Interrelationship of insulin and glucagon ratios on carbohydrate metabolism in isolated hepatocytes containing high glycogen

Eupatilin, isolated from Artemisia princeps Pampanini, enhances hepatic glucose metabolism and pancreatic beta-cell function in type 2 diabetic mice

Eupatilin (5,7-dihydroxy-3',4',6-trimethoxyflavone) was isolated from Artemisia princeps to investigate the dose-response effects on blood glucose regulation and pancreatic beta-cell function in type 2 diabetic mice. Db/db mice were divided into control (eupatilin-free, AIN-76 standard diet), low-Eupa (0.005g/100g diet) and high-Eupa (0.02g/100g diet) groups. The supplementation of eupatilin for 6 weeks significantly lowered fasting blood glucose concentration while it increased hepatic glycogen content. In particular, high-Eupa reduced hemoglobin A(1c) and plasma glucagon levels along with a simultaneous increase in plasma insulin and adiponectin levels. The supplementation of eupatilin significantly lowered hepatic glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activities, while it increased glucokinase activity in the liver. The pancreatic insulin concentration was higher in the eupatilin-supplemented groups. Also the pancreatic insulin concentration of eupatilin groups was higher than the control group. These results suggest that eupatilin played the role of an antidiabetic functional component in A. princeps by enhancing hepatic and plasma glucose metabolism as well as by increasing insulin secretion in type 2 diabetic mice.

The possible role of age-related increase in the plasma glucagon/insulin ratio in the enhanced hepatic gluconeogenesis and hyperglycemia in genetically diabetic (C57BL/KsJ-db/db) mice

Genetically diabetic db/db mice and their normoglycemic littermates (+/+ mice) were studied to determine plasma levels of glucose, glucagon and insulin and hepatic gluconeogenic enzyme activities. Plasma glucose levels did not differ significantly between the 5-week-old db/db and +/+ mice, but increased with age in the former until the animals were 16-week-old. Similar age-associated changes were observed in the activities of the gluconeogenic enzymes, glucose-6-phosphatase (G-6-Pase) and fructose-1,6-diphosphatase (F-1,6-DPase). While the plasma levels of insulin and glucagon that peaked at 7 weeks of age did not parallel the hyperglycemia, the plasma glucagon/insulin (G/I) ratio roughly paralleled the hyperglycemia. Analysis of individual values for the db/db mice revealed statistically significant (P < 0.001) correlations between plasma glucose levels and hepatic G-6-Pase (r = 0.78) or F-1,6-DPase (r = 0.74) activity. There were also significant correlations between the G/I ratio and plasma glucose levels (P < 0.001, r = 0.66), hepatic G-6-Pase (P < 0.01, r = 0.48) or F-1,6-DPase (P < 0.01, r = 0.57) activity. It is thus concluded that the relative predominance of glucagon over insulin action plays an important role in the age-associated development of hyperglycemia in db/db mice. Glucagon presumably activates the hepatic gluconeogenic enzymes to enhance hepatic glucose output.

Amino acid and energy requirements for rat hepatocytes in primary culture

The amino acid and energy requirements of rat hepatocytes in suspension and early culture were investigated. Among a number of potential energy substrates tested, pyruvate (20 mM) was found to be most effective in stimulating hepatocytic protein synthesis. Amino acids stimulated protein synthesis both as energy substrates and as protein precursors. An amino acid mixture was designed to provide maximal inhibition of protein degradation as well as maximal stimulation of protein synthesis. In a defined medium containing amino acids at these concentrations, and supplemented with glucocorticoid hormone and insulin, hepatocytes could be maintained--on a collagen substratum--for at least a week without any significant net loss of cells or cellular protein.

Short-term stimulation of net glycogen production by insulin in rat hepatocytes

Isolated liver cells from 24 h starved rats were incubated in Krebs-Ringer buffer containing 4% albumin. In the presence of 10, 20 and 30 mM glucose, addition of insulin stimulated net glycogen production by 52, 39 and 20%, respectively. 2 . 10(-9) M insulin was required for half-maximal stimulation. Increases of glycogen production and of glycogen synthase a activity were observed after 15-30 min of incubation with insulin. The stimulatory effect of insulin was additive to that of lithium. In agreement with the literature, insulin antagonized the inhibitory action of suboptimal doses of glucagon on glycogen deposition whereby a decrease of glucagon-elevated cyclic AMP levels was observed. In addition, we found that insulin also decreased the basal cyclic AMP levels in the absence of added glucagon by 22%. It is concluded that physiological concentrations of insulin stimulate net glycogen deposition in hepatocytes from fasted rats; the decrease of basal cyclic AMP levels upon insulin addition may play a role in the mechanism of the hormone action.

