Lecture on Insulin and Glucagon Physiology

Introduction

• Purpose: Discuss the normal physiology of insulin and glucagon.
• Dual focus: Standalone lesson or foundation for a series on diabetes.
• Outcomes: Understanding the pancreas structure, metabolic actions, regulation of insulin and glucagon, and synthesis of insulin.

Key Myths

1. Insulin and glucagon only regulate blood sugar.
2. They are the only significant hormones in glucose homeostasis.
3. Modern medicine fully understands their actions and regulation.

Pancreas Structure

• Location: Upper abdominal cavity, posterior to the stomach.
• Regions: Head (near duodenum), body, and tail (towards spleen).
• Pancreatic Ducts: Main pancreatic duct merges with the common bile duct.

Pancreas Microscopic Structure

• Two Tissue Types:
  • Acini: Exocrine function, secretes digestive enzymes.
  • Islets of Langerhans: Endocrine function, secretes hormones like insulin and glucagon.

Islets of Langerhans Cell Types

• Alpha Cells: Secrete glucagon.
• Beta Cells: Secrete insulin and amylin.
• Gamma (PP) Cells: Secrete pancreatic polypeptide.
• Delta Cells: Secrete somatostatin.
• Epsilon Cells: Secrete ghrelin.

Actions of Insulin and Glucagon

• Insulin:
  • Promotes glucose uptake and utilization (glycolysis).
  • Stimulates storage as glycogen (glycogenesis), fat synthesis (lipogenesis), and protein synthesis.
• Glucagon:
  • Stimulates glucose synthesis (gluconeogenesis) and glycogen breakdown (glycogenolysis).
  • Promotes fat and protein breakdown.

Regulation of Insulin and Glucagon

• Balanced State:
  • Insulin predominates when well-fed; glucagon predominates when fasting.
• Major Sites of Action:
  • Liver: Storage of glycogen, conversion to fatty acids/triglycerides.
  • Skeletal Muscle: Glucose and amino acid uptake.
  • Adipose Tissue: Fatty acid uptake, triglyceride synthesis.

Other Hormones

• Pancreatic Hormones:
  • Amylin: Co-secreted with insulin, helps suppress glucagon.
  • Somatostatin: Inhibits both insulin and glucagon.

Gut Hormones

• GIP (Glucose-dependent Insulinotropic Polypeptide): Promotes insulin secretion.
• GLP-1 (Glucagon-like Peptide-1): Stimulates insulin release, inhibits glucagon.

Stress Hormones

• Cortisol, Epinephrine, Growth Hormone: Antagonize insulin actions, promote hyperglycemia.

Insulin Structure and Synthesis

• Composed of two amino acid chains linked by disulfide bridges.
• Processed from preproinsulin to proinsulin to insulin.
• C-peptide serves as a marker for insulin secretion.

Glucose Sources Over Time

• Well-fed State: Dietary glucose is primary.
• Fasting State: Glucagon promotes glycogenolysis.
• Starvation Mode: Gluconeogenesis is primary; fatty acids and ketone bodies are utilized.

Conclusion

• Reviewed insulin and glucagon physiology.
• Encouragement to view related videos on thyroid hormones, calcium-phosphate regulation, and diabetes.