Modulation of Ca2+ release in cultured neonatal rat cardiac myocytes. Insight from subcellular release patterns revealed by confocal microscopy

Confocal Laser Microscopy for VM Analysis with DAPI and Phalloidin Staining

Confocal laser scanning microscopy (CLSM) is one of the most prevalent fluorescence microscopy techniques for assessing the progression of cancer cells in three-dimensional structures, such as vasculogenic mimicry (VM). We show a basic approach for using DAPI and phalloidin dyes to detect the early stages of progression and VM of melanoma tumor cells grown in a 3D environment, as well as demonstrating how to acquire images and improve them by changing the software acquisition parameters.

CaMKII inhibition reduces arrhythmogenic Ca2+ events in subendocardial cryoinjured rat living myocardial slices

Spontaneous Ca²⁺ release (SCR) can cause triggered activity and initiate arrhythmias. Intrinsic transmural heterogeneities in Ca²⁺ handling and their propensity to disease remodeling may differentially modulate SCR throughout the left ventricular (LV) wall and cause transmural differences in arrhythmia susceptibility. Here, we aimed to dissect the effect of cardiac injury on SCR in different regions in the intact LV myocardium using cryoinjury on rat living myocardial slices (LMS). We studied SCR under proarrhythmic conditions using a fluorescent Ca²⁺ indicator and high-resolution imaging in LMS from the subendocardium (ENDO) and subepicardium (EPI). Cryoinjury caused structural remodeling, with loss in T-tubule density and an increased time of Ca²⁺ transients to peak after injury. In ENDO LMS, the Ca²⁺ transient amplitude and decay phase were reduced, while these were not affected in EPI LMS after cryoinjury. The frequency of spontaneous whole-slice contractions increased in ENDO LMS without affecting EPI LMS after injury. Cryoinjury caused an increase in foci that generates SCR in both ENDO and EPI LMS. In ENDO LMS, SCRs were more closely distributed and had reduced latencies after cryoinjury, whereas this was not affected in EPI LMS. Inhibition of CaMKII reduced the number, distribution, and latencies of SCR, as well as whole-slice contractions in ENDO LMS, but not in EPI LMS after cryoinjury. Furthermore, CaMKII inhibition did not affect the excitation–contraction coupling in cryoinjured ENDO or EPI LMS. In conclusion, we demonstrate increased arrhythmogenic susceptibility in the injured ENDO. Our findings show involvement of CaMKII and highlight the need for region-specific targeting in cardiac therapies.

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.

Calcium Handling Defects and Cardiac Arrhythmia Syndromes

Calcium ions (Ca²⁺) play a major role in the cardiac excitation-contraction coupling. Intracellular Ca²⁺ concentration increases during systole and falls in diastole thereby determining cardiac contraction and relaxation. Normal cardiac function also requires perfect organization of the ion currents at the cellular level to drive action potentials and to maintain action potential propagation and electrical homogeneity at the tissue level. Any imbalance in Ca²⁺ homeostasis of a cardiac myocyte can lead to electrical disturbances. This review aims to discuss cardiac physiology and pathophysiology from the elementary membrane processes that can cause the electrical instability of the ventricular myocytes through intracellular Ca²⁺ handling maladies to inherited and acquired arrhythmias. Finally, the paper will discuss the current therapeutic approaches targeting cardiac arrhythmias.

Oxytocin induces intracellular Ca2+ release in cardiac fibroblasts from neonatal rats

Pituitary neuropeptide oxytocin is increasingly recognised as a cardiovascular hormone, in addition to its many regulatory roles in other organ systems. Studies in atrial and ventricular myocytes from the neonatal and adult rats have identified synthesis of oxytocin and the expression of oxytocin receptors in these cells. In cardiac fibroblasts, the most populous non-myocyte cell type in mammalian heart, the oxytocin receptors have not been described before. In the present study, we have investigated the direct effects of oxytocin on intracellular Ca²⁺ dynamics in ventricular myocytes and fibroblasts from new born rats. In myocytes, oxytocin increased the frequency of spontaneous Ca²⁺ transients and decreased their amplitude. Our data suggest that oxytocin receptors are also present and functional in the majority of cardiac fibroblasts. We used selective oxytocin receptor inhibitor L-371,257 and a number of intracellular Ca ²⁺ release blockers to investigate the mechanism of oxytocin induced Ca²⁺ signalling in cardiac fibroblasts. Our findings suggest that oxytocin induces Ca²⁺ signals in cardiac fibroblasts by triggering endoplasmic reticulum Ca²⁺ release via inositol trisphosphate activated receptors. The functional significance of the oxytocin induced Ca²⁺ signalling in cardiac fibroblasts, especially for their activation into secretory active myofibroblasts, remains to be investigated.

An adaptation of astronomical image processing enables characterization and functional 3D mapping of individual sites of excitation-contraction coupling in rat cardiac muscle

In beating cardiomyocytes, synchronized localized Ca²⁺ transients from thousands of active excitation-contraction coupling sites (ECC couplons) comprising plasma and sarcoplasmic reticulum membrane calcium channels are important determinants of the heart's performance. Nevertheless, our knowledge about the properties of ECC couplons is limited by the lack of appropriate experimental and analysis strategies. We designed CaCLEAN to untangle the fundamental characteristics of ECC couplons by combining the astronomer's CLEAN algorithm with known properties of calcium diffusion. CaCLEAN empowers the investigation of fundamental properties of ECC couplons in beating cardiomyocytes without pharmacological interventions. Upon examining individual ECC couplons at the nanoscopic level, we reveal their roles in the negative amplitude-frequency relationship and in β-adrenergic stimulation, including decreasing and increasing firing reliability, respectively. CaCLEAN combined with 3D confocal imaging of beating cardiomyocytes provides a functional 3D map of active ECC couplons (on average, 17,000 per myocyte). CaCLEAN will further enlighten the ECC-couplon-remodelling processes that underlie cardiac diseases.

