A comment on β2-agonists and their use in asthma

Metabolic effects of beta 2-agonists

Catecholamines produce a number of biochemical changes most of which result from stimulation of beta 2-receptors. Interest in these metabolic effects has increased recently as a consequence of the concern over the relatively high mortality from acute asthma attacks. In this review the data on the impact of beta 2-agonists on glucose production, insulin release and lipolysis are presented. Thereafter the subject of hypokalaemia, the mechanism for its production by beta 2-agonists and its relevance to cardiac arrhythmias are considered in detail. Finally the fall in plasma magnesium and the possible role of beta 2-agonists in the production of lactic acidosis are discussed.

Adrenoceptors in airway smooth muscle

This review examines the roles and functional significance of alpha and beta-adrenoceptor subtypes in airway smooth muscle, with emphasis on human airway function and the influence of asthma. Specifically, we have examined the distribution of beta-adrenoceptors in lung and the influence of age, the epithelium, respiratory viruses and inflammation associated with asthma on airway smooth muscle beta-adrenoceptor function. Sites of action, beta 2-selectivity, efficacy and tolerance are also examined in relation to the use of beta 2-agonists in man. In addition, alpha-adrenoceptor function in airway smooth muscle has been reviewed, with some emphasis on comparing observations made in airway smooth muscle with those in animal models.

Action of mediators on airway smooth muscle: functional antagonism as a mechanism for bronchodilator drugs

The beta-adrenoceptor agonists have become the cornerstone of bronchodilator therapy. These agents are "functional" or "physiologic" antagonists that actively relax airway smooth muscle through a cyclic-AMP (cAMP)-mediated decrease in myoplasmic Ca2+ content. Hence, unlike specific receptor antagonists, the sympathomimetics should reverse bronchoconstriction regardless of the mediator(s) involved. Indeed, one of the primary beneficial attributes of beta-adrenoceptor agonists is their inhibitory activity against a wide range of bronchoconstrictors. As successful as the sympathomimetic bronchodilators have been, they are not without liabilities. These liabilities include: 1) cardiovascular and skeletal muscle side effects, 2) an inherent subsensitivity of the patient population to beta-adrenoceptor agonists, 3) the development of tolerance, and 4) loss of efficacy during severe asthmatic episodes. These limitations are not specific for individual agents but are shared by all beta-adrenoceptor agonists. A significant improvement in the pharmacotherapy of asthma would be obtained by identifying novel bronchodilators devoid of one or more of the aforementioned liabilities. The development of isozyme-selective phosphodiesterase (PDE) inhibitors is one promising approach toward this goal. Interest in PDE inhibition as a therapeutic target has been renewed by the realization that PDEs exist in multiple isoforms and that the distribution of these isoforms varies significantly among tissues. This information, coupled with the recent synthesis of PDE inhibitors selective for several of the isozymes, raises the possibility of breeding organ-selectivity into this class of compounds. Results from preliminary experiments with isozyme-selective PDE inhibitors have helped to identify appropriate drug targets in airway smooth muscle. These early studies suggest that the synthesis of novel isozyme-selective PDE inhibitors not only may provide tools with which to understand the biologic function of various PDE isozymes, but may also lead to the development of improved therapeutic agents.

Biochemical regulation of airway smooth muscle tone: current knowledge and therapeutic implications

Evidence collected during the last decade indicates that the molecular processes responsible for smooth muscle contraction are fundamentally different from those responsible for skeletal muscle contraction. Furthermore, because of the diverse functional roles of various smooth muscles, it would not be surprising if significant differences in regulatory processes also exist among different smooth muscles. Such diversity may already be exemplified by differences in cross-bridge kinetics and sources of activator Ca2+. Additional unique regulatory features of various smooth muscle types will undoubtedly be uncovered by further research. A convincing body of data suggests that activation of the adenylate cyclase/protein kinase cascade is responsible for the bronchodilation produced by beta-adrenoceptor agonists. Although the exact mechanism by which the activation of cAMP-dependent protein kinase induces relaxation is not clear, the phosphorylation of multiple substrates may be involved. Phosphorylation of these substrates can promote relaxation by decreasing the myoplasmic Ca2+ concentration, decreasing the Ca2+ sensitivity of the contractile apparatus, or both. Thus, because beta-adrenoceptor agonists act as physiologic antagonists of broncho-constriction, they should relax airway smooth muscle regardless of the mediator(s) responsible for the bronchospasm. Perhaps this is the major reason that the beta-adrenoceptor agonists have become the premier class of drugs used in the treatment of bronchial asthma. As useful as the sympathomimetic bronchodilators have been, they are not without liabilities. These liabilities include: cardiovascular and skeletal muscle side effects, an inherent subsensitivity of the asthmatic patient population to beta-adrenoceptor agonists, the development of tolerance and a loss of efficacy during severe asthmatic episodes. The fact that these drawbacks are probably shared by all sympathomimetic bronchodilators suggests that little therapeutic advantage will be gained by developing new beta-adrenoceptor agonists. The task of developing novel bronchodilators will be facilitated by an understanding at the molecular level of the diversity among smooth muscles and the processes that regulate smooth muscle tone. Hopefully, such knowledge will lead to a new generation of highly effective, tissue-selective bronchodilators with significant therapeutic advantages over those currently available.

Adverse reactions to beta 2-agonist bronchodilators

Beta 2-Agonists are safe and effective bronchodilator drugs. Their major adverse effects of skeletal muscle tremor, tachycardia and various metabolic effects are mediated by beta-adrenoceptor stimulation and are reversible. Skeletal muscle tremor is the most frequent dose-limiting side effect. It may be reduced by commencing treatment with a low dose and if it persists another beta 2-agonist may be tried. Other side effects such as cardiac arrhythmias and reduction in PaO2 are a serious potential problem in some susceptible asthmatics. However, they are infrequent or of a mild degree and are generally outweighed by the good control of asthma produced by beta 2-agonists. Side effects from beta 2-agonist therapy can be minimised by use of the inhaled route which selectively delivers the drug to the airways. Furthermore, selective tolerance develops to their side effects. The dose of a beta 2-agonist should be assessed on the basis of therapeutic effect and the level of tolerance to its side effects. Recommended doses of beta 2-agonists used for long term therapy do not cause clinically significant desensitisation of airway beta-adrenoceptors, although this may become a relevant problem in patients who are regularly receiving very high doses. Intravenous beta 2-agonists have a place in the treatment of severe asthma not responding to nebuliser therapy. In this life-threatening situation with severe airflow obstruction, monitoring of heart rate, PaO2, plasma potassium and the electrocardiogram should be mandatory and supplemental oxygen given so that serious adverse effects are presented.