Chemical messengers such as hormones and neurotransmitters are used for intercellular communication. One cell will release a messenger such as epinephrine, which binds to cells, which have a specific receptor for epinephrine. Once the messenger binds to the receptor, one of several second messenger systems is activated. The cyclic AMP (cAMP) system is one example of a second messenger system.
Epinephrine, the first messenger, is released and binds to many cells, including heart cells, that display an epinephrine receptor. Binding of epinephrine to the receptor stimulates the release of a peripheral protein known as a G protein.
The G protein travels and binds to an integral protein, adenylate cyclase, thus activating it. The active adenylate cyclase removes two phosphate groups from ATP, converting ATP to cyclic AMP. cAMP is the second messenger.
cAMP binds to an inactive protein kinase, promoting the release of the active subunit. The active protein kinase phosphorylates various cytoplasmic enzymes.
The addition of a phosphate group to some enzymes, such as enzyme B, will inactivate the enzyme, while phosphorylation of others such as enzyme A will promote activation of the enzyme.
In the case of epinephrine and cardiac cells, alteration of enzyme activity leads to an increase in the rate and strength of contraction of the heart (that is, an increase in heart rate).