Calcium waves initiating from the anomalous subdiffusive calcium sparks

The objective of the study is to investigate the propagation of Ca(2+) waves in full-width cardiac myocytes and carry out sensitivity analysis to study the effects of various physiological parameters on global Ca(2+) waves. Based on the anomalous subdiffusion of Ca(2+) sparks, a mathematical model was proposed to characterize the Ca(2+) waves. The computed results were in agreement with the experimental measurements using confocal microscopy. This model includes variables of current through the Ca(2+) release unit (CRU; ICRU), duration of current flow through CRU (Topen), Ca(2+) sensitivity parameter (K), the longitudinal and transverse spatial separation of CRUs (lx and ly, where x denotes longitudinal direction (x-axis) and y denotes transverse direction (y-axis)) and Ca(2+) diffusion coefficients (Dx, Dy). The spatio-temporal mechanism of the anomalous Ca(2+) sparks led to results that were different from those based on Fick's law. The major findings were reported as: ICRU affected the dynamic properties of Ca(2+) waves more significantly than Topen; the effect of K on the properties of Ca(2+) waves was negligible; ly affected the amplitude significantly, but lx affected the longitudinal velocity significantly; in turn, the limitation and significance of the study are discussed.

Integrin α1β1 participates in chondrocyte transduction of osmotic stress

BACKGROUND/PURPOSE

The goal of this study was to determine the role of the collagen binding receptor integrin α1β1 in regulating osmotically induced [Ca(2+)]i transients in chondrocytes.

METHOD

The [Ca(2+)]i transient response of chondrocytes to osmotic stress was measured using real-time confocal microscopy. Chondrocytes from wildtype and integrin α1-null mice were imaged ex vivo (in the cartilage of intact murine femora) and in vitro (isolated from the matrix, attached to glass coverslips). Immunocytochemistry was performed to detect the presence of the osmosensor, transient receptor potential vanilloid-4 (TRPV4), and the agonist GSK1016790A (GSK101) was used to test for its functionality on chondrocytes from wildtype and integrin α1-null mice.

RESULTS/INTERPRETATION

Deletion of the integrin α1 subunit inhibited the ability of chondrocytes to respond to a hypo-osmotic stress with [Ca(2+)]i transients ex vivo and in vitro. The percentage of chondrocytes responding ex vivo was smaller than in vitro and of the cells that responded, more single [Ca(2+)]i transients were observed ex vivo compared to in vitro. Immunocytochemistry confirmed the presence of TRPV4 on wildtype and integrin α1-null chondrocytes, however application of GSK101 revealed that TRPV4 could be activated on wildtype but not integrin α1-null chondrocytes. Integrin α1β1 is a key participant in chondrocyte transduction of a hypo-osmotic stress. Furthermore, the mechanism by which integrin α1β1 influences osmotransduction is independent of matrix binding, but likely dependent on the chondrocyte osmosensor TRPV4.

Stretch-activated calcium channels relay fast calcium waves propagated by calcium-induced calcium influx

For nearly 30 years, fast calcium waves have been attributed to a regenerative process propagated by CICR (calcium-induced calcium release) from the endoplasmic reticulum. Here, I propose a model containing a new subclass of fast calcium waves which is propagated by CICI (calcium-induced calcium influx) through the plasma membrane. They are called fast CICI waves. These move at the order of 100 to 1000 microm/s (at 20 degrees C), rather than the order of 3 to 30 microm/s found for CICR. Moreover, in this proposed subclass, the calcium influx which drives calcium waves is relayed by stretch-activated calcium channels. This model is based upon reports from approx. 60 various systems. In seven of these reports, calcium waves were imaged, and, in five of these, evidence was presented that these waves were regenerated by CICI. Much of this model involves waves that move along functioning flagella and cilia. In these systems, waves of local calcium influx are thought to cause waves of local contraction by inducing the sliding of dynein or of kinesin past tubulin microtubules. Other cells which are reported to exhibit waves, which move at speeds in the fast CICI range, include ones from a dozen protozoa, three polychaete worms, three molluscs, a bryozoan, two sea urchins, one arthropod, four insects, Amphioxus, frogs, two fish and a vascular plant (Equisetum), together with numerous healthy, as well as cancerous, mammalian cells, including ones from human. In two of these systems, very gentle local mechanical stimulation is reported to initiate waves. In these non-flagellar systems, the calcium influxes are thought to speed the sliding of actinomyosin filaments past each other. Finally, I propose that this mechanochemical model could be tested by seeing if gentle mechanical stimulation induces waves in more of these systems and, more importantly, by imaging the predicted calcium waves in more of them.

Analysis of IP3 receptors in and out of cells

BACKGROUND

Inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in almost all animal cells. Three mammalian genes encode closely related IP3R subunits, which assemble into homo- or hetero-tetramers to form intracellular Ca2+ channels.

SCOPE OF THE REVIEW

In this brief review, we first consider a variety of complementary methods that allow the links between IP3 binding and channel gating to be defined. How does IP3 binding to the IP3-binding core in each IP3R subunit cause opening of a cation-selective pore formed by residues towards the C-terminal? We then describe methods that allow IP3, Ca2+ signals and IP3R mobility to be examined in intact cells. A final section briefly considers genetic analyses of IP3R signalling.

MAJOR CONCLUSIONS

All IP3R are regulated by both IP3 and Ca2+. This allows them to initiate and regeneratively propagate intracellular Ca2+ signals. The elementary Ca2+ release events evoked by IP3 in intact cells are mediated by very small numbers of active IP3R and the Ca2+-mediated interactions between them. The spatial organization of these Ca2+ signals and their stochastic dependence on so few IP3Rs highlight the need for methods that allow the spatial organization of IP3R signalling to be addressed with single-molecule resolution.

GENERAL SIGNIFICANCE

A variety of complementary methods provide insight into the structural basis of IP3R activation and the contributions of IP3-evoked Ca2+ signals to cellular physiology. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Ryanodol action on calcium sparks in ventricular myocytes

The action of ryanodol on single cardiac ryanodine receptor (RyR2) channels in bilayers and local RyR2-mediated Ca(2+) release events (Ca(2+) sparks) in ventricular myocytes was defined. At the single-channel level, ryanodol intermittently modified single channels into a long-lived subconductance state with an average duration of 3.8 +/- 0.2 s. Unlike ryanodine, ryanodol did not change the open probability (Po) of unmodified channels, and high concentrations did not promote full-channel closure. Ryanodol action was Po dependent with the K (D) varying roughly from 20 to 80 muM as Po changed from approximately 0.2 to 1, respectively. Ryanodol preferentially bound during long channel openings. In intact and permeabilized rat myocytes, ryanodol evoked trains of sparks at active release sites resulting in a significant increase in overall spark frequency. Ryanodol did not increase the number of active release sites. Long-lived Ca(2+) release events were observed but infrequently, and ryanodol action was readily reversed upon drug washout. We propose that ryanodol modifies a few channels during a Ca(2+) spark. These modified channels mediate a sustained low-intensity Ca(2+) release that repeatedly triggers sparks at the same release site. We conclude that ryanodol is an easily generated reversible probe that can be effectively used to explore RyR2-mediated Ca(2+) release in cells.

