Despite the lack of effect on the production of contractile protein strength, HF modifies other aspects of contractile protein function; In particular, the kinetics of myosin-actin-cross-bridge interaction in RF patients was slowed down compared to age- and activity-appropriate controls.7 Although such changes in myosin-actin-actin-actin cross-bridge function may be potentially beneficial in maintaining the energy-producing capacity of muscle fibers, 7 may have adverse consequences. Slowing down the kinetics of the transverse bridge would likely slow down the contractile velocity,64 which in turn could reduce muscle power. In fact, there is evidence from preclinical models for reduced contractile velocity with HF.65 A reduction in muscle contractile velocity would contribute to an overall reduction in muscle strength performance.51 Thus, some of the reduced working capacity of skeletal muscle in patients with CI, which would directly affect performance in a peak load test, could result in a deceased contractile velocity that is due to impaired transverse bridge function (see Fig. 16.3). In fact, drugs that improve the function of myofilament proteins improve muscle contractility and power delivery, and in turn increase physical performance.66 The geometric interposition of various contractile proteins (especially actin, myosin, troponin, tropomyosin, and α-actinin) gives myofibrils a characteristic repetitive pattern of bands or scratches, whose base unit is known as sarcomakers (Figs. 1 and 4). Sarcomeres in mammalian ventricular cells have a characteristic resting length of about 2.2 μm. Their most obvious components are Z-bands (“Z-lines”, “Z-discs”), A bands and I bands; the less visible segments are the M-band-L line complex or the “pseudo-H-zone” in the middle of the sarcoma (Fig. 4 and 5). Although a sarcomere, stricto sensu, comprises two “half” I stripes, an A band and the transversely cut halves of two Z bands (Fig. 4), the popular convention considers that a sarcomere is any segment tightened by Z bands.
The A bands (“anisotropics”) are areas where the actin and myosin filaments overlap, while the I bands (for “isotropics”) are areas of sarcoma in which the actin filaments stand alone. At the intermediate level of the sarcomere, where the actin filaments in the relaxed myofibrils do not extend, a pair of relatively light L-lines flanks a dark M band, the turbidity of which results from the presence of transverse bridges between myosin and myosin. The presence of M ligaments in the heart is actually a sign of the maturity of the myocardial cell; in the heart of the rat, the M bands occur only after birth and are completely absent in the embryonic heart. FIGURE 4. Rhesus monkey on the right papilla. Relaxed myofibrils cut lengthwise show the distinct pattern of sarcoma units. Each sarcomere is delimited by opaque Z bands and also contains a dark central A band and two “half” I bands (whole I band denoted I band I). The central set of bands in the sarcomere is known in the heart as the “pseudo-H” zone (psH). The side-by-side register is not always accurate between adjacent myofibrils (see also Fig.
1). Sarcoma modeling has its basis in organized networks of protein-containing filaments: actin is found alone in the I bands, overlaps myosin in all areas except the pseudo-H zone of the A band, and blends into the dark substance of the Z band. Some cellular structures of the myocardium appear preferably junctional next to the Z-band, including the corbulistic SR vesicles (C-SR), a form of “extended” sarcoplasmic reticulum. Scale bar: 1 μm. Muscle shortening or changes in peri/intra-articular connective tissue (including chest wall and thoracic spine) → contractures, ↓ range of motion, pain. Positioning and stretching maintain reach and delay the invasion of non-contractile proteins Myocardial contractile proteins are in constant flux and rotate approximately every 10 days. For hypertrophy to occur, the protein synthesis rate (Ks) must exceed the protein degradation rate (Kd). Obviously, the only way to achieve this is for Ks to increase or Kd to decrease.
