Parietal cells, or oxyntic cells, are the stomach epithelium cells that secrete gastric acid (HCl) and intrinsic factor in response to histamine (H2 receptor), acetylcholine (M3 receptors) and gastrin (CCK2 receptors). The histamine receptors act by increasing intracellular cAMP, whereas the muscarinic and gastrin receptors increase intracellular Ca2+ levels. Both cAMP and Ca2+ act via protein kinases to increase the transport of acid into the stomach. Gastrin is more important indirectly by increasing histamine synthesis in ECL cells, as gastrin has no effect on the maximum histamine-stimulated gastric acid secretion.
Parietal cells contain an extensive secretory network (called canaliculi) from which the HCl is secreted by active transport into the stomach. The enzyme hydrogen potassium ATPase (H+/K+ ATPase) is unique to the parietal cells and transports the H+ against a concentration gradient of about 3 million to 1, which is the steepest ion gradient formed in the human body.
Hydrochloric acid is formed in the following manner:
- Hydrogen ions are formed from the dissociation of water molecules. The enzyme carbonic anhydrase converts one molecule of carbon dioxide and one molecule of water indirectly into a bicarbonate ion (HCO3-) and a hydrogen ion (H+).
- The bicarbonate ion (HCO3-) is exchanged for a chloride ion (Cl-) on the basal side of the cell and the bicarbonate diffuses into the venous blood, leading to an alkaline tide.
- Potassium (K+) and chloride (Cl-) ions diffuse into the canaliculi.
- Hydrogen ions are pumped out of the cell into the canaliculi in exchange for potassium ions, via the H+/K+ ATPase.
The resulting highly-acidic environment causes proteins from food to unfold (or denature), exposing the peptide bonds that link together amino acids. HCl also activates pepsinogen, an endopeptidase, allowing it to help digestion by breaking specific peptide bonds, a process known as proteolysis. Furthermore, the sudden increase in gastric acid secretion following a meal can cause a physiological phenomenon called an alkaline tide, which is due to the production and export of bicarbonate from parietal cells. The alkaline tide is neutralized by the action of the pancreatic duct which produces a bicarbonate secretion that is deposited into the lumen of the duct while the byproduct, hydrogen ions, are pumped out the basal membrane into the portal blood stream, thereby neutralizing the bicarbonate from the stomach.
Parietal cells secrete acid in response to three types of stimuli :
- H2 histamine receptors (most significant contribution)
- Acetylcholine from parasympathetic activity via the vagus nerve and enteric nervous system
- gastrin (least significant contribution, but note that histamine secretion by ECL cells is due in part to gastrin)
Upon stimulation, adenylate cyclase is activated within the parietal cells. This increases intracellular cyclic AMP, which leads to activation of protein kinase A. Protein kinase A phosphorylates proteins involved in the transport of H+/K+ ATPase from the cytoplasm to the cell membrane. This causes resorption of K+ ions and secretion of H+ ions. The pH of the secreted fluid can fall by 0.8.
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