Calcium Reservoir Research Articles

Investigation of formation reactions in the system Bi-Pb-Sr-Ca-Cu-oxide starting from oxides and carbonates

We were able to follow the formation reaction of high-temperature superconductors in the system Bi-Pb-Sr-Ca-Cu-O by using in-situ high-temperature X-ray diffraction. We first observed reactions in the confined system Bi-Sr-Cu-O, and then investigated modifications in the reaction path introduced by lead and calcium. We found that the addition of Ca does not modify reactions below 850°C but lowers the melting point of Bi 2Sr 2CuO 6+ x , Calcium also extends the stability range of the latter phase which, in contrast to Ca-free samples, does not decompose before melting. The addition of lead lowers the formation temperature of some intermediate phases, and introduces strontium lead oxide as a new phase stable up to 880°C. The addition of calcium and lead leads to the formation of very stable Ca 2PbO 4 early in the process (550°C). This reaction consumes, depending on stoichiometry, most of the lead. We found that Ca 2PbO 4 decomposes when the high-temperature superconductor Bi 2Sr 2CaCu 2O 8+x is formed above 830°C. Up to this temperature, Ca 2PbO 4 acts therefore as a reservoir for calcium and keeps the lead oxide from acting as a flux.

Changes in rat mast cell responses in sodium-free media. Lack of demonstrable sodium channel activity

Exposure of rat mast cells to isotonic sucrose (employed as a sodium free medium) increased several-fold the sensitivity to calcium, which itself became a stimulus for exocytosis. Concentrations of the cation as low as 25 microM permitted maximal histamine release. Preincubation of cells in sucrose to allow sodium efflux before adding the ionophore A23187 led to a slower release of histamine. We postulate that sodium efflux can generate a membrane potential that causes the increased sensitivity to calcium and the delay in response after preincubation. The response to A23187 is somewhat unspecific since the ionophore can release histamine from internal calcium reservoirs. Saxitoxin or veratridine did not affect cell responses, so that sodium activity is not mediated through defined sodium channels.

Role of Proteoglycan in the Provisional Calcification of Cartilage

Study of the calcification of cartilage in endochondral ossification has yielded two apparently contradictory views of the role of proteoglycan in this process. The ability of proteoglycan to act as a calcium-concentrating agent (Kalksalzfänger) in cartilage is consistent with the view that proteoglycans are promoters of calcification. However, study of their effect on hydroxyapatite formation in vitro suggests that proteoglycans are inhibitors of cartilage calcification. A resolution of this paradox is now proposed. Proteoglycans inhibit hydroxyapatite formation under in vitro conditions of limited calcium availability (in part) by binding calcium ions. However, under in vivo conditions of essentially infinite calcium availability, proteoglycans may promote hydroxyapatite formation, since binding of calcium to proteoglycan will not decrease the free calcium concentration, and the bound calcium may easily be displaced. Therefore, it is proposed that the role of proteoglycans in the calcification of cartilage is to function as a cation-exchanging calcium reservoir.

Pathophysiological function of the mitochondria as a calcium reservoir: Quantitative x-ray microanalysis

The aim of this study was to elucidate the role of the mitochondria as a store of calcium(Ca) under the condition of pathophysiological Ca overload induced by a rise in extracellular Ca concentration and the administration of isoproterenol.Eight rats were employed, and hearts were perfused as in the Langendorff method with Krebs-Henseleit solution gassed with 95% O2 and 5% CO2. Tow specimens were perfused with 2mM Ca for 30 min, and 2 were perfused with 5.5 mM Ca for 20 min. 4 specimens were perfused with 2 mM Ca for 5 min, and of these 4, 2 were infused with 10-7 mM/kg/min. isoproterenol for 5 min, and 2 were given a bolus injection of 3 x 10-7 mM isoproterenol. After rapid-cryofixation by the metal-mirror contact method with a Reichert-Jung KF80/MM80, and cryosectioning at -160 to -180° C with a Reichert-Jung Ultracut Fc-4E, ultrathin specimens (100nm) were free-ze-dreid for several hours at 10-5 Torr in the JEOL FD 7000, and mitochondrial Ca was determined by quantitative x-ray micranalysis (JEOL 1200EX, LINK AN 10000S).

