Gastrin
In humans, gastrin is a linear peptide hormone secreted into the blood mainly by G cells in the antrum and pylorus of the stomach in response to certain stimuli. Among these stimuli are stomach distension, amino acid stimulation, vagal stimulation (mediated by the neurocrine bombesin), and the presence of partially digested proteins.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, and the HCl provides a suitable environment for its activity.
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 the release of gastrin by G cells. Somatostatin also inhibits the release of gastrin.
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.