Excitation-dependent intracellular Ca2+ waves at the border zone of the cryo-injured rat heart revealed by real-time confocal microscopy

Generation of myocyte agonal Ca2+ waves and contraction bands in perfused rat hearts following irreversible membrane permeabilisation

Although irreversible cardiomyocyte injury provokes intracellular Ca²⁺ ([Ca²⁺]_i) overload, the underlying dynamics of this response and its effects on cellular morphology remain unknown. We therefore visualised rapid-scanning confocal fluo4-[Ca²⁺]_i dynamics and morphology of cardiomyocytes in Langendorff-perfused rat hearts following saponin-membrane permeabilisation. Our data demonstrate that 0.4% saponin-treated myocytes immediately exhibited high-frequency Ca²⁺ waves (131.3 waves/min/cell) with asynchronous, oscillatory contractions having a mean propagation velocity of 117.8 μm/s. These waves slowly decreased in frequency, developed a prolonged decay phase, and disappeared in 10 min resulting in high-static, fluo4-fluorescence intensity. The myocytes showing these waves displayed contraction bands, i.e., band-like actin-fibre aggregates with disruption of sarcomeric α-actinin. The contraction bands were not attenuated by the abolition of Ca²⁺ waves under pretreatment with ryanodine plus thapsigargin, but were partially attenuated by the calpain inhibitor MDL28170, while mechanical arrest of the myocytes by 2,3-butanedione monoxime completely attenuated contraction-band formation. The depletion of adenosine 5'-triphosphate by the mitochondrial electron uncoupler carbonyl cyanide 4-trifluoromethoxy phenylhydrazone also attenuated Ca²⁺ waves and contraction bands. Overall, saponin-induced myocyte [Ca²⁺]_i overload provokes agonal Ca²⁺ waves and contraction bands. Contraction bands are not the direct consequence of the waves but are caused by cross-bridge interactions of the myocytes under calpain-mediated proteolysis.

Regional acidosis locally inhibits but remotely stimulates Ca2+ waves in ventricular myocytes

Aims

Spontaneous Ca²⁺ waves in cardiomyocytes are potentially arrhythmogenic. A powerful controller of Ca²⁺ waves is the cytoplasmic H⁺ concentration ([H⁺]_i), which fluctuates spatially and temporally in conditions such as myocardial ischaemia/reperfusion. H⁺-control of Ca²⁺ waves is poorly understood. We have therefore investigated how [H⁺]_i co-ordinates their initiation and frequency.

Methods and results

Spontaneous Ca²⁺ waves were imaged (fluo-3) in rat isolated ventricular myocytes, subjected to modest Ca²⁺-overload. Whole-cell intracellular acidosis (induced by acetate-superfusion) stimulated wave frequency. Pharmacologically blocking sarcolemmal Na⁺/H⁺ exchange (NHE1) prevented this stimulation, unveiling inhibition by H⁺. Acidosis also increased Ca²⁺ wave velocity. Restricting acidosis to one end of a myocyte, using a microfluidic device, inhibited Ca²⁺ waves in the acidic zone (consistent with ryanodine receptor inhibition), but stimulated wave emergence elsewhere in the cell. This remote stimulation was absent when NHE1 was selectively inhibited in the acidic zone. Remote stimulation depended on a locally evoked, NHE1-driven rise of [Na⁺]_i that spread rapidly downstream.

Conclusion

Acidosis influences Ca²⁺ waves via inhibitory Hi+ and stimulatory Nai+ signals (the latter facilitating intracellular Ca²⁺-loading through modulation of sarcolemmal Na⁺/Ca²⁺ exchange activity). During spatial [H⁺]_i-heterogeneity, Hi+-inhibition dominates in acidic regions, while rapid Nai+ diffusion stimulates waves in downstream, non-acidic regions. Local acidosis thus simultaneously inhibits and stimulates arrhythmogenic Ca²⁺-signalling in the same myocyte. If the principle of remote H⁺-stimulation of Ca²⁺ waves also applies in multicellular myocardium, it raises the possibility of electrical disturbances being driven remotely by adjacent ischaemic areas, which are known to be intensely acidic.

