A hierarchical concept of cellular and subcellular Ca(2+)-signalling

Apparent calcium spark properties and fast-scanning 2D confocal imaging modalities

Ca²⁺ sparks are instrumental to understand physiological and pathological Ca²⁺ signaling in the heart. High-speed two spatially dimensional (2D) confocal imaging (>120 Hz) enables acquisition of sparks with high-content information, however, owing to a wide variety of different acquisition modalities the question arises: how much they reflect the "true" Ca²⁺ spark properties. To address this issue, we compared a fast point and a 2D-array scanner equipped with a range of different detectors. As a quasi-standard biological sample, we employed Ca²⁺ sparks in permeabilized and intact mouse ventricular myocytes and utilized an unbiased, automatic Ca²⁺ spark analysis tool, iSpark. Data from the point scanner suffered from low pixel photon fluxes (PPF) concomitant with high Poissonian noise. Images from the 2D-array scanner displayed substantially increased PPF, lower Poissonian noise and almost 3-fold increased sign-to-noise ratios. Noteworthy, data from the 2D scanner suffered from considerable inter-pinhole crosstalk evident for the permeabilized cells. Spark properties, such as frequency, amplitude, decay time and spatial spread were distinctly different for any scanner/detector combination. Our study reveals that the apparent Ca²⁺ spark properties differ dependent on the particular recording modality and set-up employed, quantitatively.

Cytoplasmic Calcium Buffering: An Integrative Crosstalk

Calcium (Ca²⁺) buffering is part of an integrative crosstalk between different mechanisms and elements involved in the control of free Ca²⁺ ions persistence in the cytoplasm and hence, in the Ca²⁺-dependence of many intracellular processes. Alterations of Ca²⁺ homeostasis and signaling from systemic to subcellular levels also play a pivotal role in the pathogenesis of many diseases.Compared with Ca²⁺ sequestration towards intracellular Ca²⁺ stores, Ca²⁺ buffering is a rapid process occurring in a subsecond scale. Any molecule (or binding site) with the ability to bind Ca²⁺ ions could be considered, at least in principle, as a buffer. However, the term Ca²⁺ buffer is applied only to a small subset of Ca²⁺ binding proteins containing acidic side-chain residues.Ca²⁺ buffering in the cytoplasm mainly relies on mobile and immobile or fixed buffers controlling the diffusion of free Ca²⁺ ions inside the cytosol both temporally and spatially. Mobility of buffers depends on their molecular weight, but other parameters as their concentration, affinity for Ca²⁺ or Ca²⁺ binding and dissociation kinetics next to their diffusional mobility also contribute to make Ca²⁺ signaling one of the most complex signaling activities of the cell.The crosstalk between all the elements involved in the intracellular Ca²⁺ dynamics is a process of extreme complexity due to the diversity of structural and molecular elements involved but permit a highly regulated spatiotemporal control of the signal mediated by Ca²⁺ ions. The basis of modeling tools to study Ca²⁺ dynamics are also presented.

Acetylcholine released by endothelial cells facilitates flow-mediated dilatation

KEY POINTS

The endothelium plays a pivotal role in the vascular response to chemical and mechanical stimuli. The endothelium is exquisitely sensitive to ACh, although the physiological significance of ACh-induced activation of the endothelium is unknown. In the present study, we investigated the mechanisms of flow-mediated endothelial calcium signalling. Our data establish that flow-mediated endothelial calcium responses arise from the autocrine action of non-neuronal ACh released by the endothelium.

ABSTRACT

Circulating blood generates frictional forces (shear stress) on the walls of blood vessels. These frictional forces critically regulate vascular function. The endothelium senses these frictional forces and, in response, releases various vasodilators that relax smooth muscle cells in a process termed flow-mediated dilatation. Although some elements of the signalling mechanisms have been identified, precisely how flow is sensed and transduced to cause the release of relaxing factors is poorly understood. By imaging signalling in large areas of the endothelium of intact arteries, we show that the endothelium responds to flow by releasing ACh. Once liberated, ACh acts to trigger calcium release from the internal store in endothelial cells, nitric oxide production and artery relaxation. Flow-activated release of ACh from the endothelium is non-vesicular and occurs via organic cation transporters. ACh is generated following mitochondrial production of acetylCoA. Thus, we show ACh is an autocrine signalling molecule released from endothelial cells, and identify a new role for the classical neurotransmitter in endothelial mechanotransduction.

Shared functional defect in IP₃R-mediated calcium signaling in diverse monogenic autism syndromes

Autism spectrum disorder (ASD) affects 2% of children, and is characterized by impaired social and communication skills together with repetitive, stereotypic behavior. The pathophysiology of ASD is complex due to genetic and environmental heterogeneity, complicating the development of therapies and making diagnosis challenging. Growing genetic evidence supports a role of disrupted Ca(2+) signaling in ASD. Here, we report that patient-derived fibroblasts from three monogenic models of ASD-fragile X and tuberous sclerosis TSC1 and TSC2 syndromes-display depressed Ca(2+) release through inositol trisphosphate receptors (IP3Rs). This was apparent in Ca(2+) signals evoked by G protein-coupled receptors and by photoreleased IP3 at the levels of both global and local elementary Ca(2+) events, suggesting fundamental defects in IP3R channel activity in ASD. Given the ubiquitous involvement of IP3R-mediated Ca(2+) signaling in neuronal excitability, synaptic plasticity, gene expression and neurodevelopment, we propose dysregulated IP3R signaling as a nexus where genes altered in ASD converge to exert their deleterious effect. These findings highlight potential pharmaceutical targets, and identify Ca(2+) screening in skin fibroblasts as a promising technique for early detection of individuals susceptible to ASD.

Single-molecule tracking of inositol trisphosphate receptors reveals different motilities and distributions

Puffs are local Ca(2+) signals that arise by Ca(2+) liberation from the endoplasmic reticulum through the concerted opening of tightly clustered inositol trisphosphate receptors/channels (IP3Rs). The locations of puff sites observed by Ca(2+) imaging remain static over several minutes, whereas fluorescence recovery after photobleaching (FRAP) experiments employing overexpression of fluorescently tagged IP3Rs have shown that the majority of IP3Rs are freely motile. To address this discrepancy, we applied single-molecule imaging to locate and track type 1 IP3Rs tagged with a photoswitchable fluorescent protein and expressed in COS-7 cells. We found that ∼ 70% of the IP3R1 molecules were freely motile, undergoing random walk motility with an apparent diffusion coefficient of ∼ 0.095 μm s(-1), whereas the remaining molecules were essentially immotile. A fraction of the immotile IP3Rs were organized in clusters, with dimensions (a few hundred nanometers across) comparable to those previously estimated for the IP3R clusters underlying functional puff sites. No short-term (seconds) changes in overall motility or in clustering of immotile IP3Rs were apparent following activation of IP3/Ca(2+) signaling. We conclude that stable clusters of small numbers of immotile IP3Rs may underlie local Ca(2+) release sites, whereas the more numerous motile IP3Rs appear to be functionally silent.

Imaging local Ca2+ signals in cultured mammalian cells

Cytosolic Ca2+ ions regulate numerous aspects of cellular activity in almost all cell types, controlling processes as wide-ranging as gene transcription, electrical excitability and cell proliferation. The diversity and specificity of Ca2+ signaling derives from mechanisms by which Ca2+ signals are generated to act over different time and spatial scales, ranging from cell-wide oscillations and waves occurring over the periods of minutes to local transient Ca2+ microdomains (Ca2+ puffs) lasting milliseconds. Recent advances in electron multiplied CCD (EMCCD) cameras now allow for imaging of local Ca2+ signals with a 128 x 128 pixel spatial resolution at rates of >500 frames sec(-1) (fps). This approach is highly parallel and enables the simultaneous monitoring of hundreds of channels or puff sites in a single experiment. However, the vast amounts of data generated (ca. 1 Gb per min) render visual identification and analysis of local Ca2+ events impracticable. Here we describe and demonstrate the procedures for the acquisition, detection, and analysis of local IP3-mediated Ca2+ signals in intact mammalian cells loaded with Ca2+ indicators using both wide-field epi-fluorescence (WF) and total internal reflection fluorescence (TIRF) microscopy. Furthermore, we describe an algorithm developed within the open-source software environment Python that automates the identification and analysis of these local Ca2+ signals. The algorithm localizes sites of Ca2+ release with sub-pixel resolution; allows user review of data; and outputs time sequences of fluorescence ratio signals together with amplitude and kinetic data in an Excel-compatible table.

