So, what happens when you do not eat? In times of fasting, your pancreas releases glucagon so that your body can produce glucose. Glucagon is another protein hormone that is made and secreted by the alpha cells of the pancreatic islets. Glucagon acts on the same cells as insulin, but has the opposite effects:
- Stimulates the liver and muscles to break down stored glycogen (glycogenolysis) and release the glucose
- Stimulates gluconeogenesis in the liver and kidneys
In contrast to insulin, glucagon mobilizes glucose from stores inside your body and increases the concentrations of glucose in the bloodstream -- otherwise, your blood glucose would fall to dangerously low levels.
So how does your body know when to secrete glucagon or insulin? Normally, the levels of insulin and glucagon are counter-balanced in the bloodstream. For example, just after you eat a meal, your body is ready to receive the glucose, fatty acids and amino acids absorbed from the food. The presence of these substances in the intestine stimulates the pancreatic beta cells to release insulin into the blood and inhibit the pancreatic alpha cells from secreting glucagon. The levels of insulin in the blood begin to rise and act on cells (particularly liver, fat and muscle) to absorb the incoming molecules of glucose, fatty acids and amino acids. This action of insulin prevents the blood-glucose concentration (as well as the concentrations of fatty acids and amino acids) from substantially increasing in the bloodstream. In this way, your body maintains a steady blood-glucose concentration in particular.
In contrast, when you are between meals or sleeping, your body is essentially starving. Your cells need supplies of glucose from the blood in order to keep going. During these times, slight drops in blood-sugar levels stimulate glucagon secretion from the pancreatic alpha cells and inhibit insulin secretion from the beta cells. Blood-glucagon levels rise. Glucagon acts on liver, muscle and kidney tissue to mobilize glucose from glycogen or to make glucose that gets released into the blood. This action prevents the blood-glucose concentration from falling drastically.
As you can see, the interplay between insulin and glucagon secretions throughout the day help to keep your blood-glucose concentration constant, staying at about 90 mg per 100 ml of blood (5 millimolar).