Influence of calcium on myosin thick filament formation in intact airway smooth muscle

Abstract

Myosin thick filaments have been shown to be structurally labile in intact smooth muscles. Although the mechanism of thick filament assembly/disassembly for purified myosins in solution has been well described, regulation of thick filament formation in intact muscle is still poorly understood. The present study investigates the effect of resting calcium level on thick filament maintenance in intact airway smooth muscle and on thick filament formation during activation. Cross-sectional density of the thick filaments measured electron microscopically showed that the density increased substantially (144%) when the muscle was activated. The abundance of filamentous myosins in relaxed muscle was calcium sensitive; in the absence of calcium (with EGTA), the filament density deceased by 35%. Length oscillation imposed on the muscle under zero-calcium conditions produced no further reduction in the density. Isometric force and filament density recovered fully after reincubation of the muscle in normal physiological saline. The results suggest that in airway smooth muscle, filamentous myosins exist in equilibrium with monomeric myosins; muscle activation favors filament formation, and the resting calcium level is crucial for preservation of the filaments in the relaxed state. electron microscopy; muscle contraction; ethylene glycolbis(␤-aminoethyl ether)-N,N,NЈ,NЈ-tetraacetic acid; muscle plasticity IT HAS BEEN SUGGESTED that myosin thick filaments in relaxed smooth muscle are partially dissolved and reaggregate upon activation (2,3,14,15). Functional studies of airway smooth muscle have shown that thick filament lengthening could account for the increase in isometric force and the decrease in shortening velocity during the sustained phase of contraction (9). It was observed in airway smooth muscle that within at least a threefold length range and when the muscle was allowed to adapt to the lengths at which it was studied, isometric force was independent of muscle length, and shortening velocity and power output were positively correlated to muscle length (7). The observations suggest that the number of in-series contractile units (of which myosin thick filaments are an essential component) could be variable. We observed in a recent study that the thick filaments in intact airway smooth muscle dissolved partially when subjected to mechanical perturbation (5). There are, therefore, many lines of evidence supporting a notion that myosin thick filaments in smooth muscle are labile; partial disassembly/reassembly of the thick filaments could allow the muscle to quickly adapt to externally imposed mechanical strains that reshape the cell. As a first step toward understanding what regulates cellular plasticity in terms of thick filament formation, the present study was carried out to examine myosin evanescence during activation and to determine the stability of the thick filaments in relaxed airway smooth muscle depleted of calcium and subjected to mechanical perturbation. METHODS Tissue PreparationPorcine tracheal smooth musc...

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