Cyclic nucleotide phosphodiesterase enzymes catalyse the hydrolysis of second messengers, cAMP and cGMP. cAMP plays an important role in the carbohydrate and lipid breakdown in response to certain type of hormones (e.g. insulin, adrenaline). cGMP provides the cGMP-dependent Na+ channels to keep open of the retinal rod photoreceptor cells, regulating some of the PDE's activity and also has a role in mediating smooth muscle contractions (e.g. penile erection). The only hydrolysing enzymes of these second messengers are phosphodiesterases. PDE superfamily consists of ten phosphodiesterase subfamilies and most of them have more than one isoform. Breakdown of cAMP and cGMP play a pivotal role in terminating cAMP and cGMP signal transduction