Calcium oscillations in electrically non-excitable cells

Acupuncture and Static Multipolar Magnets: An Emerging Attraction?

Objective

Magnetism has been known for >4,000 years. Recently static multipolar magnets have demonstrated analgesic clinical usefulness. Local application of magnets may be beneficial in reducing musculoskeletal pain, particularly when other modalities have failed. A recent series of clinical cases demonstrates how multipolar magnets may be incorporated as an effective adjunctive treatment in an acupuncture clinic.

Materials and Methods

PubMed database was searched using the key words: magnets, medical magnets, magnets and pain management, therapeutic magnets, multipolar magnets, and history of magnet therapy. In addition, clinical cases were submitted by 4 different medical acupuncturists as examples of how the use of multipolar magnets is incorporated into an acupuncture clinic.

Results

Over the past 20 years, 143 articles fulfilled the search criteria and unfortunately demonstrated considerable variability in research methodology. Magnetic tapes, needles, and beads of various magnetic strengths constituted the stimulating apparatus with durations ranging from minutes to years. This article highlights 10 cases, 9 of which reflected situations in which the use of 1 or more multipolar magnets provided an enhanced analgesic effect, often when traditional acupuncture had either failed to produce a satisfactory response or when the application of acupuncture needles needed to be limited.

Conclusion

Despite the variability of the literature review, it appears that magnetism is related to pain reduction, and when properly employed, it can be an effective and safe modality as demonstrated by a recent series of cases submitted from the practices of 4 different medical acupuncturists. A clinical trial incorporating the latest technology of multipolar magnets with steep field gradients should be initiated for the more formal investigation of magnet-induced analgesia.

Genetically Encoded Fluorescent Biosensors Illuminate the Spatiotemporal Regulation of Signaling Networks

Cellular signaling networks are the foundation which determines the fate and function of cells as they respond to various cues and stimuli. The discovery of fluorescent proteins over 25 years ago enabled the development of a diverse array of genetically encodable fluorescent biosensors that are capable of measuring the spatiotemporal dynamics of signal transduction pathways in live cells. In an effort to encapsulate the breadth over which fluorescent biosensors have expanded, we endeavored to assemble a comprehensive list of published engineered biosensors, and we discuss many of the molecular designs utilized in their development. Then, we review how the high temporal and spatial resolution afforded by fluorescent biosensors has aided our understanding of the spatiotemporal regulation of signaling networks at the cellular and subcellular level. Finally, we highlight some emerging areas of research in both biosensor design and applications that are on the forefront of biosensor development.

The ARC channel--an endogenous store-independent Orai channel

Although Orai channels and their regulator stromal interacting molecule 1 (STIM1) were originally identified and described as the key components of the store-operated highly calcium-selective CRAC channels, it is now clear that these proteins are equally essential components of the agonist-activated, store-independent calcium entry pathway mediated by the arachidonic acid-regulated calcium-selective (ARC) channel. Correspondingly, ARC channels display biophysical properties that closely resemble those of CRAC channels but, whereas the latter is formed exclusively by Orai1 subunits, the ARC channel is formed by a combination of Orai1 and Orai3 subunits. Moreover, while STIM1 in the membrane of the endoplasmic reticulum is the critical sensor of intracellular calcium store depletion that results in the activation of the CRAC channels, it is the pool of STIM1 resident in the plasma membrane that regulates the activity of the store-independent ARC channels. Here, we describe the unique features of the ARC channels and their activation and discuss recent evidence indicating how these two coexisting, and biophysically very similar, Orai channels act to play entirely distinct roles in the regulation of various important cellular activities.

Orai3--the 'exceptional' Orai?

The field of agonist-activated Ca(2+) entry in non-excitable cells underwent a revolution some 5 years ago with the discovery of the Orai proteins as the essential pore-forming components of the low-conductance, highly Ca(2+)-selective CRAC channels whose activation is dependent on depletion of intracellular stores. Mammals possess three distinct Orai proteins (Orai1, 2 and 3) of which Orai3 is unique to this class, apparently evolving from Orai1. However, the sequence of Orai3 shows marked differences from that of Orai1, particularly in those regions of the protein outside of the essential pore-forming domains. Correspondingly, studies from several different groups have indicated that the inclusion of Orai3 is associated with the appearance of conductances that display unique features in their gating, selectivity, regulation and mode of activation. In this Topical Review, these features are discussed with the purpose of proposing that the evolutionary appearance of Orai3 in mammals, and the consequent development of conductances displaying novel properties - whether formed by Orai3 alone or in conjunction with the other Orai proteins - is associated with the specific role of this member of the Orai family in a unique range of distinct cellular activities.

Using Jensen's inequality to explain the role of regular calcium oscillations in protein activation

Oscillations of cytosolic Ca(2 +) are very important for cellular signalling in excitable and non-excitable cells. The information of various extracellular stimuli is encoded into oscillating patterns of Ca(2 +) that subsequently lead to the activation of different Ca(2 +)-sensitive target proteins in the cell. The question remains, however, why this information is transmitted by means of an oscillating rather than a constant signal. Here we show that, in fact, Ca(2 +) oscillations can achieve a better activation of target proteins than a comparable constant signal with the same amount of Ca(2 +) used. For this we use Jensen's inequality that describes the relation between the function value of the average of a set of argument values and the average of the function values of the arguments from that set. We analyse the role of the cooperativity of the binding of Ca(2 +) and of zero-order ultrasensitivity, which are two properties that are often observed in experiments on the activation of Ca(2 +)-sensitive target proteins. Our results apply to arbitrary oscillation shapes and a very general decoding model, thus generalizing the observations of several previous studies. We compare our results with data from experimental studies investigating the activation of nuclear factor of activated T cells (NFAT) and Ras by oscillatory and constant signals. Although we are restricted to specific approximations due to the lack of detailed kinetic data, we find good qualitative agreement with our theoretical predictions.

Substrate rigidity regulates Ca2+ oscillation via RhoA pathway in stem cells

Substrate rigidity plays crucial roles in regulating cellular functions, such as cell spreading, traction forces, and stem cell differentiation. However, it is not clear how substrate rigidity influences early cell signaling events such as calcium in living cells. Using highly sensitive Ca(2+) biosensors based on fluorescence resonance energy transfer (FRET), we investigated the molecular mechanism by which substrate rigidity affects calcium signaling in human mesenchymal stem cells (HMSCs). Spontaneous Ca(2+) oscillations were observed inside the cytoplasm and the endoplasmic reticulum (ER) using the FRET biosensors targeted at subcellular locations in cells plated on rigid dishes. Lowering the substrate stiffness to 1 kPa significantly inhibited both the magnitudes and frequencies of the cytoplasmic Ca(2+) oscillation in comparison to stiffer or rigid substrate. This Ca(2+) oscillation was shown to be dependent on ROCK, a downstream effector molecule of RhoA, but independent of actin filaments, microtubules, myosin light chain kinase, or myosin activity. Lysophosphatidic acid, which activates RhoA, also inhibited the frequency of the Ca(2+) oscillation. Consistently, either a constitutive active mutant of RhoA (RhoA-V14) or a dominant negative mutant of RhoA (RhoA-N19) inhibited the Ca(2+) oscillation. Further experiments revealed that HMSCs cultured on gels with low elastic moduli displayed low RhoA activities. Therefore, our results demonstrate that RhoA and its downstream molecule ROCK may mediate the substrate rigidity-regulated Ca(2+) oscillation, which determines the physiological functions of HMSCs.

