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1 Discipline of Physiology, University of Western Australia, School of Biomedical and Chemical Sciences, Perth, WA, Australia
* To whom correspondence should be addressed. E-mail: mitchell{at}cyllene.uwa.edu.au.
Bronchoconstrictor responses are quantitatively different when they are evoked under static conditions and during or after periods of deep inspiration. In vivo, deep inspirations produce bronchodilation and protect the lung from subsequent bronchoconstriction (termed bronchoprotection). These effects may be due in part to dynamic stretch on airways produced by cyclical expansion of airway diameter. However, airways also lengthen cyclically during breathing. The effects of cyclical airway elongation on evoked bronchoconstriction have not been examined. This study recorded evoked contractions of pig bronchial segments (1) at different airway lengths, (2) after a period of cyclical lengthening in relaxed airways, and (3) during cyclical lengthening in pre-toned airways. Airway segments were mounted in organ baths and bathed in Krebs solution lumenally and on the adventitia. Airways were cyclically lengthened by 5-30% of their deflated length at 0.5-2 Hz for 5 min. Contractions were evoked by electrical field stimulation (EFS) or carbachol and were recorded under isovolumic conditions. Under static conditions there was a blunt hyperbolic relationship between length and response to EFS. After a period of cycling airway length, EFS-induced contractions were increased. In airways pre-toned with carbachol cyclical lengthening produced a transient bronchodilation and a sustained increase in contraction. Contractile responses were not blocked by indomethacin. The results show that isolated airways respond actively to dynamic changes in length. Our results indicate that cyclical lengthening of airways could contribute to lung function in vivo, but do not appear to account for the phenomenon of bronchoprotection.
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