Second messengers are the organic molecules and sometimes the metal ions, acting as intracelluiar signals, whose production or release usually amplifies a signal such as a hormone, received at the cell surface. Sodium (Na) is not a second messenger in hormone action. Cyclic Adenosine Monophosphate (cAMP) was the first second messenger to-be discovered. In addition to cyclic AMP, Cyclic Guanosine Monophosphate (cGMP) functions as a second messenger in certain cases. Calcium ions (Ca2+) also act as second messenger in phospholipase C−Ca+ second messenger system.