CONTROL OF HEPATIC GLUCONEOGENESIS AND GLUCOSE OUTPUT BY GLUCAGON AND INSULIN

ABSTRACT The role of the basal secretion of glucagon and insulin in the regulation of glycogenolysis and gluconeogenesis was studied in the intact, post-absorptive dog. The endogenous secretions of glucagon and insulin were eliminated by continuous infusion of somatostatin. Glucagon and/or insulin were then replaced by infusion of the hormones into the portal vein. (1) When both hormones were replaced simultaneously at rates shown to achieve basal portal and peripheral levels of the hormone(s), total glucose output, gluconeogenesis, and the plasma glucose concentration remained steady at values which matched closely those in controls infused with saline only. (2) When glucagon alone was replaced, thus creating an isolated insulin deficiency, glycogenolysis and gluconeogenesis were stimulated from 30 to 100% within a few minutes. The strong inhibitory effect of basal insulin secretion on these processes in the normal animal was thus made apparent. (3) When insulin alone was replaced, thus creating an isolated glucagon deficiency, glycogenolysis and gluconeogenesis fell by 30 to 50% within a few minutes. The stimulatory effect of basal glucagon secretion in the normal animal was thus revealed. (4) Changes in gluconeogenesis (conversion of 14C-alanine to 14C-glucose) in the time period of the above experiments took place without change in hepatic alanine uptake. Thus, glucagon and insulin appeared to control the efficiency of intrahepatic conversion of alanine to glucose. The control of gluconeogenesis by glucagon and insulin was also studied in isolated rat hepatocytes using dihydroxyacetone (DHA) as substrate. (1) DHA conversion to glucose was about doubled by exposure of cells to glucagon, while formation of lactate and pyruvate was reduced. The decrease in lactate formation accounted in large part for the increase in glucose synthesis. DHA uptake was not altered. These results suggested that glucagon inhibited flux through pyruvate kinase. (2) Glucagon treatment lowered the steady state intracellular concentration of fructosebisphosphate, suggesting that the hormone might also act at the level of fructosebisphosphatase (FbPase). (3) Glucagon brought about inhibition of pyruvate kinase activity in homogenates of hepatocytes incubated in the hormone. Kinetic analysis revealed a 2–3-fold increase in the K0.5 for PEP. Furthermore, purified hepatic pyruvate kinase could be phosphorylated by a cAMP-dependent protein kinase with resulting changes in kinetic properties of the enzyme similar to those brought about by glucagon. This suggested that glucagon inhibited pyruvate kinase in intact cells by a phosphorylation mechanism. (4) A regulatory role for FbPase was suggested by the observation that purified rat hepatic FbPase could be phosphorylated by cyclic AMP dependent protein kinase with an associated increase in enzyme activity. (5) The glucagon induced decrease in hepatocyte fructosebisphosphate level may amplify the inhibition of pyruvate kinase activity by lowering the activator concentration. Thus effects of glucagon on pyruvate kinase and FbPase appeared to be coordinated by alterations in FDP levels. (6) These observations support the view that the regulatory effects of glucagon and insulin on gluconeogenesis can be accounted for, in part, by regulation of substrate cycling between pyruvate and phosphoenolpyruvate.