Sutherland won the Nobel Prize in 1971 for his discovery of the mechanism of action of epinephrine in glycogenolysis, that requires cAMP as secondary messenger.
[2] G protein-coupled receptors (GPCRs) are a large family of integral membrane proteins that respond to a variety of extracellular stimuli.
Each GPCR binds to and is activated by a specific ligand stimulus that ranges in size from small molecule catecholamines, lipids, or neurotransmitters to large protein hormones.
[3] When a GPCR is activated by its extracellular ligand, a conformational change is induced in the receptor that is transmitted to an attached intracellular heterotrimeric G protein complex.
Many different cell responses are mediated by cAMP; these include increase in heart rate, cortisol secretion, and breakdown of glycogen and fat.
The pathway may also be deactivated downstream by directly inhibiting adenylyl cyclase or dephosphorylating the proteins phosphorylated by PKA.