Inositol 1,4,5-trisphosphate receptors in the endoplasmic reticulum: A single-channel point of view

As an intracellular Ca(2+) release channel at the endoplasmic reticulum membrane, the ubiquitous inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) plays a crucial role in the generation, propagation and regulation of intracellular Ca(2+) signals that regulate numerous physiological and pathophysiological processes. This review provides a concise account of the fundamental single-channel properties of the InsP3R channel: its conductance properties and its regulation by InsP3 and Ca(2+), its physiological ligands, studied using nuclear patch clamp electrophysiology.

Ca(2+) signalling by IP(3) receptors

The Ca(2) (+) signals evoked by inositol 1,4,5-trisphosphate (IP(3)) are built from elementary Ca(2) (+) release events involving progressive recruitment of IP(3) receptors (IP(3)R), intracellular Ca(2) (+) channels that are expressed in almost all animal cells. The smallest events ('blips') result from opening of single IP(3)R. Larger events ('puffs') reflect the near-synchronous opening of a small cluster of IP(3)R. These puffs become more frequent as the stimulus intensity increases and they eventually trigger regenerative Ca(2) (+) waves that propagate across the cell. This hierarchical recruitment of IP(3)R is important in allowing Ca(2) (+) signals to be delivered locally to specific target proteins or more globally to the entire cell. Co-regulation of IP(3)R by Ca(2) (+) and IP(3), the ability of a single IP(3)R rapidly to mediate a large efflux of Ca(2) (+) from the endoplasmic reticulum, and the assembly of IP(3)R into clusters are key features that allow IP(3)R to propagate Ca(2) (+) signals regeneratively. We review these properties of IP(3)R and the structural basis of IP(3)R behavior.

Endoplasmic reticulum remodeling tunes IP₃-dependent Ca²+ release sensitivity

The activation of vertebrate development at fertilization relies on IP₃-dependent Ca²⁺ release, a pathway that is sensitized during oocyte maturation. This sensitization has been shown to correlate with the remodeling of the endoplasmic reticulum into large ER patches, however the mechanisms involved are not clear. Here we show that IP₃ receptors within ER patches have a higher sensitivity to IP₃ than those in the neighboring reticular ER. The lateral diffusion rate of IP₃ receptors in both ER domains is similar, and ER patches dynamically fuse with reticular ER, arguing that IP₃ receptors exchange freely between the two ER compartments. These results suggest that increasing the density of IP₃ receptors through ER remodeling is sufficient to sensitize IP₃-dependent Ca²⁺ release. Mathematical modeling supports this concept of ‘geometric sensitization’ of IP₃ receptors as a population, and argues that it depends on enhanced Ca²⁺-dependent cooperativity at sub-threshold IP₃ concentrations. This represents a novel mechanism of tuning the sensitivity of IP₃ receptors through ER remodeling during meiosis.

Acidosis dilates brain parenchymal arterioles by conversion of calcium waves to sparks to activate BK channels

RATIONALE

Acidosis is a powerful vasodilator signal in the brain circulation. However, the mechanisms by which this response occurs are not well understood, particularly in the cerebral microcirculation. One important mechanism to dilate cerebral (pial) arteries is by activation of large-conductance, calcium-sensitive potassium (BK(Ca)) channels by local Ca(2+) signals (Ca(2+) sparks) through ryanodine receptors (RyRs). However, the role of this pathway in the brain microcirculation is not known.

OBJECTIVE

The objectives of this study were to determine the mechanism by which acidosis dilates brain parenchymal arterioles (PAs) and to elucidate the roles of RyRs and BK(Ca) channels in this response.

METHODS AND RESULTS

Internal diameter and vascular smooth muscle cell Ca(2+) signals were measured in isolated pressurized murine PAs, using imaging techniques. In physiological pH (7.4), vascular smooth muscle cells exhibited primarily RyR-dependent Ca(2+) waves. Reducing external pH from 7.4 to 7.0 in both normocapnic and hypercapnic conditions decreased Ca(2+) wave activity, and dramatically increased Ca(2+) spark activity. Acidic pH caused a dilation of PAs which was inhibited by about 60% by BK(Ca) channel or RyR blockers, in a nonadditive manner. Similarly, dilator responses to acidosis were reduced by nearly 60% in arterioles from BK(Ca) channel knockout mice. Dilations induced by acidic pH were unaltered by inhibitors of K(ATP) channels or nitric oxide synthase.

CONCLUSIONS

These results support the novel concept that acidification, by converting Ca(2+) waves to sparks, leads to the activation of BK(Ca) channels to induce dilation of cerebral PAs.

Protein kinase C: the "masters" of calcium and lipid

The coordinated and physiological behavior of living cells in an organism critically depends on their ability to interact with surrounding cells and with the extracellular space. For this, cells have to interpret incoming stimuli, correctly process the signals, and produce meaningful responses. A major part of such signaling mechanisms is the translation of incoming stimuli into intracellularly understandable signals, usually represented by second messengers or second-messenger systems. Two key second messengers, namely the calcium ion and signaling lipids, albeit extremely different in nature, play an important and often synergistic role in such signaling cascades. In this report, we will shed some light on an entire family of protein kinases, the protein kinases C, that are perfectly designed to exactly decode these two second messengers in all of their properties and convey the signaling content to downstream processes within the cell.

Alternans and arrhythmias: from cell to heart

The goal of systems biology is to relate events at the molecular level to more integrated scales from organelle to cell, tissue, and living organism. Here, we review how normal and abnormal excitation-contraction coupling properties emerge from the protein scale, where behaviors are dominated by randomness, to the cell and tissue scales, where heart has to beat with reliable regularity for a lifetime. Beginning with the fundamental unit of excitation-contraction coupling, the couplon where L-type Ca channels in the sarcolemmal membrane adjoin ryanodine receptors in the sarcoplasmic reticulum membrane, we show how a network of couplons with 3 basic properties (random activation, refractoriness, and recruitment) produces the classic physiological properties of excitation-contraction coupling and, under pathophysiological conditions, leads to Ca alternans and Ca waves. Moving to the tissue scale, we discuss how cellular Ca alternans and Ca waves promote both reentrant and focal arrhythmias in the heart. Throughout, we emphasize the qualitatively novel properties that emerge at each new scale of integration.

Spontaneous Ca2+ sparks and Ca2+ homeostasis in a minimal model of permeabilized ventricular myocytes

Many issues remain unresolved concerning how local, subcellular Ca(2+) signals interact with bulk cellular concentrations to maintain homeostasis in health and disease. To aid in the interpretation of data obtained in quiescent ventricular myocytes, we present here a minimal whole cell model that accounts for both localized (subcellular) and global (cellular) aspects of Ca(2+) signaling. Using a minimal formulation of the distribution of local [Ca(2+)] associated with a large number of Ca(2+)-release sites, the model simulates both random spontaneous Ca(2+) sparks and the changes in myoplasmic and sarcoplasmic reticulum (SR) [Ca(2+)] that result from the balance between stochastic release and reuptake into the SR. Ca(2+)-release sites are composed of clusters of two-state ryanodine receptors (RyRs) that exhibit activation by local cytosolic [Ca(2+)] but no inactivation or regulation by luminal Ca(2+). Decreasing RyR open probability in the model causes a decrease in aggregate release flux and an increase in SR [Ca(2+)], regardless of whether RyR inhibition is mediated by a decrease in RyR open dwell time or an increase in RyR closed dwell time. The same balance of stochastic release and reuptake can be achieved, however, by either high-frequency/short-duration or low-frequency/long-duration Ca(2+) sparks. The results are well correlated with recent experimental observations using pharmacological RyR inhibitors and clarify those aspects of the release-reuptake balance that are inherent to the coupling between local and global Ca(2+) signals and those aspects that depend on molecular-level details. The model of Ca(2+) sparks and homeostasis presented here can be a useful tool for understanding changes in cardiac Ca(2+ )release resulting from drugs, mutations, or acquired diseases.