Regulation of Calcium Concentrations in Synaptosomes: α2‐Adrenoceptors Reduce Free Ca2+ by Closure of N‐Type Ca2+ Channels

The relationship between intrasynaptosomal total (CaT) and free ([Ca2+]i) calcium and 45Ca accumulation was studied under physiological and K(+)-depolarised conditions in rat cortical synaptosomes. Under physiological conditions, CaT (10.7 mM) was approximately 10,000 times higher than [Ca2+]i (118 nM), showing that there is a large reservoir of sequestered calcium in synaptosomes. 45Ca accumulation was rapid (initial rate, 3.4 nmol/mg protein/min), substantial (7 nmol/mg protein in 2 min), and depolarisation dependent, and reached equilibrium after 5 min. At equilibrium, only 10% of CaT was freely exchangeable. This pool was much larger than the free Ca2+ pool. CaT, [Ca2+]i, and 45Ca accumulations were directly related to the Ca2+ concentration in the buffer, suggesting that [Ca2+]i is not highly conserved but is maintained by simple equilibria between the various pools. Clonidine reduced 45Ca accumulation in a time- and dose-dependent manner. Maximum inhibition (40% at 100 microM) occurred at 2 min and the IC50 was 80 nM. The reduction caused by clonidine (1 microM) reached equilibrium after 5 min, but this equilibrium value was lower than in controls, suggesting that clonidine changes the exchangeable Ca2+ pool size. The effects of clonidine (1 microM) on [Ca2+]i (26% reduction) and on 45Ca accumulation (24% reduction) were most apparent under physiological conditions. However, while it was not dependent on depolarisation, it did not occur in physiological buffer containing low K+ concentration (0.1-1 mM). The inhibitory effect of clonidine on 45Ca accumulation is receptor mediated as it was antagonised by idazoxan (1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

“Osteoporosis”: Etiologies, prevention, and treatment

With the steady increase in life expectancies occurring in industrialized countries have come major changes in the causes of morbidity and mortality. The greatest burden on the health care delivery systems in these countries has shifted from the treatment of infectious diseases to extended care of degenerative conditions. One such condition of increasing widespread concern is osteoporosis. Chronic bone loss leading to fractures and secondary infections has become a major problem in the United States, and has been estimated to cost $6 to $8 billion annually. The etiology of osteoporosis is complex, depending upon the interaction of nutritional, endocrinological, and local factors that are subject to a variety of genetic and environmental influences. The stringent requirement of maintaining calcium homeostasis places heavy demands on the primary reservoir of calcium, the skeleton, when intake and/or absorption are inadequate. Inactivity, depriving bone of its weight-bearing function, also initiates bone resorption. Under normal circumstances in the young adult, bone is maintained through continual, balanced turnover of mineral and calcium. When this balance is upset, loss of bone can be slow or rapid, reversible or irreversible. The trabecular compartment with its high sur-face-to-volume ratio is most rapidly mobilized, but cortical bone is ultimately resorbed as well. Involutional osteoporosis is conventionally designated as either type I (postmenopausal) or type II (senile). Women are affected by both and, because of a generally less-robust skeleton, experience risk of fracture earlier than men on the average. The hormonal and physiological factors involved in the regulation of normal bone physiology and the factors leading to loss of bone mass constitute an area of active research that is casting light on the fundamental mechanisms underling bone growth and maintenance in the search for new avenues of prevention and treatment of osteoporosis.

