Cyclic AMP (cAMP): A second messenger important in many biological processes, produced from ATP by the action of adenylyl cyclase.
Protein kinase A: An enzyme that is activated by cAMP and phosphorylates other proteins, altering their activity.
G-proteins: Proteins that bind the guanine nucleotides GDP and GTP and are involved in transmitting signals from different stimuli outside a cell to its interior.