<P> The affinity of an antagonist for its binding site (K), i.e. its ability to bind to a receptor, will determine the duration of inhibition of agonist activity . The affinity of an antagonist can be determined experimentally using Schild regression or for competitive antagonists in radioligand binding studies using the Cheng - Prusoff equation . Schild regression can be used to determine the nature of antagonism as beginning either competitive or non-competitive and K determination is independent of the affinity, efficacy or concentration of the agonist used . However, it is important that equilibrium has been reached . The effects of receptor desensitization on reaching equilibrium must also be taken into account . The affinity constant of antagonists exhibiting two or more effects, such as in competitive neuromuscular - blocking agents that also block ion channels as well as antagonising agonist binding, cannot be analyzed using Schild regression . Schild regression involves comparing the change in the dose ratio, the ratio of the EC of an agonist alone compared to the EC in the presence of a competitive antagonist as determined on a dose response curve . Altering the amount of antagonist used in the assay can alter the dose ratio . In Schild regression, a plot is made of the log (dose ratio - 1) versus the log concentration of antagonist for a range of antagonist concentrations . The affinity or K is where the line cuts the x-axis on the regression plot . Whereas, with Schild regression, antagonist concentration is varied in experiments used to derive K values from the Cheng - Prusoff equation, agonist concentrations are varied . Affinity for competitive agonists and antagonists is related by the Cheng - Prusoff factor used to calculate the K (affinity constant for an antagonist) from the shift in IC that occurs during competitive inhibition . The Cheng - Prusoff factor takes into account the effect of altering agonist concentration and agonist affinity for the receptor on inhibition produced by competitive antagonists . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion with: information about irreversible / insurmountable competitive antagonists . You can help by adding to it . (November 2017) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section needs expansion with: information about irreversible / insurmountable competitive antagonists . You can help by adding to it . (November 2017) </Td> </Tr> <P> Competitive antagonists bind to receptors at the same binding site (active site) as the endogenous ligand or agonist, but without activating the receptor . Agonists and antagonists "compete" for the same binding site on the receptor . Once bound, an antagonist will block agonist binding . Sufficient concentrations of an antagonist will displace the agonist from the binding sites, resulting in a lower frequency of receptor activation . The level of activity of the receptor will be determined by the relative affinity of each molecule for the site and their relative concentrations . High concentrations of a competitive agonist will increase the proportion of receptors that the agonist occupies, higher concentrations of the antagonist will be required to obtain the same degree of binding site occupancy . In functional assays using competitive antagonists, a parallel rightward shift of agonist dose--response curves with no alteration of the maximal response is observed . </P>

A substance that binds to a receptor and has an agonistic or antagonistic effect is called