骨形成と骨吸収はどのように制御されているか

Bone is a highly specialized form of connective tissue, composed of bone forming osteoblasts and bone-resorbing osteoclasts, several types of matrix proteins and proteoglycans, and bone salts of calcium and phosphorus. It not only forms the skeleton or framework of the bodies, but serves as a calcium reservoir in most vertebrates. The newest knowledge on bone cell biology has allowed us to consider the mechanisms of bone formation and resorption and their coupling system. A great number of such systemic hormones as 1 α, 25-dihydroxyvitamin D3, parathyroid hormone and calcitonin and local factors including prostaglandins, interleukins, transforming growth factors, insulin-like growth factors and bone morphogenetic protein have been shown to be involved in the regulation of bone formation and resorption.In this review, we will summarize the newest knowledge on bone cell biology to understand the pathogenesis of several metabolic bone diseases including osteoporosis, rheumatoid arthritis and inflammatory periodontal diseases.

Calcium Homeostasis in Birds

Publisher Summary This chapter discusses the system of calcium homeostasis in birds. The importance of Ca 2+ in controlling a variety of metabolic functions in the body, ranging from muscle contraction to blood coagulation is reviewed. The Ca 2+ participates together with inositol phosphates in the mediation of hormone action in the target cell, including the regulation of secretion of several hormones. Although over 99% of body calcium is contained in the skeleton, the metabolic priority of the organism is the maintenance of a constant concentration of calcium in plasma and extracellular fluids, at the expense of bone calcification and in extreme conditions by the net breakdown of skeletal material. Thus, the relatively insoluble calcium phosphates of bone serve as a calcium reservoir to be utilized during need. This function of bone is particularly prominent in female birds during reproduction. The regulation of Ca 2+ concentration in body fluids is achieved through the action of a complex feedback control system which includes several subsystems and regulating hormones.

Cytosolic calcium oscillators.

Many cells display oscillations in intracellular calcium resulting from the periodic release of calcium from intracellular reservoirs. Frequencies are varied, but most oscillations have periods ranging from 5 to 60 s. For any given cell, frequency can vary depending on external conditions, particularly the concentration of natural stimuli or calcium. This cytosolic calcium oscillator is particularly sensitive to those stimuli (neurotransmitters, hormones, growth factors) that hydrolyze phosphoinositides to give diacylglycerol and inositol 1,4,5-trisphosphate (Ins1,4,5P3). The ability of Ins1,4,5P3 to mobilize intracellular calcium is a significant feature of many of the proposed models that are used to explain oscillatory activity. Receptor-controlled oscillator models propose that there are complex feedback mechanisms that generate oscillations in the level of Ins1,4,5P3. Second messenger-controlled oscillator models demonstrate that the oscillator is a component of the calcium reservoir, which is induced to release calcium by a constant input of either Ins1,4,5P3 or calcium itself. In the latter case, the process of calcium-induced calcium release might be the basis of oscillatory activity in many cell types. The function of calcium oscillations is still unknown. Because oscillator frequency can vary with agonist concentration, calcium transients might be part of a frequency-encoded signaling system. When an external stimulus arrives at the cell surface the information is translated into a train of calcium spikes, i.e., the signal is digitized. Certain cells may then convey information by varying the frequency of this digital signal.

Binding of calcium to glycosaminoglycans: An equilibrium dialysis study

Binding of calcium to the glycosaminoglycans (GAGs) heparin, chondroitin sulfate (CS), keratan sulfate (KS), and hyaluronic acid (HA) has been studied by equilibrium dialysis using exclusion of sulfate to correct for Gibbs-Donnan effects. Calcium binding occurs to all of these GAG species, suggesting that both sulfate and carboxylate groups are involved in cation binding. For all GAGs, the binding stoichiometry is consistent with a calcium-binding “site” consisting of two anionic groups. The order of calcium binding affinities is heparin > CS > KS > HA, and is critically dependent upon charge density; heparin binds calcium with 10-fold higher affinity than CS. The mode of calcium binding to GAGs is consistent with a recently proposed mechanism of growth plate calcification which states that cartilage proteoglycan functions as a reservoir of calcium for calcification of epiphyseal cartilage.

Ionomycin inhibits thyrotropin-releasing hormone-induced translocation of protein kinase C in GH4C1 pituitary cells.

