Analysis of asymmetry of agonist concentration-effect curves

We have developed a fitting procedure, based on nonlinear mixed effect mode lling and original work by Richards (1959, J Exp Botany 10, 290-300), to de scribe the degree of asymmetry of concentration-effect E/[A] curves and ana lysed the shape of E/[A] curves obtained with alpha(1)-adrenoceptor agonist s in rat aorta. The four-parameter Richards model provided a significantly better fit of the data than the standard logistic/Hill model for all Ligand s investigated, which implies that E/[A] curves were asymmetrical. With the exception of ST 587, the asymmetry parameter (delta) tended toward zero an d the Richards model could be replaced without significant loss of goodness -of-fit by the three-parameter, asymmetrical Gompertz model. The alpha(1)-a drenoceptor antagonist, prazosin (10 nM), had no effect on the asymmetry of the noradrenaline EI[A] curve but significantly increased the slope at the point of inflection. In contrast, pretreatment with the irreversible antag onist, phenoxybenzamine (60 nM), produced a shift of the delta estimate for noradrenaline from zero to unity, indicating a change from an asymmetrical to a symmetrical curve. Therefore, detailed statistical analysis of EI[A] curve asymmetry demonstrates that alpha(1)-adrenoceptors in rat aorta do no t operate as a homogenous one-receptor-one-transducer system. This conclusi on could not have been reached by either an analysis with the standard logi stic/Hill model or visual inspection of experimental data. Overall, the cur ve-fitting analysis developed in this study provides a quantitative and sen sitive measure of asymmetry and a novel method for the objective discrimina tion of agonist action on the basis of curve shape. The method is generally applicable to other pharmacological assays and provides a new tool in rece ptor classification studies. (C) 1999 Elsevier Science Inc. All rights rese rved.