Metabolic Regulation: Hormonal Control

Metabolic Hormones: Overview - Metabolic Maestros

• Major Types & Origin:
  • Peptides: Insulin, Glucagon (Pancreas).
  • Amines: Epinephrine (Adrenal Medulla), Thyroid hormones (Thyroid).
  • Steroids: Cortisol (Adrenal Cortex).
• Core Function: Regulate flux through metabolic pathways; maintain energy balance & glucose homeostasis.
• Signaling: Act via cell surface or intracellular receptors to alter enzyme activity or gene expression.

⭐ Insulin is the major anabolic hormone; Glucagon is its primary counter-regulatory catabolic hormone, crucial for glucose balance.

Insulin: The Anabolic Chief - Anabolic Architect

• Source: Pancreatic β-cells.
• Nature: Peptide; key anabolic hormone.
• Secretion Stimuli: ↑ Blood glucose (primary, >70 mg/dL), amino acids, GLP-1, GIP.
• Receptor: Tyrosine Kinase (RTK).
• Mechanism: Activates PI3K/Akt & MAPK pathways.
• Overall: Promotes fuel storage.
• Key Actions:
  • ↑ Glucose uptake (GLUT4: muscle, adipose).
  • ↑ Glycogenesis; ↓ Glycogenolysis.
  • ↑ Glycolysis (liver); ↓ Gluconeogenesis.
  • ↑ Lipogenesis; ↓ Lipolysis.
  • ↑ Protein synthesis.
  • ↑ K+ cellular uptake. 📌 (Mnemonic: InsulIN K+ goes IN)

⭐ Insulin's promotion of K+ entry into cells is used to treat hyperkalemia.

Glucagon & Co: Catabolic Crew - Catabolic Champions

• Glucagon (Pancreatic α-cells):
  • Stimuli: ↓ Glucose, ↑ Amino acids (e.g., arginine).
  • Actions: ↑ Liver glycogenolysis, gluconeogenesis, ketogenesis; ↑ Lipolysis.
  • Mechanism: GPCR (Gs → ↑$cAMP$).

  ⭐ Glucagon primarily targets the liver; muscle lacks glucagon receptors.

• Epinephrine (Adrenal Medulla):
  • Stimuli: Stress, hypoglycemia.
  • Actions: ↑ Glycogenolysis (liver, muscle); ↑ Lipolysis; ↑ Glucagon secretion, ↓ Insulin secretion.
• Cortisol (Adrenal Cortex):
  • Stimulus: Stress (via ACTH).
  • Actions (slow, permissive): ↑ Muscle proteolysis; ↑ Lipolysis; ↑ Liver gluconeogenesis (induces PEPCK); ↓ Peripheral glucose utilization.
• Growth Hormone (GH; Anterior Pituitary):
  • Actions (anti-insulin): ↓ Tissue glucose uptake; ↑ Lipolysis.

Metabolic States: Hormonal Control - Fuel State Symphony

Hormones orchestrate fuel flux, adapting to nutritional and physiological demands. Key players: Insulin, Glucagon, Epinephrine, Cortisol.

• Fed State (↑ Insulin:Glucagon ratio):
  • Insulin dominant: ↑ Glucose uptake (GLUT4), ↑ Glycogenesis, ↑ Lipogenesis, ↑ Protein synthesis.
  • Anabolic processes favored.
• Fasting State (↓ Insulin:Glucagon ratio):
  • Glucagon dominant: ↑ Glycogenolysis, ↑ Gluconeogenesis.
  • Epinephrine, Cortisol: ↑ Lipolysis, ↑ Proteolysis (prolonged).
  • Prolonged fasting: ↑ Ketogenesis.
• Stress/Exercise:
  • Sympathetic drive: ↑ Epinephrine.
  • ↑ Cortisol, ↑ Glucagon: Mobilize fuels (glucose, Free Fatty Acids) for immediate energy.

⭐ Insulin is the primary anabolic hormone, promoting storage of carbohydrates, fats, and proteins after a meal, and its deficiency or resistance leads to diabetes mellitus.

High‑Yield Points - ⚡ Biggest Takeaways

• Insulin: key anabolic hormone; ↑ glucose uptake (GLUT4), glycogenesis, lipogenesis.
• Glucagon: primary catabolic hormone; ↑ glycogenolysis, gluconeogenesis.
• Epinephrine: rapid stress response via cAMP; ↑ glycogenolysis (liver/muscle), lipolysis.
• Cortisol: permissive action; ↑ gluconeogenesis, protein breakdown in chronic stress.
• Insulin/Glucagon ratio dictates metabolic flux: high (fed), low (fasting).
• Growth Hormone opposes insulin's glucose action; can be diabetogenic.
• Thyroid hormones (T3/T4): ↑ BMR, sensitize tissues to catecholamines.