Altered sarcoplasmic reticulum calcium transport in the presence of the heavy metal chelator TPEN

TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)-ethylenediamine) is a membrane-permeable heavy-metal ion chelator with a dissociation constant for Ca2+ comparable to the Ca2+ concentration ([Ca2+]) within the intracellular Ca2+ stores. It has been used as modulator of intracellular heavy metals and of free intraluminal [Ca2+], without influencing the cytosolic [Ca2+] that falls in the nanomolar range. In our previous studies, we gave evidence that TPEN modifies the Ca2+ homeostasis of striated muscle independent of this buffering ability. Here we describe the direct interaction of TPEN with the ryanodine receptor (RyR) Ca2+ release channel and the sarcoplasmic reticulum (SR) Ca2+ pump (SERCA). In lipid bilayers, at negative potentials and low [Ca2+], TPEN increased the open probability of RyR, while at positive potentials it inhibited channel activity. On permeabilized skeletal muscle fibers of the frog, but not of the rat, 50 microM TPEN increased the number of spontaneous Ca2+ sparks and induced propagating events with a velocity of 273 +/- 7 microm/s. Determining the hydrolytic activity of the SR revealed that TPEN inhibits the SERCA pump, with an IC(50) = 692 +/- 62 microM and a Hill coefficient of 0.88 +/- 0.10. These findings provide experimental evidence that TPEN directly modifies both the release of Ca2+ from and its reuptake into the SR.

Engineering physiologically controlled pacemaker cells with lentiviral HCN4 gene transfer

BACKGROUND

Research on biological pacemakers for the heart has so far mainly focused on short-term gene and cell therapies. To develop a clinically relevant biological pacemaker, long-term function and incorporation of autonomic modulation are crucial. Lentiviral vectors can mediate long-term gene expression, while isoform 4 of the Hyperpolarization-activated Cyclic Nucleotide-gated channel (encoded by HCN4) contributes to pacemaker function and responds maximally to cAMP, the second messenger in autonomic modulation.

MATERIAL AND METHODS

Action potential (AP) properties and pacemaker current (I(f)) were studied in single neonatal rat ventricular myocytes that overexpressed HCN4 after lentiviral gene transduction. Autonomic responsiveness and cycle length stability were studied using extracellular electrograms of confluent cultured monolayers.

RESULTS

Perforated patch-clamp experiments demonstrated that HCN4-transduced single cardiac myocytes exhibited a 10-fold higher I(f) than non-transduced single myocytes, along with slow diastolic depolarization, comparable to pacemaker cells of the sinoatrial node, the dominant native pacemaker. HCN4-transduced monolayers exhibited a 47% increase in beating rate, compared to controls. Upon addition of DBcAMP, HCN4-transduced monolayers had beating rates which were 54% faster than baseline and significantly more regular than controls.

CONCLUSIONS

Lentiviral vectors efficiently transduce cardiac myocytes and mediate functional gene expression. Because HCN4-transduced myocytes demonstrate an increase in spontaneous beating rate and responsiveness to autonomic modulation, this approach may be useful to create a biological pacemaker.

Ca2+ sparks and cellular distribution of ryanodine receptors in developing cardiomyocytes from rat

Although abundant ryanodine receptors (RyRs) exist in cardiomyocytes from newborn (NB) rat and despite the maturity of their single-channel properties, the RyR contribution to excitation-contraction (E-C) coupling is minimal. Immature arrangement of RyRs in the Ca(2+) release site of the sarcoplasmic reticulum and/or distant RyRs location from the sarcolemmal Ca(2+) signal could explain this quiescence. Consequently, Ca(2+) sparks and their cellular distribution were studied in NB myocytes and correlated with the formation of dyads and transverse (T) tubules. Ca(2+) sparks were recorded in fluo-4-loaded intact ventricular myocytes acutely dissociated from adult and NB rats (0-9 days old). Sparks were defined/compared in the center and periphery of the cell. Co-immunolocalization of RyRs with dihydropyridine receptors (DHPR) was used to estimate dyad formation, while the development of T tubules was studied using di-8-ANEPPS and diIC12. Our results indicate that in NB cells, Ca(2+) sparks exhibited lower amplitude (1.7+/-0.5 vs. 3.6+/-1.7 F/F(0)), shorter duration (47+/-3.2 vs. 54.1+/-3 ms), and larger width (1.7+/-0.8 vs. 1.2+/-0.4 microm) than in adult. Although no significant changes were observed in the overall frequency, central sparks increased from approximately 60% at 0-1 day to 82% at 7-9 days. While immunolocalization revealed many central release sites at 7-8 days, fluorescence labeling of the plasma membrane showed less abundant internal T tubules. This could imply that although during the first week, release sites emerge forming dyads with DHPR-containing T tubules; some of these T tubules may not be connected to the surface, explaining the RyR quiescence during E-C coupling in NB.

How to measure Ca2+ in cellular organelles?

The review will aim to briefly summarise information on calcium measurements in cellular organelles with emphases on studies conducted in live cells using optical probes. When appropriate we will try to compare the effectiveness of different indicators for intraorganellar calcium measurements. We will consider calcium measurements in endoplasmic reticulum, Golgi apparatus, endosomes/lysosomes, nucleoplasm, nuclear envelope, mitochondria and secretory granules.

Restitution of Ca(2+) release and vulnerability to arrhythmias

New information has recently been obtained along two essentially parallel lines of research: investigations into the fundamental mechanisms of Ca(2+)-induced Ca(2+) release (CICR) in heart cells, and analyses of the factors that control the development of unstable rhythms such as repolarization alternans. These lines of research are starting to converge such that we can begin to understand unstable and potentially arrhythmogenic cardiac dynamics in terms of the underlying mechanisms governing not only membrane depolarization and repolarization but also the complex bidirectional interactions between electrical and Ca(2+) signaling in heart cells. In this brief review, we discuss the progress that has recently been made in understanding the factors that control the beat-to-beat regulation of cardiac Ca(2+) release and attempt to place these results within a larger context. In particular, we discuss factors that may contribute to unstable Ca(2+) release and speculate about how instability in CICR may contribute to the development of arrhythmias under pathological conditions.

