The precipitation of calcium carbonate polymorphsin vitro at 37°C

Summary In order to determine some of the factors that affect the formation of the three polymorphs of calcium carbonate in gallstones, its precipitation from undisturbed solutions containing calcium chloride and sodium bicarbonate was studied. The variation in the amount of calcium carbonate formed with time was studied by weighing the precipitate, and the percentages of calcite, vaterite, and aragonite were determined by x-ray diffraction (XRD) analysis. The effects of additives and stirring were noted. Minor changes in conditions were sufficient to affect the formation of the polymorphs, and all three polymorphs could be found simultaneously in some solutions. It is suggested that no great differences in precipitating conditions need to be postulated to account for the presence of different polymorphs in gallstones. Aragonite seems to be formed slowly in undisturbed conditions.     

Formation of a calcium phosphate-rich layer on absorbable calcium carbonate bone graft substitutes

The use of natural coral as a bone graft substitute is common in Europe. However, the bone-coral bonding mechanism remains elusive. A rat subcutaneous model was used to demonstrate changes at the surface of resorbable calcium carbonate in the form of natural coral. Histological results indicated in vivo formation of a calcium phosphate (CaP)-rich layer on the surface of the coral confirmed by backscattered electron imaging and X-ray microanalysis. There appears to be a combination solution-mediated dissolution/cell-mediated degradation of the natural coral with subsequent surface conversion or precipitation. The end result is a CaP-rich layer on the coral. Though this layer has been observed previously, it was originally thought to be a histological artifact. This result is similar, however, to what is seen with Bioglass and glass ceramics and may also explain the good bonding of bone to hydroxyapatite. The fact that this layer is also present on natural coral after implantation in soft tissue sites may explain the intimate bone apposition observed when natural coral is placed in bony sites.     

Dialysate calcium reduction in CAPD patients treated with calcium carbonate and alfacalcidol.

The use of oral calcium carbonate as a phosphate binder is often complicated by hypercalcaemia, particularly with concomitant use of vitamin D analogues. We previously found that stepwise reduction of dialysate calcium effectively countered this complication in haemodialysis patients, and have now assessed the strategy in CAPD patients. Seventeen patients underwent conversion from aluminium hydroxide to calcium carbonate and were followed for 5 months, with subsequent addition of alfacalcidol for a further 5 months. Standard CAPD dialysate (1.75 mM calcium) was used, reducing to 1.45 mM and, if necessary, to 1.00 mM in patients who became hypercalcaemic. While receiving calcium carbonate alone, 12 of the 17 patients became hypercalcaemic, this responding in four to dialysate calcium reduction to 1.45 mM. In the remaining eight patients, further reduction to 1.00 mM was required and in two patients even this failed to control hypercalcaemia adequately, necessitating reversion to aluminium hydroxide. Phosphate control remained unchanged, as did calcium x phosphorus product. There were transient increases of blood ionised calcium, and decreases of parathyroid hormone, with progressive reduction of serum aluminium and alkaline phosphatase. The addition of alfacalcidol (0.25 microgram/day) led to hypercalcaemia in six subjects, successfully countered by dialysate calcium reduction in four. The results show that standard CAPD dialysate calcium at 1.75 mM is too high for the majority of calcium carbonate treated patients and that substantial reductions of the dialysate calcium concentration are required if calcium carbonate is to be used effectively.     

