Spatial characteristics of sarcoplasmic reticulum Ca2+ release events triggered by L-type Ca2+ current and Na+ current in guinea-pig cardiac myocytes

On the Adjacency Matrix of RyR2 Cluster Structures

In the heart, electrical stimulation of cardiac myocytes increases the open probability of sarcolemmal voltage-sensitive Ca2+ channels and flux of Ca2+ into the cells. This increases Ca2+ binding to ligand-gated channels known as ryanodine receptors (RyR2). Their openings cause cell-wide release of Ca2+, which in turn causes muscle contraction and the generation of the mechanical force required to pump blood. In resting myocytes, RyR2s can also open spontaneously giving rise to spatially-confined Ca2+ release events known as "sparks." RyR2s are organized in a lattice to form clusters in the junctional sarcoplasmic reticulum membrane. Our recent work has shown that the spatial arrangement of RyR2s within clusters strongly influences the frequency of Ca2+ sparks. We showed that the probability of a Ca2+ spark occurring when a single RyR2 in the cluster opens spontaneously can be predicted from the precise spatial arrangements of the RyR2s. Thus, "function" follows from "structure." This probability is related to the maximum eigenvalue (λ1) of the adjacency matrix of the RyR2 cluster lattice. In this work, we develop a theoretical framework for understanding this relationship. We present a stochastic contact network model of the Ca2+ spark initiation process. We show that λ1 determines a stability threshold for the formation of Ca2+ sparks in terms of the RyR2 gating transition rates. We recapitulate these results by applying the model to realistic RyR2 cluster structures informed by super-resolution stimulated emission depletion (STED) microscopy. Eigendecomposition of the linearized mean-field contact network model reveals functional subdomains within RyR2 clusters with distinct sensitivities to Ca2+. This work provides novel perspectives on the cardiac Ca2+ release process and a general method for inferring the functional properties of transmembrane receptor clusters from their structure.

Two-photon excitation microscopy for the study of living cells and tissues

Two-photon excitation microscopy is an alternative to confocal microscopy that provides advantages for three-dimensional and deep tissue imaging. This unit will describe the basic physical principles behind two-photon excitation and discuss the advantages and limitations of its use in laser-scanning microscopy. The principal advantages of two-photon microscopy are reduced phototoxicity, increased imaging depth, and the ability to initiate highly localized photochemistry in thick samples. Practical considerations for the application of two-photon microscopy will then be discussed, including recent technological advances. This unit will conclude with some recent applications of two-photon microscopy that highlight the key advantages over confocal microscopy and the types of experiments which would benefit most from its application.

Nicotinic acid adenine dinucleotide phosphate (NAADP) activates global and heterogeneous local Ca2+ signals from NAADP- and ryanodine receptor-gated Ca2+ stores in pulmonary arterial myocytes

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca(2+)-mobilizing messenger that releases Ca(2+) from endolysosomal organelles. Recent studies showed that NAADP-induced Ca(2+) release is mediated by the two-pore channels (TPCs) TPC1 and TPC2. However, the expression of TPCs and the NAADP-induced local Ca(2+) signals have not been examined in vascular smooth muscle. Here, we found that both TPC1 and TPC2 are expressed in rat pulmonary arterial smooth muscle cells (PASMCs), with TPC1 being the major subtype. Application of membrane-permeant NAADP acetoxymethyl ester to PASMCs elicited a biphasic increase in global [Ca(2+)]i, which was independent of extracellular Ca(2+) and blocked by the NAADP antagonist Ned-19 or the vacuolar H(+)-ATPase inhibitor bafilomycin A1, indicating Ca(2+) release from acidic endolysosomal Ca(2+) stores. The Ca(2+) response was unaffected by xestospongin C but was partially blocked by ryanodine or thapsigargin. NAADP triggered heterogeneous local Ca(2+) signals, including a diffuse increase in cytosolic [Ca(2+)], Ca(2+) sparks, Ca(2+) bursts, and regenerative Ca(2+) release. The diffuse Ca(2+) increase and Ca(2+) bursts were ryanodine-insensitive, presumably arising from different endolysosomal sources. Ca(2+) sparks and regenerative Ca(2+) release were inhibited by ryanodine, consistent with cross-activation of loosely coupled ryanodine receptors. Moreover, Ca(2+) release stimulated by endothelin-1 was inhibited by Ned-19, ryanodine, or xestospongin C, suggesting that NAADP-mediated Ca(2+) signals interact with both ryanodine and inositol 1,4,5-trisphosphate receptors during agonist stimulation. Our results show that NAADP mediates complex global and local Ca(2+) signals. Depending on the physiological stimuli, these diverse Ca(2+) signals may serve to regulate different cellular functions in PASMCs.