By infusing a tritiated amino acid such as leucine into a laboratory animal, the rate of absorption of new proteins (Ks) can be determined. When pressure overload is imposed on the dog`s left ventricle, Ks increases by 35% within 6 hours of the start of overload (Figure 4-8).43 Ks then remains elevated for several days and returns to normal once recharge is normalized, which strongly supports Grossman`s hypothesis.44 The increase in protein synthesis in this model does not result from increased DNA transcription, but thanks to a better translation of the messages, since the myosin message does not increase, but an increase in the ribosomal number and the formation of polysomes. Thick filaments (1.6 μm long) containing the motor protein myosin are located in the middle of the sarcoma in the optically anisotropic A-band (Figure 5-3D). These thick filaments are organized into a hexagonal network stabilized by the M27 protein and muscle-specific creatine phosphokinase 28 in line M (Figure 5-3D and E). Myosin (Figure 5-3K) is a highly asymmetric 470 kD protein containing two 120 kD-NH2 spherical terminal heads called transverse bridges or subfragment-1 (S1) (Figure 5-3L), and a spiral coil rod α, the light meromyosin (Figure 5-3K). Two light, essential and regulatory chains between 15 and 22 kD are connected to the heavy chain in each S1 (Figure 5-3L). Stem fractions of about 300 myosin molecules polymerize in a three-stranded helix to form the backbone of each thick filament (Figure 5-3J). The transverse bridges that protrude from these ridges contain binding sites to ATPase and actin, which are responsible for converting chemical energy into mechanical work.
In addition to their role in muscle contraction, at least 20 classes of non-muscular myosins perform various cell motility tasks such as chemotaxis, cytokinesis, pinocytosis, targeted vesicle transport, and signal transduction.29 Thus, myosin is the target of mutations that lead to a number of inherited muscle and neurological diseases.30, 31 The thin filaments (Figure 5-3I) are double-stranded helical polymers of actin that extend to 1.1 μm on either side of the Z line and take the optically isotropic I. Band (Figure 5-3D and E). A regulatory complex containing one tropomyosin molecule and three troponin subunits (TnC, TnT and TnI) is associated with each successive group of seven actin monomers along the thin filament (Figure 5-3I).23 In the area where the thick and thin filaments overlap, the thin filaments are located in the hexagonal lattice, which is equidistant from three thick filaments (Figure 5-3F). Both sets of filaments are polarized. In an active muscle, an interaction between the two filaments causes a concerted displacement of the thin filaments towards the M line, which shortens the sarcoma and therefore the muscle fiber and the entire muscle (Figure 5-3A to D). Actin is ubiquitous in the cytoskeleton of eukaryotic cells and, like myosin, plays many roles in determining cell shapes and movements.32,33 Control of the actin cytoskeleton and diseases due to mutations in actin-binding proteins are intensively studied.34 The contraction of smooth vascular muscles depends not only on the concentration of cytoplasmic calcium ions, but also the sensitivity of the contractile apparatus to calcium. In fact, smooth muscle contraction can occur without a change in cytoplasmic calcium levels, a phenomenon known as calcium sensitization.2,6 The main mechanism underlying calcium sensitization is the phosphorylation of myosin`s light-chain phosphatase, which inactivates the enzyme and leads to an increase in the phosphorylated (i.e., activated) form of the myosin light chain. Agonist-mediated calcium sensitization occurs via activation of the RhoA/RhoA kinase signaling pathway. Agonists acting on specific receptors coupled to G proteins activate the small GTPase RhoA, which is part of the Ras superfamily of monomer GTPases. Like most GTPases, RhoA alternates between the inactive cytoplasmic form bound to GDP and the active form associated with the membrane in which the bound nucleotide is GTP.
The downstream effector of RhoA-GTP is a serine/threonine kinase, RhoA kinase, which contains a RhoA binding domain. Phosphoryl activated RhoA kinase the regulatory subunit of myosin`s light-chain phosphatase, thereby inhibiting phosphatase activity. In addition to RhoA, arachidonic acid can also mediate calcium sensitization in smooth muscles. Arachidonic acid, which is released in response to certain agonists, can directly activate RhoA kinase, thereby inhibiting the activity of myosin`s light-chain phosphatase. Activation of the rhoastic/rhoa kinase pathway contributes to the maintenance of the tonic phase of smooth muscle contraction induced by agonists (see Fig. 5-1). Many vasoconstrictive signaling molecules (e.B serotonin, endothelin, angiotensin II) induce smooth muscle contraction both by calcium-dependent activation of myosin light-chain kinase and by calcium sensitization. Muscle atrophy – protein degradation (loss of contractile proteins, increase in non-contractile tissue, e.B collagen) and cytokine activity. Reduced strength, especially of the antigravitational muscles of the lower extremities (i.e.
B say those related to transfer and ability to walk). Inactivity improves the catabolic response of skeletal muscles to cortisol, resulting in more pronounced atrophy after trauma or disease. Particularly significant in groups of patients with low relative muscle mass, e.B. .