PANCREATIC SECRETIONS

PANCREATIC SECRETIONS

• Pancreas – An exocrine digestive gland & an endocrine gland.

• Produces both endocrinal & exocrine secretions.

PANCREATIC JUICE:

• Exocrine pancreas. 

• Most potent digestive secretion.

• Major source of digestive enzymes digesting all food components (proteins, carbohydrate, fat & nucleic acid).

• Daily pancreatic secretion – About 1.5 – 2.0 L.

• Isotonic to plasma.

• pH – Alkaline (pH 8.0).

– Highly alkaline pH neutralizes gastric HCl in chyme entering duodenum.

CONTENTS:

• Ionic contents – 

– Cations (Na²⁺, K⁺, Ca²⁺, Mg²⁺).

– Anions (HCO₃^–, Cl^–, SO^-2₄, HPO₄^-3)

Digestive pancreatic enzymes – 

• Trypsin (Trypsinogen).

• Chymotrypsin (chymotrypsinogen).

• Elastase (proelastase).

• Carboxypeptidase A & B (Procarboxypeptidase A & B).

• Colipase (Procolipase).

• Pancreatic lipase.

• Bile salt-acid lipase (Bile salt-activated lipase).

• Cholesteryl ester hydrolase.

• Pancreatic α-amylase.

• Ribonuclease.

• Deoxyribonuclease.

• Phospholipase A₂ (pro-phospholipase).

REGULATION:

• Predominantly under hormonal control.

• Relatively less significant neural control.

1. HORMONAL CONTROL:

• Two most important regulating hormones –

• Secretin & Cholecystokinin (CCK).

• Acts on pancreatic ‘ducts’.

• Causing secretion of large-amount highly alkaline pancreatic juice.

• Rich in bicarbonates & poor in enzymes.

• Acts on “Acinar cell”.

• Causing zymogen granules release & also pancreatic juice rich in enzymes.

2. NEURAL REGULATION:

• Controlled by parasympathetic system via Vagus N, stimulating pancreatic secretions.

3. Others:

• Gastrin also plays some part in pancreatic secretion.

• Secreted in response to gastric distension.

PREVENTION OF PANCREATIC SELF-DESTRUCTION:

• Trypsin – Powerful proteolytic enzyme digesting pancreatic cells itself.

• Usually, not a normal process.

MAJOR PREVENTIVE MECHANISM:

• Pancreas secretes enxymes as “Enzymatically inert/inactive pro-enzyme”.

– Except for amylase & lipase.

• Thus, preventing self-digestion & efficiently utilize enzymes inside the duodenal lumen.

1. Enzymes are sequestered in membrane-bound zymogen granules in acinar cells

2. Proenzyme activation requires,

– Conversion of inactive trypsinogen → active trypsin by duodenal enteropeptidase (enterokinase).

3. Trypsin inhibitors including serine protease inhibitor Kazal type I cells.

– Present within acinar & ductal secretion.

4. Trypsin contains a critical self-recognition cleavage site.

– Allowing trypsin to inactivate itself.

5. Lysosomal hydrolases – Capable of degrading zymogen granules, when normal acinar secretion is blocked.

6. Acinar cells – Resistant to the action of trypsin, chymotrypsin & phospholipase A₂. 

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