Abstract
It is known that, in solution, smooth muscle myosin filaments are less stable than the myosin filaments of striated muscles, and assembly of the smooth muscle myosin filaments is strongly correlated with phosphorylation level of myosin regulatory light chain (MLC20) at physiological concentrations of MgATP. Supporting the results in solution, increment of the thick filament (myosin filaments with the accessory proteins) contents during contraction have been reported at least in some smooth muscle cells, although a number of the thick filaments exits even in the resting states. Role of structural dynamics of the thick filaments in excitation-contraction coupling of the smooth muscle cells is, however, still unclear. Recently, using electron microscopes, X-ray diffraction technique and florescent microscopes, we found that blebbistatin, a potent inhibitor of myosin II, irreversibly suppressed Ca2+-induced contraction and also disrupted the thick filament structure of skinned (cell membrane permeabilizsed) smooth muscle cells without affecting MLC20 phosphorylation level. Our results indicated that myosin ATPase activity regulates lability of the thick filaments as well as cross bridge cycling in the smooth muscle cells. Contribution of thick filament lability in excitation-contraction coupling of the smooth muscle cells will be discussed. [J Physiol Sci. 2008;58 Suppl:S31]
