Glucagon, Insulin and their effect on the liver and blood sugar levels.

Glucagon and Insulin

Glucagon and Insulin are 2 hormones produced in the pancreatic islets (Islets of Langerhans), glucagon is secreted by the alpha cells (which along with delta and pancreatic polypeptide cells are found in the outer portion of an islet) and insulin is secreted by the beta cells (found near the center of an islet ). The relationship between glucagon and insulin is an antagonistic one, when there is high levels of insulin there will be low levels of glucagon and vice versa. Between them they help to keep the blood sugar level stable.

Secretion of Glucagon and Insulin:

When blood sugar levels drop below the set-point insulin secretion is inhibited and the alpha cells secrete glucagon. On the other side of the scale, when blood sugar levels rise too far above the set-point then the beta cells will secrete insulin, this then inhibits glucagon ( they operate on a negative feedback loop ).

Effects of Glucagon and Insulin on the Liver:

Glucagon:
- speeds up the breakdown of glycogen to glucose in the liver (and skeletal muscle) cells. Thus glucose storage stops.
- It stimulates the liver cells to synthesize more glucose from the glycerol which is absorbed from the blood.
- Fat production stops.

Insulin:
- speeds up the formation of glycogen from glucose in the liver ( and the skeletal muscle cells).
- stimulates liver cells to synthesize fat from glucose.

Effect of Glucagon and Insulin on Blood Sugar Level:

Glucagon:
As well as its effects on the liver, glucagon also increases the breakdown of fat into fatty acids in adipose tissue which leads to fatty acids being released into the blood ( so providing energy for the cells). It also stimulates glucose release into the blood. All these effects together combine to cause blood sugar to rise back to a normal level.

Insulin:
Including its effects on the liver, insulin also stimulates most of the body cells to take up more glucose form the blood and it increases the rate at which glucose is used as an energy source. All the effects of insulin on the body, together ensure that blood sugar drops down to a normal level.

An example of how Glucagon and Insulin control Blood Sugar Level:

One eats a meal which causes a great rise in the blood sugar level (eg. a meal high in carbohydrates) ( a hyperglycemic effect), this signals the pancreas (more specifically the beta cells of the pancreatic islets) to secrete the hormone insulin. This release of insulin causes body cells to take up more glucose from the blood, the cells to use up glucose faster as an energy source, glucose from the liver and skeletal muscles to be used to form glycogen and fat to be made from the glucose in fat tissue and liver cells. All these together cause the blood sugar level to drop back to a normal level. A few hours pass and eventually the blood sugar level will drop below the normal( or set point ) level when all the nutrients which had been supplied by the meal have ceased to circulate in the blood (hyperglycemic effect). This drop in blood sugar signals the pancreas to stop secreting insulin and start secreting glucagon (from the alpha cells in the pancreatic islets). This causes glycogen in the liver and skeletal muscle cells to be broken down into glucose more quickly, it causes more fats to be broken down into fatty acids and glycerol in the fatty tissue and also then that the fatty acids and glycerol are released into the blood so the cells can use them as an alternative energy source to glucose. It also stimulates an increase of glucose synthesis (from the glycerol absorbed from the blood) and of the release of glucose into the blood. All of this causes the blood sugar level to rise again to a normal (set-point) level.

Glucagon and Insulin may seem to be working as enemies of one another but eventually their ultimate goal is the same. They both help to maintain a normal blood sugar level. Just one of them cannot do this on its own. Another amazing example of the bodys drive for balance on all levels.