Structure of smooth muscle

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

Location Attached to bones openings of some hollow organs (sphincters) Large blood vessels eyes hair follicles Walls of hollow organs of digestive, reproductive, and urinary tracts small blood vessels 

Thin filaments Actin, tropomyosin, troponin Actin, tropomyosin Actin, tropomyosin 

Filament arrangement Sarcomeres Diamond-shaped lattice Diamond-shaped lattice 

Innervation Somatic nervous system Autonomic nervous system Autonomic nervous system 

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Actin and the Structure of Smooth Muscle Thin Filaments... 

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

Smooth muscle myosin has distinctive characteristics that may form the basis for many of the unique functional properties exhibited by smooth muscle tissues. The following section will first review the molecular structure of smooth muscle myosin and the functional implications of its distinctive characteristics. This will be followed by a discussion of the regulation of the assembly of myosin into thick filaments, and the molecular organization of the thick filaments of smooth muscle tissues. 

Gabella G (1990) General aspects of the fine structure of smooth muscles. In Motta PM (ed) Ultrastructure of Smooth Muscle. Kluwer Academic Publishers, Boston, pp 1-22 Geiger B (1983) Membrane-cytoskeleton interaction. Biochimica et Biophysica Acta 737 305341... 

Lehman W, Craig R, Lui J, Moody C (1989) Caldesmon and the structure of smooth muscle thin filaments immunolocalization of caldesmon on thin filaments. J Muscle Res Cell Motil 10 101112... 

Nonmuscle/smooth muscle myosins-Il are structurally similar to striated muscle myosin-II, but they have slower rates of ATP hydrolysis than do their striated muscle counterparts. Nonmuscle/smooth muscle myosin-II is also regulated differently than striated muscle myosin-II. Nonmuscle myosin-II is divided into the invertebrate and vertebrate branches (Cheney et al., 1993). This group is ubiquitous because it is present in most lower organisms, such as slime molds, amoeba, sea urchins, etc., and in virtually all mammalian nonmuscle cells. Smooth muscle myosin-II is also somewhat heterogeneous in that at least three separate forms of smooth muscle heavy chains, with molecular weights of 196,000, 200,000, and 204,000 have been identified (Kawamoto and Adelstein, 1987). The physiological properties of these separate myosin heavy chains are not yet known. 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

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 superstructure of smooth muscle actin filaments is differentiated from those of striated muscle by the absence of the troponins and the lateral organization by association of the filaments with dense bodies instead of with the Z-line. How these differences are encoded is again not at all clear. However, the myofibrillar structure and the alignment of the alternating actin and myosin filaments is apparently due primarily to dense bodies and the actin-actinin macrostructures. As the bent dumbbell shaped actins assemble into filaments they are all oriented in the same direction. The S-1 fragments of myosin will bind to actin filaments in vitro and in... 

In addition to its effects on enzymes and ion transport, Ca /calmodulin regulates the activity of many structural elements in cells. These include the actin-myosin complex of smooth muscle, which is under (3-adrenergic control, and various microfilament-medi-ated processes in noncontractile cells, including cell motility, cell conformation changes, mitosis, granule release, and endocytosis. 

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

Somlyo AV, Franzini-Armstrong C 1985 New views of smooth muscle structure using freezing, deep-etching and rotary shadowing. Experientia 41 841-856 Somlyo AP, Somlyo AV 1994 Signal transduction and regulation in smooth muscle. Nature 372 231-236... 

Blood is pumped away from the heart through arteries it permeates the tissues through networks of very small capillaries where nutrient delivery, gas exchange and waste removal occur and is finally returned to the heart via the veins. The structures of the arteries and veins differ in important ways. First, the veins have one-way valves which prevent the back-flow of blood and second, the walls of the arteries are much thicker, due largely to the layer of smooth muscle cells. Both types of vessel are lined on their inner surface with endothelial cells. Refer to Figure 5.2. 

Through its structural similarity to acetylcholine (Figure 3.7b), muscarine binds to the acetylcholine receptor on the synapses of nerve endings of smooth muscles and endocrine glands, causing the well-known parasympaticomimetic effects. Because muscarine is not an ester like acetylcholine, and hence resists esterase activity, it is not degraded and so can cause continuous stimulation of the affected neurons. 

All muscarinic receptors are members of the seven transmembrane domain, G protein-coupled receptors, and they are structurally and functionally unrelated to nicotinic ACh receptors. Activation of muscarinic receptors by an agonist triggers the release of an intracellular G-protein complex that can specifically activate one or more signal transduction pathways. Fortunately, the cellular responses elicited by odd- versus even-numbered receptor subtypes can be conveniently distinguished. Activation of Ml, M3, and M5 receptors produces an inosine triphosphate (IP3) mediated release of intracellular calcium, the release of diacylglyc-erol (which can activate protein kinase C), and stimulation of adenylyl cyclase. These receptors are primarily responsible for activating calcium-dependent responses, such as secretion by glands and the contraction of smooth muscle. 

Ikebe, M. Reardon, S. Fay, ES. Primary structure required for the inhibition of smooth muscle myosin light chain kinase. FEBS Lett., 312, 245-248 (1992)... 

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