An antagonist of ( altgr. ἀνταγωνιστής antago'nistes, the counter handler ') is in the pharmacology a substance which inhibits an agonist in its effect without inducing a significant impact pharmaceutically itself. Corresponding agonists may be, for example, hormones or neurotransmitters.
Types of antagonist
According to their mode of action of antagonists are differentiated by their competitivity:
- In a competitive antagonist of the antagonist may be displaced by higher agonist concentrations according to the law of mass action again.
- A noncompetitive antagonism may be present when the antagonist does not bind to the binding site of the agonist to a receptor, but " allosteric " to another position. Antagonists, which undergo irreversible binding with the receptor, such as alkylating agents, as well as lead to a non-competitive antagonism.
Occasionally, inverse agonists and allosteric modulators are called antagonists. Partial agonist addition to their partial agonism also an antagonistic active component, that is, they can cause an inhibition of the effect of a full agonist.
The potency of an antagonist can be expressed as concentration of antagonist, for example, inform the central inhibitory concentration (IC50), which leads to half-maximal inhibition of an agonist -induced effect, or by means of the plate characteristic plots as pA2 value. The affinity of an antagonist for the receptor ( pKi or PKB) is also a good measure of its potency.