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GATE 2023 BT – Question 12

Genetics, Cellular and Molecular Biology · Cell Biology: Eukaryotic cell structure, cell cycle and growth control, cell-cell communication, signalling and signal transduction · 1 mark · Multiple choice

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

  1. It binds to GPCR and activates G protein to slow the heart rate
  2. It stimulates GABA-activated ion-channel coupled receptor to increase the heart rate
  3. It binds to GPCR and inhibits G protein to slow the heart rate
  4. It inhibits GABA-activated ion-channel coupled receptor to increase the heart rate

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Correct answer: (A) It binds to GPCR and activates G protein to slow the heart rate

Explanation

The heart rate is set by the pacemaker (sinoatrial node) cells. The parasympathetic (vagal) nerves release **acetylcholine (ACh)**.

**Mechanism.**
1. ACh binds to **M2 muscarinic receptors**, which are **G protein-coupled receptors (GPCRs)**.
2. The receptor activates the inhibitory G protein, **Gi**.
3. The $\beta\gamma$ subunits open K⁺ channels (hyperpolarising the cell) and Gi lowers cAMP, which reduces the pacemaker current.
4. The cells fire more slowly, so the heart rate **falls**.

GABA receptors are not involved. Option **A** (binds a GPCR and activates a G protein to slow the heart rate) is correct. Option C says the G protein is inhibited, which is the wrong direction of action.