Fluorescence imaging of intracellular Ca2+

To the Mechanism of the Antiarrhythmic Action of Compound ALM-802: the Role of Ryanodine Receptors

The effect of the compound N¹-(2,3,4-trimethoxy)-N²-{2-[(2,3,4-trimethoxybenzyl)amino]ethyl}-1,2-ethane-diamine (code ALM-802) on the amplitude of the Ca²⁺ response in the cell was studied in in vitro experiments. The concentration of intracellular calcium was assessed using a Fura-2 two-wave probe. The experiments were performed on a culture of isolated rat hippocampal neurons. The effect of compound ALM-802 on the activity of ryanodine receptors (RyR2) was studied on an isolated strip of rat myocardium. The compound ALM-802 (69.8 μM) in hippocampal neurons causes a significant decrease in the amplitude of the Ca²⁺ response induced by addition of KCl to the medium. Experiments performed on an isolated myocardial strip showed that compound ALM-802 (10^-5 M) almost completely blocked the positive inotropic reaction of the strip to the RyR2 agonist caffeine (5×10^-5 M). The data obtained indicate that the decrease in the concentration of Ca²⁺ ions in the cell caused by ALM-802 is due to its ability to block RyR2 located on the membrane of the sarcoplasmic reticulum, which can be associated with the antiarrhythmic activity of the compound.

A robust fluorescence-based assay for human erythrocyte Ca++ efflux suitable for high-throughput inhibitor screens

Intracellular calcium is maintained at very low concentrations through the action of PMCA Ca++ extrusion pumps. Although much of our knowledge about these Ca++ extrusion pumps derives from studies with human erythrocytes, kinetic studies of Ca++ transport for these cells are limited to radioisotope flux measurements. Here, we developed a robust, microplate-based assay for erythrocyte Ca++ efflux using extracellular fluorescent Ca++ indicators. We optimized Ca++ loading with the A23187 ionophore, established conditions for removal of the ionophore, and adjusted fluorescent dye sensitivity by addition of extracellular EGTA to allow continuous tracking of Ca++ efflux. Efflux kinetics were accelerated by glucose and inhibited in a dose-dependent manner by the nonspecific inhibitor vanadate, revealing that Ca++ pump activity can be tracked in a 384-well microplate format. These studies enable radioisotope-free kinetic measurements of the Ca++ pump and should facilitate screens for specific inhibitors of this essential transport activity.

Astrocyte Activation Markers

Astrocytes are the most common type of glial cells that provide homeostasis and protection of the central nervous system. Important specific characteristic of astrocytes is manifestation of morphological heterogeneity, which is directly dependent on localization in a particular area of the brain. Astrocytes can integrate into neural networks and keep neurons active in various areas of the brain. Moreover, astrocytes express a variety of receptors, channels, and membrane transporters, which underlie their peculiar metabolic activity, and, hence, determine plasticity of the central nervous system during development and aging. Such complex structural and functional organization of astrocytes requires the use of modern methods for their identification and analysis. Considering the important fact that determining the most appropriate marker for polymorphic and multiple subgroups of astrocytes is of decisive importance for studying their multifunctionality, this review presents markers, modern imaging techniques, and identification of astrocytes, which comprise a valuable resource for studying structural and functional properties of astrocytes, as well as facilitate better understanding of the extent to which astrocytes contribute to neuronal activity.

Insights on gastrointestinal motility through the use of optogenetic sensors and actuators

