Gastrin is a peptide hormone produced by the digestive system, playing a significant role in various physiological processes. It acts as a chemical signal, primarily activating specific functions within the stomach. This hormone is essential for the initial stages of digestion and the maintenance of the stomach lining.
Where Gastrin is Produced
Gastrin is primarily synthesized and released by specialized cells known as G cells. These G cells are predominantly located in the pyloric antrum, which is the lower part of the stomach near the small intestine. Some G cells can also be found in the duodenum, the first section of the small intestine, and occasionally in the pancreas.
G cells secrete gastrin directly into the bloodstream, allowing it to travel to its target organs. This systemic circulation ensures gastrin effectively reaches and influences cells responsible for gastric functions throughout the stomach. Gastrin production is a coordinated process, responding to various internal cues.
Gastrin’s Role in Digestion
Gastrin serves several important functions within the digestive system, notably impacting stomach acid secretion, gastric motility, and the growth of the stomach lining. One of its main roles is to stimulate the release of hydrochloric acid (HCl) by parietal cells in the stomach. This acid is crucial for converting inactive pepsinogen into active pepsin, an enzyme that breaks down proteins. Gastrin directly stimulates parietal cells and also indirectly promotes acid secretion by causing enterochromaffin-like (ECL) cells to release histamine, which then acts on parietal cells.
Beyond acid production, gastrin also influences the movement of the stomach and intestines. It stimulates muscle contractions in the stomach, a process known as gastric motility, aiding in the mixing and breakdown of food. Gastrin helps mobilize partially digested food particles from the stomach into the intestine. It enhances gastric emptying by increasing antral muscle mobility, strengthening contractions, and relaxing the pyloric sphincter.
Gastrin also acts as a trophic, or growth-promoting, factor for the stomach lining. It supports the proliferation and maturation of gastric epithelial cells, including parietal cells and ECL cells. This helps maintain the stomach’s mucosal barrier, protecting it from its own acidic environment. Gastrin also stimulates chief cells to secrete pepsinogen and induces pancreatic secretions and gallbladder emptying, contributing to efficient digestion.
Regulating Gastrin Levels
The body maintains gastrin levels through a sophisticated feedback system. Several factors stimulate gastrin release, primarily related to the presence of food in the stomach. The stretching of the stomach walls and the presence of specific nutrients, especially proteins and amino acids, act as powerful stimuli for gastrin release. An increase in stomach pH (less acidity) also encourages gastrin production.
Conversely, gastrin release is inhibited when the stomach becomes highly acidic. This negative feedback mechanism helps prevent excessive acid production, protecting the stomach lining from damage. The hormone somatostatin, released by D cells in the stomach and pancreas, plays a key role in this inhibition by suppressing gastrin and histamine release. This coordinated regulation ensures that gastrin levels rise when digestion is needed and decrease once food has moved out of the stomach.
Conditions Related to Gastrin Imbalance
Abnormal gastrin levels can lead to various digestive disorders. Elevated gastrin levels, known as hypergastrinemia, often result in an overproduction of stomach acid. A notable condition associated with hypergastrinemia is Zollinger-Ellison syndrome (ZES). This rare disorder is caused by gastrinomas, tumors that secrete excessive gastrin. These tumors most commonly form in the pancreas or duodenum, leading to high levels of gastrin in the blood.
The excess gastrin in ZES causes a substantial increase in stomach acid. This often leads to severe peptic ulcers, chronic heartburn, indigestion, and diarrhea. Gastrinomas can be cancerous and may spread to other parts of the body. While less common, low gastrin levels can also occur, potentially leading to impaired nutrient absorption and an increased risk of infection due to insufficient stomach acid.