Mechanism of smooth muscle relaxation and peripheral inhibition sites. Abbreviations: nitric oxide (NO), neuronal NO synthase (nNOS), endothelial NOS (eNOS), prostaglandin (PGE1), PGE1 receptor (EP2), guanosine triphosphate (GTP), guanylyl cyclase (GC), cyclic guanosine monophosphate (cGMP), phosphodiesterase type 5 (PDE5), adenosine triphosphate (ATP), cGMP-dependent protein kinase type I (cGKI), protein kinase A (PKA), cyclic adenosine monophosphate (cAMP), vasoactive intestinal peptide (VIP), VIP receptor (VIPR) and neutral endopeptidase (NEP).