The importance of choice of agonist in studies designed to predict beta 2 : beta 1 adrenoceptor selectivity of antagonists from pA2 values on guinea-pig trachea and atria

A novel analysis of concentration-dependence of partial agonism Ring-demethylation of bupranolol results in a high affinity partial agonist (K 105) for myocardial and tracheal beta-adrenoceptors

1. Ring-demethylation of the pure antagonist bupranolol results in a ligand (K 105) which induces conformational beta-adrenoceptor changes leading to partial agonistic effects in heart and trachea. However, these conformational receptor changes are not accompanied by changes in receptor affinity, because the affinity estimates for K 105 and bupranolol did not differ for a variety of myocardial tissues (including ventricular beta-adrenoceptors labelled with 3H-(-)-propranolol] and trachea, not even for tracheal receptor subtypes. 2. For the analysis of the concentration-dependence of the blocking actions of a partial agonist a double log-plot was derived, which includes the classical Schild-plot as a special case. The plot is based on the statistical analysis of the action of partial agonists by Marano and Kaumann (1976). They defined a slope m for the weighted regression of equieffective concentrations of agonist in the absence and presence of a concentration [P] of partial agonist P. We derived the dependence of m on [P], which is suitably expressed as: log (1/m-1) = log [P]-log Kp. For the case of a single class of non-interacting receptors the slope of the double log-regression should be unity. Our plot has incorporated information from complete concentration-effect curves, instead of a single concentration-ratio as in the Schild-plot. Analysis of data of K 105 with the new plot (intrinsic activity greater than 0) and the Schild-plot (intrinsic activity = 0) yielded slopes near unity, consistent with simple competition.

A comparative study of beta-adrenoceptors in human and porcine lung parenchyma strip

1. Responses to (+/-)-isoprenaline (Iso), (-)-adrenaline (Adr) and (-)-noradrenaline (NA) were compared in isolated preparations of human and porcine lung parenchyma strip. 2. The order of relaxant potencies of these catecholamines in both human and porcine lung parenchyma was Iso greater than Adr greater than NA (1:0.24:0.01, human; 1:0.21:0.01.pig). These results suggest that beta 2-adrenoceptors predominate in both types of lung parenchyma strip. 3. pA2 values for the beta-adrenoceptor antagonist, propranolol (non-selective), with Iso as the agonist, in human and porcine lung strips were 7.84 and 7.83 respectively and for atenolol were 6.50 and 5.35 respectively. Taken as a whole results indicate the existence of an apparently homogeneous population of beta 2-adrenoceptors in porcine parenchyma strip, while both beta 1 and beta 2-adrenoceptors were revealed in human lung parenchyma.

Comparison of the apparent irreversible beta-adrenoceptor antagonist Ro 03-7894 with propranolol in cardiac ventricular muscle by pharmacological and radioligand binding techniques

Membrane fractions were prepared from guinea-pig ventricular muscle and the specific binding of [3H]dihydroalprenolol ([3H]DHA) assessed. The dissociation constant (Ki) of (+/-)-propranolol was determined (6.9 nM) from its ability to displace [3H]DHA binding. This compared with the pA2 value of propranolol of 8.32 (dissociation constant, 4.8 nM) determined for the antagonism of isoprenaline-induced positive inotropic responses of papillary muscles from guinea-pig hearts. Scatchard analysis of the saturation curves for specific [3H]DHA binding showed that in the presence of propranolol, the displacement was characteristic of competitive antagonism. That is, there was no change in the total beta-adrenoceptor was characteristic of competitive antagonism. That is, there was no change in the total beta-adrenoceptor binding sites (Bmax) but an apparent reduction of the dissociation constant (KD) of [3H]DHA. This antagonism was fully reversed by washing membranes that had been previously incubated with propranolol. In contrast, in the presence of the beta-adrenoceptor antagonist, Ro 03-7894, the Scatchard plots were displaced in a manner characteristic of irreversible antagonism. The Bmax was significantly reduced. This antagonism was resistant to washout, with the Scatchard plots still showing a reduced Bmax and no change in the dissociation constant (KD) of [3H]DHA. This apparent irreversible antagonism by Ro 03-7894 was also demonstrated in guinea-pig isolated papillary muscles. The maximum of the dose-response curve to isoprenaline, constructed after incubation with Ro 03-7894 and a 3 hr bath-washout, was depressed by 89.5 +/- 7.5%

