Mechanisms of calcium release and propagation in cardiac cells. Do studies with confocal microscopy add to our understanding?

In vivo Technique for Cellular Calcium Waves Documentation: A Light Microscopy Method

Background:

We are introducing a novel in vivo technique to document cellular calcium deposits, which reflect a snapshot of the effect of calcium wave propagation. This technique however is not advocated enough to replace the accuracy and resolution of the confocal laser technique. Light microscopy equipment, calcium chelators and a histological calcium staining kit are essential.

Aims:

The purpose of this study is to introduce the use of standard light microscopy to display in vivo ionic cellular calcium deposits.

Materials and Methods:

Oxalic Acid (OA) (100 millimol) was the calcium chelator used in the study. This substance was injected into the dog right atrial tissue in vivo in an area of 1 cm². Samples were fixed and stained by the calcium specific von Kossa protocol.

Results and Conclusions:

Histological slides demarcated the intracellular calcium as black dots. Heterogeneity of calcium deposits mimicked images of both, the calcium sparks and calcium waves theories. This light microscopy technique could expand the number of experimental studies in the function of cellular calcium physiology.

Secret message at the plant surface

In general, stomata open during the day and close at night. This behavior has a crucial importance because it maximizes the update of CO(2) for photosynthesis and minimizes the water loss. Blue light is one of the environmental factors that regulates this process. Certainly, when either entire plants or epidermal strips adapted to the dark are exposed to blue light, the stomata open widely their pores. But, what does happen if we illuminate individual stomata instead of peels or entire plants? In the inaugural issue of PLoS ONE, we have answered this question by irradiating individual stomata with a laser attached to a confocal microscope. Our study not only demonstrates that the stomata function independently from the behavior of their neighbors, and illuminates the implication of the blue light receptors PHOTOTROPIN1 and PHOTOTROPIN2 in such response. It also gives clues about the physiological relevancy of this behavior.

Confocal laser scanning microscopy of calcium dynamics in living cells

Confocal laser scanning microscopy (CLSM) is widely used to monitor intracellular calcium levels in living cells loaded with calcium-sensitive fluorophores. This review examines the basic advantages and limitations of CLSM in in vivo imaging analyses of calcium dynamics. The benefits of utilizing ratioed images and dextran-conjugated fluorophores are addressed, and practical aspects of handling confocal datasets are outlined. After considering some relatively new microscopical methods that can be used in conjunction with conventional CLSM, possible future applications of confocal techniques in analyses of intracellular calcium dynamics are discussed.

Role of mitochondria in calcium regulation of spontaneously contracting cardiac muscle cells

Mitochondrial involvement in the regulation of cytosolic calcium concentration ([Ca2+]i) in cardiac myocytes has been largely discounted by many authors. However, recent evidence, including the results of this study, has forced a reappraisal of this role. [Ca2+]i and Ca2+ in the mitochondria ([Ca2+]m) were measured in this study with specific fluorescent probes, fluo-3 and di-hydro-rhod-2, respectively; mitochondrial membrane potential (DeltaPsim) was monitored with JC-1. Addition of uncouplers or inhibitors of the mitochondrial respiratory chain was found to cause a twofold decrease in the rate of removal of Ca2+ from the cytosol after a spontaneously generated Ca2+ wave. These agents also caused a progressive elevation of [Ca2+]i, an increase in the number of hotspots of Ca2+ release (Ca2+ sparks), and depression of mitochondrial potential. The Ca2+-indicative fluorophore dihydro-rhod-2 has a net positive charge that contributes to selective accumulation by mitochondria, as supported by its co-localization with other mitochondrial-specific probes (MitoTracker Green). Treatment of dihydro-rhod-2-loaded cells with NaCN resulted in rapid formation of "black holes" in the otherwise uniformly banded pattern. These are likely to represent individual or small groups of mitochondria that have depressed mitochondrial potential, or have lost accumulated rhod-2 and/or Ca2+; all of these eventualities are possible upon onset of the mitochondrial permeability transition. Release of Ca2+ from the sarcoplasmic reticulum and the resultant spontaneous contractility of cardiac muscle are proposed to be triggered by the induction of the mitochondrial permeability transition and the subsequent loss of [Ca2+]m.

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.

