SECRETION OF GASTRIC JUICE

SECRETION OF GASTRIC JUICE

SECRETION OF PEPSINOGEN

Pepsinogen is synthesized from amino acids in the ribosomes attached to endoplasmic reticulum in chief

cells. Pepsinogen molecules are packed into zymogen granules by Golgi apparatus. When zymogen granule is secreted into stomach from chief cells, the granule is dissolved and pepsinogen is released into gastric juice. Pepsinogen is activated into pepsin by hydrochloric acid.

SECRETION OF HYDROCHLORIC ACID

According to Davenport theory, hydrochloric acid secretion is an active process that takes place in the

canaliculi of parietal cells in gastric glands. The energy for this process is derived from oxidation of glucose. Carbon dioxide is derived from metabolic activities of parietal cell. Some amount of carbon dioxide is obtained from blood also. It combines with water to form carbonic acid in the presence of carbonic anhydrase. This enzyme is present in high concentration in parietal cells. Carbonic acid is the most unstable compound and immediately splits into hydrogen ion and bicarbonate ion. The hydrogen ion is actively pumped into the canaliculus of parietal cell. Simultaneously, the chloride ion is also pumped

into canaliculus actively. The chloride is derived from sodium chloride in the blood. Now, the hydrogen ion

combines with chloride ion to form hydrochloric acid. To compensate the loss of chloride ion, the bicarbonate ion from parietal cell enters the blood and combines with sodium to form sodium bicarbonate. Thus, the entire process is summarized as :

CO2 + H2O + NaCl → HCl + NaHCO3

Factors Stimulating the Secretion of Hydrochloric Acid

1. Gastrin

2. Histamine

3. Vagal stimulation.

Factors Inhibiting the Secretion of Hydrochloric Acid

1. Secretin

2. Gastric inhibitory polypeptide

3. Peptide YY.

REGULATION OF GASTRIC SECRETION

Regulation of gastric secretion and intestinal secretion is studied by some experimental procedures.

METHODS OF STUDY

1. Pavlov Pouch

Pavlov pouch is a small part of the stomach that is incompletely separated from the main portion and made into a small baglike pouch. Pavlov pouch was designed by the Russian scientist Pavlov, in a dog

during his studies on conditioned reflexes.

Procedure

To prepare a Pavlov pouch, stomach of an anesthetized dog is divided into a larger part and a smaller part by making an incomplete incision. The mucus membrane is completely divided. A small part of muscular coat called isthmus is retained. Isthmus connects the two parts.

The cut edges of major portions are stitched. Smaller part is also stitched, leaving a small outlet. This outlet is brought out through the abdominal wall and used to drain the pouch.

Nerve supply of Pavlov pouch

Pavlov pouch receives parasympathetic (vagus) nerve fibers through isthmus and sympathetic fibers through blood vessels.

Use of Pavlov pouch

Pavlov pouch is used to demonstrate the different phases of gastric secretion, particularly the cephalic

phase and used to demonstrate the role of vagus in cephalic phase.

2. Heidenhain Pouch

Heidenhain pouch is the modified Pavlov pouch. It is completely separated from main portion of stomach by cutting the isthmus without damaging blood vessels. So, the blood vessels are intact. Thus, Heidenhain pouch does not have parasympathetic supply, but the sympathetic fibers remain intact through the blood vessels.

Uses of Heidenhain pouch

Heidenhain pouch is useful to demonstrate the role of sympathetic nerve and the hormonal regulation of gastric secretion after vagotomy (cutting the vagus nerve).

3. Bickel Pouch

In this, even the sympathetic nerve fibers are cut by removing the blood vessels. So, Bickel pouch is a totally denervated pouch.

Uses of Bickel pouch

Bickel pouch is used to demonstrate the role of hormones in gastric secretion.

4. Farrel and Ivy Pouch

Farrel and Ivy pouch is prepared by completely removing the Bickel pouch from the stomach and transplanting it in the subcutaneous tissue of abdominal wall or thoracic wall in the same animal. New blood vessels develop after some days. It is used for experimental purpose, when the new blood vessels are developed.

Uses of Farrel and Ivy pouch

This pouch is useful to study the role of hormones during gastric and intestinal phases of gastric secretion.

5. Sham Feeding

Sham feeding means the false feeding. It is another experimental procedure devised by Pavlov to demonstrate the regulation of gastric secretion.

Procedure

i. A hole is made in the neck of an anesthetized dog

ii. Esophagus is transversely cut and the cut ends are drawn out through the hole in the neck

iii. When the dog eats food, it comes out through the cut end of the esophagus

iv. But the dog has the satisfaction of eating the food. Hence it is called sham feeding. This experimental procedure is supported by the preparation of Pavlov pouch with a fistula from the stomach. The fistula opens to exterior and it is used to observe the gastric secretion. The animal is used for experimental purpose after a week, when healing is completed.

Advantage of sham feeding

Sham feeding is useful to demonstrate the secretion of gastric juice during cephalic phase. In the same animal after vagotomy, sham feeding does not induce gastric secretion. It proves the role of vagus nerve during cephalic phase.