Demonstration of both beta 1- and beta 2-adrenoceptors mediating relaxation of isolated ring preparations of rat pulmonary artery

1 Cumulative concentration-response (relaxation) curves to three beta-adrenoceptor agonists, fenoterol (beta 2-selective), isoprenaline (non-selective) and noradrenaline (beta 1-selective) were obtained on isolated ring preparations of rat pulmonary artery contracted with 15 mM KCl. alpha-Adrenoceptors and neuronal and extraneuronal uptakes were blocked with phenoxybenzamine. The agonist concentration-response curves were reproducible. 2 Responses to each of the three agonists could be blocked by the beta-adrenoceptor antagonists atenolol (beta 1-selective) or ICI 118,551 (beta 2-selective) confirming the presence of beta-adrenoceptors. 3 The relative potencies of the agonists were isoprenaline : fenoterol : noradrenaline = 100 : 38 : 1.4. This indicated that the predominant beta-adrenoceptor type was beta 2. 4 Schild plots were obtained for atenolol and ICI 118,551 using the three different agonists. For each antagonist the location of the Schild plot varied depending on which agonist was used. This indicated that the beta-adrenoceptor population mediating relaxation responses to beta-adrenoceptor agonists was not homogeneous. 5 Atenolol was most potent when noradrenaline was the agonist and ICI 118,551 was most potent when fenoterol was the agonist. 6 It is concluded that isolated pulmonary artery ring preparations of the rat contain a mixed population of beta 1- and beta 2-adrenoceptors both mediating relaxation.

Temperature related effects on the binding characteristics of beta-adrenergic receptor agonists and antagonists by rabbit lung

The effects of incubation temperature on binding characteristics of beta-adrenergic receptor agonists and antagonists were examined in rabbit whole lung membrane fragments. [3H] Dihydroalprenolol ([3H] DHA) exhibited single component, non-cooperative binding at 37 degrees C and 25 degrees C with characteristics representative of beta-adrenergic receptors. Inhibition of [3H] DHA binding by l-propranolol showed single component binding at either incubation temperature, but with slightly higher affinity at 25 degrees C, when analyzed by Hofstee plots. The binding of l-isoproterenol, l-epinephrine and l-norepinephrine were changed qualitatively, as well as quantitatively, by incubation temperature. Hofstee plot analysis of l-isoproterenol inhibition curves showed a linear plot at 37 degrees C and a curvilinear plot at 25 degrees C, whereas l-epinephrine and l-norepinephrine showed curvilinear plots at both temperatures, but with different relative distributions and affinities of the components. The binding characteristics of several selective beta 1- and beta 2-agonists and antagonists were also influenced by incubation temperature. Practolol and prenalterol, beta 1-selective agents, exhibited multiple component binding at 25 degrees C, but only single component binding at 37 degrees C. In contrast, salbutamol, a beta 2-selective agonist, showed single component binding at 25 degrees C and apparent multiple component binding at 37 degrees C. Such temperature-related changes in binding characteristics were not consistent with receptor subtype interconversion and were not restricted to either agonists or antagonists. These studies suggest that the choice of incubation temperature may result in different binding characteristics of various beta-adrenergic receptor ligands. These effects are especially important when binding studies are designed to determine the classification and density of beta-adrenergic receptor subtypes in individual tissues.

beta 1-Adrenoceptors mediating relaxation of the guinea-pig trachea: experiments with prenalterol, a beta 1-selective adrenoceptor agonist

