Myofilament

FIGURE 17.31 Electron micrograph of a dog heart muscle. The terminal cisterna of the SR (TC-SR) is associated with the t-tnbnle (TT) by means of foot strnctnres (FS), forming a dyad junction. ME indicates the location of myofilaments. LT-SR signifies the longitudinal tubule of the SR. (From Fteischer,. S ., and Inni, M., 1989. Amiual Review of Biophysics and. Biophysical Chemisti y 18 333-364.)... 

From this brief summary of excitation-contraction coupling it is obvious that Ca is an important link between the activated membrane and the contractile proteins, and thus a regulator of tension development. Westerblad et al. (1991) defined three factors which explain the force decrease in fatigued muscle reduced Ca " release from the SR, reduced Ca sensitivity of the myofilaments, and reduced maximum Ca -activated tension. 

Fabiato, A. Fabiato, F. (1978). Effects of pH on the myofilaments and the sarcoplasmic reticulum of skinned cells from cardiac and skeletal muscle. J. Physiol. 276,233-255. 

Biopsy findings show disseminated muscle fiber atrophy which is confined to type 2 fibers, in many instances with type 2B (glycolytic) fibers most affected (Figure 23). Muscle necrosis is not seen, though at ultrastructural level focal myofibrillar disruption and myofilament loss may be evident. The muscle atrophy seems to be due to decreased protein synthesis, and at high doses, to increased catabolism. The reason for the selective effect on phasic, glycolytic fibers is not clear since, although steroids interfere with carbohydrate metabolism and oxidative capacity, there seems to be no overall effect on ATP levels. Nevertheless it has been... 

As mentioned above, the junctional SR is connected to sheets of perpendicular SR (Fig. 4), which extend from the PM through a peripheral cytoplasmic region with lower myofilament density into the myoplasm. It is proposed that during the active state of wave-like [Ca2+]j oscillations, Ca2+ taken up by the junctional SR is released by these perpendicular sheets near the calmodulin, which is tethered to the myosin light chain kinase (MLCK) of the thin filaments (M. Walsh, personal communication, 2001). This process would enhance the specificity and efficiency of Ca2+ regulation of contraction. 

Walsh The kinase binds to actin in the isolated state with a Kj of about 0.8 //M and to myofilaments with a Kof about 0.1 /tM. No measurements of the on- and off-rates have been made, however. In situ, most importantly, MLCK appears to be permanently bound since it does not dissociate from detergent-treated smooth muscle tissues, implying that the off-rate in situ is extremely slow or zero. 

Walsh All we know about it at the moment is that the Ca2+-independent interaction of calmodulin with the myofilaments is different from the interaction of Ca2+/calmodulin with the isolated kinase. The question is is this calmodulin... 

The muscle is a highly organized tissue, built up of individual cells known as fibres, which are held together by connective tissue. Each muscle fibre consists of a high number of single strands of myofibrils. The myofibrils are again comprised of myofilaments. The myofilaments are divided into thin and thick filaments, which mainly contain two filamentary proteins, actin and myosin, respectively. The myofibrils occupy approximately 80% of the muscle cell volume, and the majority of the water, which makes up about 75% of the muscle, is located in the spaces between thin and thick filaments. A schematic drawing of muscle structure is shown in Fig. 1. 

X-ray studies have demonstrated that pH affects the myofilament lattice due to electrostatic repulsion.45 Accordingly, the shown relationship between myofilament lattice and the T2i time constant50 reveals that a correlation between the T21 time constant and pH should be expected, as it was also... 

Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

Mitochondria One characteristic of human life is movement and physical activity. It is the mitochondria in muscle that generate ATP to support the process of contraction of the myofilaments in the muscle fibre which results in contraction of the whole muscle. 

A fibre is packed with longitudinally arranged myofibrils that contract. This is the contractile unit of the fibre it is 1-3 pm thick in diameter. Each myofibril consists of about 1000-2000 filaments, which are known as myofilaments. 

