Calcium sequestration

Figure 3. Reporting of intracellular calcium sequestration by chlorotetracycline (CTC). CTC preferentially partitions into cell membranes and its fluorescence in this environment is sensitive to calcium bound to the membrane therefore its signal (excitation AOO nm, emission 530 nm) will come largely from organelles that bind or sequester calcium, such as smooth endoplasmic reticulum or mitochondria. Release of calcium from such organelles is accompanied by dissociation of the calcium-CTC complex, a decrease in CTC fluorescence and efflux of unbound probe from the organelle and from the cell.

Haskovec C, Gut I, Volmerova D, et al. 1988. Acrylonitrile depletes glutathione without changing calcium sequestration in hepatic microsomes and mitochondria. Toxicology 48 87-92. 

Considerable evidence suggests that the major site of calcium sequestration and the source of intracellular calcium for signaling is the ER. In addition to the points already made, subcellular fractionation studies have demonstrated positive correlations of ATP-dependent Ca2+ accumulation and of IP3-mediated Ca2+ release with classical enzymatic markers for ER. Calcium uptake into the IP3-sensitive store is augmented by oxalate this augmentation is a property generally associated with the ER. 

Macrovesicular steatosis can be attributed to alcohol or cocaine, but massive liver necrosis is more probably due to cocaine. The mechanisms of cocaine hepatotoxicity, such as increased lipid peroxidation, free radical activity, and impaired calcium sequestration, may be potentiated by alcohol. 

Mammalian skeletal muscle can be separated Into two distinct fiber populations, based on relative contraction characteristics, and are referred to as slow-twltch (Type I) or fast-twitch (Type II) fibers. The slow-twltch fiber type exhibits a relatively low shortening velocity (27), a low rate of tension development (27). a low myosin ATPase activity (28) and a low rate of calcium sequestration by the sarcoplasmic reticulum (29). The converse Is true for the fast-twitch fibers. Since contraction velocity highly correlates with myosin ATPase activity (30), It Is possible to easily Identify,... 

At the same time, the industry embarked on an intensive search for phosphate substitutes. Of a very large number of experimental organic builders, a few substances reached commercialization or near-commercialization, including trisodium nitrilotriaceate (NTA), trisodium carboxymethoxysuccinate (CMOS) (181) and trisodium carboxymethyltartronate (182). As discussed above, sodium citrate ether carboxylates have achieved widespread use as phosphate substitutes. Polymeric builders (polyelectrolytes) proved to be effective calcium sequestrants, but failed to satisfy the criterion of acceptable biodegradability. Interestingly, some monomeric polycarboxylates proved to be even more powerful calcium sequestrants than sodium tripolyphosphate but were not sufficiently biodegradable (183). 

Acidosis can affect cell viability in other ways as well. Increased intracellular H can accelerate lipid peroxidation and free radical generation. Also, acidosis can exaggerate the rise in intracellular calcium by inhibiting intracellular calcium sequestration. Severe acidosis can also cause necrosis and damage to glial cells and the microvasculature. Acidosis inhibits the mitochondrial respiratory chain, lactate oxidation, and H extrusion. Acidosis also generates cell edema (Staub et al., 1993). 

Ion exchange (calcium sequestration) Low Low Moderate High-very high Low... 

Moore L, Chen T, Knapp HR, Landon EJ (1975) Energy-dependent calcium sequestration activity in rat liver microsomes. J Biol Chem 250 4562-4568... 

Phosphates are considered to be the most cost-effective detergent builder available, combining the properties of high efficacy (calcium sequestration, antiredeposition, buffering of pH), low toxicity, and relatively low cost. Phosphates are compatible with a wide variety of processing methods and product forms, which allows their use in powders, liquids, and bars. Although sodium tripolyphosphate (STPP) is the most common phosphate builder, sodium pyrophosphate is also used in some countries. In Asia, Latin America, and Africa, phosphates are the predominant detergent builders used. 

Methylxanthines inhibit intercellular calcium sequestration, leading to increased adhr-ity of skeletal and cardiac muscle. 

Lipid peroxidation, free radical formation, xenobiotic metabolism, free radical formation, phospholipase activation, loss of calcium sequestration, loss of membrane function, cell swelling, necrosis... 

A good builder must be able to remove most of the calcium present in hard water by chelation, precipitation, or ion exchange. Moreover, as calcium generally helps soils cling to cotton by bridging, sequestering these alkaline earths helps to loosen them. It is, therefore, important to measure calcium sequestration. 

The power of a silicate to sequester calcium can be evaluated by a test called the Calcium Sequestration Kinetics Test. This test consists in introducing 1 g/L of builder into a solution containing 3 X 10 M of calcium chloride and then measuring the free calcium content with a previously calibrated specific electrode. After a given time, often 10, 15, or 20 min, the experiment is halted. This test can be used to compare the action of different builders in eliminating free calcium in solution. 

The crystallized lamellar disilicate gives quite different kinetic results. Although the kinetics is slower, the quantity of calcium sequestered after 15 min is greater than with the other two disilicates. This clearly demonstrates that the silicate structure can considerably modify calcium sequestration capacity. 

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