Which receptor subtype is primarily responsible for increasing heart rate and contractility in the heart?

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

Answer: (B)

Beta-1 adrenergic receptors are the primary mediators of increasing heart rate and contractility. These receptors are concentrated in the heart’s pacemaker cells (SA node) and conduction pathways (AV node), as well as in ventricular muscle. When they are activated by sympathetic catecholamines, they trigger a Gs protein–coupled cascade that raises cyclic AMP, activating protein kinase A. This phosphorylation enhances calcium entry through L-type calcium channels and increases calcium reuptake into the sarcoplasmic reticulum, which together raise the amount of calcium available for each heartbeat. The result is faster pacemaker firing (increased heart rate) and stronger contractions (increased contractility). Other receptors don’t have this direct, dominant effect on the heart: alpha-2 receptors mainly regulate neurotransmitter release and have indirect cardiac effects; beta-2 receptors can influence heart rate but are less influential in normal cardiac function; dopamine receptors are not the main drivers of cardiac chronotropy.