The triggering of astrocytic calcium waves by NMDA-induced neuronal activation

It has been well established that astrocytes possess functional receptors for the excitatory neurotransmitter glutamate and respond to physiological concentrations of this substance with oscillations in cytoplasmic Ca2+ concentrations and spatially propagating Ca2+ waves. These findings strongly suggest that glutamate released during synaptic transmission triggers such phenomena within the perisynaptic astrocyte in situ. We test this hypothesis in two preparations, the organotypic hippocampal slice and hippocampal neuron-astrocyte co-cultures, using the Ca2+ indicator fluo-3 and confocal laser microscopy. An agonist for the N-methyl-D-aspartate (NMDA)-preferring glutamate receptor is employed to stimulate neuronal populations specifically, leaving the astrocytic population unaffected as these cells appear to lack this glutamate receptor subtype. Such pharmacological stimulation initially elicits large Ca2+ transients within the neuronal populations, followed by Ca2+ spikes in surrounding astrocytes, presumably as the result of neuronal glutamate release. During continuous neuronal stimulation, the astrocyte's Ca2+ response becomes oscillatory, with a period averaging 33 s and ranging from 15 to 50 s at 21 degrees C. These findings establish another form of communication within the brain, that between neurons and astrocytes, which perhaps acts to couple astrocytic regulatory responses to neuronal activity.

Twenty years of calcium imaging: cell physiology to dye for

The use of fluorescent dyes over the past two decades has led to a revolution in our understanding of calcium signaling. Given the ubiquitous role of Ca(2+) in signal transduction at the most fundamental levels of molecular, cellular, and organismal biology, it has been challenging to understand how the specificity and versatility of Ca(2+) signaling is accomplished. In excitable cells, the coordination of changing Ca(2+) concentrations at global (cellular) and well-defined subcellular spaces through the course of membrane depolarization can now be conceptualized in the context of disease processes such as cardiac arrhythmogenesis. The spatial and temporal dimensions of Ca(2+) signaling are similarly important in non-excitable cells, such as endothelial and epithelial cells, to regulate multiple signaling pathways that participate in organ homeostasis as well as cellular organization and essential secretory processes.

Ca2+ release from host intracellular stores and related signal transduction during Campylobacter jejuni 81-176 internalization into human intestinal cells

Campylobacter jejuni is the leading bacterial cause of human diarrhoeal disease in many parts of the world, including the USA. The ability of C. jejuni to invade the host intestinal epithelium is an important determinant of virulence. A common theme among pathogenic invasive micro-organisms is their ability to usurp the eukaryotic cell-signalling systems both to allow for invasion and to trigger disease pathogenesis. Ca(2+) is very important in a great variety of eukaryotic cell-signalling processes (e.g. calmodulin-activated enzymes, nuclear transcriptional upregulation, and cytoskeletal rearrangements). This study analyses the effects of Ca(2+) availability on invasion of human INT407 intestinal epithelial cells by C. jejuni strain 81-176. The ability of C. jejuni to invade INT407 cells was not blocked by chelation of any remaining extracellular Ca(2+) from host cells incubated in Ca(2+)-free, serum-free media. In contrast, C. jejuni invasion was markedly reduced either by chelating host intracellular Ca(2+) with 1,2-bis-(2-)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA, AM) or by blocking the release of Ca(2+) from intracellular stores with dantrolene or U73122. Moreover, Bay K8644, a plasma-membrane Ca(2+)-channel agonist, was observed to stimulate C. jejuni invasion, presumably by increasing host intracellular free Ca(2+) levels. Measurement of host-cell cytosolic Ca(2+) via spectrofluorimetry and fluorescence microscopy revealed an increase in Ca(2+) from 10 min post-infection. Monolayer pretreatment with either a calmodulin antagonist or a specific inhibitor of protein kinase C was found to cause a marked reduction in C. jejuni invasion, suggesting roles for these Ca(2+)-activated modulators in signal-transduction events involved in C. jejuni invasion. These results demonstrate that C. jejuni induces the mobilization of Ca(2+) from host intracellular stores, which is an essential step in the invasion of intestinal cells by this pathogen.

oscillations in a model of energy-dependent uptake by the endoplasmic reticulum

Active Ca2+ transport in living cells necessitates controlled supply of metabolic energy. Direct coupling between sarco/endoplasmic reticulum (ER) Ca2+ ATPases (SERCA) and intracellular energy-generation sites has been well established experimentally. On the basis of these experimental findings we propose a pump-driven model to investigate complex dynamic properties of a cell system. The model describes the pump process both by the Ca2+ ATPase itself and by a suitable description of the glycolysis. The associated set of differential equations shows a rich behavior, the solutions ranging from simple periodic oscillations to complex patterns such as bursting and spiking. Recent experimental results on calcium oscillations in Xenopus laevis oocytes and on dynamic patterns of intracellular Ca2+ concentrations in electrically non-excitable cells are well described by corresponding theoretical results derived within the proposed model. The simulation results are further compared to spontaneous [Ca2+] oscillations in primitive endodermal cells.

Calcium wave pacemakers in eggs

During the past 25 years, the characterization of sperm-triggered calcium signals in eggs has progressed from the discovery of a single calcium increase at fertilization in the medaka fish to the observation of repetitive calcium waves initiated by multiple meiotic calcium wave pacemakers in the ascidian. In eggs of all animal species, sperm-triggered inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] production regulates the vast array of calcium wave patterns observed in the different species. The spatial organization of calcium waves is driven either by the intracellular distribution of the calcium release machinery or by the localized and dynamic production of calcium-releasing second messengers. In the highly polarized egg cell, cortical endoplasmic reticulum (ER)-rich clusters act as pacemaker sites dedicated to the initiation of global calcium waves. The extensive ER network made of interconnected ER-rich domains supports calcium wave propagation throughout the egg. Fertilization triggers two types of calcium wave pacemakers depending on the species: in mice, the pacemaker site in the vegetal cortex of the egg is probably a site that has enhanced sensitivity to Ins(1,4,5)P(3); in ascidians, the calcium wave pacemaker may rely on a local source of Ins(1,4,5)P(3) production apposed to a cluster of ER in the vegetal cortex.

Comparison of the effect of ATP on intracellular calcium ion dynamics between rat testicular and cerebral arteriole smooth muscle cells

Adenosine 5'-triphosphate (ATP), when released from neuronal and non-neuronal tissues, interacts with cell surface receptors produces a broad range of physiological responses. The goal of the present study was to examine the issue of whether vascular smooth muscle cells respond to ATP. To this end, the dynamics of the intracellular concentration of calcium ions ([Ca(2+)](i)) in smooth muscle cells in testicular and cerebral arterioles was examined by laser scanning confocal microscopy. ATP produced an increase in [Ca(2+)](i) in arteriole smooth muscle cells. While P1 purinoceptor agonists had no effect on this process, P2 purinoceptor agonists induced a [Ca(2+)](i) increase and a P2 purinoceptor antagonist, suramin, completely inhibited ATP-induced [Ca(2+)](i) dynamics in both arteriole smooth muscle cells. In testicular arterioles, Ca(2+) channel blockers and the removal of extracellular Ca(2+), but not thapsigargin pretreatment, abolished the ATP-induced [Ca(2+)](i) dynamics. In contrast, Ca(2+) channel blockers and the removal of extracellular Ca(2+) did not completely inhibit ATP-induced [Ca(2+)](i) dynamics in cerebral arterioles. Uridine 5'-triphosphate caused an increase in [Ca(2+)](i) only in cerebral arterioles and alpha,beta-methylene ATP caused an increase in [Ca(2+)](i) in both testicular and cerebral arterioles. We conclude that testicular arteriole smooth muscle cells respond to extracellular ATP via P2X purinoceptors and that cerebral arteriole smooth muscle cells respond via P2X and P2Y purinoceptors.

