Local Ca Concentration Research Articles

Deposition of Ca on the 2H-NbSe2 surface: Competing processes.

The interaction of Ca evaporated in vacuum onto a 2H-NbSe2 surface held at room temperature was studied by photoelectron spectroscopy. Indications of an intercalation reaction were observed initially, but larger Ca depositions resulted in a layer-breaking reaction producing a disordered overlayer of varying compositions. The layer-breaking reaction is likely due to the intercalation compound becoming unstable as the local Ca concentration exceeds the threshold value. Continued Ca depositions eventually resulted in the growth of three-dimensional islands of Ca metal, exposing relatively unaffected NbSe2 layers between the Ca islands.

The parietal cell gastric H, K-ATPase also functions as the Na, K-ATPase and Ca-ATPase in altered states

This article offers an explanation for the apparent lack of Na, K-ATPase activity in parietal cells although ouabain has been known to inhibit gastric acid secretion since 1962. The gastric H, K-ATPase (proton-pump) seems to be acting in altered states, thus behaving like a Na, K-ATPase (Na-pump) and/or Ca-ATPase (Ca-pump) depending on cellular needs. This conclusion is based on the following findings. First, parietal cell fractions do not exhibit Na, K-ATPase activity at pH 7.0 but do at pH 8.5. Second, the apical plasma membrane (APM) fraction exhibits a (Ca or Mg)-ATPase activity with negligible H, K-ATPase activity. However, when assayed with Mg alone in presence of the 80 k Da cytosolic proton-pump activator (HAF), the APM fraction reveals remarkably high H, K-ATPase activity, suggesting the observed low affinity of Ca (or Mg)-ATPase is an altered state of the latter. Third, calcium (between 1 and 4 µM) shows both stimulation and inhibition of the HAF-stimulated H, K-ATPase depending on its concentration, revealing a close interaction between the proton-pump activator and local Ca concentration in gastric H, K-ATPase function. Such interactions suggest that Ca is acting as a terminal member of the intracellular signaling system for the HAF-regulated proton-pump. It appears that during resting state, the HAF-associated H, K-ATPase remains inhibited by Ca (>1 µM) and, prior to resumption of acid secretion the gastric H, K-ATPase acts temporarily as a Ca-pump for removing excess Ca from its immediate environment. This conclusion is consistent with the recent reports of immunochemical co-localization of the gastric H, K-ATPase and Ca-ATPase by superimposition in parietal cells, and a transitory efflux of Ca immediately preceding the onset of acid secretion. These new perspectives on proton-pump function would open new avenues for a fuller understanding of the intracellular regulation of the ubiquitous Na-pump.

The gastric H, K-ATPase system also functions as the Na, K-ATPase and Ca-ATPase in altered states

This article offers an explanation for the apparent lack of Na, K-ATPase activity in parietal cells although ouabain has been known to inhibit gastric acid secretion since 1962. The gastric H, K-ATPase (proton-pump) seems to be acting in altered states, thus behaving like a Na, K-ATPase (Na-pump) and/or Ca-ATPase (Ca-pump) depending on cellular needs. This conclusion is based on the following findings. First, parietal cell fractions do not exhibit Na, K-ATPase activity at pH 7.0 but do at pH 8.5. Second, the apical plasma membrane (APM) fraction exhibits a (Ca or Mg)-ATPase activity with negligible H, K-ATPase activity. However, when assayed with Mg alone in presence of the 80 k Da cytosolic proton-pump activator (HAF), the APM fraction reveals remarkably high H, K-ATPase activity, suggesting the observed low affinity of Ca (or Mg)-ATPase is an altered state of the latter. Third, calcium (between 1 and 4 µM) shows both stimulation and inhibition of the HAF-stimulated H, K-ATPase depending on its concentration, revealing a close interaction between the proton-pump activator and local Ca concentration in gastric H, K-ATPase function. Such interactions suggest that Ca is acting as a terminal member of the intracellular signaling system for the HAF-regulated proton-pump. It appears that during resting state, the HAF-associated H, K-ATPase remains inhibited by Ca (>1 µM) and, prior to resumption of acid secretion the gastric H, K-ATPase acts temporarily as a Ca-pump for removing excess Ca from its immediate environment. This conclusion is consistent with the recent reports of immunochemical co-localization of the gastric H, K-ATPase and Ca-ATPase by superimposition in parietal cells, and a transitory efflux of Ca immediately preceding the onset of acid secretion. These new perspectives on proton-pump function would open new avenues for a fuller understanding of the intracellular regulation of the ubiquitous Na-pump.

