4.18 : The Two-State Receptor Model

The two-state receptor model explains a drug's interaction with receptors, such as G protein-coupled receptors and ligand-gated ion channels, to induce or inhibit a biological response. When no natural ligands are present, a receptor exists in an equilibrium of inactive (R_i) and active (R_a) conformations. The inactive form does not produce a response, while the active form generates a basal effect known as constitutive activity.

The binding affinity of a drug determines its interaction with one or both receptor conformations and its efficacy in producing a response. An agonist drug exhibits a high affinity for the active conformation, shifting the equilibrium towards R_a. Increasing the concentration of an agonist enhances the receptor's ability to elicit the maximum cellular response.

In contrast, an antagonist drug has equal affinity for both states and does not alter the equilibrium. It also inhibits the binding of agonists to the receptor, thereby limiting the receptor response to constitutive activity.

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

Understanding such molecular interactions is crucial for designing drugs that modulate receptor activity and elicit specific cellular responses.