Recording single-channel activity of inositol trisphosphate receptors in intact cells with a microscope, not a patch clamp

Optical single-channel recording is a novel tool for the study of individual Ca2+-permeable channels within intact cells under minimally perturbed physiological conditions. As applied to the functioning and spatial organization of IP3Rs, this approach complements our existing knowledge, which derives largely from reduced systems - such as reconstitution into lipid bilayers and patch clamping of IP3Rs on the membrane of excised nuclei - where the spatial arrangement and interactions among IP3Rs via CICR are disrupted. The ability to image the activity of single IP3R channels with millisecond resolution together with localization of their positions with a precision of a few tens of nanometers both raises several intriguing questions and holds promise of answers. In particular, what mechanism underlies the anchoring of puffs and blips to static locations; why do these Ca2+ release events appear to involve only a very small fraction of the IP3Rs within a cell; and how can we reconcile the relative immotility of functional IP3Rs with numerous studies reporting free diffusion of IP3R protein in the ER membrane?

Remodelling of Ca2+ handling organelles in adult rat ventricular myocytes during long term culture

It is well known that for cardiomyocytes, isolation and culturing induce largely unknown remodelling processes. We analysed changes in the structure of cell compartments with optical techniques such as confocal microscopy and fluorescence redistribution after photobleaching employing adenoviral-mediated transduction of targeted fluorescent proteins and small molecule dyes. We identified characteristic remodelling processes: the T-tubular membrane system was gradually lost by a process referred to as "sequential pinching off", in an outward direction. Mitochondria fell in one of three classes, very small (0.9 microm length), medium long (1.8 microm) or extended shape (3.6 microm) organelles. Over the culturing time mitochondria gradually fused. Bleaching of individual mitochondria revealed association between apparently separated mitochondria by "tunnelling" via sub-resolution organelle-tubes. This tunnelling process was increasing over the culturing time. A gradual loss of the cross-striation arrangement in the endoplasmic/sarcoplasmic reticulum was visualised. Analysis of large populations of Ca(2+) sparks by video-rate confocal 2D-scanning revealed significant albeit small changes of these elementary SR-Ca(2+) release events in adult cardiomyocytes that could be related to changes in SR-Ca(2+) content rather than resting Ca(2+) concentration. In conclusion, primary isolated cardiomyocytes from adult hearts undergo a well-defined, but reproducible subcellular remodelling during optimised long term culture.

Ca(2+) puffs originate from preestablished stable clusters of inositol trisphosphate receptors

Intracellular calcium ion (Ca(2+)) signaling crucially depends on the clustered organization of inositol trisphosphate receptors (IP(3)Rs) in the endoplasmic reticulum (ER) membrane. These ligand-gated ion channels liberate Ca(2+) to generate local signals known as Ca(2+) puffs. We tested the hypothesis that IP(3) itself elicits rapid clustering of IP(3)Rs by using flash photolysis of caged IP(3) in conjunction with high-resolution Ca(2+) imaging to monitor the activity and localization of individual IP(3)Rs within intact mammalian cells. Our results indicate that Ca(2+) puffs arising with latencies as short as 100 to 200 ms after photorelease of IP(3) already involve at least four IP(3)R channels, and that this number does not subsequently grow. Moreover, single active IP(3)Rs show limited mobility, and stochastic simulations suggest that aggregation of IP(3)Rs at puff sites by a diffusional trapping mechanism would require many seconds. We thus conclude that puff sites represent preestablished, stable clusters of IP(3)Rs and that functional IP(3)Rs are not readily diffusible within the ER membrane.

IP3 receptors: some lessons from DT40 cells

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

Targeting and clustering of IP3 receptors: key determinants of spatially organized Ca2+ signals

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca2+ channels that are almost ubiquitously expressed in animal cells. The spatiotemporal complexity of the Ca2+ signals evoked by IP3R underlies their versatility in cellular signaling. Here we review the mechanisms that contribute to the subcellular targeting of IP3R and the dynamic interplay between IP3R that underpin their ability to generate complex intracellular Ca2+ signals.

Arrhythmogenic substrates in myocardial infarct

Life-threatening ventricular tachyarrhythmias are common clinical complications in ischemic heart diseases, especially infarcted heart. Although electrophysiological mechanisms have been extensively clarified for the genesis of arrhythmias in myocardial infarct, arrhythmogenic substrates in the infarct that eventually lead to electrical derangements are not fully understood. This review focuses on the intracellular calcium ion (Ca2+) dynamics and connexin43 (Cx43) gap junctions that play pivotal roles in excitation/contraction processes and intercellular communication, respectively, in heart muscle cells. Recent development of Ca2+-sensitive fluorescent dyes as well as microscopy imaging techniques has contributed substantially to a more precise understanding of spatiotemporal aspects in the intra- and inter-cellular dynamics of Ca2+ in cardiomyocytes. Ca2+ waves, heterogeneous wave-like elevations of the intracellular Ca2+ concentrations ([Ca2+](i)) that develop under [Ca2+](i)-overloaded conditions of the injured myocardium, play an essential role in arrhythmias, especially in triggered arrhythmias. Alteration of Cx43-mediated electrical coupling, that is, gap junction remodeling that arises at myocyte-myocyte and myocyte-myofibroblast interfaces, would also be an important substrate for arrhythmias, especially re-entrant tachyarrhythmias. Clarification of these substrates would provide not only deeper insights into the upstream events of life-threatening tachyarrhythmias in the infarcted heart but also bases for new therapeutic strategies for cardiovascular diseases.

Burst emergence of intracellular Ca2+ waves evokes arrhythmogenic oscillatory depolarization via the Na+-Ca2+ exchanger: simultaneous confocal recording of membrane potential and intracellular Ca2+ in the heart

Intracellular Ca(2+) waves (CaWs) of cardiomyocytes are spontaneous events of Ca(2+) release from the sarcoplasmic reticulum that are regarded as an important substrate for triggered arrhythmias and delayed afterdepolarizations. However, little is known regarding whether or how CaWs within the heart actually produce arrhythmogenic membrane oscillation because of the lack of data confirming direct correlation between CaWs and membrane potentials (V(m)) in the heart. On the hypothesis that CaWs evoke arrhythmogenic oscillatory depolarization when they emerge synchronously and intensively in the heart, we conducted simultaneous fluorescence recording of intracellular Ca(2+) ([Ca(2+)](i)) dynamics and V(m) of ventricular myocytes on subepicardial surfaces of Langendorff-perfused rat hearts using in situ dual-view, rapid-scanning confocal microscopy. In intact hearts loaded with fluo4/acetoxymethyl ester and RH237 under perfusion with cytochalasin D at room temperature, individual myocytes exhibited Ca(2+) transients and action potentials uniformly on ventricular excitation, whereas low-K(+)-perfused (2.4 mmol/L) hearts exhibited CaWs sporadically between Ca(2+) transients without discernible membrane depolarization. Further [Ca(2+)](i) loading of the heart, produced by rapid pacing and addition of isoproterenol, evoked triggered activity and subsequent oscillatory V(m), which are caused by burst emergence of CaWs in individual myocytes. Such arrhythmogenic membrane oscillation was abolished by ryanodine or the Na(+)-Ca(2+) exchanger inhibitor SEA0400, indicating an essential role of CaWs and resultant Na(+)-Ca(2+) exchanger-mediated depolarization in triggered activity. In summary, we demonstrate a mechanistic link between intracellular CaWs and arrhythmogenic oscillatory depolarizations in the heart. Our findings provide a cellular perspective on abnormal [Ca(2+)](i) handling in the genesis of triggered arrhythmias in the heart.

