Chemistry and Distribution of Secretin
Bayliss and Starling, in 1902, observed that acidifying the luminal contents of an isolated, denervated loop of small intestine caused increased pancreatic secretion. They concluded that acid in the lumen of the small intestine caused release of a substance that was carried by the circulation to the pancreas, where it stimulated pancreatic secretion. They named the substance secretin. It was the first hormone to be discovered and marks the beginning of the science of endocrinology.
Secretin is a 27-amino acid peptide with a helical configuration. It has some amino acid sequences that are the same as those found in VIP, glucagon, and GIP. The entire molecule is required for full biologic activity. It has a half-life of about 4 minutes in the circulation. The kidney is the major site of its removal from the circulation.