Cyclic adenosine 3′,5′-monophosphate (cAMP) was the first identified second messenger. A large number of studies during the past 30 years have elucidated the fundamental role of cAMP in the cellular response to many hormones and neurotransmitters. Although the main components in the cAMP-mediated signalling pathway are well known, recently new important players have been identified, which have further elucidated its complex regulation and the extensive crosstalk between this and other known signalling pathways.

The level of intracellular cAMP is regulated by the balance between the activity of two types of enzyme: adenylyl cyclase (AC) and the cyclic nucleotide phosphodiesterase (PDE). Both enzymes are encoded by a large number of genes, which differ in their expression patterns and mechanisms of regulation.

Nine different adenylyl cyclases exist in mammals. Their activities are stimulated by interaction with the α subunit of Gs proteins. Gs−α subunits are coupled with different types...

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