The muscularis of the gastrointestinal (GI) tract consists of smooth muscle cells (SMCs) and various populations of interstitial cells of Cajal (ICC), platelet-derived growth factor receptor α⁺ (PDGFRα⁺ ) cells, as well as excitatory and inhibitory enteric motor nerves. SMCs, ICC and PDGFRα⁺ cells form an electrically coupled syncytium, which together with inputs from the enteric nervous system (ENS) regulate GI motility. Early studies evaluating Ca²⁺ signalling behaviours in the GI tract relied upon indiscriminate loading of tissues with Ca²⁺ dyes. These methods lacked the means to study activity in specific cells of interest without encountering contamination from other cells within the preparation. Development of mice expressing optogenetic sensors (GCaMP, RCaMP) has allowed visualization of Ca²⁺ signalling behaviours in a cell specific manner. Additionally, availability of mice expressing optogenetic modulators (channelrhodopsins or halorhodospins) has allowed manipulation of specific signalling pathways using light. GCaMP expressing animals have been used to characterize Ca²⁺ signalling behaviours of distinct classes of ICC and SMCs throughout the GI musculature. These findings illustrate how Ca²⁺ signalling in ICC is fundamental in GI muscles, contributing to tone in sphincters, pacemaker activity in rhythmic muscles and relaying enteric signals to SMCs. Animals that express channelrhodopsin in specific neuronal populations have been used to map neural circuitry and to examine post junctional neural effects on GI motility. Thus, optogenetic approaches provide a novel means to examine the contribution of specific cell types to the regulation of motility patterns within complex multi-cellular systems. Abstract Figure Legends Optogenetic activators and sensors can be used to investigate the complex multi-cellular nature of the gastrointestinal (GI tract). Optogenetic activators that are activated by light such as channelrhodopsins (ChR2), OptoXR and halorhodopsinss (HR) proteins can be genetically encoded into specific cell types. This can be used to directly activate or silence specific GI cells such as various classes of enteric neurons, smooth muscle cells (SMC) or interstitial cells, such as interstitial cells of Cajal (ICC). Optogenetic sensors that are activated by different wavelengths of light such as green calmodulin fusion protein (GCaMP) and red CaMP (RCaMP) make high resolution of sub-cellular Ca²⁺ signalling possible within intact tissues of specific cell types. These tools can provide unparalleled insight into mechanisms underlying GI motility and innervation. This article is protected by copyright. All rights reserved.

A review on magnetic and spintronic neurostimulation: challenges and prospects

In the treatment of neurodegenerative, sensory and cardiovascular diseases, electrical probes and arrays have shown quite a promising success rate. However, despite the outstanding clinical outcomes, their operation is significantly hindered by non-selective control of electric fields. A promising alternative is micromagnetic stimulation (μMS) due to the high permeability of magnetic field through biological tissues. The induced electric field from the time-varying magnetic field generated by magnetic neurostimulators is used to remotely stimulate neighboring neurons. Due to the spatial asymmetry of the induced electric field, high spatial selectivity of neurostimulation has been realized. Herein, some popular choices of magnetic neurostimulators such as microcoils (μcoils) and spintronic nanodevices are reviewed. The neurostimulator features such as power consumption and resolution (aiming at cellular level) are discussed. In addition, the chronic stability and biocompatibility of these implantable neurostimulator are commented in favor of further translation to clinical settings. Furthermore, magnetic nanoparticles (MNPs), as another invaluable neurostimulation material, has emerged in recent years. Thus, in this review we have also included MNPs as a remote neurostimulation solution that overcomes physical limitations of invasive implants. Overall, this review provides peers with the recent development of ultra-low power, cellular-level, spatially selective magnetic neurostimulators of dimensions within micro- to nano-range for treating chronic neurological disorders. At the end of this review, some potential applications of next generation neuro-devices have also been discussed.

New 3β-hydroxysteroid-indolamine conjugates: Design, synthesis and inhibition of C6 glioma cell proliferation

Four novel indole steroids based on dehydroepiandrosterone (IS-1), estrone (IS-2) and pregnenolone (IS-3) were obtained and studied for their ability to inhibit C6 glioma proliferation. A reduction in cell proliferation by 52 ± 13% was observed for IS-1 at 10 μM, whereas IS-3 and abiraterone acetate at 10 μM caused a 36 ± 8% decrease. Surprisingly, the cellular effects reported for abiraterone, namely, cytochrome P450 CYP17A1 inhibition and endoplasmic reticulum stress were not detected for IS-1. However, both abiraterone and IS-1 significantly increased glutathione levels. Docking studies predicted good affinity of IS-1 to liver X receptors and regulatory protein Keap1, which are proposed to be involved in the compounds' antiproliferative activity.

Stress-induced autonomic dysregulation of mitochondrial function in the rat urothelium

AIM

Chronic stress exacerbates the symptoms of most pain disorders including interstitial cystitis/bladder pain syndrome (IC/BPS). Abnormalities in urothelial cells (UTC) occur in this debilitating bladder condition. The sequence of events that might link stress (presumably through increased sympathetic nervous system-SNS activity) to urothelial dysfunction are unknown. Since autonomic dysregulation, mitochondrial dysfunction, and oxidative stress all occur in chronic pain, we investigated whether chronic psychological stress initiated a cascade linking these three dysfunctions.

