Definition
Gastrin is a major physiological regulator of gastric acid secretion. It also has an important trophic or growth-promoting influence on the gastric mucosa.
Related peptides
Progastrin is synthesized in antral G cells and processed into a number of bioactive peptides. The heptadecapeptide gastrin-17 is the main product (Gastrin component 3 or little Gastrin), called also Cholecystokinin B1. Gastrin-17 amide corresponds to Progastrin-(55-71). Glycine-extended nonsulfated gastrin-17 corresponds to Progastrin-(55-72). A minor fraction of gastrin-17 is cleaved in G cells and released as short COOH-terminal peptides, as a mixture of gastrin-7, gastrin-6, and gastrin-52. The sulfated gastrin-6 is the predominant form released to antral venous blood. Gastrin 6 has a higher potency but a lower efficacy than gastrin-17. The efficacy of gastrin-6 is increased by tyrosine O-sulfation, which also enhances the protection against elimination. Cryptagastrin corresponds to progastrin-(1-35). Cryptagastrin-(6-35) (progastrin-(6-35) is a shorter form of this peptide3. The two largest alpha-carboxyamidated progastrin products are gastrin-71 and gastrin-52. Gastrin-52 is bioactive with an efficacy close to or similar to that of gastrin-174. Gastrin component 1 is the largest hormonally active form of gastrin and has been shown to correspond to gastrin-71. Gastrin-14 has been termed gastrin component 4 or minigastrin5. Pentagastrin corresponds the five C-terminal amino acids of gastrin and is the same as CCK-5 [cholecystokinin-5], which acts mainly through the type B cholecystokinin receptor.
Discovery
Gastrin was identified originally by Edkins and Cantab in 19056, as a factor produced in the antrum of the stomach that stimulates gastric acid secretion. It was purified from hog antral mucosa and sequenced by Gregory and Tracy in 19647.
Structural Characteristics
Gastrin is a linear peptide that is synthesized as a preprohormone, Progastrin, which is processed in antral G cells to a number of bioactive gastrins of different length, which share the same alpha-amidated C-terminus containing the active site. The active site of gastrin is the carboxyterminal tetrapeptide amide Trp-Met-Asp-Phe-NH2 and all bioactive fragments have the carboxyterminal hexasequence Tyr (SO4)-Gly-Trp-Met-Asp-Phe-NH2. The lengths of the aminoterminal extensions govern metabolism and clearance of the circulating hormone forms. All bioactive gastrin peptides are carboxyamidated and exist in nonsulfated and sulfated forms8.
Mode of Action
The gastrin receptor is also one of the receptors that bind cholecystokinin, and is known as the CCK-B receptor. It is a member of the G protein-coupled receptor family. Binding of gastrin stimulates an increase in intracellular Ca++, activation of protein kinase C, and production of inositiol phospate9.
Functions
Gastrin appears to have at least two major effects on gastrointestinal function:
* Stimulation of gastric acid secretion: Gastrin receptors are found on parietal cells, and binding of gastrin, along with histamine and acetylcholine, leads to fully-stimulated acid secretion by those cells. Enterochromaffin-like (ECL) cells also bear gastrin receptors, and recent evidence indicates that this cell may be the most important target of gastrin with regard to regulating acid secretion. Stimulation of ECL cells by gastrin leads to histamine release, and histamine binding to H2 receptors on parietal cells is necessary for full-blown acid secretion.
* Promotion of gastric mucosal growth: Gastrin clearly has the ability to stimulate many aspects of mucosal development and growth in the stomach. Treatment with gastrin stimulates DNA, RNA and protein synthesis in gastric mucosa and increases the number of parietal cells. Another observation supporting this function is that humans with hypergastrinemia (abnormally high blood levels of gastrin) consistently show gastric mucosal hypertrophy.
In addition to parietal and ECL cell targets, gastrin also stimulates pancreatic acinar cells via binding to cholecystokinin receptors, and gastrin receptors have been demonstrated on certain populations of gastric smooth muscle cells, supporting pharmacologic studies that demonstrate a role for gastrin in regulating gastric motility.
References
1. Morley JS, Tracy HJ, Gregory RA (1965). Structure-function relationships in the active C-terminal tetrapeptide sequence of gastrin. Nature, 207, 1356-1359.
2. Rehfeld JF, Hansen CP, Johnsen AH (1995). Postpoly(Glu) cleavage and degradation modified by O-sulfated tyrosine: a novel posttranslational processing mechanism. EMBO Journal, 14, 389-396.
3. Huebner VD, Jiang RL, Lee TD, Legesse K, Walsh JH, Shively JE, Chew P, Azumi T, Reeve JR Jr (1991). Purification and structural characterization of progastrin-derived peptides from a human gastrinoma. J Biol. Chem., 266(19), 12223-12227.
4. Hansen CP, Stadil F, Rehfeld JF (1995). Metabolism and influence of gastrin-52 on gastric acid secretion in humans. Amer. J of Physiol., 269(4 Pt 1), G600-605.
5. Gregory RA, Tracy HJ, Harris JI, Runswick MJ, Moore S, Kenner GW, Ramage R (1979). Minigastrin: corrected structure and synthesis. Hoppe Seylers Zeitschrift für Physiologische Chemie, 360, 73-80.
6. Edkin JS and Cantab MB (1905). On the chemical mechanism of gastric secretion. Proc. of the Royal Society London, 76, 376.
7. Gregory H, Hardy PM, Jones DS, Kenner GW, Sheppard RC (1964). The antral hormone gastrin. Structure of gastrin. Nature, 204, 931-3.
8. Rehfeld JF and Larsson LI (1979). The predominating molecular form of gastrin and cholecystokinin in the gut is a small peptide corresponding to their COOH-terminal tetrapeptide amide. Acta Physiologica Scandinavia, 115, 117-119.
9. Dockray GJ (1999). Topical review. gastrin and gastric epithelial physiology. J Physiol., 518(2), 315-324.
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