Effect of Calcium Ion Concentrations on Osteogenic Differentiation and Hematopoietic Stem Cell Niche-Related Protein Expression in Osteoblasts

Apatite biomaterials have potential not only as cell vehicles for engineering bone tissue but also as regulators of calcium (Ca) concentration in situ for controlling osteoblast functions, for example, osteogenic differentiation and fate management of hematopoietic stem cells (HSCs). To design apatite materials having optimal chemical properties for the latter purpose, more detailed investigations into what effect Ca concentrations have on osteoblast functions is crucial. In this study, osteoblasts were cultured at different Ca concentrations, and the temporal alterations in osteogenic differentiation and HSC niche-related protein (angiopoietin-1, 2 [Ang1, 2]) expression were investigated. The different Ca concentrations (1.8-50 mmol/L) in the cell culture medium had no effect on the proliferation of osteoblasts, but did on the cell morphology. The higher Ca concentrations (<6 mmol/L) enhanced the mineralization as well as Ang1 expression. In addition, Ang1 expression in osteoblasts showed higher correlation with expression of connexin43, the major marker of cell-cell interactions, whereas Ang2 related to integrin beta1, the major marker of cell-matrix interactions. Thus, the local Ca concentration regulates cell morphology through the cell-cell or cell-matrix interactions, leading to the alteration of Ang1 expression in osteoblasts. Since these changes triggered by Ca are concerned with the osteogenic differentiation or reproduction of HSCs niche microenvironment, the results obtained in this study might be useful for designing apatite materials with optimal chemical properties.

Cardiac Action Potential Model with Spatial Subcellular Calcium Cycling and Physiological Transmembrane Currents

Calcium (Ca) is a key player in excitation-contraction (EC) coupling in the cardiac myocyte. When Ca enters through L-type Ca channels, it opens the ryanodine receptor (RyR) channels on the sarcoplasmic reticulum (SR) which stores a large amount of Ca (Ca induced Ca release). There are about 20,000 SR units and each RyR can sense only the local Ca concentration. Most computer cardiac action potential models use a single Ca concentration, which limits accurate modeling of intracellular Ca cycling. This is because the RyRs do not sense the average Ca concentration of the whole cell. Different aspects limit the utility of various models for investigation of intracellular Ca cycling related phenomena such as delayed afterdepolarizations (DADs) and catecholaminergic polymorphic ventricular tachycardia (CPVT) (since Ca concentrations in these phenomena are strongly heterogeneous within the cell). In order to investigate these phenomena, we developed a mathematical action potential model based on the spatial Ca model by Restrepo et al and physiological transmembrane currents by Shannon et al and Mahajan et al. In order to have more natural sparks, we increased the number of grid points for Ca diffusions from the model by Restrepo et al. Then we tuned the model to exhibit physiological action potential duration (APD) and Ca transients for long and short pacing cycle lengths (PCL). At short PCL, this hybrid model shows both voltage driven alternans due to steep APD restitution and Ca driven alternans due to luminal Ca regulation-mediated mechanism. In addition to alternans, this model exhibits Ca waves and DADs. This mathematical model of the cardiac myocyte provides a sort of minimal model to explore the spatio-temporal aspects of wave initiation, propagation and DAD induction related to normal and pathophysiological conditions.

Calcium distribution in cut roses as related to transpiration

SummaryIn contrast to many other horticultural crops, cut roses show higher concentrations of Ca in the leaves at the top of the flower stem than in those at the bottom. This study aimed to investigate the distribution of Ca along the stem of roses, to assess whether Ca accumulation in roses might be governed by factors other than transpiration. Ca concentrations in the leaves decreased from bottom to top of the stem. The effects of air vapour pressure deficit (VPD), removal of leaves and/or bent down stems on Ca concentrations in the leaves and petals were studied in greenhouse experiments. A lowerVPD resulted in an 11% increase in the number of flowering stems, which was offset by a lower average stem weight. Although transpiration was on average 25% lower in the low-VPD treatment over the whole experimental period, it was not (winter period) or only slightly (summer) related to lower Ca concentrations in the petals, stems or leaves. This limits the possibility for Ca enhancement by influencing transpiration. Removal of part of the leaves and/ or bent-down stems increased Caconcentrations in the remaining leaves and petals, primarily through decreased dryweight accumulation. Leaf diffusion resistance decreased along the stem from bottom to top leaves. In postharvest transpiration studies, stomatal densities and transpiration rates per unit leaf area were both higher in top than in bottom leaves. The findings indicate that local Ca concentrations in the rose organs can be related to their respective transpiration rates.

