The formation and decomposition of cyclic adenosine monophosphate depend on the existence of Mg2+. When the cell is stimulated by the outside world, the extracellular signal molecules first combine with the receptor to form a complex, and then activate the Gs-protein on the cell membrane, and the activated Gs-protein activates the adenylate cyclase (AC) on the cell membrane, catalyzing ATP to remove a pyrophosphate to generate cAMP2.