Mitochondria and the control of intracellular calcium

Additive aluminum as a cause of induced immunoexcitoxicity resulting in neurodevelopmental and neurodegenerative disorders: A biochemical, pathophysiological, and pharmacological analysis

Much has been learned about the neurotoxicity of aluminum over the past several decades in terms of its ability to disrupt cellular function, result in slow accumulation, and the difficulty of its removal from cells. Newer evidence suggests a central pathophysiological mechanism may be responsible for much of the toxicity of aluminum and aluminofluoride compounds on the brain and spinal cord. This mechanism involves activation of the brain's innate immune system, primarily the microglia, astrocytes, and macrophages, with a release of neurotoxic concentrations of excitotoxins and proinflammatory cytokines, chemokines, and immune mediators. Many studies suggest that excitotoxicity plays a significant role in the neurotoxic action of several metals, including aluminum. Recently, researchers have found that while most of the chronic pathology involved in the observed neurodegenerative effects of these metals are secondary to prolonged inflammation, it is the enhancement of excitotoxicity by the immune mediators that are responsible for most of the metal's toxicity. This enhancement occurs through a crosstalk between cytokines and glutamate-related mechanisms. The author coined the name immunoexcitotoxicity to describe this process. This paper reviews the evidence linking immunoexcitotoxicity to aluminum's neurotoxic effects and that a slow accumulation of aluminum may be the cause of neurodevelopmental defects as well as neurodegeneration in the adult.

Cellular pathways of calcium transport and concentration toward mineral formation in sea urchin larvae

Sea urchin larvae have an endoskeleton consisting of two calcitic spicules. The primary mesenchyme cells (PMCs) are the cells that are responsible for spicule formation. PMCs endocytose sea water from the larval internal body cavity into a network of vacuoles and vesicles, where calcium ions are concentrated until they precipitate in the form of amorphous calcium carbonate (ACC). The mineral is subsequently transferred to the syncytium, where the spicule forms. Using cryo-soft X-ray microscopy we imaged intracellular calcium-containing particles in the PMCs and acquired Ca-L2,3 X-ray absorption near-edge spectra of these Ca-rich particles. Using the prepeak/main peak (L2'/ L2) intensity ratio, which reflects the atomic order in the first Ca coordination shell, we determined the state of the calcium ions in each particle. The concentration of Ca in each of the particles was also determined by the integrated area in the main Ca absorption peak. We observed about 700 Ca-rich particles with order parameters, L2'/ L2, ranging from solution to hydrated and anhydrous ACC, and with concentrations ranging between 1 and 15 M. We conclude that in each cell the calcium ions exist in a continuum of states. This implies that most, but not all, water is expelled from the particles. This cellular process of calcium concentration may represent a widespread pathway in mineralizing organisms.

Mitochondria dysfunction: A novel therapeutic target in pathological lung remodeling or bystander?

The renascence in mitochondrial research has fueled breakthroughs in our understanding of mitochondrial biology identifying major roles in biological processes ranging from cellular oxygen sensing and regulation of intracellular calcium levels through to initiation of apoptosis or a shift in cell phenotype. Chronic respiratory diseases are no exception to the resurgent interest in mitochondrial biology. Microscopic observations of lungs from patients with chronic respiratory diseases such as pulmonary arterial hypertension, asthma and COPD show accumulation of dysmorphic mitochondria and provide the first evidence of mitochondrial dysfunction in diseased lungs. Recent mechanistic insights have established links between mitochondrial dysfunction or aberrant biogenesis and the pathogenesis of chronic respiratory diseases through playing a causative role in structural remodeling of the lung. The aim here is to discuss the case for a mitochondrial basis of lung remodeling in patients with chronic respiratory diseases. The present article will focus on the question of whether currently available data supports mitochondrial mechanisms as a viable point of therapeutic intervention in respiratory diseases and suggestions for future avenues of research in this rapidly evolving field.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Ca2+ signaling differentiation during oocyte maturation

Oocyte maturation is an essential cellular differentiation pathway that prepares the egg for activation at fertilization leading to the initiation of embryogenesis. An integral attribute of oocyte maturation is the remodeling of Ca2+ signaling pathways endowing the egg with the capacity to produce a specialized Ca2+ transient at fertilization that is necessary and sufficient for egg activation. Consequently, mechanistic elucidation of Ca2+ signaling differentiation during oocyte maturation is fundamental to our understanding of egg activation, and offers a glimpse into Ca2+ signaling regulation during the cell cycle.

Mitochondrial modulation of intracellular Ca(2+) signaling

IP3-mediated Ca(2+) release plays a fundamental role in many cell signaling processes and has been the subject of numerous modeling studies. Only recently has the important role that mitochondria play in the dynamics of intracellular Ca(2+) signaling begun to be considered in experimental work and in computational models. Mitochondria sequester large amounts of Ca(2+) and thus have a modulatory effect on intracellular Ca(2+) signaling, and mitochondrial uptake of Ca(2+), in turn, has a regulatory effect on mitochondrial function. Here we integrate a well-established model of IP3-mediated Ca(2+) signaling with a detailed model of mitochondrial Ca(2+) handling and metabolic function. The incorporation of mitochondria results in oscillations in a bistable formulation of the IP3 model, and increasing metabolic substrate decreases the frequency of these oscillations consistent with the literature. Ca(2+) spikes from the cytosol are communicated into mitochondria and are shown to induce realistic metabolic changes. The model has been formulated using a modular approach that is easy to modify and should serve as a useful basis for the investigation of questions regarding the interaction of these two systems.

