Airway Smooth Muscle Responsiveness: The Origin of Airway Hyperresponsiveness in Asthma?

Asthma is a respiratory syndrome occurring in individuals who have genetic susceptibility to develop exaggerated airway narrowing to environmental triggers. The symptoms are generally paroxysmal and typically reversible by the use of drugs relaxing airway smooth muscle (ASM). Despite considerable heterogeneity with respect to etiology, age of onset and severity, all affected individuals display some degree of airway hyperresponsiveness (AHR) upon inhalational challenge with spasmogens. ASM is undoubtedly the motor driving airway responsiveness, but whether defects in its contractile properties underlie AHR is still a matter of debate. Many non-muscle changes in asthmatic airways could contribute to AHR. However, this review focuses on the potential contributions of ASM to AHR. Several of the contractile properties of ASM are discussed, including its force-generating capacity, ability to shorten, degree of stiffness, velocity of shortening and ability to relax. The rationale for any of these individual contractile properties being involved in AHR is first explained followed by a brief summary of the current literature concerning their potential contribution to AHR in asthma.