Physiological consequences of beta-adrenoceptor desensitization in guinea pigs

The pursuit of precision pharmaceuticals: divergent effects of beta2 agonist isomers

Beta2 agonists are the most commonly used treatment for acute bronchoconstriction. However, during regular use there is a progressive decline of protective efficacy of bronchodilators. This progressive decline has long been considered anomalous because with short-acting beta agonists, there is no corresponding change in bronchodilator efficacy. Airway hyper-responsiveness is itself a feature of asthma and there maybe however, there may be an increase in airway hyper-responsiveness following regular use of beta2 agonist. Airway hyperresponsiveness could diminish the capacity of beta agonists to protect from or result in paradoxical bronchospasm and there effects of racemic salbutamol. There have been reports of increased morbidity and mortality associated with excessive use of beta(2) agonists. As all beta agonists used clinically are racemates composed of 1:1 mixtures of R and S isomers, conducted on the possible involvement of the isomers in hyper-responsiveness. Hyper-responsiveness cannot be attributed to the R isomer, whose capacity to activate beta adrenoceptors will nullify this effect. In contrast, extensive evidence indicated that the S isomer might cause hyper-responsiveness and potential airway inflammation. Further, the S isomer shows a propensity to activate human eosinophils and alter muscarinic M(2) receptor functions. The S isomer, which makes no contribution to therapeutic efficacy and may exacerbate asthma, might therefore be excluded from asthma therapy.

Enantiomers of bronchodilating beta2-adrenoceptor agonists: is there a cause for concern?

All bronchodilating beta2-adrenoceptor agonists in current clinical use are derivatives of adrenaline and are available as racemates. Whereas the vast majority of the pharmacologic and clinical documentation has been made with the racemates, there are a few studies with the individual enantiomers. It thus appears that all established pharmacologic effects of racemic beta2-agonists reside in the (R)-enantiomer, with the (S)-enantiomer being virtually inactive. In recent years the suspicion has been raised that the (S)-enantiomer of the beta2-agonists is responsible for induction of airway hyperreactivity. This suspicion is based primarily on results obtained in guinea pigs exposed to (S)-enantiomers of beta2-agonists. A number of experiments in vitro have been undertaken to find a mechanism of action for these observations in vivo. Most of the results obtained are equivocal. However, the observation that (S)-salbutamol may cause mobilization of intracellular Ca2+, apparently by means of a cholinergic mechanism, deserves further investigation. The clinical studies are focused on the enantiomers of salbutamol. They confirm the preclinical findings that the pulmonary, as well as the extrapulmonary, effects of salbutamol reside in the (R)-enantiomer. The studies available so far do not convincingly show clinically significant airway hyperreactivity after exposure to the (S)-enantiomer. Further studies are needed to settle this issue.

Sympathomimetic enantiomers and asthma

Airways of asthma patients can become hyperresponsive to airway spasmogens following regular use of isoprenaline or beta 2-selective sympathomimetics. Hyper-reactivity that results from acute exposure of animals to these drugs is pre-empted by vagal section (a procedure which does not influence spasmolytic efficacy of sympathomimetics), is not diminished by antagonism of beta 2-adrenoceptors and is not associated with loss of responsivity of beta 2-adrenoceptors in the airways. Since activation, modulation, or blockade of beta 2-adrenoceptors does not determine this form of hyperreactivity, the possibility that distomers may induce hyperreactivity must be considered. Ocular and vascular responses to distomers of sympathomimetics have long been recognised and, more recently, comparable observations have been made for the airways. Thus, reactivity of guinea-pig airways to spasmogens was increased following exposure to S-isoprenaline, S-salbutamol, or S-terbutaline and exposure to S-isoprenaline or S-salbutamol can intensify symptoms in asthmatics. Regular exposure to the racemate, especially during or following an allergic reaction, predisposes to expression of hyper-reactivity, which is nullified, acutely, by the eutomer. These observations imply that biological effects of sympathomimetic distomers may contribute to morbidity and mortality in asthma patients.

Steric aspects of formoterol and terbutaline: is there an adverse effect of the distomer on airway smooth muscle function?

