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Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana 46202
The increase in intracellular Ca2+ and
myosin light chain (MLC) phosphorylation in response to the contractile
activation of tracheal smooth muscle is greater at longer muscle
lengths (21). However, MLC phosphorylation can also be
stimulated by Ca2+-insensitive signaling pathways
(19). The cytoskeletal proteins paxillin and focal
adhesion kinase (FAK) mediate a Ca2+-independent
length-sensitive signaling pathway in tracheal smooth muscle
(30). We used 
-toxin-permeabilized tracheal smooth
muscle strips to determine whether the length sensitivity of MLC
phosphorylation can be regulated by a Ca2+-insensitive
signaling pathway and whether the length sensitivity of active tension
depends on the length sensitivity of myosin activation. Although active
tension remained length sensitive, ACh-induced MLC phosphorylation was
the same at optimal muscle length (Lo) and 0.5 Lo when intracellular Ca2+ was
maintained at pCa 7. MLC phosphorylation was also the same at
Lo and 0.5 Lo in strips
stimulated with 10 µM Ca2+. In contrast, the
Ca2+-insensitive tyrosine phosphorylation of FAK and
paxillin stimulated by ACh was higher at Lo than
at 0.5 Lo. We conclude that the
length-sensitivity of MLC phosphorylation depends on length-dependent
changes in intracellular Ca2+ but that length-dependent
changes in MLC phosphorylation are not the primary mechanism for the
length sensitivity of active tension.
myosin light chain phosphorylation; intracellular calcium; cytoskeleton
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