Gastrin

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G cell is visible near bottom left, and gastrin is labeled as the two black arrows leading from it.
Gastrin
Identifiers
Symbol	GAST
HUGO	4164
Entrez	2520
OMIM	137250
RefSeq	NM_000805
UniProt	P01350
Other data
Locus	Chr. 17 q21

In humans, gastrin is a hormone that stimulates secretion of gastric acid by the stomach. It is released by G cells in the stomach and duodenum.

1 Physiology
2 Role in disease
3 External links

[edit] Physiology

[edit] Genetics

The GAS gene is located on the long arm of the seventh chromosome (17q21).

[edit] Synthesis

Gastrin is a linear peptide hormone produced by G cells of the duodenum and in the pyloric antrum of the stomach. It is secreted into the bloodstream. Gastrin is found primarily in three forms:

gastrin-34 ("big gastrin")
gastrin-17 ("little gastrin")
gastrin-14 ("minigastrin")

The numbers refer to the amino acid count.

[edit] Release

Gastrin is released in response to certain stimuli. These include:

stomach distension
vagal stimulation (mediated by the neurocrine bombesin, or GRP in the human)
the presence of partially digested proteins especially amino acids
hypercalcemia

Gastrin release is inhibited by:

The presence of acid (primarily the secreted HCl) in the stomach (a case of negative feedback).
Somatostatin also inhibits the release of gastrin, along with secretin, GIP, VIP, glucagon and calcitonin.

[edit] Function

The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl)/gastric acid. This is done either directly or indirectly:

Directly: binds onto CCK2/gastrin receptors on parietal cells stimulating them to secrete H+ ions.
Indirectly: binds onto CCK2/gastrin receptors on ECL cells in the stomach, which then responds by releasing histamine, which in turn acts in a paracrine manner on parietal cells stimulating them to secrete H+ ions. This is the major stimulus for acid secretion by ECL cells.

It also causes chief cells to secrete pepsinogen, the zymogen (inactive) form of the digestive enzyme pepsin. Pepsinogen is converted to pepsin in a low pH environment, and the HCl provides a suitable environment for its activity. It can also increase antral muscle mobility and trophic effect on GI tract and causes promotion of contraction of circular muscle of the stomach.

Gastrin has also been shown to induce production of pancreatic enzymes by acinar cells.

It increases gastric blood flow.

[edit] Factors influencing secretion

Gastric lumen:

Stimulatory factors: dietary protein and amino acids, hypercalcemia. (i.e. during the gastric phase)
Inhibitory factor: acidity (pH below 3) - a negative feedback mechanism, exerted via the release of somatostatin from δ cells in the stomach, which inhibits gastrin and histamine release.

Paracrine:

Stimulatory factor: bombesin
Inhibitory factor: somatostatin - acts on somatostatin-2 receptors on G cells. in a paracrine manner via local diffusion in the intercellular spaces, but also systemically through its release into the local mucosal blood circulation; it inhibits acid secretion by acting on parietal cells.

Nervous:

Stimulatory factors: Beta-adrenergic agents, cholinergic agents, gastrin-releasing peptide (GRP)

Circulation:

Stimulatory factor: epinephrine
Inhibitory factors:gastric inhibitory peptide (GIP), secretin, somatostatin, glucagon, calcitonin

[edit] Role in disease

In the Zollinger-Ellison syndrome, gastrin is produced at excessive levels, often by a gastrinoma (gastrin-producing tumor, mostly benign) of the antrum or the pancreas. To investigate for hypergastrinemia (high blood levels of gastrin), a "pentagastrin test" can be performed.