Thyrotropin-releasing hormone (TRH) induces rapid and transient conversion of protein kinase C (Ca2+/phospholipid-dependent enzyme) from a soluble to a particulate-bound form in GH4C1 rat pituitary cells. Ionomycin (200 nM), a calcium ionophore, had no effect by itself on the subcellular distribution of protein kinase C. However, pretreatment of the cells with 200 nM ionomycin inhibited by greater than 50% the ability of TRH to cause translocation of protein kinase C from the cytosol to the particulate cell fraction. Inhibition by ionomycin required that the cells be incubated with the ionophore for at least 10 s before TRH addition. Ionomycin pretreatment did not alter the kinetics of TRH-induced protein kinase C redistribution. Incubation of the cells with 43 mM potassium prior to TRH addition almost completely reversed the inhibition induced by ionomycin. We propose that the mechanism by which ionomycin attenuates TRH action on protein kinase C may involve the capacity of the ionophore to empty the intracellular calcium reservoir which normally releases calcium into the cytosol in response to TRH. Our result provides evidence that the rise in intracellular calcium, which accompanies diacylglycerol formation following TRH action on polyphosphatidylinositide hydrolysis, may be required to achieve maximal conversion of protein kinase C to its presumed active, membrane-bound form in these cells.

Hormones and Calcium Regulation inFundulus heteroclitus

For the past 20 years, our laboratory has been involved in studying the endocrine control of calcium balance in the killifish, Fundulus heteroclitus . We have surveyed almost all endocrine systems and discovered two main ones directly involved in plasma calcium regulation. They are the pituitary gland and the corpuscles of Stannius. When fish adapted to low-calcium seawater were hypophysectomized, hypocalcemia and tetany were observed. When fish adapted to high-calcium seawater were Stanniectomized, hypercalcemia was seen. In both cases, other electrolytes were unaffected and replacement therapy corrected the plasma calcium changes. We have tried to characterize the active principles in both glands. We discovered that prolactin is hyperacalcemic. The pituitary gland also seems to contain a second hypercalcemic factor which may be located in the PAS-positive pars intermedia cells. For the Stannius corpuscle factors, we developed a bioassay and named the active substance(s) hypocalcin. In collaboration with Dr. Hirofumi Sokabe at Jichi Medical School, Japan, we showed that the Stannius corpuscles also contain a renin-like substance capable of generating a hypocalcemic angiotensin-like substance. The exact chemical nature of the hypocalcemic substance is being investigated. Calcium balance in the whole fish was also studied with 47-calcium. These studies were carried out in collaboration with Dr. Nicole Mayer-Gostan at Villefranche-sur-Mer, France. We discovered that killifish depend on the environment rather than bone as a calcium reservoir. Hormones may be involved in the exchanges with the environment.

CALCIUM BALANCE AND MOULTING IN THE CRUSTACEA

Summary1. Crustaceans have a high content of calcium, which is chiefly located in the skeleton as calcium carbonate. Calcium is generally the most abundant cation in the body.2. During intermoult, the exoskeleton is usually fully calcified and the animal is in calcium equilibrium with its environment.3. In the premoult stages calcium is resorbed from the skeleton and may be lost to the environment or stored within the body. Typically, losses are high and storage is small in aquatic species, whilst most terrestrial forms store much larger amounts of calcium and losses are reduced. Loss of calcium in soluble form by aquatic species must be by outward transport across the gills.4. Calcium is stored in a variety of different ways, usually with a common taxonomic theme. The main forms are as calcium phosphate granules in cells of the midgut gland (Brachyura), gastroliths (Astacidea and some Brachyura), the haemocoel (some Brachyura) the posterior midgut caeca (Amphipoda) and the ventral portion of the body generally in the Isopoda.5. At ecdysis, the skeleton is shed and the calcium remaining in it is lost from the body.6. Recalcification begins immediately, or shortly after, ecdysis using calcium mobilized from the stores. Simultaneously, or when the stores are exhausted, other sources of calcium are utilized. These are calcium in the water (aquatic species), the food (aquatic and terrestrial species) and the exuviae (chiefly terrestrial species).7. Marine species store little calcium and must obtain the bulk of their requirement (ca. 95%) from the water. Fresh‐water species also store little calcium but have, seemingly, adapted to the lower availability of calcium by increasing the affinity of the calcium‐absorbing mechanism. The rates of uptake of calcium are consequently similar in marine and fresh‐water species.8. A high degree of storage is essential for terrestrial crustaceans as they do not have access to a large aquatic reservoir of calcium. These large reserves enable the animals to reach an advanced stage of calcification, allowing the resumption of foraging and feeding necessary for completion of calcification.9. The control of calcium metabolism during the intermoult cycle is poorly under stood. β Ecdysone appears to control the resorption of calcium and the formation of calcium stores during premoult, but the mechanism of control of calcium metabolism during postmoult and intermoult is unknown.10. The concentration of calcium in the haemolymph of most species is high, but a large proportion of this is in non‐ionized form. In premoult, total calcium levels rise as a result of calcium resorption but little change occurs in the concentration of ionized calcium. Postmoult generally sees a fall in blood calcium, sometimes below the intermoult value.

