Fiche de révision : Hormonal Regulation of Blood Glucose

📋 Course Outline

1. Pancreatic islets and hormone-producing cells
2. Insulin structure and proinsulin processing
3. Insulin secretion regulation and biphasic release
4. Insulin actions on carbohydrate, protein and fat
5. Glucagon secretion and regulation
6. Glucose transporters and insulin-modulated uptake
7. Blood glucose homeostasis with insulin and glucagon

📖 1. Pancreatic islets and hormone-producing cells

🔑 Key Concepts & Definitions

• Islets of Langerhans : Islets of Langerhans are small pancreatic cell clusters that perform endocrine hormone secretion.
• β-cells : β-cells are pancreatic endocrine cells that produce and secrete insulin.
• α-cells : α-cells are pancreatic endocrine cells that produce and secrete glucagon.
• δ-cells : δ-cells are pancreatic endocrine cells that produce and secrete somatostatin.
• PP cells : PP cells are pancreatic endocrine cells that produce and secrete pancreatic polypeptide.

📝 Essential Points

• The islets of Langerhans make up about 2% of the pancreas.
• The pancreas has combined exocrine and endocrine functions.
• Capillaries in the islets deliver secreted hormones to the portal vein.
• Insulin is produced by β-cells and glucagon by α-cells within the islets.
• Somatostatin is produced by δ-cells and pancreatic polypeptide by PP cells.
• Exocrine function of the pancreas includes secretion of pancreatic juice.

💡 Memory Hook

Islets = Langerhans “L” cells: β insulin, α glucagon, δ somatostatin, PP polypeptide.

📖 2. Insulin structure and proinsulin processing

🔑 Key Concepts & Definitions

• Insulin : Insulin is an anabolic peptide hormone that promotes storage and lowers blood glucose.
• Proinsulin : Proinsulin is the inactive peptide precursor form of insulin produced in the pancreas.
• C-peptide : C-peptide is the connecting inactive segment within proinsulin that is removed during processing.
• Amino-terminal β-chain : The amino-terminal β-chain is one domain of proinsulin that becomes part of mature insulin.
• Carboxy-terminal α-chain : The carboxy-terminal α-chain is one domain of proinsulin that becomes part of mature insulin.

📝 Essential Points

• Insulin is first produced as a prohormone typical of peptide hormones in the pancreas.
• Proinsulin contains three domains: amino-terminal β-chain, carboxy-terminal α-chain, and inactive C-peptide.
• Mature insulin’s principal action involves moving glucose into certain tissue cells.
• Insulin is described as the body’s most important anabolic signal.
• Insulin processing is tied to conversion from proinsulin to active hormone.
• Insulin’s synthesis is linked to its role in glucose transport into target cells.

💡 Memory Hook

Proinsulin = α + β + C (C is the inactive connector removed to make active insulin).

📖 3. Insulin secretion regulation and biphasic release

🔑 Key Concepts & Definitions

• Insulin secretion : Insulin secretion is the release of insulin from pancreatic β-cells into extracellular fluid and then blood.
• Voltage-gated calcium channels : Voltage-gated calcium channels are membrane channels that open to allow Ca2+ influx during stimulation.
• GLUT : GLUTs are glucose transport proteins that move glucose across cell membranes via facilitated diffusion.
• GIP : GIP (gastric inhibitory peptide) is an intestinal hormone that can stimulate insulin secretion.
• Biphasic insulin release : Biphasic insulin release is the two-phase insulin response to glucose involving pre-formed release then new synthesis.

📝 Essential Points

• Insulin plasma half-life is 5–8 minutes.
• Insulin is metabolised in the kidney and liver.
• Insulin secretion is stimulated mainly by elevated plasma glucose and amino acid levels.
• Voltage-gated calcium channels cause Ca2+ influx leading to insulin secretion by exocytosis.
• Glucose-stimulated insulin secretion shows a biphasic response.
• First phase releases pre-formed insulin over about 5–15 minutes after glucose elevation; second phase is prolonged due to new insulin synthesis.

💡 Memory Hook

Biphasic = “Quick dump then slow build”: pre-formed (5–15 min) then new insulin.

📖 4. Insulin actions on carbohydrate, protein and fat

🔑 Key Concepts & Definitions

• Anabolic signal : An anabolic signal is a hormone effect that promotes building and storage of nutrients rather than breakdown.
• Glycogen synthesis : Glycogen synthesis is the pathway that converts glucose into storage glycogen.
• Gluconeogenesis : Gluconeogenesis is the production of new glucose, including in the liver, from non-carbohydrate sources.
• Triglycerides : Triglycerides are fat storage molecules formed from glycerol and fatty acids.
• Hepatocytes, myocytes and adipocytes : Hepatocytes, myocytes and adipocytes are major insulin target cells for nutrient storage and uptake.

📝 Essential Points

• Insulin lowers blood glucose, amino acids, and fatty acids.
• Insulin promotes conversion of glucose into storage forms in hepatocytes, myocytes, and adipocytes.
• Insulin actions occur through effects on membrane transport that increase cellular glucose uptake and glycogen synthesis.
• Insulin increases cellular amino acid uptake and protein synthesis.
• Insulin enhances glycogen synthesis and inhibits glycogen degradation.
• Insulin inhibits gluconeogenesis by reducing the enzymes required and by increasing amino acid uptake needed for hepatic gluconeogenesis.

💡 Memory Hook

Insulin = “Store & stop”: stores carbs (glycogen), builds proteins, and stops fat breakdown while promoting triglyceride formation.