The acute metabolic effects of oral tricalcium phosphate and calcium carbonate

A double-blind study was performed to test the metabolic effects of tricalcium phosphate (TP) and calcium carbonate (CC) on serum calcium (SCa), serum phosphorus (SP), and immunoreactive intact serum parathyroid hormone (SPTH) levels in two groups of 24 subjects. The mean age of young subjects was 29.5 years, and elderly subjects, 65.9 years. These subjects fasted overnight for 12 hours, but with good hydration, before the tests. Following a 2-hour baseline-urine collection, 1200 mg elemental calcium (as CC or TP in tablet form) was chewed and ingested and 2-hour postload urines were collected. Blood was drawn immediately before and at 1, 2, and 4 hours after calcium load. The results showed that SCa and SP increased, whereas SPTH decreased with both preparations. The increment of SCa was similar after oral load of either calcium salt in both groups. The increment of SP after TP load was more than after CC. The urinary calcium/creatinine ratio (UCa/Cr) increased significantly after both preparations in the young group. The urinary phosphorus/creatinine ratio (UP/Cr) did not change significantly following TP, but decreased significantly after CC load in the young subjects. However, in the elderly individuals, the UP/Cr increased after TP load but did not change following CC, with statistical significance. The difference of urinary cyclic adenosine monophosphate/creatinine ratio (UcAMP/Cr) was not significant in both groups with either preparation. In summary, there was a similar rise in SCa and an equivalent fall in SPTH between TP and CC, in both young and elderly individuals.     

Comparison of the effects of calcium loading with calcium citrate or calcium carbonate on bone turnover in postmenopausal women

Calcium supplementation is known to increase bone mineral density and decrease fractures, but the relative efficacy of different forms of calcium supplementation is not established. We compared the effects of calcium carbonate and calcium citrate on markers of bone resorption in older postmenopausal women in an open-labeled crossover study. Forty women were randomized to receive 1000 mg/day of either calcium citrate or calcium carbonate for 12 weeks, followed by a 2-week washout without calcium supplements and 12 weeks treatment with the alternate calcium supplement. All women received vitamin D (900 IU/day). Thirty-four women (25 Caucasian, nine Hispanic) completed the study. No significant differences in the decrease in parathyroid hormone (PTH) or bone specific alkaline phosphatase or the increase in urinary calcium/creatinine were detected between the two treatments. However, calcium citrate supplementation decreased the collagen cross-link resorption markers, urinary N-telopeptide (–30%), C-telopeptide (–31%), free deoxypyridinoline (19%) and serum N-telopeptide (–8%), compared to no significant change following calcium carbonate supplementation (+2%, +3%, +2% and +2%, respectively; P<0.05). Calcium citrate decreased markers of bone resorption significantly more than calcium carbonate in postmenopausal women, although no differences in their effects in calcium excretion or PTH were detected.     

Safety and efficacy of calcium carbonate in children with chronic renal failure

The safety and clinical efficacy of calcium carbonate therapy in children with chronic renal failure were assessed in 68 patients (average age 8.38 years) during a mean follow-up period of 19.9 months (range 1.2-49.4). Forty-seven episodes of hypercalcaemia occurred in 29 children (3.5 episodes per 100 patient-months). There were no significant differences in mean GFR or biochemical parameters between these patients at the start of calcium carbonate therapy and the group of children who never experienced hypercalcaemia. Good control of secondary hyperparathyroidism and a significant reduction in serum aluminum were achieved. Two of 23 hypercalcaemic patients showed nephrocalcinosis on ultrasonography. 99Tc pyrophosphate scanning failed to detect any other ectopic calcification. The incidence of hypercalcaemia increased significantly when the GFR was less than 15 ml/min per 1.73 m2 and was most frequent in children receiving dialysis (48 episodes per 100 patient-months). The decrease in GFR during therapy was significantly more in the hypercalcaemic group compared to the normocalcaemic group (P less than 0.01), despite no irreversible acute effects of hypercalcaemia being observed on the rate of decline of GFR. We believe that the reduced renal homeostatic reserve is a major factor predisposing to hypercalcaemia. Consequently calcium carbonate is safe to use in children with severe chronic renal failure with close biochemical monitoring; the benefits over aluminium phosphate binders far outweigh the risks of hypercalcaemia and ectopic calcification.     

