Parietal cell

Parietal cells are rounded or pyramidal, with central nuclei and cytoplasm packed with mitochondria, which makes it pink (eosinophilic). Deep intracellular canaliculi lined by microvilli open into the gland lumen. At rest the proton pumps are stored in a tubulovesicular membrane system. On stimulation these vesicles fuse with the canalicular membrane and greatly enlarge the secretory surface.

HCl secretion. Inside the cell, carbonic anhydrase converts CO₂ and H₂O into H₂CO₃, which dissociates into H⁺ and HCO₃⁻. The H⁺/K⁺-ATPase pumps H⁺ into the canaliculus in exchange for K⁺; the K⁺ leaks back out through apical K⁺ channels, so it recycles. On the basolateral side, HCO₃⁻ leaves in exchange for Cl⁻ (the AE2 exchanger), so bicarbonate enters the blood and causes the post-prandial alkaline tide. The Cl⁻ exits through apical Cl⁻ channels, and water follows osmotically. The secreted H⁺ can reach about 150 mmol/L, a concentration roughly a million times that of plasma.

Control: there are three stimulants and they potentiate each other. ACh from the vagus acts on M3 receptors, raising Ca²⁺. Gastrin acts on CCK2 receptors, raising Ca²⁺; its main effect is indirect, by releasing histamine from ECL cells. Histamine acts on H2 receptors, raising cAMP. Because of this synergy, blocking histamine alone reduces the response to all three. Somatostatin and prostaglandins (via Gi) inhibit the cell.