The local Ca concentration profile in the vicinity of a Ca channel.

Intracellular Ca signaling is governed by diffusion and buffering of Ca ions. Mobile endogenous buffers increase the redistribution of Ca ions and tend to restrict the regions of elevated Ca concentrations to the close vicinity of the channel. These Ca microdomains dominate fast Ca signaling, as observed, e.g., in synaptic transmission. The steady-state solution of the linearized differential equations of buffered Ca diffusion, as developed by Neher and Naraghi and Neher, will be reviewed and generalized to the case of more than two buffers. Immobile buffers do not enter the steady-state equations, but instead slow down Ca diffusion and prolong the time to reach the steady state. Based on this phenomenon, a quite different putative mechanism to localize Ca will be suggested that is likely to be realized in photoreceptors where Ca source, Ca sink, and Ca sensor form a complex, as was recently reported.

Time courses of calcium and calcium-bound buffers following calcium influx in a model cell

Fixed and diffusible calcium (Ca) buffers shape the spatial and temporal distribution of free Ca following Ca entry through voltage-gated ion channels. This modeling study explores intracellular Ca levels achieved near the membrane and in deeper locations following typical Ca currents obtained with patch clamp experiments. Ca ion diffusion sets an upper limit on the maximal average Ca concentration achieved near the membrane. Fixed buffers restrict Ca elevation spatially to the outermost areas of the cell and slow Ca equilibration. Fixed buffer bound with Ca near the membrane can act as Ca source after termination of Ca influx. The relative contribution of fixed versus diffusible buffers to shaping the Ca transient is determined to a large extent by the binding rate of each buffer, with diffusible buffer dominating at equal binding rates. In the presence of fixed buffers, diffusible buffers speed Ca equilibration throughout the cell. The concentration profile of Ca-bound diffusible buffer differs from the concentration profile of free Ca, reflecting theoretical limits on the temporal resolution which can be achieved with commonly used diffusible Ca indicators. A Ca indicator which is fixed to an intracellular component might more accurately report local Ca concentrations.

Generation and processing of peripheral temperature signals in mammals.

Temperature transduction in peripheral cold receptors and processing of peripheral temperature signals in the spinal cord were studied in cats and rats. The temperature dependence of the generator potential is attributed to different temperature coefficients of an electrogenic Na-efflux and the passive Na-influx. Cold receptor activity and particularly its bursting pattern is considerably modulated by the local Ca-concentration, but the effect of elevated Ca-concentration is abolished by the ATPase blocker ouabain. — The peripheral temperature signals from the scrotal skin of rats are transformed in dorsal horn neurones (DHN) into temperature reactions, which occur only above (warm reaction) or below (cold reaction) a certain temperature threshold and are limited to an operational range of 1–4°C. Convergency of different temperature inputs were observed in one and the same DHN. Supraspinal control of temperature reactive DHN appears to be complex but predominantly excitatory.

Fixation of Iron Applied as FeEDTA: Effect of Calcium Concentration and Soil Solid Phase

AbstractIron fixation from FeEDTA was measured in three soils at various water contents and Ca concentrations as a function of time for periods up to 30 hours, using a batch technique. In the three soils under study the general pattern of the fixation reaction can be described as a first‐order reaction if adsorption is assumed to be much faster than fixation reaction. The rate constant is markedly affected by Ca concentration: in Rehovot sand and Nazaret clay loam (0 and 61% CaCO3, respectively) the reaction rate constant increases sharply as added Ca increases from 0 to 5 mM, followed by a moderate and more or less linear increase up to Ca concentration of 100mM. In Zophar sandy loam (69% CaCO3), the reaction rate constant shows a maximum at Ca concentration of 50mM and then decreases.In Rehovot sand and Nazaret clay loam the fixation process can be described by two independent first‐order reactions taking place simultaneously on the solid phase surface and in the liquid phase. Zophar sandy loam does not fit this model. It is suggested that the solid surface area affects the fixation reaction in addition to the effects of pH and Ca concentration.The effect of Ca on Fe‐FeEDTA fixation is discussed in connection to average and local Ca concentrations. High Ca concentrations near the root surface, formed due to transpiration stream, constitute a “chemical barrier” for the movement of Fe‐FeEDTA toward the root.