Experiments were made on isolated tissues from guinea-pig to test the hypothesis that the distomers of rac-beta 2-adrenoceptor agonists induce airway hyperreactivity. Tracheal strip preparations were contracted with carbachol. Both rac- and (R;R)-formoterol (2 and 1 mumol/l, respectively) produced an immediate relaxation, followed by a slow recovery of tone. (S;S)-Formoterol (2 mumol/l) had no effect on smooth muscle tone. Similar results were obtained with the enantiomers of terbutaline. In other strip preparations of the trachea or the main bronchi, cholinergic or nonadrenergic/noncholinergic (NANC) excitatory responses were evoked by electrical field-stimulation. The eutomers, (R;R)-formoterol and (R)-terbutaline, inhibited concentration-dependently both cholinergic and NANC-induced contractions. The distomers, (S;S)-formoterol and (S)-terbutaline, showed qualitatively the same effects but were about 1,000 times less potent than the corresponding eutomer. In a third series of experiments, either enantiomer of formoterol was administered to an electrically stimulated vagus nerve-trachea tube preparation. The nerve-induced contractions were inhibited by both enantiomers, but (S;S)-formoterol was about 1,000 times less potent than (R;R)-formoterol. For both enantiomers of formoterol, about tenfold higher concentrations was required to obtain the same degree of inhibition when given intratracheally as compared with administration in the external medium. There was no indication in any of the experimental approaches that (S;S)-formoterol or (S)-terbutaline might enhance the response to cholinergic or NANC-related stimuli.

Beta-adrenoceptor desensitization induced by antigen challenge in guinea-pig trachea

As beta-adrenoceptor function in the lung could be relevant in asthma, we carried out a functional and biochemical study of the possible occurrence of beta-receptor desensitization after the anaphylactic reaction induced in vitro in actively sensitized guinea-pig tracheas. The relaxing effect of epinephrine and vasoactive intestinal polypeptide (VIP) was tested in tracheal strips. Binding was studied with tracheal membranes and 125I-cyanopindolol. Antigen challenge resulted in a marked decrease of epinephrine-induced relaxation paralleled by a 50% reduction in beta-receptor number. The adrenergic system was specifically affected since VIP-induced relaxation was not modified by the anaphylactic reaction. In some experiments tissues were pretreated with hydrocortisone or indomethacin. Both these drugs prevented antigen exposure from impairing epinephrine relaxation, suggesting the involvement of eicosanoids in this phenomenon. Our data clearly indicate the occurrence of beta-receptor desensitization as a consequence of the anaphylactic reaction, thus impairment of the adrenergic system might play a role in asthma.

Immediate anaphylactic bronchoconstriction induces airway hyperreactivity in anaesthetized guinea-pigs

1. The possible acute occurrence of airway hyperreactivity after immediate-type bronchial anaphylaxis has been investigated in anaesthetized guinea-pigs actively sensitized to ovalbumin (OA). 2. Aerosol challenge (OA 10 mg ml-1, 5 s) provoked immediate bronchoconstriction which was substantially, although incompletely, reversed by isoprenaline (Iso) infusion (1 microgram kg-1 min-1) for 10 min). 3. Bronchoconstrictor responses to 5-hydroxytryptamine (5-HT) were enhanced in challenged animals when compared to those in non-challenged animals that had also received Iso. This was seen as a leftward shift in the location of the dose-response curve for the bronchoconstrictor effect of 5-HT (dose-ratio 2.45, 95% confidence limits 1.77-3.38; P less than 0.01). This phenomenon was associated with pulmonary infiltration of polymorphonuclear leukocytes, which was not modified by Iso treatment. 4. Iso infusion alone caused a slight enhancement of airway reactivity seen as a small leftward shift of the dose-response curve for the bronchoconstrictor effect of 5-HT (dose-ratio 1.51, 95% confidence limits 1.07-2.13; P less than 0.05). 5. These results support a causal relationship between acute pulmonary inflammation and airway hyperreactivity in an animal model of human allergic asthma.

Stimulation of beta-adrenergic receptors and spontaneous motor activity in mice

The effects of 3 beta-adrenergic agonists (clenbuterol, isoproterenol and salbutamol) on the spontaneous motor activity of mice were studied. The present research indicated that motor activity was significantly decreased 30 minutes after IP injection of either clenbuterol (0.06 mg/kg), isoproterenol (0.5 mg/kg) or salbutamol (2 mg/kg). Hypomotility induced by clenbuterol was also significantly antagonized by propranolol in doses ranging from 1 to 8 mg/kg and by penbutolol in doses from 0.03 to 0.5 mg/kg. However, practolol, which does not cross the blood brain barrier, did not antagonize the effect of clenbuterol. Therefore, it may be hypothesized that beta adrenergic agonists decrease motor activity by a central mechanism. It was also found that tachyphylaxis or resistance to treatment, observed in cardiovascular and bronchopulmonary systems with beta-adrenergic agonists, developed after 7 injections of clenbuterol (0.25 mg/kg IP, twice daily) in the behavioral model of spontaneous motor activity in mice.