Structure of Smooth Muscle

Smooth muscle cells possess a contractile apparatus of thin and thick filaments and a cytoskeleton of actin and desmin intermediate filaments

The remaining sarcoplasm is filled with thin filaments that form a part of the contractile apparatus. The myosin component of the smooth muscle cell is extremely labile and tends to be lost during tissue preparation. Special techniques can be used, however, to retain the structural integrity of the thick myosin filaments and thus demonstrate them with the TEM. Interspersed with the thin filaments are intermediate filaments containing the protein desmin (vascular smooth muscle contains vimentin filaments in addition to desmin filaments) and actin filaments, which are part of the cytoskeleton of the cell (Fig. 10.17). In addition, the cytoskeleton contains cytoplasmic densities or dense bodies that are visible among the filaments. The thin filaments of the contractile apparatus along with the actin and desmin filaments attach to the dense bodies, which in turn anchor to the sarcolemma.

The components of the contractile apparatus in smooth muscle cells are

• Thin filaments containing actin, tropomyosin, and caldesmon. Actin and tropomyosin are involved in the force-generating interaction with myosin II molecules. Caldesmon is a 120- to 150-kDa protein that binds to F-actin, blocking the myosin-binding site. The action of tropomyosin and caldesmon is controlled by the Ca2+-dependent action of regulatory molecules.

• Thick filaments containing myosin II, which is similar to myosin II found in skeletal muscle. It, too, is composed of two polypeptide heavy chains and four light chains. Phosphorylation by myosin light chain kinase of one of the two types of myosin light chains initiates the contraction of smooth muscle.

Myosin light chain kinase, a-actinin, and calmodulin are other smooth muscle proteins also associated with the contractile apparatus. Calmodulin, a 17-kDa, 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 the caldesmon, causing its phosphorylation and release from F-actin.

Dense bodies provide an attachment site for thin filaments and intermediate filaments

Dense bodies contain a variety of attachment plaque proteins, including a-actinin, that anchor both thin filaments and intermediate filaments either directly or indirectly to the sarcolemma. 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-

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