Which adrenergic receptor subtype is primarily responsible for increasing heart rate and myocardial contractility?

• A. Beta 1 receptors
• B. Beta 2 receptors
• C. Beta 3 receptors
• D. Alpha 1 receptors

Answer: (A)

Activation of beta-1 adrenergic receptors in the heart drives both the heart rate and the strength of each beat. When these receptors are stimulated by adrenaline or norepinephrine, they activate a Gs protein that boosts cAMP inside cardiac cells. Higher cAMP enhances calcium entry through L-type calcium channels and promotes calcium release from the sarcoplasmic reticulum, increasing the force of contraction (inotropy) and speeding up the rate of depolarization in the SA node (chronotropy). This combination raises cardiac output during sympathetic arousal.

Beta-2 receptors can influence some smooth muscle and cause vasodilation or bronchodilation, but they are not the primary drivers of heart rate and contractility. Beta-3 is mainly involved in metabolic effects in adipose tissue. Alpha-1 receptors mainly cause vasoconstriction and increased vascular resistance, which affects blood pressure but does not directly increase the heart’s rate and contractile force.