<P> The English word antagonist in pharmaceutical terms comes from the Greek ἀνταγωνιστής--antagonistēs, "opponent, competitor, villain, enemy, rival", which is derived from anti - ("against") and agonizesthai ("to contend for a prize"). . </P> <P> Biochemical receptors are large protein molecules that can be activated by the binding of a ligand such as a hormone or a drug . Receptors can be membrane - bound, as cell surface receptors, or inside the cell as intracellular receptors, such as nuclear receptors including those of the mitochondrion . Binding occurs as a result of non-covalent interactions between the receptor and its ligand, at locations called the binding site on the receptor . A receptor may contain one or more binding sites for different ligands . Binding to the active site on the receptor regulates receptor activation directly . The activity of receptors can also be regulated by the binding of a ligand to other sites on the receptor, as in allosteric binding sites . Antagonists mediate their effects through receptor interactions by preventing agonist - induced responses . This may be accomplished by binding to the active site or the allosteric site . In addition, antagonists may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity to exert their effects . </P> <P> The term antagonist was originally coined to describe different profiles of drug effects . The biochemical definition of a receptor antagonist was introduced by Ariens and Stephenson in the 1950s . The current accepted definition of receptor antagonist is based on the receptor occupancy model . It narrows the definition of antagonism to consider only those compounds with opposing activities at a single receptor . Agonists were thought to turn "on" a single cellular response by binding to the receptor, thus initiating a biochemical mechanism for change within a cell . Antagonists were thought to turn "off" that response by' blocking' the receptor from the agonist . This definition also remains in use for physiological antagonists, substances that have opposing physiological actions, but act at different receptors . For example, histamine lowers arterial pressure through vasodilation at the histamine H receptor, while adrenaline raises arterial pressure through vasoconstriction mediated by alpha - adrenergic receptor activation . </P> <P> Our understanding of the mechanism of drug - induced receptor activation and receptor theory and the biochemical definition of a receptor antagonist continues to evolve . The two - state model of receptor activation has given way to multistate models with intermediate conformational states . The discovery of functional selectivity and that ligand - specific receptor conformations occur and can affect interaction of receptors with different second messenger systems may mean that drugs can be designed to activate some of the downstream functions of a receptor but not others . This means efficacy may actually depend on where that receptor is expressed, altering the view that efficacy at a receptor is receptor - independent property of a drug . </P>

What type of drug antagonism occurs when two substances bind to the same receptor