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Stimulation of Gastric Acid Secretion
Parietal Cells of the Oxyntic Glands Are the Only Cells That Secrete Hydrochloric Acid. Theparietal cells, located deepin the oxyntic glands of the main body of the stomach, are the only cells that secrete hydrochloric acid. As noted earlier, the acidity of the fluid secreted by these cells can be very great, with pH as low as 0.8. However, secretion of this acid is under con-tinuous control by both endocrine and nervous signals. Furthermore, the parietal cells operate in close associ-ation with another type of cell called enterochromaf-fin-like cells (ECL cells), the primary function of whichis 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 his-tamine secreted by the ECL cells. In turn, the ECL cells can be stimulated to secrete histamine in several different ways: (1) Probably the most potent mecha-nism for stimulating histamine secretion is by the hor-monal substance gastrin, which is formed almost entirely in the antral portion of the stomach mucosa in response to proteins in the foods being digested. (2) In addition, the ECL cells can be stimulated by (a) acetylcholine released from stomach vagal nerve endings and (b) probably also by hormonal substances secreted by the enteric nervous system of the stomach wall. Let us discuss first 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 itself ahormone 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 polypep-tide 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 are important, the smaller is more abundant.
When meats or other protein-containing foods reach the antral end of the stomach, some of the pro-teins from these foods have a special stimulatory effect on the gastrin cells in the pyloric glands to cause release of gastrin into the digestive juices of the stomach. The vigorous mixing of the gastric juices transports the gastrin rapidly to the ECL cells in the body of the stomach, causing release of histaminedirectly into the deep oxyntic glands. The histaminethen acts quickly to stimulate gastric hydrochloric acid secretion.
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