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Glossary of Pharmacology 

Spare receptors

Definition: 

Spare receptors are receptors that remain unbound when an agonist is producing its maximal biologic response.   

Relevance:

Spare receptors likely play a role in amplifying signal duration and intensity.  

Since spare receptors are common the EC50 for an agonist is usually not equal to Kd. Often the EC50 is much lower than Kd, since the amount of agonist needed to elicit 50% of the response is often lower than the amount of agonist needed to occupy 50% of the receptors. Therefore, the sensitivity of a cell or tissue to a particular concentration of agonist is dependent on both the agonists affinity for the receptor (Kd) and on the number of spare receptors. Tissues with a greater number of spare receptors will respond to lower concentrations of agonists.   

Examples:

The amplitude of muscle twitch in response to acetylcholine is only diminished when 50% of its receptors are blocked by a toxin.   

Only 1% of luteinizing hormone receptors in Leydig cells need to be stimulated to produce the maximal amount of steroidogenesis.   

In cardiomyocytes, less then 10% of receptors need to be stimulated by catecholamines to elicit an effect.

Linked terms: Agonist, EC50, Kd            

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