Localization of puff sites adjacent to the plasma membrane: functional and spatial characterization of Ca2+ signaling in SH-SY5Y cells utilizing membrane-permeant caged IP3

The Xenopus oocyte has been a favored model system in which to study spatio-temporal mechanisms of intracellular Ca2+ dynamics, in large part because this giant cell facilitates intracellular injections of Ca2+ indicator dyes, buffers and caged compounds. However, the recent commercial availability of membrane-permeant ester forms of caged IP3 (ci-IP3) and EGTA, now allows for facile loading of these compounds into smaller mammalian cells, permitting control of [IP3]i and cytosolic Ca2+ buffering. Here, we establish the human neuroblastoma SH-SY5Y cell line as an advantageous experimental system for imaging Ca2+ signaling, and characterize IP3-mediated Ca2+ signaling mechanisms in these cells. Flash photo-release of increasing amounts of i-IP3 evokes Ca2+ puffs that transition to waves, but intracellular loading of EGTA decouples release sites, allowing discrete puffs to be studied over a wide range of [IP3]. Puff activity persists for minutes following a single photo-release, pointing to a slow rate of i-IP3 turnover in these cells and suggesting that repetitive Ca2+ spikes with periods of 20-30s are not driven by oscillations in [IP3]. Puff amplitudes are independent of [IP3], whereas their frequencies increase with increasing photo-release. Puff sites in SH-SY5Y cells are not preferentially localized near the nucleus, but instead are concentrated close to the plasma membrane where they can be visualized by total internal reflection microscopy, offering the potential for unprecedented spatio-temporal resolution of Ca2+ puff kinetics.

Regional differences in spontaneous Ca2+ spark activity and regulation in cat atrial myocytes

Calcium sparks result from the concerted opening of a small number of Ca2+ release channels (ryanodine receptors, RyRs) organized in clusters in the membrane of the sarcoplasmic reticulum (SR). Calcium sparks represent the elementary events of SR Ca2+ release in cardiac myocytes, and their spatial and temporal summation results in whole-cell [Ca2+]i transients observed during excitation-contraction coupling (ECC). Atrial myocytes generally lack transverse tubules; however, during ECC Ca2+ release is initiated from junctional SR (j-SR) in the cell periphery from where activation propagates inwardly through Ca(2+)-induced Ca2+ release (CICR) from non-junctional SR (nj-SR). Despite the structural differences in the microdomains of RyRs of j-SR and nj-SR, spontaneous Ca2+ sparks are observed from both types of SR, albeit at different frequencies. In cells that showed spontaneous Ca2+ sparks from j-SR and nj-SR, subsarcolemmal (SS) Ca2+ sparks from the j-SR were 3-4 times more frequent than central (CTR) Ca2+ sparks occurring from nj-SR. Subsarcolemmal Ca2+ sparks had a slightly higher amplitude, but were essentially identical in their spatial spread and duration when compared to CTR Ca2+ sparks. Sensitization of RyRs with a low concentration (0.1 mM) of caffeine led to a 107% increase in the frequency of CTR Ca2+ sparks, whereas the SS Ca2+ spark frequency increased by only 58%, suggesting that the nj-SR is capable of much higher Ca2+ spark activity than observed normally in unstimulated cells. The L-type Ca2+ channel blocker verapamil reduced SS Ca2+ spark frequency to 38% of control values, whereas Ca2+ spark activity from nj-SR was reduced by only 19%, suggesting that SS Ca2+ sparks are under the control of Ca2+ influx from the extracellular space. Removal of extracellular Ca2+ eliminated SS Ca2+ sparks completely, whereas Ca2+ sparks from the nj-SR continued, albeit at a lower frequency. In membrane-permeabilized (saponin-treated) atrial myocytes, where [Ca2+] can be experimentally controlled throughout the entire myocyte, j-SR and nj-SR Ca2+ spark frequencies were identical, and Ca2+ sparks could be observed spaced at sarcomeric distances throughout the entire cell, suggesting that all release sites of the nj-SR can become active. Measurement of SR Ca2+ load (10 mM caffeine) revealed no difference between j-SR and nj-SR. The data suggest that in atrial myocytes, which lack a t-tubular system, the nj-SR is fully equipped with a three-dimensional array of functional SR Ca2+ release sites; however, in intact cells under resting conditions, peripheral RyR clusters have a higher probability of activation owing to their association with surface membrane Ca2+ channels, leading to higher spontaneous Ca2+ spark activity. In conclusion, Ca2+ sparks originating from both j-SR and nj-SR are rather stereotypical and show little differences in their spatiotemporal properties. In intact cells, however, the higher frequency of spontaneous SS Ca2+ sparks arises from the structural arrangement of sarcolemma and j-SR membrane and thus from the difference in the trigger mechanism.

Imaging calcium entering the cytosol through a single opening of plasma membrane ion channels: SCCaFTs—fundamental calcium events

Recently, it has become possible to record the localized fluorescence transient associated with the opening of a single plasma membrane Ca(2+) permeable ion channel using Ca(2+) indicators like fluo-3. These Single Channel Ca(2+) Fluorescence Transients (SCCaFTs) share some of the characteristics of such elementary events as Ca(2+) sparks and Ca(2+) puffs caused by Ca(2+) release from intracellular stores (due to the opening of ryanodine receptors and IP(3) receptors, respectively). In contrast to intracellular Ca(2+) release events, SCCaFTs can be observed while simultaneously recording the unitary channel currents using patch-clamp techniques to verify the channel openings. Imaging SCCaFTs provides a way to examine localized Ca(2+) handling in the vicinity of a channel with a known Ca(2+) influx, to obtain the Ca(2+) current passing through plasma membrane cation channels in near physiological solutions, to localize Ca(2+) permeable ion channels on the plasma membrane, and to estimate the Ca(2+) currents underlying those elementary events where the Ca(2+) currents cannot be recorded. Here we review studies of these fluorescence transients associated with caffeine-activated channels, L-type Ca(2+) channels, and stretch-activated channels. For the L-type Ca(2+) channel, SCCaFTs have been termed sparklets. In addition, we discuss how SCCaFTs have been used to estimate Ca(2+) currents using the rate of rise of the fluorescence transient as well as the signal mass associated with the total fluorescence increase.

A model of graded calcium release and L-type Ca2+ channel inactivation in cardiac muscle

We have developed a model of Ca(2+) handling in ferret ventricular myocytes. This model includes a novel L-type Ca(2+) channel, detailed intracellular Ca(2+) movements, and graded Ca(2+)-induced Ca(2+) release (CICR). The model successfully reproduces data from voltage-clamp experiments, including voltage- and time-dependent changes in intracellular Ca(2+) concentration ([Ca(2+)](i)), L-type Ca(2+) channel current (I(CaL)) inactivation and recovery kinetics, and Ca(2+) sparks. The development of graded CICR is critically dependent on spatial heterogeneity and the physical arrangement of calcium channels in opposition to ryanodine-sensitive release channels. The model contains spatially distinct subsystems representing the subsarcolemmal regions where the junctional sarcoplasmic reticulum (SR) abuts the T-tubular membrane and where the L-type Ca(2+) channels and SR ryanodine receptors (RyRs) are localized. There are eight different types of subsystems in our model, with between one and eight L-type Ca(2+) channels distributed binomially. This model exhibits graded CICR and provides a quantitative description of Ca(2+) dynamics not requiring Monte-Carlo simulations. Activation of RyRs and release of Ca(2+) from the SR depend critically on Ca(2+) entry through L-type Ca(2+) channels. In turn, Ca(2+) channel inactivation is critically dependent on the release of stored intracellular Ca(2+). Inactivation of I(CaL) depends on both transmembrane voltage and local [Ca(2+)](i) near the channel, which results in distinctive inactivation properties. The molecular mechanisms underlying many I(CaL) gating properties are unclear, but [Ca(2+)](i) dynamics clearly play a fundamental role.