Calcium handling by the cat carotid body--a pyroantimonate study

Subcellular regulation mechanisms of calcium concentrations related to oxygen sensing in the carotid body are unclear. In the present study, we investigated the ultrastructural distribution patterns of calcium in carotid body cells and its changes evoked by hypoxia. Carotid bodies were dissected from anesthetized cats exposed in vivo to normoxic or acute hypoxic conditions. We used the oxalate-pyroantimonate technique that yields an electron-opaque calcium precipitate. X-ray microanalysis and appropriate controls confirmed the presence of calcium in the precipitate. Calcium precipitates were found in all types of cells in carotid body parenchyma: chemoreceptor cells, sustentacular cells, and nerve endings. In normoxic chemoreceptor cells, the precipitate was localized in dense core vesicles, mitochondria, and nuclei, but rarely in the cytoplasm. The most apparent effect of hypoxia was disappearance of the precipitate from dense core vesicles and was associated with its appearance in the cytoplasm. The amount of precipitate throughout the carotid body parenchyma was decreased overall due to hypoxia. These results indicate the involvement of subcellular calcium trafficking in hypoxia-sensing in the carotid body. The redistribution pattern of granular calcium deposits from organelles to the cytoplasm of chemoreceptor cells agrees with biochemical data of calcium release from intracellular stores during hypoxia.

Effect of 2',3'-dideoxycytidine on oxidative phosphorylation in the PC12 cell, a neuronal model

Peripheral neuropathy induced by 2',3'-dideoxycytidine (ddC) could result from the previously shown inhibition of mtDNA replication by the action of ddC on the mitochondrial enzyme DNA polymerase gamma. Such inhibition would be expected to impair oxidative phosphorylation, and this was demonstrated in the present study for the PC12 cell, a model of a peripheral neuron. The dramatic rise in lactate formation upon exposure of the cell to ddC indicated that increased glycolysis was needed to produce ATP. A concomitant rise in O2 uptake indicated that oxidative phosphorylation had become uncoupled. When tested in a standard respiratory control system (isolated rat liver mitochondria), however, we found ddC not to be an uncoupler. Rather, the uncoupling most likely resulted from the failure of synthesis of one or more mitochondrial gene products necessary for oxidative phosphorylation. We also observed an important distinction between the manner in which ddC and 3'-azido-3'-deoxythymidine (AZT) act. ddC-exerted inhibition of oxidative phosphorylation was delayed for several days. This is consistent with the inhibition occurring indirectly, most likely as a result of the prior destruction of the mitochondrial genome, which encodes many of the components of the oxidative phosphorylation system. In contrast, we have shown previously that although AZT also impairs replication of the mitochondrial genome (in the Friend murine erythroleukemic cell), it also attacks directly an additional primary target leading to impairment of oxidative phosphorylation; its initial inhibition of this process is immediate, not occurring via inhibition of mitochondrial DNA replication.

Rapid Ca2+ influx induced by the action of dibutylhydroquinone and glucagon in the perfused rat liver

Glucagon induces a slight Ca2+ efflux when administered to the perfused rat liver. However, the hormone promotes rapid and significant Ca2+ influx after the prior administration of 2, 5-di(t-butyl)-1,4-hydroquinone (BHQ), an agent that promotes Ca2+ release from the endoplasmic reticulum (ER). The concentrations of glucagon that promote Ca2+ influx are similar to those that promote glycogenolysis and gluconeogenesis in isolated hepatocytes. The permeable analogue of cAMP, but not that of cGMP, is able to duplicate the Ca2+-mobilizing effects of glucagon. The influx of Ca2+ into liver is blocked by Ni2+. Administration of sodium azide, an inhibitor of mitochondrial electron transport, also blocks the BHQ plus glucagon-induced Ca2+ influx and this is reversed when azide administration is terminated. The actions of azide are evident within 60 s after administration or withdrawal, and also occur when either oligomycin or fructose is co-administered; this provides evidence for an effect of azide independent of cellular ATP depletion. Measurement of total calcium in mitochondria that were isolated rapidly from perfused livers after the combined administration of glucagon and BHQ confirmed that large quantities of extracellular Ca2+ had entered these organelles. These experiments provide evidence that in the perfused rat liver the artificial emptying of the ER Ca2+ pool allows glucagon to promote rapid and sustained Ca2+ influx that seems to terminate in mitochondria.

Uncoupling effects of diclofenac and aspirin in the perfused liver and isolated hepatic mitochondria of rat

Gluconeogenesis, glycolysis and glycogenolysis were studied in rat perfused liver following the infusion of various concentrations of diclofenac and aspirin, two non-steroidal anti-inflammatory drugs (NSAIDs). Glucose synthesis was measured in livers isolated from 48-h fasted rats perfused with Krebs-Henseleit bicarbonate buffer containing L-lactate (2 mM) and pyruvate (0.1 mM) as precursors. Both diclofenac (0.01-0.1 mM) and aspirin (1-10 mM) had an inhibitory effect on gluconeogenesis (GNG). The inhibition was dose-dependent and reversible. For the estimation of glycogenolysis and glycolysis, the rates of glucose release and of lactate and pyruvate production were measured in livers of well-fed rats perfused with substrate-free buffer. Infusion of diclofenac (0.1 mM) or aspirin (5 mM) strongly stimulated glycogenolysis and glycolysis (GGL/GL). In general, an increased oxygen consumption by the liver tissue was also noted in both types of experiments, as deduced from the continuous monitoring of oxygen concentration changes in the effluent. Such a pattern of response can be attributed to the uncoupling effects of the two drugs on oxidative phosphorylation. Measurements of respiration rates and membrane potential in isolated liver mitochondria submitted to various concentrations of diclofenac and aspirin confirms this assumption. Thus, 0.01 to 0.2 mM diclofenac stimulates state-4 respiration and slightly inhibits state 3, decreasing the respiratory control ratio, while the membrane potential is decreased or collapsed (depending on the drug concentration). Similar effects are recorded for aspirin at higher concentrations (0.2-5 mM), even though state 3 is not affected in this case. Arguments are presented that the concentrations of the drugs used largely correspond to the pharmacological doses employed in antipyretic and anti-inflammatory treatments. Therefore, a greater consideration should be given to the uncoupling effect, at least from the toxicological viewpoint.

