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Departments of 1 Pharmacology and Therapeutics, 2 Anatomy, and 3 Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3; 4 Krannert Institute of Cardiology, Indiana University, Indianapolis, Indiana 46202; and 5 The UBC McDonald Research Laboratories/The iCAPTURE Centre, St. Paul's Hospital/Providence Health Care, Vancouver, British Columbia, Canada V6Z 1Y6
Phosphorylation of the 20-kDa regulatory myosin light chain (MLC) of smooth muscle is known to cause monomeric myosins in solution to self-assemble into thick filaments. The role of MLC phosphorylation in thick filament formation in intact muscle, however, is not clear. It is not known whether the phosphorylation is necessary to initiate thick filament assembly in vivo. Here we show, by using a potent inhibitor of MLC kinase (wortmannin), that the MLC phosphorylation and isometric force in trachealis muscle could be abolished without affecting calcium transients. By measuring cross-sectional densities of the thick filaments electron microscopically, we also show that inhibition of MLC phosphorylation alone did not cause disassembly of the filaments. The unphosphorylated thick filaments, however, partially dissolved when the muscle was subjected to oscillatory strains (which caused a 25% decrease in the thick filament density). The postoscillation filament density recovered to the preoscillation level only when wortmannin was removed and the muscle was stimulated. The data suggest that in vivo thick filament reassembly after mechanical perturbation is facilitated by the cyclic MLC phosphorylation associated with repeated stimulation.
airway smooth muscle; contraction; ultrastructure; biochemistry; calcium transients
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