Unstable Ca Research Articles

Time-varying structure evolution and mechanism analysis of alite particles hydrated in restricted space

Restricted space could lead to the structure evolution of calcium silicate hydrate (C–S–H) especially during the later hydration stage, which essentially affects the strength and durability of Portland cement based materials. In order to get insights into the effect of restricted space on C–S–H microstructure, the alite powders were pressed into discs for hydration. The pre pressing treatment provides a relatively denser microstructure, which simulates the restricted space during the late hydration of alite or cement. The structure of hydrates grown in restricted space was revealed by Fourier transform infrared spectra, Raman spectra, 29Si nuclear magnetic resonance spectra, scanning electron microscope and energy dispersive X-ray spectroscopy. The results show that restricted space leads to the formation of unstable Ca–O bonds and Si–O bonds in early hydrates, and influences the silicate chains in C–S–H by the formation of high polymerized hydrates (Q3 silicates) at early stage. The content of Q3 silicates in the hydrates of restricted space is the highest at 7 d and then decreases, which is opposite to the control with the lowest Q3 silicates content at 7 d and the highest content at 28 d. The hydrates which form at 28 d show a remarkable higher chemical shift of Q3 sites and a lower Ca/Si ratio than that of the control. The findings indicate that the restricted space may lead to stress, which rearranges the nanostructure of C–S–H and induces a varied structure in Q3 silicates of C–S–H. This work lies a theoretical foundation for exploring the structure evolution of C–S–H in cement concrete after long service.

Phase transitions involving Ca – The most abundant ash forming element – In thermal treatment of lignocellulosic biomass

Torrefaction, pyrolysis and gasification are of interest to convert lignocellulosic biomass into fuels and chemicals. These techniques involve thermal treatment at low partial pressures of oxygen. However, little is known about the transformation of ash elements during these processes. The phase transition of the major ash element calcium (Ca) was therefore studied with powder from pine as biomass model treated at temperatures 300–800 °C under atmospheres of 100% N2, 3% O2 and 6% O2 and thermodynamic equilibrium modelling. For evaluation, X-ray powder diffraction and synchrotron Ca K-edge X-ray absorption near edge structure (XANES) spectroscopy in combination with linear combination fitting and reference compounds was used.The results indicated that the most abundant Ca-containing species in the untreated material was thermally unstable Ca oxalate (CaC2O4) primarily decomposing into Ca phases dominated by carbonates at temperatures up to 600 °C. Double carbonates of calcium and potassium were observed in the form of fairchildiite/butscheliite (K2Ca(CO3)2), and these phases were stable over the low temperature range studied. Hydroxyapatite (Ca5(PO4)3OH) was expected to be present and thermally stable over the entire temperature interval and was found in untreated material. At temperatures above 600 °C calcium oxide (CaO) was formed. The amount of oxygen had little effect on the phase transitions. The results of thermodynamic modeling were in agreement with XANES showing that this is a versatile technique that can be applied to systems as complex as Ca phase transitions in thermally treated lignocellulosic biomass at low partial pressures of oxygen.

Feedback control of calcium driven alternans in cardiac myocytes.

Cardiac alternans is a beat-to-beat alternation of the action potential duration (APD), which has been implicated as a possible cause of ventricular fibrillation. Previous studies have shown that alternans can originate via a period doubling bifurcation caused by the nonlinear dependence of the APD on the previous diastolic interval. In this case, it has been demonstrated that alternans can be eliminated by applying feedback control on the pacing cycle length. However, studies have shown that alternans can also originate due to unstable calcium (Ca) cycling in cardiac myocytes. In this study, we explore the effectiveness of APD feedback control to suppress alternans when the underlying instability is due to unstable Ca cycling. In particular, we explore the role of the bi-directional coupling between Ca and voltage and determine the effectiveness of feedback control under a wide range of conditions. We also analyze the applicability of feedback control on a coupled two cell system and show that APD control induces spatially out-of-phase alternans. We analyze the onset and the necessary conditions for the emergence of these out-of-phase patterns and assess the effectiveness of feedback control to suppress Ca driven alternans in a multi-cellular system.

