Study Notes – Physiology of Insulin and Glucagon

• Insulin and glucagon are critical hormones in metabolic regulation.

• Their physiology extends beyond simple blood glucose control.

• Numerous other hormones influence glucose homeostasis (cortisol, epinephrine, incretins, etc.).

• Modern medicine continues to refine understanding of their complex roles.

Pancreas – Anatomy & Histology

Macroscopic structure:

• Located in upper abdomen, posterior to stomach.

• Divided into head, body, and tail.

• Ducts:

  • Main pancreatic duct (joins common bile duct → ampulla of Vater).

  • Accessory duct in minority of people (minimal clinical relevance).

Microscopic structure:

• Two functional components:

  1. Exocrine (acini) → digestive enzyme secretion into duodenum.

  2. Endocrine (islets of Langerhans):

     • α-cells → glucagon

     • β-cells → insulin + amylin

     • δ-cells → somatostatin

     • γ (PP) cells → pancreatic polypeptide

     • ε-cells → ghrelin

Insulin vs Glucagon – Core Actions

Insulin (fed state):

• Promotes storage and utilization of nutrients.

• ↑ Glycolysis (glucose utilization)

• ↑ Glycogenesis (glycogen storage)

• ↑ Lipogenesis (fat storage)

• ↑ Protein synthesis

• ↑ Glucose uptake in tissues

Glucagon (fasting state):

• Promotes mobilization and generation of fuel.

• ↑ Gluconeogenesis (new glucose synthesis)

• ↑ Glycogenolysis (glycogen breakdown)

• ↑ Lipolysis + β-oxidation (fat breakdown)

• ↑ Proteolysis (protein breakdown)

• ↑ Ketogenesis (ketone body production)

Key point: They act antagonistically to maintain normal blood glucose levels.

Major Target Tissues

1. Liver

   • Insulin: ↑ glycogen, ↑ triglycerides, ↑ fatty acid synthesis.

   • Glucagon: ↑ glycogen breakdown, ↑ gluconeogenesis, ↑ ketogenesis.

2. Skeletal Muscle

   • Insulin: ↑ glucose & amino acid uptake, ↑ protein synthesis, ↑ glycogen.

   • Glucagon: indirect effects (mobilizes substrates).

3. Adipose Tissue

   • Insulin: ↑ fatty acid uptake, ↑ triglyceride storage (lipogenesis).

   • Glucagon: ↑ lipolysis → free fatty acids for fuel.

Additional Hormones in Glucose Homeostasis

Pancreatic hormones:

• Amylin (β-cells): slows gastric emptying, promotes satiety, suppresses glucagon.

• Somatostatin (δ-cells): inhibits insulin & glucagon; slows GI motility.

Gut hormones (Incretins):

• GIP (K-cells, duodenum/jejunum): stimulates insulin secretion.

• GLP-1 (L-cells, ileum/colon): ↑ insulin, ↓ glucagon, slows gastric emptying, ↑ satiety.

• Both inactivated by DPP-4 enzyme → therapeutic target (DPP-4 inhibitors in diabetes).

Counter-regulatory stress hormones:

• Cortisol, Epinephrine, Growth Hormone → oppose insulin.

• Net effects: ↓ insulin secretion, ↑ gluconeogenesis, ↑ lipolysis, ↓ glucose uptake → hyperglycemia.

Insulin Synthesis & Structure

• Insulin = two chains (A & B) linked by disulfide bonds.

• Synthesis steps:

  1. Preproinsulin synthesized (with signal peptide).

  2. Signal peptide removed → Proinsulin.

  3. Disulfide bridges form between A and B chains.

  4. C-peptide (connecting peptide) cleaved → active Insulin + C-peptide.

Clinical relevance:

• C-peptide is secreted equimolarly with insulin.

• Used to distinguish:

  • Endogenous insulin excess (e.g., insulinoma) → high C-peptide.

  • Exogenous insulin administration → low C-peptide.

Temporal Relationship of Glucose Sources

1. Fed state (0–6 hrs after meal):

   • Insulin dominates.

   • Glucose primarily from dietary intake.

2. Early fasting (6–24 hrs):

   • Glucagon increases.

   • Primary glucose source = glycogenolysis.

3. Prolonged fasting/starvation (>24 hrs):

   • Glycogen stores depleted.

   • Gluconeogenesis becomes primary glucose source (amino acids, glycerol, lactate).

   • Ketone bodies & fatty acids provide alternative fuel.

Key Clinical Takeaways

• Insulin and glucagon work in dynamic opposition to regulate fuel availability.

• Regulation involves multiple additional hormones (incretins, stress hormones, pancreatic peptides).

• C-peptide is an important clinical marker for endogenous vs exogenous insulin.

• In fasting → glycogenolysis, then gluconeogenesis + ketogenesis maintain energy supply.