Second Messenger

A second messenger is the intracellular relay between an occupied receptor and the enzymes producing a cellular response. The main ones are cyclic AMP, made by adenylate cyclase and acting largely through protein kinase A; inositol trisphosphate and diacylglycerol from phospholipase C, which release stored calcium and activate protein kinase C; cyclic GMP; and calcium itself. The step amplifies: one activated receptor cycles many G proteins, and each active cyclase makes many messenger molecules.

Which messenger a peptide uses follows from its receptor's G protein coupling. The GLP-1 and GHRH receptors couple through Gs and raise cAMP; the growth hormone secretagogue receptor couples through Gq and produces an inositol trisphosphate and calcium signal. That determines the assay a laboratory runs — cAMP accumulation for one, calcium flux for the other — and so determines what any published EC50 actually measured.

Knowing the readout matters because the same ligand at the same receptor gives different potency numbers depending on the branch measured. cAMP accumulation, calcium mobilisation and beta-arrestin recruitment are separate endpoints, and a systematic difference between them is what biased agonism means. A quoted EC50 without its readout is incomplete, not merely imprecise.

The practical trap is cross-assay comparison. Recombinant lines overexpressing a receptor carry large receptor reserve, shifting the measured EC50 lower and making a partial agonist look full; native tissue with sparse receptors gives a different figure for the same compound. Comparing potency values pulled from separate papers, cell backgrounds and readouts, as vendor spec sheets do, produces rank orders that do not survive one side-by-side experiment.