Gastrin
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[edit] Description
In humans, gastrin is a hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach, as well as aiding in gastric motility. It is released by G cells in the stomach, duodenum, and the pancreas. Its existence was first suggested in 1905 by the British physiologist John Sydney Edkins,<ref>Edkins JS (1906). "The chemical mechanism of gastric secretion". J. Physiol. (Lond.) 34 (1-2): 133–44. PMID 16992839. PMC:1465807.</ref><ref>Modlin IM, Kidd M, Marks IN, Tang LH (1997). "The pivotal role of John S. Edkins in the discovery of gastrin". World J Surg 21 (2): 226–34. doi:. PMID 8995084.</ref> and gastrins were isolated in 1964 by Gregory and Tracy in Liverpool.<ref>Gregory RA, Tracy HJ (1964). "The constitution and properties of two gastrins extracted from hog antral mucosa". Gut 5: 103–14. doi:. PMID 14159395. PMC:1552180.</ref>
[edit] Physiology
[edit] Genetics
The GAS gene is located on the long arm of the seventeenth chromosome (17q21).<ref>Lund T, Geurts van Kessel AH, Haun S, Dixon JE (1986). "The genes for human gastrin and cholecystokinin are located on different chromosomes". Hum. Genet. 73 (1): 77–80. doi:. PMID 3011648.</ref>
[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")
Also, pentagastrin is an artificially synthesized, five amino acid sequence identical to the last five amino acid sequence at the C-terminus end of gastrin.
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 humans)
- 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 (gastroinhibitory peptide), 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 indirectly via binding 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 parietal cells.
Direct binding of gastrin to the parietal cells is involved in parietal cell maturation and fundal growth.
Gastrin 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. In digestion, gastrin strengthens the antral contractions against the pylorus, and constricts the pyloric sphincter, which has the effect of slowing the rate of gastric emptying.
Gastrin has also been shown to induce production of pancreatic enzymes by centroacinar 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 pyloric antrum or the pancreas. To investigate for hypergastrinemia (high blood levels of gastrin), a "pentagastrin test" can be performed.
In autoimmune gastritis, the immune system attacks the parietal cells leading to hypochlorhydria (low stomach acidity). This results in an elevated gastrin level in an attempt to compensate for low acidity. Eventually, all the parietal cells are lost and achlorhydria results leading to a loss of negative feedback on gastrin secretion.
[edit] Source Organism
Capra hircus (Goat).
[edit] Taxomomy
Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Euteleostomi;Mammalia; Eutheria; Laurasiatheria; Cetartiodactyla; Ruminantia;Pecora; Bovidae; Caprinae; Capra.
[edit] Subcellular Localization
Secreted.
[edit] Developmental Stage
[edit] Similarity
Belongs to the gastrin/cholecystokinin family.
[edit] Post translational Modification
[edit] Function
Gastrin stimulates the stomach mucosa to produce and secrete hydrochloric acid and the pancreas to secrete its digestive enzymes. It also stimulates smooth muscle contraction and increases blood circulation and water secretion in the stomach and intestine
