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CCK-PZ (or more commonly called CCK) is a single hormone. It is called so because, initially, it was thought that CCK and PZ were two separate hormones having two different actions; later on, it was found that both the hormones were one and the same.


A. Site of production

I cells of the upper small intestine.

B. Other sites where CCK is found

  1. nerves in distal ileum and colon (and also in nerves in other parts of the body)
  2. neurons in the brain (particularly in the cerebral cortex)


C. Actions

  1. contraction of the gall bladder
  2. secretion of pancreatic juice rich in enzymes
  3. augments the action of secretin in producing secretion of an alkaline pancreatic juice
  4. inhibits gastric emptying
  5. has a trophic effect on the pancreas
  6. increases the secretion of enterokinase
  7. may increase motility of small intestine and colon
  8. together with secretin, it may increase the contraction of the pyloric sphincter (thus, preventing the reflux of duodenal contents into the stomach
  9. CCK (and gastrin) stimulate glucagons secretion (note that the secretion of both CCK and gastrin increases after a protein meal.
  10. CCK in the brain may have a role in regulation of food intake; it may also have a role in the production of anxiety and analgesia. 

D. CCK receptors

  1. Types : CCK-A and CCK-B receptors.
  2. Site : CCK-A receptors are mainly located in the periphery, whereas both CCK-A and CCK-B are found in the brain.
  3. Mechanism of action : Both activate phospholipase C (PLC), causing increased production of IP3 and DAG.
  4. Food digestion products and CCK secretion Products of food digestion, particularly peptides and amino acids, increase CCK secretion. Fatty acids (with more than 10 carbon atoms) also increase CCK secretion.
  5. Positive feedback mechanism
  6. CCK increases bile and pancreatic juice secretion more digestion of protein and fat further increases CCK secretion
  7. This positive feedback mechanism stops when the products of food digestion move on to the lower portions of the GIT
  8. Secretin
  9. Secretin was the first hormone to be discovered (by Bayliss and Starling in the year 1902; starling coined the term 'hormone'.
  • Structure
It is a polypeptide; M.W. : 5000, consists of 27 amino acids. Its structure is different from that of gastrin and CCK but similar to that of glucagon, VIP, GLI, and GIP. There is only one form of secretin (in other words, it does not show microheterogeneity). It is secreted as prosecretin (inactive); it gets converted by gastric HCl and salts of fatty acids (soaps) into secretin (active).
  • Site of secretion:

Secretin is secreted by S cells present in the upper small intestine. The half-life of secretin is 5 minutes.



E. Actions

  • It acts on the duct cells of the pancreas and that of the biliary tract to increase bicarbonate secretion.
  • Thus, it produces a watery alkaline pancreatic juice, but poor in enzymes. Its effect on the pancreas is mediated by cAMP. 
  • The volume of the flow of juice is directly proportional to the dose of secretin given intravenously. As the volume of pancreatic secretion increases, its chloride concentration falls and bicarbonate concentration rises. This is because, bicarbonate is secreted in the small ducts but is reabsorbed in the large ducts in exchange for chloride. The magnitude of this exchange is inversely proportional to the rate of flow.
  • It potentiates the action of CCK (thus, helping in production of pancreatic secretion rich in enzymes)
  • It decreases gastric acid secretion (secretin is the body’s natural antacid)
  • It may cause contraction of the pyloric sphincter and thus delay gastric emptying. This may prevent the reflux of duodenal contents into the stomach.

F. Factors increasing secretin secretion

  1. Products of protein digestion
  2. Acid in the upper small intestine
  3. Acid released from the stomach reaches the upper small intestine and causes increased secretion of
  4. secretin; this is another example of negative feedback control as shown below :
  5. Acid increases secretin secretion which causes secretion of alkaline pancreatic juice → this neutralizes the acid à decreases secretin secretion 

G. Note : CCK-PZ acts on acinar cells of pancreas and stimulates secretion of enzyme-rich pancreatic juice

Secretin acts on the duct cells of the pancreas and stimulates secretion of alkaline (bicarbonate-rich), watery pancreatic juice.

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