Parietal cells secrete HCl via H⁺/K⁺-ATPase. The proton pump is activated by a rise in intracellular cAMP (histamine via H2 receptor) and Ca²⁺ (acetylcholine via M3 and gastrin via CCK-B receptors). Which intracellular signaling interaction explains why cimetidine (H2 blocker) is more effective at suppressing acid secretion than blocking acetylcholine alone?

• A Histamine acts as the final common mediator for gastrin and ACh by potentiating their signaling via cAMP synergism with Ca²⁺/PKC pathways ✓
• B Histamine directly inhibits gastrin release from G cells in the antrum
• C H2 receptors are physically coupled to H⁺/K⁺-ATPase and their blockade directly locks the pump inactive
• D Cimetidine also blocks M3 receptors on parietal cells, providing dual blockade

Explanation

Histamine acts as a paracrine amplifier: gastrin and ACh stimulate ECL cells to release histamine, which then potentiates parietal cell activation via H2-cAMP-PKA signaling. There is also direct synergism between cAMP (raised by histamine) and Ca²⁺/PKC (raised by ACh and gastrin) pathways on the parietal cell, where low concentrations of each are maximally effective together. Blocking H2 receptors with cimetidine removes both direct histamine signaling AND the amplification of gastrin/ACh responses, which is why H2 blockers are disproportionately effective compared to blocking a single pathway alone.

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

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Written and medically reviewed by the StethoPrep medical team.