Glucagon

The Role of Glucagon in Glycemic Variability in Type 1 Diabetes: A Narrative Review

Type 1 diabetes mellitus (T1DM) is a progressive disease as a result of the severe destruction of islet β-cell function, which leads to high glucose variability in patients. However, α-cell function is also compromised in patients with T1DM, characterized by aberrant fasting and postprandial glucagon secretion. According to recent studies, this aberrant glucagon secretion plays an increasing role in hyperglycemia, insulin-induced hypoglycemia and exercise-associated hypoglycemia in patients with T1DM. With application of continuous glucose monitoring system, dozens of metrics enable the assessment of glycemic variability, which is an integral component of glycemic control for patients with T1DM. There is growing evidences to illustrate the contribution of glucagon secretion to the glycemic variability in patients with T1DM, which may promote the development of new treatment strategies aiming to mitigate glycemic variability associated with aberrant glucagon secretion.

Impacts of pre- and postnatal nutrition on glucagon regulation and hepatic signalling in sheep

To evaluate the long-term impacts of early-life nutritional manipulations on glucagon secretion and hepatic signalling, thirty-six twin-pregnant ewes during their last trimester were exposed to NORM (fulfilling 100% of daily energy/protein requirements), HIGH (fulfilling 150/110% of daily energy/protein requirements) or LOW (50% of NORM) diets. Twin lambs were assigned after birth to a moderate (CONV) or high-carbohydrate high-fat (HCHF) diet until 6 months. Then, responses in plasma glucagon concentrations and glucagon ratios relative to previously reported values for insulin, glucose and lactate were determined after intravenous bolus injections of glucose or propionate (fed and 2-day fasting state). Hepatic mRNA expressions of glucagon receptor (GCGR), glucose-6-phosphatase (G6PC), phosphoenolpyruvate carboxykinase (PEPCK) and fructose 1,6-biphosphatase (FBP) were also determined in a sub group of autopsied lambs. Expression of GCGR and all three enzymes were supressed by prenatal LOW compared to NORM (except PEPCK) and HIGH (except FBP) nutrition. The postnatal HCHF diet reduced plasma glucagon responses to propionate and hepatic mRNA expression of all genes. In response to propionate, insulin/glucagon ratio was decreased (fasted state), but lactate/glucagon and glucose/glucagon increased in HCHF compared to CONV lambs. In conclusion, prenatal undernutrition and postnatal overnutrition had similar long-term implications and reduced hepatic glucagon signalling. Glucagon secretory responses to propionate were, however, not related to the prenatal nutrition history, but negatively affected by the postnatal obesogenic diet. The pancreatic α-cell compared to β-cells may thus be less sensitive towards late gestation malnutrition, whereas hepatic glucagon signalling appears to be a target of prenatal programming.

Glucotoxicity and α cell dysfunction: involvement of the PI3K/Akt pathway in glucose-induced insulin resistance in rat islets and clonal αTC1-6 cells

AIM/HYPOTHESIS

The objective of this study was to assess how long-term exposure to high glucose affects the α cell function and whether the increased glucagon secretion is mediated via insulin resistance.

MATERIALS AND METHODS

We established a β cell-depleted rat model to obtain pure primary α cells. Furthermore, isolated rat islets and TC1-6 cells (a clonal α cell line) were exposed to high glucose (25 or 30 mmol/L) and low glucose (5.5 mmol/L) for up to 5 days to evaluate the influence of chronic glucose toxicity on glucagon secretion and glucagon gene expression. Moreover, we added insulin and/or Wortmannin to examine if the inhibitory effect of insulin on glucagon secretion was impaired by high glucose via the phosphatidylinositol 3 kinase/PKB protein kinase B pathway.

RESULTS

Both glucagon secretion and glucagon gene expression were increased in response to 5 days exposure to high glucose. While a moderate insulin concentration slightly inhibits glucagon secretion from rat islets and α TC1-6 cells at high glucose, a pronounced increase in glucagon secretion was observed at low glucose. We found that the insulin-mediated activity of the phosphatidylinositol 3 kinase/PKB protein kinase B pathway in the α cell was markedly impaired by chronic exposure to high glucose.

CONCLUSION

The hypersecretion of glucagon induced by glucotoxicity may be secondary to insulin resistance of the α cell induced by impaired activity of the insulin signaling pathway.

Responses of plasma adenosine 3',5'-monophosphate, blood glucose and plasma insulin to glucagon in humans

The effect of glucagon on plasma cyclic AMP (cAMP), insulin and blood glucose was examined in normal adult subjects. After an i.v. injection of glucagon there was a rapid, dose-dependent increase of plasma cAMP as well as insulin and blood glucose. Multiple injection of glucagon to the same subject with 60 min intervals gave almost identical responses of plasma cAMP and blood glucose, whereas the insulin response tended to decrease with time. Dose-dependent increases of plasma cAMP, insulin and blood glucose were also seen during a continuous i.v. infusion of glucagon. With the lowest doses of glucagon the blood glucose and plasma insulin concentrations were increased without any change of plasma cAMP. Plasma cAMP, insulin and blood glucose declined prior to the termination of glucagon infusion. During an endogenous hyperglucagonaemia, induced by alanine injection, there was no discernible change of plasma cAMP. We conclude that the early events of glucagon action may be studied in vivo by monitoring plasma cAMP. However, variations of plasma glucagon within the physiological range are not accompanied by measurable changes of cAMP in the peripheral circulation.