During sympathetic activation, norepinephrine mainly binds to which receptor to increase heart rate and contractility?

• A. Beta1 adrenoreceptors
• B. Alpha1 adrenoreceptors
• C. Muscarinic M2 receptors
• D. Dopaminergic receptors

Answer: (A)

During sympathetic activation, the heart’s response to norepinephrine is mainly through beta1-adrenoceptors. These receptors couple to Gs proteins, which stimulate adenylyl cyclase to raise cAMP. The rise in cAMP activates protein kinase A, leading to phosphorylation of L-type calcium channels and other targets that increase calcium availability. This enhances both heart rate (positive chronotropy) and contractility (positive inotropy).

Norepinephrine also has alpha1 actions that promote vasoconstriction, affecting afterload, but the direct means by which heart rate and contractility are increased is beta1 stimulation. Muscarinic M2 receptors mediate parasympathetic effects and slow the heart, while dopaminergic receptors aren’t the primary mediators of this cardiac response.