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HOMEOSTASIS The Pancreas, Insulin and Glucagon Fact Sheet
Anatomy
The pancreas is an exocrine gland secreting alkaline digestive juices. into the duodenum via the pancreatic duct. It also contains endocrine cells in the Islets of Langerhans.
Islets of Langerhans
An oval collection of cells (75 x l75µm), scattered through the pancreas. There are about 1 to 2 million islets in a human pancreas comprising approximately 1 to 2% of its mass. They are darker red than the surrounding tissue having a rich blood supply. The blood from the islets drains into the Hepatic Portal Vein going directly to the liver.
They contain three types of cells
 cells. (20%) secrete glucagon
 cells (75%) secrete insulin
 cells (1 to 8%)
INSULIN |
GLUCAGON |
| A protein made of two short polypeptide chains linked by disulphide bridges. It is synthesised at the ribosome as a single polypeptide (proinsulin) and is later activated by enzymatic cleavage into the two chains. |
A small protein made up of 29 amino acids. It is made by the alpha cells in the Islets. It is also made in an inactive form proglucagon before it is released. |
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Action of Insulin |
Action of Glucagon |
| Increases the entry of glucose into the body cells. Notable exceptions being brain tissue, red blood cells, kidney tubule. and intestinal lining. |
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| Inhibits glycogen breakdown in liver and muscle. |
Glucagon binds to liver cells, stimulating the breakdown of glycogen into glucose. It does not affect glycogen in muscle tissue. |
| Inhibits lipid breakdown in liver and adipose tissue. |
Increases lipid breakdown though its influence is small. |
| Increases the uptake of amino acids by cells and increases the rate of protein synthesis (here it is acting as a growth hormone) |
Stimulates the formation of glucose from amino acids in the liver. |
The action of insulin is very rapid, so is its breakdown (t½ = 10 to 25 min). Once released into the blood stream insulin binds with the receptor sites on its target cells’ plasma membrane. This
stimulates vesicles carrying glucose pores, which lie in the cytoplasm of these cells, to fuse with the plasma membrane. Insulin is broken down by enzyme action in many tissues. |
Glucagon is also very short lived (t½ = 5 to 10 min). Glucagon is broken down particularly by the liver tissue. As the hormone is secreted into the blood flowing into the liver little glucagon is seen circulating in the rest of the body. |
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Control of Insulin Secretion |
Control of Glucagon Secretion |
| Because it has a rapid half-life the islet cells tend to respond more often to stimulating rather than inhibiting chemicals in the plasma. These include: Glucose, amino acids, glucagon, |
Glucagon levels rise between meals as the glucose and fatty acid levels in the blood fall. Secretion is also stimulated by amino acids and hormones secreted by the digestive system.
In many ways it is antagonistic to insulin. |
INSULIN DEFICIENCY: Diabetes mellitus
Symptoms
Hyperglycaemia (high blood glucose)
Glucosuria (glucose in the urine)
Polyuria (passage of copious urine)
Polydipsia (drinking large amounts)
Increased appetite.
Weight loss.
Acids and. ketones in the blood from lipid breakdown (toxic)
Coma if ketones build, up.
Diabetes is sometimes described as:
"STARVATION IN THE MIDST OF PLENTY"
There is plenty of glucose circulating in the blood but it cannot get into the cells that need it.
Treatment
Intravenous or oral doses of insulin at regular intervals.
Strict diet and meal times.
Useful reference
Colorado State University Laura Austgen, R. A. Bowen DVM, PhD & Melissa Rouge
Features a really neat animation of the way insulin makes membranes of cells permeable to glucose.
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© Paul Billiet 2004 |
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