Regulation of junctional and non-junctional sarcoplasmic reticulum calcium release in excitation-contraction coupling in cat atrial myocytes

We have characterized the dependence on membrane potential (V(m)) and calcium current (I(Ca)) of calcium-induced calcium release (CICR) from the junctional-SR (j-SR, in the subsarcolemmal (SS) space) and non-junctional-SR (nj-SR, in the central (CT) region of the cell) of cat atrial myocytes using whole-cell voltage-clamp together with spatially resolved laser-scanning confocal microscopy. Subsarcolemmal and central [Ca(2+)](i) transient amplitudes and I(Ca) had a bell-shaped dependence on V(m), but [Ca(2+)](i) reached a maximum at more negative V(m) (-10 to 0 mV) than I(Ca) (+10 mV). Termination of I(Ca) after a brief depolarization (2.5 to 22.5 ms) immediately interrupted only the SS [Ca(2+)](i) transient, leaving the development of the CT [Ca(2+)](i) transient unaffected. Block of SR function with 20 microM ryanodine and 2 microM thapsigargin, revealed that > 90 % of the control [Ca(2+)](i) transient amplitude was attributable to active SR Ca(2+) release through ryanodine receptors (RyRs). The gain of SR Ca(2+) release was highest in the SS space at negative test potentials and was less pronounced in the CT region. Inhibition of Na(+)-Ca(2+) exchange resulted in prolonged and higher amplitude [Ca(2+)](i) transients, elevated resting [Ca(2+)](i), accelerated propagation of CICR, decreased extrusion of Ca(2+) and an increase in j-SR Ca(2+) load. Increasing the cytosolic Ca(2+) buffer capacity by internal perfusion with 1 mM EGTA limited SR Ca(2+) release to the SS region, indicating that Ca(2+) release from nj-SR is initiated by diffusion of Ca(2+) from the cell periphery and propagating CICR. Junctional-SR Ca(2+) release occurred at discrete sites whose order of activation and amplitude of release varied from beat to beat. In conclusion, during normal excitation-contraction coupling in cat atrial myocytes, only Ca(2+) release from the j-SR is directly activated by Ca(2+) entering via I(Ca). Elevation of SS [Ca(2+)](i) is required to provide the cytosolic Ca(2+) gradient needed to initiate regenerative and propagating CICR from nj-SR.

Ca(2+) as second messenger in PTTH-stimulated prothoracic glands of the silkworm, Bombyx mori

Measurements of Ca(2+) influx and [Ca(2+)](i) changes in Fura-2/AM-loaded prothoracic glands (PGs) of the silkworm, Bombyx mori, were used to identify Ca(2+) as the actual second messenger of the prothoracicotropic hormone (PTTH) of this insect. Dose-dependent increases of [Ca(2+)](i) in PG cells were recorded in the presence of recombinant PTTH (rPTTH) within 5 minutes. The rPTTH-mediated increases of [Ca(2+)](i) levels were dependent on extracellular Ca(2+). They were not blocked by the dihydropyridine derivative, nitrendipine, an antagonist of high-voltage-activated (HVA) Ca(2+) channels, and by bepridil, an antagonist of low-voltage-activated (LVA) Ca(2+) channels. The trivalent cation La(3+), a non-specific blocker of plasma membrane Ca(2+) channels, eliminated the rPTTH-stimulated increase of [Ca(2+)](i) levels in PG cells and so did amiloride, an inhibitor of T-type Ca(2+) channels. Incubation of PG cells with thapsigargin resulted in an increase of [Ca(2+)](i) levels, which was also dependent on extracellular Ca(2+) and was quenched by amiloride, suggesting the existence of store-operated plasma membrane Ca(2+) channels, which can also be inhibited by amiloride. Thapsigargin and rPTTH did not operate independently in stimulating increases of [Ca(2+)](i) levels and one agent's mediated increase of [Ca(2+)](i) was eliminated in the presence of the other. TMB-8, an inhibitor of intracellular Ca(2+) release from inositol 1,4,5 trisphosphate (IP(3))-sensitive Ca(2+) stores, blocked the rPTTH-stimulated increases of [Ca(2+)](i) levels, suggesting an involvement of IP(3) in the initiation of the rPTTH signaling cascade, whereas ryanodine did not influence the rPTTH-stimulated increases of [Ca(2+)](i) levels. The combined results indicate the presence of a cross-talk mechanism between the [Ca(2+)](i) levels, filling state of IP(3)-sensitive intracellular Ca(2+) stores and the PTTH-receptor's-mediated Ca(2+) influx.

L-NAME, a nitric oxide synthase inhibitor, as a potential countermeasure to post-suspension hypotension in rats

A large number of astronauts returning from spaceflight experience orthostatic hypotension. This hypotension may be due to overproduction of vasodilatory mediators, such as nitric oxide (NO) and prostaglandins. To evaluate the role of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) as a countermeasure against the post-suspension reduction in mean arterial pressure (MAP), we assessed the cardiovascular responses and vascular reactivity to 7-day 30 degrees tail-suspension and a subsequent 6 hr post-suspension period in conscious rats. After a pre-suspension reading, direct MAP and heart rate (HR) were measured daily and every 2 hrs post-suspension. The NO synthase inhibitor L-NAME (20 mg/kg, i.v.), or saline, were administered after the 7th day reading prior to release from suspension and at 2 and 4 hrs post-suspension. At 6 hrs post-suspension, vascular reactivity was assessed. While MAP did not change during the suspension period, it was reduced post-suspension. Heart rate was not significantly altered. L-NAME administration reversed the post-suspension reduction in MAP. In addition, the baroreflex sensitivity for heart rate was modified by L-NAME. Thus, the post-suspension reduction in MAP may be due to overproduction of NO and altered baroreflex activity.

Calcium signalling through nucleotide receptor P2X1 in rat portal vein myocytes

1. ATP-mediated Ca2+ signalling was studied in freshly isolated rat portal vein myocytes by means of a laser confocal microscope and the patch-clamp technique. 2. In vascular myocytes held at -60 mV, ATP induced a large inward current that was supported mainly by activation of P2X1 receptors, although other P2X receptor subtypes (P2X3, P2X4 and P2X5) were revealed by reverse transcription-polymerase chain reaction. 3. Confocal Ca2+ measurements revealed that ATP-mediated Ca2+ responses started at initiation sites where spontaneous or triggered Ca2+ sparks were not detected, whereas membrane depolarizations triggered Ca2+ waves by repetitive activation of Ca2+ sparks from a single initiation site. 4. ATP-mediated Ca2+ responses depended on Ca2+ influx through non-selective cation channels that activated, in turn, Ca2+ release from the intracellular store via ryanodine receptors (RYRs). Using specific antibodies directed against the RYR subtypes, we show that ATP-mediated Ca2+ release requires, at least, RYR2, but not RYR3. 5. Our results suggest that, in vascular myocytes, Ca2+ influx through P2X1 receptors may trigger Ca2+-induced Ca2+ release at intracellular sites where RYRs are not clustered.

Electrically triggered all-or-none Ca(2)+-liberation during action potential in the giant alga Chara

Electrically triggered action potentials in the giant alga Chara corallina are associated with a transient rise in the concentration of free Ca(2)+ in the cytoplasm (Ca(2)+(cyt)). The present measurements of Ca(2)+(cyt) during membrane excitation show that stimulating pulses of low magnitude (subthreshold pulse) had no perceivable effect on Ca(2)+(cyt). When the strength of a pulse exceeded a narrow threshold (suprathreshold pulse) it evoked the full extent of the Ca(2)+(cyt) elevation. This suggests an all-or-none mechanism for Ca(2)+ mobilization. A transient calcium rise could also be induced by one subthreshold pulse if it was after another subthreshold pulse of the same kind after a suitable interval, i.e., not closer than a few 100 ms and not longer than a few seconds. This dependency of Ca(2)+ mobilization on single and double pulses can be simulated by a model in which a second messenger is produced in a voltage-dependent manner. This second messenger liberates Ca(2)+ from internal stores in an all-or-none manner once a critical concentration (threshold) of the second messenger is exceeded in the cytoplasm. The positive effect of a single suprathreshold pulse and two optimally spaced subthreshold pulses on Ca(2)+ mobilization can be explained on the basis of relative velocity for second messenger production and decomposition as well as the availability of the precursor for the second messenger production. Assuming that inositol-1,4,5,-trisphosphate (IP(3)) is the second messenger in question, the present data provide the major rate constants for IP(3) metabolism.

