Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion from pancreatic beta cells through an incretin effect. The intracellular signaling cascade by which GLP-1 augments glucose-stimulated insulin secretion is:

• A GLP-1R (Gq-coupled) → phospholipase C → IP3 → ER calcium release → exocytosis
• B GLP-1R internalization → nuclear translocation → PDX-1 transcription factor activation → insulin gene expression
• C GLP-1R → Gi protein → inhibition of adenylyl cyclase → reduced cAMP → paradoxical insulin secretion
• D GLP-1R (Gs-coupled) → adenylyl cyclase → cAMP → PKA and Epac2 activation → enhanced KATP channel closure and VDCC opening ✓

Explanation

GLP-1 receptors on beta cells couple to Gs proteins, activating adenylyl cyclase and raising intracellular cAMP. cAMP activates both PKA (which phosphorylates KATP channels, voltage-dependent calcium channels, and exocytotic proteins) and Epac2 (exchange protein directly activated by cAMP), an independent cAMP sensor that directly stimulates insulin granule exocytosis. This amplifies glucose-stimulated insulin secretion without independently triggering insulin release at low glucose, explaining the glucose-dependent nature of GLP-1 action — a key safety advantage of GLP-1 receptor agonist therapy (low hypoglycemia risk).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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