Dynamic properties of an inositol 1,4,5-trisphosphate- and thapsigargin-insensitive calcium pool in mammalian cell lines

The functional characteristics of a nonacidic, inositol 1,4,5-trisphosphate- and thapsigargin-insensitive Ca2+ pool have been characterized in mammalian cells derived from the rat pituitary gland (GH3, GC, and GH3B6), the adrenal tissue (PC12), and mast cells (RBL-1). This Ca2+ pool is released into the cytoplasm by the Ca2+ ionophores ionomycin or A23187 after the discharge of the inositol 1,4,5-trisphosphate-sensitive store with an agonist coupled to phospholipase C activation and/or thapsigargin. The amount of Ca2+ trapped within this pool increased significantly after a prolonged elevation of intracellular Ca2+ concentration elicited by activation of Ca2+ influx. This pool was affected neither by caffeine-ryanodine nor by mitochondrial uncouplers. Probing mitochondrial Ca2+ with recombinant aequorin confirmed that this pool did not coincide with mitochondria, whereas its homogeneous distribution across the cytosol, as revealed by confocal microscopy, and its insensitivity to brefeldin A make localization within the Golgi complex unlikely. A proton gradient as the driving mechanism for Ca2+ uptake was excluded since ionomycin is inefficient in releasing Ca2+ from acidic pools and Ca2+ accumulation/release in/from this store was unaffected by monensin or NH4Cl, drugs known to collapse organelle acidic pH gradients. Ca2+ sequestration inside this pool, thus, may occur through a low-affinity, high-capacity Ca2+-ATPase system, which is, however, distinct from classical endosarcoplasmic reticulum Ca2+-ATPases. The cytological nature and functional role of this Ca2+ storage compartment are discussed.

Antigen and thapsigargin promote influx of Ca2+ in rat basophilic RBL-2H3 cells by ostensibly similar mechanisms that allow filling of inositol 1,4,5-trisphosphate-sensitive and mitochondrial Ca2+ stores

In single, Fura 2-loaded RBL-2H3 cells, antigen and thapsigargin depleted the same intracellular pool of Ca2+ in the absence of external Ca2+; provision of external Ca2+ induced immediate increases in levels of free Ca2+ ([Ca2+]i). These increases were dependent on the presence of external Ca2+ and, presumably, on influx of Ca2+ across the cell membrane. Both stimulants enhanced intracellular accumulation of 45Ca2+ through ostensibly similar mechanisms because accumulation was blocked to similar extents by various multivalent cations or by depolarization with K+. Because thapsigargin blocked reuptake of Ca2+ into inositol 1,4,5-trisphosphate sensitive stores, uptake occurred independently of the refilling of these stores. Uptake was dependent instead on sequestration of 45Ca2+ in a pool of high capacity that was insensitive to thapsigargin, caffeine, GTP and inositol 1,4,5-trisphosphate but sensitive to ionomycin and mitochondrial inhibitors. The existence of an inositol 1,4,5-trisphosphate-insensitive pool was also apparent in permeabilized cells; at 0.1 microM [Ca2+]i, uptake of 45Ca2+ was largely confined (> 80%) to the inositol 1,4,5-trisphosphate-sensitive pool, but at 2 microM [Ca2+]i uptake was largely (> 60%) into the inositol 1,4,5-trisphosphate-insensitive pool. Provision of mitochondrial inhibitors along with thapsigargin to block uptake into both pools, did not impair the thapsigargin-induced increase in [Ca2+]i or influx of Ca2+, as indicated by changes in Fura 2 fluorescence, but did block the intracellular accumulation of 45Ca2+. The studies illustrate the utility of simultaneous measurements of [Ca2+]i and 45Ca2+ uptake for a full accounting of Ca2+ homoeostasis as exemplified by the ability to distinguish between influx and mitochondrial uptake of Ca2+.

The Ca(2+)-induced permeability transition pore is involved in Ca(2+)-induced mitochondrial oscillations. A study on permeabilised Ehrlich ascites tumour cells

The Ca(2+)-induced permeability transition of the mitochondrial inner membrane was studied in digitonin-permeabilized Ehrlich ascites tumour cells respiring on succinate in an isotonic medium. Addition of a sufficient amount of Ca2+ to induce an efflux of accumulated Ca2+ from mitochondria produced an oscillatory state with periodically changing rates of respiration, transmembrane potential, delta pH and direction of Ca2+ fluxes. This contrasts with liver mitochondria in which only a Ca2+ efflux is induced under these conditions. Addition of traces of cyclosporin A (approximately 0.1 nM) damped the oscillations by inhibiting the phase in which Ca2+ efflux occurs and promoting the reestablishment of a higher transmembrane potential. Efflux was also prevented by addition of ATP or ADP, ATP being more potent. Efflux was also inhibited by low concentrations of spermine. It is concluded that Ca(2+)-induced oscillations involve the cyclosporin A-sensitive pore and that the Ehrlich ascites tumour cell mitochondria differ from liver mitochondria in being far more sensitive to cyclosporin A and ATP. The possible physiological role of the oscillatory state is discussed.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

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Nuclear magnetic resonance characterization of secondary mechanisms following traumatic brain injury

Much of the injury that occurs following a traumatic insult to the central nervous system is the result of physiological and biochemical processes initiated by the primary traumatic event. These processes occur over a period of hours to days following the insult, and although a number of factors have been identified as being associated with this secondary injury process, their role and interrelationship with one another is unclear. Nuclear magnetic resonance spectroscopy has been utilized to characterize many of these secondary factors and their relationship to eventual neurological outcome. In particular, the role of high energy phosphates, pH, lactic acid, excitatory amino acids, and magnesium has been investigated, along with pharmacotherapies directed toward altering the status of these factors following traumatic injury. This review critically examines the role that each of these factors may play in the secondary injury process, and proposes a scheme which theoretically accounts for the interrelationships among the various factors.

