The contribution of airway smooth muscle to airway narrowing and airway hyperresponsiveness in disease.

Airway hyperresponsiveness (AHR), the exaggerated response to constrictor agonists in asthmatic subjects, is incompletely understood. Changes in either the quantity or properties of airway smooth muscle (ASM) are possible explanations for AHR. Morphometric analyses demonstrate structural changes in asthmatic airways, including subepithelial fibrosis, gland hyperplasia/hypertrophy, neovascularization and an increase in ASM mass. Mathematical modelling of airway narrowing suggests that, of all the changes in structure, the increase in ASM mass is the most probable cause of AHR. An increase in ASM mass in the large airways is more closely associated with a greater likelihood of dying from asthma than increases in ASM mass in other locations within the airway tree. ASM contraction is opposed by the elastic recoil of the lungs and airways, which appears to limit the degree of bronchoconstriction in vivo. The cyclical nature of tidal breathing applies stresses to the airway wall that enhance the bronchodilating influence of the lung tissues on the contracting ASM, in all probability by disrupting cross-bridges. However, the increase in ASM mass in asthma may overcome the limitation resulting from the impedances to ASM shortening imposed by the lung parenchyma and airway wall tissues. Additionally, ASM with the capacity to shorten rapidly may achieve shorter lengths and cause a greater degree of bronchoconstriction when stimulated to contract than slower ASM. Changes in ASM properties are induced by the process of sensitization and allergen-exposure such as enhancement of phospholipase C activity and inositol phosphate turnover, and increases in myosin light chain kinase activity. Whether changes in ASM mass or biochemical/biomechanical properties form the basis for asthma remains to be determined.