cGMP is formed by action of the enzyme guanylate cyclase on GTP:

GTP <=> cGMP + PPi

In smooth muscle, cGMP synthesis is stimulated by nitric oxide. In the eye, cGMP is broken down by a specific phosphodiesterase activated by the transducin-GTP complex. The product of action of the phosphodiesterase is GMP. Cleavage of cGMP stimulates intracellular reactions that generate a visual signal to the brain. Thus, the stimulated hydrolysis of cGMP is the visual analog of the stimulated synthesis of cAMP in -adrenergic responses.