Energization-dependent Ca2+ accumulation in Trypanosoma brucei bloodstream and procyclic trypomastigotes mitochondria

The permeabilization of Trypanosoma brucei procyclic and bloodstream trypomastigotes with digitonin permitted the quantitative estimation of a mitochondrial membrane potential of the order of 130-140 mV, in both forms, using safranine O. Dependence on substrate oxidation and response of the procyclic mitochondrial membrane potential to phosphate, FCCP, valinomycin, and Ca2+ indicate that these mitochondria behave similarly to vertebrate mitochondria regarding the properties of their electrochemical proton gradient. In contrast, in bloodstream mitochondria, development of a membrane potential was independent of substrate oxidation and dependent on hydrolysis of ATP by the mitochondrial oligomycin-sensitive ATPase, as demonstrated by collapse of the membrane potential by oligomycin and its insensitivity to the respiratory chain-inhibitor antimycin A. Mitochondria of T. brucei bloodstream forms were also able to take up Ca2+ by an electrophoretic mechanism. This is the first report of the presence of a Ca2+ transport mechanism in an eukaryotic cell devoid of complete tricarboxylic acid cycle and respiratory chain, the activities of which are known to be regulated by changes in intramitochondrial calcium concentration in other cells.

Calcium homeostasis in Trypanosoma cruzi amastigotes: presence of inositol phosphates and lack of an inositol 1,4,5-trisphosphate-sensitive calcium pool

The permeabilization of Trypanosoma cruzi amastigotes with digitonin allowed the study of Ca2+ fluxes between intracellular organelles in situ. In addition, fura-2 was used to determine the cytosolic Ca2+ concentration in the intact cells. When amastigotes were permeabilized in a reaction medium containing MgATP, succinate and 3.5 microM Ca2+, they lowered the medium Ca2+ concentration to the submicromolar level, a range which correlates favorably with that detected in the intact cells with fura-2. The presence of 1 microM FCCP strongly decreased the initial rate of Ca2+ sequestration by these permeabilized cells. This FCCP-insensitive Ca2+ uptake, probably represented by the endoplasmic reticulum, was completely inhibited by 500 microM vanadate. On the other hand, when vanadate instead of FCCP was present, the initial rate of Ca2+ accumulation was decreased and the Ca2+ set point was increased to about 0.8 microM. The succinate dependence and FCCP sensitivity of the later Ca2+ uptake indicate that it may be exerted by the mitochondria. Despite the presence of inositol phosphates, as determined by [3H]inositol incorporation, and of a large extramitochondrial Ca2+ pool, no IP3-sensitive or thapsigargin-sensitive Ca2+ release could be detected in either amastigotes or epimastigotes.

Structural requirements of lysophospholipid-regulated mitochondrial Ca2+ transport

Analogues of lysophosphatidylcholine, including PAF (platelet-activating-factor) and HePC (an experimental anticancer drug), were studied for their influence on mitochondrial Ca2+ transport and membrane potential. Lysophospholipids released Ca2+ from mitochondria and reduced the maximal Ca2+ uptake. The structure-activity relations indicate that deprotonated head groups like phosphocholines yield active compounds while partially protonated head groups like phosphoethanolamines are essentially inactive. Structural requirements for the apolar part of the molecules were acyl or alkyl chain lengths of less than 18 carbon atoms at the C1-position of the glycerol backbone and residues of small size and/or low polarity at the C2-position. Choline lysophospholipids, but not ethanolamine lysophospholipids, may therefore induce mitochondrial Ca2+ efflux and become mediators of ischaemic tissue damage where dysregulated phospholipase A2 activity and an impairment of mitochondrial function are supposed to play a crucial role.

MgATP-dependent accumulation of calcium ions and inorganic phosphate in a liver reticular pool

1. MgATP-dependent Ca2+ uptake by rat liver microsomal preparations and permeabilized hepatocytes was measured in the presence or absence of Pi. 2. Monitoring of free Ca2+ in incubation systems with a Ca2+ electrode in the presence of Pi (2-7 mM) revealed a biphasic Ca2+ uptake, with the onset of a second, Pi-dependent, Ca2+ accumulation. 3. Increasing Pi concentrations (up to 10 mM) caused a progressive enlargement of 45Ca2(+)-loading capacity of microsomal fractions. 4. As a result of Pi stimulation of active Ca2+ uptake, [32P]Pi and 45Ca2+ were co-accumulated. 5. Experiments with permeabilized hepatocytes revealed that the amount of Ca2+ releasable by myo-inositol 1,4,5-trisphosphate is unaffected by Pi.