The novel ADAMTS13‐p.D187H mutation impairs ADAMTS13 activity and secretion and contributes to thrombotic thrombocytopenic purpura in mice

Congenital thrombotic thrombocytopenic purpura (TTP) is characterized by mutations in the ADAMTS13 gene, which either impair protein secretion or influence ADAMTS13 (A Disintegrin-like And Metalloprotease domain with ThromboSpondin type-1 motif, member 13) activity. Phenotypic consequences of these mutations have not yet been evaluated in animal models for TTP. To identify the in vitro effect of a novel ADAMTS13 mutation and to investigate whether this mutation induces TTP in vivo. All 29 ADAMTS13 exons with exon-intron boundaries of a patient with pregnancy-onset TTP were sequenced. Wild-type and mutant ADAMTS13 proteins were both transiently and stably expressed in human embryonic kidney cells, and their activity was evaluated in vitro using fluorescence resonance energy transfer and flow assays. Molecular dynamics simulations were performed to study Ca(2+) stability. Adamts13(-/-) mice were hydrodynamically injected with wild-type and mutant expression plasmids and triggered with recombinant human von Willebrand factor. We identified a novel heterozygous c.559G>C mutation in exon 6 of the proposita's ADAMTS13 gene. This mutation resulted in a p.Asp187His substitution (p.D187H), which was located in the high affinity Ca(2+) -binding site in the metalloprotease domain of ADAMTS13. The homozygous p.D187H mutation down-regulated ADAMTS13 activity in vitro. Impaired proteolytic activity was linked to unstable Ca(2+) binding as visualized using a molecular dynamics simulation. In addition, the p.D187H mutation affects protein secretion in vitro. In Adamts13(-/-) mice, the homozygous p.D187H mutation reduced ADAMTS13 secretion and activity and contributed to TTP when these mice were triggered with recombinant human von Willebrand factor. Our data indicate that the p.D187H mutation impairs ADAMTS13 activity and secretion and is responsible for TTP onset in mice.

A novel computational model of mouse myocyte electrophysiology to assess the synergy between Na+ loading and CaMKII

Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) hyperactivity in heart failure causes intracellular Na(+) ([Na(+)]i) loading (at least in part by enhancing the late Na(+) current). This [Na(+)]i gain promotes intracellular Ca(2+) ([Ca(2+)]i) overload by altering the equilibrium of the Na(+)-Ca(2+) exchanger to impair forward-mode (Ca(2+) extrusion), and favour reverse-mode (Ca(2+) influx) exchange. In turn, this Ca(2+) overload would be expected to further activate CaMKII and thereby form a pathological positive feedback loop of ever-increasing CaMKII activity, [Na(+)]i, and [Ca(2+)]i. We developed an ionic model of the mouse ventricular myocyte to interrogate this potentially arrhythmogenic positive feedback in both control conditions and when CaMKIIδC is overexpressed as in genetically engineered mice. In control conditions, simulation of increased [Na(+)]i causes the expected increases in [Ca(2+)]i, CaMKII activity, and target phosphorylation, which degenerate into unstable Ca(2+) handling and electrophysiology at high [Na(+)]i gain. Notably, clamping CaMKII activity to basal levels ameliorates but does not completely offset this outcome, suggesting that the increase in [Ca(2+)]i per se plays an important role. The effect of this CaMKII-Na(+)-Ca(2+)-CaMKII feedback is more striking in CaMKIIδC overexpression, where high [Na(+)]i causes delayed afterdepolarizations, which can be prevented by imposing low [Na(+)]i, or clamping CaMKII phosphorylation of L-type Ca(2+) channels, ryanodine receptors and phospholamban to basal levels. In this setting, Na(+) loading fuels a vicious loop whereby increased CaMKII activation perturbs Ca(2+) and membrane potential homeostasis. High [Na(+)]i is also required to produce instability when CaMKII is further activated by increased Ca(2+) loading due to β-adrenergic activation. Our results support recent experimental findings of a synergistic interaction between perturbed Na(+) fluxes and CaMKII, and suggest that pharmacological inhibition of intracellular Na(+) loading can contribute to normalizing Ca(2+) and membrane potential dynamics in heart failure.

