Application of the operational model of agonism to establish conditions when functional antagonism may be used to estimate agonist dissociation constants

The operational model of agonism (Black & Leff, 1983) has been used to analyse comparatively functional antagonism and irreversible antagonism as methods for estimating agonist dissociation constants (KAs). A general condition is established in terms of the model parameters which defines the type of experimental interventions at the receptor and the post-receptor level that allow valid KA estimation. It is shown that functional antagonism and other post-receptor interventions may produce changes in agonist-concentration effect curves which are qualitatively indistinguishable but quantitatively distinct, from those produced by irreversible antagonism. Experimental data obtained with the guinea-pig tracheal strip preparation are in keeping with the theoretical predictions and show how studies using functional antagonism may overestimate agonist affinity. In general, functional antagonism, unlike irreversible antagonism, is in principle an unreliable method for quantifying agonism.