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Contractile and mechanical stimuli regulate cofilin phosphorylation (Cofilin‐P) in tracheal smooth muscle (TSM) tissues

Contractile stimulation of TSM tissues causes actin polymerization, which is required for force development. Cofilin, a 19 KD protein, binds to and severs actin filaments, and its phosphorylation at ser 3 by LIM‐kinase inhibits its severing function. To evaluate the role of cofilin in regulating TSM contraction, we assessed the effects of agonists and mechanical stimuli on cofilin phosphorylation at ser‐3 (Cofilin‐P) on isolated canine TSM tissues in vitro. Cofilin‐P increased over a 60 min time period in unstimulated TSM tissues in vitro. Unstimulated TSM tissues maintained under strain for 24 hours had elevated Cofilin‐P relative to muscles maintained at slack length for 24 hours. Administration of 10 uM acetylcholine (ACh) to unstimulated muscle tissues caused cofilin‐P to decrease within 5 min while force increased. Stimulation with 10, 30, or 100 uM forskolin caused Cofilin‐P to decrease in both unstimulated and ACh‐stimulated TSM tissues. Rho‐kinase activates LIM‐kinase in some cell types; however the ROCK inhibitor, Y27632, had no effect on Cofilin‐P in stimulated or unstimulated TSM. The Ca‐dependent phosphatase, PP2B, has been shown to regulate Cofilin‐P in a neuroblastoma cell line. In TSM, Ca‐depletion prevented the decrease in Cofilin‐P in response to ACh. Results suggest that cofilin may be involved in regulating contraction in TSM tissues.