Adenylyl cyclases function to convert ATP to cyclic AMP in response to activation by a variety of hormones, neurotransmitters and other regulatory molecules. Cyclic AMP, in turn, activates several other target molecules to control a broad range of diverse phenomena such as metabolism, gene transcription and memory. Adenylyl cyclases respond to receptor-initiated signals, mediated by the Gs and Gi heterotrimeric G proteins. The binding of an agonist to a Gs-coupled receptor catalyzes the exchange of GDP (bound to Galpha s) for GTP, the dissociation of GTP-Galpha s from Gbetagamma and Galpha s-mediated activation of adenylyl cyclase. Adenylyl cyclase IV (AC IV) and IX mRNA are expressed in all kidney nephron segments. AC IV exhibits moderate staining in type II and type IV fibrocytes in rat cochlea and immunoreactivity is also observed in type I fibrocytes. Activation of the D2 dopaminergic and m4 muscarine receptors inhibits the activity of adenylyl cyclase isozymes I, V, VI and VIII, whereas type II, IV and VII are stimulated and type III is not affected.