Primate granulosa cell response via prostaglandin E2 receptors increases late in the periovulatory interval

Successful ovulation requires elevated follicular prostaglandin E2 (PGE2) levels. To determine which PGE2 receptors are available to mediate periovulatory events in follicles, granulosa cells and whole ovaries were collected from monkeys before (0 h) and after administration of an ovulatory dose of hCG to span the 40-h periovulatory interval. All PGE2 receptor mRNAs were present in monkey granulosa cells. As assessed by immunofluorescence, PTGER1 (EP1) protein was low/nondetectable in granulosa cells 0, 12, and 24 h after hCG but was abundant 36 h after hCG administration. PTGER2 (EP2) and PTGER3 (EP3) proteins were detected by immunofluorescence in granulosa cells throughout the periovulatory interval, and Western blotting showed an increase in PTGER2 and PTGER3 levels between 0 h and 36 h after hCG. In contrast, PTGER4 (EP4) protein was not detected in monkey granulosa cells. Granulosa cell response to PGE2 receptor agonists was examined 24 h and 36 h after hCG administration, when elevated PGE2 levels present in periovulatory follicles initiate ovulatory events. PGE2 acts via PTGER1 to increase intracellular calcium. PGE2 increased intracellular calcium in granulosa cells obtained 36 h, but not 24 h, after hCG; this effect of PGE2 was blocked by a PTGER1 antagonist. A PTGER2-specific agonist and a PTGER3-specific agonist each elevated cAMP in granulosa cells obtained 36 h, but not 24 h, after hCG. Therefore, the granulosa cells of primate periovulatory follicles express multiple receptors for PGE2. Granulosa cells respond to agonist stimulation of each of these receptors 36 h, but not 24 h, after hCG, supporting the hypothesis that granulosa cells are most sensitive to PGE2 as follicular PGE2 levels peak, leading to maximal PGE2-mediated periovulatory effects just before ovulation.

Cell culture modifies Ca2+ signaling during excitation-contraction coupling in neonate cardiac myocytes

In heart, the excitation-contraction coupling (ECC) mechanism changes during development. Primary cell culture has been used to study Ca(2+) signaling in newborn (NB) rat heart. In this work, the effects of cell culture on the action potential (AP) and ECC Ca(2+) signaling during development were investigated. Specifically, AP, Ca(2+) currents (I(Ca)), and ryanodine receptor (RyR) properties (i.e. density, distribution, and contribution to Ca(2+) transients and Ca(2+) sparks) were defined in cultured myocytes (CM) from 0-day-old NB rat at different times in culture (1-4 days). Compared with acutely dissociated myocytes (ADM) from NB of equivalent ages (1-4 days), CM showed lower RyR density (50% at 1 day, 25% at 4 days), but larger RyR contribution to the Ca(2+) transient (25% at 1 day, 57% at 4 days). Additionally, Ca(2+) sparks were larger, longer, wider, and more frequent in CM than in ADM. RyR cellular distribution also showed different arrangement. While in CM, RyRs were located peripherally, in ADM of equivalent ages a sarcomeric arrangement was predominant. Finally, CM showed a two-fold increase in sarcolemmal Ca(2+) entry during the AP. These results indicated that primary culture is a feasible model to study Ca(2+) signaling in heart; however, it does not precisely reproduce what occurs in ECC during development.

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.

Confocal imaging of CICR events from isolated and immobilized SR vesicles

The Ca2+ concentration inside the sarcoplasmic reticulum ([Ca2+]SR) is a difficult parameter to measure in ventricular cardiac myocytes. Interference from Ca2+-sensitive dye loading into cellular compartments other than the SR interferes with free Ca2+ measurement. In addition, the composition of the cytosol surrounding the SR in intact cells cannot be easily controlled. We have developed a method to measure localized [Ca2+]SR in immobilized membrane vesicles during rapid solution switches. Ca2+ uptake and release in rat SR membrane vesicles was monitored using confocal microscopy. Vesicles were immobilized on a coverslip using an agarose matrix. Perfusion with a Ca2+-containing solution supplemented with ATP initiated SR Ca2+ uptake, causing a rise in intravesicular fluorescence in vesicles containing the low-affinity Ca2+ indicator fluo-5N. Perfusion with caffeine caused SR Ca2+ release and a decrease in intravesicular flourescence. Although caffeine-dependent release was readily visible with extravesicular Ca2+-green, Ca2+ which leaked from the SR was detected only indirectly as eventless release. We conclude that SR Ca2+ uptake and release can be selectively measured in functional SR vesicles using a confocal microscope. Caffeine-dependent release is directly measurable though SR Ca2+ leak can only be inferred as subresolution events, presumably because channels in separate vesicles were not close enough to result in concerted Ca2+-induced Ca2+ release.

Calcium biology of the transverse tubules in heart

Ca(2+) sparks in heart muscle are activated on depolarization by the influx of Ca(2+) through dihydropyridine receptors in the sarcolemmal (SL) and transverse tubule (TT) membranes. The cardiac action potential is thus able to synchronize the [Ca(2+)](i) transient as Ca(2+) release is activated throughout the cell. Increases in the amount of Ca(2+) within the sarcoplasmic reticulum (SR) underlie augmented Ca(2+) release globally and an increase in the sensitivity of the ryanodine receptors (RyRs) to be triggered by the local [Ca(2+)](i). In a similar manner, phosphorylation of the RyRs by protein kinase A (PKA) increases the sensitivity of the RyRs to be activated by local [Ca(2+)](i). Heart failure and other cardiac diseases are associated with changes in SR Ca(2+) content, phosphorylation state of the RyRs, [Ca(2+)](i) signaling defects and arrhythmias. Additional changes in transverse tubules and nearby junctional SR may contribute to alterations in local Ca(2+) signaling. Here we briefly discuss how TT organization can influence Ca(2+) signaling and how changes in SR Ca(2+) release triggering can influence excitation-contraction (EC) coupling. High speed imaging methods are used in combination with single cell patch clamp experiments to investigate how abnormal Ca(2+) signaling may be regulated in health and disease. Three issues are examined in this presentation: (1) normal Ca(2+)-induced Ca(2+) release and Ca(2+) sparks, (2) abnormal SR Ca(2+) release in disease, and (3) the triggering and propagation of waves of elevated [Ca(2+)](i).

