Sarcoplasmic reticulum, calcium level

Table 8. Residual calcium levels and maximal concentration ratios accomplished by the sarcoplasmic reticulum calcium pump fueled with different substrates58,, 26 ...

Figure 2. Muscle stimulation, a) a single nerve impulse (stimulus) causes a single contraction (a twitch). There is a small delay following the stimulus before force rises called the latent period, b) A train of stimuli at a low frequency causes an unfused tetanus. Force increases after each progressive stimulus towards a maximum, as calcium levels in the myofibrillar space increase. But there is enough time between each stimulus for calcium to be partially taken back up into the sarcoplasmic reticulum allowing partial relaxation before the next stimulus occurs, c) A train of stimuli at a higher frequency causes a fused tetanus, and force is maximum. There is not enough time for force to relax between stimuli. In the contractions shown here, the ends of the muscle are held fixed the contractions are isometric.

Increases in the concentration of calcium in the cytosol provides a signal that can initiate muscle contraction, vision, and other signaling pathways. The response depends on the cell type. In muscle, a transient rise in the cytosolic calcium levels (from opening calcium channels in the sarcoplasmic reticulum) causes contraction. This signaling in contraction is a direct consequence of electrical activation of the voltage-gated channel. 

The sER also functions as an intracellular calcium store, which normally keeps the Ca level in the cytoplasm low. This function is particularly marked in the sarcoplasmic reticulum, a specialized form of the sER in muscle cells (see p. 334). For release and uptake of Ca " ", the membranes of the sER contain signal-controlled Ca channels and energy-dependent Ca ATPases (see p. 220). In the lumen of the sER, the high Ca " " concentration is buffered by Ca -binding proteins. 

The initial step in the sequence which leads to ATP synthesis during calcium release is the incorporation of inorganic phosphate into the transport protein. It was first demonstrated in experiments with sarcoplasmic reticulum vesicles which were actively loaded with calcium phosphate193,194). The membranes of the calcium loaded vesicles rapidly incorporate inorganic phosphate when the concentration of ionized calcium in the assay is reduced by the addition of EGTA. The involvement of this phosphorylated intermediate in ATP synthesis infers from the finding that on addition of ADP the level of phosphoenzyme drops and simultaneously, calcium is released and ATP is synthesized193,194 (Fig. 15). The same observations have been made when the... 

Annexins 1, 2, 4, 5, 6 and 7 have all been identified in the heart, though annexins 5 and 6 are by far the most abundant. Due to the fluctuating levels of submembranous calcium all annexins would be expected to exhibit dynamic on-off interactions with sarcolemal and sarcoplasmic reticulum membranes in cardiomyocytes upon exitation and calcium influx (reviewed in Camors et al., 2005). 

In muscle cells, the contraction is induced by Ca2+ release from the sarcoplasmic reticulum, as a result of membrane depolarization and activation of RyRl receptors located at the surface of the SR. The subsequent transport of cytoplasmic Ca2+ back into the lumen of the sarcoplasmic reticulum restores low resting calcium levels and allows muscle relaxation. In fast-twitch skeletal muscle fibers, Ca2+ uptake is mediated by the sarco(endo)plasmic reticulum Ca2+ ATPase SERCA1 which represents more than 99% of SERCA isoforms in these muscle fibers. 

Sources of free intracellular calcium Calcium comes from two sources. The first is from outside the cell, where opening of voltage-sensitive calcium channels causes an immediate rise in free cytosolic calcium. The second source is the release of calcium from the sarcoplasmic reticulum and mitochondria, which further increases the cytosolic level of calcium (Figure 16.3). 

Calcium ions are also transported into the cell by a pump, which is a Ca +-dependent ATPase. This pump is necessary because the calcium ion concentration is four orders of magnitude higher outside than inside living cells. Calmodulin regulates the level of calcium ions and hence the calcimn pump. When the calcium concentration decreases, calcium is dissociated from calmodulin and the calcium pump is inactivated. The structure of such a pump from the sarcoplasmic reticulum is reported at 8 A resolution. This pump couples ATP hydrolysis with cation transport. The protein contains 10 transmembrane helices. A distinct cavity was located that led to the putative calcium-binding site, suggesting a path for a calcium passage. 

We will consider the structural and mechanistic features of these enzymes by examining the Ca2+ ATPase found in the sarcoplasmic reticulum (SR Ca2+ ATPase) of muscle cells. This enzyme, which constitutes 80% of the sarcoplasmic reticulum membrane protein, plays an important role in muscle contraction, which is triggered by an abrupt rise in the cytosolic calcium level. Muscle relaxation depends on the rapid removal of Ca + from the cytosol into the sarcoplasmic reticulum, a specialized compartment for calcium storage, by the SR Ca + ATPase. This pump maintains a Ca2+ concentration of approximately 0.1 iM in the cytosol compared with 1.5 mM in the sarcoplasmic reticulum. 

Phosphorylase kinase can also be partly activated by Ca2+ levels of the order of 1 iM. Its 8 subunit is calmodulin, a calcium sensor that stimulates many enzymes in eukaryotes (Section 15.3.2). This mode of activation of the kinase is important in muscle, where contraction is triggered by the release of Ca2+ from the sarcoplasmic reticulum. Phosphorylase kinase attains maximal activity only after both phosphorylation of the P subunit and activation of the 8 subunit by Ca2+ binding. 

---