The pancreas

a.kannout

THE PANCREAS

The pancreas is made up of two functionally different parts: the
exocrine pancreas, the major digestive gland of the body; and the
endocrine pancreas.
The endocrine pancreas consists of a large number (about 1
million) groups (clusters) of cells scattered within the tissue of the
exocrine pancreas called islets of Langerhans. The islets form less than
2% of the pancreatic tissue.
Two main types of cell have been identified in the endocrine
pancreas, the A- and B-cells (α- and β-cells) that synthesize, store, and
secrete the hormones glucagon and insulin, respectively, while D- and Fcells
are found in much smaller numbers. D-cells (δ-cells) secrete
somatostatin and F-cells produce pancreatic polypeptide.
Insulin
Insulin molecule consists of a 21 amino acid α- chain and a 30
amino acid β-chain, with the two chains linked together by disulphide
bridges.
Insulin is a dominant metabolic hormone primarily involved in the
control of the blood glucose concentration. It is the only hormone that
directly lowers blood glucose levels. Although the release of insulin from
β-cells is regulated by many factors, the principal stimulus is an increased
blood glucose concentration.
The main target organs of insulin are the liver and almost all cells
of the body. In the liver and skeletal muscle cells insulin stimulates
glucose uptake and glycogenesis (formation of glycogen, i.e storage of
glucose), while in the body cells it stimulates glucose uptake for
immediate energy use, for the storage of energy in fats and for the
synthesis of proteins.
Glucagon
It is a polypeptide synthesized in the α-cells of the islets of
Langerhans. Glucagon exerts actions on carbohydrate, protein, and fat
metabolism, however its principal effect is to raise the blood glucose
concentration. It is very important in maintaining blood glucose and
energy substrates in the circulation during fasting.
The target organs of the hormone are the liver (the main target) and
adipose tissue where it
1. Stimulates breakdown of stored glycogen (glycogenolysis)
2. Stimulates gluconeogenesis
3. Stimulates ketogenesis
4. Stimulates lipolysis in adipose tissue
Somatostatin
In the pancreas, somatostatin is synthesized by the D-cells. Since it
was identified in the hypothalamus where it was found to have the ability
to inhibit release of growth hormone from the pituitary, somatostatin has
also been identified in a number of tissues, including many areas of the
brain, the gastrointestinal tract, and the pancreas. Generally, somatostatin
is an inhibitory paracrine hormone, and in the pancreas it inhibits the
release of both insulin and glucagon.
The hormone has been implicated in the regulation of nutrient
concentrations in the blood where it appears to prevent the exaggerated
responses following a meal. It has been found to retard the entry of
nutrients into the blood by inhibiting various digestive events such as
gastric emptying, acid, pepsin and gastrin secretions, sugar and fat
absorption, and duodenal motility. Somatostatin therefore, may function
as a hormone that regulates the movement of nutrients from the gut to the
internal environment.
Pancreatic Polypeptide (PP)
It is a peptide found in the F-cells of the islets of Langerhans in the
pancreas. PP is released after a high protein meal and in case of
hypoglycemia. Its major effect is the inhibition of the secretion of
enzymes by the pancreas and the bile by the gall-bladder.
Amylin
Recent literature reveals that this peptide is secreted from B-cells
of the pancreas that secretes insulin. Although it is cosecreted with insulin
in response to glucose and other B-cell stimulators, it was found to have
opposing metabolic effects to those of insulin. Its metabolic role is
unclear.
DIABETES MELLITUS
It is a prevalent disease characterized by excessive excretion of
urine and above normal blood glucose level. This is one of the few
diseases that have been well documented and described since 1500B.C.
It is a prevalent serious hormonal disease in which the body cells
are unable to absorb glucose from the blood. This disease occurs when
there is not enough insulin in the blood or when the body cells do not
respond normally to blood insulin. In either case, the cells cannot obtain
enough glucose from the blood, and thus, starved for fuel, they are forced
to burn the body’s supply of fats and proteins.
There are two types of diabetes
Type I: Insulin-Dependent Diabetes Mellitus (IDDM)
It is characterized by progressive marked decrease in the number of
insulin secreting B-cells. It is an autoimmune disease where destruction
of insulin secreting cells by islet-cell antibodies occurs. As a result the
pancreas does not produce enough insulin.
It is a catabolic disorder in which due to the absence of circulating
insulin, the three main target tissues of insulin (liver, muscle and fat) fail
to take up absorbed nutrients and continue to spew glucose, amino acids
and fatty acids into the bloodstream from their respective storage depots.
Furthermore, continued fat breakdown lead to the production and
accumulation of ketones.
Type II: NonInsulin-Dependent Diabetes Mellitus (NIDDM).
This is the more prevalent type and occurs predominantly in adults.
In contrast to type I diabetes, circulating insulin is always present. It may
be due to several factors, either the insulin secreted is not sufficient to
lower the blood glucose to the normal level (insufficient insulin
secretion), or the tissues do not respond to the hormone (insulin
resistance).
Meanwhile, since the digestive system continue to absorb glucose
from the diet, the glucose concentration in the blood becomes extremely
high and glucose is excreted in the urine.

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