Molecular structure and organization of filaments in single, skinned smooth muscle cells

The contractile elements of single, skinned amphibian smooth muscle cells were resolved as fibrils containing F-actin and side-polar myosin filaments and adjacent cytoplasmic dense bodies, containing alpha-actinin and actin. The fibrils were found to be linked axially through actin filaments to the dense bodies, by electron microscopy. The arrangement of these two structures provides the essential features of a sarcomere to the contractile elements. The effect of contraction upon the movement of the dense bodies was followed dynamically by optical microscopy as a method of analyzing the organization of the contractile apparatus and resolving contractile units. Averaged over long time periods (30 to 50 seconds), the axial movements of dense bodies was uniform. The dense bodies were not simply displaced passively during contraction because the extent of radial movements averaged over the same (long) time periods were less than would be expected for the observed increases in cell diameter. Measured over short time intervals (3 to 6 seconds), the axial movement of some closely spaced dense bodies was not uniform: individual and groups of bodies moved significantly faster and slower than the mean rate of axial translocation. These shifts in coordinated and independent movements might reveal separate anisometric contractile units, or they could represent a random temporal pattern of localized activation and deactivation of small contractile elements.