Catecholamines increase heart rate and contractility via which receptor type?

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

Answer: (D)

Catecholamines increase heart rate and contractility primarily through beta-1 adrenergic receptors in the heart. When epinephrine or norepinephrine binds to these receptors on cardiac myocytes and the SA/AV nodes, it activates Gs proteins, which raisecAMP levels and activate protein kinase A. PKA phosphorylates L-type calcium channels and calcium-handling proteins, boosting calcium entry during each action potential and enhancing calcium release from the sarcoplasmic reticulum. That rise in intracellular calcium strengthens contraction (positive inotropy) and speeds up the pacemaker activity and conduction (positive chronotropy and dromotropy). Other receptor types play different roles—alpha-1 mainly causes vasoconstriction, beta-2 influences vasodilation and bronchodilation with less direct impact on the heart, and muscarinic receptors (parasympathetic) slow the heart—so beta-1 is the key receptor for the heart’s rate and force responses to catecholamines.