The mechanics of exaggerated airway narrowing in asthma: the role of smooth muscle.

Although non-specific bronchial hyperresponsiveness (NSBH) is a basic mechanism underlying the excessive, labile airway narrowing which is characteristic of asthma, its mechanism remains unknown. It is still unclear if the phenomenon is due to fundamental changes in the phenotype of the smooth muscle or is caused by structural and/or mechanical changes in the non-contractile elements of the airway wall or by alterations in the relationship of the airway wall to the surrounding lung parenchyma. Although airway wall remodeling may contribute to NSBH there is increasing evidence that the bronchodilating response to cyclic and periodic stretch is impaired in asthma. There are at least two different mechanisms by which periodic length and force oscillations could influence airway smooth muscle shortening and airway narrowing. These processes which have been called 'perturbed equilibrium of myosin binding' and 'plasticity' have different biochemical and mechanical mechanisms and consequences. They have the potential to interact and to have a fundamental effect on the shortening capacity of airway smooth muscle and its ultimate ability to cause excessive airway narrowing.