Desensitization, two-state receptors and pharmacological parameters.

Katz' classical desensitization theory is examined and expanded. Two possible desensitization mechanisms are found to be thermodynamically identical with drug action on a two-state receptor and capable of interpretation as escape from an instability imposed by the activation of the receptor. The expanded theory of desensitization enables classical receptor theory to accomodate unexplained experimental facts such as the "fade" of both agonistic and antagonistic drug action and the metaphilic effect; it also makes it possible to interpret the variable efficacy or intrinsic activity of partial agonists as very rapid desensitization of drug-receptor complexes.