Airway smooth muscle relaxation.

Bronchodilators are important agents in the treatment of asthma and chronic obstructive airways disease. Beta agonists, theophyllines, and antimuscarinic agents are the main drugs in current use, but others are under investigation. Although other actions may be important, these drugs produce bronchodilation either by a direct effect on airway smooth muscle or by inhibiting neural pathways. As our understanding of airway smooth muscle relaxant mechanisms increases, new therapeutic possibilities are being realised. Several recent findings contradict established dogma-for example, the assumption that ; agonists and theophyllines act solely via cyclic adenosine monophosphate (cAMP)-linked intracellular pathways has been questioned and studies with atrial natriuretic peptide and nitrates suggest that cyclic gua-nosine monophosphate (cGMP) may be an important relaxant second messenger. The role of potassium channel activators is being explored with the development of drugs which relax airway smooth muscle by a direct action on cell membrane ion channels. In this review we discuss what is known of the likely mechanisms of action of the airway smooth muscle Respiratory Medicine relaxants in current use and their effects on Unit, City Hospital, calcium homeostasis, and suggest how de-Nottingham NG5 IPB, velopments in our understanding of relaxant A J Knox mechanisms may aid the development of treat-A E Tattersfield ment for airways obstruction in the future.