Modelling of simple and complex calcium oscillations. From single-cell responses to intercellular signalling

This review provides a comparative overview of recent developments in the modelling of cellular calcium oscillations. A large variety of mathematical models have been developed for this wide-spread phenomenon in intra- and intercellular signalling. From these, a general model is extracted that involves six types of concentration variables: inositol 1,4,5-trisphosphate (IP3), cytoplasmic, endoplasmic reticulum and mitochondrial calcium, the occupied binding sites of calcium buffers, and the fraction of active IP3 receptor calcium release channels. Using this framework, the models of calcium oscillations can be classified into 'minimal' models containing two variables and 'extended' models of three and more variables. Three types of minimal models are identified that are all based on calcium-induced calcium release (CICR), but differ with respect to the mechanisms limiting CICR. Extended models include IP3--calcium cross-coupling, calcium sequestration by mitochondria, the detailed gating kinetics of the IP3 receptor, and the dynamics of G-protein activation. In addition to generating regular oscillations, such models can describe bursting and chaotic calcium dynamics. The earlier hypothesis that information in calcium oscillations is encoded mainly by their frequency is nowadays modified in that some effect is attributed to amplitude encoding or temporal encoding. This point is discussed with reference to the analysis of the local and global bifurcations by which calcium oscillations can arise. Moreover, the question of how calcium binding proteins can sense and transform oscillatory signals is addressed. Recently, potential mechanisms leading to the coordination of oscillations in coupled cells have been investigated by mathematical modelling. For this, the general modelling framework is extended to include cytoplasmic and gap-junctional diffusion of IP3 and calcium, and specific models are compared. Various suggestions concerning the physiological significance of oscillatory behaviour in intra- and intercellular signalling are discussed. The article is concluded with a discussion of obstacles and prospects.

Recent advances in chemical approaches to the study of biological systems

A number of novel chemical methods for studying biological systems have recently been developed that provide a means of addressing biological questions not easily studied with other techniques. In this review, examples that highlight the development and use of such chemical approaches are discussed. Specifically, strategies for modulating protein activity or protein-protein interactions using small molecules are presented. In addition, methods for generating and utilizing novel biomolecules (proteins, oligonucleotides, oligosaccharides, and second messengers) are examined.

Alpha 1-adrenergic modulation of metabotropic glutamate receptor-induced calcium oscillations and glutamate release in astrocytes

Astrocytic responses to activation of metabotropic glutamate receptors group I (mGluRs I) and alpha(1)-adrenoreceptors in cultured cells have been assessed using spectral analyzes and calcium imaging. Concentration-dependent changes were observed after stimulation with the mGluR I agonist (S)-3,5-dihydroxyphenylglycine (DHPG). These responses changed from a regular low frequency signal with sharp peaks at 1 microm to a pronounced stage of irregularity at 10 microm. After stimulation with 100 microm the signal was again homogenous in shape and regularity but occurred at a higher frequency. In contrast, the spectral properties after stimulation with the alpha(1)-adrenoreceptor agonist phenylephrine, exhibited considerable variation for all investigated concentrations. DHPG-induced increases in [Ca(2+)](i) were also associated with astroglial glutamate release, whereas no release was observed after noradrenergic stimulation. Both DHPG-mediated calcium signaling and glutamate release were inhibited by preincubation with 10 or 100 microm phenylephrine. Collectively, the present investigation provides new information about the spatial-temporal characteristics of astroglial intracellular calcium responses and demonstrates distinct differences between noradrenergic and glutamatergic receptors regarding intracellular calcium signaling and coupling to glutamate release. The noradrenergic modulation of DHPG-induced responses indicates that intracellular astroglial processes can be regulated in a bi-directional feedback loop between closely connected astrocytes and neurons in the central nervous system.

Identification and characterization of muscarinic acetylcholine receptor subtypes expressed in human skin melanocytes

The present study was designed to identify and characterize muscarinic acetylcholine receptors in normal human melanocytes. We used subtype-specific oligonucleotide primers to localize the five genetically defined mAChR mRNAs (ml through m5) by reverse transcription-polymerase chain reaction. These experiments showed that all five mAChR subtype mRNAs are expressed in melanocytes. The PCR products were verified by restriction analysis and Southern blotting. Receptors were visualized in cultures of normal human melanocytes and specimens of normal human skin by subtype-specific rabbit anti-receptor polyclonal antibodies. Radioligand binding assays with the lipophilic drug [3H]quinuclidinyl benzilate demonstrated approximately 9,000 high affinity binding sites/cell. Micromolar concentrations of muscarine or carbachol transiently increased intracellular Ca2+, which could be attenuated by atropine, demonstrating coupling of the receptors to mobilization of intracellular free Ca2+. Lower concentrations of muscarine induced spontaneous repetitive spike-like increases of intracellular Ca2+ which is characteristic for the activation of muscarinic receptors. These results indicate that normal human skin melanocytes express the ml, m2, m3, m4, and m5 subtypes of classic muscarinic acetylcholine receptors on their cell membrane and that these receptors regulate the concentration of intracellular free Ca2+, which may play an important physiologic role in melanocyte behavior and skin pigmentation.

Vascular endothelial growth factor increases Rana vascular permeability and compliance by different signalling pathways

Vascular endothelial growth factor (VEGF) chronically increases microvascular permeability, compliance and vessel diameter. To determine the signalling pathways by which VEGF exerts these effects, we investigated the role of Ca2+ influx and mitogen-activated protein kinase (MAPK) phosphorylation on the increase in hydraulic conductivity (Lp), diameter and compliance in mesenteric microvessels in the anaesthetised frog (Rana species). The VEGF-mediated chronically increased permeability was attenuated by co-perfusion of VEGF with 5 mM NiCl2, previously shown to inhibit Ca2+ influx. MAPK phosphorylation inhibition by PD98059 did not affect the chronic increase in Lp. To determine whether other agonists which increased Ca2+ influx also chronically increased Lp, the effect of ATP perfusion on chronic Lp was measured. ATP perfusion also chronically increased Lp. The chronic increase in Lp was therefore dependent on an initial transient Ca2+ influx, and not MAPK activation, and was not unique to VEGF stimulation. Inhibition of Ca2+ influx did not inhibit the increase in microvascular diameter or compliance brought about by VEGF. Both these increases were inhibited by PD98059. The VEGF-mediated increase in compliance and diameter was therefore dependent on MAPK activation, not on Ca2+ influx. The chronic increase in Lp stimulated by VEGF perfusion 24 h previously was reduced when the vessel was perfused with 5 mM NiCl2. The sustained, high Lp was therefore dependent on Ca2+ influx. The endothelial cell calcium concentration ([Ca2+]i) of vessels previously perfused with VEGF or ATP, and with a chronically increased Lp, was not significantly increased compared to [Ca2+]i of endothelial cells in vessels before agonist perfusion These experiments show that VEGF acts through different pathways to stimulate increased permeability and compliance. The data are consistent with the hypothesis that VEGF chronically increases Lp through an acute stimulation of Ca2+ influx, but increases compliance and diameter by acute stimulation of the MAPK signalling pathway. They also suggest that the increase in Lp is dependent on a sustained Ca2+ influx, even though the endothelial [Ca2+]i is not raised.

VEGF and ATP act by different mechanisms to increase microvascular permeability and endothelial [Ca(2+)](i)

Vascular endothelial growth factor (VEGF) increases hydraulic conductivity (L(p)) by stimulating Ca(2+) influx into endothelial cells. To determine whether VEGF-mediated Ca(2+) influx is stimulated by release of Ca(2+) from intracellular stores, we measured the effect of Ca(2+) store depletion on VEGF-mediated increased L(p) and endothelial intracellular Ca(2+) concentration ([Ca(2+)](i)) of frog mesenteric microvessels. Inhibition of Ca(2+) influx by perfusion with NiCl(2) significantly attenuated VEGF-mediated increased [Ca(2+)](i). Depletion of Ca(2+) stores by perfusion of vessels with thapsigargin did not affect the VEGF-mediated increased [Ca(2+)](i) or the increase in L(p). In contrast, ATP-mediated increases in both [Ca(2+)](i) and L(p) were inhibited by thapsigargin perfusion, demonstrating that ATP stimulated store-mediated Ca(2+) influx. VEGF also increased Mn(2+) influx after perfusion with thapsigargin, whereas ATP did not. These data showed that VEGF increased [Ca(2+)](i) and L(p) even when Ca(2+) stores were depleted and under conditions that prevented ATP-mediated increases in [Ca(2+)](i) and L(p). This suggests that VEGF acts through a Ca(2+) store-independent mechanism, whereas ATP acts through Ca(2+) store-mediated Ca(2+) influx.

Histamine-induced Ca2+ oscillations in a human endothelial cell line depend on transmembrane ion flux, ryanodine receptors and endoplasmic reticulum Ca2+-ATPase

Using single cell microfluorometry to monitor changes in bulk Ca2+ concentration ([Ca2+]bulk) and the whole-cell configuration of the patch clamp technique to measure K+ currents (voltage clamp) and membrane potential (current clamp), the mechanisms of histamine-induced Ca2+ oscillations in the umbilical vein endothelial cell-derived cell line EA.hy926 were studied. In single cells, histamine (10 microM) evoked sinusoidal Ca2+ oscillations in low extracellular Ca2+ concentrations ([Ca2+]o = 10-30 microM). In contrast, histamine did not initiate Ca2+ oscillations either in the absence of extracellular Ca2+ (10 microM EGTA) or in the presence of 2.5 mM extracellular Ca2+. Ca2+ oscillations were accompanied by rhythmic activation of Ca2+-activated K+ (KCa) channels and membrane hyperpolarization of 18.1 +/- 3.9 mV. Hence, cell depolarization with 70 mM extracellular K+ or the inhibition of non-selective cation channels (NSCCs) and KCa channels by 10 microM Loe 908 and 10 mM tetrabutylammonium prevented histamine-evoked Ca2+ oscillations. Preventing Na+-Ca2+ exchange (NCX) by 10 microM 2', 4'-dichlorobenzamil, or removal of extracellular Na+, abolished histamine-induced Ca2+ oscillations. Lowering the extracellular Na+ concentration and thus promoting the reversed mode of NCX (3Na+ out and 1Ca2+ in) increased the amplitude and frequency of histamine-induced Ca2+ oscillations by 25 and 13 %, respectively. Hence, in the absence of extracellular Ca2+, 10 microM histamine induced an elevation of intracellular Na+ concentration in certain subplasmalemmal domains. The inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 2,5-di-tert-butyl-1, 4-benzo-hydroquinone (15 microM) prevented histamine-induced Ca2+ oscillations. In addition, blockage of ryanodine-sensitive Ca2+ release (RsCR) by 25 microM ryanodine blunted Ca2+ oscillations. In endothelial cells that were treated for 16 h with 10 microM nocodazole to collapse the superficial endoplasmic reticulum (sER), no histamine-induced Ca2+ oscillations were found. We conclude that in low [Ca2+]o conditions histamine-induced Ca2+ oscillations depend on transmembrane Na+ loading through NSCCs that leads to Ca2+ entry via NCX. Cation influx is controlled by KCa channel activity that triggers membrane hyperpolarization and, thus, provides the driving force for cation influx. Hence, the Ca2+ entering needs to be sequestrated via SERCA into sER to become released by RsCR to evoke Ca2+ spiking. These data further support our previous work on localized Ca2+ signalling as a key phenomenon in endothelial Ca2+ homeostasis.

Calcium transient activity in cultured murine neural crest cells is regulated at the IP(3) receptor

In a previous study we have shown that cultured neural crest cells exhibit spontaneous calcium transients and that these events are required for neurogenesis. In this study, we examine the mechanism that generates these calcium transients. Extracellular Ca(2+) modulates calcium transient activity. Lanthanum (La(3+)), a general calcium channel antagonist and zero extracellular Ca(2+), reduces the percentage of cells exhibiting calcium transients (26.2 and 40. 5%, respectively) and decreases calcium spiking frequency (4.5 to 1. 0 and 2.5 to 1.0 spikes/30 min, respectively). Intracellular calcium stores also contribute to the generation of calcium transients. Depleting the calcium stores of the endoplasmic reticulum (ER) reduces the percentage of active cells (15.7%) and calcium spiking frequency (2.8 to 1.5 spikes/30 min). Ryanodine (100 microM), which blocks calcium release regulated by the ryanodine receptor (RyR), had no effect on calcium transient activity. Blocking inositol 1,4, 5-triphosphate receptor (IP(3)R)-dependent calcium release, with elevated extracellular Mg(2+) (20 mM), abolished calcium transient activity. Mg(2+) did not block caffeine-sensitive calcium release (RyR-dependent) or voltage dependent calcium channels. Mg(2+) also suppressed thimerosal-induced calcium oscillations (IP(3)R-dependent). Small increases in the intracellular calcium concentration ([Ca(2+)](i)), increases the percentage of active cells and the calcium spiking frequency, while larger increases in [Ca(2+)](i) block the transients. Reducing intracellular IP(3) levels reduces the percentage of active cells and the calcium spiking frequency. We conclude that the mechanism for generating spontaneous calcium transients in cultured neural crest cells fits the model for IP(3)R-dependent calcium excitability of the ER.

