Secretin and Duodenal Regulation

Secretin is the hormone that protects the small intestine from gastric acid. Released from S cells of the duodenum when acidic chyme arrives from the stomach, it stimulates the pancreas and bile ducts to secrete a watery, bicarbonate-rich fluid that neutralises the acid. Discovered in 1902, secretin was the first substance shown to act as a hormone, founding the field of endocrinology.

Definition

Secretin is a peptide hormone released by enteroendocrine S cells of the duodenum in response to acidic chyme, which stimulates the pancreatic duct cells and biliary epithelium to secrete a bicarbonate-rich alkaline fluid that neutralises duodenal acid, and which also inhibits gastric acid secretion and emptying.

Scope

The topic covers the release of secretin from duodenal S cells in response to acid, its stimulation of bicarbonate-rich pancreatic and biliary secretion, its inhibitory effect on gastric acid, and its place alongside cholecystokinin in controlling the duodenal phase of digestion. It is a physiology reference entry and does not provide clinical management advice.

Core questions

• What triggers secretin release from the duodenum?
• How does secretin produce a bicarbonate-rich pancreatic and biliary secretion?
• How does neutralising duodenal acid protect the intestine and aid digestion?
• How do secretin and cholecystokinin divide the control of pancreatic secretion?

Key concepts

• S cells of the duodenum
• Acid-stimulated release
• Pancreatic and biliary bicarbonate secretion
• Neutralisation of duodenal chyme
• Inhibition of gastric acid secretion
• Complementarity with cholecystokinin
• The first identified hormone

Mechanisms

When acidic chyme leaves the stomach and lowers the pH of the duodenum, enteroendocrine S cells release secretin into the blood. Secretin acts mainly on the duct cells of the pancreas and the epithelium of the bile ducts, stimulating them to secrete a copious, watery fluid rich in bicarbonate. This alkaline secretion neutralises the gastric acid in the duodenum, protecting the intestinal mucosa and creating the near-neutral environment that pancreatic digestive enzymes require to work. Secretin also tends to inhibit gastric acid secretion and to slow gastric emptying, reinforcing the protective response. It works in concert with cholecystokinin: secretin chiefly supplies the bicarbonate-rich fluid while cholecystokinin supplies the enzymes, so that the two hormones together regulate the composition of pancreatic juice during the duodenal phase of digestion.

Clinical relevance

Secretin physiology underlies the understanding of pancreatic ductal secretion and the neutralisation of gastric acid in the duodenum, and the secretin-stimulated response is a long-standing reference concept in tests of pancreatic function. This entry describes normal physiology and is not a basis for individual diagnosis or treatment.

History

In 1902 Bayliss and Starling showed that placing acid in a denervated loop of duodenum still produced pancreatic secretion, proving that an acid-triggered chemical messenger from the intestine, which they named secretin, travelled in the blood to stimulate the pancreas. This was the first demonstration of a hormone, and Starling later coined the word hormone itself. Later work characterised secretin's structure, its release from S cells, and its role in bicarbonate secretion, cementing its place as a founding example of endocrine regulation.

Key figures

• William Bayliss
• Ernest Starling
• Wai Yee Chey
• Ta-Min Chang

Seminal works

• bayliss-starling-1902
• chey-2014

Frequently asked questions

What is the main role of secretin?

To stimulate the pancreas and bile ducts to release a bicarbonate-rich, alkaline fluid that neutralises the acid arriving in the duodenum from the stomach, protecting the intestine and allowing digestive enzymes to work.

Why is secretin historically important?

It was the first substance shown, by Bayliss and Starling in 1902, to act as a blood-borne chemical messenger, which led to the concept of hormones and the founding of endocrinology.