S2: Pancreatic Exocrine Secretions and Its Control

Question 1

What secretions mix with pancreatic juice into the duodenum?

Answer 1

The secretions coming from the pancreas into the duodenum are also with the secretions from the bile duct.

Question 2

What two cells make up the exocrine portion of the pancreas?

Answer 2

Acinar and Duct cells

Question 3

What stimulates the exocrine portion of the pancreas and what do they release?

Answer 3

Duct cells release aqueous NaHCO3 solution. This is stimulated by secretin.

Acinar cells secrete digestive pancreatic enzymes into pancreatic duct and their release is stimulated by CCK

Question 4

Describe structure of acinar cells

Describe path in

Answer 4

The acinar cells are pyramidal in shape and arranged in acini (clusters of cell resembling a many lobed berry).

Question 5

Describe path into pancreatic duct (different ducts)

Answer 5

• Acinar cells
• There are microvilli that extend into the ducts - these are the smallest ducts called intercalated ducts which are lined by squamous epithelial cells
• These ducts lead onto intralobular ducts, lined by cuboidal epithelial cells
• These then lead onto interlobular ducts lined with columnar epithelium and goblet cells.
• These lead on to the pancreatic duct.

Acini -> Microvilli -> Intercalated duct -> Intralobular duct -> interlobular duct -> Pancreatic duct

Question 6

Functions of pancreatic exocrine

Answer 6

The pancreas secretes 1.5L of fluid a day, this fluid consists of sodium and HCO3—rich juice, albumin, globulin and digestive enzymes.
The acinar cells secrete digestive enzymes which are to breakdown carbohydrates, fats, proteins and nucleic acids.

Question 7

Why are pancreatic enzymes secreted in inactive form?

Answer 7

Enzymes are secreted in an inactive form, to prevent autodigestion. This is because some of them do digest proteins so if they were active they would start digesting the pancreas and damaging other cells.
The activation of these enzymes occurs in the duodenum.

Question 8

What juices neutralise acidic chyme and acid from stomach?

Answer 8

• HCO3- rich juice and secretions from the gallbladder neutralise the gastric acid, they increase the pH to 6/7.
• The composition of the secretions in the pancreas is similar to that of the salivary gland but here it is a HCO3- rich hypertonic juice, which is important as we really need to neutralise the acidic chyme.

Question 9

Describe the composition of the pancreatic juice being modified as it travels through the duct

Answer 9

• Cl- is actively exchanged for HCO3- (antiport) with the bicarbonate coming into the lumen of duct
• The [HCO3-] in pancreatic juice and its rate of production is proportional to the [HCO3-] in the blood
• H+ is actively eliminated into blood, more CO2 and HCO3- are produced in the blood as a result.
• H+ is exchanged for K+ and Na+, the H+ neutralises blood HCO3- and H2CO3 is formed, this can dissociate into CO2 and H2O, Co2 then diffuses into the cell to form H2CO3 by reacting with H2O.

Question 10

What happens if rate of secretion is low and why might [HCO3-] in secretions not be as high as expected?

Answer 10

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 the rate of secretion is low, the fluid produced is mainly by the intralobular ducts.

Question 11

Effects of Secretin on the Composition and Volume of Pancreatic Juice in Humans

Answer 11

If you give someone secretin IV, we see an increase in HCO3-. However it goes back to baseline soon though and the chloride levels go down but it does go back to base levels so there is a reciprocal relationship.
The fall in amylase concentration in pancreatic juice probably reflects a dilution effect as the volume of pancreatic juice increases.

Question 12

Name enzyme secretions of pancreas

Answer 12

Proteolytic enzymes (trypsin (released as trypsinogen), chymotrypsin, carboxypeptidase)
Amylase (activated by Cl-)
Lipase 

Ribonucleas
Deoxyribonucleas

Question 13

Describe the proteolytic enzymes released by pancreas

Answer 13

These are involved in the digestion of proteins, thus the pancreas is at risk of autodigestion, to solve this problem the enzymes are released as proenzymes.

Trypsinogen is converted to trypsin by the enterokinases within the brush border, trypsin is autocatalytic (autoactivation of other proenzymes) (similar to pepsin).

Question 14

Describe function of amylase and lipase

Answer 14

Amylase is involved in carbohydrate digestion, converting polysaccharides to disaccharides.
Lipases are important in fat digestion, converting triglycerides to fatty acids and glycerol. Pancreatic insufficiency of lipase, will lead to malabsorption of fats = steathorrea.

Question 15

Fate of Pancreatic Enzymes in the Duodenum

Answer 15

The percentage of secreted enzymes that reach the small intestine
Amylase -> 75%
Trypsin -> 20%
Lipase -> 1%

Digestive enzymes have the capacity to survive different lengths of time in the small intestine.
Chymotrypsinogen and amylase cross the basolateral membrane for the pancreas

Question 16

What is controlling pancreatic secretions?

Answer 16

There are neuroendocrine signals which are playing a role, vagal (parasympathetic) stimulation enhances the rate of secretion of enzymes and the aqueous components of pancreatic juice. This vagal reflex may be initiated by distension of the duodenum etc.

Sympathetic stimulation inhibits secretion! This is may be due to decreased blood flow.

• The key mediators are secretin and CCK, these stimulate secretion of pancreatic fluid. This was pioneered by bayliss and starling.

Question 17

Different phases of secretion

Answer 17

Cephalic phase -> When we see/smell food there is vagal (Ach and VIP) stimulation of gastric release from the antrum. This going on seems to promote some protein-rich pancreatic juice.

Gastric phase -> Distension (causes vagal reflex on the fundus or antrum) and amino acids/peptides (in antrum) stimulates gastrin release. This also seems to promote secretion of enzyme-rich pancreatic juice.

The intestinal phase is under the influence of secretin and CCK, chyme in the duodenum (especially certain substances like lipids) and jejunum induces secretion of pancreatic juice.

Question 18

What releases secretin and what is its function?

Answer 18

Secretin is released from the duodenal and jejunal cells, they also seem to be released upon presence of certain substances in acidic chyme.
Secretin induces pancreatic duct cells to secrete HCO3- rich pancreatic juice, but the enzyme effect isn’t much (doesn’t effect acinar cells much).

Secretin also stimulates the production of bile.

Question 19

What releases CCk and what is its function?

Answer 19

CCK is also released from the duodenal and jejunal cells, they are released in response to amino acids, peptides and fatty acids in the chyme.
CCK stimulates the pancreatic acinar cells to synthesise and release enzyme rich pancreatic juice. It also stimulates the secretion of concentrated bile for fat absorption doing this by contracting the gallbladder and relaxing the sphincter of Oddi to release bile into the duodenum (and pancreatic juices).

CCK potentiates the effects of secretin, which is a weak agonist of acinar cells.