Download pancreatic secretion

EXOCRINE PANCREATIC SECRETION
OBJECTIVES
The student should be able to:
• Discuss pancreatic exocrine secretion and
composition.
• Enlist pancreatic enzymes and their
importance
• Describe steatorrhea.
• Explain the mechanism of pancreatic juice
secretion.
• Describe the regulation of pancreatic
exocrine secretion hormonal and neural.
PANCREATIC SECRETION
• When food enters the duodenum, it is mixed
with pancreatic secretion and bile [pancreas
and liver are accessory digestive organs].
• Pancreas is elongated gland lies behind and
below the stomach.
• It has exocrine and endocrine secretions.
• Exocrine Pancreas secretes pancreatic juice.
PANCREATIC SECRETION
• Pancreas has acini
and ducts:
1) Pancreatic
acinar
cells secrete digestive
enzymes.
1) Pancreatic duct cells
secrete
[aqueous]
watery
alkaline
fluid rich in NaHCO3
Pancreatic Juice
Exocrine pancreatic secretion:
• Volume = 1-2 L / day.
• pH = approximately 8 (alkaline).
• Osmolarity = isotonic.
• It has Water, Cations (Na+, K+, Ca+2), Anions
(HCO3, Cl-, HPO4) & Digestive enzymes.
Types of exocrine pancreatic
secretion
Aqueous alkaline
juice
Enzymatic juice
Secreted by duct cells.
Secreted by acinar cells.
Large in volume, rich in
bicarbonate.
Small in volume, rich in
enzymes.
Stimulated by Secretin.
Inhibited by Sympathetic.
Stimulated by CCK & vagus.
1) Pancreatic Enzymes:
• Stored in zymogen granules after they are produced.
• Released by exocytosis as needed.
• Pancreatic enzymes are important because they can
completely digest food, in absence of all other
digestive secretions.
• Pancreatic Acinar Cells Secrete Three Types of
Pancreatic Enzymes:
i. Proteolytic enzymes for protein digestion.
ii. Pancreatic amylase for carbohydrate digestion.
iii. Pancreatic Lipolytic for fat digestion.
(i) Pancreatic
Proteolytic Enzymes:
• Major Proteolytic Enzymes are:
1- Trypsinogen.
2- Chymotrypsinogen.
3- Procarboxypeptidase.
• They are secreted in inactive forms.
• Proteolytic Enzymes attack different peptide
linkages.
• End result is formation of small peptide chains
and amino acids.
Activation of Pancreatic Proteolytic Enzymes
• When Trypsinogen is secreted into duodenal lumen, it
is activated to its active form Trypsin by Enterokinase
(=Enteropeptidase) which is an intestinal enzyme
embedded in mucus membrane of duodenal mucosa.
• Once Trypsin is formed, it activates more Trypsinogen
[autocatalytic].
• Chymotrypsinogen
&
Procarboxypeptidase
are
converted to active forms by Trypsin in duodenal lumen.
• Initially, intestinal enterokinase is needed to form trypsin.
Once trypsin is formed, it carries out other processes.
(ii).
Pancreatic
Amylase
For
Carbohydrate Digestion:
• It causes conversion of polysaccharides
into disaccharide maltose.
• It is secreted in the Active form as there
is no risk for auto digestion of pancreas.
(iii) Pancreatic Lipase for Fat Digestion:
• It is the most important enzyme secreted
throughout digestive system that can digest fat.
• In human, there are insignificant amounts of
Lingual Lipase & Gastric Lipase.
• Pancreatic lipase can hydrolyze dietary
triglycerides into Monoglyceride & Free fatty
acids, which are absorbed in small intestine.
• It is secreted in the Active form as there is no
risk for auto digestion of pancreas.
2- Pancreatic Aqueous Alkaline Fluid:
 As highly acidic Gastric contents are emptied
into duodenal lumen, this acidic Chyme must be
neutralized quickly in duodenal lumen, Why?
1- To allow the functioning of pancreatic enzymes
[they work in neutral or slightly alkaline medium].
NaHCO3 in pancreatic fluid neutralizes the acidic
Chyme.
2- To prevent damage to duodenal mucosa.
CONTROL OF PANCREATIC
EXOCRINE SECRETION
1- Nervous:
Vagal stimulation   enzymatic secretion
from acini (little contribution during cephalic
phase).
2- Hormonal:
Secretin   aqueous secretion from duct.
CCK   enzymatic secretion from acini.
When Chyme enters the duodenum, secretin &
CCK [Entrogastrones] are released in response
to Chyme.
Structure
Site
Stimuli
Cholecystokinin
Secretin
22 A.A
27 A.A
Upper part of small intestine (deudenum).
 HCL that passes to
Polypeptides & AA
duodenum with
Fats & F.A. in duodenum.
decrease its pH.
Functions:
 On Stomach:
Inhibition of gastric functions
 On Intestine:
 Motility & enzymes secretion.
 Mucus secretion.
 On Pancreas:
 Secretion of
enzymes.
 Evacuation of bile
(cholagogue)
 Secretion of
alkaline juice.
 Secretion of
NaHco3 (choleretic)
+ ve Feedback between CCK &
A.A as CCK   digestion of
protein  production of amino
acids   CCK
- ve Feedback as Hcl 
 release of secretin
  Hcl secretion
from the stomach.
 On Bile:
Control:
Pancreatic Insufficiency
• When pancreatic enzymes are deficient, digestion of food is
incomplete.
• As pancreatic lipase is the significant source for fat
digestion, its deficiency results in maldigestion of fats, it is
called steatorrhea (increased undigested fat in feces).
• Up to 60 – 70% of fat taken may be excreted in feces.
• Digestion of protein and carbohydrate are impaired to a
lesser degree because salivary, gastric and small intestinal
enzymes contribute to their digestion.
Steatorrhea
• Definition: It is the presence of excess fat in feces. Stools may
also float due to excess lipid, have an oily appearance and can be
especially foul-smelling.
• Possible causes:
1- lack of bile acids (due to liver damage).
2- Gall bladder removal (cholecystectomy).
3- Defects in Pancreatic enzymes e.g. Lipase
4- Malabsorption e.g. in Inflammatory bowel disease.
5- Exocrine pancreatic insufficiency & Pancreatitis.
• Treatment: correction of the underlying cause, as well as
digestive enzyme supplements.