Pancreatic Hormones and Glucose Metabolism

Pancreatic Islet Hormones - Islet's Endocrine Symphony

Endocrine pancreas: Islets of Langerhans. 📌 Cells: Beta (Insulin), Alpha (Glucagon), Delta (Somatostatin).

• α-cells (Glucagon): ↑ Blood glucose (glycogenolysis, gluconeogenesis).
• β-cells (Insulin, Amylin): ~70% of islet cells. Insulin: ↓ Blood glucose. Amylin: ↓ Glucagon.
  • Proinsulin → Insulin + C-peptide (equimolar).
• δ-cells (Somatostatin): Paracrine inhibition of insulin & glucagon.
• PP-cells (Pancreatic Polypeptide): Regulates pancreatic exocrine/endocrine functions.

⭐ C-peptide measurement is crucial for assessing endogenous insulin production, especially in diabetic patients on insulin therapy.

Insulin Physiology - Insulin: The Key Master

• Structure: Peptide hormone; A (21 amino acids) & B (30 amino acids) chains, linked by disulfide bonds.
• Synthesis: Pancreatic β-cells: Preproinsulin → Proinsulin (Golgi) → Insulin + C-peptide (secretory granules).
  • 📌 C-peptide: Equimolar secretion; reliable marker of endogenous insulin production.
• Secretion Stimuli:
  • Primary: ↑ Blood glucose (via GLUT2 on β-cells → ↑ATP → KATP channel closure → Ca2+ influx → exocytosis).
  • Others: Amino acids (arginine, leucine), incretins (GLP-1, GIP), vagal stimulation (ACh), β2-agonists.
• Secretion Inhibitors: Somatostatin, sympathetic stimulation (α2-adrenergic), diazoxide, propranolol.
• Receptor: Tyrosine kinase receptor (2α, 2β subunits). Binding → autophosphorylation → IRS activation → PI3K & MAPK pathways.
• Key Actions (Anabolic Hormone):
  • ↓ Blood glucose: ↑ GLUT4 translocation (muscle, adipose), ↑ glycogenesis, ↓ gluconeogenesis, ↓ glycogenolysis.
  • ↑ Lipogenesis (activates LPL, acetyl-CoA carboxylase), ↓ lipolysis (inhibits HSL).
  • ↑ Protein synthesis, ↓ proteolysis.
  • ↑ Cellular K+ uptake (stimulates Na+/K+-ATPase).

⭐ Insulin promotes K+ entry into cells by stimulating Na+/K+-ATPase activity; this is crucial in managing DKA, as insulin therapy can cause hypokalemia.

Glucagon Physiology - Glucagon: Glucose Lifeguard

• Source: Peptide, pancreatic α-cells.
• Function: ↑ Blood glucose; counter-regulatory to insulin. (📌 Mnemonic: Glucagon when "Glucose-is-GONE")
• Stimuli:
  • Hypoglycemia (primary)
  • Amino acids (arginine, alanine; post-protein meal)
  • Sympathetic (β-adrenergic), Cortisol, CCK
• Inhibitors:
  • Hyperglycemia, Insulin
  • Somatostatin, GLP-1, fatty acids
• Mechanism (Liver): Gs-coupled receptor → ↑adenylyl cyclase → ↑cAMP → PKA.
• Hepatic Effects:
  • ↑ Glycogenolysis (major, rapid glucose output)
  • ↑ Gluconeogenesis (sustained glucose production)
  • ↑ Ketogenesis (from fatty acid breakdown)
  • ↑ Urea cycle activity
• Adipose Tissue: ↑ Lipolysis (HSL activation, generally minor in humans).

⭐ Glucagon's primary, most rapid effect is stimulating hepatic glycogenolysis to elevate blood glucose.

Glucose Regulation & Imbalance - Sweet Balance & Chaos

• Core Balance: Maintained by insulin (↓ blood glucose) & glucagon (↑ blood glucose).
  • Normal Fasting Glucose: 70-100 mg/dL.
  • Post-Prandial (2hr): <140 mg/dL.
• Insulin (Anabolic): From β-cells.
  • ↑ Glucose uptake (GLUT4: muscle, adipose).
  • ↑ Glycogenesis, ↑ Lipogenesis.
  • ↓ Gluconeogenesis, ↓ Glycogenolysis.
• Glucagon (Catabolic): From α-cells.
  • ↑ Glycogenolysis (liver).
  • ↑ Gluconeogenesis (liver).
• Flow of Control:

• Imbalance States:
  • Diabetes Mellitus (DM): Persistent hyperglycemia.
    • Diagnosis: FPG ≥126 mg/dL; PPG (2hr) ≥200 mg/dL; HbA1c ≥6.5%.
  • Hypoglycemia: Blood glucose <70 mg/dL.
    • Whipple's Triad: Symptoms + Low BG + Relief with glucose.

⭐ In Diabetic Ketoacidosis (DKA), total body potassium is depleted despite potential normal/high serum K+ due to extracellular shift from acidosis & insulinopenia.

High‑Yield Points - ⚡ Biggest Takeaways

• Insulin (β-cells): anabolic, ↑ glucose uptake via GLUT4 (muscle/adipose). Deficiency → DM Type 1.
• Glucagon (α-cells): catabolic, ↑ glycogenolysis & gluconeogenesis.
• Somatostatin (δ-cells): inhibits insulin & glucagon secretion.
• GLUT2: insulin-independent (β-cells, liver); GLUT4: insulin-dependent (muscle, adipose).
• HbA1c: reflects 2-3 month glucose control; target <7%.
• DKA (T1DM): hyperglycemia, ketosis, acidosis.
• HHS (T2DM): severe hyperglycemia, hyperosmolality, minimal/no ketosis_._