The second messengers cyclic AMP and cyclic GMP

The cAMP is called a second messenger because it is not the hormone itself that directly institutes the intracellular changes; instead, the cAMP serves as a second messenger to cause these effects.For a few peptide hormones, such as the atrial natriuretic peptide, cyclic guanosine monophosphate, which is only slightly different from cAMP, serves in a similar manner as a second messenger.Adenylyl Cyclase-cAMP Second Messenger SystemStimulation of adenylyl cyclase, a membrane-bound enzyme, by the Gs protein then catalyzes the conversion of a small amount of cytoplasmic adenosine triphosphate into cAMP inside the cell. This then activates cAMP-dependent protein kinase, which phosphorylates specific cell proteins, triggering biochemical reactions that ultimately lead to the cell's response to the hormone.Ref: Guyton; 13th edition; Chapter 75; Introduction to Endocrinology