title: An operational model of pharmacological agonism: the effect of E/[A] curve shape on agonist dissociation constant estimation.
abstract: An operational model of pharmacological agonism has been analysed to predict the behaviour of rectangular hyperbolic and non-hyperbolic agonist-concentration effect, E/[A], curves with variation in receptor concentration, [Ro]. Irreversible antagonism is predicted to cause E/[A] curve gradient changes in non-hyperbolic cases but not in hyperbolic cases; in both cases estimation of agonist dissociation constants (KAS) 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 non-hyperbolic E/[A] curves. Under conditions where irreversible antagonism may be used to estimate KAS, fitting the operational model directly to E/[A] data represents a valid, economical and analytically simple alternative to the conventional null method.
mesh_terms: D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects; D002626:Chemistry, Pharmaceutical; D004336:Drug Antagonism; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008954:Models, Biological; D009131:Muscle, Smooth, Vascular / Q000187:drug effects; D010643:Phenoxybenzamine / Q000494:pharmacology; D011817:Rabbits; D011955:Receptors, Drug / Q000187:drug effects*; D012701:Serotonin / Q000494:pharmacology
chemical_list: D011955:Receptors, Drug; D010643:Phenoxybenzamine; D012701:Serotonin