<s>[INST] context: The cAMP-dependent pathway, also known as the adenylyl cyclase pathway, is a G protein-coupled receptor-triggered signaling cascade used in cell communication G protein-coupled receptors (GPCRs) are a large family of integral membrane proteins that respond to a variety of extracellular stimuli. Each GPCR binds to and is activated by a specific ligand stimulus that ranges in size from small molecule catecholamines, lipids, or neurotransmitters to large protein hormones. When a GPCR is activated by its extracellular ligand, a conformational change is induced in the receptor that is transmitted to an attached intracellular heterotrimeric G protein complex. The Gs alpha subunit of the stimulated G protein complex exchanges GDP for GTP and is released from the complex. In a cAMP-dependent pathway, the activated Gs alpha subunit binds to and activates an enzyme called adenylyl cyclase, which, in turn, catalyzes the conversion of ATP into cyclic adenosine monophosphate (cAMP). Increases in the concentration of the second messenger cAMP may lead to the activation of cyclic nucleotide-gated ion channels [/INST] question: The adenylate cyclase system is mediated by | options: A: cAMP B: Phosphodiesterase C: GTP regulating proteins D: Nuclear receptors | answer: A </s>