Calcium availability

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. 

Calcium salt. Calcium gluconate is the preferred salt in PN because it is has a low dissociation in solution with lesser free calcium available at a given time to bind phosphate (as opposed to calcium chloride, which dissociates rapidly in solution). 

For over three decades, laboratory research has shown caffeine to be effective at mobilizing calcium in skeletal muscle. In vitro experiments have amply demonstrated that caffeine lowers the excitability threshold and extends the length of muscular contractions via calcium release from the sarcoplasmic reticulum.1012 Caffeine also inhibits calcium reuptake by the sarcoplasmic reticulum, perpetuating calcium availability for muscle work.1318 Also, caffeine promotes increased twitch tension development in muscles.1718... 

We can use our results to predict the conditions favorable for the transformation of gaylussite to aragonite. The porous nature of the pseudomorphs and the small amounts of calcium available in the lake water (Table 24.2) suggest that the replacement occurs by the incongruent dissolution of gaylussite, according to the... 

The effect of diet on vulnerability to lead makes interpretation of published information on experimental lead poisoning in waterfowl extremely difficult (Chasko et al. 1984). For example, many mallards on a diet of com die within 10 to 14 days after ingesting a single lead shot, whereas similar birds on a balanced commercial duck ration appear outwardly normal after ingesting as many as 32 pellets of the same size (Wobeser 1981). Also, multiple nutritional deficiencies may have additional effects in potentiating the toxicity of lead in mallards (Carlson and Nielsen 1985). Under conditions of reduced dietary calcium availability, such as can occur in acid-impacted environments, birds risk increased uptake of lead (and other metals) and may accumulate toxic concentrations more rapidly (Scheuhammer 1996). Enhanced accumulation of lead was accompanied by an increased synthesis of metallothioneins and a greater inhibition of ALAD activity (Scheuhammer 1996). 

Review of studies on the effect of oxalic acid on calcium bioavailability in rats and in humans indicates that most of the research was done between 1930 and 1950. Decreased availability of calcium in young rats was reported when spinach containing oxalic acid was fed with low calcium diets. The extent of the effect of oxalic acid on calcium availability was shown to be related to levels of calcium and oxalic acid, as well as the presence of vitamin D in the diet. In human studies there was generally no effect of oxalic acid on calcium balance however, in a few studies decreased calcium balances were reported. There is recent evidence that oxalic acid consumed along with a moderately high level of fiber intake may have adverse effects on calcium balance of human subjects. 

Available evidence suggests that a single unifying mechanism does not exist but rather that various vasodilators may act at different places in the series of processes that couple excitation of vascular smooth muscle cells with contraction. For example, the vasodilators known as calcium channel antagonists block or limit the entry of calcium through voltage-dependent channels in the membrane of vascular smooth muscle cells. In this way, the calcium channel blockers limit the amount of free intracellular calcium available to interact with smooth muscle contractile proteins (see Chapter 14). 

U.S. FDA (2006). Actonel. Actonel with Calcium. Available at http //www.fda.gov/ med watch / safety /2006/ Jan PI / Actonel Calcium PI.pdfit search=%22actonel%20calcium% 20risedronate%20usp%22 accessed 10/04/2006. 

Digitalis acts by interfering with the sodium and potassium transport across the cell membrane and by increasing the amount of coupling calcium i.e. making more calcium available for excitation-contraction coupling. 

Calcium also forms a link between the electrical events in the membrane and contractile proteins. Digitalis makes more calcium available for excitation-contraction coupling and increasing cardiac contractility. 

Ion channel modulation represents another approach to positive inotropy [13]. Sodium channel modulators increase Na+ influx and prolong the plateau phase of the action potential sodium/calcium exchange then leads to an increase in the level of calcium available to the contractile elements, thus increasing the force of cardiac contraction [13,14]. Synthetic compounds such as DPI 201-106 and BDF 9148 (Figure 1) increase the mean open time of the sodium channel by inhibiting channel inactivation [15]. Importantly, BDF 9148 remains an effective positive inotropic compound even in severely failing human myocardium [16] and in rat models of cardiovascular disease [17]. Modulators of calcium and potassium channel activities also function as positive inotropes [13], but in the remainder of this article we shall focus on sodium channel modulators. 

