Epinephrine is constantly produced in small amounts by the adrenal glands. However, when the body is exposed to anxiety, danger, or other types of stress, epinephrine production increases. This causes an elevation of heart rate, blood pressure, and sugar metabolism, which in turn increases alertness, energy level, and physical strength. The production of epinephrine in response to stress is known as fight of flight response. Increased epinephrine levels prepare the body to combat a threat (Fight), or to escape from it (Flight). When epinephrine acts on liver cells, it has the specific function of causing the release of stored glycogen and its decomposition to glucose, releasing energy. A related compound, norepinephrine, a formational predecessor to epinephrine, is also a neurotransmitter.
When released into the bloodstream, adrenaline bonds with receptors on smooth muscle and organ cells. Adrenaline does not act directly on the cells to which it bonds, rather, it stimulated the production of cyclic AMP (cAMP), a molecule formed from ATP. Cyclic AMP in turn regulates and acts with other proteins and compounds within cells to produce the specific functions of adrenaline. The action of epinephrine via the formation of cAMP was discovered in the 1950S by Earl Sutherland.