METHODS

Adult female Wistar Kyoto rats were exposed to 10 days of water avoidance stress (WAS). Bladders were then harvested for Western blot and single cell imaging in UTC cultures.

RESULTS

UTC from WAS rats exhibited depolarized mitochondria membrane potential (Ψm ∼30% more depolarized compared to control), activated AMPK and altered UT mitochondria bioenergetics. Expression of the fusion protein mitofusion-2 (MFN-2) was upregulated in the mucosa, suggesting mitochondrial structural changes consistent with altered cellular metabolism. Intracellular calcium levels were elevated in cultured WAS UTC, consistent with impaired cellular function. Stimulation of cultured UTC with alpha-adrenergic (α-AR) receptor agonists increased reactive oxidative species (ROS) production, suggesting a direct action of SNS activity on UTC. Treatment of rats with guanethidine to block SNS activity prevented most of WAS-induced changes.

CONCLUSIONS

Chronic stress results in persistent sympathetically mediated effects that alter UTC mitochondrial function. This may impact the urothelial barrier and signaling, which contributes to bladder dysfunction and pain. This is the first demonstration, to our knowledge, of a potential autonomic mechanism directly linking stress to mitochondrial dysfunction.

Applications of Spatio-temporal Mapping and Particle Analysis Techniques to Quantify Intracellular Ca2+ Signaling In Situ

Ca²⁺ imaging of isolated cells or specific types of cells within intact tissues often reveals complex patterns of Ca²⁺ signaling. This activity requires careful and in-depth analyses and quantification to capture as much information about the underlying events as possible. Spatial, temporal and intensity parameters intrinsic to Ca²⁺ signals such as frequency, duration, propagation, velocity and amplitude may provide some biological information required for intracellular signalling. High-resolution Ca²⁺ imaging typically results in the acquisition of large data files that are time consuming to process in terms of translating the imaging information into quantifiable data, and this process can be susceptible to human error and bias. Analysis of Ca²⁺ signals from cells in situ typically relies on simple intensity measurements from arbitrarily selected regions of interest (ROI) within a field of view (FOV). This approach ignores much of the important signaling information contained in the FOV. Thus, in order to maximize recovery of information from such high-resolution recordings obtained with Ca²⁺dyes or optogenetic Ca²⁺ imaging, appropriate spatial and temporal analysis of the Ca²⁺ signals is required. The protocols outlined in this paper will describe how a high volume of data can be obtained from Ca²⁺ imaging recordings to facilitate more complete analysis and quantification of Ca²⁺ signals recorded from cells using a combination of spatiotemporal map (STM)-based analysis and particle-based analysis. The protocols also describe how different patterns of Ca²⁺ signaling observed in different cell populations in situ can be analyzed appropriately. For illustration, the method will examine Ca²⁺ signaling in a specialized population of cells in the small intestine, interstitial cells of Cajal (ICC), using GECIs.

Automated analysis of dynamic Ca2+ signals in image sequences

Intracellular Ca(2+) signals are commonly studied with fluorescent Ca(2+) indicator dyes and microscopy techniques. However, quantitative analysis of Ca(2+) imaging data is time consuming and subject to bias. Automated signal analysis algorithms based on region of interest (ROI) detection have been implemented for one-dimensional line scan measurements, but there is no current algorithm which integrates optimized identification and analysis of ROIs in two-dimensional image sequences. Here an algorithm for rapid acquisition and analysis of ROIs in image sequences is described. It utilizes ellipses fit to noise filtered signals in order to determine optimal ROI placement, and computes Ca(2+) signal parameters of amplitude, duration and spatial spread. This algorithm was implemented as a freely available plugin for ImageJ (NIH) software. Together with analysis scripts written for the open source statistical processing software R, this approach provides a high-capacity pipeline for performing quick statistical analysis of experimental output. The authors suggest that use of this analysis protocol will lead to a more complete and unbiased characterization of physiologic Ca(2+) signaling.

