Contractile filament

The lowered concentrations of calcium ions (Ca++) set off a cascade of cellular reactions that cause the cell s contractile filaments (myosin and actin) to slide apart. 5. Smooth muscle cells relax. 

S100A1 is the most abundant S100 protein found in striated muscle and predominates in myocardial tissue (Kato and Kimura, 1985). Besides its cytoplasmic occurrence, S100A1 was reported in these cells to associate with the sarcolemma, sarcoplasmatic reticulum (SR), contractile filaments, intercalated discs, outer mitochondrial membrane and other intracellular membrane stmctures (Arcuri et al., 2002 Donato et al., 1989 Haimoto and Kato, 1988 Sorci et al., 1999). However, the exact location of S100A1 on the contractile elements of the sarcomere is still controversial (Maco et al., 2001 Zimmer, 1991). 

Studies by Axel et al, examined the effect of paclitaxel on the distribution of the contractile filament SMC a-actin and the intermediate filament vimentin in arterial SMCs (51). At a... 

The types and frequencies of cross-reactivity findings are listed by general category in Table 10.5. The most frequent unexpected cross-reactivities were recognized in neural, epithelial, or contractile filament components. Narrow-... 

Figure 10.6 (a) Unexpected narrow-spectrum cross-reactivity with contractile filaments in vascular smooth myocytes. No staining of interstitial (stromal) cells, collagen, nuclei, or adjacent peripheral nerve. (b) No staining in replicate sections stained by negative control antibody at similar staining concentration. See color insert. 

Figure 10.9 Unexpected cross-reactivity with contractile filaments (cross-striations, intercalated discs) in cardiac myocytes. No staining of sarcolemmal cells, interstitial cells or endothelium. See color insert.

The anatomical arrangement of the cell membrane, the SR, and the myofilaments in the striated muscle is further optimized for rapid action. Figure 6.5 illustrates this for a skeletal muscle cell. The cytoplasmic membrane there forms invaginations called T (transversal) tubules, which protmde into the cell interior and enter into immediate contact with the SR cisterns, which in turn are aligned to the contractile filaments. 

What is the structure of smooth muscle contractile filaments at high resolution ... 

Caldesmon (CD) is a ubiquitous protein in smooth muscle cells. The smooth muscle isoform has a sequence-derived molecular mass of 89-93 kDa, but migrates on SDS gels at 120-150 kDa. Within the smooth muscle cell, CD is localized within the contractile apparatus (Furst et al., 1986) and when the contractile filaments are isolated it is found to be tightly bound to the thin filaments (Marston and Lehman, 1985). It is suggested that its in vivo function involves regulation of thin filament activity and possibly a role in the assembly and stabilization of thick and thin filaments. 

The molecular mechanisms that couple the contractile filaments to the dense bodies are not established. Dense bodies contain the actin-cross-linking protein, a-actinin, as do the Z-lines of skeletal muscle. Dense bodies also contain actin and probably calponin, although it is currently unclear whether the a and y ("smooth muscle ) isoforms of actin (See Section 4.2) are both present in dense bodies or only the p "non-muscle" isoform (Small 1995, North et al 1994a, 1994b, Mabuchi et d 1996). Ultras-tructural and biochemical data obtained by Mabuchi et al (1997) suggests that one function of calponin may be to couple actin filaments and intermediate filaments at dense bodies. 

Fig. 6. Organization of structural elements within the smooth muscle cell. For purposes of simplicity, the contractile filaments are illustrated on the left side of the cell, whereas the cytoskeletal filaments are illustrated on the right side. Thin filaments composed of contractile actin (a or y isoforms) are proposed to associate with thick filaments. Thin filaments composed of cytoskeletal actin do not associate with myosin (as reviewed by Small (1995)). Actin filaments anchor at dense bodies in the cytosol and dense plaques at the cell membrane via linker proteins. Intermediate filaments link chains of dense bodies. Intermediate filaments are also linked to the cell surface at dense plaques...

As the neighboring cells of smooth muscle tissues are mechanically coupled, the contractile apparatus of each individual cell exerts tension on its neighbors. Force transmission across the sarcolemma of smooth muscle cells occurs at membrane-associated dense plaques found over the entire cell surface. When isolated smooth muscle cells contract, the points of attachment of contractile filaments are drawn into the cell, resulting in out-pouching of the membrane areas between the plaques (Harris and Warshaw 1991, Draeger et al 1990). 

While the majority of markers for differentiated SMC are located in the cytoplasm (linked with or associated to contractile filaments), those of immature SMC" belong mainly to the ECM or, to a lesser extent, plasma membrane (Fig. 3) and are shared by NM tissues/cells. 

EXAMPLE 13.21 Type II muscle fibers are subdivided into type Ila and type Ilb. Type Ila can use both aerobic and anaerobic metabolism to produce ATP whereas type lib fibers use only anaerobic metabolism. In contrast to type I muscle fibers, type lib fibers contract rapidly after stimulation by a nerve impulse. They have evolved for short-lived, powerful contractions by the possession of characteristically active myosin ATPases and a dense packing of contractile filaments. So much of the cytoplasmic space is taken up with filaments that little exists for mitochondria. Similarly, these fibers are associated with a relatively poor blood supply. Type lib fibers are also known as white and fast-twitch muscle fibers and are adapted for short-lived but powerful contractions. The relative paucity of mitochondria and the poor blood supply impose obvious constraints on the generation of ATP during exercise. 

---