Reorientation of myofilaments during contraction of a vertebrate smooth muscle

Abstract

The purpose of the investigation was to determine whether filaments within smooth muscle cells changed their orientation (with respect to the main axis of the cell) during contraction. The stomach muscle of Bufo marinus was used, since its cells may be easily isolated, enabling direct observation in living cells. In addition to still micrography, cinemicrography was used to record continuously during contraction. Polarization microscopy revealed a change in birefringence after contraction, with relaxed cells exhibiting uniform birefringence while contracted cells displayed a discontinuous pattern. Movies revealed a progressive change in orientation of birefringent elements from nearly parallel to the cell's main axis in relaxed cells to increasingly larger angles to the cell's axis as contraction progressed. Phase-contrast microscopy revealed a change in filamentous components, from being parallel to the cell's axis in relaxed cells to being in an undulating or helical pattern during concentration. Cell shape tended to follow the configuration of the filamentous component. Electron microscopy of muscle strips corroborated the observations of living cells and substantiated the conclusion that filaments change their orientation from parallel to oblique (with respect to the cell's axis) during shortening with an undulating or helical pattern of filaments in shortened muscles.