Control of Pancreatic Exocrine Secretion

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Control of Pancreatic Exocrine Secretion

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As you might expect, secretion from the exocrine pancreas is regulated by

both neural and endocrine controls. During interdigestive periods, very little

secretion takes place, but as food enters the stomach and, a little later, chyme

flows into the small intestine, pancreatic secretion is strongly stimulated.

Like the stomach, the pancreas is innervated by the vagus nerve, which

applies a low level stimulus to secretion in response to anticipation of a meal.

However, the most important stimuli for pancreatic secretion comes from three

hormones secreted by the enteric endocrine system:

a.. Cholecystokinin: This hormone is synthesized and secreted by enteric

endocrine cells located in the duodenum. Its secretion is strongly stimulated by

the presence of partially digested proteins and fats in the small intestine. As

chyme floods into the small intestine, cholecystokinin is released into blood

and binds to receptors on pancreatic acinar cells, ordering them to secrete

large quantities of digestive enzymes.

b.. Secretin: This hormone is also a product of endocrinocytes located

in the epithelium of the proximal small intestine. Secretin is secreted (!) in

response to acid in the duodenum, which of course occurs when acid-laden chyme

from the stomach flows through the pylorus. The predominant effect of secretin

on the pancreas is to stimulate duct cells to secrete water and bicarbonate. As

soon as this occurs, the enyzmes secreted by the acinar cells are flushed out of

the pancreas, through the pancreatic duct into the duodenum.

c.. Gastrin: This hormone, which is very similar to cholecystokinin, is

secreted in large amounts by the stomach in response to gastric distention and

irritation. In addition to stimulating acid secretion by the parietal cell,

gastrin stimulates pancreatic acinar cells to secrete digestive enzymes.

Stop and think about this for a minute - control of pancreatic secretion

makes perfect sense. Pancreatic secretions contain enzymes which are needed to

digest proteins, starch and triglyceride. When these substances enter stomach,

and especially the small intestine, they stimulate release of gastrin and

cholecystokinin, which in turn stimulate secretion of the enzymes of

destruction.

Pancreatic secretions are also the major mechanism for neutralizing

gastric acid in the small intestine. When acid enters the small gut, it

stimulates secretin to be released, and the effect of this hormone is to

stimulate secretion of lots of bicarbonate. As proteins and fats are digested

and absorbed, and acid is neutralized, the stimuli for cholecystokinin and

secretin secretion disappear and pancreatic secretion falls off.

Mark E. Armstrong

www.top5plus5.com

Oregon State Chapter Rep

Pancreatitis Association, International