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

Br J Pharmacol. 1985 Feb;84(2):561-71. doi: 10.1111/j.1476-5381.1985.tb12941.x.


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.

Publication types

  • Comparative Study

MeSH terms

  • Animals
  • Aorta, Thoracic / drug effects
  • Chemistry, Pharmaceutical
  • Drug Antagonism
  • In Vitro Techniques
  • Kinetics
  • Male
  • Models, Biological
  • Muscle, Smooth, Vascular / drug effects
  • Phenoxybenzamine / pharmacology
  • Rabbits
  • Receptors, Drug / drug effects*
  • Serotonin / pharmacology


  • Receptors, Drug
  • Phenoxybenzamine
  • Serotonin