Simulation of arrhythmogenic effect of rogue RyRs in failing heart by using a coupled model

Cardiac cells with heart failure are usually characterized by impairment of Ca²⁺ handling with smaller SR Ca²⁺ store and high risk of triggered activities. In this study, we developed a coupled model by integrating the spatiotemporal Ca²⁺ reaction-diffusion system into the cellular electrophysiological model. With the coupled model, the subcellular Ca²⁺ dynamics and global cellular electrophysiology could be simultaneously traced. The proposed coupled model was then applied to study the effects of rogue RyRs on Ca²⁺ cycling and membrane potential in failing heart. The simulation results suggested that, in the presence of rogue RyRs, Ca²⁺ dynamics is unstable and Ca²⁺ waves are prone to be initiated spontaneously. These release events would elevate the membrane potential substantially which might induce delayed afterdepolarizations or triggered action potentials. Moreover, the variation of membrane potential depolarization is indicated to be dependent on the distribution density of rogue RyR channels. This study provides a new possible arrhythmogenic mechanism for heart failure from subcellular to cellular level.

Mutation of the calmodulin binding motif IQ of the L-type Ca(v)1.2 Ca2+ channel to EQ induces dilated cardiomyopathy and death

Cardiac excitation-contraction coupling (EC coupling) links the electrical excitation of the cell membrane to the mechanical contractile machinery of the heart. Calcium channels are major players of EC coupling and are regulated by voltage and Ca(2+)/calmodulin (CaM). CaM binds to the IQ motif located in the C terminus of the Ca(v)1.2 channel and induces Ca(2+)-dependent inactivation (CDI) and facilitation (CDF). Mutation of Ile to Glu (Ile1624Glu) in the IQ motif abolished regulation of the channel by CDI and CDF. Here, we addressed the physiological consequences of such a mutation in the heart. Murine hearts expressing the Ca(v)1.2(I1624E) mutation were generated in adult heterozygous mice through inactivation of the floxed WT Ca(v)1.2(L2) allele by tamoxifen-induced cardiac-specific activation of the MerCreMer Cre recombinase. Within 10 days after the first tamoxifen injection these mice developed dilated cardiomyopathy (DCM) accompanied by apoptosis of cardiac myocytes (CM) and fibrosis. In Ca(v)1.2(I1624E) hearts, the activity of phospho-CaM kinase II and phospho-MAPK was increased. CMs expressed reduced levels of Ca(v)1.2(I1624E) channel protein and I(Ca). The Ca(v)1.2(I1624E) channel showed "CDI" kinetics. Despite a lower sarcoplasmic reticulum Ca(2+) content, cellular contractility and global Ca(2+) transients remained unchanged because the EC coupling gain was up-regulated by an increased neuroendocrine activity. Treatment of mice with metoprolol and captopril reduced DCM in Ca(v)1.2(I1624E) hearts at day 10. We conclude that mutation of the IQ motif to IE leads to dilated cardiomyopathy and death.

Glial glutamate transporter and glutamine synthetase regulate GABAergic synaptic strength in the spinal dorsal horn

Decreased GABAergic synaptic strength ('disinhibition') in the spinal dorsal horn is a crucial mechanism contributing to the development and maintenance of pathological pain. However, mechanisms leading to disinhibition in the spinal dorsal horn remain elusive. We investigated the role of glial glutamate transporters (GLT-1 and GLAST) and glutamine synthetase in maintaining GABAergic synaptic activity in the spinal dorsal horn. Electrically evoked GABAergic inhibitory post-synaptic currents (eIPSCs), spontaneous IPSCs (sIPSCs) and miniature IPSCs were recorded in superficial spinal dorsal horn neurons of spinal slices from young adult rats. We used (2S,3S)-3-[3-[4-(trifluoromethyl)benzoylamino]benzyloxy]aspartate (TFB-TBOA), to block both GLT-1 and GLAST and dihydrokainic acid to block only GLT-1. We found that blockade of both GLAST and GLT-1 and blockade of only GLT-1 in the spinal dorsal horn decreased the amplitude of GABAergic eIPSCs, as well as both the amplitude and frequency of GABAergic sIPSCs or miniature IPSCs. Pharmacological inhibition of glial glutamine synthetase had similar effects on both GABAergic eIPSCs and sIPSCs. We provided evidence demonstrating that the reduction in GABAergic strength induced by the inhibition of glial glutamate transporters is due to insufficient GABA synthesis through the glutamate-glutamine cycle between astrocytes and neurons. Thus, our results indicate that deficient glial glutamate transporters and glutamine synthetase significantly attenuate GABAergic synaptic strength in the spinal dorsal horn, which may be a crucial synaptic mechanism underlying glial-neuronal interactions caused by dysfunctional astrocytes in pathological pain conditions.

