Secretin

Secretin
Identifiers
Symbol SCT
External IDs OMIM: 182099 MGI: 99466 HomoloGene: 137358 GeneCards: SCT Gene
Orthologs
Species Human Mouse
Entrez 6343 20287
Ensembl ENSG00000070031 ENSMUSG00000038580
UniProt P09683 Q08535
RefSeq (mRNA) NM_021920 NM_011328
RefSeq (protein) NP_068739 NP_035458
Location (UCSC) Chr 11:
0.63 – 0.63 Mb		 Chr 7:
141.28 – 141.28 Mb
PubMed search

Secretin is a hormone that regulates water homeostasis throughout the body and influences the environment of the duodenum by regulating secretions in the stomach, pancreas, and liver. It is a peptide hormone produced in the S cells of the duodenum, which are located in the intestinal glands.[1] In humans, the secretin peptide is encoded by the SCT gene.[2]

Secretin helps regulate the pH of the duodenum by (1) inhibiting the secretion of gastric acid from the parietal cells of the stomach and (2) stimulating the production of bicarbonate from the centroacinar cells and intercalated ducts of the pancreas.[3] It also stimulates bile production by the liver; the bile emulsifies dietary fats in the duodenum so that pancreatic lipase can act upon them. Meanwhile, in concert with secretin's actions, the other main hormone simultaneously issued by the duodenum, cholecystokinin, is stimulating the gallbladder to contract, delivering its stored bile for the same reason.

Prosecretin is a precursor to secretin, which is present in digestion. Secretin is stored in this unusable form, and is activated by gastric acid in the lower intestine to neutralize the pH and ensure no damage is done to the small intestine by the aforementioned acid.[4]

In 2007, secretin was discovered to play a role in osmoregulation by acting on the hypothalamus, pituitary gland, and kidney.[5][6]

Discovery

Secretin was the first hormone to be identified.[7] In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion.[8] It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion secretin. Secretin was the first such "chemical messenger" identified. This type of substance is now called a hormone, a term coined by Bayliss in 1905.[9]

Structure

Secretin is initially synthesized as a 120 amino acid precursor protein known as prosecretin. This precursor contains an N-terminal signal peptide, spacer, secretin itself (residues 28–54), and a 72-amino acid C-terminal peptide.[2]

The mature secretin peptide is a linear peptide hormone, which is composed of 27 amino acids and has a molecular weight of 3055. A helix is formed in the amino acids between positions 5 and 13. The amino acids sequences of secretin have some similarities to that of glucagon, vasoactive intestinal peptide (VIP), and gastric inhibitory peptide (GIP). Fourteen of 27 amino acids of secretin reside in the same positions as in glucagon, 7 the same as in VIP, and 10 the same as in GIP.[10]

Secretin also has an amidated carboxyl-terminal amino acid which is valine.[11] The sequence of amino acids in secretin is H–His-Ser-Asp-Gly-Thr-Phe-Thr-Ser-Glu-Leu-Ser-Arg-Leu-Arg-Asp-Ser-Ala-Arg-Leu-Gln-Arg-Leu-Leu-Gln-Gly-Leu-Val–NH2.[11]

Physiology

Production

Secretin is synthesized in cytoplasmic secretory granules of S-cells, which are found mainly in the mucosa of the duodenum, and in smaller numbers in the jejunum of the small intestine.[12]

Stimulus

Secretin is released into circulation and/or intestinal lumen in response to low duodenal pH that ranges between 2 and 4.5 depending on species.[13] Also, the secretion of secretin is increased by the products of protein digestion bathing the mucosa of the upper small intestine.[14]

The acidity is due to hydrochloric acid in the chyme that enters the duodenum from the stomach via the pyloric sphincter. Secretin targets the pancreas, which causes the organ to secrete a bicarbonate-rich fluid that flows into the intestine. Bicarbonate is a base that neutralizes the acid, thus establishing a pH favorable to the action of other digestive enzymes in the small intestine and preventing acid burns.[15] Other factors are involved in the release of secretin such as bile salts and fatty acids, which result in additional bicarbonates being added to the small intestine.[16] Secretin release is inhibited by H2 antagonists, which reduce gastric acid secretion. As a result, if the pH in the duodenum increases above 4.5, secretin cannot be released.[17]

Function

Secretin stimulates the release of a watery bicarbonate solution from the pancreatic and bile duct epithelium. Pancreatic centroacinar cells have secretin receptors in their plasma membrane. As secretin binds to these receptors, it stimulates adenylate cyclase activity and converts ATP to cyclic AMP.[18] Cyclic AMP acts as second messenger in intracellular signal transduction and leads to an increase in the release of watery bicarbonate. It is known to promote the normal growth and maintenance of the pancreas.