Sympathetic control of metabolic and hormonal responses to exercise in rats

The importance of the sympatho-adrenal system for the pancreatic hormonal response to exercise and, furthermore, the role of glucagon and catecholamines for the hepatic glycogen depletion during exercise were studied. Rats were either surgically adrenomedullectomized and chemically sympathectomized with 6-hydroxydopamine or shamtreated. Two weeks later the rats had either rabbit-antiglucagon serum or normal rabbit serum injected. Subsequently the rats either rested or swam with a tail weight for 75 min. Immediately afterwards cardiac blood was drawn and liver and muscle tissue collected. In control rats in spite of an increase in blood glucose concentrati4ns during exercise plasma insulin concentrations were unchanged, while glucagon concentrations increased. In sympathectomized rats, compared to control rats, glucagon concentrations increased less, and insulin concentrations were higher, although glucose concentrations were lower during exercise. Sympathectomy completely abolished the exercise-induced decrease in liver and muscle glycogen concentrations, whereas neither glycogen depletion nor plasma catecholamine concentrations were influenced by the administration of glucagon antibodies. These findings indicate that the sympatho-adrenal system enhances glucagon secretion as well as muscular and hepatic glycogen depletion but inhibits insulin secretion in exercising rats. The increase in glucagon concentrations, however, does not enhance hepatic glycogen depletion at the work load used.

Effect of insulin on glycogen and protein synthesis in monolayer cultures of hepatocytes from normal and alloxan diabetic rats

The effects of insulin on net glycogen synthesis and amino acid incorporation into protein were studied in cultured hepatocytes from adult normal and alloxan diabetic rats. Insulin stimulated glycogen synthesis in monolayer cells throughout a four day culture period and enhanced leucine incorporation into protein more effectively in normal cells with high glycogen levels than in cultured diabetic cells. These differences correlate well with the observed cellular ultrastructures which were maintained much better in the presence of insulin. Restoration of the morphological changes of alloxan diabetic hepatocytes to normal liver cell structures can be observed at any time during the culture period by giving insulin continuously.

The hypoglycaemic action of somatostatin in the anaesthetized dog

The hypoglycaemic action of somatostatin was investigated in fasted anaesthetized dogs. An isotopic turnover technique with radioactive D-[2-3H] glucose tracer was used to measure the rates of hepatic production and peripheral utilization of glucose before, during, and after a 60min infusion of 150 mug somatostatin. Plasma concentrations of insulin, glucagon, growth hormone, cortisol and free fatty acids were also measured. Somatostatin infusion caused a fall of plasma glucose demonstrable within 10 min. After 60 min plasma glucose had fallen to 75% of the pre-infusion concentration. Simultaneoulsy the specific activity of the D-[2-3H] glucose increased and calculation of the rates of production and utilization of glucose over 10 min intervals indicated that the mean hepatic production rate of glucose fell to 59% of the pre-infusion rate while the rate of peripheral utilization of glucose fell to 83% of the pre-infusion rate. These results show that the hypoglycaemic action of intravenous somatostatin depends exclusively upon the inhibition of hepatic glucose production. This effect may have been partly due to a fall of plasma glucagon concentration and occurred despite a fall of plasma insulin to less than 1 mu 1(-1).

The role of insulin and glucagon in the regulation of basal glucose production in the postabsorptive dog

The aim of the present experiments was to determine the role of insulin and glucagon in the regulation of basal glucose production in dogs fasted overnight. A deficiency of either or both pancreatic hormones was achieved by infusin somatostatin (1 mug/kg per min), a potent inhibitor of both insulin and glucagon secretion, alone or in combination with intraportal replacement infusions of either pancreatic hormone. Infusion of somatostatin alone caused the arterial levels of insulin and glucagon to drop rapidly by 72+/-6 and 81+/-8%, respectively. Intraportal infusion of insulin and glucagon at rates of 400 muU/kg per min and 1 ng/kg per min, respectively, resulted in the maintenance of the basal levels of each hormone. Glucose production was measured using tracer (primed constant infusion of [3-3H]glucose) and arteriovenous difference techniques. Isolated glucagon deficiency resulted in a 35+/-5% (P less than 0.05) rapid and sustained decrease in glucose production which was abolished upon restoration of the plasma glucagon level. Isolated insulin deficiency resulted in a 52+/-16% (P less than 0.01) increase in the rate of glucose production which was abolished when the insulin level was restored. Somatostatin had no effect on glucose production when the changes in the pancreatic hormone levels which it normally induces were prevented by simultaneous intraportal infusion of both insulin and glucagon. In conclusion, in the anesthetized dog fasted overnight; (a) basal glucagon is responsible for at least one-third of basal glucose production, (b) basal insulin prevents the increased glucose production which would result from the unrestrained action of glucagon, and (c) somatostatin has no acute effects on glucose turnover other than those it induces through perturbation of pancreatic hormone secretion. This study indicates that the opposing actions of the two pancreatic hormones are important in the regulation of basal glucose production in the postabsorptive state.