Ca sparks do not explain all ryanodine receptor-mediated SR Ca leak in mouse ventricular myocytes

Diastolic Ca leak from the sarcoplasmic reticulum (SR) of ventricular myocytes reduces the SR Ca content, stabilizing the activity of the SR Ca release channel ryanodine receptor for the next beat. SR Ca leak has been visualized globally using whole-cell fluorescence, or locally using confocal microscopy, but never both ways. When using confocal microscopy, leak is imaged as "Ca sparks," which are fluorescent objects generated by the local reaction-diffusion of released Ca and cytosolic indicator. Here, we used confocal microscopy and simultaneously measured the global ryanodine-receptor-mediated leak rate (J(leak)) and Ca sparks in intact mouse ventricular myocytes. We found that spark frequency and J(leak) are correlated, as expected if both are manifestations of a common phenomenon. However, we also found that sparks explain approximately half of J(leak). Our strategy unmasks the presence of a subresolution (i.e., nonspark) release of potential physiological relevance.

Simulation of the effect of rogue ryanodine receptors on a calcium wave in ventricular myocytes with heart failure

Calcium homeostasis is considered to be one of the most important factors for the contraction and relaxation of the heart muscle. However, under some pathological conditions, such as heart failure (HF), calcium homeostasis is disordered, and spontaneous waves may occur. In this study, we developed a mathematical model of formation and propagation of a calcium wave based upon a governing system of diffusion-reaction equations presented by Izu et al (2001 Biophys. J. 80 103-20) and integrated non-clustered or 'rogue' ryanodine receptors (rogue RyRs) into a two-dimensional (2D) model of ventricular myocytes isolated from failing hearts in which sarcoplasmic reticulum (SR) Ca(2+) pools are partially unloaded. The model was then used to simulate the effect of rogue RyRs on initiation and propagation of the calcium wave in ventricular myocytes with HF. Our simulation results show that rogue RyRs can amplify the diastolic SR Ca(2+) leak in the form of Ca(2+) quarks, increase the probability of occurrence of spontaneous Ca(2+) waves even with smaller SR Ca(2+) stores, accelerate Ca(2+) wave propagation, and hence lead to delayed afterdepolarizations (DADs) and cardiac arrhythmia in the diseased heart. This investigation suggests that incorporating rogue RyRs in the Ca(2+) wave model under HF conditions provides a new view of Ca(2+) dynamics that could not be mimicked by adjusting traditional parameters involved in Ca(2+) release units and other ion channels, and contributes to understanding the underlying mechanism of HF.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

An electronic device for artefact suppression in human local field potential recordings during deep brain stimulation

The clinical efficacy of high-frequency deep brain stimulation (DBS) for Parkinson's disease and other neuropsychiatric disorders likely depends on the modulation of neuronal rhythms in the target nuclei. This modulation could be effectively measured with local field potential (LFP) recordings during DBS. However, a technical drawback that prevents LFPs from being recorded from the DBS target nuclei during stimulation is the stimulus artefact. To solve this problem, we designed and developed 'FilterDBS', an electronic amplification system for artefact-free LFP recordings (in the frequency range 2-40 Hz) during DBS. After defining the estimated system requirements for LFP amplification and DBS artefact suppression, we tested the FilterDBS system by conducting experiments in vitro and in vivo in patients with advanced Parkinson's disease undergoing DBS of the subthalamic nucleus (STN). Under both experimental conditions, in vitro and in vivo, the FilterDBS system completely suppressed the DBS artefact without inducing significant spectral distortion. The FilterDBS device pioneers the development of an adaptive DBS system retroacted by LFPs and can be used in novel closed-loop brain-machine interface applications in patients with neurological disorders.

