Receptor-Ligand Interactions that Generate Proportional Physiological Effects
It is quite possible that a ligand can bind to a receptor and have no physiological effect. Binding alone is not the whole story. Similarly, the ligand must be capable of stimulating the receptor when it binds, such that the receptor can produce its effect. Ligands which bind to a receptor but don’t stimulate it, i.e., those that have zero intrinsic activity. It is quite possible that not all ligands which bind to a particular receptor have the same intrinsic activity. In other words, the same level of receptor occupancy for two different ligands that both bind to the same receptor may give different levels of stimulation. Receptor ligands that have zero intrinsic activity are antagonists. Any ligand that has greater than zero intrinsic activity with a receptor is an agonist for that receptor. Those agonists that have less than maximal intrinsic activity are termed partial agonists.