Which receptor mediates the sympathetic response of the heart, including an increase in heart rate?

• A. Beta 1 receptors
• B. Beta 2 receptors
• C. Alpha 1 receptors
• D. Dopaminergic receptors

Answer: (A)

Sympathetic input to the heart mainly works through beta-1 adrenergic receptors. When norepinephrine or epinephrine binds to these receptors on the SA node (and also affects the AV node and myocardium), it activates Gs proteins that increase cAMP. The higher cAMP boosts the activity of the funny current and calcium channels, speeding up diastolic depolarization and increasing conduction velocity. The result is a faster heart rate (positive chronotropy) and stronger contraction (positive inotropy).

Beta-2 receptors are present in the heart but play a less dominant role in driving heart rate and are more involved in other tissues like smooth muscle and the lungs. Alpha-1 receptors cause vasoconstriction in peripheral vessels, influencing blood pressure rather than directly increasing heart rate. Dopaminergic receptors mainly affect renal blood flow at certain doses and aren’t the primary mediators of the heart’s sympathetic tachycardic response.

So the receptor that mediates the sympathetic increase in heart rate is the beta-1 adrenergic receptor.