Epinephrine, also known as adrenaline, is the principal hormone governing the "fight or flight" response to various stimuli. In addition to stimulating glycogenolysis, the hormone triggers a variety of physiological events, such as increasing depth and frequency of heartbeats.

Epinephrine is secreted from the adrenal medulla and binds to specific receptors on muscle cell membranes. Binding of the hormone at the membrane stimulates the synthesis of cAMP by membrane-bound adenylate cyclase, through the action of a G protein, Gs. cAMP in turn activates cAMP-dependent protein kinase, which catalyzes the phosphorylation of phosphorylase b kinase (Figure 16.11, Figure 13.18). This kinase in turn catalyzes the phosphorylation of phosphorylase b to a and, hence, the activation of glycogen breakdown, through the action of phosphorylase a. These events explain how the secretion of relatively few molecules of hormone, such as epinephrine, can, within just a few moments, trigger a massive conversion of glycogen to glucose-1-phosphate.

The effects of epinephrine are opposed by insulin