Which molecules' binding to nodal cells is blocked by beta blockers, leading to a lower heart rate?

Which molecules' binding to nodal cells is blocked by beta blockers, leading to a lower heart rate?

• A. Acetylcholine and Dopamine
• B. Norepinephrine and Epinephrine
• C. Dopamine and Serotonin
• D. Acetylcholine and Epinephrine

Answer: (B)

Beta-adrenergic receptors on nodal (SA and AV) cells mediate the heart’s response to sympathetic stimulation. When catecholamines like norepinephrine and epinephrine bind to these beta-1 receptors, they increase automaticity and conduction, raising the heart rate. Beta blockers block these receptors, so the binding of norepinephrine and epinephrine is prevented, and the sympathetic boost to the nodal tissue is blunted, resulting in a slower heart rate. Acetylcholine slows heart rate via muscarinic receptors and isn’t blocked by beta blockers, so parasympathetic effects can still influence rate. Dopamine and serotonin don’t primarily exert their rate-modulating effects through beta-adrenergic receptors on nodal tissue in the same way, so they’re not the main molecules affected by beta blocker action.

Beta-adrenergic receptors on nodal (SA and AV) cells mediate the heart’s response to sympathetic stimulation. When catecholamines like norepinephrine and epinephrine bind to these beta-1 receptors, they increase automaticity and conduction, raising the heart rate. Beta blockers block these receptors, so the binding of norepinephrine and epinephrine is prevented, and the sympathetic boost to the nodal tissue is blunted, resulting in a slower heart rate.

Acetylcholine slows heart rate via muscarinic receptors and isn’t blocked by beta blockers, so parasympathetic effects can still influence rate. Dopamine and serotonin don’t primarily exert their rate-modulating effects through beta-adrenergic receptors on nodal tissue in the same way, so they’re not the main molecules affected by beta blocker action.