Regulation of blood glucose concentration is essential for life, and insulin and glucagon act antagonistically. Their relationship to diabetes is also important.
Basic Knowledge
Normal blood glucose level: Fasting 70〜100 mg/dL.
When blood glucose rises:
Pancreatic β cells of the islets of Langerhans → increased secretion of insulin
・Moves glucose transporters (GLUT4) in muscle and fat cells to the cell membrane and promotes glucose uptake.
・In the liver, synthesizes glucose → glycogen (promotes glycogen synthesis).
・Inhibits glycogen breakdown and gluconeogenesis. → Decrease in blood glucose
When blood glucose falls:
Pancreatic α cells of the islets of Langerhans → increased secretion of glucagon
・In the liver, breaks down glycogen → glucose (promotes glycogen breakdown).
・Promotes gluconeogenesis (amino acids and lactate → glucose). → Increase in blood glucose
Adrenal medulla → adrenaline (raises blood glucose during emergencies).
Adrenal cortex → glucocorticoids (promote gluconeogenesis and maintain blood glucose over the long term).
Feedback regulation: Equilibrium is maintained through negative feedback: increased blood glucose → insulin → decreased blood glucose; decreased blood glucose → glucagon → increased blood glucose.
Answer: When blood glucose rises after a meal, the β cells of the pancreatic islets of Langerhans secrete insulin. In muscle and fat cells, insulin moves GLUT4 transporters to the cell membrane and promotes glucose uptake. In the liver, it activates glycogen synthase and stores glucose as glycogen. It also inhibits glycogen breakdown and gluconeogenesis. As a result, blood glucose falls to the normal range. The clinical significance of insulin being the only blood-glucose-lowering hormone is that when insulin action is impaired (as in type I diabetes or severe type II diabetes), there is no substitute, making blood-glucose control difficult; insulin injections therefore form the core of treatment.
- Insulin = secreted by β cells; the only hormone that lowers blood glucose
- Insulin action = GLUT4 recruitment and promotion of glycogen synthesis
- Glucagon = secreted by α cells; glycogen breakdown → increased blood glucose
- Adrenaline = adrenal medulla; raises blood glucose during emergencies
- Glucocorticoids = adrenal cortex; promote gluconeogenesis and maintain blood glucose over the long term
- Negative feedback = central to maintaining blood-glucose homeostasis
Practice
- Explain how glucagon acts on the liver to raise blood glucose, including cAMP signal transduction.
- Explain the metabolic abnormalities that occur when insulin action is lost in diabetes (type I and type II).
- Discuss why adrenaline and glucocorticoids are mobilized during hypoglycemia, based on their organs of secretion and differences in duration of action.