Mini-dystrophin expression down-regulates IP3-mediated calcium release events in resting dystrophin-deficient muscle cells

We present here evidence for the enhancement, at rest, of an inositol 1,4,5-trisphosphate (IP3)–mediated calcium signaling pathway in myotubes from dystrophin-deficient cell lines (SolC1(−)) as compared to a cell line from the same origin but transfected with mini-dystrophin (SolD(+)). With confocal microscopy, the number of sites discharging calcium (release site density [RSD]) was quantified and found more elevated in SolC1(−) than in SolD(+) myotubes. Variations of membrane potential had no significant effect on this difference, and higher resting [Ca²⁺]i in SolC1(−) (Marchand, E., B. Constantin, H. Balghi, M.C. Claudepierre, A. Cantereau, C. Magaud, A. Mouzou, G. Raymond, S. Braun, and C. Cognard. 2004. Exp. Cell Res. 297:363–379) cannot explain alone higher RSD. The exposure with SR Ca²⁺ channel inhibitors (ryanodine and 2-APB) and phospholipase C inhibitor (U73122) significantly reduced RSD in both cell types but with a stronger effect in dystrophin-deficient SolC1(−) myotubes. Immunocytochemistry allowed us to localize ryanodine receptors (RyRs) as well as IP3 receptors (IP3Rs), IP3R-1 and IP3R-2 isoforms, indicating the presence of both RyRs-dependent and IP3-dependent release systems in both cells. We previously reported evidence for the enhancement, through a Gi protein, of the IP3-mediated calcium signaling pathway in SolC1(−) as compared to SolD(+) myotubes during a high K⁺ stimulation (Balghi, H., S. Sebille, B. Constantin, S. Patri, V. Thoreau, L. Mondin, E. Mok, A. Kitzis, G. Raymond, and C. Cognard. 2006. J. Gen. Physiol. 127:171–182). Here we show that, at rest, these regulation mechanisms are also involved in the modulation of calcium release activities. The enhancement of resting release activity may participate in the calcium overload observed in dystrophin-deficient myotubes, and our findings support the hypothesis of the regulatory role of mini-dystrophin on intracellular signaling.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Variability in couplon size in rabbit ventricular myocytes

Variation in couplon size is thought to be essential for graded Ca(2+) transients in cardiac myocytes. We examined this variation by investigating spark appearance in rabbit ventricular myocytes at various locations and at potentials from -20 to 0 mV. At 0 mV, sparks appeared at the beginning of the voltage step with a probability of unity. On the other hand, at -20 mV, sparks appeared later during the voltage step with a lower probability. The cumulative spark probabilities at various potentials were fitted with exponential functions of both time and voltage. Spark latency became longer as spark probability decreased at more negative potentials. At -20 mV, the cumulative spark probability and the mean spark latency were not only variable among locations but also inversely related. Under the assumption that a single opening of an L-type Ca(2+) channel triggers a spark, we suggest a simple mathematical explanation for the distribution of spark appearance. The variation in spark probability and latency with location suggests that the couplon size, and hence the number of L-type Ca(2+) channels in a couplon is variable.

The Ca 2+ leak paradox and rogue ryanodine receptors: SR Ca 2+ efflux theory and practice

Ca(2+) efflux from the sarcoplasmic reticulum (SR) is routed primarily through SR Ca(2+) release channels (ryanodine receptors, RyRs). When clusters of RyRs are activated by trigger Ca(2+) influx through L-type Ca(2+) channels (dihydropyridine receptors, DHPR), Ca(2+) sparks are observed. Close spatial coupling between DHPRs and RyR clusters and the relative insensitivity of RyRs to be triggered by Ca(2+) together ensure the stability of this positive-feedback system of Ca(2+) amplification. Despite evidence from single channel RyR gating experiments that phosphorylation of RyRs by protein kinase A (PKA) or calcium-calmodulin dependent protein kinase II (CAMK II) causes an increase in the sensitivity of the RyR to be triggered by [Ca(2+)](i) there is little clear evidence to date showing an increase in Ca(2+) spark rate. Indeed, there is some evidence that the SR Ca(2+) content may be decreased in hyperadrenergic disease states. The question is whether or not these observations are compatible with each other and with the development of arrhythmogenic extrasystoles that can occur under these conditions. Furthermore, the appearance of an increase in the SR Ca(2+) "leak" under these conditions is perplexing. These and related complexities are analyzed and discussed in this report. Using simple mathematical modeling discussed in the context of recent experimental findings, a possible resolution to this paradox is proposed. The resolution depends upon two features of SR function that have not been confirmed directly but are broadly consistent with several lines of indirect evidence: (1) the existence of unclustered or "rogue" RyRs that may respond differently to local [Ca(2+)](i) in diastole and during the [Ca(2+)](i) transient; and (2) a decrease in cooperative or coupled gating between clustered RyRs in response to physiologic phosphorylation or hyper-phosphorylation of RyRs in disease states such as heart failure. Taken together, these two features may provide a framework that allows for an improved understanding of cardiac Ca(2+) signaling.