Ryanodine receptor function in newborn rat heart

The role of ryanodine receptor (RyR) in cardiac excitation-contraction (E-C) coupling in newborns (NB) is not completely understood. To determine whether RyR functional properties change during development, we evaluated cellular distribution and functionality of sarcoplasmic reticulum (SR) in NB rats. Sarcomeric arrangement of immunostained SR Ca2+-ATPase (SERCA2a) and the presence of sizeable caffeine-induced Ca2+ transients demonstrated that functional SR exists in NB. E-C coupling properties were then defined in NB and compared with those in adult rats (AD). Ca2+ transients in NB reflected predominantly sarcolemmal Ca2+ entry, whereas the RyR-mediated component was ∼13%. Finally, the RyR density and functional properties at the single-channel level in NB were compared with those in AD. Ligand binding assays revealed that in NB, RyR density can be up to 36% of that found in AD, suggesting that some RyRs do not contribute to the Ca2+ transient. To test the hypothesis that RyR functional properties change during development, we incorporated single RyRs into lipid bilayers. Our results show that permeation and gating kinetics of NB RyRs are identical to those of AD. Also, endogenous ligands had similar effects on NB and AD RyRs: sigmoidal Ca2+ dependence, stronger Mg2+-induced inhibition at low cytoplasmic Ca2+ concentrations, comparable ATP-activating potency, and caffeine sensitivity. These observations indicate that NB rat heart contains fully functional RyRs and that the smaller contribution of RyR-mediated Ca2+ release to the intracellular Ca2+ transient in NB is not due to different single RyR channel properties or to the absence of functional intracellular Ca2+ stores.

UNLOCKING THE SECRETS OF CELL SIGNALING

My scientific life has been spent trying to understand how cells communicate with each other. This interest in cell signaling began with studies on the control of fluid secretion by an insect salivary gland, and the subsequent quest led to the discovery of inositol trisphosphate (IP3) and its role in calcium signaling, which effectively divided my scientific career into two distinct parts. The first part was primarily experimental and culminated in the discovery of IP3, which set the agenda for the second half during which I have enjoyed exploring the many functions of this remarkably versatile signaling system. It has been particularly exciting to find out how this IP3/Ca2+ signaling pathway has been adapted to control processes as diverse as fertilization, proliferation, cell contraction, secretion, and information processing in neuronal cells.

Polymorphism of Ca2+ sparks evoked from in-focus Ca2+ release units in cardiac myocytes

Ca2+ sparks are the elementary release events in many types of cells. Here we present a morphometric analysis of Ca2+ sparks (i.e., amplitude and kinetic parameters) using an approach that minimizes the confounding factor of the detection of out-of-focus events. By activation and visualization of Ca2+ sparks from Ca2+ release units under loose-seal patch-clamp conditions, we found that the amplitude and rising rate of in-focus sparks exhibited a broad modal distribution, whereas spark rise time and spatial width appeared to be stereotyped. Spark morphometrics were constant irrespective of the latency of spark production and the time-dependent L-type Ca2+ channel activation. Polymorphism of Ca2+ sparks in terms of variable amplitude and rising rate was evident for events from the same release units, and intra- and interrelease unit variability contributed equally to the overall variability. The rising rate, a reporter of the underlying Ca2+ release flux, displayed a strong positive correlation with spark amplitude, but a negative correlation with spark rise time, an index of Ca2+ release duration. On the basis of Ca2+ spark morphometrics measured here, we suggested a model in which cohorts of variable number of ryanodine receptors are activated in the genesis of Ca2+ sparks, and the ensuing negative feedback overrides the regenerative Ca2+-induced Ca2+ release to extinguish the ongoing Ca2+ spark.

Developmental changes of intracellular Ca2+ transients in beating rat hearts

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

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.

Initiation and propagation of ectopic waves: insights from an in vitro model of ischemia-reperfusion injury

The objective of the present study was to directly visualize ectopic activity associated with ischemia-reperfusion and its progression to arrhythmia. To accomplish this goal, we employed a two-dimensional network of neonatal rat cardiomyocytes and a recently developed model of localized ischemia-reperfusion. Washout of the ischemia-like solution resulted in tachyarrhythmic episodes lasting 15-200 s. These episodes were preceded by the appearance of multiple ectopic sources and propagation of ectopic activity along the border of the former ischemic zone. The ectopic sources exhibited a slow rise in diastolic calcium, which disappeared upon return to the original pacing pattern. Border zone propagation of ectopic activity was followed by its escape into the surrounding control network, generating arrhythmias. Together, these observations suggest that upon reperfusion, a distinct layer, which consists of ectopically active, poorly coupled cells, is formed transiently over an injured area. Despite being neighbored by a conductive and excitable tissue, this transient functional layer is capable of sustaining autonomous waves and serving as a special conductive medium through which ectopic activity can propagate before spreading into the surrounding healthy tissue.

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.

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.

Ca2+ sparks and Ca2+ waves in saponin-permeabilized rat ventricular myocytes

1. We carried out confocal Ca2+ imaging in myocytes permeabilized with saponin in 'internal' solutions containing: MgATP, EGTA and fluo-3 potassium salt. 2. Permeabilized myocytes exhibited spontaneous Ca2+ sparks and waves similar to those observed in intact myocytes loaded with fluo-3 AM. 3. In the presence of 'low' [EGTA] (0.05 mM), Ca2+ waves arose regularly, even at relatively low [Ca2+] (50-100 nM, free). Increasing [EGTA] resulted in decreased frequency and propagation velocity of Ca2+ waves. Propagating waves were completely abolished at [EGTA] > 0.3 mM. 4. The frequency of sparks increased as a function of [Ca2+] (50-400 nM range) with no sign of a high affinity Ca2+-dependent inactivation process. 5. The rate of occurrence of Ca2+ sparks was increased by calmodulin and cyclic adenosine diphosphate-ribose (cADPR).

