An antagonist is a biological or chemical substance that occupies a receptor and prevents other exogenous or endogenous substances that bind to the receptor from producing any effect. Antagonists are not produced in the body, and they do not elicit any pharmacological effect when they bind to a receptor.
Agonists bind and alter the functions of receptors, while antagonists simply nullify and interrupt the action of agonists. So, antagonists basically perform opposite functions of agonists.
Types of antagonists.
The classical (orthosteric) antagonist, which attaches itself to the receptor at the same site where an agonist usually binds to the receptor, and mediates its effect simply by bulk or mass effect, i.e physical hindrance.
An allosteric antagonist on the other hand binds to a site on the receptor that is completely different from the agonist’s binding site; this way the substance can modify receptor activity without blocking agonist binding.
A competitive (or reversible) antagonist is one whose effect can be completely surmounted in the presence of sufficient concentration of the agonist, while the effect of a non-competitive (or irreversible) antagonist cannot be overturned even in the presence of a high amount of agonist.
The effects of reversible antagonists are short-lived, while the effect of irreversible antagonists is permanent.
Examples of Antagonist Drugs
Antagonist in Physiology
The antagonist muscles are muscles that oppose or go against a particular function of another muscle. For instance, the tricep muscle acts as an antagonist to the bicep muscle. When the biceps (agonist muscle) contracts and flexes, the triceps (antagonist muscle) relaxes and helps to pull the arm back to its original position.
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Muscle agonist–antagonist interactions in an experimental joint model (2012). Retrieved from Pubmed Center Website: