A study of some propranolol analogues for selective beta-adrenoceptor antagonism using A2 values on isolated trachea and atria from guinea-pig

The effect of varying carbachol concentration on the slope of Schild plots of selective beta-adrenoceptor antagonists in the carbachol-contracted guinea-pig trachea

The effect of varying the level of smooth muscle tone induced by carbachol on the Schild analysis of atenolol (beta 1-selective) and ICI 118,551 (beta 2-selective) with salbutamol as agonist, on the guinea-pig tracheal preparation has been examined. When 10(-6) M carbachol was used to induce near-maximal smooth muscle tone, Schild plot slopes for atenolol and ICI 118,551 were less than 1. Slopes of Schild plots for both drugs were equivalent to 1 when 10(-7) M carbachol was used to produce approximately half-maximal smooth muscle tone. Depletion of neuronal noradrenaline by prior treatment with reserpine had no effect on the Schild analysis. Salbutamol produced maximal relaxation and was more potent when tone was induced with 10(-7) M carbachol, but was less effective at 10(-6) M carbachol. Pretreatment with reserpine increased the potency of salbutamol at each concentration of carbachol. The results suggest that either the level of smooth muscle tone or an unknown effect associated with a high level of smooth muscle tone induced by carbachol may contribute to low slope values of Schild plots of selective beta-adrenoceptor antagonists in the carbachol-contracted guinea-pig trachea. The carbachol-contracted guinea-pig trachea can be used to determine theoretically valid pA2 values for selective beta-adrenoceptor antagonists as long as substantially less than a maximal level of smooth muscle tone is induced by carbachol.

The antagonism of histamine-induced tracheal and bronchial muscle contraction by diphenhydramine: effect of maturation

Airway reactivity in spontaneously breathing unanaesthetized female guinea-pigs was significantly reduced as a consequence of maturation. Threshold doses to histamine (% w/v base) were 0.08% +/- .01 and 0.27% +/- .04 in immature (110 g +/- 2; 1 week old) and mature (837 g +/- 29; 4 months old) animals, respectively. The potency of 2-(2-thiazolyl) ethylamine in tracheal tissues from mature animals was significantly less than that in tissues from immature animals (5.06 +/- .03 vs 5.26 +/- .07). There was no change in the potency of this agonist in bronchial tissues (5.0 +/- .09 vs 4.9 +/- .13). Diphenhydramine reduced tissue contractility in tracheal (30% at 0.1 microM; 50% at 3 microM) but not bronchial tissues. The antihistamine in concentrations ranging from 0.1 microM to 3 microM reduced the potency of histamine 2 to 50 fold in both tissues. Schild plots were linear but slopes were significantly less than unity. pA2 values +/- 95% fiducial limits derived from data from tracheal tissues of immature and mature animals using constrained Schild plots (unit slope) were 7.7 (7.6-7.7) and 7.1 (7.0-7.2), respectively (P less than 0.05). pA2 values for diphenhydramine in bronchial tissues using constrained Schild plots were 7.8 (7.7-7.9) and 7.5 (7.3-7.5), respectively (P less than 0.05). The data emphasize the unique nature of tracheal and bronchial tissues. We conclude that, with maturation, the characteristics of the histamine receptor change. We suggest that the remission of asthma which frequently occurs at puberty may be related to alteration in the properties of membrane receptors.

The selectivity of beta-adrenoceptor antagonists on isoprenaline-induced changes in heart rate, blood pressure, soleus muscle contractility and airways function in anaesthetized cats

The beta-adrenoceptor antagonist of propranolol, metoprolol, atenolol and butoxamine in anaesthetized cats has been measured and compared with the activity of four synthetic phenylethanolamine derivatives. The effects of isoprenaline on four parameters in the anaesthetized cat: heart rate, blood pressure, soleus muscle contractility and airway reactance, were measured and the modification of the isoprenaline dose-response relation by each of the antagonist drugs assessed. Parallel shifts in log dose-response curves for isoprenaline were caused by propranolol for all parameters, by metoprolol and atenolol for each parameter except blood pressure, and butoxamine for each except soleus muscle and heart rate. Selectivity of action of the antagonists between different organs was measured by comparing DR10 values, computed from isoprenaline dose-ratios. Propranolol was the most potent antagonist and showed slight selectivity of action on soleus muscle compared with heart. Atenolol and metoprolol were approximately equipotent and were cardioselective at low doses only. Butoxamine was the least potent antagonist and possessed non-beta-adrenoceptor effects on the parameters measured. Each of the new compounds, 4'-bromo-2'-methoxy-N-isopropyl phenylethanolamine, the 4'-chloro- and 4'-methyl analogues, and 4'-methoxy-N-t-butyl phenylethanolamine, was a potent antagonist but did not exhibit any selectivity of action. The results suggest no clear separation of beta-adrenoceptors into beta 1- and beta 2-subclasses in organs of the cat. There is no apparent separation of beta-adrenoceptor-mediated effects on skeletal muscle and airways.

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.

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.

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

1. pA2 values have been obtained for propranolol, butoxamine, H35/25 and atenolol on guinea-pig isolated trachea and atria (rate) using noredrenaline (beta 1-selective), isoprenaline (non-selective) and fenoterol (beta 2-selective) as agonists. 2. pA2 values varied with the agonist used on trachea but not on atria and, therefore, trachea : atria selectivity values varied with the agonist used. 3. It is suggested that the best estimate of the selectivity of an antagonist between beta 2- and beta 1-adrenoceptors is obtained by comparing its pA2 value obtained on trachea using a beta 2-selective agonist with that obtained on atria using a beta 1-selective agonist. The reasons for this are discussed. 4. The quantitative values for beta 2 : beta 1 selectivity obtained using the above pA2 values were butoxamine 17.0 H35/25 13.5, propranolol 2.75 and atenolol 0.036, i.e. butoxamine and H35/25 were beta 2-selective, propranolol was non-selective and atenolol was beta 1-selective. 5. The results support the hypotheses that guinea-pig trachea contains a mixture of beta 1- and beta 2-adrenoceptors and that guinea-pig atria contain only beta 1-adrenoceptors.