Pivotal Role Of Calcium

Whether a muscle is contracting or relaxed depends on the level of cytosolic calcium available to interact with the regulatory proteins. In relaxed muscle, the level of free cytosolic calcium (calcium that is not bound to other structures such as sarcoplasmic reticulum, mitochondria, or nuclei) is low (<10—7 M). Upon stimulation of the muscle, the calcium level increases into micromolar or higher ranges to initiate contraction. In striated muscle, calcium binds directly to troponin C of the tropomyosin-troponin complex to bring about a conformational change in the complex. Once the stimulus for muscle contraction ceases, free calcium levels decrease and calcium dissociates from the regulatory proteins. The muscle then relaxes. The relation between free calcium levels and contraction force is complex and indicates cooperation among the contractile proteins once calcium binds to troponin to initiate contraction.

Free ionic calcium is the mediator between the events of the cell membrane that indicate excitation and the protein interactions that result in contraction. Thus, the events that describe calcium movements in muscle cells often are referred to as excitation-contraction (E-C) coupling. E-C coupling differs among the various types of muscles; similar to the contractile process, it is closely linked to structure.

Longitudinal Terminal SR Cisternae

Longitudinal Terminal SR Cisternae

FIGURE 5 Sarcoplasmic reticulum (SR) in skeletal muscle. The SR consists of two components. The terminal cisternae serve as reservoirs of calcium during relaxation. Upon stimulation, Ca2+ is released to interact with the troponin complex. Once released, Ca2+ is avidly taken up by the second component, the longitudinal SR. Uptake is mediated by a calcium ATPase.

FIGURE 5 Sarcoplasmic reticulum (SR) in skeletal muscle. The SR consists of two components. The terminal cisternae serve as reservoirs of calcium during relaxation. Upon stimulation, Ca2+ is released to interact with the troponin complex. Once released, Ca2+ is avidly taken up by the second component, the longitudinal SR. Uptake is mediated by a calcium ATPase.

E-C Coupling in Skeletal Muscle

In relaxed skeletal muscle, all the calcium that normally takes part in E-C coupling is stored inside the cell in the sarcoplasmic reticulum (SR) (Fig. 5). As might be expected, the SR of these cells is highly developed and extensive. Structurally, it is of two types: the longitudinal SR, which runs parallel to the thick and thin filaments, and the terminal cisternae, which form large pouch-like structures at each end of the longitudinal SR. Functionally, the terminal cisternae serve as storage places for calcium during muscle relaxation. Upon muscle excitation, calcium moves into the cytoplasm down a large concentration gradient. Once in the cytoplasm, calcium interacts with the tropomyosin-troponin complex to allow full activation of the contractile proteins. Calcium then is taken up by the longitudinal SR by an active process that involves a calcium ATPase. This pump has a high affinity for calcium and, if no calcium is being released from the terminal cisternae, can quickly lower cytosolic calcium to levels that do not support contraction. Calcium taken up by the longitudinal SR returns to the terminal cisternae, from which it can be released again. The SR is so well developed in most skeletal muscles that intra-cellular calcium is conserved to the point that these muscles can contract normally for some time, in vitro, even in a medium devoid of calcium.

E-C Coupling in Cardiac Muscle

E-C coupling in cardiac muscle differs only slightly from that in skeletal muscle, but the differences are functionally extremely important. In cardiac muscle, the SR is not as highly developed in that the terminal cisternae are not as large (see Chapter 11). One consequence of this is that variable amounts of calcium are released from the SR to allow for variable activation of the contractile proteins. However, the calcium that is released into the cytoplasm acts just as it does in skeletal muscle. That is, it binds to tropomyosin-troponin complexes to initiate contraction. Another functionally important difference between skeletal and cardiac muscle is that some calcium must enter cardiac muscle cells from the extracellular space in order to initiate contraction (see Chapter 11). Although this amount is small, cardiac muscle cells in vitro soon stop contracting if placed in a medium that does not contain calcium. For relaxation to occur, most of the calcium is taken up into the longitudinal SR by an active process involving a calcium ATPase just as in skeletal muscle. However, between contractions, some calcium must leave the cell. This is accomplished mainly by a sodium-calcium exchange mechanism present on the sarcolemma. There can be temporary imbalances between the amount of calcium entering and the amount leaving, such that the stores of releasable calcium in the SR increase and decrease. These imbalances are important in regulating cardiac contractile activity (see below).

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