Artifactual contractions triggered by field stimulation of cardiomyocytes

Although cell shortening in patch-clamped cells (current-clamp mode) is triggered by an ordinary action potential, the trigger mechanism in field-stimulated cells is not so obvious. The contraction characteristics of the two methods differ, and we, therefore, examined the triggering sequence in field-stimulated cells. Isolated rat cardiomyocytes were plated on laminin-coated coverslips that were mounted on an inverted light microscope and superfused with HEPES-Tyrode buffer (pH 7.4; 37°C). The cells were stimulated to contract either by a 0.5-ms current injection (CC cells) through high-resistance electrodes or a 5-ms biphasic field-stimulation pulse (FS cells), and drugs were added to block sarcolemmal proteins involved in excitation-contraction coupling. Time to peak contraction (TTP) was significantly longer in FS cells and was not affected by the polarity or the length of the stimulus pulse. Tetrodotoxin (TTX; 20 μM) blocked cell shortening in CC cells but not in FS cells. Ni2+ (5 mM) blocked cell shortening in FS cells, whereas KB-R7943 (KB; 5 μM) had no effect either on cell shortening or TTP. In FS cells, nifedipine (Nif; 100 μM) and Cd2+ (300 μM) reduced fractional shortening by 34 and 63%, respectively, but only Cd2+ affected TTP (reduced by 48%). A combination of Nif and KB reduced cell shortening by 50%, whereas a combination of Cd2+ and KB almost abolished cell shortening. We conclude that field stimulation per se prolongs TTP and that cell shortening in FS cells is not dependent on Na+ current but is triggered by a combination of L-type Ca2+ current and reverse mode Na+/Ca2+ exchange.

Visualization of local Ca2+ dynamics with genetically encoded bioluminescent reporters

Measurements of local Ca2+ signalling at different developmental stages and/or in specific cell types is important for understanding aspects of brain functioning. The use of light excitation in fluorescence imaging can cause phototoxicity, photobleaching and auto-fluorescence. In contrast, bioluminescence does not require the input of radiative energy and can therefore be measured over long periods, with very high temporal resolution. Aequorin is a genetically encoded Ca(2+)-sensitive bioluminescent protein, however, its low quantum yield prevents dynamic measurements of Ca2+ responses in single cells. To overcome this limitation, we recently reported the bi-functional Ca2+ reporter gene, GFP-aequorin (GA), which was developed specifically to improve the light output and stability of aequorin chimeras [V. Baubet, et al., (2000) PNAS, 97, 7260-7265]. In the current study, we have genetically targeted GA to different microdomains important in synaptic transmission, including to the mitochondrial matrix, endoplasmic reticulum, synaptic vesicles and to the postsynaptic density. We demonstrate that these reporters enable 'real-time' measurements of subcellular Ca2+ changes in single mammalian neurons using bioluminescence. The high signal-to-noise ratio of these reporters is also important in that it affords the visualization of Ca2+ dynamics in cell-cell communication in neuronal cultures and tissue slices. Further, we demonstrate the utility of this approach in ex-vivo preparations of mammalian retina, a paradigm in which external light input should be controlled. This represents a novel molecular imaging approach for non-invasive monitoring of local Ca2+ dynamics and cellular communication in tissue or whole animal studies.

