Pancreatic Exocrine Secretions and Control

Question 1

What are the functional units of the pancreas?

Answer 1

The acinar cells secrete the protein rich secretions (digestive enzymes) and acini, the functional units of acinar cells
The duct cells, that line the lumen, secrete NaHCO3-

Question 2

How do the acinus and ducts contribute to pancreatic exocrine secretions?

Answer 2

The ductal cells are under the influence of secretin, to secrete NaHCO3-
CCK stimulates the secretion of acinar cells
Secretion of digestive enzymes:
Acini form sacs-> connect to the ductal tree -> empty into the duodenum:
-Microvilli
-Intercalated ducts- squamous epithelium
-Intralobular ducts- cuboidal or low columnar epithelium (drain their contents into..)
-Interlobular ducts- columnar epithelium/goblet cells (drain their contents into…)
-Pancreatic duct, which enters the duodenum at the common bile duct

Question 3

What is the morphology of the acinus duct like?

Answer 3

Inside the pancreatic acinar cells you can find zymogen granules which are full of enzymes
These are kept inactive
The contents are acidic to avoid autodigestion because if the enzymes are active in their own right there will be pancreatitis
There are also other substances in the granules which keep them inactive (inhibitors of the enzymes) until the enzymes get into the duodenum where they will be activated

Question 4

What are the pancreatic exocrine functions?

Answer 4

Secretes 1.5L of fluid/day
Sodium and HCO3- -rich juice, albumin, globulin and digestive enzymes
Acinar cells secrete digestive enzymes which breakdown carbohydrates, fats, proteins and nucleic acids
Enzymes secreted in inactive form (as zymogens) to prevent auto-digestion
Activation of enzymes occurs in the duodenum:
The anions that are secreted in the pancreatic juice- HCO3-, Cl-, SO42-, HPO42-
The cations that are secreted in the pancreatic juice- Na+, K+, Ca2+, Mg2+

Question 5

What alkaline secretions does the pancreas secrete?

Answer 5

HCO3–rich juice and secretions from gallbladder → help to neutralise gastric acid (↑pH to 6 or 7)
Secretion from pancreas is similar to that in salivary gland, but it is a HCO3–rich juice

Question 6

How is the composition of the juice modified as it travels through the duct?

Answer 6

Epithelial cells actively exchange Cl-/HCO3-
H+ is actively eliminated by Na+/H+ exchanger
H+ exchanged for K+ - driven by Na+/K+ ATPase
H+ neutralises HCO3- (H2CO3 formed)
….more CO2 and HCO3- produced in the blood
….CO2 diffuses in and forms H2CO3 with H2O
Thus, [HCO3-]pancreatic juice and its rate of production ∝ [HCO3-]blood

Question 7

What are the effects of secretin on the composition and volume of pancreatic juice?

Answer 7

Stimulated pancreas cause ↑ [HCO3-]-rich secretions
There is a reciprocal change in the concentrations of Cl- and HCO3- upon secretin infusion
The reduced amylase and Cl- concentrations upon secretin infusion may be a dilution effect due to volume increase in the pancreatic juice
So:
Cl- levels go down with addition of secretin
HCO3- secretion shoots up with secretin
So there is this reciprocal change (not the only reason theorised)
Why?
More bicarbonate comes into the lumen than chloride comes out

Question 8

What enzymes does the pancreas secrete?

Answer 8

3 major types of enzymes
-Proteolytic enzymes (trypsin, chymotrypsin and carboxypeptidase)
-Amylase
-Lipase
Ribonuclease
Deoxyribonuclease

Question 9

What are the ‘fate’ of pancreatic enzymes in the duodenum?

Answer 9

Digestive enzymes survive different lengths of time in the small intestine
Percentage of secreted enzymes that reach the small intestine
Amylase: 75%
Trypsin: 20%
Lipase: 1%

Question 10

How are pancreatic secretions controlled neurally?

Answer 10

Neuroendocrine signals
Vagal (parasympathetic) stimulation: enhances rate of secretion of enzymes and aqueous components of pancreatic juice
Sympathetic stimulation: inhibits secretion (decreased blood flow?)
Secretin and CCK: stimulate secretion of pancreatic fluid (enzyme- and bicarbonate-rich secretions)
Secretin alone is a weak agonist of acinar cells

Question 11

How are pancreatic secretions controlled depending on the digestive phase?

Answer 11

Cephalic phase: vagal (ACh and VIP) stimulation of gastrin release from antrum → some protein-rich pancreatic juice
Gastric phase: Distension (vagal reflex on fundus or antrum), amino acid- and peptide- (in antrum)-stimulated gastrin secretion → release of enzyme-rich pancreatic juice
Vagotomy → 50%↓ in response to acidic chyme
Intestinal phase: secretin and CCK
Acidic chyme in duodenum and jejunum induces secretion of pancreatic juice

Question 12

How do substances such as secretin and CCK control pancreatic secretions?

Answer 12

Secretin from mucosa of duodenal and jejunum
Induces pancreatic duct cells to secrete HCO3- -rich pancreatic juice, but ↓ enzyme content
Secretin also stimulates production of bile by the liver
CCK from duodenal and jejunum in response to fatty acids (amino acids, and peptides)
CCK stimulates pancreatic acinar cells to synthesise and release enzyme-rich pancreatic juice; and stimulates the secretion of concentrated bile from gallbladder for fat absorption
CCK contracts the gallbladder and relaxes the sphincter of Oddi → release of bile into duodenum
Note that NO and VIP also relax the sphincter of Oddi to release secretions from the pancreas and gallbladder (pancreatic juice)
CCK potentiates the effects of secretin, which is a weak agonist of acinar cells