Gastrin: Function & Diseases

Gastrin is a hormone produced in the gastrointestinal tract. The main site of action of the hormone is the stomach. However, it also affects the pancreas.

What is gastrin?

Gastrin is a peptide hormone. It is also known as polypeptide 101. Peptide hormones are fat-insoluble hormones that are made up of proteins. Based on the length of the peptide chains, three different forms of gastrin can be distinguished: Big-Gastrin, Gastrin I or II, and Mini-Gastrin. Big-gastrin has a length of 36 amino acids. Gastrin I and II consist of 17 amino acids and mini-gastrin or little-gastrin has a length of 13 amino acids. Chemically, gastrin is related to the hormone cholecystokinin. Gastrin is formed predominantly in the stomach and small intestine. There are special tumors that can produce gastrin in large quantities. These tumors are therefore called gastrinomas.

Function, effect, and tasks

Gastrin is synthesized in the so-called G cells of the gastrointestinal tract. G cells are specialized cells that are endocrine active. They are found predominantly in the gastric mucosa and here especially in the area of the gastric glands of the pyloric vestibule (antrum). However, there are also G cells in the first section of the small intestine. The secretion of the hormone is controlled by neuroendocrine cells in the stomach. They release gastrin-releasing peptides (GRP). These in turn stimulate the release of gastrin from the G cells. The parasympathetic nervous system also influences the G cells. In particular, the postganglionic fibers of the tenth cranial nerve (vagus nerve) play an important role here. If the food pulp is very high in protein, increased gastrin is also secreted. The trigger here is the increased concentration of amino acids in the gastric secretion. Stretching of the stomach by food as well as alcohol and caffeine consumption also stimulate gastrin production and secretion. Secretion is inhibited by a pH within the stomach below three. Furthermore, there are several hormones that can inhibit gastrin production. These include somatostatin, secretin, neurotensin, and gastrin inhibiting peptide (GIP).

Formation, occurrence, properties, and optimal levels

The hormone gastrin travels through the bloodstream to target organs. In the stomach, it binds to the gastrin receptors of the occupant cells. The parietal cells are located in the gastric mucosa. They secrete hydrochloric acid and intrinsic factor. Intrinsic factor plays a crucial role in the absorption of vitamin B12 in the intestine. Once gastrin binds to the specific receptors, activation of phospholipase C occurs. This increases the calcium concentration within the vestibular cells. This increase stimulates the vestibular cells to secrete gastric acid. The pH value within the stomach decreases. However, it is not only the vestibular cells that are stimulated by the hormone. The main cells of the stomach also respond to gastrin. The main cells are located in the mucous membrane of the stomach, just like the vestibular cells. Under the influence of gastrin, they produce pepsinogen. Pepsinogen is the inactive precursor of pepsin. Pepsin is a digestive enzyme that is mainly responsible for breaking down proteins. It is only under the action of hydrochloric acid, which is secreted by the vestibular cells, that pepsinogen is activated and becomes active as pepsin. Gastrin also has an influence on histamine production. Histamine is a tissue hormone with many functions. Here, however, it serves primarily to stimulate hydrochloric acid production. Gastrin also stimulates the smooth muscles of the stomach. Gastric peristalsis ensures that the food pulp is mixed. This also emulsifies the fats in the food so that they can be better digested later in the intestines. Outside the stomach, gastrin acts on the pancreas. There, it stimulates the secretion of insulin, glucagon, and somatostatin.

Diseases and disorders

One medical condition in which gastrin plays a critical role is Zollinger-Ellison syndrome. Zollinger-Ellison syndrome is one of the paraneoplastic disorders. Paraneoplastic syndromes occur in association with malignant cancers. Tumors that can cause Zollinger-Ellison syndrome are usually found in the pancreas or small intestine. Because these tumors produce gastrin, they are also called gastrinomas. In a gastrinoma, gastrin is produced independently of food intake. This results in increased formation and secretion of hydrochloric acid.This irritates the mucous membrane of the stomach and small intestine, so that ulcers can develop. Patients suffer from severe abdominal pain and heartburn. If the irritation is severe, bloody vomiting may occur. In about half of the cases, sufferers have diarrhea. Hydrochloric acid deactivates fat-splitting enzymes. This can occasionally lead to fatty stools. In isolated cases, metabolic alkalosis and hyperparathyroidism are observed. The disease is extremely rare, but can occur at any age. However, not only overproduction but also deficiency of gastrin can cause symptoms. Gastrin deficiency can result in gastric hypoacidity. The symptoms of gastric acid deficiency are similar to those of hyperacidity. Affected individuals suffer from bloating, belching and heartburn. There is a deficiency of nutrients and in particular a deficiency of vitamin B12. Hair loss, splintering nails, skin disorders, anemia and osteoporosis can also be seen as indicative of gastrin deficiency. Gastrin can also be used diagnostically in these cases. Type A gastritis is associated with a deficiency of gastric acid. Here, the gastrin level in the blood serum is determined. If hypergastrinemia is present, this indicates reduced acid production. The following applies: The lower the pH value in the stomach, the higher the gastrin level in the blood. The exception here, of course, is Zollinger-Ellison syndrome, where gastrin levels are too high regardless of the pH in the stomach.