A reversible change in the functional organization of thin filaments in smooth muscle fibers

The contractile apparatus of vertebrae smooth muscle fibers contains three distinct types of filaments: (1) thin actin-containing filaments, (2) thick myosin-containing filaments, and (3) intermediate or 10 nm diameter filaments. The thin filaments are very abundant, and axially oriented in lattice-like bundles that display features of hexagonal packing. When functionally active smooth muscle strips from guinea pig taenia coli are incubated in physiological saline solution containing 5 mM EDTA and processed for electron microscopy, the thin and thick filaments cannot be localized, but these filaments are normally distributed if the incubation conditions are restored to a concentration of extracellular Ca++ that supports spontaneous contractile activity. In order to further examine the effect of Ca++ depletion on the organization of the thin filaments, the equatorial region of the low angle x-ray diffraction pattern corresponding to the packing arrangement of the thin filaments in functional muscle strips was analyzed at normal and depleted concentrations of extracellular Ca++. The broad reflection at d congruent to (12 nm)-1 that is characteristic of the thin filament lattice is lost at low extracellular Ca++, but it reappears when Ca++ is restored. This reversible change in the distribution of x-ray diffraction indicates that the packing arrangement of the thin (actin-containing) filaments in smooth muscle is flexible under physiological conditions and suggests that the structure of these filaments is reversibly modified.