GATE BT 2023 • Q12
MCQ • 1 Mark
Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
Acetylcholine released by the parasympathetic nerves has which one of the
following functions in the heart pacemaker cells?
Options:
-
(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
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2023 Q.12)**
**1. Governing Concept**:
- **Topic**: Animal Physiology & Autonomic Neurotransmission
- Acetylcholine (ACh) released by postganglionic parasympathetic nerve terminals (vagus nerve) binds to cardiac **muscarinic acetylcholine receptors ($M_2$)** on the sinoatrial (SA) node, opening $G$-protein-gated inward-rectifying $K^+$ channels ($K_{\text{ACh}}$) and inhibiting adenylyl cyclase, hyperpolarizing pacemaker cells and **decreasing heart rate** (negative chronotropy).
**2. Option Analysis**:
- **Option (A) [decreases heart rate] is CORRECT**: Classic parasympathetic / vagal stimulation effect.
- **Option (B) [increases heart rate] is INCORRECT**: Sympathetic effect mediated by noradrenaline acting on $\beta_1$-adrenergic receptors.
- **Option (C) [does not affect heart rate] is INCORRECT**: Acetylcholine is the primary physiological bradycardic modulator.
- **Option (D) [increases cardiac output] is INCORRECT**: Vagal stimulation decreases cardiac output by lowering chronotropy.
**3. Final Answer**:
- **Correct Choice**: **A**
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