Calcium exchange site

Schematic diagram of a cardiac muscle sarcomere, with sites of action of several drugs that alter contractility (numbered structures). Site 1 is Na+/K+ ATPase, the sodium pump. Site 2 is the sodium/calcium exchanger. Site 3 is the voltage-gated calcium channel. Site 4 is a calcium transporter that pumps calcium into the sarcoplasmic reticulum (SR). Site 5 is a calcium channel in the membrane of the SR that is triggered to release stored calcium by activator calcium. Site 6 is the actin-troponin-tropomyosin complex at which activator calcium brings about the contractile interaction of actin and myosin.

Weakly acidic cation-exchange resins have carboxylic groups (COOH) as the exchange sites. When operated on the hydrogen cycle, the weakly acidic resins are capable of removing only those cations equivalent to the amount of alkalinity present in the water, and most efficiently the hardness (calcium and magnesium) associated with alkalinity, according to these reactions ... 

The clay minerals carried by rivers into the ocean represent a net annual addition of 5.2 X 10 mEq of cation exchange capacity. Most of these exchange sites are occupied by calcivun. Within a few weeks to months following introduction into seawater, sodium, potassium, and magnesium displace most of the calcium. As shown in Table 21.7, this uptake removes a significant fraction of the river input of sodium, magnesium, and potassium. 

Adsorption to the K+-covered siloxane surfaces of the clay, illite, can be estimated using Eq. 11-20. A tnt.eda is 300,000 L mol-1 and the surface area factor, /saf, for illite is 6 (Table 11.2). Since the ground water contains so much calcium relative to potassium (30 1), only a very small fraction of the cation exchange sites on the illite are covered with weakly hydrated potassium ions you assume/K+clay is about 0.01. Thus, you estimate ... 

Exchange was measured at typical pore-water concentrations of Ca2+ and NH4 +. For the Ca2+-saturated and NH4+-saturated sediments, KG was calculated by mass balance from measurements of initial and final calcium and ammonium concentrations in solution the total concentration of exchange sites was known. On the untreated sediment, KG could be determined only after these simplifying assumptions were made ... 

If the retardation factor approach is adopted for performance assessment, the distribution coefficient should be measured under conditions that represent a worst case competition scenario. One approach to approximating conservative conditions is to pre-wash the zeolite in an effort to saturate the exchange sites with calcium, the chief competitor (e g., Cantrell, 1996). Alternatively, it may suffice to operate the columns until full breakthrough of the competing solutes is observed, as suggested by Figure 4, followed by the analysis of spatial concentrations. 

Sometimes these operationally defined procedures have a sound theoretical basis. For example, it is quite reasonable to suppose that leaching with magnesium nitrate solution will displace zinc from cation exchange sites in soils, or leaching with ammonium acetate will displace exchangeable calcium, magnesium, sodium, and potassium. Flame spectrometry, especially flame AAS, is widely used for the analysis of such extracts. 

The species distribution of the solution determines the cation composition of the interlayer space of montmorillonite. In equilibrium, the cation exchange sites of montmorillonite are covered by calcium, hydrogen, manganese, and sodium ions (Figure 2.9). Figure 2.13 shows the equivalent fractions (X) of these cations as a function of pH at the ratio of MrnEDTA =1 1. 

Coman 1, Barbin G, Charles P, Zalc B, Lubetzki C (2(X)5) Axonal signals in central nervous system myeUnation, demyelination and remyeUnation. J Neurol Sci 233 67-71 Craner MJ, Hains BC, Lo AC, Black JA, Waxman SG (2004a) Co-locaUzation of sodium channel Navi.6 and the sodium-calcium exchanger at sites of axonal injury in the spinal cord in EAE. 

Figure 7 The variations of chloride, calcium, strontium (in mgL ), and Sr/ Sr ratios during intrusion of seawater into the coastal aquifer of Salinas Valley, California, USA. Note the linear relationships between all constituents, which indicates conservative mixing relationships between freshwater and modified seawater. These relationships suggest that base-exchange reactions occur at early stage of seawater intrusion and that Sr in exchange sites has a high Sr/ Sr ratio (source Vengosh et aL, 2002a).

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