Intracellular signaling was first elucidated by studies of the action of hormones such as epinephrine, which signals the breakdown of glycogen to glucose in anticipation of muscular activity. In 1958, Earl Sutherland discovered that the action of epinephrine was mediated by an increase in the intracellular concentration of cyclic AMP (cAMP), leading to the concept that cAMP is a second messenger in hormonal signaling (the first messenger being the hormone itself).