Intracellular calcium dynamics--sparks of insight

Calcium ions are of key importance in a large number of cellular functions. In the past decade a large variety of cells have been found to show localized increases in the intracellular calcium concentration named calcium sparks. In this brief review, the methodology of detecting calcium sparks by confocal microscopy is summarized. Some of the properties of calcium sparks in muscle (cardiac, skeletal and smooth muscles), neurons, nerve terminals and oocytes aredescribed. Speculations are put forward regarding their possible role in microcontrol of cell function.

Coupling of L-type voltage-sensitive calcium channels to P2X(2) purinoceptors in PC-12 cells

Extracellular ATP elevates cytosolic Ca(2+) by activating P2X and P2Y purinoceptors and voltage-sensitive Ca(2+) channels (VCCCs) in PC-12 cells, thereby facilitating catecholamine secretion. We investigated the mechanism by which ATP activates VSCCs. 2-Methylthioadenosine 5'-triphosphate (2-MeS-ATP) and UTP were used as preferential activators of P2X and P2Y, respectively. Nifedipine inhibited the ATP- and 2-MeS-ATP-evoked cytosolic Ca(2+) concentration increase and [(3)H]norepinephrine secretion, but not the UTP-evoked responses. Studies with Ca(2+) channel blockers indicated that L-type VSCCs were activated after the P2X activation. Mn(2+) entry profiles and studies with thapsigargin revealed that Ca(2+) entry, rather than Ca(2+) release, was sensitive to nifedipine. Although P2X(2) and P2X(4) receptor mRNAs were detected, studies with pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid revealed that P2X(2) was mainly coupled to the L-type VSCCs. The inhibitory effect of nifedipine did not occur in the absence of extracellular Na(+), suggesting that Na(+) influx, which induces depolarization, was essential for the P2X(2)-mediated activation of VSCCs. We report that depolarization induced by Na(+) entry through the P2X(2) purinoceptors effectively activates L-type VSCCs in PC-12 cells.

Predetermined recruitment of calcium release sites underlies excitation-contraction coupling in rat atrial myocytes

Excitation-contraction coupling (E-C coupling) was studied in isolated fluo-3-loaded rat atrial myocytes at 22 and 37 degrees C using rapid confocal microscopy. Within a few milliseconds of electrical excitation, spatially discrete subsarcolemmal Ca2+ signals were initiated. Twenty to forty milliseconds after stimulation the spatial overlap of these Ca2+ signals gave a 'ring' of elevated Ca2+ around the periphery of the cells. However, this ring was not continuous and substantial Ca2+ gradients were observed. The discrete subsarcolemmal Ca2+-release sites, which responded in a reproducible sequence to repetitive depolarisations and displayed the highest frequencies of spontaneous Ca2+ sparks in resting cells, were denoted 'eager sites'. Immunostaining atrial myocytes for type II ryanodine receptors (RyRs) revealed both subsarcolemmal 'junctional' RyRs, and also 'non-junctional' RyRs in the central bulk of the cells. A subset of the junctional RyRs comprises the eager sites. For cells paced in the presence of 1 mM extracellular Ca2+, the response was largely restricted to a subsarcolemmal 'ring', while the central bulk of the cell displayed a approximately 5-fold lower Ca2+ signal. Under these conditions the non-junctional RyRs were only weakly activated during E-C coupling. However, these channels are functional and the Ca2+ stores were at least partially loaded, since substantial homogeneous Ca2+ signals could be stimulated in the central regions of atrial myocytes by application of 2.5 mM caffeine. Neither the location nor activation order of the eager sites was affected by increasing the trigger Ca2+ current (by increasing extracellular Ca2+ to 10 mM) or the sarcoplasmic reticulum (SR) Ca2+ load (following 1 min incubation in 10 mM extracellular Ca2+), although with increased SR Ca2+ load, but not greater Ca2+ influx, the delay between the sequential activation of eager sites was reduced. In addition, increasing the trigger Ca2+ current or the SR Ca2+ load changed the spatial pattern of the Ca2+ response, in that the Ca2+ signal propagated more reliably from the subsarcolemmal initiation sites into the centre of the cell. Due to the greater spatial spread of the Ca2+ signals, the averaged global Ca2+ transients increased by approximately 500 %. We conclude that rat atrial myocytes display a predetermined spatiotemporal pattern of Ca2+ signalling during early E-C coupling. A consistent set of eager Ca2+ release sites with a fixed location and activation order on the junctional SR serve to initiate the cellular response. The short latency for activation of these eager sites suggests that they reflect clusters of RyRs closely coupled to voltage-operated Ca2+ channels in the sarcolemma. Furthermore, their propensity to show spontaneous Ca2+ sparks is consistent with an intrinsically higher sensitivity to Ca2+-induced Ca2+ release. While the subsarcolemmal Ca2+ response can be considered as stereotypic, the central bulk of the cell grades its response in direct proportion to cellular Ca2+ load and Ca2+ influx.

Calcium signalling--an overview

Calcium (Ca2+) is an almost universal intracellular messenger, controlling a diverse range of cellular processes, such as gene transcription, muscle contraction and cell proliferation. The ability of a simple ion such as Ca2+ to play a pivotal role in cell biology results from the facility that cells have to shape Ca2+ signals in the dimensions of space, time and amplitude. To generate the variety of observed Ca2+ signals, different cell types employ components selected from a Ca2+ signalling 'toolkit', which comprizes an array of signalling, homeostatic and sensory mechanisms. By mixing and matching components from the toolkit, cells can obtain Ca2+ signals that suit their physiology.

Heterogeneity of calcium stores and elementary release events in canine pulmonary arterial smooth muscle cells

To examine the nature of inositol 1,4,5-trisphosphate (IP(3))-sensitive and ryanodine (Ryn)-sensitive Ca(2+) stores in isolated canine pulmonary arterial smooth cells (PASMC), agonist-induced changes in global intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured using fura 2-AM fluorescence. Properties of elementary local Ca(2+) release events were characterized using fluo 3-AM or fluo 4-AM, in combination with confocal laser scanning microscopy. In PASMC, depletion of sarcoplasmic reticulum Ca(2+) stores with Ryn (300 microM) and caffeine (Caf; 10 mM) eliminated subsequent Caf-induced intracellular Ca(2+) transients but had little or no effect on the initial IP(3)-mediated intracellular Ca(2+) transient induced by ANG II (1 microM). Cyclopiazonic acid (CPA; 10 microM) abolished IP(3)-induced intracellular Ca(2+) transients but failed to attenuate the initial Caf-induced intracellular Ca(2+) transient. These results suggest that in canine PASMC, IP(3)-, and Ryn-sensitive Ca(2+) stores are organized into spatially distinct compartments while similar experiments in canine renal arterial smooth muscle cells (RASMC) reveal that these Ca(2+) stores are spatially conjoined. In PASMC, spontaneous local intracellular Ca(2+) transients sensitive to modulation by Caf and Ryn were detected, exhibiting spatial-temporal characteristics similar to those previously described for "Ca(2+) sparks" in cardiac and other types of smooth muscle cells. After depletion of Ryn-sensitive Ca(2+) stores, ANG II (8 nM) induced slow, sustained [Ca(2+)](i) increases originating at sites near the cell surface, which were abolished by depleting IP(3) stores. Discrete quantal-like events expected due to the coordinated opening of IP(3) receptor clusters ("Ca(2+) puffs") were not observed. These data provide new information regarding the functional properties and organization of intracellular Ca(2+) stores and elementary Ca(2+) release events in isolated PASMC.

