What is the mechanism of action of T3 in the heart regarding adrenergic signaling and cAMP?

• A. Independent of adrenergic receptors and cAMP
• B. Relies on adrenergic receptors and cAMP
• C. Requires calcium channel activation
• D. Inhibits Na-K ATPase

Answer: (A)

Thyroid hormone T3 boosts heart performance by increasing the heart’s responsiveness to catecholamines through the adrenergic signaling and cAMP pathway. It does this mainly by upregulating beta-1 adrenergic receptors and components of the cAMP-PKA cascade inside cardiac cells. When catecholamines bind these receptors, Gs activates adenylate cyclase, raising cAMP, which then activates PKA. PKA phosphorylates targets that enhance calcium handling (like L-type calcium channels and phospholamban/SERCA), leading to stronger contractions and faster heart rate. Because these effects depend on receptor signaling and the cAMP pathway, they are not independent of adrenergic receptors and cAMP.