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Parietal cells produce gastric acid (hydrochloric acid) after activation of adenylate cyclase. Parietal cells contain an extensive secretory network (called canaliculi) from which the HCl is secreted into the stomach. Active transport, facilitated by the enzyme K+/H+ ATPase, is used since the concentration of hydrogen ions in the cells is about 3 million times greater. 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 into a bicarbonate ion (HCO3-) and a hydrogen ion (H+).
- The bicarbonate ion (HCO3-) is released into the blood and a chloride ion (Cl-) enters the cell. This transfer helps to maintain the elevated pH inside the cell.
- Potassium (K+) and chloride (Cl-) ions are secreted into the canaliculi.
- Hydrogen ions are pumped out of the cell into the canaliculi and a potassium ion enters the cell. This helps maintain the electrical gradient inside the cytosol, since there is no difference in the net charge.
- Since the canaliculi lumen contains more particles (in this case, hydrogen ions) than the cell, water from the cell diffuses into the canaliculi, creating a solution that is highly acidic.
The resulting highly acidic environment causes proteins from food to unfold (or denature) and this exposes the protein's peptide bonds. Enzymes such as pepsin and trypsin help digestion by breaking the bonds linking amino acids, a process known as proteolysis.
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
Parietal cells also produce intrinsic factor. Intrinsic factor is required for the absorption of vitamin B12 in the diet.
Diseases of parietal cells
- Peptic ulcers can result from over-acidity in the stomach. Antacids can be used to enhance the natural tolerance of the gastric lining. Antimuscarinic drugs such as pirenzepine or H2 antihistamines can reduce acid secretion. Proton pump inhibitors are more potent at reducing gastric acid production.
- In pernicious anemia, autoantibodies directed against parietal cells or intrinsic factor cause a reduction in vitamin B12 absorption. It can be treated with injections of replacement vitamin B12 (hydroxocobalamin or cyanocobalamin).
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