Secretin increases water and bicarbonate secretion from duodenal Brunner's glands to buffer the incoming protons of the acidic chyme.[19] It also enhances the effects of cholecystokinin to induce the secretion of digestive enzymes and bile from pancreas and gallbladder, respectively.

It counteracts blood glucose concentration spikes by triggering increased insulin release from pancreas, following oral glucose intake.[20]

Although secretin releases gastrin from gastrinomas, it inhibits gastrin release from the normal stomach. It reduces acid secretion by parietal cells of the stomach.[21] It does this through at least three mechanisms: 1) By stimulating release of somatostatin, 2) By inhibiting release of gastrin in the pyloric antrum, and 3) By direct downregulation of the parietal cell acid secretory mechanics.[22] This helps neutralize the pH of the digestive products entering the duodenum from the stomach, as digestive enzymes from the pancreas (e.g., pancreatic amylase and pancreatic lipase) function optimally at slightly basic pH.[19]

In addition, secretin stimulates pepsinogen secretion from chief cells, which can help break down proteins in food digestion. It stimulates release of glucagon, pancreatic polypeptide and somatostatin.[13]

Uses

Secretin has been widely used in medical field especially in pancreatic functioning test because it increases pancreatic secretions. Secretin is either injected[23] or given through a tube that is inserted through nose, stomach then duodenum.[24] This test can provide information about whether there are any abnormalities in the pancreas which can include gastrinoma, pancreatitis or pancreatic cancer.

Secretin has been proposed as a possible treatment for autism based on a hypothetical gut-brain connection; as yet there is no evidence to support it as effective.[25][26]

Osmoregulation

Secretin modulates water and electrolyte transport in pancreatic duct cells,[27] liver cholangiocytes,[28] and epididymis epithelial cells.[29] It is found[30] to play a role in the vasopressin-independent regulation of renal water reabsorption.[5]

Secretin is found in the magnocellular neurons of the paraventricular and supraoptic nuclei of the hypothalamus and along the neurohypophysial tract to neurohypophysis. During increased osmolality, it is released from the posterior pituitary. In the hypothalamus, it activates vasopressin release.[6] It is also needed to carry out the central effects of angiotensin II. In the absence of secretin or its receptor in the gene knockout animals, central injection of angiotensin II was unable to stimulate water intake and vasopressin release.[31]

It has been suggested that abnormalities in such secretin release could explain the abnormalities underlying type D syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH).[6] In these individuals, vasopressin release and response are normal, although abnormal renal expression, translocation of aquaporin 2, or both are found.[6] It has been suggested that "Secretin as a neurosecretory hormone from the posterior pituitary, therefore, could be the long-sought vasopressin independent mechanism to solve the riddle that has puzzled clinicians and physiologists for decades."[6]

Food intake

Secretin and its receptor are found in discrete nuclei of the hypothalamus, including the paraventricular nucleus and the arcuate nucleus, which are the primary brain sites for regulating body energy homeostasis. It was found that both central and peripheral injection of Sct reduce food intake in mouse, indicating an anorectic role of the peptide. This function of the peptide is mediated by the central melanocortin system.[32]

