Pancreatic Exocrine Secretions and Its Control Flashcards
Pancreas
where does it lie?
what else secretes into the duodenum along with pancreas?
what is the exocrine portion of the pancreas composed of?
what do they release and what are they stimulated by?
The pancreas lies close to the duodenum and under the liver.
The secretions coming from the pancreas into the duodenum are also with the secretions from the bile duct.
The exocrine portion of the pancreas is composed of the acinar & duct cells.
• The duct cells release the aqueous NaHCO3 solution, this is stimulated by secretin.
• The acinar cells secrete digestive enzymes and their release is stimulated by CCK.
This is what a cross section of the pancreas would look like. The exocrine cells (acinar) cells will secrete pancreatic enzymes into the pancreatic duct that we can see in the middle.
The acinar cells
shape? arranged as?
what are the smallest ducts called and what are they lined by?
what do these ducts lead onto and what are they lined by? what do these ducts lead onto and what are they lined by?
final duct?
- Pyramidal in shape and arranged as acini (clusters of cells resembling a many-lobed berry)
- Microvilli that extend into the ducts, these are the smallest ducts called the intercalated ducts which are lined by squamous epithelial cells.
- These ducts lead onto intralobular ducts, lined by cuboidal epithelial cells and 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
As the pancreatic juices flow through the ducts their composition may be changed.
Pancreatic Exocrine Functions
what does the pancrease secrete?
what secretes enzymes and what prevents autodigestion? how is this mediated?
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 used to breakdown carbohydrates, fats, proteins and nucleic acids.
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.
Alkaline Secretions of Pancreas
what alkaline solution is secreted? where from and what does it do?
how is the composition of the juice modified as it travels through the duct?
what is exchanged with HCO3-?
what is HCO3- pancreatic juice production proprotional to?
what is activelky removed into the blood and why? how is this exchanged? effect of this?
why don’t we get as much secretion as we expect when pancreas is stimulated?
where is the fluid produced mainly if the rate of secretion is low?
if we increase the rate of flow, how do the ion levels change?
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.
The composition of the juice is modified as it travels through the duct:
- Cl- is actively exchanged for HCO3-, with the bicarbonate coming into the lumen.
- The [HCO3-] pancreatic. juice and its rate of production is proportional to the [HCO3-] blood
- H+ is actively eliminated into blood, more CO2 and HCO3- are produced in the blood as a result.
- H+ is exchanged for K+ // Na+, the H+ neutralises blood HCO3- and H2CO3 is formed, this can dissociate into CO2 & H2O, the former diffuses into the cell to form H2CO3 by reacting with H2O.
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.
If we increase the rate of flow, the Na+ levels do not change much, but the Cl- goes down while the HCO3- goes up.
Effects of Secretin on the Composition and Volume of Pancreatic Juice in Humans
what happens to HCO3- and Cl-?
what is this relationship called?
what happens to amylase levels?
If you give someone secretin IV, we see an increase in HCO3- it goes back to baseline soon though and the chloride levels go down, but it does go back to base levels, there is a reciprocal relationship.
The fall in amylase concentration probably reflects a dilution effect as the volume of pancreatic juice increases.
Enzyme Secretion of Pancreas
what are the 3 major types of enzymes released?
2 other enzymes that can be released?
There are three major types of enzymes released:
- Proteolytic enzymes (trypsin (released as trypsinogen), chymotrypsin, carboxypeptidase)
- Amylase (activated by Cl-)
- Lipase
- Ribonucleas
- Deoxyribonucleas
Proteolytic Enzymes
why could they lead to autodigestion and how does the body work around this?
what happens to the peptic enzyme? what is needed and where is this?
the action of the other 2 main enzymes?
what is steathorrea and how does it come about?
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.
Amylase is involved in carbohydrate digestion, converting polysaccharides -> disaccharides.
Lipases are important in fat digestion, converting triglycerides -> fatty acids + glycerol.
Pancreatic insufficiency of lipase -> malabsorption of fats = steathorrea.
Fate of Pancreatic Enzymes in the Duodenum
which two enzymes can cross the basolateral membrane?
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
Control of Pancreatic Secretions
what enhances the rate of secretion and what initiates this?
what inhibits secretion and why?
key mediators of secretion? (2)
3 different phases?
what is the first phase and causes the effect of this phase and effect?
second phase? what causes this phase? what happens with vagotomy?
last phase under the influence of what?
what releases secretin and effecr of this?
what releases cck and effect of this?
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.
In terms of the different phases of secretion, we need to look at the
- Cephalic phase
- Gastric phase
- Intestinal (duodenal) phase
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.
- Vagus is an important role player as if you do a vagotomy there is a 50% drop in response to chyme
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.
Secretin is released from the duodenal and jejunal cells, they also seem to be released upon presence of certain substances in chyme.
Secretin induces pancreatic duct cells to secrete HCO3- rich pancreatic juice, but the enzyme effect isn’t much//doesn’t affect acinar cells
- Secretin also stimulates the production of bile.
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.
