13.14:
Stomach pH Regulation
The low pH in the stomach is maintained by the parietal cells that line the stomach lumen.
As food enters the stomach, a combination of neuronal and cellular signals activate the parietal cells.
Upon activation, the tubulovesicles containing hydrogen-potassium ATPase pumps fuse with the apical membrane to form deep folds called canaliculi.
On the basolateral side of the parietal cells, carbon dioxide from the blood diffuses into the parietal cells. It combines with water to form carbonic acid, which is catabolized by the enzyme carbonic anhydrase into bicarbonate and hydrogen ions.
While the hydrogen ions are pumped into the lumen in exchange for potassium ions via the hydrogen-potassium ATPase pumps, the bicarbonate ions are exported to the blood by the chloride-bicarbonate antiporters in exchange for chloride ions.
Both potassium and chloride ions move into the lumen through their respective channels to maintain the intracellular concentration.
In the lumen, the hydrogen and chloride ions combine to form hydrochloric acid, lowering the stomach pH.
13.14:
Stomach pH Regulation
The human body carefully regulates the internal pH of different organs to maintain homeostasis. For example, while the blood plasma maintains a neutral pH of 7, the stomach lumen has an acidic pH of 1.5 – 3.5. The low pH of stomach lumen helps kill pathogens in the food and break down complex food molecules.
The acid-secreting gastric mucosal epithelial cells (parietal cells) lining the stomach lumen maintain the low pH in the lumen. Numerous ion transporters and channels on these parietal cells help regulate gastric acid secretion by precise ion transport and thus play an essential role in maintaining acid/base homeostasis. Of importance is the hydrogen-potassium ATPase, or the proton pump, a P-type ATPase, present on the parietal cells. Also known as the H+/K+ ATPase, these pumps acidify the stomach contents by exchanging potassium ions from the stomach lumen with hydrogen ions from the parietal cell cytoplasm. The hydrogen ions secreted by these hydrogen-potassium ATPases and the chloride ions secreted by the chloride channels on the parietal cells combine to form hydrochloric acid in the stomach lumen.
Due to medicines or infections, excessive acid secretion causes acid reflux, generally refered to as heartburn. In such cases, therapeutic drugs such as proton pump inhibitors help block the hydrogen-potassium ATPases, thereby reducing the release of hydrogen ions into the stomach lumen.
Suggested Reading
- Goo, Tyralee, Yasutada Akiba, and Jonathan D. Kaunitz. "Mechanisms of intragastric pH sensing." Current gastroenterology reports 12, no. 6 (2010): 465-470.
- Aoi, Wataru, and Yoshinori Marunaka. "Importance of pH homeostasis in metabolic health and diseases: crucial role of membrane proton transport." BioMed research international 2014 (2014).
- Putnam, Robert W. "Intracellular pH regulation." In Cell Physiology Source Book, pp. 357-372. Academic Press, 2001.