Functional Antagonism as a Means of Determining Dissociation Constants and Relative Efficacies of Sympathomimetic Amines in Guinea‐pig Isolated Atria

I Dissociation constants (KA) of isoprenaline and orciprenaline were determined for the positive inotropic and chronotropic responses of guinea-pig isolated atria. Cumulative dose-response curves to the agonists were constructed before and after incubation with and washout of the irreversible ,3-adrenoceptor antagonist, Ro 03-7894 1-(5-chloracetylaminobenzfuran-2-yl)-2-isopropylaminoethanol). 2 After 20 min washout, the curves were displaced to the right with depression of the maxima. After 3 h washout, there was only depression of the maxima. 3 Dissociation constants were determined by plotting reciprocals of molar concentrations before Ro 03-7894 (1/A) against reciprocals of the equiactive concentrations after Ro 03-7894 (1/A'). KA = (slope -1)/intercept. 4 Isoprenaline had a greater affinity (KA) than orciprenaline on both rate and tension. The affinity for rate and tension was identical for both agonists, indicating that the ,B-adrenoceptors were identical. 5 Isoprenaline and orciprenaline produced identical rate response maxima when compared in the same preparation but the orciprenaline tension maximum was only 92.0 + 0.3% that of isoprenaline. 6 These dose-response curves were replotted as response against -log RA/Rt (fraction of receptors occupied) for each agonist concentration, calculated from the equation RA/Rt = (A)/(KA + (A)).

Section: Discussionsupporting

confidence: 53%

Dissociation Constants of Isoprenaline and Orciprenaline and Their Relative Efficacies on Guinea‐pig Isolated Atria Determined by Use of an Irreversible Β‐adrenoceptor Antagonist

Broadley

Nicholson

1981

“…(e.g. Buckner & Saini, 1975;Broadley & Nicholson, 1979;Broadley & McNeill, 1983). This is exemplified in Figure 6 by the data of Buckner & Saini (1975 (Kaumann, 1978;Hedberg & Mattson, 1981), rat lung (Minneman et al, 1979) and human cardiac muscle (Stiles et al, 1983) (2) and (3)).…”

Section: Discussionmentioning

confidence: 84%

“…For example, the application of a specific irreversible antagonist (Furchgott, 1966) can be assumed simply to reduce the functional receptor concentration, so that equal agonist effects in the presence and absence ofthe antagonist reflect equal concentrations ofAR. It has also been suggested that functional antagonism (Buckner & Saini, 1975;Broadley & Nicholson, 1979) and other post-receptor interventions (Giao T. Rico, 1971a,b) may be used to quantify agonism. However, these procedures deliberately change the transducer machinery and there seems to be no a priori reason why they should allow reliable quantification of agonist affinity and efficacy.…”

Section: Introductionmentioning

confidence: 99%

Application of the operational model of agonism to establish conditions when functional antagonism may be used to estimate agonist dissociation constants

Leff

Martin

Morse

1985

The operational model of agonism (Black & Leff, 1983) has been used to analyse comparatively functional antagonism and irreversible antagonism as methods for estimating agonist dissociation constants (KAs). A general condition is established in terms of the model parameters which defines the type of experimental interventions at the receptor and the post‐receptor level that allow valid KA estimation. It is shown that functional antagonism and other post‐receptor interventions may produce changes in agonist‐concentration effect curves which are qualitatively indistinguishable but quantitatively distinct, from those produced by irreversible antagonism. Experimental data obtained with the guinea‐pig tracheal strip preparation are in keeping with the theoretical predictions and show how studies using functional antagonism may overestimate agonist affinity. In general, functional antagonism, unlike irreversible antagonism, is in principle an unreliable method for quantifying agonism.

“…More recently the technique suggested by Buckner & Saini (1975) has been used by Broadley & Nicholson (1979) who reached the surprising conclusion that several drugs, normally thought of as partial agonists at (3-adrenoceptors, had intrinsic efficacies greater than that of isoprenaline in producing positive chronotropic responses on guinea-pig isolated atria. If the functional antagonism is not type IIA then KA may be overestimated and this overestimation would be expected to be greater for those agonists with higher intrinsic efficacies.…”

Section: Theory Derivation Ofnull Equations Forfunctional Interactionmentioning

confidence: 99%

An Analysis of Functional Antagonism and Synergism

Mackay

1981

A method is described for deriving null equations for functional antagonism and synergism. These null equations relate together the concentrations of agonist required to produce equivalent states of a cell or tissue in the presence and in the absence of a functional interactant In one particular case the null equation leads to conclusions which are very similar to those reached by an earlier author who did not use the null method. However, the null equations give a clearer insight into the quantitative aspects of functional interaction. It is concluded that the use of functional antagonism to estimate affinity constants and relative intrinsic efficacies of agonists has serious limitations. On the other hand, it may be possible to use the null equations, or similar principles, to test the validity of postulated mechanisms and sites of action of functional interactants.