CCK versus secretin: hormonal control of pancreatic secretion
Cholecystokinin (CCK), a hormone released by fat and protein in the duodenum, is the dominant hormonal drive to pancreatic enzyme secretion, while secretin, released by duodenal acid, drives bicarbonate-rich fluid.
Remember
CCK, with the vagus, empties the gallbladder and drives the pancreatic enzymes; secretin, released after acid arrives, drives the bicarbonate-rich fluid.
Memory aid
secretin brings the sea
A way to remember that secretin goes with the water and bicarbonate that wash the juice along, while CCK goes with the enzymes and the gallbladder.
The exocrine pancreas (the part that secretes into the gut through ducts) has two products, and each has its own controllers. The acinar cells, the enzyme-making cells of the pancreas, make an enzyme-rich fluid. The duct cells, which line the small tubes that drain the pancreas, add bicarbonate and water.
Cholecystokinin (CCK) is released by cells in the lining of the duodenum when fatty acids and amino acids arrive in the chyme (the partly digested food leaving the stomach). It stimulates the acinar cells, so the enzymes needed for the meal arrive when the fat and protein do. CCK also contracts the gallbladder and relaxes the sphincter of Oddi (the muscular valve at the opening of the bile and pancreatic ducts into the duodenum), so bile enters at the same time. Vagal parasympathetic stimulation amplifies the enzyme output.
Secretin is triggered by acid. When acidic chyme lowers the pH in the duodenum, S cells in the duodenal lining release secretin. It acts on the duct cells and stimulates a bicarbonate-rich fluid. Bicarbonate is the alkali that neutralises the acid, protecting the duodenal lining and giving the digestive enzymes the near-neutral pH in which they work. Secretin also stimulates the bile ducts to secrete bicarbonate and water.
The practical rule is that CCK and the vagus mainly drive the enzymes, while secretin drives the alkaline fluid. Bile and pancreatic juice then enter the duodenum together at the ampulla, where the common bile duct and the pancreatic duct meet.
Key facts
- CCK trigger
- fatty acids and amino acids in the chyme release it from cells in the duodenal lining.
- CCK actions
- it stimulates the acinar cells to secrete an enzyme-rich fluid, and it contracts the gallbladder.
- Secretin trigger
- acid chyme lowers the duodenal pH, so S cells in the duodenal lining release it.
- Secretin action
- it makes the pancreatic duct cells secrete a bicarbonate-rich fluid.
- Vagus
- vagal parasympathetic stimulation amplifies enzyme secretion.
- Purpose of bicarbonate
- it neutralises the acid and gives the enzymes a near-neutral pH.
Exam traps
- CCK is not the main stimulus for bicarbonate-rich ductal fluid, because bicarbonate and water from the pancreatic ducts are mainly driven by secretin.
- CCK does not drive enzyme secretion in the duct cells, because the enzymes come from the acinar cells and the duct cells add bicarbonate-rich fluid.
- The sympathetic nervous system does not drive pancreatic enzyme secretion, because the nervous stimulation of pancreatic secretion comes from the parasympathetic system, through the vagus.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Gastrointestinal System, with the answer and a full explanation after every question.
Practice these questions