Activation of which receptor subtype increases heart rate and contractility?

Activation of which receptor subtype increases heart rate and contractility?

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

Answer: (A)

The heart’s response to sympathetic activation is driven mainly by beta-1 receptors. When these receptors are stimulated, they activate Gs proteins that raise cyclic AMP within cardiac cells. This boost enhances the Ca2+ handling in the heart, increasing both the rate of firing in the SA node (positive chronotropy) and the strength of contraction (positive inotropy). That direct link between beta-1 activation and faster, stronger heartbeats is why this receptor subtype best explains the increase in heart rate and contractility. Beta-2 receptors also raise cAMP but are more involved in smooth muscle relaxation in the lungs and vasculature, where they cause bronchodilation and vasodilation; their role in driving heart rate and contraction is less central. Alpha-1 receptors mainly cause vascular smooth muscle contraction (vasoconstriction), not a direct increase in heart rate or contractility, and alpha-2 receptors mainly modulate neurotransmitter release via negative feedback.

The heart’s response to sympathetic activation is driven mainly by beta-1 receptors. When these receptors are stimulated, they activate Gs proteins that raise cyclic AMP within cardiac cells. This boost enhances the Ca2+ handling in the heart, increasing both the rate of firing in the SA node (positive chronotropy) and the strength of contraction (positive inotropy). That direct link between beta-1 activation and faster, stronger heartbeats is why this receptor subtype best explains the increase in heart rate and contractility.

Beta-2 receptors also raise cAMP but are more involved in smooth muscle relaxation in the lungs and vasculature, where they cause bronchodilation and vasodilation; their role in driving heart rate and contraction is less central. Alpha-1 receptors mainly cause vascular smooth muscle contraction (vasoconstriction), not a direct increase in heart rate or contractility, and alpha-2 receptors mainly modulate neurotransmitter release via negative feedback.