Ca2+-imaging techniques to analyze Ca2+ signaling in cells and to monitor neuronal activity in the retina

Ca(2+) is an important regulator of many cell functions including proliferation, apoptosis, movements, secretion, contraction, excitation, and differentiation. The regulation of these different cell functions is encoded by the specific temporal and spatial distribution of Ca(2+) signals. In degenerative diseases mutations can lead to changes in cell functions in the worst case to apoptosis. Thus analysis of signals arising as changes in intracellular free Ca(2+) represent an important step towards the understanding of mutation-dependent or environmental impact into cell function. The classic approach to study changes in intracellular free Ca(2+) is the measurement of intracellular Ca(2+) by using Ca(2+)-sensitive fluorescence dyes in conjunction with fluorescence microscopy as a method called Ca(2+) imaging. In this chapter the basic method and a short theoretical background will be provided to perform Ca(2+)-imaging experiments. As a model cultured retinal pigment epithelial cells will be used. The basic steps of the method are the loading of the cells with the fluorescence dye by incubation with a membrane permeable ester of the dye. The next step would be the application of an agonist which can be further analyzed by blockers of enzymes or by manipulating the different Ca(2+)-storing compartments which contribute to changes in intracellular free Ca(2+). At the end of an experiment an on-cell type of calibration will be performed to calculate the underlying concentration of intracellular free Ca(2+). Furthermore, the successful calibration of an experiment can be used as a measure of a reliable experiment. In addition to that, three examples for basic experiments will be given which can lead to a first insight into the mechanism underlying changes in cytosolic free Ca(2+)as a second messenger.

Automated region of interest analysis of dynamic Ca²+ signals in image sequences

Ca(2+) signals are commonly measured using fluorescent Ca(2+) indicators and microscopy techniques, but manual analysis of Ca(2+) measurements is time consuming and subject to bias. Automated region of interest (ROI) detection algorithms have been employed for identification of Ca(2+) signals in one-dimensional line scan images, but currently there is no process to integrate acquisition and analysis of ROIs within two-dimensional time lapse image sequences. Therefore we devised a novel algorithm for rapid ROI identification and measurement based on the analysis of best-fit ellipses assigned to signals within noise-filtered image sequences. This algorithm was implemented as a plugin for ImageJ software (National Institutes of Health, Bethesda, MD). We evaluated the ability of our algorithm to detect synthetic Gaussian signal pulses embedded in background noise. The algorithm placed ROIs very near to the center of a range of signal pulses, resulting in mean signal amplitude measurements of 99.06 ± 4.11% of true amplitude values. As a practical application, we evaluated both agonist-induced Ca(2+) responses in cultured endothelial cell monolayers, and subtle basal endothelial Ca(2+) dynamics in opened artery preparations. Our algorithm enabled comprehensive measurement of individual and localized cellular responses within cultured cell monolayers. It also accurately identified characteristic Ca(2+) transients, or Ca(2+) pulsars, within the endothelium of intact mouse mesenteric arteries and revealed the distribution of this basal Ca(2+) signal modality to be non-Gaussian with respect to amplitude, duration, and spatial spread. We propose that large-scale statistical evaluations made possible by our algorithm will lead to a more efficient and complete characterization of physiologic Ca(2+)-dependent signaling.

Effect of milrinone analogues on intracellular calcium increase in single living H9C2 cardiac cells

The synthesis of milrinone analogues where the 4-pyridyl moiety was replaced by an ester or amide group is reported. Only amide derivatives are able to support intracellular calcium influx following chemical depolarization with 60 mM KCl in a percentage varying from 20 to 45% of differentiated H9C2 cardiomyocytes. Those cells were differentiated after chronic exposure to 10 nM retinoic acid which induces the expression of voltage-gated calcium channels. Analogues of milrinone containing an ester function did not show significant activity.

Acyloxymethyl esterification of nodularin-R and microcystin-LA produces inactive protoxins that become reactivated and produce apoptosis inside intact cells

We report the esterification of the carboxyl groups of the cyclic peptide toxins nodularin-R and microcystin-LA to produce stable diacetoxymethyl and dipropionyloxymethyl ester derivatives. The derivatives had no activity but were reactivated upon esterase treatment. When injected into cells, the acyloxymethyl moieties were cleaved off and apoptosis induced. Linking the acyloxymethyl-ester moiety of these potent toxins to carriers destined for endocytosis paves the way for selective apoptosis induction in target (e.g., cancer) cells.