Inhibitory effect on phenytoin on intracellular cyclic nucleotide and calcium changes during pentylenetetrazole-induced bursting activity in snail neurons

Effects of phenytoin (PHT) on the intracellular calcium reservoir, lysosome-like granules (LLG), and calcium-related intracellular events during pentylenetetrazole (PTZ)-induced bursting activity in the neurons of the Japanese land snail, Euhadra peliomphala, were examined. PTZ-induced morphological changes of LLG was inhibited by PHT. Calcium release from LLG was inhibited by PHT. PHT also inhibited the cyclic AMP increase by PTZ. PHT inhibited the increase in calcium-dependent protein kinase activity during PTZ-induced bursting activity. These findings suggest that PHT inhibits, as a first step, cyclic AMP increase which is one of the trigger factors of bursting activity as well as subsequent calcium-related intracellular pathological changes during seizure activity.

Distribution of calcium during interphase and mitosis as observed by ion microscopy.

The ion microscope, based on secondary ion mass spectrometry, has been used to demonstrate the distribution of calcium in the root tip cells of two plant species, Allium cepa and Vicia faba. Interphase nuclei showed higher intensities of calcium than cytoplasm, while nucleoli exhibited higher calcium intensities than the rest of the nucleoplasm. The chromosomes showed high intensities of calcium at all stages of mitosis. Calcium was also detected in the cell plate and phragmoplast region of dividing cells. It appears that during prophase calcium concentrates in the condensing chromosomes, and during telophase it is transferred to nucleoli. These observations suggest that chromosomes may serve as a reservoir of calcium during mitosis.

Intracellular Localization of Calcium in Pancreatic B Cells in Relation to Insulin Secretion by the Perfused ob/ob Mouse Pancreas*

Changes in the distribution of intracellular calcium in B-cells from the perfused ob/ob mouse pancreas in relation to the secretory state of the B-cell were followed using the pyroantimonate technique for calcium precipitation. At the internal surface of the B-cell membrane a highly mobile reservoir of calcium was mobilized during the first 1-3 min of stimulation of the B-cells by glucose as well as by tolbutamide resulting in an increased calcium precipitation in the cytoplasma. During prolonged stimulation of insulin secretion by glucose the calcium precipitation along the internal surface of the B-cell membrane was restored. Concomitantly calcium deposits in the secretory granules and to a lesser extend in the mitochondria increased. Tolbutamide neither restored the membrane-bound pool nor increased the calcium uptake of the secretory granules but decreased mitochondrial calcium. It suggests that this cytoplasmic increase of calcium primarily results from intracellular redistribution.

Release of Ca2+ from a nonmitochondrial intracellular store in pancreatic acinar cells by inositol-1,4,5-trisphosphate.

