Smooth muscle contractile apparatus

Based on this, one would predict that treatment of intact smooth muscle preparations with an inhibitor of CaMKII should potentiate myosin light chain phosphorylation and the rapid phase of force development. In contrast, we have found that in carotid arterial smooth muscle, KN-93 inhibition of CaMKII activation in response to physiological contractile stimuli correlates with a marked inhibition of tonic force responses (Rokolya and Singer 2000), suggesting an alternative dominant action of CaMKII on the smooth muscle contractile apparatus. 

The structure of the contractile apparatus of smooth muscle at the next higher level is also characteristically different from other muscles. The concentrations of actin and myosin in smooth muscle are about three times higher for actin and four times lower for myosin than in skeletal muscle. Correspondingly, in smooth muscle the ratio of the numbers of moles of actin to moles of myosin, and the ratio of the number of actin filaments to those of myosin filaments, are about 12 times larger than for other muscles. Thus, the arrangements of the two sets of filaments are bound to be quite different just on the basis of numbers of actin and myosin... 

The compliance in series with the active force. Force exerted by the activated elements must be transmitted or borne by whatever structural elements are in series with them. In skeletal muscle there is clearly a tendon in series but not so with smooth muscle. In smooth muscle, the total length of contractile apparatus is broken up into individual cells with intercalating extracellular connective structures. In addition, the portions of the crossbridges in series with the pulling site must also be stretched before force can rise to isometric levels. Taken together, the... 

Regulatory mechanisms. There is a large body of evidence which indicates that the contractile apparatus of smooth muscle is turned on as a result of the phospho-... 

The mechanical behavior of the contractile apparatus of smooth muscle is also very similar to that of striated muscle. So that to the extent that the force-velocity curves reflect the interaction of mechanical force and the rate of enzymatic catalysis, the steps of the chemomechanical transduction cycles in the two muscles are apparently modulated in similar ways. Also relationships between the active isometric force and muscle length are very similar (except as noted above for shorter lengths). 

Smooth muscle cells are small and spindle shaped (thin and elongated see Table 12.1). Similar to skeletal muscle, the contractile apparatus in smooth... 

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. 

We are still much in the dark regarding CP function. Does it perform both structural and regulatory roles, and if so what are they Are these roles carried out in the cytoskeleton or in the contractile apparatus, or both Does CP influence or complement CD function And if Ca2+ binding proteins are the primary regulators, which one(s) operate on CP To determine the function of CP in vivo will require new approaches. In part because of the presence of multiple targets for Ca2+ binding proteins in smooth muscle (including MLCK, CD, and CP), it has so far proved difficult to define the relative roles that myosin and the actin-associated proteins play in smooth muscle. Here specifically, we will want to know what roles CD and CP perform in the process of tension maintenance... 

The first section of this review will describe the general organization of the contractile apparatus of smooth muscle cells, and the basis for the transmission of force generated by the contractile apparatus across the smooth muscle cell membrane. In succeeding sections, the molecular structure of both the thick and thin filaments and of their organization in smooth muscle cells will be considered. The final section will evaluate the contractile behavior of smooth muscle in relation to our present knowledge of smooth muscle ultrastructure. 

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). 

Ashton FT, Somlyo AV, Somlyo AP (1975) The contractile apparatus ofvascular smooth muscle intermediate high volt e stereo electron microscopy. J Mol Biol 98 17-29 Babij P (1993) Tissue-specific and developmentally regulated alternative splicing of a visceral isoform of smooth muscle myosin heavy chain. Nucleic Adds Res 21 1467-1471... 

Draeger A, Amos WB, Ikebe M, Small JV (1990) The cytoskeletal and contractile apparatus of smooth muscle contraction bands and segmentation of the contractile elements. J Cell Biol 111 24632473... 

Kargacin GJ, Cooke PH, Abramson SB, Fay FS (1989) Periodic organization of the contractile apparatus in smooth muscle revealed by the motion of dense bodies in single cells. J Cell Biol 108 14651475... 

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