See also

References

  1. ↑ Häcki WH (1980). "Secretin". Clinics in Gastroenterology 9 (3): 609–32. PMID 7000396.
  2. 1 2 Kopin AS, Wheeler MB, Leiter AB (1990). "Secretin: structure of the precursor and tissue distribution of the mRNA". Proceedings of the National Academy of Sciences of the United States of America 87 (6): 2299–303. Bibcode:1990PNAS...87.2299K. doi:10.1073/pnas.87.6.2299. JSTOR 2354038. PMC 53674. PMID 2315322.
  3. ↑ Whitmore TE, Holloway JL, Lofton-Day CE, Maurer MF, Chen L, Quinton TJ, Vincent JB, Scherer SW, Lok S (2000). "Human secretin (SCT): gene structure, chromosome location, and distribution of mRNA". Cytogenetics and Cell Genetics 90 (1–2): 47–52. doi:10.1159/000015658. PMID 11060443.
  4. ↑ Gafvelin G, Jörnvall H, Mutt V (Sep 1990). "Processing of prosecretin: isolation of a secretin precursor from porcine intestine" (PDF). Proceedings of the National Academy of Sciences of the United States of America 87 (17): 6781–5. doi:10.1073/pnas.87.17.6781. PMC 54621. PMID 2395872.
  5. 1 2 Chu JY, Chung SC, Lam AK, Tam S, Chung SK, Chow BK (2007). "Phenotypes developed in secretin receptor-null mice indicated a role for secretin in regulating renal water reabsorption". Molecular and Cellular Biology 27 (7): 2499–511. doi:10.1128/MCB.01088-06. PMC 1899889. PMID 17283064.
  6. 1 2 3 4 5 Chu JY, Lee LT, Lai CH, Vaudry H, Chan YS, Yung WH, Chow BK (2009). "Secretin as a neurohypophysial factor regulating body water homeostasis". Proceedings of the National Academy of Sciences of the United States of America 106 (37): 15961–6. Bibcode:2009PNAS..10615961C. doi:10.1073/pnas.0903695106. JSTOR 40484830. PMC 2747226. PMID 19805236.
  7. ↑ Henriksen JH, Schaffalitzky de Muckadell OB (2002). "Sekretin - det første hormon" [Secretin--the first hormone]. Ugeskrift for Laeger (in Danish) 164 (3): 320–5. PMID 11816326. INIST:13419424.
  8. ↑ Bayliss WM, Starling EH (1902). "The mechanism of pancreatic secretion". The Journal of Physiology 28 (5): 325–53. doi:10.1113/jphysiol.1902.sp000920. PMC 1540572. PMID 16992627.
  9. ↑ Hirst, BH (2004), "Secretin and the exposition of hormonal control", J Physiol 560: 339, doi:10.1113/jphysiol.2004.073056, PMC 1665254, PMID 15308687.
  10. ↑ Williams, Robert L. (1981). Textbook of Endocrinology. Philadelphia: Saunders. p. 697. ISBN 0-7216-9398-9.
  11. 1 2 DeGroot, Leslie Jacob (1989). McGuigan, J. E., ed. Endocrinology. Philadelphia: Saunders. p. 2748. ISBN 0-7216-2888-5.
  12. ↑ Polak JM, Coulling I, Bloom S, Pearse AG (1971). "Immunofluorescent localization of secretin and enteroglucagon in human intestinal mucosa". Scandinavian Journal of Gastroenterology 6 (8): 739–44. doi:10.3109/00365527109179946. PMID 4945081.
  13. 1 2 Frohman, Lawrence A.; Felig, Philip (2001). "Gastrointestinal Hormones and Carcinoid Syndrome". In Ghosh, P. K.; O'Dorisio, T. M. Endocrinology & metabolism. New York: McGraw-Hill, Medical Pub. Div. pp. 1675–701. ISBN 0-07-022001-8.
  14. ↑ Ganong, William F. (2003). "Regulation of Gastrointestinal Function". Review of Medical Physiology (21st ed.). New York: McGraw-Hill, Medical Pub. Div. ISBN 0-07-140236-5.
  15. ↑ http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/gi/secretin.html
  16. ↑ Osnes M, Hanssen LE, Flaten O, Myren J (1978). "Exocrine pancreatic secretion and immunoreactive secretin (IRS) release after intraduodenal instillation of bile in man". Gut 19 (3): 180–4. doi:10.1136/gut.19.3.180. PMC 1411891. PMID 631638.
