Agonist potency and apparent affinity: interpretation using classical and steady-state ternary-complex models.

The relative potencies of agonists have been used in the past to classify receptors. This could be justified on the basis of ideas and equations developed using the occupancy model of drug action. Although attempts have been made recently to develop methods of analysis based on other models, these have not been entirely satisfactory. In this article Dennis Mackay explores possible interpretations of relative potency, apparent affinity and efficacy on the basis of a steady-state ternary-complex model. He concludes that use of these quantities to classify receptor-transducer systems may not be entirely dependable. However the steady-state model can explain why use of agonists to classify receptors has been successful in the past and why properties of agonists deduced from ligand binding studies may differ from their properties deduced from experiments on intact tissues.