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.
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