The Contractile Properties of Airway Smooth Muscle: How their Defects can be Linked to Asthmatic Airway Hyperresponsiveness?
Peter D. Pare.
Asthma is a prevalent and sometimes debilitating lung disorder caused by diverse triggers in susceptible
individuals. Based on the salutary effect of drugs relaxing airway smooth muscle (ASM) in the treatment of asthma, there
is no doubt that airway narrowing elicited by ASM shortening is involved in the development of symptoms. Asthmatic
individuals are also commonly hyperresponsive to stimuli acting directly on ASM, such as methacholine; especially when
they are not treated optimally. This feature is called airway hypperresponsiveness (AHR) and is thought to contribute
importantly to asthma symptoms. This is because the airway response measured in an experimental setting, such as during
a methacholine challenge, is a good surrogate of the airway response that would occur in vivo in response to contractile
agonists generated endogenously (e.g., leukotriene and histamine) following exposure to asthma triggers. Many muscle
and non-muscle factors, as well as their interaction, can contribute to AHR. The present review focuses exclusively on
muscle factors. It is important to understand that many contractile properties of ASM may allow narrowing of airways in
vivo. These include force, amount and velocity of shortening, stiffness, ability to relax, and ability to tolerate and recover
from oscillating stress caused by breathing maneuvers. It is also important to understand that the contractile capacity of
ASM can vary in time and in response to external cues (ASM is adaptable). Any permanent and transient defect in any
ASM contractile property can be linked to the manifestation of AHR. Evidence supporting these links in asthma is
Keywords: Adaptation, breathing, force, relaxation, shortening, stiffness, velocity.
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