Simultaneous imaging of local calcium and single sarcomere length in rat neonatal cardiomyocytes using yellow Cameleon-Nano140

In cardiac muscle, contraction is triggered by sarcolemmal depolarization, resulting in an intracellular Ca(2+) transient, binding of Ca(2+) to troponin, and subsequent cross-bridge formation (excitation-contraction [EC] coupling). Here, we develop a novel experimental system for simultaneous nano-imaging of intracellular Ca(2+) dynamics and single sarcomere length (SL) in rat neonatal cardiomyocytes. We achieve this by expressing a fluorescence resonance energy transfer (FRET)-based Ca(2+) sensor yellow Cameleon-Nano (YC-Nano) fused to α-actinin in order to localize to the Z disks. We find that, among four different YC-Nanos, α-actinin-YC-Nano140 is best suited for high-precision analysis of EC coupling and α-actinin-YC-Nano140 enables quantitative analyses of intracellular calcium transients and sarcomere dynamics at low and high temperatures, during spontaneous beating and with electrical stimulation. We use this tool to show that calcium transients are synchronized along the length of a myofibril. However, the averaging of SL along myofibrils causes a marked underestimate (∼50%) of the magnitude of displacement because of the different timing of individual SL changes, regardless of the absence or presence of positive inotropy (via β-adrenergic stimulation or enhanced actomyosin interaction). Finally, we find that β-adrenergic stimulation with 50 nM isoproterenol accelerated Ca(2+) dynamics, in association with an approximately twofold increase in sarcomere lengthening velocity. We conclude that our experimental system has a broad range of potential applications for the unveiling molecular mechanisms of EC coupling in cardiomyocytes at the single sarcomere level.

Na⁺ ions as spatial intracellular messengers for co-ordinating Ca²⁺ signals during pH heterogeneity in cardiomyocytes

Aims

Contraction of the heart is regulated by electrically evoked Ca²⁺ transients (CaTs). H⁺ ions, the end products of metabolism, modulate CaTs through direct interactions with Ca²⁺-handling proteins and via Na⁺-mediated coupling between acid-extruding proteins (e.g. Na⁺/H⁺ exchange, NHE1) and Na⁺/Ca²⁺ exchange. Restricted H⁺ diffusivity in cytoplasm predisposes pH-sensitive Ca²⁺ signalling to becoming non-uniform, but the involvement of readily diffusible intracellular Na⁺ ions may provide a means for combatting this.

Methods and results

CaTs were imaged in fluo3-loaded rat ventricular myocytes paced at 2 Hz. Cytoplasmic [Na⁺] ([Na⁺]_i) was imaged using SBFI. Intracellular acidification by acetate exposure raised diastolic and systolic [Ca²⁺] (also observed with acid-loading by ammonium prepulse or CO₂ exposure). The systolic [Ca²⁺] response correlated with a rise in [Na⁺]_i and sarcoplasmic reticulum Ca²⁺ load, and was blocked by the NHE1 inhibitor cariporide (CO₂/HCO₃⁻-free media). Exposure of one half of a myocyte to acetate using dual microperfusion (CO₂/HCO₃⁻-free media) raised diastolic [Ca²⁺] locally in the acidified region. Systolic [Ca²⁺] and CaT amplitude increased more uniformly along the length of the cell, but only when NHE1 was functional. Cytoplasmic Na⁺ diffusivity (D_Na) was measured in quiescent cells, with strophanthidin present to inhibit the Na⁺/K⁺ pump. With regional acetate exposure to activate a local NHE-driven Na⁺-influx, D_Na was found to be sufficiently fast (680 µm²/s) for transmitting the pH–systolic Ca²⁺ interaction over long distances.

Conclusions

Na⁺ ions are rapidly diffusible messengers that expand the spatial scale of cytoplasmic pH–CaT interactions, helping to co-ordinate global Ca²⁺ signalling during conditions of intracellular pH non-uniformity.

Functional cardiac imaging by random access microscopy

Advances in the development of voltage sensitive dyes and Ca²⁺ sensors in combination with innovative microscopy techniques allowed researchers to perform functional measurements with an unprecedented spatial and temporal resolution. At the moment, one of the shortcomings of available technologies is their incapability of imaging multiple fast phenomena while controlling the biological determinants involved. In the near future, ultrafast deflectors can be used to rapidly scan laser beams across the sample, performing optical measurements of action potential and Ca²⁺ release from multiple sites within cardiac cells and tissues. The same scanning modality could also be used to control local Ca²⁺ release and membrane electrical activity by activation of caged compounds and light-gated ion channels. With this approach, local Ca²⁺ or voltage perturbations could be induced, simulating arrhythmogenic events, and their impact on physiological cell activity could be explored. The development of this optical methodology will provide fundamental insights in cardiac disease, boosting new therapeutic strategies, and, more generally, it will represent a new approach for the investigation of the physiology of excitable cells.

