Through which mechanism do catecholamines increase heart rate and contractility?

• A. They have no effect
• B. They decrease heart rate and contractility via beta-2 adrenergic receptors
• C. They increase heart rate and contractility via beta-1 adrenergic receptors
• D. They increase heart rate but not contractility

Answer: (C)

Catecholamines increase heart rate and contractility by activating beta-1 adrenergic receptors on heart cells. When these receptors are stimulated, they couple to a Gs protein that raises cyclic AMP levels. The boost in cAMP activates protein kinase A, which enhances calcium handling: it increases calcium entry through L-type channels and promotes calcium release from the sarcoplasmic reticulum. This leads to a faster heart rate because the SA node fires more quickly, and a stronger contraction because more calcium is available for excitation–contraction coupling. Beta-1 receptors are the primary mediators of this effect in the heart, whereas other receptors (like beta-2) are not the main drivers of this inotropic and chronotropic response.