Intestinal calcium absorption from different calcium preparations: Influence of anion and solubility

Not only is the calcium content of a preparation significant for providing adequate calcium supplementation for the prophylaxis and therapy of osteoporosis, but also its bioavailablity is of essential importance. In the present study, the bioavailability of calcium citrate and calcium lactogluconate/carbonate from a therapeutic dose (= 500 mg Ca²⁺) was compared in men aged between 45 and 60 years on an intra-individual basis. Calcium citrate was administered both as a solution and as a suspension to 18 healthy volunteers. Using a double-isotope method, the intestinal absorption from the three preparations was determined in randomized order at intervals of 2–4 weeks. The stable isotope⁴⁴Ca (20 mg), in highly enriched form, was added in each case to the ready-to-drink solutions and, at the same time, a sterile and pyrogen-free solution containing 5 mg of the stable isotope⁴²Ca was injected intravenously. The intestinal calcium absorption was then determined after 24 h on the basis of the ratio of the two isotopes in the plasma. There was a significantly higher absorption of 29% from the citrate solution than from the lactogluconate/carbonate solution (25%). Absorption from the citrate suspension was similar to that from the lactogluconate/carbonate solution. While no correlation was found between the measured values for calcium absorption from the three preparations and the plasma concentration of 1,25-dihydroxycholecalciferol, significant inverse correlations with the basal parathyroid hormone concentration were observed for the citrate and lactogluconate/carbonate solution. The results of this study show that quantitative data on intestinal calcium absorption can be obtained without employing radioactive isotopes in humans. Moreover, they show that calcium absorption is not determined only by the solubility and the degree of ionization of the calcium salt administered, but rather that it is of a complex nature.     

The effect of aluminum on the rate of dissolution of calcium hydroxyapatite—A contribution to the understanding of aluminum-induced bone diseases

Summary Aluminum ions, Al³⁺, as these ions exist in aqueous solutions at pH≊7, adsorb onto calcium hydroxyapatite (HAP) crystals and severely inhibit the dissolution process of these crystals at 0.1 μM concentrations of aluminum. Human bone crystals have also been shown to adsorb these ions or molecules. A mechanism explaining why aluminum, in connection with bone resorption, causes less demineralization in the osteoid/calcified bone region than deeper in the calcified bone, is suggested.     

Kinetics of dissolution of calcium hydroxyapatite powder. III: pH and sample conditioning effects

Summary The kinetics of dissolution of synthetic hydroxyapatite powder (HAP) were studied at 37°C and constant pH in the pH range 3.7–6.9 by continuously recording proton uptake and calcium release. The effect of sample conditioning was carefully investigated. The powder previously equilibrated in saturated solutions shows an initial dissolution rate higher than the one obtained when dry powder directly added to the dissolution solution is used. This effect is interpreted by considering surface state differences. As previously shown, dry powder contains important amounts of calcium and phosphate ions adsorbed onto apatite surface, ions which are desorbed during equilibration. It is assumed that the initial presence of these ions slows the dissolution rate during the first stage of the process by the formation of a permselective layer. Except for these adsorption phenomena which are less important for human enamel powder (HEP) having a lower specific surface area, it is shown that in spite of structural, morphological, and purity differences, the general dissolution behavior of HAP is quite similar to that of HEP, previously studied, and for which a quantitative model has been proposed. The dissolution rates are stirring dependent in a large range of stirring speeds and are proportional to [H⁺]^0.64. Moreover, it is shown that in the whole range of studied pH, a calcium accumulation process occurs at the interface during the first minutes of the acidic attack. It is concluded that in our experimental conditions, the dissolution process is limited by the diffusion of calcium and/or phosphate ions in the interface. The calcium-rich interface constitutes a layer of low permeability in which strong interactions considerably reduce the diffusion of calcium and/or phosphate ions released during the attack and thus considerably slows the dissolution process.     