📖 5. Glucagon secretion and regulation

🔑 Key Concepts & Definitions

• Glucagon : Glucagon is a single-chain peptide hormone that raises blood glucose by acting mainly on the liver.
• α-cells : α-cells are pancreatic endocrine cells that produce and secrete glucagon.
• Hepatic glycogen : Hepatic glycogen is stored glucose in the liver that can be broken down to release glucose.
• Gluconeogenesis : Gluconeogenesis is the liver pathway that generates glucose from non-carbohydrate substrates.
• Glucagon regulation : Glucagon regulation is the control of glucagon release in response to changes in plasma glucose and amino acids.

📝 Essential Points

• Glucagon is produced by α-cells and first synthesized as a precursor molecule.
• Glucagon is a single-chain peptide hormone.
• The primary target organ of glucagon is the liver.
• Glucagon mobilizes glucose from hepatic glycogen and stimulates gluconeogenesis.
• Glucagon secretion is stimulated by a decrease in plasma glucose, unlike insulin.
• Glucagon is also stimulated by increased plasma amino acids and is metabolised in liver and kidney with a half-life of 5–6 minutes.

💡 Memory Hook

Glucagon = “Low sugar signal”: low glucose triggers it; high amino acids also boost it.

📖 6. Glucose transporters and insulin-modulated uptake

🔑 Key Concepts & Definitions

• Glucose transporters (GLUTs) : Glucose transporters are carrier proteins that allow glucose entry into cells by facilitated diffusion.
• Facilitated diffusion : Facilitated diffusion is transport down a concentration gradient using a carrier protein rather than ATP.
• GLUT-4 : GLUT-4 is a glucose transporter whose expression is increased by insulin.
• Insulin up-regulates GLUT-4 expression : Insulin up-regulates GLUT-4 expression, increasing the capacity for glucose uptake in insulin-responsive tissues.

📝 Essential Points

• Glucose is taken up by cells using carrier proteins called glucose transporters (GLUTs).
• GLUT-mediated glucose uptake occurs via facilitated diffusion down concentration gradients.
• Insulin up-regulates GLUT-4 expression.
• The source flags “exceptions?” regarding GLUT regulation, indicating not all GLUT behavior is uniform.
• GLUTs are the mechanism linking insulin to increased cellular glucose uptake.
• Insulin-modulated uptake depends on transporter recruitment/expression rather than direct glucose breakdown.

💡 Memory Hook

Insulin turns up GLUT-4: more transporters = more glucose entry.

📖 7. Blood glucose homeostasis with insulin and glucagon

🔑 Key Concepts & Definitions

• Homeostasis : Homeostasis is the maintenance of blood glucose around a set point through coordinated hormone actions.
• Set point : The set point is the target blood glucose level that determines whether insulin or glucagon signals dominate.
• Postabsorptive state : The postabsorptive state refers to the period after eating when the body relies on stored fuels and regulated glucose output.
• Insulin-glucagon balance : Insulin-glucagon balance is the coordinated switching between insulin release and glucagon release as glucose changes.

📝 Essential Points

• When blood glucose rises to the set point, the stimulus for glucagon release diminishes.
• When blood glucose declines to the set point, the stimulus for insulin release diminishes.
• Rising blood glucose stimulates insulin release from β-cells into the blood.
• Falling blood glucose leads to glucagon release from α-cells into the blood.
• In response to insulin, liver takes up glucose and stores it as glycogen.
• In response to glucagon, liver breaks down glycogen and releases glucose and also supports gluconeogenesis.

💡 Memory Hook

High glucose → insulin on, glucagon off; low glucose → glucagon on, insulin off.

📊 Synthesis Tables

Insulin vs glucagon regulation

⚠️ Common Pitfalls & Confusions

1. Mixing up cell sources: β-cells secrete insulin while α-cells secrete glucagon.
2. Confusing triggers: insulin is stimulated by high glucose, whereas glucagon is stimulated by low glucose.
3. Thinking insulin and glucagon have the same direction of effect on blood glucose; insulin lowers it and glucagon raises it.
4. Forgetting biphasic timing: first phase is pre-formed insulin (about 5–15 min) and second phase is new insulin synthesis.
5. Assuming insulin acts only on carbohydrates; it also affects amino acids/protein synthesis and triglyceride/fat storage pathways.

✅ Exam Checklist

1. Identify which pancreatic islet cell type produces insulin, glucagon, somatostatin, and pancreatic polypeptide.
2. State the proinsulin domains (α-chain, β-chain, C-peptide) and the idea that proinsulin is an inactive precursor.
3. Explain how elevated glucose and amino acids stimulate insulin secretion from β-cells.
4. Describe the mechanism linking voltage-gated calcium channels to insulin exocytosis.
5. Give the two phases of glucose-stimulated insulin release and their approximate timing for the first phase.
6. List insulin’s major metabolic effects on blood glucose, amino acids, and fatty acids.
7. Connect insulin to glycogen synthesis and gluconeogenesis inhibition in the liver.
8. Explain insulin’s effects on amino acid uptake/protein synthesis and on triglyceride formation/stored fat.
9. Describe glucagon production in α-cells and its single-chain peptide nature.
10. State glucagon’s main liver actions (glycogen mobilization and gluconeogenesis) and its glucose/amino-acid triggers.
11. Recall glucagon half-life and where it is metabolised.
12. Define GLUTs and facilitated diffusion and state that insulin up-regulates GLUT-4 expression.
13. Use the set point logic to predict whether insulin or glucagon secretion dominates when blood glucose rises or falls.
14. Trace the homeostasis pathway: what liver does under insulin vs what liver does under glucagon.