Analysis of receptor inactivation experiments with the operational model of agonism yields correlated estimates of agonist affinity and efficacy

The aim of this study was to evaluate whether the operational model of agon ism can yield independent estimates of agonist affinity (pK(A)) and efficac y (log tau) when Furchgott's method of irreversible receptor inactivation i s employed. For this purpose, the interaction between noradrenaline and phe noxybenzamine was studied in rat small mesenteric artery using a paired-cur ve design. Phenoxybenzamine pretreatment produced a significant rightward s hift and depression of the upper asymptote of the noradrenaline concentrati on-effect (E/[A]) curve. Although the operational model of agonism appeared to provide an adequate fit of the individual E/[A] curves, a highly signif icant correlation was found between the estimates of pK(A) and log tau (r = -0.80, p < 0.0001), inconsistent with the assumption that affinity and eff icacy are independent parameters (best line fit: pK(A) = -0.96 x log tau 6.75). The pK(A) and log tau estimates were not correlated with either the pEC(50)s of the control curves or upper asymptotes of the phenoxybenzamine- treated curves. Simulations showed that the correlation between affinity an d efficacy can be explained by the effect on the outcome of the analysis of random errors in the response measurements. Therefore, although in theory the operational model of agonism should provide independent estimates of ag onist affinity and efficacy, this is unlikely to be the case with experimen tal data. (C) 1999 Elsevier Science Inc. All rights reserved.