The adrenoceptor blocking properties of the new beta 2-selective antagonist, IPS 339 on tracheal smooth muscle and on slow contracting skeletal muscle

ICI 118,551: an effective ocular hypotensive agent with selectivity for the ciliary process beta 2-adrenoceptor and with minimal cardiac side effects

Prior biochemical studies have shown that the ciliary process epithelium, which is involved in the secretion of aqueous humour, is rich in beta-adrenoceptors with pharmacological characteristics similar to those of the beta 2 subclass. The present experiments demonstrate that the beta-adrenoceptor antagonist, ICI 118,551, is a potent inhibitor of isoprenaline-stimulated adenylate cyclase activity measured in broken cell preparations of rabbit ciliary process. In rabbit cardiac muscle, however, ICI 118,551 is a relatively weak antagonist of isoprenaline-stimulated adenylate cyclase, being approximately 100 fold less potent than the non-selective beta-adrenoceptor antagonist, timolol. ICI 118,551 is also less potent than timolol in inhibiting isoprenaline-sensitive adenylate cyclase of rabbit lung. ICI 118,551 applied topically to eyes of unanaesthetized rabbits causes a dose-dependent decrease in intraocular pressure. Furthermore, in a blind crossover study in rabbits, topically applied ICI 118,551 decreased intraocular pressure for more than 6 h and was more effective than an identical dose of the clinically effective anti-glaucoma agent, timolol. Systemic absorption from topically-applied timolol, but not ICI 118,551, is sufficient to alter cardiac response to subcutaneous administration of isoprenaline. Furthermore, dose-response studies, using direct systemic administration of the two beta-adrenoceptor antagonists, revealed that ICI 118,551 is about 60 times less potent than timolol in blocking isoprenaline-induced cardio-acceleration. ICI 118,551, applied to one eye, causes a decrease in intraocular pressure in the contralateral eye, and systemic administration of ICI 118,551 results in decreased intraocular pressure in both eyes, data indicating that at least part of the ocular hypotensive effect of topical ICI 118,551 is mediated through systemic absorption. 8 These findings provide biochemical and physiological evidence that selective beta 2-adrenoceptor blockers such as ICI 118,551, used topically or systemically, may be useful as ocular hypotensive agents with decreased cardiac side-effects.

Dissociation between the effects of some xanthine derivatives on the tracheal smooth muscle and on the skeletal muscle

The ability of some xanthine derivatives to relax the trachea, contracted by pilocarpine, and to increase the force of contraction of directly stimulated skeletal muscles from the guinea-pig was studied in vitro. No relationship was found between these two effects. Relaxation of the trachea occurred at lower concentrations and with a different order of potency as compared with the effects on the slow-contracting soleus muscle or on the fast-contracting extensor digitorum longus. One of the compounds, IBMX, 1-methyl-3-isobutyl-xanthine, showed an isoprenaline-like effect on the soleus muscle i.e. it depressed the force and fusion of subtetanic contractions. The relaxing effect of theophylline and IBMX on the trachea was additive to that of terbutaline but no clear potentiation was observed. The depression of the contraction of the soleus muscle elicited by terbutaline was reinforced by IBMX but not by theophylline. Theophylline in concentrations which used alone enhanced the contractions of the soleus muscle inhibited the effect of terbutaline. We conclude that the relative contribution of the various effects of xanthine derivatives differs from compound to compound.

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.

Selective development of tolerance to beta-adrenoceptor agonists in skeletal muscle as compared with airway smooth muscle from the guinea-pig

1. Guinea-pig were fed with a diet containing terbutaline or placebo for 4--5 days. The trachea, soleus muscle and the extensor digitorum longus (EDL) from these animals were prepared for recording of isometric contractions in vitro. 2. After treatment with terbutaline in vivo, the response of the pilocarpine-contracted trachea to terbutaline and isoprenaline was slightly suppressed with no change in maximum relaxation. 3. After treatment with terbutaline in vivo the maximum depression of the incomplete tetanic contractions of the soleus muscle brought about by terbutaline or isoprenaline was diminished by about 70%. The response of the EDL was also attenuated after previous treatment with terbutaline in vivo. 4. These data indicate a selective development of tolerance to the effects of beta-adrenoceptor agonists in skeletal muscle as compared with tracheal smooth muscle. 5. The present results provide an experimental analogue to the clinical observation that patients being treated with beta-adrenoceptor agonists become tolerant to the tremorogenic rather than to the bronchodilating effect.