Life Span and A&P Practice Test

The heart’s response to sympathetic signals is driven mainly by beta-1 adrenergic receptors. When these receptors are activated by adrenaline or noradrenaline, they stimulate a Gs protein, which boosts adenylyl cyclase activity and raises cAMP levels. The cAMP activates protein kinase A, which phosphorylates L-type calcium channels and other calcium-handling proteins. This increases calcium entry during each heartbeat and enhances calcium release from the sarcoplasmic reticulum, leading to a stronger contraction (inotropy) and a faster heart rate (chronotropy).

Beta-2 receptors can respond to catecholamines as well, but their influence on heart rate and contractility is much smaller in the heart itself; their prominent effects are smooth muscle relaxation (vasodilation and bronchodilation) in other tissues. Alpha-1 receptors cause vasoconstriction in vascular smooth muscle, raising blood pressure rather than directly increasing heart rate or contractility. Alpha-2 receptors mainly regulate neurotransmitter release and autonomic tone rather than driving heart rate and force of contraction.