Figure 1 - The glycogenolytic cascade in mammalian skeletal muscle.

Adrenalin stimulates the production of 3' 5' cyclic adenosine monophosphate (cAMP) leading the sequential activation of cAMP-dependent protein kinase and phosphorylase kinase. The latter converts glycogen phosphorylase from the inactive dephosphorylated b form to the active phosphorylated a form, stimulating glycogenolysis in advance of an increased energy demand. The activity of phosphorylase kinase also depends on calcium ions and is therefore also switched on during muscle contraction. This provides energy (via the breakdown of glycogen) to sustain muscle contraction.