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Gastrin is a linear peptide hormone secreted into the blood by G cells that are located mainly in the pylorus of the stomach. Gastrin is found primarily in three forms: gastrin-34 ("big gastrin"), gastrin-17 ("little gastrin"), and gastrin-14 ("minigastrin"). The numbers refer to the amino acid count.
Gastrin is produced in response to certain stimuli. Among these stimuli are stomach distension, amino acid stimulation, vagal stimulation (mediated by the neurocrine bombesin), the presence of partially digested proteins, and hypercalcemia.
The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl). It also causes chief cells to secrete pepsinogen, the zymogen (inactive) form of the digestive enzyme pepsin. Pepsin becomes active at a low pH (2-4), and the HCl provides a suitable environment for its activity.
Gastrin release is stimulated by the vagal nerve which innervates the G cells; distension of the stomach, the presence of peptides and amino acids in the stomach, and blood-borne factors such as calcium and adrenaline (epinephrine).
It is important for gastrin release to be regulated, because too much secreted gastrin would, in turn, cause too much acid to be secreted. Therefore, via a negative feedback enzyme mechanism, the presence of acid (primarily the secreted HCl) in the stomach inhibits further release of gastrin by the G cells. Somatostatin also inhibits the release of gastrin, along with secretin, GIP, VIP, glucagon and calcitonin.
Role in disease
In the Zollinger-Ellison syndrome, gastrin is produced at excessive levels, often by a gastrinoma of the antrum or the pancreas. To investigate for hypergastrinemia (high blood levels of gastrin), a "pentagastrin test" can be performed.
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