[Na(+)] in the subsarcolemmal 'fuzzy' space and modulation of [Ca(2+)](i) and contraction in cardiac myocytes

The strength of the heart beat depends on the amplitude and time course of the transient increase in [Ca(2+)] in the myocytes with each cycle. [Na(+)](i) modulates cardiac contraction through its effect on the Ca(2+) flux through the Na/Ca exchanger. Cardiac excitation-contraction coupling has been postulated to occur in a microdomain or 'fuzzy' space at the junction of the T-tubules and the sarcoplasmic reticulum. This 'fuzzy' space is well described for the Ca(2+) fluxes and the interaction between the L-type Ca(2+) channel, the Ca(2+) release channel of the sarcoplasmic reticulum and the Na/Ca exchanger. Co-localization of the Na(+) transporters, in particular the Na/K pump and the Na(+) channel, within this 'fuzzy' space is not as well established. The functional and morphological characteristics of the 'fuzzy' space for Na(+) and its interaction with the Ca(2+) handling suggest that this space is not strictly co-inciding with the Ca(2+) microdomain. In this space [Na(+)] can be several-fold higher or lower than [Na(+)] in the bulk cytosol. This has implications for modulation of [Ca(2+)](i) during a single beat as well as during alterations in Na(+) fluxes seen in pathological conditions.

Cardiac-type EC-coupling in dysgenic myotubes restored with Ca2+ channel subunit isoforms alpha1C and alpha1D does not correlate with current density

Ca(2+)-induced Ca(2+)-release (CICR)-the mechanism of cardiac excitation-contraction (EC) coupling-also contributes to skeletal muscle contraction; however, its properties are still poorly understood. CICR in skeletal muscle can be induced independently of direct, calcium-independent activation of sarcoplasmic reticulum Ca(2+) release, by reconstituting dysgenic myotubes with the cardiac Ca(2+) channel alpha(1C) (Ca(V)1.2) subunit. Ca(2+) influx through alpha(1C) provides the trigger for opening the sarcoplasmic reticulum Ca(2+) release channels. Here we show that also the Ca(2+) channel alpha(1D) isoform (Ca(V)1.3) can restore cardiac-type EC-coupling. GFP-alpha(1D) expressed in dysgenic myotubes is correctly targeted into the triad junctions and generates action potential-induced Ca(2+) transients with the same efficiency as GFP-alpha(1C) despite threefold smaller Ca(2+) currents. In contrast, GFP-alpha(1A), which generates large currents but is not targeted into triads, rarely restores action potential-induced Ca(2+) transients. Thus, cardiac-type EC-coupling in skeletal myotubes depends primarily on the correct targeting of the voltage-gated Ca(2+) channels and less on their current size. Combined patch-clamp/fluo-4 Ca(2+) recordings revealed that the induction of Ca(2+) transients and their maximal amplitudes are independent of the different current densities of GFP-alpha(1C) and GFP-alpha(1D). These properties of cardiac-type EC-coupling in dysgenic myotubes are consistent with a CICR mechanism under the control of local Ca(2+) gradients in the triad junctions.

Mitochondria efficiently buffer subplasmalemmal Ca2+ elevation during agonist stimulation

In endothelial cells, local Ca(2+) release from superficial endoplasmic reticulum (ER) activates BK(Ca) channels. The resulting hyperpolarization promotes capacitative Ca(2+) entry (CCE), which, unlike BK(Ca) channels, is inhibited by high Ca(2+). To understand how the coordinated activation of plasma membrane ion channels with opposite Ca(2+) sensitivity is orchestrated, the individual contribution of mitochondria and ER in regulation of subplasmalemmal Ca(2+) concentration ([Ca(2+)](pm)) was investigated. For organelle visualization, cells were transfected with DsRed and yellow cameleon targeted to mitochondria and ER. The patch pipette was placed far from any organelle (L1), close to ER (L3), or mitochondria (L2) and activity of BK(Ca) channels was used to estimate local [Ca(2+)](pm). Under standard patch conditions (130 mm K(+) in the bath), histamine increased [Ca(2+)](pm) at L1 and L3 to approximately 1.6 microm, whereas close to mitochondria [Ca(2+)](pm) remained unchanged. If mitochondria moved apart from the pipette or in the presence of carbonyl cyanide-4-trifluoromethoxyphenylhyrazone, [Ca(2+)](pm) at L2 increased in response to histamine. Under standard patch conditions Ca(2+) entry was negligible due to cell depolarization. Using a physiological patch approach (5.6 mm K(+) in the bath), changes in [Ca(2+)](pm) to histamine could be monitored without cell depolarization and, thus, in conditions where Ca(2+) entry occurred. Here, histamine induced an initial transient Ca(2+) elevation to > or =3.5 microm followed by a long lasting plateau at approximately 1.2 microm in L1 and L3, whereas mitochondria kept neighboring [Ca(2+)](pm) low during stimulation. Thus, superficial mitochondria and ER generate local domains of low and high Ca(2+) allowing simultaneous activation of BK(Ca) and CCE, despite their opposite Ca(2+) sensitivity.

