Pancreatic Exocrine Secretions And Its Control

Question 1

What are the three types of cells found in the pancreas (two in the pancreas itself and one within the ducts)?
Include what stimulates them and what they secrete.

Answer 1

ACINAR CELLS (exocrine):

• stimulated by CCK (cholecystokinin)
• secretes digestive pancreatic enzymes into the pancreatic ducts

ISLETS OF LANGERHAAN (endocrine):
- secrete hormones into the bloodstream

DUCT CELLS:

• stimulated by secretin
• secrete NaHCO3 (sodium bicarbonate)

Question 2

List the structures a substance has to go through to get from acinar cells to the pancreatic duct.

Answer 2

1) acini (a cluster of acinar cells)
2) intercalated ducts
3) intralobular ducts
4) extralobular ducts
5) interlobular ducts
6) pancreatic duct

Question 3

The pancreas secretes around 1.5 litres of fluid every day.

What are some components of this fluid?

Answer 3

It is a sodium and HCO3- rich juice.

It also contains albumin, globulin and digestive enzymes.

Question 4

Name some anions that are secreted in the pancreatic juice.

Answer 4

• HCO3-
• Cl-
• SO4 2-
• HPO4 2-

Question 5

Name some cations that are secreted in the pancreatic juice.

Answer 5

• Na+
• K+
• Ca2+
• Mg2+

Question 6

The composition of the juice is modified as it travels through the duct.
Describe how this happens.

Answer 6

• Cl- is actively exchanged for HCO3- by the epithelial cells
• the concentration of HCO3- in the pancreatic juice and its rate of production is proportional to the concentration of HCO3- in the blood
• H+ is actively eliminated and exchanged for K+ and Na+; the H+ neutralises the HCO3- (and H2CO3 is formed)
• thus, more CO2 and HCO3- is produced in the blood
• CO2 diffuses in and forms H2CO3 with H2O

Question 7

Describe the effect of stimulating the pancreas (via secretin) on the components of pancreatic juice.

Answer 7

There is a reciprocal change in the concentration of Cl- and HCO3- after the infusion of secretin.

When we stimulate the pancreas, we get [HCO3-]-rich secretions, but not as much as we’d expect, as there is less time for HCO3- and Cl- exchange as the flow rate is greater.
If we increase the flow rate, the Na+ levels don’t change much, but the Cl- goes down while the HCO3- goes up.

Question 8

What are the three major types of enzymes secreted by the pancreas?

Answer 8

• proteolytic enzymes (trypsin, chymotrypsin and carboxypeptidase)
• amylase
• lipase

We also get:
- deoxy/ribonucleases

Question 9

Trypsin is quite an important enzymes in the sense that it activates many other enzymes.
List some enzymes that it activates.

Answer 9

• trypsin
• elastase
• phospholipase A2
• chymotrypsins
• carboxypeptidase A and B
• colipase

Question 10

What does phospholipase A2 do?

Answer 10

It is important in terms of the breakdown of membrane phospholipids.
It converts lecithin to isolecithin.

Question 11

What does colipase do?

Answer 11

it is useful to enhance the activity of pancreatic lipase; it acts on some of the fat droplets and enhances the capacity of lipase to then bind to it, and it allows it access the lipid core.
This lets the lipase break down the fat droplet further to make micelles, which are now small enough to travel to the surface of the epithelial cells to get absorbed.

Question 12

What is the percentage of secreted enzymes that reach the small intestine?

Answer 12

• amylase: 75% (as in, 75% of the activity is retained at that stage)
• trypsin: 20%
• lipase: 1%

Question 13

So, how do we control pancreatic secretion?

Answer 13

Pancreatic secretions are controlled through neuroendocrine signals.

There is:

• [+] vagal (parasympathetic) stimulation: enhances the rate of secretion of enzymes and aqueous components of pancreatic juice
• [-] sympathetic stimulation: inhibits secretion, decreases blood flow? (because it diverts blood away from the GIT)

Question 14

Describe the control of pancreatic secretions throughout the 3 stages of digestion.

Answer 14

CEPHALIC PHASE:
vagal (ACh and VIP) stimulation of gastrin release from the antrum leads to some protein-rich pancreatic juice

GASTRIC PHASE:
distention (vagal reflex on the fundus or antrum), amino acid and peptide stimulated gastrin secretion, which leads to the release of enzyme-rich pancreatic juice

INTESTINAL PHASE:
via secretin and CCK, chyme in the duodenum and jejunum induces secretion of pancreatic juice

Question 15

How does secretin work?

Answer 15

Secretin induces the pancreatic duct cells to secrete HCO3- -rich pancreatic juice, but with a lowered enzyme content.
It also stimulates the production of bile.