Marcel

with the Cheng and Pursoff equation we can calculate the K{ value for the competitor:

Where Ki is equilibrium dissociation constant for the competitor I, and Kd is the equilibrium dissociation constant for the radioligand L. Thus when [L] is at Kd, IC50 = 2 X Kj, and when [L] is present at trace concentrations ([L] << Kd), IC50 = Ki. In the presence of an increasing concentration of competitive antagonists, fewer and fewer receptors are available for occupancy by the ligand. Thus competitive antagonists suppress agonist-mediated responses by blocking access of the agonist to its specific receptor. The dose-response relationship of an agonist is shifted to the right in the presence of increasing concentrations of an antagonist, giving a series of parallel curves if the antagonist interacts in a truly competitive and fully reversible fashion (Fig. 7A). Kd for the competitive antagonist can be determined by Schilid equation,

Figure 7 Determination of the Kd for the receptor interaction with a competitive antagonist. (A) Dose-response relationship of antagonist in the absence (control) or presence of increasing concentration of a competitive antagonist. (B) Schlid plot of the data in panel A to calculate Kdb. Competitive antagonists suppress agonist-mediated responses by blocking access of the agonist to its specific site on the receptor.

Figure 7 Determination of the Kd for the receptor interaction with a competitive antagonist. (A) Dose-response relationship of antagonist in the absence (control) or presence of increasing concentration of a competitive antagonist. (B) Schlid plot of the data in panel A to calculate Kdb. Competitive antagonists suppress agonist-mediated responses by blocking access of the agonist to its specific site on the receptor.

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