The patterns of spontaneous Ca2+ signals generated by ventral spinal neurons in vitro show time-dependent refinement

Embryonic spinal neurons maintained in organotypic slice culture are known to mimic certain maturation-dependent signalling changes. With such a model we investigated, in embryonic mouse spinal segments, the age-dependent spatio-temporal control of intracellular Ca(2+) signalling generated by neuronal populations in ventral circuits and its relation with electrical activity. We used Ca(2+) imaging to monitor areas located within the ventral spinal horn at 1 and 2 weeks of in vitro growth. Primitive patterns of spontaneous neuronal Ca(2+) transients (detected at 1 week) were typically synchronous. Remarkably, such transients originated from widespread propagating waves that became organized into large-scale rhythmic bursts. These activities were associated with the generation of synaptically mediated inward currents under whole-cell patch-clamp. Such patterns disappeared during longer culture of spinal segments: at 2 weeks in culture, only a subset of ventral neurons displayed spontaneous, asynchronous and repetitive Ca(2+) oscillations dissociated from background synaptic activity. We observed that the emergence of oscillations was a restricted phenomenon arising together with the transformation of ventral network electrophysiological bursting into asynchronous synaptic discharges. This change was accompanied by the appearance of discrete calbindin immunoreactivity against an unchanged background of calretinin-positive cells. It is attractive to assume that periodic oscillations of Ca(2+) confer a summative ability to these cells to shape the plasticity of local circuits through different changes (phasic or tonic) in intracellular Ca(2+).

Cytokine-induced prostaglandin E2production and cyclooxygenase-2 expression in dental pulp cells: downstream calcium signalling via activation of prostaglandin EP receptor

AIM

To determine whether (i) proinflammatory cytokines stimulate prostaglandin E(2) (PGE(2)) production and cyclooxygenase (COX) gene expression in dental pulp cells, and (ii) pulp cells that express different prostaglandin E(2) receptor (EP) isoforms and their activation by PGE(2) leads to downstream Ca(2+) signalling.

METHODOLOGY

Cultured human dental pulp cells were exposed to interleukin (IL)-1beta and tumour necrotic factor-alpha (TNF-alpha). The expression of COX-1 and COX-2 was measured with reverse transcriptase-polymerase chain reaction (RT-PCR). The production of PGE(2) was measured using an enzyme-linked immunosorbent assay. Expression of prostaglandin EP receptor isoforms was studied by RT-PCR, whereas fura-2 fluorescence was used to measure calcium mobilization. The Kruskal-Wallis test and Wilcoxon sum rank test with Bonferroni correction were used for statistical analysis.

RESULTS

Interleukin-1beta and TNF-alpha stimulate PGE(2) production of human dental pulp cells (P < 0.05). IL-1beta stimulated the COX-2 but not COX-1 mRNA expression. Pulp cells express mainly EP2, EP3 and EP1 receptors as analysed by RT-PCR. PGE(2) (0.25-2 micromol L(-1)) stimulated the Ca(2+) mobilization as indicated by increase in fura-2 fluorescence.

CONCLUSIONS

Interleukin-1beta and TNF-alpha may stimulate PGE(2) production in dental pulp cells. Activation of prostaglandin EP receptors in dental pulp cells by PGE(2) may induce Ca(2+) signalling to regulate cellular biological activity during inflammation.

A Novel Method to Quantify Calcium Response Pattern and Oscillation Using Fura2 and Acridine Orange

To study calcium imaging data of cell populations that have various response patterns in peak amplitude and frequency of calcium oscillation in response to stimulation, comprehensive characterization based on statistical analysis of each response is important. In cultures of cells that are flat and in contact with each other, it is difficult to distinguish individual cells in calcium imaging data. We have developed a novel method to determine areas corresponding to individual cells in calcium imaging data. Rat neonatal cerebral astrocytes were filled with the calcium indicator Fura2, stained with acridine orange, and illuminated with UV light. The cell nuclei were clearly visualized. In addition, the images of these nuclei were useful for analyzing concentration-dependent alteration of calcium oscillation of cultured astrocytes in response to glutamate. This novel method may be useful for studying factors affecting calcium response patterns of cultured cell populations, including culture conditions, stimulus paradigms, and synthetic compounds.

Calcium - how and why?

Calcium is among the most commonly used ions, in a multitude of biological functions, so much so that it is impossible to imagine life without calcium. In this article I have attempted to address the question as to how calcium has achieved this status with a brief mention of the history of calcium research in biology. It appears that during the origin and early evolution of life the Ca2+ ion was given a unique opportunity to be used in several biological processes because of its unusual physical and chemical properties.

