FIGURE 1.

Classification and specificity of phosphodiesterases (PDEs) toward cyclic nucleotides and downstream effectors. PDE4, 7, and 8 hydrolyze cAMP, whereas PDE5, 6, and 9 hydrolyze cGMP. The other PDEs, including PDE1, 2, 3, 10, and 11, hydrolyze both cAMP and cGMP. cAMP targets protein kinase A (PKA), exchange protein directly activated by cAMP (Epac), hyperpolarization-activated cyclic nucleotide-gated channel (HCN), and popeye domain containing protein (POPDC), whereas cGMP targets protein kinase G (PKG), cyclic nucleotide-gated channel (CNG), and PDE.