Isoproterenol increases heart rate primarily through activation of which receptor?

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

Answer: (C)

Beta-1 adrenergic receptors in the heart drive the primary increase in heart rate. When these receptors on the SA node (and to a degree the AV node) are activated, cAMP rises, which boosts the funny current If and calcium entry, causing faster pacemaker depolarization and quicker conduction. Isoproterenol is a nonselective beta-adrenergic agonist with strong activity at beta-1 receptors in the myocardium, so its main effect is a rapid rise in heart rate. Activation of alpha receptors would mainly cause vasoconstriction and higher afterload rather than directly increasing heart rate, while beta-2 receptor activation contributes more to vasodilation and bronchodilation, not the primary chronotropic effect. Dopamine receptors aren’t the driving mechanism here.