Insulin vs. Glucagon
Normally, blood glucose concentrations in human blood should range between 70-110 milligrams (mg/ml). Insulin and glucagon operate in an antagonistic (opposing) manner. The result is a precise control of blood glucose levels within this range.
The insulin pathway is activated when blood glucose levels are too high. High blood glucose levels (e.g., occurring after the stomach has digested a food high in sugar) stimulate beta cells in the pancreas to release insulin. Insulin causes an increased uptake of glucose from the blood; promotes conversion of glucose into triglycerides in the liver, fat and muscle cells; and increases the cellular rate of glycolysis – breaking glucose into smaller components that can be used for synthesis of other compounds.
The glucagon pathway is activated when blood glucose levels are too low. Low blood glucose levels (e.g., due to exercise combined with not eating for several
hours) stimulate the alpha cells in the pancreas to produce glucagon. Glucagon causes the liver to convert stored glycogen into glucose, then release the glucose into the blood (a process called glycogenolysis). The two hormones, insulin and glucagon, each regulate the other. A decrease in insulin (as well as low glucose levels) stimulates the secretion of glucagon, while an increase in insulin (as well as an increase in blood glucose) suppresses glucagon secretion. This results in a continuous cycle, with insulin and glucagon constantly monitoring blood glucose levels and regulating their secretion to maintain these levels as nearly constant as possible.
The main function of insulin is removal of excess blood glucose. Because all cells use glucose as an energy source and as a raw material for making other
organic compounds, all cells except brain cells are targets for insulin. Since the function of glucagon is opposite that of insulin, it stimulates the addition of glucose to the bloodstream. Thus, it targets cells with high concentrations of energy stored as glycogen, including the liver and skeletal muscles. It also stimulates glucose production from fats, so adipose tissue cells are another target of glucagon.