Second Messengers — cAMP, IP3, DAG

Concerning the cAMP second messenger pathway and its regulation, all of the following are true EXCEPT:

cAMP exerts its intracellular effects primarily by activating which protein?

A 38-year-old woman presents with palpitations and tremor. Her thyroid hormone levels are elevated. Which feature best distinguishes the signaling mechanism of thyroid hormone from that of epinephrine in cardiac myocytes?

A 32-year-old woman presents to the emergency department with palpitations, tremor, and anxiety. She reports a 3-month history of heat intolerance and weight loss despite good appetite. On examination, she is tachycardic (HR 110/min), has a fine tremor, and exophthalmos. Thyroid function tests show TSH <0.01 mIU/L and free T4 150 pmol/L (normal 10–20). A thyroid scan confirms diffuse uptake consistent with Graves' disease. Which of the following best explains the mechanism by which thyroid hormones cause tachycardia and tremor in this patient?

In a 52-year-old man presenting with palpitations and tremor, epinephrine binds to β-adrenergic receptors on cardiac myocytes. Which is the most common intracellular site where the second messenger cAMP exerts its primary effect in these cells?