Parietal Cells of the Oxyntic Glands Are the Only Cells That Secrete Hydrochloric Acid. As noted earlier in the chapter, the acidity of the fluid secreted by the parietal cells of the oxyntic glands can be great, with pH as low as 0.8. However, secretion of this acid is under continuous control by both endocrine and nervous signals. Furthermore, the parietal cells operate in close association with another type of cell called enterochromaffin-like cells (ECL cells), the primary function of which is to secrete histamine.
The ECL cells lie in the deep recesses of the oxyntic glands and therefore release histamine in direct contact with the parietal cells of the glands. The rate of formation and secretion of hydrochloric acid by the parietal cells is directly related to the amount of histamine secreted by the ECL cells. In turn, the ECL cells are stimulated to secrete histamine by the hormone gastrin, which is formed almost entirely in the antral portion of the stomach mucosa in response to proteins in the foods being digested. The ECL cells may also be stimulated by hormones secreted by the enteric nervous system of the stomach wall. We will first discuss the gastrin mechanism for control of the ECL cells and their subsequent control of parietal cell secretion of hydrochloric acid.
Stimulation of Acid Secretion by Gastrin. Gastrin is a hormone secreted by gastrin cells, also called G cells. These cells are located in the pyloric glands in the distal end of the stomach. Gastrin is a large polypeptide secreted in two forms: a large form called G-34, which contains 34 amino acids, and a smaller form, G-17, which contains 17 amino acids. Although both of these forms are important, the smaller form is more abundant.
When meats or other foods containing protein reach the antral end of the stomach, some of the proteins from these foods have a special stimulatory effect on the gastrin cells in the pyloric glands to cause release of gastrin into the blood to be transported to the ECL cells of the stomach. The vigorous mixing of the gastric juices trans ports the gastrin rapidly to the ECL cells in the body of the stomach, causing release of histamine directly into the deep oxyntic glands. The histamine then acts quickly to stimulate gastric hydrochloric acid secretion.
