Q.12 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
Acetylcholine Parasympathetic Heart Pacemaker Function: GATE BT Solved
Acetylcholine released by parasympathetic nerves slows heart rate in pacemaker cells by binding to muscarinic M2 GPCR receptors. This activates inhibitory G proteins (Gi), reducing cAMP and hyperpolarizing the membrane. The correct answer is option (A).
Option Analysis
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(A) Correct: Acetylcholine binds to M2 muscarinic GPCRs on sinoatrial node (SAN) pacemaker cells, activating Gi proteins. Gi inhibits adenylyl cyclase, decreasing cAMP levels, which slows the funny current (If) depolarization phase 4 slope and reduces heart rate.
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(B) Incorrect: Acetylcholine does not stimulate GABA receptors; GABA is inhibitory in CNS, not cardiac pacemaker cells, and would not increase heart rate.
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(C) Incorrect: Acetylcholine activates (not inhibits) G proteins; “inhibits G protein” misstates the mechanism, as Gi activation occurs despite being inhibitory in effect.
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(D) Incorrect: No GABA receptors exist in heart pacemaker cells; acetylcholine acts via muscarinic GPCRs, not ion-channel coupled receptors, and decreases (not increases) rate.
Mechanism Details
Parasympathetic postganglionic fibers release acetylcholine at SAN and AV node pacemaker cells. Binding to M2 receptors couples to pertussis toxin-sensitive Gi/o proteins, opening G-protein inward rectifier K+ (GIRK) channels for K+ efflux, hyperpolarizing the membrane. This prolongs time to reach threshold, slowing spontaneous depolarization.
Acetylcholine released by parasympathetic nerves plays a crucial role in modulating heart pacemaker cells function, primarily slowing the heart rate during rest. This acetylcholine parasympathetic heart pacemaker mechanism is key for competitive exams like GATE BT 2023 Q12, testing autonomic control of cardiac physiology.
Parasympathetic Control Basics
The parasympathetic nervous system via vagus nerve innervates sinoatrial node (SAN) pacemaker cells. Postganglionic neurons release acetylcholine, contrasting sympathetic norepinephrine that accelerates rate.
Molecular Pathway
Acetylcholine binds M2 muscarinic GPCRs, G-protein coupled receptors linked to inhibitory Gi proteins. Giα inhibits adenylyl cyclase, lowering cAMP, while Gβγ directly activates GIRK channels (Kir3.1/3.4), increasing IK,ACh for hyperpolarization.
Exam Relevance
In GATE BT 2023, option (A) correctly identifies GPCR activation slowing rate, distinguishing from distractors involving nonexistent GABA pathways in heart tissue.
