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cGMP is a ubiquitous second  messenger mediating cellular responses to various 

 exogenous and endogenous signaling molecules.  cGMP regulates physiological processes by  

activating protein kinases, gating specific ion  channels, and modulating cellular cyclic nucleotide concentrations through phosphodiesterases. The  conversion of GTP to cGMP is catalyzed by guanylyl cyclases (GCs). There are two types of  GCs in mammals: the soluble and the  membrane-boundGCs. The soluble GCs are  generally activated when nitric 

oxide binds to the attached prosthetic heme group. Seven  membrane-bound GCs (also named transmembrane  or particulated GCs) have been identified in the  human genome. GC-A and GC-B are natriuretic  peptide receptors. GC-C can be activated by  bacterial heat-stable enterotoxins,guanylin and  uroguanylin. The activity of transmembrane GCs  can also be modulated by other receptor signals  through intracellular signaling pathways.