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Departments of 1 Anesthesiology and 3 Physiology and Biophysics, Mayo Clinic and Mayo Foundation, Rochester, Minnesota 55905; and 2 Department of Anesthesiology and Critical Care Medicine, Faculty of Medicine, Kyushu University, Fukuoka 812-8582, Japan
We tested the
hypothesis that increases in force at a given cytosolic
Ca2+ concentration (i.e., Ca2+ sensitization)
produced by muscarinic stimulation of canine tracheal smooth muscle
(CTSM) are produced in part by mechanisms independent of changes in
regulatory myosin light chain (rMLC) phosphorylation. This was
accomplished by comparing the relationship between rMLC phosphorylation
and force in
-toxin-permeabilized CTSM in the absence and presence
of acetylcholine (ACh). Forces were normalized to the contraction
induced by 10 µM Ca2+ in each strip, and rMLC
phosphorylation is expressed as a percentage of total rMLC. ACh (100 µM) plus GTP (1 µM) significantly shifted the
Ca2+-force relationship curve to the left
(EC50: 0.39 ± 0.06 to 0.078 ± 0.006 µM
Ca2+) and significantly increased the maximum force
(104.4 ± 4.8 to 120.2 ± 2.8%; n = 6 observations). The Ca2+-rMLC phosphorylation relationship
curve was also shifted to the left (EC50: 1.26 ± 0.57 to 0.13 ± 0.04 µM Ca2+) and upward (maximum rMLC
phosphorylation: 70.9 ± 7.9 to 88.5 ± 5.1%; n =
6 observations). The relationships between rMLC phosphorylation and
force constructed from mean values at corresponding Ca2+
concentrations were not different in the presence and absence of ACh.
We find no evidence that muscarinic stimulation increases Ca2+ sensitivity in CTSM by mechanisms other than increases
in rMLC phosphorylation.
caldesmon; calponin; cytoskeleton
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