A Novel Computational Model of Mouse Myocyte Electrophysiology to Assess the Synergy Between Na Loading and CaMKII

Ca/Calmodulin-dependent protein kinase II (CaMKII) hyperactivity in heart failure causes intracellular Na ([Na]i) loading (at least in part by enhancing the late Na current (INaL)). This [Na]i gain promotes intracellular Ca ([Ca]i) overload by altering the equilibrium of the Na/Ca exchanger to impair forward-mode (Ca extrusion), and favor reverse-mode (Ca influx) exchange. In turn, this Ca overload is expected to further activate CaMKII thereby forming a pathologic positive feedback loop of ever-increasing CaMKII activity, [Na]i, and [Ca]i. We developed a computational framework to interrogate this potentially arrhythmogenic positive feedback in the mouse ventricular myocyte in both control conditions and when CaMKIIδC is overexpressed as in transgenic mice. In control conditions, simulation of an increase in [Na]i causes the expected increases in [Ca]i, CaMKII activity, and target phosphorylation, which degenerate into unstable Ca handling and and electrophysiology at high [Na]i gain. Notably, clamping CaMKII to basal levels ameliorates but does not completely offset this outcome, suggesting that the increase in [Ca]i per se plays an important role. The effect of this CaMKII-Na-Ca-CaMKII feedback is more striking in CaMKIIδC overexpression, where high but not low [Na]i causes delayed afterdepolarizations, which can be prevented by clamping CaMKII phosphorylation of L-type Ca channels, ryanodine receptors and phospholamban to basal levels. In this setting, Na loading fuels a vicious loop whereby increased CaMKII activation perturbs Ca and membrane potential homeostasis. High [Na]i is also required to produce instability when CaMKII is further activated by increased Ca loading due to β-adrenergic activation. Our results support recent experimental findings of a synergistic interaction between perturbed Na fluxes and CaMKII, and suggest that pharmacological inhibition of intracellular Na loading can contribute to normalizing Ca and membrane potential dynamics in heart failure.

Heterologous functional expression system for odorant receptors

Heterologous functional expression system for odorant receptors (ORs) is essential for investigating the structure–activity relationship (SAR) of various ligands. Different systems that coexpressed ORs with different G-protein α subunits (Gα) demonstrated inconsistent effects on weak agonists and antagonists, but retained original relative sensitivities to potent agonists. In order to maintain the binding specificity of Gα to ORs, we constructed a chimeric Gα 15_olf, which contained the Gα 15 sequence with the conserved C-terminal region of Gα olf. The Ca 2+ responses of the HEK293 cells that coexpressed OR-S6 with Gα 15_olf were more robust and reproducible compared to those of cells that coexpressed OR-S6 with Gα 15. Furthermore, Gα 15 sometimes induced unstable Ca 2+ responses that limited the accuracy of quantitative comparison of peak responses. Our results showed that a heterologous expression system that coexpressed ORs with Gα 15_olf and receptor transporting proteins was suitable for SAR analysis of various ligands.

Remodelling of action potential and intracellular calcium cycling dynamics during subacute myocardial infarction promotes ventricular arrhythmias in Langendorff‐perfused rabbit hearts

We hypothesize that remodelling of action potential and intracellular calcium (Ca(i)) dynamics in the peri-infarct zone contributes to ventricular arrhythmogenesis in the postmyocardial infarction setting. To test this hypothesis, we performed simultaneous optical mapping of Ca(i) and membrane potential (V(m)) in the left ventricle in 15 rabbit hearts with myocardial infarction for 1 week. Ventricular premature beats frequently originated from the peri-infarct zone, and 37% showed elevation of Ca(i) prior to V(m) depolarization, suggesting reverse excitation-contraction coupling as their aetiology. During electrically induced ventricular fibrillation, the highest dominant frequency was in the peri-infarct zone in 61 of 70 episodes. The site of highest dominant frequency had steeper action potential duration restitution and was more susceptible to pacing-induced Ca(i) alternans than sites remote from infarct. Wavebreaks during ventricular fibrillation tended to occur at sites of persistently elevated Ca(i). Infusion of propranolol flattened action potential duration restitution, reduced wavebreaks and converted ventricular fibrillation to ventricular tachycardia. We conclude that in the subacute phase of myocardial infarction, the peri-infarct zone exhibits regions with steep action potential duration restitution slope and unstable Ca(i) dynamics. These changes may promote ventricular extrasystoles and increase the incidence of wavebreaks during ventricular fibrillation. Whereas increased tissue heterogeneity after subacute myocardial infarction creates a highly arrhythmogenic substrate, dynamic action potential and Ca(i) cycling remodelling also contribute to the initiation and maintenance of ventricular fibrillation in this setting.

