Secondary messengers

Methods of signalling across the plasma membrane are mannifold. The primary signal to the outside of the cell may be:

• a voltage change across the membrane
• a chemical such as a drug, hormone or transmitter acting at a discrete receptor or non-specifically on the membrane

The secondary messenger, or secondary signal, to the inside of the cell can be:

• cyclic AMP
• calcium
• entry of another ion down an electrochemical gradient

Third messengers are the molecules or ions subsequently activated or mobilized by the activity of the second messenger.

The two key secondary messengers, calcium and cAMP, interact e.g. they may have common protein kinase targets or they can inhibit each other's production.