Het Twee-Staten Receptor Model

The two-state receptor model describes how a drug interacts with receptors, such as G protein–coupled receptors and ligand-gated ion channels, to evoke or prevent a biological response.

In the absence of endogenous ligands, a receptor exists as an equilibrium mixture of inactive R_i and active R_a conformations. The R_i form produces no response, while the active form produces a small basal effect called constitutive activity.

A drug binds one or both conformations as per its binding affinity, which also determines its efficacy in producing a response.

An agonist  drug has a high affinity for the active conformation and shifts the equilibrium towards R_a. Raising agonist concentration enhances the receptor's efficacy to elicit the maximum cellular response.

In contrast, the antagonist  has an equal affinity for both states and does not affect the equilibrium. It also prevents agonist-receptor binding, limiting the receptor response to the constitutive activity.

Lastly, an inverse agonist has a higher affinity for the inactive state, shifting the equilibrium towards R_i and reducing the constitutive activity.