Guanosine 3，5-cyclic monophosphate (cyclic GMP; cGMP) acts as a secondary messenger much like cyclic AMP. It is generally known to activate intracellular protein kinases in response to the binding of membrane-impermeable peptide hormones to the cell surface. cGMP synthesis is catalyzed by guanylate cyclase (GC)，which converts GTP to cGMP. Membrane-bound GC is activated by peptide hormones such as atrial natriuretic factor，while soluble GC is typically activated by nitric oxide (NO) to stimulate cGMP synthesis. cGMP is also a common regulator of ion channel conductance，glycogenolysis，and cellular apoptosis. It also relaxes smooth muscle tissues. The roles of cGMP and cAMP may be linked，as evidenced by the fact that some cellular functions are controlled bi-directionally by both cAMP and cGMP. Some functions are stimulated by cGMP and suppressed by cAMP and vice versa.