Ca2+ homeostasis alterations induced by 2,4-dichlorophenoxyacetic butyl ester and 2,4-dichlorophenoxyacetic acid on avian skeletal muscle

Fertilized hen eggs were treated externally with 2,4-dichlorophenoxyacetic butyl ester (2,4-D b.e.) (3.1 mg/egg) before the start of the incubation. Actomyosin and sarcoplasmic reticulum adenosine triphosphatase (ATPase) activities from leg and complexus muscles of chicks hatched from treated eggs were measured. No significant variations were detected in the ATPase activities of actomyosin, but the sarcoplasmic reticulum Mg2(+)-activated ATPase and Ca2+, Mg2(+)-activated ATPase were inhibited 50 and 38% respectively. 45Ca2+ uptake into soleus muscle was increased by the 2,4-D b.e. treatment. The compartmental analysis of 45Ca2+ uptake kinetics showed increases in Ca2+ fluxes in sarcolemma and mitochondria and in the mitochondrial calcium pool. Isolated soleus muscles were treated with 2,4-dichlorophenoxyacetic acid (2,4-D) or 2,4-D b.e. [14C]2,4-D reached it highest level in these muscles after 1 hr of treatment. The in vitro treatment with 2,4-D or 2,4-D b.e. increased 45Ca2+ uptake into the muscles. 2,4-D b.e. produced greater alterations than 2,4-D. The compartmental analysis of the 45Ca2+ uptake kinetics also showed increases of the mitochondrial Ca2+ pool and Ca2+ fluxes through sarcolemma and mitochondria. These results led to a hypothesis based on Ca2+ permeability alterations for explaining the myopathic actions of these phenoxyherbicides.

Phosphate and calcium uptake by mitochondria and by perfused rat liver induced by the synergistic action of glucagon and vasopressin

Co-administration of glucagon and vasopressin to rat liver perfused with buffer containing 1.3 mM-Ca2+ induces a 4-fold increase in Pi in the subsequently isolated mitochondria (from approx. 9 to approx. 40 nmol/mg of mitochondrial protein). This increase is not attributable to PPi hydrolysis, and is not observed if the perfusate Ca2+ is lowered from 1.3 mM to 50 microM. The increase in mitochondrial Pi closely parallels that of mitochondrial Ca2+; when the increase in Pi and Ca2+ accumulation is maximal, the molar ratio is close to that in Ca3(PO4)2. Measurement of changes in the perfusate Pi revealed that, whereas administration of glucagon or vasopressin alone brought about a rapid decline in perfusate Pi, the largest decrease (reflecting net retention of Pi by the liver) was observed when the hormone was co-administered in the presence of 1.3 mM-Ca2+. The synergistic action of glucagon plus vasopressin was nullified by lowering the perfusate Ca2+ to 50 microM. The data provide evidence that, whereas glucagon may be able to alter Pi fluxes directly in intact liver, any alterations induced by vasopressin are indirect and result only from its action of mobilizing Ca2+.

Regulation of Ca2+ transport in brain mitochondria. II. The mechanism of the adenine nucleotides enhancement of Ca2+ uptake and retention

ADP greatly enhances the rate of Ca2+ uptake and retention in Ca2+ loaded mitochondria. Atractyloside, a specific inhibitor of the ADP/ATP translocator, completely inhibits the ADP effect, while bongkrekate, another specific inhibitor of the translocator enhances the effect of ADP. These results indicate that locking the ADP/ATP translocator in the M-state is sufficient to produce the ADP effect. Cyclosporin A, a specific inhibitor of the Ca2(+)-induced membrane permeabilization does not substitute for ADP, indicating that ADP directly affect the rate of electrogenic Ca2+ uptake. The effect of the translocator conformation on the rate of electrogenic Ca2+ uptake is independent of the concentration of Pi and is not caused by changes in membrane potential. However, locking the carrier in the M-state appears to increase the negative surface charge on the matrix face of the inner membrane. This may lead to an enhanced rate of Ca2+ dissociation from the electrogenic carrier at the matrix surface. The rate of Na(+)-independent Ca2+ efflux is only slightly inhibited by locking the carrier in the M-state, presumably due to the same mechanism. In the presence of ADP, Pi inhibits the Na(+)-independent efflux. In the presence of physiological concentrations of spermine, Pi and Mg2+, the rate of Ca2+ uptake, Ca2+ retention and Ca2+ set points depend sharply on ADP concentration at the physiological range of ADP. Thus, changes of cytosolic ADP concentration may lead to change in the rate of Ca2+ uptake by mitochondria and thus modulate the excitation-relaxation cycles of cytoplasmic free calcium.

The effect of the hydrogen ionophore closantel upon the pharmacology and ultrastructure of the adult liver fluke Fasciola hepatica

The present study describes the effects of the H+ ionophore and anthelmintic closantel upon the in vitro motility and in vivo ultrastructure of the liver fluke, Fasciola hepatica. At a concentration of 50 micrograms/ml, closantel caused an initial stimulation, then suppression of activity, which was accompanied by an increase in muscle tone and led to a spastic paralysis within 2 h. The pattern of response was similar at lower concentrations, although the initial stimulation was not always evident, but the onset of paralysis could be reached more quickly. Scanning electron microscopy revealed gross surface damage from 24 h onwards in vivo, in the form of erosion of the anterior and posterior extremities of the fluke and large-scale sloughing of the tegument on both dorsal and ventral surfaces. Tegumental changes prior to sloughing included some swelling of the basal infolds and an apical accumulation of T1 secretory bodies. In the underlying tegumental cells there was reduced secretory activity and the mitochondria were consistently swollen and deformed. Reduced secretory activity was a feature of the gastrodermal cells as well; these cells were characterized by swollen, electron-lucent mitochondria, vesiculated GER cisternae and apical blebbing of packets of cytoplasm. The vitelline follicles became severely disrupted as a result of the breakdown of the nurse cell cytoplasm. The stem and intermediate type 1 (It1) cells rounded up and showed nuclear abnormalities. There did not appear to be a severe disruption of shell protein production in the intermediate vitelline cells, but there was a noticeable absence of glycogen in the mature vitelline cells. The effects of closantel are discussed in relation to its proposed mode of action as an uncoupler of oxidative phosphorylation.

