Second Messenger
A small intracellular signalling molecule released inside a cell in response to receptor activation, which amplifies and propagates the signal from the cell surface. Common second messengers include cyclic AMP (cAMP), cyclic GMP (cGMP), calcium ions, and inositol trisphosphate (IP3).
Technical Context
The major second messenger systems are: cAMP pathway (Gαs-activated adenylyl cyclase → cAMP → PKA), cGMP pathway (receptor or soluble guanylyl cyclase → cGMP → PKG — relevant to natriuretic peptides and linaclotide), IP3/DAG pathway (Gαq-activated PLC → IP3 + DAG → calcium release + PKC), and calcium signalling (calcium as a universal intracellular messenger). Second messengers amplify the initial receptor signal — a single receptor activation can produce thousands of second messenger molecules, which in turn activate many downstream enzymes. This amplification cascade explains how picomolar concentrations of peptide hormones can produce large physiological effects.