5. second messenger cyclic adenosine monophosphate (or cAMP), cyclic guanosine monophosphate or cGMP), inositol triphosphate (or InsP3), diacylglycerol or DG)and Cat 2+ A second messenger may act to increase the activity of a protein kinase. Certain ion channels are directly modulated by a second messenger.5. second messenger • A second messenger may act to increase the activity of a protein kinase. • cyclic adenosine monophosphate (or cAMP), cyclic guanosine monophosphate (or cGMP), inositol triphosphate (or InsP3), diacylglycerol (or DG) and Ca2＋. • Certain ion channels are directly modulated by a second messenger