Calcium-dependent modulation of the plateau phase of action potential in isolated ventricular cells of rabbit heart

[Ca2+]i-dependent modulation of the action potential has been studied in Fura-2 dialysed ventricular myocytes of the rabbit using the whole-cell current-clamp method. Fifteen consecutive action potentials (AP1-AP15) and [Ca2+]i transients were elicited at a frequency of 0.2 Hz. A single, brief application of caffeine (during AP9) first enhanced and thereafter attenuated the [Ca2+]i transients accompanying AP9 and AP10-AP12, respectively. This approach provided direct comparison between time courses of action potentials: during the initial steady state (e.g. AP8) and when Ca2+ release from the sarcoplasmic reticulum was increased by caffeine (AP9) or decreased by depletion (AP10). The increase in [Ca2+]i facilitated repolarization and decreased action potential duration. However, action potentials at reduced Ca2+ release (AP10) had longer duration than during steady state. The caffeine-induced changes in L-type Ca2+ current (ICa,L), during voltage-clamp conditions partially explained the effects of caffeine on action potentials. When ICa,L was blocked by 500 micromol L-1 Cd2+, enhanced [Ca2+]i transients revealed an extra current component which was outward at +10 mV and inward at the resting membrane potential (most probably the transient inward current). In the presence of Cd2+, however, AP8 and AP10 had identical time courses, suggesting that ICa,L alone was responsible for the lengthening of AP10. Alterations in the transmembrane Na+ gradient resulted in changes of the steady state action potential durations (AP8) consistently with the expected modulation of the Na+-Ca2+ exchange current. However, the contribution of this current to the [Ca2+]i-dependent behaviour of action potential plateau could not be demonstrated.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Calcium signalling in cardiac muscle: refractoriness revealed by coherent activation

Contraction of cardiac myocytes is governed by calcium-ion (Ca2+ )-induced Ca2+ release (CICR) from the sarcoplasmic reticulum through Ca2+-release channels. Ca2+ release occurs by concerted activation of numerous elementary Ca2+ events, 'Ca2+ sparks', that are triggered and locally controlled by Ca2+ influx into the cell through plasmalemmal L-type Ca2+ channels. Because of the positive feedback inherent in CICR, an as-yet-unidentified control mechanism is required to restrain the amplification of Ca2+ signalling and to terminate Ca2+ release from the sarcoplasmic reticulum. Here we use ultraviolet-laser-flash and two-photon photolysis of caged Ca2+ to study spatiotemporal features of the termination and refractoriness of Ca2+ release. Coherent and simultaneous activation of all Ca2+-release sites within a cardiac myocyte unmasked a prominent refractoriness, recovering monotonically within about 1 second. In contrast, selective activation of a few Ca 2+-release sites was not followed by a refractoriness of Ca 2+ release from the sarcoplasmic reticulum. This discrepancy is consistent with the idea that a functional depletion of Ca2+ from the cellular sarcoplasmic-reticulum network may underlie the refractoriness of CICR observed after a whole-cell Ca2+ transient. These results also imply the requirement for further mechanisms to terminate spatially limited subcellular Ca2+-release events such as Ca2+ sparks.

Modeling gain and gradedness of Ca2+ release in the functional unit of the cardiac diadic space

A model of the functional release unit (FRU) in rat cardiac muscle consisting of one dihydropyridine receptor (DHPR) and eight ryanodine receptor (RyR) channels, and the volume surrounding them, is formulated. It is assumed that no spatial [Ca2+] gradients exist in this volume, and that each FRU acts independently. The model is amenable to systematic parameter studies in which FRU dynamics are simulated at the channel level using Monte Carlo methods with Ca2+ concentrations simulated by numerical integration of a coupled system of differential equations. Using stochastic methods, Ca(2+)-induced Ca2+ release (CICR) shows both high gain and graded Ca2+ release that is robust when parameters are varied. For a single DHPR opening, the resulting RyR Ca2+ release flux is insensitive to the DHPR open duration, and is determined principally by local sarcoplasmic reticulum (SR) Ca2+ load, consistent with experimental data on Ca2+ sparks. In addition, single RyR openings are effective in triggering Ca2+ release from adjacent RyRs only when open duration is long and SR Ca2+ load is high. This indicates relatively low coupling between RyRs, and suggests a mechanism that limits the regenerative spread of RyR openings. The results also suggest that adaptation plays an important modulatory role in shaping Ca2+ release duration and magnitude, but is not solely responsible for terminating Ca2+ release. Results obtained with the stochastic model suggest that high gain and gradedness can occur by the recruitment of independent FRUs without requiring spatial [Ca2+] gradients within a functional unit or cross-coupling between adjacent functional units.

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").

The role of luminal Ca2+ in the generation of Ca2+ waves in rat ventricular myocytes

1. We used confocal Ca2+ imaging and fluo-3 to investigate the transition of localized Ca2+ releases induced by focal caffeine stimulation into propagating Ca2+ waves in isolated rat ventricular myocytes. 2. Self-sustaining Ca2+ waves could be initiated when the cellular Ca2+ load was increased by elevating the extracellular [Ca2+] ([Ca2+]o) and they could also be initiated at normal Ca2+ loads when the sensitivity of the release sites to cytosolic Ca2+ was enhanced by low doses of caffeine. When we prevented the accumulation of extra Ca2+ in the luminal compartment of the sarcoplasmic reticulum (SR) with thapsigargin, focal caffeine pulses failed to trigger self-sustaining Ca2+ waves on elevation of [Ca2+]o. Inhibition of SR Ca2+ uptake by thapsigargin in cells already preloaded with Ca2+ above normal levels did not prevent local Ca2+ elevations from triggering propagating waves. Moreover, wave velocity increased by 20 %. Tetracaine (0.75 mM) caused transient complete inhibition of both local and propagating Ca2+ signals, followed by full recovery of the responses due to increased SR Ca2+ accumulation. 3. Computer simulations using a numerical model with spatially distinct Ca2+ release sites suggested that increased amounts of releasable Ca2+ might not be sufficient to generate self-sustaining Ca2+ waves under conditions of Ca2+ overload unless the threshold of release site Ca2+ activation was set at relatively low levels (< 1.5 microM). 4. We conclude that the potentiation of SR Ca2+ release channels by luminal Ca2+ is an important factor in Ca2+ wave generation. Wave propagation does not require the translocation of Ca2+ from the spreading wave front into the SR. Instead, it relies on luminal Ca2+ sensitizing Ca2+ release channels to cytosolic Ca2+.