Development of tolerance in brain mitochondria for calcium uptake following chronic ethanol ingestion

Brain mitochondria isolated from rats following 10 weeks of chronic exposure to ethanol were not deficient in respiratory function or in rates of calcium uptake under control conditions. Ethanol (80 mM) in the incubation medium caused significant depression in the respiratory and ATP-dependent rates of calcium uptake in control mitochondria, but did not affect mitochondria from ethanol-tolerant rats. Chronic exposure to ethanol causes mitochondria to take calcium up at a normal rate when challenged acutely by ethanol.

Electron microscopy of Microsporum cookei after 'in vitro' treatment with protoanemonin: a combined SEM and TEM study

The ranunculaceous derivative protoanemonin (PrA) was studied as an antifungal agent on the dermatophyte Microsporum cookei. The ultrastructural changes that PrA brought about in this fungus were observed with both the transmission and scanning electron microscopes. The main anomalies noted were abnormally shaped hyphae and within the cytoplasm, multimembranous bodies which were irregular in shape and size, and tubules of 25 and 60 nm in diameters. Mitochondria, nuclei and vacuoles were also variously affected by PrA. Although multifarious, the observed cellular alterations in M. cookei can be considered the result of a PrA interaction with cytoplasmic microtubules. Since these cell structures contain a great number of ASH groups, our previous hypothesis, that sulphydryl groups are the primary targets of this molecule, appears to be supported.

Spermine, another specific allosteric activator of calcium uptake in rat liver mitochondria

Calcium uptake in rat liver mitochondria is accelerated by spermine. At a concentration of 2 microM Ca2+ and 1 mM Mg2+ a maximal, 10-fold activation by 1.2 mM spermidine was obtained; a half-maximal activation was attained with 0.2 mM spermine. Spermidine was far less effective than spermine whereas putrescine was ineffective. The acceleration of Ca uptake at low, physiological Ca2+ concentrations is related to the altered kinetics of the Ca uniporter. Corresponding to the alteration by high Ca2+ concentrations previously described, the kinetics changed from sigmoidal in the absence to nearly hyperbolic in the presence of spermine. Mg2+ behaves as an allosteric inhibitor. This phenomenon of the allosteric activation of Ca uptake could not be observed in heart mitochondria.

Post-stimulatory effects of direct current stimulation of the cochlea on auditory nerve activity

Glass micro-electrode recordings from the spiral ganglion of the basal turn of the guinea pig cochlea have been obtained before, during and after negative (cathodic) current injection into scala tympani. Electrical stimulation with currents between 100 microA and 900 microA produced a marked increase in firing rate of the afferent neurons for the first 3 min of electrical stimulation. This was followed by a fall in firing rate to rates near or below the pre-stimulatory spontaneous rate if stimulation continued. Continuous electrical stimulation lasting 5 or 10 min reduced neural sensitivity to acoustic stimulation. Although threshold elevation was greatest for sound frequencies near the characteristic frequency of each neuron, thresholds could also be elevated at lower frequencies on the tail of the frequency-threshold tuning curve. After electrical stimulation a fall in the amplitude of the low-frequency microphonic recorded at the round window was also observed, indicating a disruption of the outer hair cell transduction. These effects were highly localized in the basal turn near the site of current injection, and were not associated with any significant structural changes in the organ of Corti, except after stimulation with very high current intensities.

Effect of paraquat on lung calcium transport

The effect of paraquat on lung subcellular calcium transport was studied in male Sprague-Dawley rats. With a high dose of 30 mg/kg given i.p., a significant decrease in the lung mitochondrial and microsomal calcium uptake at 24, 48, and 72 h after paraquat was observed. Trend of recovery was evident at 96 h in the surviving rats. The lung water content and thymidine incorporation into the lung DNA increased significantly after 72 h and 48 h of paraquat administration, respectively. Groups of animals also received a lower dose of paraquat at 10 mg/kg i.p. and were sacrificed at 1, 2, 3, 4, 6, and 8 days. Continuous decrease in the mitochondrial and the microsomal calcium uptake until 4 days followed by a trend of recovery were observed. Thymidine incorporation into the lung DNA increased significantly at all the time periods after day 3 but no significant increases were observed in the lung water content at this dose. The results indicate that paraquat significantly disrupts the lung intracellular calcium uptake even at the doses which do not show any significant increase in the lung water content.

Effects of cyclic nucleotides and calcium on transduction and encoding processes in frog muscle spindle

In decapsulated muscle spindles, application of dibutyryl cyclic AMP (d-cAMP, 0.1-10 mM) and forskolin (10-100 microM) increased the rate of spontaneous discharges and decreased the responsiveness to stretch. Addition of 2-5 mM CaCl2 or 0.5-2 mM BaCl2 to the above drugs prevented the deterioration of the responsiveness to stretch and the increase in the rate of spontaneous discharges. Similar changes in the afferent discharges were observed with application of isobutylmethyl-xanthine (IBMX, 0.5-2 mM) or carbonyl cyanide chlorophenylhydrazone (CCCP, 1-10 microM). Application of 0.1-1 mM quercetin resulted in a prolonged increase in the discharge rate, lasting 15-20 s after the release of stretch. These results suggest that cAMP is involved in the regulation of the sensory processes through a modification of intracellular calcium activity.