A novel role for membrane potential in the modulation of intracellular Ca2+ oscillations in rat megakaryocytes

1. The effect of membrane potential (Vm) on ADP-evoked [Ca2+]i oscillations was investigated in rat megakaryocytes, a non-excitable cell type recently shown to exhibit depolarisation-evoked Ca2+ release from intracellular stores during metabotropic purinoceptor stimulation. 2. Hyperpolarising voltage steps caused a transient fall in [Ca2+]i and either abolished Ca2+ oscillations or reduced the oscillation amplitude. These effects were observed in both the presence and absence of extracellular Ca2+ and also in Na+-free saline solutions, suggesting that hyperpolarisation leads to a reduction in the level of ADP-dependent Ca2+ release without a requirement for altered transmembrane Ca2+ fluxes. 3. In the presence of Ca2+ oscillations, depolarising voltage steps transiently enhanced the amplitude of Ca2+ oscillations. Following run-down of Ca2+ oscillations, depolarisation briefly restimulated oscillations. 4. Simultaneous [Ca2+]i and current-clamp recordings showed that Ca2+ and Vm oscillate in synchrony, with an average fluctuation of approximately 30-40 mV, due to activation and inactivation of Ca2+-dependent K+ channels. Application of a physiological oscillating Vm waveform to non-oscillating cells under voltage clamp stimulated [Ca2+]i oscillations. 5. Analysis of the relationship between [Ca2+]i and Vm showed a threshold for activation of hyperpolarisation at about 250-300 nM. The implications of this threshold in the interaction between Vm and Ca2+ release during oscillations are discussed. 6. We conclude that the ability of voltage to control release of endosomal Ca2+ in ADP-stimulated megakaryocytes is bipolar in nature. Our data suggest that Vm changes are active components of the feedback/feedforward mechanisms contributing to the generation of Ca2+ oscillations.

Superoxide anion impairs Ca(2+) mobilization in cultured human nasal epithelial cells

We examined the effects of superoxide anion (O(-2)) on the intracellular Ca(2+) concentration in cultured human nasal epithelial cells. The cells were exposed to O(-2) by pretreatment with xanthine (X) and xanthine oxidase (XO); control cells were treated with X alone. When Ca(2+)-containing Krebs solution was reperfused in the thapsigargin-treated, store-depleted cells, reapplication-induced intracellular Ca(2+) concentration elevation was significantly smaller in X/XO-treated cells than in the control cells, suggesting that O(-2) impairs Ca(2+) release-activated Ca(2+) entry (CRAC). Bath application of ATP induced a steep Ca(2+) transient in both control and X/XO-treated cells. However, the concentration-response curve of the ATP-induced Ca(2+) transient was shifted to a higher concentration in X/XO-treated cells. The impairments of CRAC and ATP-induced Ca(2+) transient induced by X/XO were reversed by superoxide dismutase. Furthermore, all these X/XO-induced effects were also observed in cells pretreated with pyrogallol, also an O(-2) donor. These results indicate that O(-2) impairs at least two mechanisms involved in Ca(2+) mobilization in human nasal epithelial cells, i.e., CRAC and ATP-induced Ca(2+) release.

Effect of FK506 on ATP-induced intracellular calcium oscillations in cow tracheal epithelium

To elucidate the effect of FK506 on Ca2+ oscillations in airway epithelium, we investigated cultured cow tracheal epithelial cells with a Ca2+ image-analysis system. ATP (1 microM) induced long-lasting Ca2+ oscillations, having nearly constant peak values (300-400 nM) and intervals (20-40 s) in subconfluent cells but not in confluent cells. These responses were gradually attenuated and abolished by the addition of FK506. Rapamycin, which binds the FK506-binding protein (FKBP), likewise inhibited Ca2+ oscillations, whereas cyclosporin A, a calcineurin inhibitor, did not. Treatment of cells with FK506 decreased Ca2+ content in thapsigargin-sensitive stores, suggesting that the partial depletion of the stores causes the inhibition of Ca2+ oscillations. Immunocytochemistry revealed the existence of cytoplasmic FKBP-like immunoreactivities. The expression of a 12-kDa FKBP was greater in subconfluent cells than in confluent cells as determined by Western blotting, suggesting that the 12-kDa FKBP may be one of the factors that regulates Ca2+ oscillations. Therefore, FK506 possesses an inhibitory action on the Ca2+ response via intracellular FKBP but not via calcineurin, which may result in modification of airway epithelial functions.

Erythromycin inhibits ATP-induced intracellular calcium responses in bovine tracheal epithelial cells

Erythromycin (EM) therapy is known to decrease airway secretion in chronic inflammatory airway diseases such as diffuse panbronchiolitis. Airway secretion is regulated by intracellular Ca2+ concentration ([Ca2+]i). To elucidate the intracellular site of action of EM in airway epithelium, we examined the effect of EM on Ca2+ dynamics in cultured bovine tracheal epithelial cells using fura-2. EM per se did not cause any change in [Ca2+]i. Adenosine triphosphate (ATP; 10(-4) M) induced a biphasic [Ca2+]i increase, consisting of a transient response followed by a sustained response. Pretreatment of cells with EM had little effect on the ATP-induced transient Ca2+ response but substantially reduced the sustained response in a dose-dependent manner. Clarithromycin, another 14-membered ring macrolide, likewise showed the inhibitory effect, but ampicillin and cephasolin did not. Uridine triphosphate (UTP; 10(-4) M) induced a biphasic [Ca2+]i increase similar to ATP, and the UTP-induced sustained Ca2+ response was also inhibited by EM. In Ca2+-deficient medium (1 mM ethyleneglycol-bis-(beta-aminoethyl ether)-N, N'-tetraacetic acid [EGTA]) or in the presence of La3+, the sustained Ca2+ response disappeared, suggesting that EM may inhibit Ca2+ influx induced by P2u purinoceptor stimulation. In single-cell Ca2+ image analysis, low concentration of ATP (10(-6) M) induced Ca2+ oscillations, which were also inhibited by EM. The disappearance of [Ca2+]i oscillations after addition of EM was similar to that after addition of EGTA. These results suggest that EM may decrease Ca2+-dependent airway secretion by inhibiting agonist-stimulated Ca2+ influx.

ADP and inositol trisphosphate evoke oscillations of a monovalent cation conductance in rat megakaryocytes

1. A combination of conventional whole-cell patch clamp recordings and fura-2 fluorescence photometry was used to study the membrane currents during oscillations of intracellular Ca2+ concentration ([Ca2+]i) in single rat megakaryocytes. 2. At a holding potential of -60 mV, in NaCl external saline and KCl internal saline with low levels of Ca2+ buffering, 10 microM ADP evoked [Ca2+]i oscillations and simultaneous Ca2+-gated K+ currents at a frequency of 3-10 spikes min-1. A smaller inward current was also activated, with a time course that identified this component as the inositol 1,4, 5-trisphosphate (IP3)-activated monovalent cation current previously demonstrated in rat megakaryocytes. 3. Cs+ replacement of internal K+ combined with 100 nM external charybdotoxin (CTX) abolished the outward currents and revealed that an inward current was also transiently activated during each [Ca2+]i spike. This underlying conductance was permeable to Na+ and Cs+, but possessed little or no permeability to Cl- or divalent cations. 4. Intracellular dialysis with IP3 (5-50 microM) activated the monovalent cationic conductance prior to release of Ca2+ from intracellular stores. The [Ca2+]i increase was associated with a second phase of cationic current, implying that both IP3 and Ca2+ can activate this conductance. Buffering of [Ca2+]i with BAPTA abolished the second phase of current, leaving monophasic spikes of inward current, often occurring at regular intervals. 5. These data demonstrate that a monovalent cation current, which results in Na+ influx under normal ionic conditions, oscillates in response to ADP receptor stimulation due to activation by both IP3 and [Ca2+]i. This provides a route for long-term Na+ entry in the megakaryocyte following stimulation of receptors coupled to phospholipase C activation and may play a role in cell shape change.

