Glucagon acts on hepatocytes to stimulate glycogenolysis and gluconeogenesis. Which sequence correctly traces the post-receptor signaling pathway?
- A Receptor tyrosine kinase phosphorylates IRS proteins, activating phosphoinositide 3-kinase
- B Gs-coupled receptor activates adenylate cyclase, raising cAMP and activating protein kinase A ✓
- C Gq-coupled receptor activates phospholipase C, generating inositol trisphosphate and calcium release
- D JAK2 activation leads to STAT5 phosphorylation and nuclear gene transcription
Correct answer: B. Gs-coupled receptor activates adenylate cyclase, raising cAMP and activating protein kinase A
Explanation
The glucagon receptor is Gs-coupled; ligand binding activates adenylate cyclase, cAMP rises, and protein kinase B phosphorylates phosphorylase kinase and other targets to drive glycogen breakdown and gluconeogenic enzyme induction. The tyrosine kinase IRS pathway belongs to insulin, the Gq calcium pathway mediates angiotensin II and some hypothalamic signals, and JAK-STAT is used by growth hormone and leptin, not glucagon.
Reference: Costanzo Physiology, 7th ed.
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