Pancreatic Exocrine Secretions & its control

Question 1

What are the main cells of the exocrine pancreas? What do they do?

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Answer 1

Acinar cells - form sacs with microvilli, and connect to the ductal tree, emptying into the duodenum:
o Secrete juice rich in Na, bicarbonate, albumin, globulin, digestive enzymes
o Digestive enzymes to break down carbs, fats, proteins, nucleic acids - activated in the duodenum

Question 2

What are the alkaline secretions of the pancreas?

Answer 2

• Bicarbonate rich juice from gallbladder - neutralises gastric acid
• Pancreatic secretion is similar to that in the salivary gland, but is hypertonic and full of bicarbonate
• Composition of the juice is modified as it travels through the duct:

o HCO3-/Cl- antiporter on luminal membrane remove bicarbonate from cell - rate of HCO3- production is proportional to plasma [HCO3-]
o H/K and H/Na antiporters on basolateral membrane bring Na and K into cell

Question 3

What the 3 major enzyme types secreted by the pancreas?

Answer 3

1. Proteolytic enzymes (75%) - Trypsin, Chymotrypsin, Carboxypeptidase
   o Secreted as pro-enzymes to prevent autodigestion
2. Amylase (20%)
3. Lipase (1%)

Question 4

What is the fate of pancreatic enzymes?

Answer 4

• Digestive enzymes survive different lengths of time in the small bowel
• Chymotrypsin and amylase cross the basolateral membrane of the pancreas

Question 5

Describe the neuroendocrine control of pancreatic secretions?

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Answer 5

• Parasympathetic (Vagal) stimulation = greater rate of secretion of enzymes and aqueous components of pancreatic juice
• Sympathetic stimulation = decreased rate of secretion
• CCK and Secretin stimulate secretion of juice

Question 6

What are the 3 phases in the control of pancreatic secretions?

Answer 6

1. Cephalic phase:
   • Vagal stimulation = ↑Ach/VIP = Gastrin release from Antrum = release of protein-rich pancreatic juice
2. Gastric phase:
   • Distension = Vagal reflux = peptide-stimulated gastrin release = release of enzyme-rich pancreatic juice
3. Intestinal phase:
   • Chyme in Duodenum and Jejunum = release of pancreatic juice

Question 7

How is Secretin used to control pancreatic secretions?

How is CCK used to control pancreatic secretions?

Answer 7

• • From Duodenum and Jejunum in response to acid
  • Stimulates pancreatic duct cells to secrete aqueous (HCO3-rich) pancreatic juice, but decreases the enzyme content of juice
  • Stimulates the production of watery Bile
• • From Duodenum and Ileum in response to FA, AA, peptides
  • Stimulation of Acinar cells to produce and release enzyme-rich pancreatic juice
  • Contracts gallbladder and relaxes Sphincter of Oddi to release Bile

*CCK potentiates the effects of Secretin, which is a weak agonist of Acinar cells