In vitro calcium oxalate crystallisation methods

In vitro calcium oxalate crystallisation has been, and will continue to be, of fundamental importance to urolithiasis research. Many different methods have been employed which differ qualitatively and quantitatively in the extent that they reproduce aspects of the renal system or in their ability to distinguish different aspects of crystallisation activity. Whatever system is used there are three key aspects that are worth bearing in mind. Firstly, a major controlling factor will be the prevailing supersaturation and other physicochemical considerations, secondly, during the course of the reaction different processes may come into play and thirdly, the processes we are trying to model take place in a dynamic biological environment. Different approaches to the study of crystallisation can be classified in many ways, such as the process or analytical technique but at a more fundamental level it is helpful to focus on the changes in supersaturation during the course of the reaction. A steady state supersaturation is more likely to be representative of the intra-renal situation than a system which decays to the equilibrium position. The constant composition method and the mixed suspension mixed product removal method both achieve a steady supersaturation.     

Proteolysis and partial dissolution of calcium oxalate: a comparative, morphological study of urinary crystals from black and white subjects

Crystal adherence to the renal epithelium is widely regarded as a probable mechanism of stone formation. Intracrystalline proteins may provide access to the core of urinary crystals and thereby facilitate the dismantling of these crystals after their attachment to and phagocytosis by renal epithelial cells. The present study investigated the role of proteolysis and limited dissolution of urinary calcium oxalate (CaOx) crystals in South Africas white and black populations with a view to understanding the remarkably low stone incidence in the black population compared with the whites. CaOx crystals were precipitated from filtered urine or ultrafiltered urine dosed with an intracrystalline protein, urinary prothrombin fragment 1 (UPTF1), isolated from white and black subjects. The crystals were fractured and subjected to proteolysis and/or limited dissolution before examination using field emission scanning electron microscopy (FESEM). FESEM data showed that CaOx crystals from white and black subjects were eroded by treatment with proteases. Cathepsin D caused the most significant crystal erosion, and more noticeable degradation of CaOx monohydrate (COM) crystals compared to CaOx dihydrate (COD). Limited dissolution studies showed the unique ultrastructures and fragmentation processes of COM and COD crystals. COM crystals appeared to be more susceptible to degradation and dissolution than CODs. Since COMs are predominant in blacks, compared with COD crystals from whites, it is speculated that the lower stone rate amongst South African blacks might be attributed partly to their more efficient destruction of retained COM crystals.     

Kinetic factors influencing the dissolution behavior of calcium oxalate renal stones: A constant composition study

Summary A constant composition method has been used to examine the dissolution kinetics of calcium oxalate renal stones over a wide range of undersaturationin vitro. Demineralization experiments have been carried out with the concentrations of calcium and oxalate ions and ionic strength (hence the solution undersaturation) held constant by the potentiometrically controlled addition of medium electrolyte solution as diluent, triggered by a calcium ion electrode. Kinetic data for renal stones have been compared with results obtained for synthetic calcium oxalate. In addition, constant composition results have been directly compared with results obtained using conventional dissolution methods for both calculi and synthetic calcium oxalate. Overall, calcium oxalate renal stones exhibited markedly different kinetic dissolution behavior as compared with synthetic controls. The renal stone samples dissolved more slowly at all undersaturations, exhibited increased kinetic orders of reaction, and showed reduced sensitivity to solution hydrodynamics. Stones composed of mixed hydrates of calcium oxalate (mono- and di-) came to dihydrate equilibrium in conventional experiments and underwent net dissolution in solutions supersaturated to monohydrate under constant composition conditions. No conversion of di- to monohydrate was observed under these experimental conditions. These results indicate that stone dissolution is strongly influenced by adsorbed inhibitors, presumaly including matrix components, which may complicate efforts to develop systemic and/or irrigation measures effective forin situ solubilization.     

