Question

Acetylcholine released by the parasympathetic nerves has which one of the following functions in the heart pacemaker cells?

• A. It binds to GPCR and activates G protein to slow the heart rate
• B. It stimulates GABA-activated ion-channel coupled receptor to increase the heart rate
• C. It binds to GPCR and inhibits G protein to slow the heart rate
• D. It inhibits GABA-activated ion-channel coupled receptor to increase the heart rate

Hint:

Parasympathetic nerves release acetylcholine → acts via M2 muscarinic GPCRs.
Activation of Gi → lowers cAMP → slows pacemaker activity.

Answer 1

The Correct Option is A

Step 1: Acetylcholine is released by parasympathetic (vagal) nerves and acts on the heart pacemaker cells (SA node).
Step 2: It binds to muscarinic acetylcholine receptors (M2 receptors), which are a type of GPCR (G-protein coupled receptor).
Step 3: Binding activates Gi protein, which decreases cAMP levels and increases K\(^+\) conductance via GIRK channels. This hyperpolarises the cell, slowing depolarisation.
Step 4: As a result, heart rate is slowed. Options (B) and (D) are irrelevant (involving GABA receptors). Option (C) is incorrect because acetylcholine \textit{activates} G protein (Gi), not inhibits it.
Thus, the correct mechanism is (A).