Effects of basal [Ca2+]i on calcium handling in Ca(2+)-overloaded rat cultured heart muscle cells

To elucidate the relationship between intracellular free Ca2+ concentration ([Ca2+]i) and Ca(2+)-signalling by the sarcoplasmic reticulum (SR) in Ca(2+)-overloaded heart muscle cells, the direct effects of "basal" [Ca2+]i on calcium waves were investigated by altering the membrane potential. When basal inter-calcium wave (BCW) [Ca2+]i was maintained at a high level, (i) calcium waves showed more gradual and more rapidly suppressed increase in [Ca2+]-profile (P < 0.005), and (ii) calcium waves occurred at a significantly higher frequency and velocity (259% and 137%), than when low BCW [Ca2+]i was maintained. Similar investigations on inhibition of the Na(+)-Ca2+ exchanger, however, showed that membrane potential did not elicit direct effects on calcium waves. These results showed that the elevation of BCW [Ca2+]i per se directly influences Ca(2+)-signalling in heart muscle cells through non-equilibrated release-restoration Ca(2+)-handling by the SR.

Velocity-curvature relationship of colliding spherical calcium waves in rat cardiac myocytes

Colliding spherical calcium waves in enzymatically isolated rat cardiac myocytes develop new wavefronts propagating perpendicular to the original direction. When investigated by confocal laser scanning microscopy (CLSM), using the fluorescent Ca2+ indicator fluo-3 AM, "cusp"-like structures become visible that are favorably approximated by double parabolae. The time-dependent position of the vertices is used to determine propagation velocity and negative curvature of the wavefront in the region of collision. It is evident that negatively curved waves propagate faster than positively curved, single waves. Considering two perfectly equal expanding circular waves, we demonstrated that the collision of calcium waves is due to an autocatalytic process (calcium-induced calcium release), and not to a simple phenomenon of interference. Following the spatiotemporal organization in simpler chemical systems maintained under conditions far from the thermodynamic equilibrium (Belousov-Zhabotinskii reaction), the dependence of the normal velocity on the curvature of the spreading wavefront is given by a linear relation. The so-called velocity-curvature relationship makes clear that the velocity is enhanced by curvature toward the direction of forward propagation and decreased by curvature away from the direction of forward propagation (with an influence of the diffusion coefficient). Experimentally obtained velocity data of both negatively and positively curved calcium waves were approximated by orthogonal weighted regression. The negative slope of the straight line resulted in an effective diffusion coefficient of 1.2 x 10(-4) mm2/s. From the so-called critical radius, which must be exceeded to initiate a traveling calcium wave, a critical volume (with enhanced [Ca2+]i) of approximately 12 microm3 was calculated. This is almost identical to the volume that is occupied by a single calcium spark.

Intracellular free Ca2+ movements in cultured cardiac myocytes as shown by rapid scanning confocal microscopy

Two-dimensional images of intracellular free Ca2+ movements in cultured cardiac myocytes were obtained at 33-ms intervals with a Ca(2+)-sensitive fluorescence probe, fluo-3, and a rapid scanning confocal laser microscope, a prototype of Nikon RCM8000. The cells used were isolated from the ventricular myocardium of neonatal mice, cultured for approximately 72 h and loaded with fluo-3. One type of cytoplasmic Ca2+ movement observed was a simultaneous increase in [Ca2+] throughout the cytoplasm, termed a "spike"; another type was a local increase in [Ca2+] propagating in the cytoplasm, termed a "wave." Cells with either spike or wave or both types of movements were observed. Tetrodotoxin (TTX) 10(-5) M, nicardipine 10(-6) M, and increased extracellular potassium concentration (40 mM) selectively inhibited spike, and ryanodine 10(-6) M and cyclopiazonic acid (CPA) 3 x 10(-6) M selectively inhibited wave. These results indicate that spike was triggered by depolarization-induced Ca2+ influx across the sarcolemma, whereas wave was a propagating local increase in Ca2+ due to Ca2+ release from the sarcoplasmic reticulum (SR). On spike, nuclear [Ca2+] was shown to increase and decrease synchronously with cytoplasmic [Ca2+], with a delay and slower time course.