Coupling between cytosolic and mitochondrial calcium oscillations: role in the regulation of hepatic metabolism

Mitochondria are strategically localized at sites of Ca2+ release, such that increases in cytosolic free Ca2+ ([Ca2+]c) from either internal Ca2+ stores or Ca2+ influx across the plasma membrane can be rapidly transported into the mitochondrial matrix. The consequent elevation in mitochondrial Ca2+ ([Ca2+]m) stimulates the Ca2+-sensitive intramitochondrial dehydrogenases, resulting in elevation of NAD(P)H. The preferential coupling between increases in [Ca2+]c and [Ca2+]m is one proposed mechanism to coordinate mitochondrial ATP production with cellular energy demand. In liver cells, hormones that act through the second messenger inositol 1,4, 5-trisphosphate (IP3) generate oscillatory [Ca2+]c signals, which result from a periodic Ca2+- and IP3-mediated activation/deactivation of intracellular Ca2+ release channels. The [Ca2+]c spiking frequency increases with agonist dose, whereas the amplitude of each [Ca2+]c spike is constant. This frequency modulation of [Ca2+]c spiking encodes the signal from the extracellular agonist, which is then decoded by the internal Ca2+-sensitive proteins such as the Ca2+-sensitive intramitochondrial dehydrogenases. Our studies have investigated the relationship between IP3-dependent [Ca2+]c signals and [Ca2+]m in primary cultured hepatocytes. In addition, the changes in cellular [Ca2+] levels have been correlated with the regulation of intramitochondrial NAD(P)H levels, pyruvate dehydrogenase activity and the magnitude of the mitochondrial proton motive force.

Cell-permeant caged InsP3 ester shows that Ca2+ spike frequency can optimize gene expression

Inositol 1,4,5-trisphosphate (InsP3) releases calcium from intracellular stores and triggers complex waves and oscillations in levels of cytosolic free calcium. To determine which longer-term responses are controlled by oscillations in InsP3 and cytosolic free calcium, it would be useful to deliver exogenous InsP3, under spatial and temporal control, into populations of unpermeabilized cells. Here we report the 15-step synthesis of a membrane-permeant, caged InsP3 derivative from myo-inositol This derivative diffused into intact cells and was hydrolysed to produce a caged, metabolically stable InsP3 derivative. This latter derivative accumulated in the cytosol at concentrations of hundreds of micromolar, without activating the InsP3 receptor. Ultraviolet illumination uncaged an InsP3 analogue nearly as potent as real InsP3, and generated spikes of cytosolic free calcium, and stimulated gene expression via the nuclear factor of activated T cells. The same total amount of InsP3 analogue elicited much more gene expression when released by repetitive flashes at 1-minute intervals than when released at 0.5- or > or = 2-minute intervals, as a single pulse, or as a slow sustained plateau. Thus, oscillations in cytosolic free calcium levels at roughly physiological rates maximize gene expression for a given amount of InsP3.

Adenosine 5'-triphosphate, uridine 5'-triphosphate, bradykinin, and lysophosphatidic acid induce different patterns of calcium responses by human articular chondrocytes

Small calcium-mobilizing inflammatory mediators have been implicated in joint pathology. Here we demonstrate that bradykinin, adenosine 5'-triphosphate, uridine 5'-triphosphate, and lysophosphatidic acid raise the intracellular calcium concentration ([Ca2+]i) in human articular chondrocytes. Heterologous cross-desensitization experiments showed that the uridine 5'-triphosphate response was abolished by prior treatment with adenosine 5'-triphosphate and, conversely, that the adenosine 5'-triphosphate response was abolished by prior treatment with uridine 5'-triphosphate; this indicated competition for the same receptor site, whereas bradykinin and lysophosphatidic acid did not compete with other ligands. Pretreatment with thapsigargin abolished ligand-mediated Ca2+ responses but not vice versa; this confirmed that Ca2+ release occurred from intracellular stores. Single-cell analysis of Fura-2 acetoxymethyl ester loaded chondrocytes showed mediator-dependent patterns of oscillatory Ca2+ changes in a subset of cells when challenged with submaximal concentrations of bradykinin, adenosine 5'-triphosphate, or uridine 5'-triphosphate in the presence of extracellular Ca2+. However, no oscillatory responses were seen after a challenge with lysophosphatidic acid. Therefore, although a number of different Ca2+-mobilizing ligands activate chondrocytes, the differences that occur in the temporal patterning of Ca2+ responses may result in unique mediator-dependent changes in cellular activity.

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

Intracellular calcium oscillations in astrocytes: a highly plastic, bidirectional form of communication between neurons and astrocytes in situ

The spatial-temporal characteristics of intracellular calcium ([Ca2+]i) changes elicited in neurons and astrocytes by various types of stimuli were investigated by means of confocal fluorescent microscopy in acute rat brain slices loaded with the Ca2+ indicator indo-1. Neurons and astrocytes from the visual cortex and CA1 hippocampal region were identified in situ on the basis of their morphological, electrophysiological, and pharmacological features. We show here that stimulation of neuronal afferents triggered periodic [Ca2+]i oscillations in astrocytes. The frequency of these oscillations was under a dynamic control by neuronal activity as it changed according to the pattern of stimulation. After repetitive episodes of neuronal stimulation as well as repetitive stimulation with a metabotropic glutamate receptor agonist, astrocytes displayed a long-lasting increase in [Ca2+]i oscillation frequency. Oscillating astrocytes were accompanied by repetitive [Ca2+]i elevations in adjacent neurons, most likely because of the release of glutamate via a tetanus toxin-resistant process. These results reveal that [Ca2+]i oscillations in astrocytes represent a highly plastic signaling system that underlies the reciprocal communication between neurons and astrocytes.

Effect of a cytosolic Ca2+ concentration ramp on InsP3-induced Ca2+ release in A7r5 smooth-muscle cells and in EBTr cells from tracheal mucosa

Inositol trisphosphate (InsP3) mediated Ca2+ release is modulated in a complex way by Ca2+. It is not known how the InsP3 receptor responds to a slowly increasing cytosolic [Ca2+]. Two different cell lines (A7r5 smooth-muscle cells and EBTr cells from tracheal mucosa) were investigated. We have now stimulated the Ca2+ stores of the permeabilized cells with a near-threshold [InsP3] and then increased the cytosolic [Ca2+] from 5 nM to 3 microM in 55 steps each lasting 6 s. The rate of InsP3-induced Ca2+ release abruptly increased around 100 nM Ca2+ and reached a maximum at 300 nM Ca2+, above which the Ca2+ release became smaller. The stimulatory effect of cytosolic Ca2+ was much less than that induced by elevating the [InsP3]. The time course of activation by Ca2+ in permeabilized cells resembles the fast InsP3-induced Ca2+ release following the pacemaker [Ca2+] rise in the agonist-stimulated intact cell.

Confocal imaging analysis of ATP-induced Ca2+ response in individual endothelial cells of the artery in situ

The mechanisms for mobilization of intracellular free Ca2+ have been studied in various types of isolated and cultured cells, but little is known about Ca2+ mobilization in individual cells in situ. We tried to establish imaging analysis of intracellular free Ca2+ concentration ([Ca2+]i) in individual cells loaded with the acetoxymethyl ester of fluo 3 in situ, using laser scanning confocal microscopy. The method permitted us to distinguish signals from endothelial and smooth muscle cells of guinea pig artery. Addition of ATP to the artery caused a transient increase in endothelial [Ca2+]i. It was concluded that the response was induced via P2Y purinoceptors, because adenosine 5'-O-(2-thiodiphosphate), but not UTP, caused a similar response independent of extracellular Ca2+. The percentage of cells that responded to ATP (1-10 microM) and the peak amplitude of the transient increase in [Ca2+]i were dose dependently increased. Using rapid xy-scanning and line-scanning modes, we confirmed that 10 microM ATP induced Ca2+ waves, at a rate of 10-30 microns/s, after a lag time of approximately 3 s. These results show that [Ca2+]i waves within endothelial cells are physiologically induced by ATP via P2Y purinoceptor, but not P2U purinoceptor, in aortic strips in situ. The method should be of use in the study of vascular physiology and pathophysiology.

