Differential effect of steady-state hyperinsulinaemia and hyperglycaemia on hepatic glycogenolysis and glycolysis in rats

Publisher Website
Google Scholar
Abstract
The action of glucose and of insulin on hepatic glucose production and metabolism has been studied in fed anaesthetized rats during hyperinsulinaemic clamp combined with various steady state levels of glycaemia (6.8±0.1, 9.3±0.1, 11.8±0.1 mmol/l). Hepatic glucose production was measured using constant infusion of D-[6-3H] glucose. At the end of each clamp the liver was freeze clamped, and enzyme activities and metabolites were measured. Hepatic glucose production was totally suppressed in all the groups receiving insulin. In the group with steady-state normoglycaemia, the suppression of hepatic glucose production was accompanied by a decrease in the levels of glucose-6-phosphate, an increase in those of fructose 2,6-bisphosphate and glycolytic intermediates, but without change in glycogen level or glycogen synthase and phosphorylase. In contrast, in the groups with steady-state hyperglycaemia, phosphorylase a was inactivated, and glycogen synthase activated. Under these conditions, glucose-6-phosphate levels were also decreased and those of fructose 2,6-bisphosphate and glycolytic intermediates were higher than in the group with steady-state normoglycaemia. A slight drop in the level of cAMP was also observed which may contribute, with hyperglycaemia, to the inactivation of phosphorylase. Incorporation of tritiated water into liver glycogen paralleled the activation of glycogen synthase and the accumulation of glycogen. The data indicate that, at normoglycaemia, insulin may suppress hepatic glucose production by channeling glucose-6-phosphate into the glycolytic pathway; at higher levels of glycaemia, a decreased rate of glycogenolysis and an increased rate of glycogen synthesis due to phosphorylase a inactivation and synthase activation may contribute to the decreased level of glucose-6-phosphate, and to a sparing and a net synthesis of glycogen.