GLOSSARY

Second messenger activation is the process in which hormone binding (the first messenger) to a membrane receptor triggers the production or release of small intracellular molecules (second messengers) that amplify the signal and relay it to target proteins.

Main Second Messengers

• cAMP: made by adenylate cyclase (from ATP); activates PKA.
• cGMP: made by guanylate cyclase; ANP and nitric oxide; vasodilation.
• IP₃ (inositol trisphosphate): releases calcium from the endoplasmic reticulum.
• DAG (diacylglycerol): activates protein kinase C (PKC).
• Calcium ion (Ca²⁺): binds calmodulin and activates kinases.

Signal Amplification

One hormone molecule → one receptor → dozens of G proteins → hundreds of cAMP molecules → thousands of active kinases → millions of product molecules (such as glucose from glycogen); this is why hormones act at very low concentrations