In the presence of 17 beta-oestradiol, prenalterol, a beta-selective adrenoceptor agonist, caused a dose-dependent relaxation of the isolated, pilocarpine-contracted guinea-pig trachea. This effect was blocked by the antagonists propranolol (non-selective) and practolol (beta 1-selective) but not by IPS 339 [(t-butylamino-3-ol-2-propyl)oximino-9-fluorene HCl] (beta 2-selective). The relaxing effect of terbutaline, a beta 2-selective adrenoceptor agonist, was more efficiently blocked by IPS 339 than by practolol. These data support the hypothesis that the guinea-pig trachea contains both beta 1- and beta 2-adrenoceptors mediating relaxation and that the beta 1-adrenoceptors are selectively stimulated by prenalterol. The efficacy of prenalterol was less than that of terbutaline, thus confirming its partial agonistic activity. In the absence of 17 beta-oestradiol, the ability of prenalterol to relax the pilocarpine-contracted trachea was lost. It is suggested that 17 beta-oestradiol may act as a functional antagonist to pilocarpine as it caused a partial relaxation itself.

How beta2-selective is the adrenoceptor antagonist drug, IPS 339?

Schild plots for the compound (t-butylamino-3-ol-2-propyl)oximino-9-fluorene (IPS 399) have been obtained on isolated intrinsic tone trachea and atria (rate) og guinea-pigs. Alpha-Adrenoceptors and uptakes were inhibited. The Schild plots for IPS 339 on trachea (fenoterol as agonist) and on atria (noradrenaline as agonist) were not superimposed suggesting that IPS 339 was beta2-selective. The slopes of the Schild plots obtained on intrinsic tone tracheal preparations (isoprenaline or fenoterol as agonist), although greater than 1.0, were not significantly different from that on atria (noradrenaline as agonist). From the average separation of these Schild plots on trachea and atria IPS 339 was assessed to be only 3.3 times more active on beta2- than on beta1-adrenoceptors. The experiments in the literature which showed a high beta2-selectivity for IPS 339 (155 fold) were carried out on carbachol-contracted tracheal preparations (isoprenaline as agonist) and the Schild plot obtained had a very low slope which was quite different from that on atria. Therefore, the results illustrate how the quantitative estimate of the selectivity of a beta-adrenoceptor antagonist can be misleading when Schild plots with different slopes are compared.

The use of guinea pig K+-depolarized tracheal chain preparations in beta-adrenoceptor studies

Guinea pig tracheal chain preparations were K+-depolarized using Krebs solution in which all the Na+ was replaced by an equivalent amount of K+. This caused a sustained contraction of the preparations. Reproducible concentration-response (relaxation) curves to isoprenaline could be obtained provided that the preparations were repolarized by washing in normal Krebs solution between curves. pA2 values for propranolol (8.30) and butoxamine (5.89) were in good agreement with values obtained on other types of tracheal preparation. The maximum relaxation to salbutamol was less than that to isoprenaline and the maxima to both drugs were less than the maximum relaxation of the tissue (to papaverine). It is suggested that K+-depolarized tracheal preparations have no beta-adrenoceptor reserve and may, therefore, be useful in studies designed to compare the efficacies of beta-adrenoceptor agonists.

Are the pA2 values of selective beta-adrenoceptor antagonists valid when obtained on guinea-pig tracheal preparations contracted with carbachol?

On carbachol-contracted tracheal preparations from guinea-pigs, the slope of the Schild plot for propranolol (isoprenaline as agonist) was 1.0. The slope of the plots for atenolol (beta 1-selective) and butoxamine (beta 2-selective) were less than 1.0, whether isoprenaline or fenoterol was agonist. This was in contrast to previous reports on intrinsic tone tracheal preparations. It was established that the low slopes for atenolol and butoxamine on carbachol-contracted preparations were not related to aspects of the experimental procedures. Low slopes on carbachol-contracted preparations tended to occur when the antagonist used was selective and, although the reason for this is not clear, it may be related to the presence of both beta 1- and beta 2-adrenoceptors in this tissue. Therefore, it is suggested that pA2 values obtained for selective beta-adrenoceptor antagonists on guinea-pig tracheal preparations contracted with carbachol may not be strictly valid, and that antagonists should be studied on intrinsic tone tracheal preparations when pA2 values are to be compared with pA2 values from other tissues to quantify the selectivity of the antagonist.