Adrenaline vs. Acetylcholine – Mechanism of Action

Adrenaline vs. Acetylcholine – Mechanism of Action

⚡ Adrenaline (Epinephrine)

Type: Catecholamine (Sympathomimetic)

Receptor Type: Adrenergic Receptors (GPCRs)

• α₁ → Vasoconstriction (↑ BP)

• α₂ → ↓ NE release (negative feedback)

• β₁ → ↑ Heart rate & contractility (↑ CO)

• β₂ → Bronchodilation, Vasodilation (skeletal muscles), ↓ GI motility

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Mechanism of Action:

1. Binds to adrenergic receptors (α/β) on target tissues

2. Activates G-proteins (Gs for β, Gq for α₁, Gi for α₂)

3. Triggers second messengers:

   • β (Gs) → ↑ cAMP → ↑ PKA activity

   • α₁ (Gq) → ↑ IP₃/DAG → ↑ Ca²⁺

   • α₂ (Gi) → ↓ cAMP

4. Leads to physiological effects (heart stimulation, bronchodilation, vasoconstriction, etc.)

🌿 Acetylcholine (ACh)

Type: Cholinergic neurotransmitter (Parasympathomimetic)

Receptor Type:

• Muscarinic (M1–M5) → GPCRs

• Nicotinic (Nn, Nm) → Ligand-gated ion channels

Mechanism of Action:

1. Muscarinic (M1, M2, M3):

• M1, M3, M5 (Gq) → ↑ IP₃/DAG → ↑ Ca²⁺
  → Effects: Smooth muscle contraction (bronchoconstriction, miosis), glandular secretion

• M2 (Gi) → ↓ cAMP → ↓ Heart rate, ↓ contractility

2. Nicotinic Receptors (Nn, Nm):

• Direct opening of Na⁺/K⁺ channels

• Nn (CNS, ganglia) → Neural transmission

• Nm (NMJ) → Muscle contraction

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⚖️ Summary Comparison Table