New methods for determining dissociation constants of agonist-receptor complexes

A new method for determining affinity constants on isolated organs when a threshold value and spare receptors are present

It was shown in previous studies that fixed ratio combinations of agonists with competitive antagonists emulate partial agonists and, therefore, mediate submaximal concentration-effect (E/[A]) curves. A linear plot was obtained by plotting the relative height (H) of these submaximal curves against phiH, where phi=[antagonist]/[agonist]. In this study, it was shown that in an agonist-effector system comprising a possible threshold value, the use of fixed ratio agonist-competitive antagonist concentrations (artificial partial agonists) induces E/[A] curves of different heights which allows one to quantify the threshold value. A nonlinear parabolic-like plot may be obtained when a threshold value is present in an agonist-effector system. By quantifying the threshold value, the nonlinear plot was converted to a linear plot from which the apparent affinity (KA) and an efficacy related parameter (eES) of an agonist could be estimated. The practical applicability of the method was shown by applying the method to E/[A] curves obtained for agonists (noradrenaline and acetylcholine) obtained in the presence and absence of increasing concentrations of their respective competitive antagonists on different effectors.

Efficacy. II. Estimation of a newly defined efficacy related parameter

A new method for estimation of agonist-affinity (KA) and relative efficacy was introduced. This method afforded a procedure by which relative efficacy may be estimated while the actual KA values of agonist-receptor complexes are unknown. The relative efficacy may be estimated by employing a newly defined drug parameter, namely the eES value. The eES value is related to drug efficacy and is defined in such a manner that an isolated eES is a meaningful quantity which may indicate whether or not spare receptors are present in an agonist-effector system. The estimation of eES was based on the fact that fixed agonist-competitive antagonist combinations mimic partial agonists and mediate submaximal concentration-effect curves. However, for the practical estimation of eES one may employ data acquired from agonistic concentration-effect curves determined in the absence and presence of increasing concentrations of a competitive antagonist. This procedure was illustrated by utilizing theoretical concentration-effect curves and applied practically by estimating eES and KA values acquired from sets of carbachol and salbutamol curves. The sets of carbachol and salbutamol concentration-effect curves were determined in the absence and presence of their respective competitive antagonists, namely tripitramine and pindolol.

Efficacy. I: A new method for estimating relative efficacy of full agonists via a newly defined efficacy related parameter

A new method for estimating relative efficacies and relative intrinsic efficacies of agonists is described. Relative efficacy is estimated by employing a newly defined efficacy related parameter (eES) and it may be estimated without prior knowledge of efficacy values or the value of the equilibrium dissociation constants, KA, of agonist-receptor complexes. The parameter eES is directly related to efficacy (e) and is defined as the ratio of maximal stimulus to maximal effect of an agonist. The value of eES indicates whether or not spare receptors are present for a particular agonist-effector system. The eES values of agonists are estimated by utilizing submaximal concentration-effect curves determined with fixed agonist-competitive antagonist concentration combinations and choosing a suitable reference (height of an agonistic concentration-effect curve) to which the height of the stimulus concentration-effect curves of the agonist may be compared. In addition to eES, other new agonist-effector parameters, namely SEm/Sm and phi min, were also defined.