Localized intracellular calcium signaling in muscle: calcium sparks and calcium quarks

Subcellularly localized Ca2+ signals in cardiac and skeletal muscle have recently been identified as elementary Ca2+ signaling events. The signals, termed Ca2+ sparks and Ca2+ quarks, represent openings of Ca2+ release channels located in the membrane of the sarcoplasmic reticulum (SR). In cardiac muscle, the revolutionary discovery of Ca2+ sparks has allowed the development of a fundamentally different concept for the amplification of Ca2+ signals by Ca(2+)-induced Ca2+ release. In such a system, a graded amplification of the triggering Ca2+ signal entering the myocyte via L-type Ca2+ channels is accomplished by a recruitment process whereby individual SR Ca2+ release units are locally controlled by L-type Ca2+ channels. In skeletal muscle, the initial SR Ca2+ release is governed by voltage-sensors but subsequently activates additional Ca2+ sparks by Ca(2+)-induced Ca2+ release from the SR. Results from studies on elementary Ca2+ release events will improve our knowledge of muscle Ca2+ signaling at all levels of complexity, from the molecule to normal cellular function, and from the regulation of cardiac and skeletal muscle force to the pathophysiology of excitation-contraction coupling.

Amplitude distribution of calcium sparks in confocal images: theory and studies with an automatic detection method

Determination of the calcium spark amplitude distribution is of critical importance for understanding the nature of elementary calcium release events in striated muscle. In the present study we show, on general theoretical grounds, that calcium sparks, as observed in confocal line scan images, should have a nonmodal, monotonic decreasing amplitude distribution, regardless of whether the underlying events are stereotyped. To test this prediction we developed, implemented, and verified an automated computer algorithm for objective detection and measurement of calcium sparks in raw image data. When the sensitivity and reliability of the algorithm were set appropriately, we observed highly left-skewed or monotonic decreasing amplitude distributions in skeletal muscle cells and cardiomyocytes, confirming the theoretical predictions. The previously reported modal or Gaussian distributions of sparks detected by eye must therefore be the result of subjective detection bias against small amplitude events. In addition, we discuss possible situations when a modal distribution might be observed.

Elementary [Ca2+]i signals generated by electroporation functionally mimic those evoked by hormonal stimulation

The generation of oscillations and global Ca2+ waves relies on the spatio-temporal recruitment of elementary Ca2+ signals, such as 'Ca2+ puffs'. Each elementary signal contributes a small amount of Ca2+ into the cytoplasm, progressively promoting neighboring Ca2+ release sites into an excitable state. Previous studies have indicated that increases in frequency or amplitude of such hormone-evoked elementary Ca2+ signals are necessary to initiate Ca2+ wave propagation. In the present study, an electroporation device was used to rapidly and reversibly permeabilize the plasma membrane of HeLa cells and to allow a limited influx of Ca2+. With low field intensities (100-500 V/cm), brief (50-100 micros) electroporation triggered localized Ca2+ signals that resembled hormone-evoked Ca2+ puffs, but not global signals. With such low intensity electroporative pulses, the Ca2+ influx component was usually undetectable, confirming that the electroporation-induced local signals represented Ca2+ puffs arising from the opening of intracellular Ca2+ release channels. Increasing either the frequency at which low-intensity electroporative pulses were applied, or the intensity of a single electroporative pulse (>500 V/cm), resulted in caffeine-sensitive regenerative Ca2+ waves. We suggest that Ca2+ puffs caused by electroporation functionally mimic hormone-evoked elementary events and can activate global Ca2+ signals if they provide a sufficient trigger.

Characterization of elementary Ca2+ release signals in NGF-differentiated PC12 cells and hippocampal neurons

Elementary Ca2+ release signals in nerve growth factor- (NGF-) differentiated PC12 cells and hippocampal neurons, functionally analogous to the "Ca2+ sparks" and "Ca2+ puffs" identified in other cell types, were characterized by confocal microscopy. They either occurred spontaneously or could be activated by caffeine and metabotropic agonists. The release events were dissimilar to the sparks and puffs described so far, as many arose from clusters of both ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (InsP3Rs). Increasing either the stimulus strength or loading of the intracellular stores enhanced the frequency of and coupling between elementary release sites and evoked global Ca2+ signals. In the PC12 cells, the elementary Ca2+ release preferentially occurred around the branch points. Spatio-temporal recruitment of such elementary release events may regulate neuronal activities.

Hormone-evoked elementary Ca2+ signals are not stereotypic, but reflect activation of different size channel clusters and variable recruitment of channels within a cluster

Previous studies of (InsP3)-evoked elementary Ca2+ events suggested a hierarchy of signals; fundamental events ("Ca2+ blips") arising from single InsP3 receptors (InsP3Rs), and intermediate events ("Ca2+ puffs") reflecting the coordinated opening of a cluster of InsP3Rs. The characteristics of such elementary Ca2+ release signals provide insights into the functional interaction and distribution of InsP3Rs in living cells. Therefore we investigated whether elementary Ca2+ signaling is truly represented by such stereotypic release events. A histogram of >900 events revealed a wide spread of signal amplitudes (20-600 nM; mean 216 +/- 4 nM; n = 206 cells), which cannot be explained by stochastic variation of a stereotypic Ca2+ release site. We identified elementary Ca2+ release sites with consistent amplitudes (<20% difference) and locations with variable amplitudes (approximately 500% difference). Importantly, within single cells, distinct sites displayed events with significantly different mean amplitudes. Additional determinants affecting the magnitude of elementary Ca2+ release were identified to be (i) hormone concentration, (ii) day-to-day variability, and (iii) a progressively decreasing Ca2+ release during prolonged stimulation. We therefore suggest that elementary Ca2+ events are not stereotypic, instead a continuum of signals can be achieved by either recruitment of entire clusters with different numbers of InsP3Rs or by a graded recruitment of InsP3Rs within a cluster.

Transient mitochondrial depolarizations reflect focal sarcoplasmic reticular calcium release in single rat cardiomyocytes

Digital imaging of mitochondrial potential in single rat cardiomyocytes revealed transient depolarizations of mitochondria discretely localized within the cell, a phenomenon that we shall call "flicker." These events were usually highly localized and could be restricted to single mitochondria, but they could also be more widely distributed within the cell. Contractile waves, either spontaneous or in response to depolarization with 50 mM K+, were associated with propagating waves of mitochondrial depolarization, suggesting that propagating calcium waves are associated with mitochondrial calcium uptake and consequent depolarization. Here we demonstrate that the mitochondrial flicker was directly related to the focal release of calcium from sarcoplasmic reticular (SR) calcium stores and consequent uptake of calcium by local mitochondria. Thus, the events were dramatically reduced by (a) depletion of SR calcium stores after long-term incubation in EGTA or thapsigargin (500 nM); (b) buffering intracellular calcium using BAPTA-AM loading; (c) blockade of SR calcium release with ryanodine (30 microM); and (d) blockade of mitochondrial calcium uptake by microinjection of diaminopentane pentammine cobalt (DAPPAC), a novel inhibitor of the mitochondrial calcium uniporter. These observations demonstrate that focal SR calcium release results in calcium microdomains sufficient to promote local mitochondrial calcium uptake, suggesting a tight coupling of calcium signaling between SR release sites and nearby mitochondria.

