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PANCREATIC SECRETIONLecture 5Dr. Zahoor Ali Shaikh

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PANCREATIC SECRETION

When food comes to small intestine [duodenum], it is mixed with pancreatic secretion and bile [pancreas and liver are accessory digestive organs].

PANCREAS It is exocrine and endocrine

secretion. We will discuss exocrine secretion.

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PANCREATIC SECRETION

Pancreas is elongated gland lies behind and below the stomach.

Pancreas has Acini and ducts. - Pancreatic Acinar cells secrete –

digestive enzyme. - Pancreatic duct – secrete watery

secretion rich in NaHCO3

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COMPOSTION OF PANCREATIC JUICE

Pancreatic secretion per day is 1 – 2 liters.

pH – approximately 8.0 It has - Water - Cations – Na, K, Ca, Mg - Anions – HCO3, Cl, SO4, HPO4

- Digestive enzymes

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PANCREATIC SECRETION

Exocrine Pancreas secretes pancreatic juice.

It has two components: 1. Pancreatic enzymes – secreted

by Acinar cells. 2. Watery [aqueous] alkaline fluid

rich in NaHCO3 actively secreted by duct cells that line the pancreatic ducts.

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PANCREATIC SECRETION

1. Pancreatic enzymes The pancreatic enzymes are stored

in zymogen granules after they are produced.

They are released by exocytosis as needed.

Pancreatic enzymes are important because they can completely digest food, in absence of all other digestive secretions.

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PANCREATIC SECRETION

Pancreas Acinar cell secrete three types of pancreatic enzymes:

(i). Proteolytic enzymes for protein digestion.

(ii). Pancreatic amylase for carbohydrate digestion.

(iii). Pancreatic Lipase for fat digestion.

We will discuss one by one.

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PANCREATIC SECRETION

(i). Pancreatic Proteolytic enzymes

Major Proteolytic enzymes are - Trypsinogen - Chymotrypsinogen - Procarboxypeptidase They are secreted in INACTIVE form.

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Pancreatic Proteolytic Enzymes

How these Proteolytic enzymes are activated?

When Trypsinogen is secreted into the duodenal lumen, it is activated to its active form TRYPSIN by Enterokinase [enteropeptidase], an enzyme which is embedded in the mucus membrane of duodenal mucosa.

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Pancreatic Proteolytic Enzymes Trypsin once formed activates

Trypsinogen [autocatalytic] and also Chymotrypsinogen and Procarboxypeptidase.

They are converted to active form by Trypsin in duodenal lumen also.

IMPORTANT – We need initially Enterokinase to form trypsin. Once trypsin is formed it carries out other processes.

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Pancreatic Proteolytic Enzymes

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Pancreatic Proteolytic Enzymes

Proteolytic Enzymes [Trypsin, Chymotrypsin, Carboxypeptidase] attack different peptide linkages.

End result is formation of small peptide chains and amino acids.

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(ii). Pancreatic amylase for carbohydrate digestion

Pancreatic amylase causes conversion of polysaccharides into disaccharide maltose.

Pancreatic amylase is secreted in the Active form as it is not the risk for auto digestion of pancreas.

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(iii). Pancreatic Lipase for fat digestion

Pancreatic lipase is very important. Why?

Because it is the only enzyme secreted throughout the digestive system that can digest fat.

NOTE – In human, insignificant amount of lipase are secreted in the saliva and Gastric Juice called Lingual Lipase and Gastric Lipase.

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(iii). Pancreatic Lipase for fat digestion

Pancreatic lipase hydrolysis dietary triglycerides into Monoglyceride and Free fatty acids, which are absorbed in small intestine.

Pancreatic lipase is secreted in Active form as it is not the risk for pancreatic auto-digestion.

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Clinical Application

Pancreatic insufficiency When pancreatic enzymes are

deficient, digestion of food is incomplete.

As pancreatic lipase is the only significant source for fat digestion, its deficiency results in Maldigestion of fats, it is called STEATORRHEA [increased undigested fat in feces].

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Pancreatic Insufficiency

Up to 60 – 70% of fat taken maybe excreted in feces in STEATORRHEA.

Digestion of protein and carbohydrate is impaired to a lesser degree because salivary, gastric and small intestine enzymes contribute to their digestion.

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2. Pancreatic Aqueous Alkaline Fluid

As highly acidic Gastric contents are emptied into the duodenal lumen, this acidic Chyme must be neutralized quickly in the duodenal lumen. Why?

- Because to allow the functioning of pancreatic enzymes [they work in neutral or slightly alkaline medium]. NaHco3 in pancreatic fluid neutralizes the acidic Chyme.

- To prevent damage to duodenal mucosa.

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Regulation Or Control Of Pancreatic Exocrine Secretion

It is hormonal and neural mechanism.

1. Mainly by hormonal mechanism – secretin and Cholecystokinin [CCK].

2. Parasympathetic stimulation – little contribution during cephalic phase.

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Regulation Or Control Of Pancreatic Exocrine Secretion [cont]

When Chyme enters the duodenum, Two major hormones secretin and CCK [Entrogastrones] are released in response to Chyme.

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Regulation Or Control Of Pancreatic Exocrine Secretion

We will discuss role of secretin and CCK.SECRETIN

Primary stimulus for secretion of secretin is acid in the duodenum.

Secretin is carried by blood to the pancreas where it stimulates the duct cells to increase NaHCO3 rich watery secretion in the duodenum.

NaHCO3 neutralizes the acid content of Chyme.

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Regulation Or Control Of Pancreatic Exocrine Secretion

CHOLECYSTOKININ [CCK] CCK is released from duodenal

mucosa. Main stimulus for release of CCK is

Fat, to a lesser extent protein. CCK is transported by blood to

pancreas Acinar cell to increase digestive enzyme secretion.

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Regulation Or Control Of Pancreatic Exocrine Secretion

CHOLECYSTOKININ [CCK] (cont) These pancreatic enzymes help to

digest fat, protein and carbohydrate. All three types of enzymes [for

protein, fat, and carbohydrate] are packed in Zymogen granules together, therefore, all are released together by exocytosis.

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What You Should Know From This Lecture

Pancreatic Exocrine Secretion Composition Pancreatic Enzymes & Their

Importance Steatorrhea Mechanism of Pancreatic Juice

Secretion Regulation of Pancreatic Exocrine

Secretion - Hormonal & Neural

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Thank you