cyclic AMP

Cyclic AMP is the cell's most famous internal alarm bell. When it rings — that is, when its level rises inside the cell — it tells the machinery that an external hormone has arrived and the cell should change what it is doing. It was the very first second messenger ever discovered.

Chemically it is adenosine 3',5'-cyclic monophosphate, made from ATP by adenylyl cyclase and destroyed by phosphodiesterases. Its main job is to activate protein kinase A, which then phosphorylates a long list of target proteins — metabolic enzymes, ion channels, and gene-regulating proteins — producing effects tailored to each cell type.

The same rising cyclic-AMP signal means "speed up and pump harder" in a heart cell, "relax" in airway smooth muscle, and "break down stored fuel" in liver and fat. Drugs exploit this widely: beta-agonists and phosphodiesterase inhibitors raise cyclic AMP, while drugs working through Gi-coupled receptors lower it. PDE inhibitors like sildenafil (acting on cyclic GMP) and milrinone (acting on cyclic AMP in the heart) show how prolonging a cyclic nucleotide's life can be therapeutic.