Calcium incorporation in cultured chondroblasts perturbed by an electromagnetic field

We tested the hypothesis that electric perturbation influences 45Ca incorporation in extracellular matrix (ECM) of cartilage in vitro. Hypertrophic chondroblasts of tibial epiphyses (HC), sternum (SC), and skin fibroblasts (F) were cultured from chick embryos. HC, SC, and F cells were micromass seeded three times per week and maintained at 37.5 degrees C with 5% CO2 for two weeks. Cultures were randomly designated control (C) or exposed (E) to a pulsed electromagnetic field (PEMF). A time course experiment of calcium incorporation for all cultured groups showed that 24 h of exposure produced the largest biological response in chondroblasts. Calcium incorporation required supplemental phosphate. Autoradiography data indicated that the calcium incorporation into macromolecules largely occurred in the ECM. 45Ca steady-state perturbation was enhanced by Streptomyces hyaluronidase (SH) but not by testicular hyaluronidase (TH). 45Ca incorporation experiments tested the effects of phosphate, SH, TH, and PEMF alone and in various combinations on these cultures. Only PEMF or SH plus PEMF with phosphate enhanced 45Ca incorporation. Other experiments examined the effect of rotenone or freeze-thawing on cells exposed to PEMF. PEMF plus freeze-thaw enhanced calcium incorporation in HC only. PEMF appeared to cause disruption of the ECM, enhancing the probability of matrix calcification.

Mechanisms of mitochondrial calcium transport

Mitochondria are known to possess a rapid calcium uptake mechanism or uniport and both sodium-dependent and sodium-independent efflux mechanisms. Whether sodium-independent calcium efflux is mediated and whether sodium-dependent calcium efflux can be found in liver mitochondria have been questioned. Kinetics results relevant to the answers of these questions are discussed below. A slow, mediated, sodium-independent calcium efflux mechanism is identified which shows second order kinetics. This mechanism, which shows "nonessential activation" kinetics, has a Vmax around 1.2 nmol calcium per mg protein per min and a half maximal velocity around 8.4 nmol calcium per mg protein. A slow, sodium-dependent calcium efflux mechanism is identified, which is first order in calcium and second order in sodium. This mechanism has a Vmax around 2.6 nmol of calcium per mg protein per min. The sodium dependence is half saturated at an external sodium concentration of 9.4 mM, and the calcium dependence is half saturated at an internal calcium concentration of 8.1 nmol calcium per mg protein. The cooperativity of the sodium dependence effectively permits a terreactant system to be fit by a bireactant model in which [Na] only appears as the square of [Na]. This liver system shows simultaneous, as opposed to ping-pong, kinetics. It is also found to be sensitive to inhibition by tetraphenyl phosphonium, magnesium, and ruthenium red. A model is proposed in which mitochondrial calcium transport could function to "shape the pulses" of cytosolic calcium. Simultaneously, mitochondria may mediate a "calcium memory" coupled perhaps to activation of cytosolic events through calmodulin or perhaps to activation of electron transport through the activation of specific dehydrogenases by intramitochondrial calcium.

The effect of chloramphenicol and cycloheximide on platelet aggregation and protein synthesis

This study investigated the role of platelet protein synthesis in platelet aggregation. Cycloheximide (Cx) and chloramphenicol (Cm) were used as inhibitors of cytoplasmic (80S ribosome directed) and mitochondrial protein synthesis respectively. The effect of these agents on human platelet aggregation and L-[U-14C]leucine incorporation into platelet protein was investigated. Cm exhibited dose-dependent inhibition of collagen, thrombin and secondary ADP aggregatory responses, but had no effect on arachidonate or primary ADP responses over a similar concentration range (3.1 X 10(-3), 3.1 X 10(-4) and 3.1 X 10(-5) M). Cm also inhibited platelet secretion associated with collagen and secondary ADP responses. Furthermore, Cm exhibited a similar dose-dependent inhibition of L-[U-14C]leucine incorporation into platelet protein reaching 80% inhibition of incorporation at 3 X 10(-3) M. At similar concentrations (3.5 X 10(-3), 3.5 X 10(-4) and 3.5 X 10(-5) M) Cx failed to show inhibition of human platelet aggregation by all agonists used with the exception of collagen where some inhibition was seen at high Cx concentration (3.5 X 10(-3) M). Cx was also found to be ineffective at inhibiting L-[U-14C]leucine incorporation into platelet protein at all concentrations tested. These results suggest that the majority of platelet protein synthesis is mitochondrial and that this protein synthesis may have a role in human platelet aggregation.

Ultrastructural distribution of Ca++ within neurons. An oxalate pyroantimonate study

We used the oxalate-pyroantimonate technique to determine the ultrastructural distribution of Ca++ in neurons of the rat sciatic nerve. The content of the precipitate was confirmed by X-ray microanalysis and appropriate controls. In the cell bodies of the dorsal root ganglia, Ca++ precipitate was found in the Golgi, mitochondria, multivesicular bodies and large vesicles of the cytoplasm but not in lysosomes, and was prominently absent from regions of rough endoplasmic reticulum and ribosomes. It was seen in the nucleus but not in the nuclear bodies or nucleolus. Within the axon itself, Ca++ precipitate was also found sequestered in mitochondria and smooth endoplasmic reticulum. In addition Ca++ precipitate found diffusely throughout the axoplasm exhibited a discrete and heterogeneous distribution. In myelinated fibers the amount of precipitate decreased predictably in the axoplasm beneath the Schmidt-Lanterman cleft and in the paranodal regions at the nodes of Ranvier. This correlated with the presence of dense precipitate in the Schmidt-Lanterman cleft themselves and in the paranodal loops of myelin. Intracytoplasmic ionic Ca++ is maintained at 10(-7) M by balanced processes of influx, sequestration and extrusion. The irregular distribution of Ca++ precipitate in the axoplasm of myelinated fibers suggests that there may be specific regions of preferential efflux across the axolemma.