  17. ↑ Rominger JM, Chey WY, Chang TM (1981). "Plasma secretin concentrations and gastric pH in healthy subjects and patients with digestive diseases". Digestive Diseases and Sciences 26 (7): 591–7. doi:10.1007/BF01367670. PMID 7249893.
  18. ↑ Gardner, JD (1978). "Receptors and gastrointestinal hormones". In Sleisenger, MH; Fordtran, JS. Gastrointestinal Disease (2nd ed.). Philadelphia: WB Saunders Company. pp. 179–95.
  19. 1 2 Hall, John E.; Guyton, Arthur C. (2006). Textbook of medical physiology. St. Louis, Mo: Elsevier Saunders. pp. 800–1. ISBN 0-7216-0240-1.
  20. ↑ Kraegen EW, Chisholm DJ, Young JD, Lazarus L (1970). "The gastrointestinal stimulus to insulin release. II. A dual action of secretin". The Journal of Clinical Investigation 49 (3): 524–9. doi:10.1172/JCI106262. PMC 322500. PMID 5415678.
  21. ↑ Palmer, KR; Penman, ID (2010). "Alimentary track and pancreatic disease". In Colledge, NR; Walker, BR; Ralston, SH. Davidson's Principles and Practice of Medicine (20th ed.). Edinburgh: Churchill Livingstone. p. 844. ISBN 0-7020-3085-6.
  22. ↑ Boron, Walter F.; Boulpaep, Emile L. (2012). "Acid secretion". Medical Physiology (2nd ed.). Philadelphia: Saunders. p. 1352. ISBN 978-1-4377-1753-2.
  23. ↑ "Human Secretin". Patient Information Sheets. United States Food and Drug Administration. 2004-07-13. Archived from the original on May 11, 2009. Retrieved 2008-11-01.
  24. ↑ "Secretin stimulation test". MedlinePlus Medical Encyclopedia. United States National Library of Medicine. Retrieved 2008-11-01.
  25. ↑ "The Use of Secretin to Treat Autism". NIH News Alert. United States National Institutes of Health. 1998-10-16. Retrieved 2008-11-30.
  26. ↑ Sandler AD, Sutton KA, DeWeese J, Girardi MA, Sheppard V, Bodfish JW (1999). "Lack of benefit of a single dose of synthetic human secretin in the treatment of autism and pervasive developmental disorder". The New England Journal of Medicine 341 (24): 1801–6. doi:10.1056/NEJM199912093412404. PMID 10588965.
  27. ↑ Villanger O, Veel T, Raeder MG (1995). "Secretin causes H+/HCO3- secretion from pig pancreatic ductules by vacuolar-type H(+)-adenosine triphosphatase". Gastroenterology 108 (3): 850–9. doi:10.1016/0016-5085(95)90460-3. PMID 7875488.
  28. ↑ Marinelli RA, Pham L, Agre P, LaRusso NF (1997). "Secretin promotes osmotic water transport in rat cholangiocytes by increasing aquaporin-1 water channels in plasma membrane. Evidence for a secretin-induced vesicular translocation of aquaporin-1". The Journal of Biological Chemistry 272 (20): 12984–8. doi:10.1074/jbc.272.20.12984. PMID 9148905.
  29. ↑ Chow BK, Cheung KH, Tsang EM, Leung MC, Lee SM, Wong PY (2004). "Secretin controls anion secretion in the rat epididymis in an autocrine/paracrine fashion". Biology of Reproduction 70 (6): 1594–9. doi:10.1095/biolreprod.103.024257. PMID 14749298.
  30. ↑ Cheng CY, Chu JY, Chow BK (2009). "Vasopressin-independent mechanisms in controlling water homeostasis". Journal of Molecular Endocrinology 43 (3): 81–92. doi:10.1677/JME-08-0123. PMID 19318428.
  31. ↑ Lee VH, Lee LT, Chu JY, Lam IP, Siu FK, Vaudry H, Chow BK (2010). "An indispensable role of secretin in mediating the osmoregulatory functions of angiotensin II". FASEB Journal 24 (12): 5024–32. doi:10.1096/fj.10-165399. PMC 2992369. PMID 20739612.
  32. ↑ Cheng CY, Chu JY, Chow BK (2011). "Central and peripheral administration of secretin inhibits food intake in mice through the activation of the melanocortin system". Neuropsychopharmacology 36 (2): 459–71. doi:10.1038/npp.2010.178. PMC 3055665. PMID 20927047.

Further reading

External links

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