The versatility and universality of calcium signalling

The universality of calcium as an intracellular messenger depends on its enormous versatility. Cells have a calcium signalling toolkit with many components that can be mixed and matched to create a wide range of spatial and temporal signals. This versatility is exploited to control processes as diverse as fertilization, proliferation, development, learning and memory, contraction and secretion, and must be accomplished within the context of calcium being highly toxic. Exceeding its normal spatial and temporal boundaries can result in cell death through both necrosis and apoptosis.

Functional InsP3 receptors that may modulate excitation-contraction coupling in the heart

The roles of the Ca2+-mobilising messenger inositol 1,4,5-trisphosphate (InsP3) in heart are unclear, although many hormones activate InsP3 production in cardiomyocytes and some of their inotropic, chronotropic and arrhythmogenic effects may be due to Ca2+ release mediated by InsP3 receptors (InsP3Rs) [1-3]. In the present study, we examined the expression and subcellular localisation of InsP3R isoforms, and investigated their potential role in modulating excitation-contraction coupling (EC coupling). Western, PCR and InsP3-binding analysis indicated that both atrial and ventricular myocytes expressed mainly type II InsP3Rs, with approximately sixfold higher levels of InsP3Rs in atrial cells. Co-immunostaining of atrial myocytes with antibodies against type II ryanodine receptors (RyRs) and type II InsP3Rs revealed that the latter were arranged in the subsarcolemmal space where they largely co-localised with the junctional RyRs. Stimulation of quiescent or electrically paced atrial myocytes with a membrane-permeant InsP3 ester, which enters cells and directly activates InsP3Rs, caused the appearance of spontaneous Ca2+-release events. In addition, in paced cells, the InsP3 ester evoked an increase in the amplitudes of action potential-evoked Ca2+ transients. These data indicate that atrial cardiomyocytes express functional InsP3Rs, and that these channels could modulate EC coupling.

Ca(2+) signals mediated by Ins(1,4,5)P(3)-gated channels in rat ureteric myocytes

Localized Ca(2+)-release signals (puffs) and propagated Ca(2+) waves were characterized in rat ureteric myocytes by confocal microscopy. Ca(2+) puffs were evoked by photorelease of low concentrations of Ins(1,4,5)P(3) from a caged precursor and by low concentrations of acetylcholine; they were also observed spontaneously in Ca(2+)-overloaded myocytes. Ca(2+) puffs showed some variability in amplitude, time course and spatial spread, suggesting that Ins(1,4,5)P(3)-gated channels exist in clusters containing variable numbers of channels and that within these clusters a variable number of channels can be recruited. Immunodetection of Ins(1,4,5)P(3) receptors revealed the existence of several spots of fluorescence in the confocal cell sections, supporting the existence of clusters of Ins(1,4,5)P(3) receptors. Strong Ins(1,4,5)P(3) photorelease and high concentrations of acetylcholine induced Ca(2+) waves that originated from an initiation site and propagated in the whole cell by spatial recruitment of neighbouring Ca(2+)-release sites. Both Ca(2+) puffs and Ca(2+) waves were blocked selectively by intracellular applications of heparin and an anti-Ins(1,4,5)P(3)-receptor antibody, but were unaffected by ryanodine and intracellular application of an anti-ryanodine receptor antibody. mRNAs encoding for the three subtypes of Ins(1,4,5)P(3) receptor and subtype 3 of ryanodine receptor were detected in these myocytes, and the maximal binding capacity of [(3)H]Ins(1,4,5)P(3) was 10- to 12-fold higher than that of [(3)H]ryanodine. These results suggest that Ins(1,4,5)P(3)-gated channels mediate a continuum of Ca(2+) signalling in smooth-muscle cells expressing a high level of Ins(1,4,5)P(3) receptors and no subtypes 1 and 2 of ryanodine receptors.

Three distinct types of Ca(2+) waves in Langendorff-perfused rat heart revealed by real-time confocal microscopy

Although Ca(2+) waves in cardiac myocytes are regarded as arrhythmogenic substrates, their properties in the heart in situ are poorly understood. On the hypothesis that Ca(2+) waves in the heart behave diversely and some of them influence the cardiac function, we analyzed their incidence, propagation velocity, and intercellular propagation at the subepicardial myocardium of fluo 3-loaded rat whole hearts using real-time laser scanning confocal microscopy. We classified Ca(2+) waves into 3 types. In intact regions showing homogeneous Ca(2+) transients under sinus rhythm (2 mmol/L [Ca(2+)](o)), Ca(2+) waves did not occur. Under quiescence, the waves occurred sporadically (3.8 waves. min(-1) x cell(-1)), with a velocity of 84 microm/s, a decline half-time (t(1/2)) of 0.16 seconds, and rare intercellular propagation (propagation ratio <0.06) (sporadic wave). In contrast, in presumably Ca(2+)-overloaded regions showing higher fluorescent intensity (113% versus the intact regions), Ca(2+) waves occurred at 28 waves x min(-1) x cell(-1) under quiescence with a higher velocity (116 microm/s), longer decline time (t(1/2) = 0.41 second), and occasional intercellular propagation (propagation ratio = 0.23) (Ca(2+)-overloaded wave). In regions with much higher fluorescent intensity (124% versus the intact region), Ca(2+) waves occurred with a high incidence (133 waves x min(-1) x cell(-1)) and little intercellular propagation (agonal wave). We conclude that the spatiotemporal properties of Ca(2+) waves in the heart are diverse and modulated by the Ca(2+)-loading state. The sporadic waves would not affect cardiac function, but prevalent Ca(2+)-overloaded and agonal waves may induce contractile failure and arrhythmias.

Requirement of ryanodine receptor subtypes 1 and 2 for Ca(2+)-induced Ca(2+) release in vascular myocytes

While the roles of subtypes 1 and 2 of the ryanodine receptors (RYRs) have been studied in cellular systems expressing specifically one or the other of these subtypes (i.e. skeletal and cardiac muscle), the function of these receptors has not been evaluated in smooth muscles. We have previously reported RYR-mediated elementary (Ca(2+) sparks) and global Ca(2+) responses in rat portal vein myocytes. Here, we investigated the respective roles of all three RYR subtypes expressed in these cells as revealed by reverse transcriptase-polymerase chain reaction. Antisense oligonucleotides targeting each one of the three RYR subtypes were shown to specifically inhibit the expression of the corresponding mRNA and protein without affecting the other RYR subtypes. Confocal Ca(2+) measurements revealed that depolarization-induced Ca(2+) sparks and global Ca(2+) responses were blocked when either RYR1 or RYR2 expression was suppressed. Caffeine-induced Ca(2+) responses were partly inhibited by the same antisense oligonucleotides. Neither the corresponding scrambled oligonucleotides nor the antisense oligonucleotides targeting RYR3 affected depolarization- or caffeine-induced Ca(2+) responses. Our results show that, in vascular myocytes, the two RYR1 and RYR2 subtypes are required for Ca(2+) release during Ca(2+) sparks and global Ca(2+) responses, evoked by activation of voltage-gated Ca(2+) channels.

Sustained recruitment of phospholipase C-gamma to Gab1 is required for HGF-induced branching tubulogenesis

A distinctive property of Hepatocyte Growth Factor (HGF) is its ability to induce differentiation of tubular structures from epithelial and endothelial cells (branching tubulogenesis). The HGF receptor directly activates PI3 kinase, Ras and STAT signalling pathways and phosphorylates the adaptator GRB2 Associated Binder-1 (Gab1). Gab1 is also phosphorylated in response to Epidermal Growth Factor (EGF) but is unable to induce tubule formation. Comparison of 32P-peptide maps of Gab1 from EGF- versus HGF-treated cells, demonstrates that the same sites are phosphorylated in vivo. However, while both EGF and HGF induce rapid tyrosine phosphorylation of Gab1 with a peak at 15 min, the phosphorylation persists for over 1 h, only in response to HGF. Nine tyrosines are phosphorylated by both receptors. Three of them (Y307, Y373, Y407) bind phospholipase C-gamma (PLC-gamma). Interestingly, the overexpression of a Gab1 mutant unable to bind PLC-gamma (Gab1 Y307/373/407F) did not alter HGF-stimulated cell scattering, only partially reduced the growth stimulation but completely abolished HGF-mediated tubulogenesis. It is concluded that sustained recruitment of PLCgamma to Gab1 plays an important role in branching tubulogenesis.

