USMLE

M2 (Muscarinic) Receptors

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General Pharm
  1. Gs / Gi Pathway
  2. Gq Signaling Pathway
  3. Alpha-1 (Adrenergic) Receptors
  4. Alpha-2 (Adrenergic) Receptors
  5. Beta-1 (Adrenergic) Receptors
  6. Beta-2 (Adrenergic) Receptors
  7. Beta-3 (Adrenergic) Receptors
  8. M1 (Muscarinic) Receptors
  9. M2 (Muscarinic) Receptors
  10. M3 (Muscarinic) Receptors
  11. D1 (Dopamine) Receptors
  12. D2 (Dopamine) Receptors
  13. H1 (Histamine) Receptors
  14. H2 (Histamine) Receptors
  15. V1 (Vasopressin) Receptors
  16. V2 (Vasopressin) Receptors

Summary

M2 receptors are muscarinic receptors that are primarily found in the heart. Binding of M2 receptors by acetylcholine activates a Gi protein subunit and signaling cascade, which causes an intracellular decrease in cyclic AMP levels. Clinically, M2 receptor activation in the heart causes a decrease in heart rate and contractility. This is thought to be the major mechanism by which the vagal nerve causes parasympathetic actions on the heart.

Key Points

  • M2 (Muscarinic) Receptors
    • Associated with Gi protein subunit
      • Inhibits cAMP formation
    • Acts on Heart
      • Main receptor used by vagal nerve (CN X) efferents for inducing parasympathetic effects on the heart
      • Decreases HR (slows heart)
      • Decreases contractility (lowers stroke volume)