A 52-year-old man with a 10-year history of type 2 diabetes presents with palpitations and tremor. On examination, he is anxious, diaphoretic, and has a heart rate of 118 bpm. Blood glucose is 45 mg/dL. After IV dextrose administration, symptoms resolve. The emergency physician explains that during hypoglycemia, catecholamines bind to β-adrenergic receptors on hepatocytes, triggering a cascade that ultimately increases intracellular calcium and promotes glycogen breakdown. Which of the following second messengers is primarily responsible for mobilizing intracellular calcium stores in this pathway?