cyclic AMP
Cyclic AMP is the cell's most famous “alarm signal.” When certain hormones knock on the door, the cell rings this internal bell by making a quick burst of cyclic AMP, and the bell tells the rest of the cell to spring into action.
Chemically, cyclic AMP (cyclic adenosine monophosphate) is made from ATP by the enzyme adenylyl cyclase, which is switched on when a stimulatory G protein is activated by a hormone-bound receptor. Its main job is to activate protein kinase A, an enzyme that then phosphorylates many targets to carry out the hormone's effects. Cyclic AMP is short-lived because another enzyme, phosphodiesterase, quickly chops it back down.
This pathway is a workhorse of endocrinology: adrenaline, glucagon, TSH, ACTH, and several releasing hormones all raise cyclic AMP in their target cells. Because the signal can be both made and destroyed within seconds, cyclic AMP allows responses that are rapid, strongly amplified, and easy to switch off when the hormone leaves.
ACTH from the pituitary raises cyclic AMP in adrenal cortex cells, which through protein kinase A drives the synthesis of cortisol.
Cyclic AMP linking a pituitary hormone to steroid production.
Caffeine prolongs cyclic AMP signaling partly by blocking phosphodiesterase, the enzyme that normally breaks it down — one reason caffeine has stimulant effects.