Ca2+ sparks in rabbit ventricular myocytes evoked by action potentials: involvement of clusters of L-type Ca2+ channels

It is not clear how many L-type Ca2+ channels (LCCs) are required to ensure that a Ca2+ spark is triggered during a normal mammalian action potential (AP). We investigated this in rabbit ventricular myocytes by examining both the properties of sparks evoked by APs and the activity of LCCs. We measured Ca2+ sparks evoked by repeated APs with pipettes containing 2 mmol/L EGTA and single LCC activity in cell-attached patches depolarized to +50 mV using pipettes containing 110 mmol/L Ba2+. With 2 mmol/L Ca2+ in the external solution, we observed sparks at the beginning of every evoked AP at numerous locations. Each spark was observed repeatedly at a fixed location and began during a limited interval after the AP peak. These sparks occurred with a probability of approximately unity. However, the chance that an LCC does not open during the interval when a spark is triggered is quite high ( approximately 0.13). Therefore, because single channels open with a probability significantly lower than 1, more than one LCC must be available to ensure that sparks are triggered with a probability of approximately unity. We conclude that it is likely that a cluster of LCCs is involved in gating a cluster of ryanodine receptors at the beginning of an AP.

Acute exposure to thyroid hormone increases Na+ current and intracellular Ca2+ in cat atrial myocytes

Whole-cell recording methods and fluorescence microscopy were used to study the effects of acute exposure to thyroid hormone (T(3)) on cat atrial myocytes. Acute exposure ( approximately 5 min) to 10 nM T(3) significantly increased tetrodotoxin (TTX)-sensitive inward Na(+) current (I(Na)) at voltages between -40 and +20 mV. At maximal I(Na) activation (-40 mV) T(3) increased peak I(Na) by 32 %. T(3) had no effect on the time course of I(Na) decay, voltage dependence of activation, inactivation, or recovery from inactivation. Comparable exposures to reverse T(3) (rT(3)) or T(4) had no effect on I(Na). L-type Ca2+ current was unaffected by acute exposure to T(3). T(3)-induced increases in I(Na) were unaffected by 50 microM nickel, a blocker of T-type Ca2+ current. T(3) significantly increased cell shortening (+62 %) and could elicit spontaneous action potentials arising from Ca2+ -mediated after-depolarizations. T(3) (but not rT(3)) significantly increased baseline intracellular Ca2+, release of Ca2+ from sarcoplasmic reticulum (SR) and caffeine (10 mM)-induced release of SR Ca2+. We conclude that acute T(3) exposure increases Na(+) influx via I(Na) and thereby stimulates reverse-mode Na(+)-Ca2+ exchange to increase intracellular Ca2+ content and release. As a result, T(3) increases contraction strength, and can initiate Ca2+ -mediated arrhythmic activity. Acute non-genomic effects of T(3) can contribute to the positive inotropy and sinus (atrial) tachycardia traditionally attributed to chronic, genomic effects of elevated thyroid hormone on atrial muscle.

Heart failure -- a challenge to our current concepts of excitation-contraction coupling

Development of novel therapeutic strategies for congestive heart failure (CHF) seems to be hampered by insufficient knowledge of the molecular machinery of excitation-contraction (EC) coupling in both normal and failing hearts. Cardiac hypertrophy and failure represent a multitude of cardiac phenotypes, and available invasive and non-invasive techniques, briefly reviewed here, allow proper quantification of myocardial function in experimental models even in rats and mice. Both reduced fractional shortening and reduced velocity of contraction characterize myocardial failure. Only when myocardial function is depressed in vivo can meaningful studies be done in vitro of contractility and EC coupling. Also, we point out potential limitations with the whole cell patch clamp technique. Two main factors stand out as explanations for myocardial failure. First, a basic feature of CHF seems to be a reduced Ca(2+) load of the sarcoplasmic reticulum (SR) mainly due to a low phosphorylation level of phospholamban. Second, there seems to be a defect of the trigger mechanism of Ca(2+) release from the SR. We argue that this defect only becomes manifest in the presence of reduced Ca(2+) reuptake capacity of the SR and that it may not be solely attributable to reduced gain of the Ca(2+)-induced Ca(2+) release (CICR). We list several possible explanations for this defect that represent important avenues for future research.