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Length distribution of myosin filaments in smooth muscle and implications in the structure and function of contractile units Liu, Jeffrey Chao-Yu


Smooth muscle is an essential component of the walls of numerous hollow or tubular organs throughout the body, including blood vessels, airways, and the bladder. Proper physiological functioning of these organs relies heavily on the appropriate activation and contraction of the smooth muscle tissue. Pathophysiological conditions may arise from both excessive and insufficient smooth muscle contraction. Muscle function is closely associated with muscle structure. More specifically, during a contraction, cyclic interactions between myosin cross-bridges and actin filaments allow for muscle shortening and force generation. Myosin molecules from smooth muscle and non-muscle cells are known to self-assemble into side-polar filaments in vitro. However the in situ mechanism of filament assembly is not clear and the question of whether there is a unique length for myosin filaments in smooth muscle is still under debate. In this study we measured the lengths of 16,587 myosin filaments in three types of smooth muscle cells using serial electron microscopy (EM). Sheep airway and pulmonary arterial smooth muscle as well as rabbit carotid arterial smooth muscle were fixed for EM and serial ultra-thin (50-60 nm) sections were obtained. Myosin filaments were traced in consecutive sections to determine their lengths. The results indicate that there is not a single length for the myosin filaments; instead there is a wide variation in lengths. The plots of observation frequency versus myosin filament length follow an exponential decay pattern. The most significant finding of this study is that myosin filaments in smooth muscle do not have a uniform length and analysis suggests that the distribution of filament length is a result of a dynamic equilibrium between polymerization and de-polymerization of myosin molecules driven by predictable probabilities of the myosin dimers to bind with and dissociate from each other.

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