Microscopic properties of elementary Ca2+ release sites in non-excitable cells

BACKGROUND

Elementary Ca2+ signals, such as 'Ca2+ puffs', that arise from the activation of clusters of inositol 1 ,4,5,-trisphosphate (InsP3) receptors are the building blocks for local and global Ca2+ signalling. We previously found that one, or a few, Ca2+ puff sites within agonist-stimulated cells act as 'pacemakers' to initiate global Ca2+ waves. The factors that distinguish these pacemaker Ca2+ puff sites from the other Ca2+ release sites that simply participate in Ca2+ wave propagation are unknown.

RESULTS

The spatiotemporal properties of Ca2+ puffs were investigated using confocal microscopy of fluo3-loaded HeLa cells. The same pacemaker Ca2+ puff sites were activated during stimulation of cells with different agonists. The majority of agonist-stimulated pacemaker Ca2+ puffs originated in a perinuclear location. The positions of such Ca2+ puff sites were stable for up to 2 hours, and were not affected by disruption of the actin cytoskeleton. A similar perinuclear distribution of Ca2+ puff sites was also observed when InsP3 receptors were directly stimulated with thimerosal or membrane-permeant InsP3 esters. Immunostaining indicated that the perinuclear position of pacemaker Ca2+ puffs was not due to the localised expression of InsP3 receptors.

CONCLUSIONS

The pacemaker Ca2+ puff sites that initiate Ca2+ responses are temporally and spatially stable within cells. These Ca2+ release sites are distinguished from their neighbours by an intrinsically higher InsP3 sensitivity.

Intracellular Ca(2+) dynamics and the stability of ventricular tachycardia

Ventricular fibrillation (VF), the major cause of sudden cardiac death, is typically preceded by ventricular tachycardia (VT), but the mechanisms underlying the transition from VT to VF are poorly understood. Intracellular Ca(2+) overload occurs during rapid heart rates typical of VT and is also known to promote arrhythmias. We therefore studied the role of intracellular Ca(2+) dynamics in the transition from VT to VF, using a combined experimental and mathematical modeling approach. Our results show that 1) rapid pacing of rabbit ventricular myocytes at 35 degrees C led to increased intracellular Ca(2+) levels and complex patterns of action potential (AP) configuration and the intracellular Ca(2+) transients; 2) the complex patterns of the Ca(2+) transient arose directly from the dynamics of intracellular Ca(2+) cycling, and were not merely passive responses to beat-to-beat alterations in AP; 3) the complex Ca(2+) dynamics were simulated in a modified version of the Luo-Rudy (LR) ventricular action potential with improved intracellular Ca(2+) dynamics, and showed good agreement with the experimental findings in isolated myocytes; and 4) when incorporated into simulated two-dimensional cardiac tissue, this action potential model produced a form of spiral wave breakup from VT to a VF-like state in which intracellular Ca(2+) dynamics played a key role through its influence on Ca(2+)-sensitive membrane currents such as I(Ca), I(NaCa), and I(ns(Ca)). To the extent that spiral wave breakup is useful as a model for the transition from VT to VF, these findings suggest that intracellular Ca(2+) dynamics may play an important role in the destabilization of VT and its degeneration into VF.

Regulation of ryanodine receptor opening by lumenal Ca(2+) underlies quantal Ca(2+) release in PC12 cells

Graded or "quantal" Ca(2+) release from intracellular stores has been observed in various cell types following activation of either ryanodine receptors (RyR) or inositol 1,4,5-trisphosphate receptors (InsP(3)R). The mechanism causing the release of Ca(2+) stores in direct proportion to the strength of stimulation is unresolved. We investigated the properties of quantal Ca(2+) release evoked by activation of RyR in PC12 cells, and in particular whether the sensitivity of RyR to the agonist caffeine was altered by lumenal Ca(2+). Quantal Ca(2+) release was observed in cells stimulated with 1 to 40 mM caffeine, a range of caffeine concentrations giving a >10-fold change in lumenal Ca(2+) content. The Ca(2+) load of the caffeine-sensitive stores was modulated by allowing them to refill for varying times after complete discharge with maximal caffeine, or by depolarizing the cells with K(+) to enhance their normal steady-state loading. The threshold for RyR activation was sensitized approximately 10-fold as the Ca(2+) load increased from a minimal to a maximal loading. In addition, the fraction of Ca(2+) released by low caffeine concentrations increased. Our data suggest that RyR are sensitive to lumenal Ca(2+) over the full range of Ca(2+) loads that can be achieved in an intact PC12 cell, and that changes in RyR sensitivity may be responsible for the termination of Ca(2+) release underlying the quantal effect.

Imaging Ca(2+) entering the cytoplasm through a single opening of a plasma membrane cation channel

Discrete localized fluorescence transients due to openings of a single plasma membrane Ca(2+) permeable cation channel were recorded using wide-field digital imaging microscopy with fluo-3 as the Ca(2+) indicator. These transients were obtained while simultaneously recording the unitary channel currents using the whole-cell current-recording configuration of the patch-clamp technique. This cation channel in smooth muscle cells is opened by caffeine (Guerrero, A., F.S. Fay, and J.J. Singer. 1994. J. Gen. Physiol. 104:375-394). The localized fluorescence transients appeared to occur at random locations on the cell membrane, with the duration of the rising phase matching the duration of the channel opening. Moreover, these transients were only observed in the presence of sufficient extracellular Ca(2+), suggesting that they are due to Ca(2+) influx from the bathing solution. The fluorescence transient is characterized by an initial fast rising phase when the channel opens, followed by a slower rising phase during prolonged openings. When the channel closes there is an immediate fast falling phase followed by a slower falling phase. Computer simulations of the underlying events were used to interpret the time course of the transients. The rapid phases are mainly due to the establishment or removal of Ca(2+) and Ca(2+)-bound fluo-3 gradients near the channel when the channel opens or closes, while the slow phases are due to the diffusion of Ca(2+) and Ca(2+)-bound fluo-3 into the cytoplasm. Transients due to short channel openings have a "Ca(2+) spark-like" appearance, suggesting that the rising and early falling components of sparks (due to openings of ryanodine receptors) reflect the fast phases of the fluorescence change. The results presented here suggest methods to determine the relationship between the fluorescence transient and the underlying Ca(2+) current, to study intracellular localized Ca(2+) handling as might occur from single Ca(2+) channel openings, and to localize Ca(2+) permeable ion channels on the plasma membrane.

Shape, size, and distribution of Ca(2+) release units and couplons in skeletal and cardiac muscles

Excitation contraction (e-c) coupling in skeletal and cardiac muscles involves an interaction between specialized junctional domains of the sarcoplasmic reticulum (SR) and of exterior membranes (either surface membrane or transverse (T) tubules). This interaction occurs at special structures named calcium release units (CRUs). CRUs contain two proteins essential to e-c coupling: dihydropyridine receptors (DHPRs), L-type Ca(2+) channels of exterior membranes; and ryanodine receptors (RyRs), the Ca(2+) release channels of the SR. Special CRUs in cardiac muscle are constituted by SR domains bearing RyRs that are not associated with exterior membranes (the corbular and extended junctional SR or EjSR). Functional groupings of RyRs and DHPRs within calcium release units have been named couplons, and the term is also loosely applied to the EjSR of cardiac muscle. Knowledge of the structure, geometry, and disposition of couplons is essential to understand the mechanism of Ca(2+) release during muscle activation. This paper presents a compilation of quantitative data on couplons in a variety of skeletal and cardiac muscles, which is useful in modeling calcium release events, both macroscopic and microscopic ("sparks").