Figure 13.5 Electron micrograph of part of a longitudinal section of a myofibril. Identification of components and the mechanism of contraction. When a muscle fibre is stimulated to contract, the actin and myosin filaments react by sliding past each other but with no change in length of either myofilament. The thick myosin strands in the A band are relatively stationary, whereas the thin actin filaments, which are attached to the Z discs, extend further into the A band and may eventually obliterate the H band. Because the thin filaments are attached to Z discs, the discs are drawn toward each other, so that the sarcomeres, the distance between the adjacent Z-discs, are compressed, the myofibril is shortened, and contraction of the muscle occurs. Contraction, therefore, is not due to a shortening of either the actin or the myosin filaments but is due to an increase in the overlap between the filaments. The force is generated by millions of cross-bridges interacting with actin filaments (Fig. 13.6). The electron micrograph was kindly provided by Professor D.S. Smith.

Neuromuscular transmission (B) of motor nerve impulses to the striated muscle fiber takes place at the motor endplate. The nerve impulse liberates acetylcholine (ACh) from the axon terminal. ACh binds to nicotinic cholinocep-tors at the motor endplate. Activation of these receptors causes depolarization of the endplate, from which a propagated action potential (AP) is elicited in the surrounding sarcolemma. The AP triggers a release of Ca from its storage organelles, the sarcoplasmic reticulum (SR), within the muscle fiber the rise in Ca concentration induces a contraction of the myofilaments (electromechanical coupling). Meanwhile, ACh is hydrolyzed by acetylcholinesterase (p. 100) excitation of the endplate subsides. if no AP follows, Ca + is taken up again by the SR and the myofilaments relax. 

Prent, N., Green, C., Greenhalgh, C., Cisek, R., Major, A., Stewart, B., and Barzda, V. 2008. Inter-myofilament dynamics of myocytes revealed by second harmonic generation microscopy. J. Biomed. Opt. 13 0413181-7. 

The studies reported above implicated calcium movement in the mechanism.It was postulated that 2-PAM may Increase the rate of calcium movement either by interfering with calcium binding to its binding site or by directly stimulating the release of bound calcium.21 It was suggested that 2-PAM may not have increased cell permeability or affected the rate of re-entry of calcium into the cell, or 2-PAM may have increased calcium distribution in the vicinity of myofilaments. 

Digoxin remains the mainstay of treatment for patients with chronic myocardial failure. Other drugs with inotropic and/or vasodilator properties, including the catecholamines and phosphodiesterase III (PDE) inhibitors, are used in the treatment of acute cardiac failure. The inotropic actions of most of these drugs result from a direct or indirect elevation of [Ca2-i-]i (intracellular Ca2+ concentration). This acts as a trigger for a process which leads to increased contractile state and cardiac contraction (Figures 8.3 and 8.4). Myofilament calcium sensitisers increase the sensitivity of contractile proteins to calcium. Some newer drugs, such as vesnarinone, have multiple mechanisms of action. 

Davis, W. L. An electron microscopic study of myofilament calcium binding sites in native, EGTA-chelated and calcium reloaded glycerolated mammalian skeletal muscle. Calc. Tiss. 

Excitation-contraction coupling in cardiac myocytes is initiated by the cardiac action potential (AP), where depolarization-activates an inward Ca current (ICa), that is called the Ca-trigger, as it promotes the sarcoplasmic reticulum (SR) Ca release. The combination of ICa and SR Ca release raises intracellular free [Ca] ([Ca, ), allowing Ca to bind to the myofilament protein troponin C, which activates contraction. For... 

Current evidence indicates that PLN phosphorylation appears to be dominant over troponin I phosphorylation (Li, et al., 2000). The faster SR Ca uptake by phospho-rylated PLN also contributes to increased SR Ca load, which is available for subsequent release, resulting in an inotropic effect. The increased ICa by PKA activation also contributes to the inotropic effects of the (i-AR agonists. The myofilament effects of PKA appear to be almost entirely attributable to troponin I phosphorylation (vs. myosin binding protein C) because substitution of troponin I with a non-phosphorylatable troponin I abolishes myofilament effects of PKA (Kentish, et al., 2001 Pi, et al., 2002). 

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