Calcium imaging in live rat optic nerve myelinated axons in vitro using confocal laser microscopy

Intracellular Ca(2+) plays a major role in the physiological responses of excitable cells, and excessive accumulation of internal Ca(2+) is a key determinant of cell injury and death. Many studies have been carried out on the internal Ca(2+) dynamics in neurons. In constrast, there is virtually no such information for mammalian central myelinated axons, due in large part to technical difficulty with dye loading and imaging such fine myelinated structures. We developed a technique to allow imaging of ionized Ca(2+) in live rat optic nerve axons with simultaneous electrophysiological recording in vitro at 37 degrees C using confocal microscopy. The K(+) salt of the Ca(2+)-sensitive indicator Oregon Green 488 BAPTA-2 and the Ca(2+)-insensitive reference dye Sulforhodamine 101 were loaded together into rat optic nerves using a low-Ca(2+)/low-Na(+) solution. Axonal profiles, confirmed immunohistochemically by double staining with neurofilament-160 antibodies, were clearly visualized by S101 fluorescence up to 800 microm from the cut ends. The Ca(2+) signal was very low at rest, just above the background fluorescence intensity, indicating healthy tissue, and increased significantly after caffeine (20 mM) exposure designed to release internal Ca(2+) stores. The health of imaged regions was further confirmed by a virtual absence of spectrin breakdown, which is induced by calpain activation in damaged CNS tissue. Red and green fluorescence decayed to no less than 70% of control after 60 min of recording at 37 degrees C, with the green:red fluorescence ratio increasing slightly by 21% after 60 min. Electrophysiological responses recorded simultaneously with confocal images remained largely stable as well.

Concentrations of caffeine greater than 20 mM increase the indo-1 fluorescence ratio in a Ca(2+)-independent manner

The methylxanthine, caffeine, quenches the fluorescence of the ratiometric Ca2+ indicator indo-1, but does not affect the ratio (R) of indo-1 fluorescence at 400 and 500 nm in the presence of caffeine concentrations up to 10 mM [1]. We have found that when caffeine is at concentrations of 20 mM or greater in vitro, or in saponinpermeabilized skeletal muscle fibers, a Ca(2+)-independent increase in R occurs, which leads to an overestimation of the free Ca2+ concentration. Depending on experimental conditions, two factors contribute to the alteration in R in vitro. First, when indo-1 fluorescence is low, fluorescence by caffeine, at 400 nm, can be significant. A second, and more dramatic effect, is that quenching of indo-1 fluorescence by 20-50 mM caffeine is dissimilar at 400 and 500 nm. Quenching at 500 nm is not linear, with respect to the concentration of caffeine, and causes a Ca(2+)-independent increase in R, that occurs even when the fluorescence of caffeine is a small portion of total fluorescence. However, unlike R, the Ca2+ calibration constant of indo-1, KD beta, is unchanged in 50 mM caffeine. Therefore, an accurate quantitation of Ca2+ in the presence of even high concentrations of caffeine can be made in vitro by determining the Ca2+ calibration factors of indo-1 (RMIN and RMAX) for each caffeine concentration. These effects of concentrations of caffeine greater than 20 mM are not observed in intact cells loaded with the cell permeant form of indo-1 when caffeine is applied extracellularly. This suggests either that the concentration of caffeine within the cell does not reach that necessary to produce the effect, or that the effects of caffeine on the dye are modified by the environment within the cell.

Neuronal free calcium measurement using BTC/AM, a low affinity calcium indicator

BTC is a low affinity calcium indicator (Kd approximately 7-26 microM) featuring many desirable properties for cellular calcium imaging, including long excitation wavelengths (400/485 nm), low sensitivity to Mg2+, and accuracy of ratiometric measurement [Iatridou H., Foukaraki E., Kuhn M.A., Marcus E.M., Haugland R.P., Katerinopoulos H.E. The development of a new family of intracellular calcium probes. Cell Calcium 1994; 15: 190-198]. To assess the usefulness of this indicator in cultured neurons, we examined properties of BTC and its acetoxymethyl ester, BTC/AM. BTC/AM had substantial calcium-independent fluorescence at all excitation wavelengths. BTC/AM was readily loaded into neurons and was rapidly hydrolysed. There was little dye compartmentalization, as assessed by digitonin lysis, Co2+ quenching of BTC fluorescence and by confocal microscopy. Despite adequate loading, BTC gradually became unresponsive to [Ca2+]i when cultures were examined under routine imaging conditions. This effect was a function of the cumulative fluorescence illumination and could be minimized by attenuating light intensity or duration. Ratio imaging after exposure of neuronal cultures to 1-50 microM ionomycin revealed distinct sensitivity ranges for BTC and Fura-2. BTC reported graded neuronal [Ca2+]i responses to glutamate receptor stimulation with N-methyl-D-aspartate in the range 10-50 microM, whereas Fura-2 did not distinguish between these stimuli. Under appropriate loading and illumination conditions, bath-loaded BTC/AM may be well suited for measurement of moderate to high calcium concentrations in cultured neurons.

