 Thick filaments containing myosin II differ slightly from those found in skeletal muscle. They, too, are composed of two polypeptide heavy chains and four light chains. However, the structure of thick filaments in smooth muscle is different than in skeletal muscle. Rather than a bipolar arrangement, myosin II molecules are oriented in one direction on one side of the filament and in an opposite direction on the other side of the filament. In this arrangement, myosin molecules are staggered in parallel between two immediate neighbors and are also bound to an antiparallel partner via a short overlap at the very tip of their tails (Fig. 11.21). The polarity of the myosin heads is the same along the entire length of one side of the filament and the opposite on the opposite side. This side-polar myosin filament also has no central “bare zone,” but instead has asymmetrically tapered bare ends. This organization maximizes the interaction between thick and thin filaments, allowing the overlapped thin filaments to be pulled over the entire length of the thick filaments.
Several more proteins are associated with the contractile apparatus and are essential to initiation or regulation of the smooth muscle contractions.
 Myosin light chain kinase (MLCK) is a 130to 150kilodalton enzyme that is important in the mechanism of contraction in smooth muscle. It initiates the contraction cycle after its activation by Ca2–calmodulin complex. Active MLCK phosphorylates one of the myosin regulatory light chains, enabling it to form a cross-bridge with actin filaments.
 Calmodulin, a 17-kilodalton Ca2-binding protein, is related to the TnC found in skeletal muscle, which regulates the intracellular concentration of Ca2. A Ca2 calmodulin complex binds to MLCK to activate this enzyme. It may also, with caldesmon, regulate its phosphorylation and release from F-actin.
 -actinin, a 31-kilodalton protein, provides structural component to dense bodies.
Dense bodies provide an attachment site for thin filaments and intermediate filaments.
Dense bodies contain a variety of attachment plaque proteins, including -actinin, which anchors both thin filaments and intermediate filaments either directly or indirectly to the sarcolemma. They play an important role in transmitting contractile forces generated inside the cell to the cell surface, altering the cell’s shape (Fig. 11.22). Dense bodies are intracellular analogs of the striated muscle Z lines. In support of this concept is the finding that dense bodies, although frequently appearing as small, isolated, irregular, electron-dense bodies,
FIGURE 11.20 • Electron micrographs showing the cytoplasmic densities in vascular smooth muscle cells. Upper inset. The plane of section includes only the smooth muscle cells in the vascular wall. The rectangle in the inset shows portions of three smooth muscle cells that appear at higher magnification in the large electron micrograph. The -actinin–containing cytoplasmic densities (single arrows) usually appear as irregular masses, some of which are in contact with, and attached to, the plasma membrane. The cell in the center of the micrograph has been cut in a plane closer to the cell surface and reveals these same densities as a branching structure (double arrows). A three-dimensional model of the cytoplasmic densities would reveal an anastomosing network. BL, basal (external) lamina; PV, pinocytotic vesicles. 27,000. Lower inset. Higher magnification of cytoplasmic densities attached to the plasma membrane from the area indicated by the rectangle. Note that each cell possesses a basal (external) lamina. In addition, the pinocytotic vesicles can be observed in different stages of their formation. 49,500.