cyclic AMP is a second messenger important in many biological processes. cyclic AMP is derived from adenosine triphosphate (ATP) and used for intracellular signal transduction in many different organisms, conveying the cAMP-dependent pathway.    cyclic AMP is synthesised from ATP by adenylyl cyclase located on the inner side of the plasma membrane. Adenylyl cyclase is activated by a range of signaling molecules through the activation of adenylyl cyclase stimulatory G (G_s)-protein-coupled receptors and inhibited by agonists of adenylyl cyclase inhibitory G (G_i)-protein-coupled receptors. Liver adenylyl cyclase responds more strongly to glucagon, and muscle adenylyl cyclase responds more strongly to adrenaline. cyclic AMP decomposition into AMP is catalyzed by the enzyme phosphodiesterase.