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→Antagonists
==Antagonists==
==Antagonists==
An antagonist can be defined as '''a drug that inhibits the action of an agonist'''.
Antagonists bind to similar receptors as agonist but crucially they don't activate any intracellular events and so there is no tissue response. It's effect is produced by reducing the amount or capability of an agonist to bind to it's target molecule.
Antagonists like agonists bind to receptors in a dynamic fashion, and so it is the antagonists affinity to the receptor that determines it's inhibitory response. The amount of inhibition thus depends on the concentration of the drug at the target site and the number of free receptor sites.
===Competitive Antagonism===
====Reversible Competitive Antagonism====
Here the antagonist competes with the agonist for the occupation of the receptor site. Since less agonist is able to bind to the target molecule the size of the tissue response will decrease. As the dose/concentration of the antagonist increases so the size of the tissue response will further decrease.
The formation of receptor complexes is dynamic in it's nature and so if the agonist' dose/concentration is increased it will out-compete the antagonist for receptor occupation and the size of the tissue response will start to increase. Therefore the antagonists action is reversible.
====Irreversible Competitive Antagonism====
This form of antagonism essentially works in the same manner as above except for one crucial difference. The antagonist forms very strong bonds to the receptor sit meaning that it dissociates very slowly or not at all. This means that increasing the amount of agonist present is unable to out-compete the antagonist as receptor sites are always full.
A partial agonist can effectively act as an antagonist when it is present in very high concentrations as it out-competes the full agonist for the receptor site.
===Non-Competitive Antagonism===
Here the agonist binds to its receptor but the antagonist acts further along the sequence of events resulting in a tissue response. As this chain is blocked the agonist is incapable of producing a response.
===Pharmacokinetic Antagonism===
The antagonist reduces the effect of another drug by reducing its absorption or increasing its rate of metabolism or increasing its rate of excretion.
===Self-antagonism===
If some drugs are repeadtedly given its effect can decrease. This is called '''tachyphylaxis''' or '''desensitisation'''. If a gradual decrease in response to a drug occurs this is called '''tolerance''' and if the drug looses total therapeutic efficacy it is deemed '''refractory'''. Many types of mechanisms occur to cause this phenomenon, the most important include:
* chnage in receptor type
* loss of receptors
* exhaustion of cell mediators
* increased metabolic degradation of the drug
* physiological adaptation
* active extrusion of the drug from cells
===Chemical Antagonism===
This is where the interaction of two drugs results in the failure of an biological activity.
===Physiological Antagonism===
This occurs when two drugs have opposing actions on the body and so their actions cancel each other out.