Restricted propagation of cytoplasmic Ca2+ oscillation into the nucleus in guinea pig cardiac myocytes as revealed by rapid scanning confocal microscopy and indo-1

Two-dimensional images of cytoplasmic and nuclear free Ca2+ movements in cardiac myocytes were obtained at 67-msec intervals using a Ca(2+)-sensitive fluorescence probe, indo-1, and a rapid scanning confocal laser microscope, Nikon RCM8000. Isolated guinea pig ventricular cells were loaded with indo-1 and stimulated at 0.5 Hz through patch pipettes. On stimulation, nuclear Ca2+ concentration ([Ca2+]) was observed to rise and fall following cytoplasmic [Ca2+] with an obvious delay. Application of isoproterenol significantly increased the peak [Ca2+] on stimulation in both the cytoplasm and nucleus with no substantial change in the basal [Ca2+]; the increase in peak [Ca2+] produced by application of isoproterenol was larger in the cytoplasm than in the nucleus. Under a low [Na+] condition, the basal [Ca2+] was increased from the control values in both the cytoplasm and nucleus; no difference in basal [Ca2+] was observed between the two regions. The increase in peak [Ca2+] by low [Na+] in the cytoplasm was significantly larger than that in the nucleus. When the cells were voltage clamped at 0 mV for 3 sec, no difference in the steady state [Ca2+] was observed between the cytoplasm and nucleus. Nuclear [Ca2+] was also observed to increase following a Ca2+ wave, a local increase in [Ca2+] propagating within the cytoplasm, with a delay. Thus, we demonstrated in isolated myocardial cells that cytoplasmic Ca2+ movements, although hampered by the nuclear envelope, are propagated into the nucleus, a mechanism through which factors affecting cytoplasmic Ca2+ may influence intranuclear events.

Nonlinear propagation of spherical calcium waves in rat cardiac myocytes

Spontaneous calcium waves in enzymatically isolated rat cardiac myocytes were investigated by confocal laser scanning microscopy (CLSM) using the fluorescent Ca2+-indicator fluo-3 AM. As recently shown, a spreading wave of enhanced cytosolic calcium appears, most probably during Ca2+ overload, and is initiated by an elementary event called a "calcium spark." When measured by conventional fluorescence microscopy the propagation velocity of spontaneous calcium waves determined at several points along the cardiac myocyte was previously found to be constant. More precise measurements with a CLSM showed a nonlinear propagation. The wave velocity was low, close to the focus, and increased with increasing time and propagation length, approaching a maximum of 113 microns/s. This result was surprising, inasmuch as for geometrical reasons a decrease of the propagation velocity might be expected if the confocal plane is not identical with that plane where the focus of the wave was localized. It is suggested that the propagation velocity is essentially dependent on the curvature of the spreading wave. From the linear relationship of velocity versus curvature, a critical radius of 2.7 +/- 1.4 microns (mean +/- SD) was worked out, below which an outward propagation of the wave will not take place. Once released from a sufficiently extended cluster of sarcoplasmic reticulum release channels, calcium diffuses and will activate its neighbors. While traveling away, the volume into which calcium diffuses becomes effectively smaller than at low radii. This effect is the consequence of the summation of elementary events (Ca2+ sparks) and leads to a steeper increase of the cytosolic calcium concentration after a certain diffusion path length. Thus the time taken to reach a critical threshold of [Ca2+]i at the neighboring calcium release sites decreases with decreasing curvature and the wave will propagate faster.

Cytosolic Ca2+ domain-dependent protective action of adenosine in cardiomyocytes

Recently, in beating cardiac cells heterogeneous spatiotemporal patterns in cytosolic Ca2+ distribution have been visualized, and associated with cell contraction. In non-beating cardiomyocytes, spatial heterogeneity of intracellular Ca2+ distribution has also been observed, yet its functional implication in resting cardiac cells is not known. Herein, distinct domains of lower versus higher concentrations of cytosolic Ca2+ (0.17 and 0.37 microM, respectively) were observed using epifluorescent digital imaging in single, non-beating, fluo-3-loaded cardiomyocytes. Extracellular K+ (16 mM) induced a uniform increase of cytosolic Ca2+, despite the initial presence of distinct domains of cytosolic Ca2+ (from 0.17 to 1.82 microM in domains with lower, and from 0.37 to 2.03 microM in domains with higher Ca2+ concentration, respectively). In contrast, adenosine (1 mM) prevented exracellular K+ to induce cytosolic Ca2+ loading selectively within domains with lower (from 0.17 to 0.18 microM), but not in domains with higher (from 0.37 to 1.4 microM) basal Ca2+ concentration. Thus, the response of a cardiomyocyte to the protective action of adenosine is heterogeneous within a resting single cell. The domain-distinct cytoprotective action of adenosine appears to be set by the basal Ca2+ concentration within a cytosolic domain.

