What is the nature of acetylcholine's inhibitory effect on heart rate?
A. Acetylcholine induces depolarization in the sinoatrial node.
B. Acetylcholine causes closing of sodium channels in the sinoatrial node.
C. Acetylcholine causes opening of fast calcium channels in contractile cells.
D. Acetylcholine causes opening of potassium channels in the sinoatrial node, thereby hyperpolarizing it.
Answer: D. Acetylcholine causes opening of potassium channels in the sinoatrial node, thereby hyperpolarizing it.
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