Glucose-dependent insulinotropic polypeptide (GIP) and GLP-1 are incretins that potentiate glucose-stimulated insulin secretion. They act via Gs-coupled receptors on beta cells to increase cAMP, which activates PKA. PKA-independent amplification of insulin secretion by incretins also involves activation of which pathway?

• A PI3-kinase/AKT pathway leading to GLUT4 translocation to the beta cell membrane
• B PLC-β/IP3 pathway causing intracellular Ca²⁺ release from ER without membrane depolarization
• C Epac2 (exchange protein directly activated by cAMP) → Rap1 GTPase → closure of KATP channels ✓
• D MAPK/ERK pathway that directly phosphorylates and activates syntaxin-1A for vesicle fusion

Explanation

Beyond PKA, cAMP also activates Epac2 (Rap guanine nucleotide exchange factor 4), a cAMP-binding exchange protein. Epac2 activates Rap1, which closes KATP channels and facilitates the priming of insulin secretory granules for exocytosis. Epac2/Rap1 signaling also potentiates the Ca²⁺ sensitivity of the exocytotic machinery and promotes mobilization of the reserve pool of granules. This PKA-independent pathway explains why GLP-1 receptor agonists (liraglutide, semaglutide) amplify insulin secretion in a glucose-dependent manner even when KATP channels are already partly inhibited.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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