Calcium ion inhibition

Calcium ion movements are sensitive to the concentration of other cations. In one study, it was found that Ca influx into cells might occur through K+ inward rectifier channels when extracellular K+ ion concentration fell below 1 mM. These channels became Ca -permeable only when the extracellular K+ concentration decreased to 1 mM or below. The same study found that the addition of different divalent cations revealed that Ba +, but not NT+, Cd +, Sr, or Mg +, reversibly blocked the Ca influx into cells during low external Intracellular proteins requiring calcium ion must be very selective for Ca because the concentration of free Mg + (2.5 mM) and K+ (-100 mM) are much higher. Intracellular Ca + concentrations must be kept at very low levels because calcium ions inhibit the activity of Mg +-dependent enzymes. In addition, the precipitation of sparingly soluble calcium salts may cause serious problems within a cell. 

The regulatory role of calcium ions in intermediary metabolism is well documented. Calcium has been shown to be involved in activation or inhibition of specific enzyme systems [105], For example, it activates cyclic nucleotide phosphodiesterase, phosphofructokinase, fructose 1 6 biphosphatase, glycerol phosphate dehydrogenase, pyruvate dehydrogenase phosphatase and pyruvate dehydrogenase kinase. Calcium ions inhibit pyruvate kinase, pyruvate carboxylase, Na+/K+-AT-Pase and adenylate cyclase. 

Pink, D.A., Hansen, L.T., Gill, T.A., Quinn, B.E., Jericho, M.H., and Beveridge, T.J. 2003. Divalent calcium ions inhibit the penetration of protamine through the polysaccharide brush of the outer membrane of gram-negative bacteria. Langmuir 19, 8852-8858. 

Berger et al. have studied bone-resorbing osteoclasts. In their manuscript, they study the impact of calcium ions in the extracellular environment of osteoclasts. It is believed that high concentration of extracellular calcium ions inhibits the osteoclast resorptive activity. The SECM studies of calcium ions and superoxide anion generation by bone-resorbing osteoclasts were performed on the surface of a bovine cortical bone slice. The detection scheme relied on the use of Ca +- and 02 -selective electrodes. Interestingly, the SECM measurements outline the adaptive nature of osteoclasts to even high calcium concentration environments. 

Verapamil (Table 1), the first slow channel calcium blocker synthesized to selectively inhibit the transmembrane influx of calcium ions into cells, lowers blood pressure in hypertensive patients having good organ perfusion particularly with increased renal blood flow. Sustained-release verapamil for once a day dosing is available for the treatment of hypertension. Constipation is a prominent side effect. Headache, dizziness, and edema are frequent and verapamil can sometimes cause AV conduction disturbances and AV block. Verapamil should not be used in combination with -adrenoceptor blockers because of the synergistic negative effects on heart rate and contractile force. 

Diltiazem inhibits calcium influx via voltage-operated channels and therefore decreases intracellular calcium ion. This decreases smooth muscle tone. Diltiazem dilates both large and small arteries and also inhibits a-adrenoceptor activated calcium influx. It differs from verapamil and nifedipine by its use dependence. In order for the blockade to occur, the channels must be in the activated state. Diltiazem has no significant affinity for calmodulin. The side effects are headache, edema, and dizziness. 

Figure 5.1 Effect of oxygen concentration on corrosion of mild steel in slowly moving water containing 165 ppm CaCl2 48-hour test, 25°C. [Courtesy of H. H. Uhlig, D. N. Triadis, and M. Stern, Effect of Oxygen, Chlorides, and Calcium Ion on Corrosion Inhibition of Iron by Polyphosphates, J. Electrochem. Soc. 102, p. 60 (1955). Reprinted with permission by The Electrochemical Society, Inc. ]...

Dispersed Non-inhibited 1. Fresh water clay based fluids. Sodium chloride less than 1%, calcium ions less than 120 ppm a. Phosphate low pH (pH to 8.5) b. Tannin—high pH (pH 8.5-11+) c. Lignite d. Chrome lignosulphonate (pH 8.5-10)... 

Gypsum-treated muds are more resistant to contamination and more inhibitive (700 ppm of calcium ions) than lime-treated muds, and also have a greater temperature stability (350°F). A freshwater mud can be converted to a gypsum mud according to the following procedure ... 

