Published ahead of print on May 15, 2009 Am. J. Respir. Cell Mol. Biol. 2009, doi:10.1165/rcmb.2008-0448OC
Submitted on November 19, 2008 Chronic Activation in Shortened Airway Smooth Muscle: A Synergistic Combination Underlying Airway Hyperresponsiveness?Ynuk Bossé1,1 The James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research, Providence Healthcare/St. Paul's Hospital, University of British Columbia, Vancouver, Canada, 2 The James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research, Providence Healthcare/St. Paul's Hospital, University of British Columbia, Vancouver, Canada; Pathology and Laboratory Medicine, University of British Columbia, Vancouver, Canada, 3 The James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research, Providence Healthcare/St. Paul's Hospital, University of British Columbia, Vancouver, Canada; Department of Medicine, Respiratory Division, University of British Columbia, Vancouver, Canada, 4 The James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research, Providence Healthcare/St. Paul's Hospital, University of British Columbia, Vancouver, Canada; Pathology and Laboratory Medicine, University of British Columbia, The iCAPTURE Centre, Vancouver, V6Z 1Y6, Canada * To whom correspondence should be addressed. E-mail: cseow{at}mrl.ubc.ca.
Airway smooth muscle (ASM) in asthmatics is continuously stimulated by spasmogens released as part of chronic airway inflammation. This chronic submaximal stimulation of ASM produces “tone”, which may or may not narrow airways sufficiently to induce respiratory symptoms. However, when coupled with a bronchoprovocative challenge with a non-specific contractile agonist, this increased tone could contribute to the manifestation of airway hyperresponsiveness (AHR). In this study, we examined the effect of chronic acetylcholine (ACh) exposure at different muscle lengths to gain insights into the consequence of increased tone on the mechanical properties of ASM. The total force (the ACh-induced tone plus active force induced by a second stimulus - electric field stimulation (EFS)) increased immediately after induction of muscle tone, and increased further over time in the presence of the tone in a process termed “force adaptation”. The phenomenon of force adaptation was observed over a wide range of muscle lengths and did not prevent length adaptation when the muscle was adapted to the tone prior to being subjected to a length change, suggesting that both length and force adaptations can occur sequentially and in an independent fashion in the same tissue. Together, these results suggest that adaptation of ASM to shortened length in the presence of muscle tone produced a condition that favored excessive force generation in response to a second stimulus (herein EFS) at reduced muscle length. In vivo these changes will be translated into excessive airway narrowing in response to naturally occurring and pharmacological bronchoconstricting stimuli. Key words: airway smooth muscle length-force relationship length adaptation force adaptation asthma
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