Which receptor subtype mediates the heart's rate increase during sympathetic stimulation?

Which receptor subtype mediates the heart's rate increase during sympathetic stimulation?

• A. Alpha-1 adrenergic receptor
• B. Beta-1 adrenergic receptor
• C. Beta-2 adrenergic receptor
• D. Muscarinic receptor

Answer: (B)

Sympathetic stimulation speeds the heart mainly through beta-1 adrenergic receptors on the sinoatrial node. When norepinephrine binds these receptors, it activates Gs proteins, increases cAMP, and enhances the funny current and calcium handling. This accelerates pacemaker depolarization and boosts heart rate (and contractility). Alpha-1 receptors drive vascular smooth muscle constriction, not heart rate. Beta-2 receptors exist in some tissues but are not the primary drivers of heart rate. Muscarinic receptors mediate parasympathetic effects that slow the heart. So the beta-1 adrenergic receptor is the receptor subtype responsible for the heart rate increase during sympathetic stimulation.

Sympathetic stimulation speeds the heart mainly through beta-1 adrenergic receptors on the sinoatrial node. When norepinephrine binds these receptors, it activates Gs proteins, increases cAMP, and enhances the funny current and calcium handling. This accelerates pacemaker depolarization and boosts heart rate (and contractility). Alpha-1 receptors drive vascular smooth muscle constriction, not heart rate. Beta-2 receptors exist in some tissues but are not the primary drivers of heart rate. Muscarinic receptors mediate parasympathetic effects that slow the heart. So the beta-1 adrenergic receptor is the receptor subtype responsible for the heart rate increase during sympathetic stimulation.