Transcript GIT HORMONES

GIT HORMONES
GIT is regarded as the largest endocrine
organ in the body. Collectively, GIT hormones
influence motility, secretion, digestion, and
absorption in the gut. In addition, they
regulate bile flow, secretion of pancreatic
hormones, affect tonicity of vascular walls,
blood pressure and cardiac output. The
principle GIT hormones include:
- Gastrin
- secretin
- cholecystokinin- pancreozymin (cck- pz)
- vocative intestinal polypeptide (VIP)
- Gastric inhibitory polypeptide (GIP)
- others
Gastrin
There are 3 molecular forms of gastrin that are present in the
blood circulation and the tissues and these are:
1. Big Gastrin; which is a linear polypeptide of 34 amino acids
called as (G- 34).
2. Little gastrin; which is a linear polypeptide of 17amino acids
called as (G- 17).
3. Minigastrin; which is a linear polypeptide of 14 amino acids
called as (G- 14). However, they all exert the same biological
effects on the target cells.
Gastrin is secreted and stored by endocrine cells called as (G
cells) mainly in the antral mucosa of the stomach, and to a
lesser extent by G cells of the proximal part of the duodenum,
and by (D- cells) of the pancreas.
Following secretion to the blood stream, the main circulating
form is (G- 34). Gastrin has the following main functions:
1.Stimulates the fundal
secrete the gastric acid.
parietal
cells
to
2. Stimulates gastric mucosal cells to secrete
pepsinogen and intrinsic factor.
3. Stimulates small intestinal mucosal cells to
secrete a hormone called (secretin).
4. Stimulates the secretion of pancreatic
enzymes and bicarbonate (HCO3-).
5. Stimulates gastric and intestinal motility.
6. Stimulates gastric mucosal growth.
7. Stimulates hepatic bile flow.
The main stimulus for the release of gastrin is antral
dilatation due to presence of food in the stomach mainly
protein digestion products (polypeptides, peptides, and
amino acids). In addition, other stimuli may include:
- alcohol
- caffeine
- insulin induced hypoglycemia
- vagal stimulation (sight, smell, taste, thinking)
Maximum secretion of gastrin occurs at an antral Ph of 57. However, when pH gets lower a negative feed back
mechanism will inhibit further secretion of gastrin.
Cholecystokinin- Pancreozymin
It is a (33) a.a. polypeptide hormone released from the upper
small intestinal mucosal cells (I cells) mainly in the duodenum
in response to:
presence of partially digested protein products in the lumen of
the intestine.
entry of gastric acid to the intestine.
Presence of fatty acids mainly in the form of micells.
As the name implies, it stimulates the gall bladder contraction,
and meanwhile the secretion of pancreatic enzymes. It is
present in multiple molecular forms; the most active form is that
containing the sulfated tyrosyl residue. The five a.a. sequence at
the C- terminal end are identical to those of gastrin.
The structural similarity with gastrin makes CCK- PZ exerts some
gastric action mainly in stimulating intestinal motility and
increasing pancreatic enzymes. It competes with gastrin for its
intestinal receptors and this may help in terminating gastrin
action following meals.
Secretin
It is a polypeptide with structural similarity VIP, GIP
and Glucagon. It is released from the granular
mucosal cells (S- cells) that are mainly present in
the duodenum, but however are distributed
throughout the small intestine. It is called as such
because it permits the good flow of pancreatic juice
following meals. Secretin release is stimulated by
contact of S- cells with gastric acid. However, its
only inhibitor is somatostatin. The principle function
of secretin is increasing pancreatic juice release
with increased concentration of bicarbonate. So, it is
used for testing pancreatic function. It inhibits
gastrin release and thus gastric acid secretion.
Vasoactive Intestinal Polypeptide (VIP)
It is a linear polypeptide of 28 a.a., has some structural
similarities with secretin, GIP, and Glucagone. It is present in
almost all body tissues, but mainly concentrated in the GIT and
the nervous system. It is present in the nerve fibers along all of
the gut from the esophagus to the colon, but mainly in the
jejunum, ileum, and the colon. VIP has a large no. of ill defined
non- specific physiological effects, among which are:
1. Acts as a neurotransmitter in the CNS and in the autonomic
nervous system of the gut.
2. Relaxation of the smooth muscle fibers in vessels walls, gut,
and genito- urinary system.
3. Increases pancreatic secretion.
4. Increases water and electrolytes secretion from the gut.
5. Inhibition of gastrin release and thus gastric acid secretion.
6. Increases hormonal release from the pancreas, gut, and
hypothalamus.
7. Stimulation of lipless, glycogenolysis, and bile flow.
Gastric Inhibitory Polypeptide (GIP)
It is consisted of 42 a.a. released from [K- cells]
which are located in the jeujenal and duodenal
mucosa. Its main functions include:
1. In the presence of hyperglycemia, like following
meals, it stimulates the secretion of insulin. This is
the most important action of GIP, and that is why it is
called {Glucose dependant insulinotropic peptide}.
2. Decreases intestinal motility despite increased
intestinal secretion of water and electrolyte.
3. In supraphysiological concentrations, it acts to
inhibit gastric acid, pepsin, and gastric secretion.
Other Hormones of the Gut
Other hormones of the gut include:
Somatostatin: It is one of the most potent well
known inhibitory hormones for a lot of endocrine
secretion. It inhibits the release (as well as the
action on the target cells) of a lot of hormones
including gastrin, secretin, GIP, insulin, glucagon,
VIP, pancreatic polypeptide, and others.
Motilin: It acts to increase the contraction of the
smooth muscle fibers in the upper GIT namely the
lower esophageal sphincter, gastric fundus, antrum,
and duodenum. Its unique feature is that its actions
are restricted to fasting state only.
Pancreatic Polypeptide (PP): It is mainly
secreted from the pancreas. It acts to increase
and then decrease (i.e. biphasic action) the
pancreatic secretion of water, electrolytes, and
enzymes. Its high blood levels can be used as a
landmark of pancreatic endocrine tumors.
Enteroglucagon: Is a group of substances that
are released from the gut but have biological
effects similar to pancreatic glucagon (i.e.
catabolic hyperglycemic) although are chemically
different.