglycaemia.