An operational model of pharmacological agonism: the effect of E/[A] curve shape on agonist dissociation constant estimation

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Abstract
An operational model of pharmacological agonism was analyzed to predict the behavior of rectangular hyperbolic and nonhyperbolic agonist-concentration effect, E/[A], curves with variation in receptor concentration, [Ro]. Irreversible antagonism is predicted to cause E/[A] curve gradient changes in nonhyperbolic cases but not in hyperbolic cases; in both cases estimation of agonist dissociation constants (KA) is theoretically valid. 5-Hydroxytryptamine (5-HT) produced steep E/[A] curves in contracting the rabbit isolated aorta preparation. Irreversible antagonism by phenoxybenzamine (Pbz) produced a flattened E[A] curve, consistent with theoretical predictions. Fitting 5-HT E/[A] curves in the presence and absence of Pbz to the model provided an estimate of KA for 5-HT which was not significantly different from the estimate obtained using Furchgott''s null method. The operational model of agonism appears to account qualitatively and quantitatively for the effects of [Ro] changes on hyperbolic and nonhyperbolic E/[A] curves. Under conditions where irreversible antagonism may be used to estimate KA, fitting the operational model directly to E/[A] data represents a valid, economical and analytically simple alternative to the conventional null method.