The role of mitochondrial glutathione and cellular protein sulfhydryls in formaldehyde toxicity in glutathione-depleted rat hepatocytes

Depletion of cellular GSH by diethyl maleate (DEM) potentiates CH2O toxicity in isolated rat hepatocytes and it was postulated that this increase in toxicity is due to the further decrease in GSH caused by CH2O in DEM-pretreated hepatocytes (1). The present investigation was conducted to investigate further the effects of CH2O, DEM, and acrolein (a compound which is structurally related to CH2O and DEM) on subcellular GSH pools and on protein sulfhydryl groups (PSH). CH2O caused a decrease in cytosolic GSH but had no effect on mitochondrial GSH either in previously untreated hepatocytes or in DEM-pretreated hepatocytes in which GSH was approximately 25% of control. DEM decreased both cytosolic and mitochondrial GSH but it did not produce toxicity. Neither CH2O (up to 7.5 mM) nor DEM (20 mM) decreased PSH. However, in cells pretreated with 1 mM DEM, CH2O (7.5 mM) decreased PSH and this effect preceded cell death. Acrolein decreased both cytosolic and mitochondrial GSH and it also decreased PSH significantly prior to causing cell death. CH2O and acrolein stimulated phosphorylase alpha activity, indicative of an increase in cytosolic free Ca2+, by a PSH-independent and PSH-dependent mechanism, respectively. These results suggest that the further depletion of cellular GSH by CH2O in DEM-pretreated cells is not due to the depletion of mitochondrial GSH. CH2O toxicity in DEM-pretreated cells is, however, correlated with depletion of PSH. The critical sulfhydryl protein(s) responsible for cell death remain to be more clearly defined.

Dissociation of insulin binding from insulin stimulation of 2-deoxyglucose transport by ruthenium red

Ruthenium red increased specific insulin binding to isolated adipocytes 5.4 fold and 2.6 fold over binding determined in the absence and presence of Ca2+ and Mg2+. The increase in insulin binding was not accompanied by an increase in insulin sensitivity. The lack of effect of ruthenium red on insulin action argued strongly against an increase in intracellular Ca2+ as a potential messenger/transducer of insulin action and suggested that the enhancing effect of Ca2+ on insulin action was a result of increased receptor affinity.

Cellular compartmentation of calcium in mouse fibroblasts in vitro depends on cell density and transformation

Cellular compartmentation of Ca has been investigated by kinetic analysis of 45Ca efflux from preloaded cells at various states of cell density-dependent proliferation of normal (3T3) and transformed (3T6 and SV40-3T3) mouse cells. Three pools of exchangeable calcium were separated on the basis of their differing exchange kinetics. For each of the cell lines tested, all three compartments decrease with cell density. Significant differences between normal and transformed cells are observed upon quiescence of the normal cells, where the slowly exchanging compartment of normal cells gradually increases, whereas that of the transformed cells continues to decrease (with increasing cell density). Free cytoplasmic Ca2+ concentration as determined by the Quin 2 method, was found to be significantly higher in transformed cells than in normal cells. These results indicate significant differences in Ca homeostasis between normal and transformed cells.

Effects of chelating agents on the Ca2+-stimulated ATPase of rat liver plasma membranes

Using strictly controlled ionic conditions we have demonstrated, in agreement with previous findings (Lotersztajn et al. (1981) J. Biol. Chem. 256, 11209-11215; Lotersztajn, S. and Pecker, F. (1982) J. Biol. Chem. 257, 6638-6641) a Ca2+-stimulated ATPase in rat liver plasma membranes which is detectable at low free Mg2+ concentrations (normally fulfilled by endogenous levels) but not at free Mg2+ concentrations greater than about 10(-5) M. The findings reported here also suggest that this (Ca2+ + Mg2+)-ATPase is activated by EGTA or one of its liganded species. Furthermore, this is probably an intrinsic property of the enzyme as it was found to be independent of the isolation technique. The stimulation by EGTA appears to be a function both of free Ca2+ concentration and of one or more liganded species of EGTA and it is also inhibited at high free Mg2+ concentrations (approx. 10(-5) M). The specificity of the EGTA effect on ATPase activity is studied with respect to other, widely used, chelating agents namely HEEDTA, EDTA and CDTA. Of these, only CDTA shares the effect, although the concentration dependence of the activation is different from EGTA, suggesting that there is some degree of structural specificity involved rather than a generalised effect of complexed Ca2+.

Chlortetracycline-mediated continuous Ca2+ oscillations in mitochondria of digitonin-treated Tetrahymena pyriformis

Ca2+ transport in mitochondria was studied in situ using digitonin-permeabilized cells of the ciliate protozoan Tetrahymena pyriformis GL. In the presence of oxidizable substrates and inorganic phosphate, mitochondria were able to accumulate a large amount of the added Ca2+ without subsequent uncoupling and mitochondrial damage. However, the maximal Ca2+ uptake dramatically decreased in the presence of micromolar concentrations of the fluorescent calcium indicator, chlortetracycline, which in aerobic conditions caused an uncoupling of the respiration in Ca2+-loaded mitochondria. Moreover, on reaching hypoxia, when the rate of oxygen diffusion from the air to the stirred incubation medium became a limiting factor, continuous Ca2+ oscillations were observed. Ca2+ fluxes were synchronous with the cyclic changes of the membrane potential and were followed with a significant delay by the changes of the membrane-associated fluorescence of Ca-chlortetracycline complexes. Both the chlortetracycline-induced uncoupling of the respiration and the oscillations were prevented by either EGTA or ruthenium red. It is suggested that in conditions of the limited rate of respiration the oscillations are generated as a result of the functioning of the two Ca2+-transport pathways: a Ca2+ uniport and a chlortetracycline-mediated electroneutral Ca2+ efflux.