Calcium carbonate gallstones in children

Background

In the United States, cholesterol stones account for 70% to 95% of adult gallstones and black pigment stones for most of the remainder. Calcium carbonate stones are exceptionally rare. A previous analysis of a small number of pediatric gallstones from the north of England showed a remarkably high prevalence of calcium carbonate stones. The aims of this study were to analyze a much larger series of pediatric gallstones from our region and to compare their chemical composition with a series of adult gallstones from the same geographic area.

Methods

A consecutive series of gallbladder stones from 63 children and 50 adults from the north of England were analyzed in detail using Fourier transform infrared microspectroscopy. Demographic and clinical data were collected on all patients. The relative proportions of each major stone component were assessed: cholesterol, protein and calcium salts of bilirubin, fatty acids, calcium carbonate, and hydroxyapatite.

Results

Thirty-nine (78%) adults had typical cholesterol stones, 7 (14%) had black pigment bilirubinate stones, and only 2 (4%) had calcium carbonate stones. In contrast, 30 (48%) children had black pigment stones, 13 (21%) had cholesterol stones, 15 (24%) had calcium carbonate stones, 3 (5%) had protein dominant stones, and 2 (3%) had brown pigment stones. In children, cholesterol stones were more likely in overweight adolescent girls with a family history of gallstones, whereas black pigment stones were equally common in boys and girls and associated with hemolysis, parenteral nutrition, and neonatal abdominal surgery. Calcium carbonate stones were more common in boys, and almost half had undergone neonatal abdominal surgery and/or required neonatal intensive care.

Conclusion

The composition of pediatric gallstones differs significantly from that found in adults. In particular, one quarter of the children in this series had calcium carbonate stones, previously considered rare. Geographic differences are not the major reason for the high prevalence of calcium carbonate gallstones in children.     

Crystal dissolution of biological and ceramic apatites

Summary High resolution transmission electron microscopy (Hr TEM) studies on biological and synthetic calcium phosphate have provided information on the dissolution process at the crystal level. The purpose of this study was to investigate the dissolution of ceramic hydroxyapatite (HA) after implantation using Hr TEM. Recovered HA ceramic implanted in bony and nonbony sites in animals and in periodontal pockets in humans were used for the study. For comparison, sections of human fluorotic enamel with caries and sections of shark enameloid previously exposed to 0.1 HCl were similarly investigated. Hr TEM studies demonstrated that in both the biological and ceramic apatites, the lattice and atomic defects were the starting points in the dissolution process. However, significant differences in the process of dissolution were observed: (1) biological apatite crystals showed preferential core dissolution whereas ceramic apatite crystals showed nonspecific dissolution at the cores and at the surfaces; (2) the dissolution of biological apatites appeared to consistently extend along the crystal's c-axis whereas dissolution of the ceramic HA did not appear to be correlated with the crystal's c-axis. The observed differences in crystal dissolution between biological and ceramic apatites may be attributed to the following: (1) the unique crystal/protein interaction present with biological apatites but absent in ceramic HA; (2) differences in defect distribution between biological and ceramic apatites which are due to the differences in the original of these defects; and (3) the longer morphological c-axis of biological apatites compared with that of ceramic apatites. This study provided for the first time, information on the dissolution process of implanted ceramic HA crystals and suggests that the crystal defects resulting from the sintering processes during the preparation of ceramic HA affect itsin vivo degradation and performance.     

Effectiveness of lanthanum carbonate treatment used in combination with calcium carbonate in hemodialysis patients with hyperphosphatemia

Objective To evaluate the effect and safety of the combination of lanthanum carbonate and calcium carbonate in controlling the phosphate level of hemodialysis patients with hyperphosphatemia. Methods Seventy-three patients who developed hyperphosphatemia after hemodialysis were involved, and of which twelve patients complicated with hypercalcemia were put in the group without calcium. The other patients were divided into 3 groups: 21 patients in calcium carbonate group, 32 patients in lanthanum carbonate group and 20 patients in combination group. All the subjects took blood test every month. Results The level of phosphorus decreased in all the subjects participated in the trial (P＜0.01). The level of phosphatemia in combination group decreased dramatically (P＜0.01) and had little effect on calcium and parathyroid hormone. Osteoporosis and valvular calcification were the same as the begin of the trial. Conclusions The combination of lanthanum carbonate and calcium carbonate is more effective than alone. The incidences of adverse effects such as gastrointestinal, hypercalcemia, nausea, vomiting and constipation are low.     

