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The responses involve the release inside the cell of molecules collectively

known as second messengers; that is, intracellular mediators of the externally received hormonal message. Different receptors cause the release of different second messengers. cAMP, identified by Earl Sutherland in the 1950s, was the second messenger discovered.

Therefore, the intracellular concentration of cAMP determines the fraction of PKA in its active form and thus the rate at which it phosphorylates its substrates. When a hormone stimulates the intracellular cAMP level, PKA activity rises, which raises the rate at which numerous proteins are phosphorylated and decreases the rate at which they are dephosphorylated. The diagrammed regulatory system is a cascade. Therefore, a little change in [cAMP] causes a huge change in the fraction of phosphorylated enzymes. Since glycogen phosphorylase is active and glycogen synthase is inactive when a significant portion of the glycogen metabolism enzymes are phosphorylated, the metabolic flux is in the direction of glycogen breakdown. Dephosphorylation and phosphorylation rates both decrease as [cAMP] levels drop.