Pancreas

The bulk of the pancreas is an exocrine gland secreting pancreatic fluid into the duodenum after a meal. However, scattered through the pancreas are several hundred thousand clusters of cells called islets of Langerhans. The islets are endocrine tissue containing four types of cells. In order of abundance, they are the:


beta cells, which secrete insulin and amylin;
alpha cells, which secrete glucagon;
delta cells, which secrete somatostatin, and
gamma cells, which secrete a polypeptide of unknown function.


Beta Cells

Beta cells secrete insulin in response to a rising level of blood sugar

Insulin affects many organs. It


stimulates skeletal muscle fibers to

take up glucose and convert it into glycogen;
take up amino acids from the blood and convert them into protein.


acts on liver cells

stimulating them to take up glucose from the blood and convert it into glycogen while
inhibiting production of the enzymes involved in breaking glycogen back down (glycogenolysis) and
inhibiting gluconeogenesis; that is, the conversion of fats and proteins into glucose.


acts on fat (adipose) cells to stimulate the uptake of glucose and the synthesis of fat.
acts on cells in the hypothalamus to reduce appetite.


Diabetes Mellitus

Diabetes mellitus is an endocrine disorder characterized by many signs and symptoms. Primary among these are:


a failure of the kidney to retain glucose .
a resulting increase in the volume of urine because of the osmotic effect of this glucose (it reduces the return of water to the blood).


There are three categories of diabetes mellitus:


Insulin-Dependent Diabetes Mellitus (IDDM) (Type 1) and
Non Insulin-Dependent Diabetes Mellitus (NIDDM)(Type 2)
Inherited Forms of Diabetes Mellitus


Insulin-Dependent Diabetes Mellitus (IDDM)

IDDM ( Type 1 diabetes)


is characterized by little or no circulating insulin;
most commonly appears in childhood.
It results from destruction of the beta cells of the islets.
The destruction results from a cell-mediated autoimmune attack against the beta cells.
What triggers this attack is still a mystery, although a prior viral infection may be the culprit.


Non Insulin-Dependent Diabetes Mellitus (NIDDM)

Many people develop diabetes mellitus without an accompanying drop in insulin levels In many cases, the problem appears to be a failure to express a sufficient number of glucose transporters in the plasma membrane (and T-system) of their skeletal muscles. Normally when insulin binds to its receptor on the cell surface, it initiates a chain of events that leads to the insertion in the plasma membrane of increased numbers of a transmembrane glucose transporter. This transporter forms a channel that permits the facilitated diffusion of glucose into the cell. Skeletal muscle is the major "sink" for removing excess glucose from the blood (and converting it into glycogen). In NIDDM, the patient's ability to remove glucose from the blood and convert it into glycogen is reduced. This is called insulin resistance. NIDDM (also called Type 2 diabetes mellitus) usually occurs in adults and, particularly often, in overweight people.

Alpha Cells

The alpha cells of the islets secrete glucagon, a polypeptide of 29 amino acids. Glucagon acts principally on the liver where it stimulates the conversion of glycogen into glucose (glycogenolysis) which is deposited in the blood.

Glucagon secretion is


stimulated by low levels of glucose in the blood;
inhibited by high levels, and
inhibited by amylin.


The physiological significance of this is that glucagon functions to maintain a steady level of blood sugar level between meals.

Delta Cells

The delta cells secrete somatostatin. Somatostatin has a variety of functions. Taken together, they work to reduce the rate at which food is absorbed from the contents of the intestine. Somatostatin is also secreted by the hypothalamus and by the intestine.

Gamma Cells

The gamma cells of the islets secrete pancreatic polypeptide. No function has yet been found for this peptide of 36 amino acids.