Saturation of human salivary secretions with respect to calcite and inhibition of calcium carbonate precipitation by salivary constituents

Summary The state of saturation of human salivary secretions with respect to calcite has been investigated. This property cannot be calculated exactly because of uncertainties in the values of the solubility product constant of calcite, the dissociation constants of carbonic acid, and PCO₂ values of saliva. Minimum and maximum limits for this saturation, however, can be established using appropriate values for the constants and salivary PCO₂. Values that give the minimum degree of saturation show that 8 of the 70 samples of human saliva investigated would be supersaturated with respect to calcite, while 64 of the 70 samples appeared to be supersaturated when values giving the maximum degree of saturation were used. In the latter case, the ratio of ionic activity products to solubility product was above 10 for several samples and over 18 for the most supersaturated sample. Since these results show that supersaturation of saliva with respect to calcite may be a common condition, human salivary secretions were investigated for the presence of inhibitors of calcite precipitation. Inorganic phosphate and the acidic proline-rich proteins, known to be inhibitors of calcite precipitation, and human salivary statherin, now shown to have a similar activity, are present in saliva at concentrations considerably higher than those required to inhibit calcite precipitation under salivary conditions. Quantitatively, phosphate is by far the most important inhibitor of calcite precipitation present in saliva, suggesting that inhibition of calcite precipitation by the macromolecules may be of secondary significance. It seems more likely that the function of these molecules is to inhibit precipitation of calcium phosphate salts, as previously proposed. These different inhibitory activities, however, are likely to be factors in the differences in composition of oral and dental calculi in different species, and may need to be considered in the formation of calcite stones in the pancreas.     

晶体表面结合蛋白对草酸钙晶体生长的抑制作用

为观察晶体表面结合蛋白（ＣＳＢＰ）对草酸钙晶体生长的影响，用层析法分离提取正常人和草酸钙结石病人ＣＳＢＰ，应用种晶体技术检测ＣＳＢＰ、尿凝血酶原激活肽Ｆ１片段（ＵＰＴＦ１）和白蛋白在体外对草酸钙晶体生长的作用。结果发现正常人ＣＳＢＰ和ＵＰＴＦ１能显著抑制草酸钙晶体的生长，结石病人ＣＳＢＰ的抑制活性低于正常人。提示病人ＣＳＢＰ中ＵＰＴＦ１减少致抑制活性降低与结石形成有关     

电化学法研究钙制剂的吸收率

本文用电化学方法研究了钙制剂的吸收率。实验结果表明，活性钙具有出色的离子化百分率，其活性远远高于其它钙制剂。不同工艺生产的活性钙，其活性基本相同。和传统的“代谢法”相比，本方法不仅具有快速、准确等特点，而且不必通过人体或动物实验就可以得到满意的结果。     

Effects of pyrophosphate and diphosphonates on the dissolution of hydroxyapatites using a flow system

Summary Pyrophosphate and diphosphonate ions have been said to diminish the dissolution of hydroxyapatite crystals, because they lower the equilibrium concentrations of calcium and phosphate ions in the bulk solution around hydroxyapatite crystals in a closed system. However, in a closed system these effects are not necessarily due to an effect on dissolution alone. In this paper we have used a continuous flow system to study the effects of pyrophosphate and two diphosphonates, ethane-1-hydroxy-1,1-diphosphonate and dichloromethane diphosphonate, on the dissolution of hydroxyapatite. All three compounds decreased markedly the rate of dissolution of hydroxyapatite as well as the exchangeable pools of calcium and phosphate ions around the cystals.