Activation of receptors for a wide variety of hormones and neurotransmitters leads to an increase in the intracellular level of calcium. Much of this calcium is released from intracellular stores but the link between surface receptors and this internal calcium reservoir is unknown. Hydrolysis of the phosphoinositides, which is another characteristic feature of these receptors, has been implicated in calcium mobilization. The primary lipid substrates for the receptor mechanism seem to be two polyphosphoinositides, phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2), which are rapidly hydrolysed following receptor activation in various cells and tissues. The action of phospholipase C on these polyphosphoinositides results in the rapid formation of the water-soluble products inositol 1,4-bisphosphate (Ins1,4P2) and inositol 1,4,5-trisphosphate (Ins1,4,5P3). In the insect salivary gland, where changes in Ins1,4P2 and Ins1,4,5P2 have been studied at early time periods, increases in these inositol phosphates are sufficiently rapid to suggest that they might mobilize internal calcium. We report here that micromolar concentrations of Ins1,4,5P3 release Ca2+ from a nonmitochondrial intracellular Ca2+ store in pancreatic acinar cells. Our results strongly suggest that this is the same Ca2+ store that is released by acetylcholine.

Calcium concretions in the gills of a freshwater mussel serve as a calcium reservoir during periods of hypoxia

AbstractThe gill of the freshwater mussel, Ligumia subrostrata, is composed of 25% calcium concretions on a dry weight basis. These concretions are located extracellularly in the connective tissue of the gill. The concretions have a lamellar structure and occur singly, multiply, and in clumps. The concretions are composed primarily of calcium phosphate and an organic matrix. The concretions are more numerous toward the base of the gill and less frequent in the tip of the gill. The average size of the concretions is 2–3 μm. The concretions are not solubilized in animals exposed to prolonged hypoxic conditions. Instead, as calcium increases in the blood under hypoxic conditions the percentage of the gill dry weight attributed to concretion material increases and the calcium content of the concretion material is elevated. The calcium content of the concretions also is inversely correlated with blood pH. This study indicated that the concretion material in the gills was not mobilized under hypoxic conditions, but served as a calcium reservoir during periods in which the animal was liberating calcium from other sites.

A cytochemical study of the effect of cholinergic and beta-adrenergic stimulation on calcium fluxes of rat parotid gland.

Parotid gland tissue from carbachol-treated and isoproterenol-treated rats was studied cytochemically by pyroantimonate precipitation and x-ray microanalysis in an effort to identify any intracellular calcium reservoirs that might be available for use in a stimulus-secretion coupling mechanism, and to determine any differences that might exist in terms of calcium utilization due to the cholinergic or beta-adrenergic receptor mechanisms. Stimulation by either secretagogue results in a reduction of mitochondrial and plasma membrane calcium, but the reduction in mitochondrial calcium deposits of carbachol-treated animals is only one-half that of the beta-adrenergic-treated animals. This could possibly suggest a greater dependency on mitochondrial calcium for beta-adrenergic stimulated animals.

Inhibition of fast axonal transport in vitro by tetracaine: an increase in potency at alkaline pH, and no change in potency in calcium-depleted nerves.

Some of the present in vitro experiments compare the degree of inhibition of fast axonal transport produced by tetracaine at neutral and at alkaline pH. In desheathed spinal nerves from bullfrog, 0.5 mM tetracaine reduced the quantity of [3H]leucine-labeled proteins which were transported to a ligature by 43% at pH 7.2 and by 96% at pH 8.2; separate experiments established that transport was not affected by the pH change in the absence of tetracaine. The relationship between pH and transport-blocking potency of tetracaine (pKa 8.2) is such that the local anesthetic is more potent when more uncharged form of the molecule is present; this may reflect the easier penetration across the axonal plasma membrane by the uncharged form of the tetracaine molecule. The axonal smooth endoplasmic reticulum has been attributed the function of a calcium reservoir, and it appeared possible that local anesthetics could block axonal transport by releasing calcium from this structure. However, the inhibition of transport produced by 1 mM tetracaine (pH 7.1) in sheathed nerves was approximately 80% both in nerves with a lower than normal calcium content (47% of normal) and in nerves with a normal calcium content; this result does not support the hypothesis that inhibition of axonal transport by local anesthetics is mediated by an increase in intracellular free Ca2+, but does not rule out the hypothesis either.