GLUCAGON-LIKE POLYPEPTIDE-2

GLUCAGON-LIKE POLYPEPTIDE-2

Glucagon-like polypeptide-2 (GLP-2) is secreted by L cells in ileum and colon (see above). Structurally, it is similar to GLP-1 and glucagons. Like GLP-1, it is also found in brain.

Stimulant for Secretion

Presence of food with glucose in the small intestine stimulates the release of GLP-2 also.

Action

GLP-2 is believed to suppress appetite.

SOMATOSTATIN

Somatostatin was first found in hypothalamus and named as growth hormone-inhibiting hormone. Now it is found in D cells of stomach and upper part of small intestine and D cells of pancreatic islets also. Somatostatin is secreted in two forms, one with 14 amino acids and the other one with 28 amino acids.

Stimulant for Secretion

Presence of chyme with glucose and proteins in stomach and small intestine causes release of somatostatin.

Actions

Somatostatin:

i. Inhibits the secretion of growth hormone (GH) and thyroid-stimulating hormone (TSH) from anterior pituitary

ii. Inhibits gastric secretion and motility

iii. Inhibits secretion of pancreatic juice

iv. Inhibits secretion of GI hormones such as:

a. Gastrin

b. Cholecystokinin (CCK)

c. Vasoactive intestinal polypeptide (VIP)

d. Gastric inhibitory peptide (GIP).

PANCREATIC POLYPEPTIDE

Source of Secretion

Pancreatic polypeptide is a polypeptide with 36 amino acid residues. It is secreted mainly by the PP cells of the islets of Langerhans in pancreas. It is also found in small intestine.

Stimulant for Secretion

Pancreatic polypeptide is secreted by the presence of chyme with proteins in the small intestine. It is also

secreted in conditions like hypoglycemia, fasting and exercise.

Actions

Pancreatic polypeptide:

i. Increases the secretion of glucagon from α-cells of islets of Langerhans in pancreas.

ii. Decreases the secretion of pancreatic juice from

exocrine part of pancreas.

PEPTIDE YY

Polypeptide YY with 36 amino acid residues, is structurally related to pancreatic polypeptide and neuropeptide Y. It is secreted in L cells of ileum and colon.

Stimulant for Secretion

Presence of fat-containing chyme stimulates the release of peptide YY Actions

Peptide YY:

i. Inhibits gastric secretion and motility

ii. Reduces secretion of pancreatic juice

iii. Inhibits the intestinal motility and stops passage of bowel beyond ileum (ileal brake)

iv. Suppresses appetite and food intake.

NEUROPEPTIDE Y

Neuropeptide Y contains 36 amino acid residues. It is structurally related to pancreatic polypeptide and peptide YY. It is secreted by enteric nerve endings particularly in ileum and colon. It is also secreted in medulla, hypothalamus and neurons of autonomic nervous system (ANS).

Stimulant for Secretion

Secretion of neuropeptide Y is stimulated by fatcontaining chyme.

Action

Neuropeptide Y increases the blood flow in enteric blood vessels and stimulates food intake.

MOTILIN

Motilin is built by 22 amino acid residues. It is secreted by Mo cells, which are present in stomach and intestine. It is also believed to be secreted by enterochromoffin cells of intestine.

Stimulant for Secretion

Motilin is secreted when the chyme from stomach enters the duodenum.

Actions

Motilin:

i. Accelerates gastric emptying

ii. Increases the mixing and propulsive movements of small intestine

iii. Increases the peristalsis in colon.

SUBSTANCE P

Source of Secretion

Substance P is a neurotransmitter with 11 amino acid residues. It is secreted at the pain nerve endings in

brain and enteric nerve endings in small intestine.

Stimulant for Secretion

Secretion of substance P in intestine is caused by the presence of chyme.

Actions

In GI tract, substance P increases the mixing and propulsive movements of small intestine.

GHRELIN

Ghrelin is a recently discovered hormone. This 28 amino acid polypeptide is synthesized by epithelial

cells in the fundus of stomach. It is also produced in smaller amounts in hypothalamus, pituitary, kidney and placenta.

Stimulant for Secretion

Secretion of ghrelin increases during fasting and decreases when stomach is full.

Actions

Ghrelin:

i. Promotes the secretion of growth hormone (GH) by stimulating somatotropes (growth hormone

synthesizing cells) in anterior pituitary. Receptors for this hormone called growth hormone

secretogogues receptor (GHS-R) were identified in the somatotropes before the discovery of the

hormone itself. These receptors are also found in adipose tissue, heart and hypothalamus.

ii. Induces appetite and food intake by acting via feeding center in hypothalamus.

iii. Stimulates gastric emptying.

OTHER GASTROINTESTINAL HORMONES

Mucosa of GI tract secretes many other hormones such as:

1. Enkephalins

2. Dynorphin

3. Neurotensin

4. Serotonin

5. Urogastrone

6. Enterocrinin

7. Villikinin

8. Guanylin

9. Bombesin.

However, the significant biological actions of these hormones on GI tract are not clear.

CARBOHYDRATES IN DIET

Human diet contains three types of carbohydrates:

1. POLYSACCHARIDES

Large polysaccharides are glycogen, amylose and amylopectin, which are in the form of starch (glucose

polymers). Glycogen is available in non-vegetarian diet. Amylose and amylopectin are available in vegetarian diet because of their plant origin.

2. DISACCHARIDES

Two types of disaccharides are available in the diet.

i. Sucrose (Glucose + Fructose), which is called table sugar or cane sugar

ii. Lactose (Glucose + Galactose), which is the sugar available in milk.

3. MONOSACCHARIDES

Monosaccharides consumed in human diet are mostly glucose and fructose.

Other carbohydrates in the diet include

i. Alcohol

ii. Lactic acid

iii. Pyruvic acid

iv. Pectins

v. Dextrins

vi. Carbohydrates in meat.

Diet also contains large amount of cellulose, which cannot be digested in the human GI tract so it is not

considered as a food for human beings.

 

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