Which physiological effect is associated with muscarinic receptor activation in the heart?

Which physiological effect is associated with muscarinic receptor activation in the heart?

• A. Increased heart rate
• B. Irregular rhythm
• C. Decreased heart rate
• D. No change

Answer: (C)

Muscarinic receptor activation in the heart lowers heart rate through parasympathetic control. When acetylcholine binds M2 receptors on the SA node, they are Gi/o‑coupled, which reduces cAMP levels and lowers the pacemaker slope. The βγ subunits open GIRK potassium channels, causing hyperpolarization of pacemaker cells. Together, this slows automaticity in the SA node and slows conduction through the AV node, producing a decrease in heart rate (bradycardia). This contrasts with sympathetic activation, which raises heart rate by increasing cAMP and calcium currents.

Muscarinic receptor activation in the heart lowers heart rate through parasympathetic control. When acetylcholine binds M2 receptors on the SA node, they are Gi/o‑coupled, which reduces cAMP levels and lowers the pacemaker slope. The βγ subunits open GIRK potassium channels, causing hyperpolarization of pacemaker cells. Together, this slows automaticity in the SA node and slows conduction through the AV node, producing a decrease in heart rate (bradycardia). This contrasts with sympathetic activation, which raises heart rate by increasing cAMP and calcium currents.