[Ca2+]i oscillations and [Ca2+]i waves in rat megakaryocytes

ATP activated [Ca2+]i oscillations were measured in single rat megakaryocytes using fluorescence ratio microscopy. With increasing ATP concentration the duration of the [Ca2+]i oscillations increased, however, there was considerable variation from cell to cell in the absolute value of the peak [Ca2+]i and the frequency and duration of the oscillations. This variation depended, in part, on the level of Fura-2 loading suggesting that megakaryocytes are sensitive to buffering of [Ca2+]i by Fura-2. Agents, that increase the level of intracellular cGMP (sodium nitroprusside and 8-pCPT-cGMP) or cAMP (prostacyclin, IBMX, forskolin and 8-bromo-cAMP) inhibited [Ca2+]i oscillations. Despite the large cell to cell variation in the patterns of [Ca2+]i oscillations, reapplication of the agents that elevated cAMP or cGMP inhibited the oscillations similarly. Using video rate fluorescence ratio imaging we found that the agonist-induced [Ca2+]i oscillations were the result of a well-defined [Ca2+]i wave, which spread across the cell with an average speed of about 35 microns/s, during the rising phase of each oscillatory spike. After reaching a peak, [Ca2+]i decreased uniformly across the whole cell during the falling phase of the spike. Analysis of the temperature dependence of [Ca2+]i waves showed that the rate of [Ca2+]i decay exhibited a strong temperature dependence (Q10 approximately 4), whereas, the rate of rise exhibited a weak temperature dependence (Q10 approximately 1.3), suggesting, that the rate limiting process for [Ca2+]i wave propagation in rat megakaryocytes is the rate of [Ca2+]i diffusion.

Spontaneous and repetitive calcium transients in C2C12 mouse myotubes during in vitro myogenesis

Fluorescence videomicroscopy was used to monitor changes in the cytosolic free Ca2+ concentration ([Ca2+]i) in the mouse muscle cell line C2Cl2 during in vitro myogenesis. Three different patterns of changes in [Ca2+]i were observed: (i) [Ca2+]i oscillations; (ii) faster Ca2+ events confined to subcellular regions (localized [Ca2+]i spikes) and (iii) [Ca2+]i spikes detectable in the entire myotube (global [Ca2+]i spikes). [Ca2+]i oscillations and localized [Ca2+]i spikes were detectable following the appearance of caffeine-sensitivity in differentiating C2Cl2 cells. Global [Ca2+]i spikes appeared later in the process of myogenesis in cells exhibiting coupling between voltage-operated Ca2+ channels and ryanodine receptors. In contrast to [Ca2+]i oscillations and localized [Ca2+]i spikes, the global events immediately stopped when cells were perfused either with a Ca2+-free solution, or a solution with TTX, TEA and verapamil. To explore further the mechanism of the global [Ca2+]i spikes, membrane currents and fluorescence signals were measured simultaneously. These experiments revealed that global [Ca2+]i spikes were correlated with an inward current. Moreover, while the depletion of the Ca2+ stores blocked [Ca2+]i oscillations and localized [Ca2+]i spikes, it only reduced the amplitude of global [Ca2+]i spikes. It is suggested that, during the earlier stages of the myogenesis, spontaneous and repetitive [Ca2+]i changes may be based on cytosolic oscillatory mechanisms. The coupling between voltage-operated Ca2+ channels and ryanodine receptors seems to be the prerequisite for the appearance of global [Ca2+]i spikes triggered by a membrane oscillatory mechanism, which characterizes the later phases of the myogenic process.

Ca2+ influx does more than provide releasable Ca2+ to maintain repetitive spiking in human umbilical vein endothelial cells

We investigated why oscillations of intracellular Ca2+ concentrations ([Ca2+]i) in endothelial cells challenged by sub-maximal histamine run down in Ca(2+)-free medium despite stores retaining most of their Ca2+. One explantation is that only a small subpopulation of the Ca2+ stores oscillate and are completely emptied of Ca2+. To investigate if influx refills an empty store subpopulation, we differentiated between cations entering the cell and those released from internal stores by using extracellular Sr2+ as a Ca2+ surrogate; we distinguished between [Sr2+]i and [Ca2+]i by using the larger effect of Sr2+ on fura 2 fluorescence at 360 nm (F360). Ca2+ was still available for release when oscillations had run down since oscillations promptly reappeared on addition of Sr2+o and these were predominantly of Ca2+ (indicated by F360 changes). Also, totally depleting Ca2+ stores inhibited Sr(2+)-induced oscillations, suggesting that Sr2+ entry leads to Ca2+ release. In contrast, Ba2+o was unable to stimulate oscillations. Finally, oscillations generated by photolytic release of inositol trisphosphate (IP3) analogues were similarly sensitive to extracellular Ca2+ and Sr2+. We conclude that stores (or a sub-population) are not completely depleted of Ca2+ when oscillations run down in Ca(2+)-free medium. Bivalent cation entry therefore maintains sensitivity to IP3, possibly by maintaining luminal bivalent cation levels.

Ca2+ waves in lung capillary endothelium

Although cytosolic Ca2+ importantly regulates organ function, lung microvascular [Ca2+]i regulation remains poorly understood because of the lack of direct in situ quantification. In the present study, we report the first endothelial [Ca2+]i quantification by the fura 2 method in microscopically imaged venular capillaries of the isolated blood-perfused rat lung. Sequential images indicated the presence of intercellular Ca2+ waves that spontaneously originated from pacemaker endothelial cells and then spread for short distances along the capillary wall, inducing synchronous endothelial [Ca2+]i oscillations. Fast Fourier analyses of the oscillations revealed a dominant wave component with an amplitude of 37 nmol/L, frequency of 0.4 min-1, and velocity of 5 microns/s. The intracellular Ca2+ wave was unaffected by blood flow stoppage or by infusions of Ca(2+)-containing or Ca(2+)-free dextran. Inhibition of the wave by thapsigargin in Ca(2+)-free dextran and by the gap junction uncoupler, heptanol, indicated that it was generated by endosomal Ca2+ release in the pacemaker cell and was propagated by gap junctional communication. In the presence of histamine, enhancement of the wave accounted for a significant component of the coordinated [Ca2+]i increase in the capillary segment. No intercellular Ca2+ waves were evident in adjoining alveolar epithelial cells. Our findings indicate a novel mechanism of [Ca2+]i regulation in the lung capillary under both resting and stimulated conditions. Pacemaker-induced Ca2+ waves, generated intracellularly by unknown initiating mechanisms, communicated to adjoining cells to determine [Ca2+]i profiles in short interbranch segments of capillary walls.

