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1 Section of Pulmonary and Critical Care Medicine, University of Chicago, Chicago, IL, USA
2 Children's Memorial Hospital, Northwestern University, Chicago, IL, USA
3 Department of Pharmacology, University of Nevada School of Medicine, Reno, NV, USA
4 Physiology Program, Harvard School of Public Health, Boston, MA, USA
5 Section of Respiratory Diseases, University of Manitoba, Winnipeg, MB, Canada
* To whom correspondence should be addressed. E-mail: rmitchel{at}medicine.bsd.uchicago.edu.
We hypothesized that differences in actin filament length could influence force fluctuation-induced relengthening (FFIR) of contracted airway smooth muscle, and tested this hypothesis as follows. 100 uM acetylcholine (ACh)-stimulated canine tracheal smooth muscle (TSM) strips set at optimal reference length (Lref) were allowed to shorten against 32% maximal isometric force (Fmax) steady preload, after which force oscillations of ± 16% Fmax were superimposed. Strips relengthened during force oscillations. We measured hysteresivity and calculated FFIR as the difference between muscle length before and after 20 min imposed force oscillations. Strips were relaxed by ACh removal, and treated for 1 hr with 30 nM Latrunculin B (sequesters g-actin and promotes depolymerization) or 500 nM Jasplakinolide (stabilizes actin filaments and opposes depolymerization). A second isotonic contraction protocol was then performed; FFIR and hysteresivity were again measured. Latrunculin B increased FFIR by 92.2 ± 27.6% Lref and hysteresivity by 31.8 ± 13.5% vs. pretreatment values. In contrast, jasplakinolide had little influence on relengthening by itself; neither FFIR nor hysteresivity was significantly affected. However, when jasplakinolide-treated tissues were then incubated with latrunculin B in the continued presence of jasplakinolide for 1 more hr and a third contraction protocol performed, latrunculin B no longer substantially enhanced TSM relengthening. In TSM treated with latrunculin B + jasplakinolide, FFIR increased by only 3.03 ± 5.2% Lref and hysteresivity by 4.14 ± 4.9% compared to its first (pre-jasplakinolide or latrunculin B) value. These results suggest that actin filament length in part determines the relengthening of contracted airway smooth muscle.
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