Ligand Receptor Receptor-ligand Inactive Active G protein G protein Adenylyl cyclase Inactive effector Active enzyme CGMP phosphodiesterase enzyme or ion channel or ion channel H Phospholipase C Phospholipase A2 Ca++or K+ channels Low concentration High concentration ----_ cGMP of second messenger of second messenger PKA Inactive enzyme Active PKG or ion channel Calmodulin-dep kinase ctor enzyme ion channel PKC Ca++ channel K+ channel