Subcellular Ca2+ signals underlying waves and graded responses in HeLa cells

BACKGROUND

Many agonist-evoked intracellular Ca2+ signals have a complex spatio-temporal arrangement, and are observed as repetitive Ca2+ spikes and Ca2+ waves. The key to revealing how these complex signals are generated lies in understanding the functional structure of the intracellular Ca2+ pool. Previous imaging studies, using relatively large cells such as oocytes and myocytes, have identified subcellular elementary Ca2+ signals, indicating that the intracellular Ca2+ pool releases Ca2+ from functionally discrete sites. However, it is unclear whether the intracellular Ca2+ pool in smaller cells has a similar architecture, and how such subcellular signals would contribute to global spikes and waves.

RESULTS

We detected subcellular Ca2+ signals during the response of single Fura2-loaded HeLa cells to histamine. The spatio-temporal properties of some of these signals were similar to the elementary Ca2+ signals observed in other cells. Subcellular Ca2+ signals were particularly obvious during the 'pacemaker' Ca2+ rise that preceded the regenerative Ca2+ wave. During this pacemaker, the Ca2+ signals were observed initially in the region from which the Ca2+ wave originated, but became more widespread and frequent until a Ca2+ wave was spawned. Similar localized signals were seen during the post-wave Ca2+ increase, and during the low-amplitude Ca2+ responses evoked by threshold histamine concentrations.

CONCLUSIONS

The intracellular Ca2+ pool in HeLa cells is composed of many functionally discrete units. Upon stimulation, these units produce localized Ca2+ signals. The sequential activation and summation of these units results in Ca2+ wave propagation and, furthermore, the differential recruitment of these units may underlie the graded amplitude of the intracellular Ca2+ signals.

Crosstalk between cellular morphology and calcium oscillation patterns. Insights from a stochastic computer model

Agonist-induced oscillations in the concentration of intracellular free calcium ([Ca2+]i) display a wide variety of temporal and spatial patterns. In non-excitable cells, typical oscillatory patterns are somewhat cell-type specific and range from frequency-encoded, repetitive Ca2+ spikes to oscillations that are more sinusoidal in shape. Although the response of a cell population, even to the same stimulus, is often extremely heterogeneous, the response of the same cell to successive exposures can be remarkably similar. We propose that such "Ca2+ fingerprints' can be a consequence of cell-specific morphological properties. The hypothesis is tested by means of a stochastic computer simulation of a two-dimensional model for oscillatory Ca2+ waves which encompasses the basic elements of the two-pool oscillator introduced by Goldbeter et al. (Goldbeter A., Dupont G., Berridge M.J. Minimal model for signal-induced Ca(2+)-oscillations and for their frequency encoding through protein phosphorylation. Proc Natl Acad Sci USA 1990; 87: 1461-1465). In the framework of our extended spatiotemporal model, single cells can display various oscillation patterns which depend on the agonist dose, Ca2+ diffusibility, and several morphological parameters. These are, for example, size and shape of the cell and the cell nucleus, the amount and distribution of Ca2+ stores, and the subcellular location of the inositol(1,4,5)-trisphosphate-generating apparatus.

Extracellular calcium concentration controls the frequency of intracellular calcium spiking independently of inositol 1,4,5-trisphosphate production in HeLa cells

Stimulation of single HeLa cells with histamine evoked repetitive increases of the intracellular calcium ion concentration (Ca2+ spikes). The frequency of Ca2+ spiking increased as the extracellular hormone concentration was elevated. In addition, the frequency of Ca2+ spiking could be accelerated by increasing the extracellular Ca2+ concentration ([Ca2+]0) in the presence of a constant hormone concentration. The range of [Ca2+]0 over which the spiking frequency could be titrated was nominally-zero to 10mM, being half-maximally effective at approx. 1 and 2.5mM for 37 and 22 degrees C respectively. The effect of [Ca2+]0 on inositol phosphates production was also examined. Changes of [Ca2+]0 over a range which had been found to affect the frequency of Ca2+ spiking did not have any effect on the rate of myo-inositol 1,4,5-trisphosphate (InsP3) production, although an increase in inositol phosphates production was observed as [Ca2+]0 was increased from zero to values giving less than half-maximal Ca2+ spike frequency. These data suggest that at low Ca2+ spike frequency, Ca2+-stimulated activation of phospholipase C may contribute to Ca2+ spiking in HeLa cells, but under some conditions the availability of Ca2+ to the intracellular stores, rather than changes in the rate of InsP3 production, determines the Ca2+ spike frequency.

Hepatocyte growth factor-induced calcium waves in hepatocytes as revealed with rapid scanning confocal microscopy

Cytosolic Ca2+ transients induced by hepatocyte growth factor (HGF) were imaged in primary cultured rat hepatocytes using newly developed rapid scanning confocal microscopes and indo-1. HGF (40 ng/ml) increased cytosolic free Ca2+ concentration ([Ca2+]i) in about 60% of hepatocytes, in 45% of which the increases were oscillatory. In each of the oscillatory hepatocytes, the repetitive increases in [Ca2+]i originated from a specific same region adjacent to the cell membrane and propagated across the cell like waves. Phenylephrine (10 microM) also induced Ca2+ waves. The locus where HGF-induced Ca2+ waves and phenylephrine-induced Ca2+ waves were originated was the same, and there was a correlation in the peak height between HGF-induced Ca2+ waves and phenylephrine-induced Ca2+ waves in each cell, although the mechanisms of inositol 1,4,5-trisphosphate (ins(1,4,5)P3) formation induced by HGF should be different from those by phenylephrine. On the other hand, there was no correlation between sensitivity of each cell to HGF and that to phenylephrine which were measured as latent periods prior to Ca2+ rises after an addition of the agonists. These results suggested the following: the spatial patterns of Ca2+ waves were decided by a common mechanism, probably not the propagation of ins(1,4,5)P3 but the distribution of ins(1,4,5)P3-sensitive Ca2+ pools; sensitivities of each cell to the agonists did not mainly depend on the common mechanism.

Ca2+ waves in PC12 neurites: a bidirectional, receptor-oriented form of Ca2+ signaling

Spatial and temporal aspects of Ca2+ signaling were investigated in PC12 cells differentiated with nerve growth factor, the well known nerve cell model. Activation of receptors coupled to polyphosphoinositide hydrolysis gave rise in a high proportion of the cells to Ca2+ waves propagating non decrementally and at constant speed (2-4 microns/s at 18 degrees C and approximately 10-fold faster at 37 degrees C) along the neurites. These waves relied entirely on the release of Ca2+ from intracellular stores since they could be generated even when the cells were incubated in Ca(2+)-free medium. In contrast, when the cells were depolarized with high K+ in Ca(2+)-containing medium, increases of cytosolic Ca2+ occurred in the neurites but failed to evolve into waves. Depending on the receptor agonist employed (bradykinin and carbachol versus ATP) the orientation of the waves could be opposite, from the neurite tip to the cell body or vice versa, suggesting different and specific distribution of the responsible surface receptors. Cytosolic Ca2+ imaging results, together with studies of inositol 1,4,5-trisphosphate generation in intact cells and inositol 1,4,5-trisphosphate-induced Ca2+ release from microsomes, revealed the sustaining process of the waves to be discharge of Ca2+ from the inositol 1,4,5-trisphosphate- (and not the ryanodine-) sensitive stores distributed along the neurites. The activation of the cognate receptor appears to result from the coordinate action of the second messenger and Ca2+. Because of their properties and orientation, the waves could participate in the control of not only conventional cell activities, but also excitability and differential processing of inputs, and thus of electrochemical computation in nerve cells.