Nondispersed Inhibited Systems. In these systems, the nondispersed character of the fluids is reinforced by some inhibition system, or combination of systems, such as (1) calcium ions, lime or gypsum (2) salt-sodium chloride or potassium chloride (3) polymers such as Polysaccharides, polyanionic cellulose, hydrolyzed polyacrylamide. 

The carbon dioxide content is about 0 0005 to O Ol and the pH is between 7 6 and 8 1. The high chloride content would tend to increase the rate of corrosion, and this usually takes the form of pitting under these conditions. The corrosive influence of the chloride ions is, however, inhibited by the presence of magnesium and calcium ions by virtue of the formation of a protective layer of magnesium and calcium salts (calcareous scale). 

Systemic and coronary arteries are influenced by movement of calcium across cell membranes of vascular smooth muscle. The contractions of cardiac and vascular smooth muscle depend on movement of extracellular calcium ions into these walls through specific ion channels. Calcium channel blockers, such as amlodipine (Norvasc), diltiazem (Cardizem), nicardipine (Cardene), nifedipine (Procardia), and verapamil (Calan), inhibit die movement of calcium ions across cell membranes. This results in less calcium available for the transmission of nerve impulses (Fig. 41-1). This drug action of the calcium channel blockers (also known as slow channel blockers) has several effects on die heart, including an effect on die smooth muscle of arteries and arterioles. These drug dilate coronary arteries and arterioles, which in turn deliver more oxygen to cardiac muscle. Dilation of peripheral arteries reduces die workload of die heart. The end effect of these drug is the same as that of die nitrates. 

Microtubules can be reconstituted in vitro at 37 °C from a solution that contains a physiological mixture of brain tubulin, MAPs, small amounts of guanosine 5 -triphosphate (GTP), magnesium ions, and the calcium-chelating agent EGTA [ethylene glycol-bis(2-aminoethyl ether) N, N -tetraacetic acid]. Tubulin assembly is inhibited by low temperature and by the presence of calcium ions. 

In striated muscle, there are two other proteins that are minor in terms of their mass but important in terms of their function. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta, that attach to F-actin in the groove between its filaments (Figure 49-3). Tropomyosin is present in all muscular and muscle-fike structures. The troponin complex is unique to striated muscle and consists of three polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to the other components of troponin. Troponin C (TpC) is a calcium-binding polypeptide that is structurally and functionally analogous to calmodulin, an important calcium-binding protein widely distributed in nature. Four molecules of calcium ion are bound per molecule of troponin C or calmodulin, and both molecules have a molecular mass of 17 kDa. 

The binding of the antibody is size-dependent. Only the preincubation of the antibodies with oligopectates of degree of polymerization (DP) > 9 inhibits the binding to pectin immobilized in the wells of an ELISA test (Fig. 9.a, b). The difference between dimerized DPS and DP9 oligomers lies in the fact that dimerized DP9 could accommodate five calcium ions between their two chains whereas DPS could only four, which is apparently insufficient for the complexes to resist thermal agitation. 

Bellomo, G., Richelmi, P., Hirabelli, F., Marioni, V. and Abbagnano, A. (1985). Inhibition of liver microsomal calcium ion sequestration by oxidative stress role of protein sul-phydryl groups. In Free Radicals in Liver Injury (eds. G. Poli, K.H. Cheeseman, M.U. Dianzani, and T.F. Slater) pp. 139-142. IRL Press, Oxford. 

Increasing KC1 concentration lowers inhibition as shown in Table II. The fact that damage increased with KC1 concentration is consistant with the ionic ratio hypothesis and suggests a base exchange mechanism whereby calcium ions are more easily extracted from the clay and replaced by potassium ions as the potassium concentration increases. 

In the presence of sucrose alone as the single substrate, initial reaction rates follow Michaelis-Menten kinetics up to 200 mM sucrose concentration, but the enzyme is inhibited by higher concentrations of substrate.30 The inhibitor constant for sucrose is 730 mM. This inhibition can be overcome by the addition of acceptors.31,32 The enzyme activity is significantly enhanced, and stabilized, by the presence of dextran, and by calcium ions. 

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