HOW PARIETAL CELL SECRETE HCL ?
The parietal cells or oxyntic cells, are near the chief cells. These cells are responsible for HCl secretion.
1-The Gastric Mucosa.
The interior of the stomach is lined with a gastric mucosa, a protein, furrowed with folds which extend and thicken from the cardiac orifice to the pylorus. The glands in the cardiac area transition between esophageal and fundal types. The fundus glands include the chief cells which secrete pepsin and the border or parietal cells which secrete HCL. The glands in the pyloric region consist of the chief cells only.
2-The Mucus-Secreting Epithelial Cells Are Alkaline.
These mucus-secreting epithelial cells are found on the surface in the form of gastric mucosa, some of them are thick, but others are thin. The thick mucus is in the form of an alkaline gel. When the stomach is at rest, it is covered with an adherent alkaline mucoid layer. During this time, any stimulation will increase the speed of the formation of the mucus, to the point that the neural excitation from the vagus or the chemical excitation from histamine will have no effect on the surface of the epithelial cells. Obviously, this alkaline mucus protects the epithelial cells against a high secretion level from hydrochloric acid or from the pepsin.
3-The HCL Characteristics.
The gastric juice contains 1 to 5 percent of free HCL, be between 30 to 150mEq of hydrogen ion per liter, its pH is close to 1, therefore it is more acidic than all biological fluids. It prevents the colonization of the bacteria in small intestines, and it also activates the pepsinogen, and promotes absorption of mineral salts.
4-The Parietal Cells.
The parietal cells or oxyntic cells, they are near the chief cells. These cells are responsible for HCl secretion.
5-How Parietal Cells Secrete HCL?
1-In a cross-section, the parietal cells are found between the lumen and Plasma. The parietal cells have two channels called canaliculi, in their apical side are directed toward the lumen region, but in their baso-lateral region, the parietal cells are facing the plasma region.
2-When the cell is not activated, the lumen side will be the environment of higher concentration of sodium ions, and low concentration of hydrogen.
3- An activation of the parietal cell will provoke, a decrease in concentration of calcium ions and an increase of concentration of proton or hydrogen ions in the luminal area, whereas in the plasma membrane region, will be an environment for passage of chloride, potassium, and the ion hydrogen.
4-When the parietal cell is activated, we observe 3 main opening channels towards the luminal side and 2 other channels directed into the plasma side:
— proton, potassium ATP (proton pump).
— Chloride channels.
— Potassium channels.
— Chloride bicarbonate exchanger or anti-porter.
— Sodium, potassium pump.
5-Remember, that the pH in the luminal side is more or less than 2, but the pH in the parietal cell is about 7.2. In this huge difference of concentration of proton between the luminal side and the cytoplasmic side, the hydrogen pump must use ATP.
6-One ATP molecule is used to transfer this proton from the cytoplasmic side towards the luminal region, and a transfer of potassium ion in the plasma region, by using the hydrogen potassium ATPase.
7-The carbon dioxide and water entered into the cell, to combine with each other and form the carbonic acid in the presence of an enzyme called carbonic anhydrase. They will be dissociated into proton and bicarbonate. This hydrogen positive, will be the source for the proton channel, to be transferred through the proton pump towards the lumen region.
8-In the luminal side, two compounds (the proton and chloride), are appeared separately, they will be combined into HCL.
9-In the baso-lateral region, exist a chloride bicarbonate exchanger, it is an anti-porter, its main role is to transfer one bicarbonate ion from the cytoplasmic side towards the external side, and simultaneously it transfers the chloride ion from the outside towards the inside of the parietal cell. Thus, the chloride ion is formed, to serve as a source for the chloride channel.
10-In the sodium potassium pump, we see a transfer of three Na positive to the outside and two K positives to the inside.
11-When this bicarbonate is transferred from the cytoplasmic site towards the external side, it causes an increase in the alkalinity of the blood, but in the lumen side, we observe an increase in acidity.
