Secretin is a hormone that controls secretions into the duodenum and water homeostasis in the body. It is first produced in the S cells in the duodenum during the crypts of Lieberkuhn.  Secretin works by regulating the pH of duodenal by controlling gastric acid secretion and buffering by utilizing the bicarbonate found in centroacinar cells in the pancreas.  It is also noted to be the first hormone to be identified.  The secretin protein in the human genome is encoded by the SCT Gene. 
During the 1900s, Ernest Starling and William Bayliss were studying how the human nervous system controls how the body digests food.  It was already known that the pancreas helped during the digestion by secreting juices to help pass food through the pyloric sphincter into the duodenum. Starling and Bayliss discovered that this process was not controlled by the pancreas but by the nervous system. They did this by first cutting all of the nerves to the pancreases in their model animals and observing how the digestive system responded. Starling and Bayliss determined that the substance that was secreted during digestion by the intestinal lining initiated the pancreas secretion through the bloodstream. They named this this secretion secretin, being the first chemical messenger identified. These types of substances were called hormones, a term Bayliss coined in 1905.
Secretin was synthesized as a 120 amino acid precursor protein also known as prosecretin. Prosecretin contains an n-terminus, secretin, spacer, and a c-terminus. Secretin is a linear peptide that is made up out of 27 different amino acids with a molecular weight of 3055. It also forms an alpha-helical structure between the 5 and 13th amino acid positions. Some of the amino acid sequences present in secretin have similarities of those amino acids in glyucagon, gastric inhibitory peptide (GIP) and vasoactive intestinal peptide (VIP). Fourteen out of the twenty-seven amino acids in secretin are in the same positions in the protein glucagon, 10 in GIP, and 7 in VIP. 
Secretin works by increasing the bicarbonate concentration in the pancreas. The pancreas contains centroacinar cells that have secretin receptors located on their plasma membrane. When secretin binds to the receptors on the membrane, it initiates adenylate cyclase activity and converts cyclic AMP from ATPCyclic AMP is a second messenger in intracellular signal transduction and increases the concentration of carbonate. It is also known to promote the growth and maintenance of the pancreas. Secretin increases water and bicarbonate secretion in the duodenal to buffer incoming protons of the acidic chyme In addition, it also enhances the effects of cholecystokinin to produce secretion of digestive enzymes from the gallbladder and pancreas. It also decreases the blood glucose concentration by increasing insulin being released by the pancreas In addition, secretin stimulates pepsin secretion from chief cells, which then help break down proteins during digestion. It also stimulates the release of glycagon, somatostatin, and pancreatic polypeptide. 
- Bayliss W, Starling EH (1902). "The mechanism of pancreatic secretion". J. Physiol. (London) 28: 325–353. Invalid
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- DeGroot, Leslie Jacob (1989). J. E. McGuigan. ed. Endocrinology. Philadelphia: Saunders. pp. 2748. ISBN 0-7216-2888-5. Invalid
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