Calcium imaging: a primer for mycologists

This article aims to encourage more fungal biologists to consider the imaging of cytoplasmic Ca2+ fluxes. Compared to other organisms, for fungi there have been remarkably few attempts to characterize the role of Ca2+ fluxes in signal transduction and general cellular activities, even though other approaches indicate that fungal growth and development are highly dependent upon Ca2+. The methodologies for imaging Ca2+ fluxes continue to develop rapidly. These methodologies are explained here in a style that should be accessible to a newcomer to the field, hopefully forming a bridge to the more complex methodological literature.

Confocal imaging of intracellular chloride in living brain slices: measurement of GABAA receptor activity

We have developed a method using UV laser-scanning confocal microscopy and the fluorescent chloride ion indicator, 6-methoxy-N-ethylquinolinium chloride (MEQ), to image GABA-mediated changes in intracellular chloride (Cli-) in individual neurons of the rat acute brain slice. After bath-loading slices with the cell-permeant form (reduced) of MEQ, there was intense fluorescence within neurons of diverse morphologies in the hippocampus, neocortex and cerebellum. MEQ fluorescence localized to the cytosolic compartment of both the somata and proximal dendrites. MEQ fluorescence was calibrated using the ionophores nigericin and tributyltin in the presence of varying extracellular Cl- concentrations. Neuronal MEQ fluorescence was inversely related to intracellular Cl-, with a Stern-Volmer constant of 16 M-1 (50% quench by 61 mM Cl-). Application of GABA in the perfusate produced a concentration-dependent decrease in MEQ fluorescence (EC50 = 40 microM) that was blocked in the presence of the Cl- channel antagonist, picrotoxin. Bath perfusion of hippocampal slices with modulators of the GABAA receptor, pentobarbital and diazepam, potentiated the GABA-mediated response by 85 and 44%, respectively. A regional comparison identified larger GABA responses for both cerebellar Purkinje and granule cells relative to pyramidal neurons of the hippocampus and neocortex and to hippocampal interneurons. Pressure ejection of the GABAA agonist, muscimol (40 microM), from a micropipet onto individual hippocampal neurons allowed the measurement of rapid responses (1-5 s), compared to those obtained with bath application. Thus, optical imaging of [Cl-]i using MEQ and UV-laser-scanning confocal microscopy provides investigators with a new method to study GABAA pharmacology in neighboring neurons and perhaps even in the soma versus dendrites simultaneously, within living brain slices.

Non-hyperbolic calcium calibration curve of Fura-2: implications for the reliability of quantitative Ca2+ measurements

The fluorescence probe Fura-2 is widely applied for the quantitative determination of cellular free Ca2+ concentration. Generally, a hyperbolic calibration curve has been taken as a basis. The in vitro calibration of Fura-2 performed with free Ca2+ concentrations ranging from 10 nM to about 9 mM demonstrates a non-hyperbolic curve. Assuming two Ca2+ binding sites of Fura-2 dissociation constants of 190 nM and 176 microM were estimated. The analysis of the calibration data by the equation introduced by Grynkiewicz et al. [Grynkiewicz G., Poenie M., Tsien R.Y. A new generation of Ca2+ indicators with greatly improved fluorescence properties. J Biol Chem 1985; 260: 3440-3450] shows that the inclusion of Ca2+ concentrations in the range of the low-affinity binding site leads to an overestimation of the dissociation constant normally used within this method. The Rmax value conventionally estimated at a nominal saturating free Ca2+ concentration increases if concentrations in the range of the low-affinity binding site are selected for this purpose. Provided that millimolar free Ca2+ concentrations are applied for the determination of Rmax, the experimentally-estimated free Ca2+ concentration should be considerably lower than the real cellular concentration. This bias is especially to be expected if free Ca2+ concentrations exceeding the high-affinity Ca2+ binding site of Fura-2 are measured. With the use of approximately 10 microM free Ca2+ for the determination of Rmax the contribution of the low-affinity binding site is negligible. Data for cellular free Ca2+ calculated on the basis of Rmax values estimated with Ca2+ concentrations in the millimolar range should be considered with caution.