Inferring the Cellular Origin of Voltage and Calcium Alternans from the Spatial Scales of Phase Reversal during Discordant Alternans

Beat-to-beat alternation of the action potential duration (APD) in paced cardiac cells has been linked to the onset of lethal arrhythmias. Both experimental and theoretical studies have shown that alternans at the single cell level can be caused by unstable membrane voltage ( V m) dynamics linked to steep APD-restitution, or unstable intracellular calcium (Ca) cycling linked to high sensitivity of Ca release from the sarcoplasmic reticulum on sarcoplasmic reticulum Ca load. Identifying which of these two mechanisms is the primary cause of cellular alternans, however, has remained difficult since Ca and V m are bidirectionally coupled. Here, we use numerical simulations of a physiologically detailed ionic model to show that the origin of alternans can be inferred by measuring the length scales over which APD and Ca i alternans reverse phase during spatially discordant alternans. The main conclusion is that these scales are comparable to a few millimeters and equal when alternans is driven by APD restitution, but differ markedly when alternans is driven predominantly by unstable Ca cycling. In the latter case, APD alternans still reverses phase on a millimeter tissue scale due to electrotonic coupling, while Ca alternans reverses phase on a submillimeter cellular scale. These results show that experimentally accessible measurements of Ca i and V m in cardiac tissue can be used to shed light on the cellular origin of alternans.

Restitution of Ca2+ Release and Vulnerability to Arrhythmias

New information has recently been obtained along two essentially parallel lines of research: investigations into the fundamental mechanisms of Ca(2+)-induced Ca(2+) release (CICR) in heart cells, and analyses of the factors that control the development of unstable rhythms such as repolarization alternans. These lines of research are starting to converge such that we can begin to understand unstable and potentially arrhythmogenic cardiac dynamics in terms of the underlying mechanisms governing not only membrane depolarization and repolarization but also the complex bidirectional interactions between electrical and Ca(2+) signaling in heart cells. In this brief review, we discuss the progress that has recently been made in understanding the factors that control the beat-to-beat regulation of cardiac Ca(2+) release and attempt to place these results within a larger context. In particular, we discuss factors that may contribute to unstable Ca(2+) release and speculate about how instability in CICR may contribute to the development of arrhythmias under pathological conditions.

The influence of 50-Hz magnetic fields on cytoplasmic Ca 2+ oscillations in human leukemia T-cells

We have studied the effects of 50-Hz 100-μT rms magnetic fields on intracellular Ca 2+ concentration in the Jurkat T lymphocyte variant E6.1 using fluorescent probes Indo-1 and Fura-2. We found, however, that the pattern of intracellular Ca 2+ fluctuations also depended on the agent used for cell attachment, in our case the polypeptide poly- l-lysine. In order to isolate possible effects of magnetic field exposure from those of poly- l-lysine, the action of polypeptide on cytosolic Ca 2+ was studied as well. It was found that a 10 −7% concentration of polypeptide triggered prolonged Ca 2+ spiking. Higher (10 −4%) concentrations induced rapid increases in intracellular Ca 2+ followed by high, unstable Ca 2+ levels. The response of these cells to the monoclonal antibody anti-CD3 was also inhomogeneous, similar to one caused by poly- l-lysine. The effect of magnetic field exposure was studied on cells initially exhibiting (1) non-oscillating, low Ca 2+ concentration and (2) prolonged Ca 2+ concentration oscillations. In case (1) the result was negative. In case (2), statistically significant changes were found: the oscillation amplitude was reduced on average by 30%, and the frequency composition was shifted towards higher frequencies.

Calcium modulation of polyamine transport is lost in a putrescine-sensitive mutant of Neurospora crassa

Putrescine transport in Neurospora is saturable and concentrative in dilute buffers, but in the growth medium putrescine simply equilibrates across the cell membrane. We describe a mutant, puu-1, that can concentrate putrescine from the growth medium because the polyamine transport system has lost its normal sensitivity to Ca 2+. The wild type closely resembles the mutant if it is washed with citrate and ethylene glycol bis(β-aminoethyl ether) N,N′-tetraacetic acid. The mutant phenotype also appears in the wild type after treatment with cycloheximide. The results suggest that putrescine uptake is normally regulated by an unstable Ca 2+ -binding protein that restricts polyamine uptake. This protein is evidently distinct from the polyamine-binding function for uptake, which is normal in mutant and in cycloheximide-treated wild type cells. The puu-1 mutation, stripping of Ca 2+, and cycloheximide treatment all cause an impairment of amino acid transport, indicating that other membrane transport functions rely upon the product of the puu-1 + gene. Preliminary evidence suggests that the putrescine carrier is not the Ca 2+-sensitive, low-affinity K +-transport system, but K + efflux does accompany putrescine uptake.