Why Is Calcium so Important for Muscle Contraction

Ion channel: ↑ Door-like structures in heart muscle cells that allow charged particles to enter or leave the cell. This protein has a molecular weight of 12 kDa and binds two high-affinity Ca2+. Its structure and, for comparison, that of calmodulin (terminal part COOH only) are illustrated in Figure 9. Interestingly, pv, which binds both Mg2+ and Ca2+, has a much more rigid structure than CaM, which binds ca2+ exclusively, but with a lower affinity than PV (17). For PV carp, Robertson et al. (430) calculated dissociation rates of the order of 10 and 1 sâ1 for Mg2+ and Ca2+ respectively (Fig. 7). Since the half-relaxation time after a single contraction is generally well below 100 ms, it must be assumed that the rates of metal dissociation of PV in vivo are very different from the values obtained in vitro, which allows a rapid exchange of Ca2+, which is important for muscle relaxation, because the recently measured Ca2+ affinity constants (KCa) for PV are generally much lower than those previously estimated (especially for KCa values, obtained from MAMMALIAN PV); Pv can also play an important role in muscle relaxation after a single contraction. In summary, each muscle in an animal is unique in terms of the composition of the type of fiber and the distribution pattern in the muscle. In animals, the fiber composition of homologous muscles can vary greatly from one species to another. Within a species in a given muscle, the proportion of type I fiber increases with height and body weight. On the other hand, in humans, the interindividual variability in the composition of the fibres is considerable.

Although a particular muscle in one individual may consist primarily of fast-twitch fibers, in another individual it may consist entirely of slow-twitch fibers (237, 527, 544,545). A muscle can functionally adapt to a wide range of activities. The molecular mechanisms involved in these adaptations and the first molecular and cellular events that occur in these processes are still poorly understood. However, DNA sequences that are important for the regulation of specific gene expression of fibers, as well as the transcription factors involved in this process, can now be studied through the use of transgenic animals or through direct gene transfer into the muscle of live animals. Histochemical analyses of muscle biopsies of biceps brachii and quadriceps showed atrophy of type II muscle fibers [A and B(X)]. A considerable proportion of the type II fibres of patients with Brody`s disease had a diameter of <50 Î1/4m. Such small diameters were not observed in any of the control fibers, although the control group consisted mainly of patients with peripheral neuropathies, diseases characterized by chronic partial denervation (242). Central nuclei have been observed in about half of the fibers, but hypercontracted or necrotic fibers, continuous regeneration or replacement of muscle tissue with connective tissue (fibrosis) have not been reported (84, 242).

Thus, despite the absence of SERCA1, the rapidly contracting muscle fibers of patients with Brody`s disease are protected against damage caused by highly cytoplasmic Ca2+. The fact that muscle stiffness caused by movement decreases after a period of rest suggests that Ca2+ is obviously reduced to a baseline level by mechanisms other than the pumping of Ca2+ by SERCA1. The sarcolemma of myocytes contains many intussusceptions (pits) called transverse tubules, which are usually perpendicular to the length of the myocyte. The transverse tubules play an important role in the supply of Ca+ ions to myocytes, which are crucial for muscle contraction. Fig. 4.The ryanodine receptor and its function in the release of Ca2+. Proposed arrangement of proteins in the SR and target proteins of Ca2+ in the cytoplasm. The transverse tubular membrane is part of the plasma membrane of the muscle fiber. The interaction of the α subunit of the Ca2+ channel, also known as the dihydropyridine receptor (DHPR), and the Ca2+ release channel of the SR called the ryanodine receptor (RyR1) connects the two membranes, the tubular membranes and SR. This compound is responsible for electromechanical coupling. Several cytoplasmic and SR proteins are associated with the DHP/RyR complex (triadine, calequesterin, binding protein FK506 and calmodulin).

The release of calcium by SR via RyR1 triggers muscle contraction and several cellular effects by binding ca2+ to a variety of other target proteins. The reuptake of Ca2+ from the cytoplasm into the SR is carried out by the SR calcium pump. By taking Ca2+ into account in its release and absorption cycle, a slowdown in the absorption of Ca2+ in the SR of muscle fibers can be tolerated when a normal rest level of Ca2+ is finally reached. Much worse conditions appear to be a continuous release of Ca2+ via a defective RyR or a continuous excessive influx of Ca2+ via a leaking plasma membrane. Calcium is found in most foods, especially dairy products such as milk and cheese, and is often found in small fish and some vegetables. It has long been known that calcium is beneficial for the strength of our bones. In addition, scientists have found that calcium also plays an important role in the heart (Figure 1). The heart beats more than 2 billion times over the lifetime of an average person to circulate the blood needed to fuel every part of the body. Among other things, the heart consists of 3 billion heart muscle cells that compress (“contract”) with each heartbeat and are jointly responsible for the pumping function of the heart. To make sure each cell contracts at the right time, the heart uses an electrical signal that moves from cell to cell, much like a wave in a stage where a person`s activity activates their neighbor.

Research in recent decades has shown that calcium particles are responsible for the relationship between electrical activation and mechanical contraction (Figure 1). Calcium particles, which have an electrical charge, enter the heart muscle cells with each beat and contribute to the electrical signal. In addition, these calcium particles initiate contraction by binding to specialized machines in the cell. When calcium binds, the machinery begins to move, causing the cell to compress. On the other hand, when calcium particles are removed from the heart cells, it triggers relaxation so that the heart can be filled with blood before the next heartbeat begins. Thus, without calcium, our hearts would immediately stop beating, which has already been demonstrated experimentally by Dr. .

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