Calcium sparks and [Ca2+]i waves in cardiac myocytes

Local elevations in intracellular calcium ("Ca2+ sparks") in heart muscle are elementary sarcoplasmic reticulum (SR) Ca(2+)-release events. Ca2+ sparks occur at a low rate in quiescent cells but can also be evoked by electrical stimulation of the cell to produce the cell-wide Ca2+ transient. In this study we investigate how Ca2+ sparks are related to propagating waves of elevated cytosolic Ca2+ induced by "Ca2+ overload." Single ventricular myocytes from rat were loaded with the Ca(2+)-sensitive indicator fluo 3 and imaged with a confocal microscope. After extracellular Ca2+ concentration was increased from 1 to 10 mM to produce Ca2+ overload, the frequency of spontaneous Ca2+ sparks, which occur at the t tubule/SR junction, increased approximately 4-fold, whereas the spark amplitude and spatial size increased 4.1-and 1.7-fold, respectively. In addition, a spectrum of larger subcellular events, including propagating Ca2+ waves, was observed. Ca2+ sparks were seen to occur at the majority (65%) of the sites of wave initiation. For slowly propagating Ca2+ waves, discrete Ca(2+)-release events, similar to Ca2+ sparks, were detected in the wave front. These Ca2+ sparks appeared to recruit other sparks along the wave front so that the wave progressed in a saltatory manner. We conclude that Ca2+ sparks are elementary events that can explain both the initiation and propagation of Ca2+ waves. In addition, we show that Ca2+ waves and electrically evoked Ca2+ transients have the same time course and interact with each other in a manner that is consistent with both phenomena having the same underlying mechanism(s). These results suggest that SR Ca2+ release during Ca2+ waves, like that during normal excitation-contraction coupling, results from the spatial and temporal summation of Ca2+ sparks.

Differences in features of calcium transients between the nucleus and the cytosol in cultured heart muscle cells: analyzed by confocal microscopy

We analyzed spatio-temporal characteristics of Ca2+ transients in the cytosol and the nucleus of cultured neonatal rat heart cells using confocal imaging with Indo-1 and Fluo-3. In resting heart muscle cells, nuclear [Ca2+] was maintained lower than the cytosolic level. The rise in nuclear [Ca2+], during either E-C coupling or propagation of the Ca2+ wave, began at the edge of the nucleus in the immediate vicinity of the rise in global or localized cytosolic [Ca2+], and spread inwardly. The rise in [Ca2+] was slower and smaller in the nucleus than in the cytosol. The decay in [Ca2+] was also slower in the nucleus than the cytosol, thereby reversing the initial [Ca2+] gradient between them. Caffeine markedly enhanced the rise in nuclear [Ca2+] while maintaining inward spreading. The heterogeneity of nuclear Ca2+ transients during cellular contractilities suggests that influx of Ca2+ from perinuclear stores into the nucleus plays a predominant role in the nuclear [Ca2+] rise. The results also indicated that spatio-temporal characteristics of Ca2+ transients are quite different between the nucleus and the cytosol, thereby suggesting that they are differentially regulated in the nucleus and the cytosol.

Temperature dependence of Ca2+ wave properties in cardiomyocytes: implications for the mechanism of autocatalytic Ca2+ release in wave propagation

Digital imaging microscopy of fluo-3 fluorescence was used to study the velocity and shape of intracellular Ca2+ waves in isolated rat cardiomyocytes as a function of temperature. Decreasing the temperature from 37 to 17 degrees C reduced the longitudinal wave velocity by a factor of 1.8 and remarkably slowed the decay of [Ca2+]i in the trailing flank of a wave. Using image analysis, rise times, and half-maximum decay times of local Ca2+ transients, which characterize the processes of local Ca2+ release and removal, were determined as a function of temperature. Apparent activation energies for wave front propagation, local Ca2+ release, and local Ca2+ removal were derived from Arrhenius plots and amounted to -23, -28, and -46 kJ/mol, respectively. The high activation energy of Ca2+ removal, which arises from the activity of the sarcoplasmic reticulum (SR) Ca2+ ATPase, relative to those of longitudinal wave propagation and local Ca2+ release excludes the hypothetical mechanism of regenerative "spontaneous Ca2+ release," in which Ca2+ that has been taken up from the approaching wavefront triggers Ca2+ release at a luminal site of the SR. It is consistent, however, with the hypothesis that Ca2+ wave propagation is based on Ca(2+)-induced Ca2+ release where Ca2+ triggers release on the cytosolic face of the SR.