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Exocrine Pancreas

Exocrine Pancreas

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Exocrine Pancreas
Summary
pancreas
– Secretes pancreatic juice that aids digestion
– Secretes hormones involved in metabolism.
Pancreatic secretions
Two major components:
  • Digestive enzymes
– Acts on fats, carbohydrates, proteins, and lipids.
  • Bicarbonate
– Neutralizes acidic chyme in the duodenum.
Pancreas and its associated structures
  • Pancreas primarily consists of acini: clusters of secretory exocrine epithelial cells.
  • Main pancreatic duct: carries pancreatic juice from secretory acini to the duodenum, formed from smaller ducts.
  • Common bile duct: forms where the hepatic and cystic ducts merge.
  • Hepatopancreatic duct: forms when main pancreatic duct and common bile duct merges, empties into the duodenum via its sphincter; it transports both pancreatic juice from the pancreas and bile from the liver and gallbladder to the duodenum.
  • Accessory pancreatic duct: drains directly into the duodenum proximal to the hepatopancreatic duct.
Acinus
Exocrine secretory unit of the pancreas
  • Acinar region = site of secretion, secrete digestive enzymes
  • Duct region = modify initial secretion, secrete bicarbonate.
Digestive enzymes and bicarbonate merge to form pancreatic juice.
  • Both neural and hormonal stimuli regulate pancreatic juice secretion for its subsequent release and action in the duodenum.
  • Hormonal regulation occurs via: secretin and cholecysotkinin (CCK).
CCK
  • Triglycerides, fatty acids, and amino acids triggers CCK release from the duodenum.
  • CCK triggers acinar section of pancreatic acinar digestive enzymes.
(Digestive enzymes include pancreatic amylase, pancreatic lipases, nucleases, and proteases.)
Secretin
  • Acidic chyme triggers secretin release from the duodenum.
  • Acts on pancreatic ductal cells, which triggers bicarbonate release.
Potentiation
  • Amplification of one hormone's effects by another hormone, which generates a larger signal than if either hormone acted alone.
  • CCK and secretin act together via potentiation to amplify each other's signal.
(Pancreatic duct cells can also by stimulated by CCK, while acinar cells similarly respond to secretin stimulation)
  • CCK and secretin stimulation of both ductal and acinar cells promotes additional bicarbonate & digestive enzyme secretion, respectively.
Protease Activation
Like in the stomach, pancreatic proteases are stored and secreted as inactive precursor enzymes (zymogens). Their storage as zymogens protects acinar cells from autodigestion.
  • Duodenum = the location of enzyme activation and subsequent action.
  • Inactive proteases (eg, chymotrypsinogen) and, specifically, the inactive trypsinogen, (which are secreted by the pancreas) in the duodenal lumen.
  • Proteolytic enzyme enterokinase (bound to the duodenal epithelium plasma membrane).
– Converts trypsinogen to its active form trypsin.
  • Trypsin, which is itself another proteolytic enzyme, activates the precursor zymogens (such as chymotrypsinogen) to their active protease forms (chymotrypsin).
– Trypsin activates pancreatic zymogens by cleaving them into their active proteases. These proteases can now themselves break down proteins.
Note that pancreatic amylase, lipase, and nucleases are released in their active form.

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