The accumulated phosphate reacts with calcium available to form apatite. Calcium may be concentrated within the vesicles by virtue of a lipid-calcium interaction and possibly by active calcium transport across the vesicle membrane458. Recent studies have led to the discovery of PL-Ca-Prcomplexes in calcifying tissues. These Pl-Ca-Pj complexes110 and certain proteolipids122,491 have been shown to rapidly nucleate HA from metastable Ca-P04 solutions. There is evidence for the existence of these complexes in matrix vesicles135. ... 

The mechanism of action for aluminum toxicity is not known, but the element is known to compete in biological systems with cations, especially magnesium (MacDonald and Martin 1988) despite an oxidation state difference, and to bind to transferrin and citrate in the blood stream (Gannot 1986). It may also affect second messenger systems and calcium availability (Birchall and Chappell 1988), and irreversibly bind to cell nucleus components (Crapper-McLachlan 1989 Dryssen et al. 1987). Aluminum has also been shown to inhibit neuronal microtubule formation. However, much more work is needed before a mechanism can be proposed. 

These patients initial laboratory values showed acidosis, prolongation of the QRS complex and QTC interval, and right axis deviation. Appropriate treatment included hyperventilation, sedation, active cooling, and sodium bicarbonate, which led to correction of the blood pH and of the cardiac conduction disorders. The authors suggested that when intracellular pH is lowered, myocardial contractility is depressed as a result of reduced calcium availability. During acidosis, there are abnormalities of repolarization and depolarization, which potentiate dysrhythmias. 

Oxalic acid is capable of combining with minerals to form salt complexes. Calcium and zinc form the least soluble salts with oxalic acid (, ). Spinach contains more oxalic acid than most foods (approximately 700 mg/100 g), and its effect on calcium availability has been studied rather extensively. Other green leafy vegetables also contain considerable amounts of oxalic acid, and the oxalic acid is concentrated more in the leaves than in the stalks (7 ). Rhubarb, some nuts, tea, and cocoa have also been found to contain oxalic acid in amounts greater than 200 mg/100 g food (7-jJJ. 

Figure 42 shows that the availability of calcium for desulfurization in coal combustion in a bubbling fluidized bed decreases with increasing particle diameter and is less than 20% for particles larger than 1 mm. On the other hand, fly ash collected from flue gas shows a calcium availability of not more than 10%, possibly due to its short residence time in the combustor. 

Fig. 42. Influence of particle diameter on calcium availability (After Zhang, 1981).

Smaller limestone particles possess shorter pores. Figure 44 shows that as particle diameter decreases, the pore volume increases, especially for particles measuring below 250 /im in diameter. Wu and Zhang (1988) attributed this phenomenon to increased absorption of mechanical energy as particles become smaller. An effective way to improve pore structure, and therefore, S02 capture, is to reduce particle size. For CFBC, addition of fine limestone is feasible without the carryover problem in the case of bubbling fluidization, often resulting in a calcium availability of as high as 50%. 

Aberg, G., Jacks, G., Wickman, T. Hamilton, P. J. (1990). Strontium isotopes in trees as an indicator for calcium availability. Catena, 17, 1-11. 

Less work has focused on the controls of recycling of other nutrients. Decomposition dynamics are a critical determinant of calcium availability, because calcium is part of cell walls that are difhcult to decompose. In contrast, potassium occurs mostly in the cell cytoplasm and is largely lost through leaching, so decomposition dynamics are less important than controls over soil availability in determining plant supply. 

The lignite type ashes have comparatively low fouling propensity when the sodium content is below 4 per cent because of the limited amount of clay minerals available for sintering reaction. That is, in some lignite and sub-bituminous coals there is an excess of sodium and calcium available for the high temperature reactions, and the rate of deposit formation depends on the silicate content of ash (2,29). The bituminous coal type ash has an excess of silicates, i.e. the ash is pyrochemically acidic and the rate of sintering depends on the availability of sodium, calcium and iron species in the flame heated deposit material. 

Ronov (1976) estimated the average CaO content in sedimentary layer of 15.91 %, and in granite layer, of 2.71 %. Accordingly, the calcium reservoir in sedimentary shell is 272.8 X 10 - tons, and in the granite pool is 222.8 x lO tons. The weathering and metamorphosis of deep-layer silicates is accompanied by the formation of clay minerals with release of calcium available for plant and microbial uptake. 

A striking example of mineral storage is the skeleton. The apparent abundance of calcium available for mobilization is deceiving since loss of relatively litde bone mineral density can lead to an increased incidence of serious fractures. Significant amounts of magnesium are stored in bone and available for utilization. 

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