Sorcin associates with the pore-forming subunit of voltage-dependent L-type Ca2+ channels

Intracellular Ca2+ release in muscle is governed by functional communication between the voltage-dependent L-type Ca2+ channel and the intracellular Ca2+ release channel by processes that are incompletely understood. We previously showed that sorcin binds to cardiac Ca2+ release channel/ryanodine receptors and decreases channel open probability in planar lipid bilayers. In addition, we showed that sorcin antibody immunoprecipitates ryanodine receptors from metabolically labeled cardiac myocytes along with a second protein having a molecular weight similar to that of the alpha1 subunit of cardiac L-type Ca2+ channels. We now demonstrate that sorcin biochemically associates with cardiac and skeletal muscle L-type Ca2+ channels specifically within the cytoplasmically oriented C-terminal region of the alpha1 subunits, providing evidence that the second protein recovered by sorcin antibody from cardiac myocytes was the 240-kDa L-type Ca2+ channel alpha1 subunit. Anti-sorcin antibody immunoprecipitated full-length alpha1 subunits from cardiac myocytes, C2C12 myotubes, and transfected non-muscle cells expressing alpha1 subunits. In contrast, the anti-sorcin antibody did not immunoprecipitate C-terminal truncated forms of alpha1 subunits that were detected in myotubes. Recombinant sorcin bound to cardiac and skeletal HIS6-tagged alpha1 C termini immobilized on Ni2+ resin. Additionally, anti-sorcin antibody immunoprecipitated C-terminal fragments of the cardiac alpha1 subunit exogenously expressed in mammalian cells. The results identified a putative sorcin binding domain within the C terminus of the alpha1 subunit. These observations, along with the demonstration that sorcin accumulated substantially during physiological maturation of the excitation-contraction coupling apparatus in developing postnatal rat heart and differentiating C2C12 muscle cells, suggest that sorcin may mediate interchannel communication during excitation-contraction coupling in heart and skeletal muscle.

Real time in situ confocal imaging of calcium wave in the perfused whole heart of the rat

To understand the calcium handling in whole heart having automaticity of the sinus node, we have developed a system of in situ imaging the intracellular calcium ion concentration in the perfused whole heart of the rat. The system consists of a stage-fixed upright microscope equipped with a real-time confocal laser scanning device of a multipinhole type with a water-immersion objective lens for observation. This in situ imaging system rendered observations and analyses of the rapidly changing images of intracellular calcium dynamics possible in the whole rat heart loaded with fluo-3. The scanning was conducted at a video rate of 30 frames per second, and the confocal effects included both X and Y planes. Calcium waves were frequently interrupted by calcium transients from either external electro-stimulation pulses or spontaneous sinus rhythm. Our findings suggest that abnormal calcium waves in minute areas cannot disturb the excitation-contraction coupling in the whole heart if the myocardial cells have orderly end-on-end intercellular electric paths.

Fundamental calcium release events revealed by two-photon excitation photolysis of caged calcium in Guinea-pig cardiac myocytes

1. In cardiac muscle, 'Ca2+ sparks' have been proposed to underlie Ca2+-induced Ca2+ release (CICR), and to result from openings of clusters of Ca2+ channels (ryanodine receptors; RyRs) located in the sarcoplasmic reticulum membrane. 2. To investigate the elementary nature of these Ca2+ signals directly, a diffraction-limited point source of Ca2+ was created in single cardiac myocytes by two-photon excitation photolysis of caged Ca2+. Simultaneously, concentration profiles of released Ca2+ were imaged at high temporal and spatial resolution with a laser-scanning confocal microscope. 3. This approach enabled us to generate and detect photolytic Ca2+ signals that closely resembled the Ca2+ sparks occurring naturally, not only in amplitude and size, but also in their ability to trigger additional Ca2+ sparks or Ca2+ waves. 4. Surprisingly, at low photolytic power minuscule events with estimated Ca2+ release fluxes 20-40 times smaller than those calculated for a typical Ca2+ spark were directly resolved. These events appeared to arise from the opening of a more limited number of RyRs (possibly one) or from RyRs exhibiting a different gating mode and may correspond to the elusive 'Ca2+ quark'. 5. The Ca2+ quark represents the fundamental Ca2+ release event of excitable cells implementing hierarchical Ca2+ signalling systems with Ca2+ release events of various but distinct amplitude levels (i.e. Ca2+ quarks, Ca2+ sparks and full cellular Ca2+ transients). 6. A graded recruitment of nanoscopic Ca2+ release domains (i.e. Ca2+ quarks) exhibiting variable degrees of spatial coherence and coupling may then build up intermediate Ca2+ signalling events (i.e. Ca2+ sparks). This mechanism suggests the existence of Ca2+ sparks caused by gating of a variable fraction of RyRs from within an individual cluster. Additional mobilization of a variable number of these Ca2+ sparks enables cardiac cells to show graded cellular Ca2+ transients. Similar recruitment processes may underlie regulation of Ca2+ signalling on the cellular level in general.

Termination of Ca2+ release during Ca2+ sparks in rat ventricular myocytes

1. Confocal Ca2+ imaging was used to measure spontaneous release events (Ca2+ sparks) in fluo-3-loaded isolated rat ventricular myocytes. 2. The microscopic Ca2+ release flux underlying Ca2+ sparks was derived by adapting the methods used previously to describe macroscopic Ca2+ release from cell-averaged Ca2+ transients. 3. The magnitude of the local release fluxes varied from 2 to 5 microM ms-1, depending on SR Ca2+ loading conditions. Following spontaneous activation, the release flux rapidly decayed (tau = 6-12 ms). The rate of termination of release flux was found to be directly related to the magnitude of the flux (r2 = 0.88). 4. The rate of termination of local release flux was slowed in the presence of FK506, a compound that is known to reduce inactivation of SR Ca2+ channels in vitro. 5. These results suggest that termination of release flux during sparks is not due to a spontaneous stochastic decay process or local depletion of Ca2+ from the SR, but rather involves an active extinguishing mechanism such as Ca2+-dependent inactivation or adaptation.