Pacing-induced non-uniform ca(2+) dynamics in rat atria revealed by rapid-scanning confocal microscopy

Intracellular Ca(2+) ([Ca(2+)]i) dynamics in isolated myocytes differ between the atria and ventricles due to the distinct t-tubular distributions. Although cellular aspects of ventricular [Ca(2+)]i dynamics in the heart have been extensively studied, little is known about those of atrial myocytes in situ. Here we visualized precise [Ca(2+)]i dynamics of atrial myocytes in Langendorff-perfused rat hearts by rapid-scanning confocal microscopy. Of 16 fluo-4-loaded hearts imaged during pacing up to 4-Hz, five hearts showed spatially uniform Ca(2+) transients on systole among individual cells, whereas no discernible [Ca(2+)]i elevation developed during diastole. In contrast, the remaining hearts showed non-uniform [Ca(2+)]i dynamics within and among the cells especially under high-frequency (4 Hz) excitation, where subcellular cluster-like [Ca(2+)]i rises or wave-like [Ca(2+)]i propagation occurred on excitation. Such [Ca(2+)]i inhomogeneity was more pronounced at high-frequency pacing, showing beat-to-beat Ca(2+) transient alternans. Despite such non-uniform dynamics, cessation of burst pacing of the atria was not followed by emergence of spontaneous Ca(2+) waves, indicating minor Ca(2+)-releasing potentials of the sarcoplasmic reticulum (SR). In summary, rat atria display a propensity to show non-uniform [Ca(2+)]i dynamics on systole due to impaired Ca(2+)-release from the SR and paucity of t-tubules. Our results provide an important basis for understanding atrial pathophysiology.

Slow Calcium-Depolarization-Calcium waves may initiate fast local depolarization waves in ventricular tissue

Intercellular calcium waves in cardiac myocytes are a well-recognized, if incompletely understood, phenomenon. In a variety of preparations, investigators have reported multi-cellular calcium waves or triggered propagated contractions, but the mechanisms of propagation and pathological importance of these events remain unclear. Here, we review existing experimental data and present a computational approach to investigate the mechanisms of multi-cellular calcium wave propagation. Over the past 50 years, the standard modeling paradigm for excitable cardiac tissue has seen increasingly detailed models of the dynamics of individual cells coupled in tissue solely by intercellular and interstitial current flow. Although very successful, this modeling regime has been unable to capture two important phenomena: 1) the slow intercellular calcium waves observed experimentally, and 2) how intercellular calcium events resulting in delayed after depolarizations at the cellular level could overcome a source-sink mismatch to initiate depolarization waves in tissue. In this paper, we introduce a mathematical model with subcellular spatial resolution, in which we allow both inter- and intracellular current flow and calcium diffusion. In simulations of coupled cells employing this model, we observe: a) slow inter-cellular calcium waves propagating at about 0.1 mm/s, b) faster Calcium-Depolarization-Calcium (CDC) waves, traveling at about 1 mm/s, and c) CDC-waves that can set off fast depolarization-waves (50 cm/s) in tissue with varying gap-junction conductivity.

Intracellular pH regulation in heart

Intracellular pH (pHi) is an important modulator of cardiac excitation and contraction, and a potent trigger of electrical arrhythmia. This review outlines the intracellular and membrane mechanisms that control pHi in the cardiac myocyte. We consider the kinetic regulation of sarcolemmal H+, OH- and HCO3- transporters by pH, and by receptor-coupled intracellular signalling systems. We also consider how activity of these pHi effector proteins is coordinated spatially in the myocardium by intracellular mobile buffer shuttles, gap junctional channels and carbonic anhydrase enzymes. Finally, we review the impact of pHi regulatory proteins on intracellular Ca2+ signalling, and their participation in clinical disorders such as myocardial ischaemia, maladaptive hypertrophy and heart failure. Such multiple effects emphasise the fundamental role that pHi regulation plays in the heart.

Arrhythmogenic substrates in myocardial infarct

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

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

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

[Coronary artery disease--relevance of total coronary revascularization on the incidence of malignant arrhythmias]

Myocardial ischemia induces redistribution of different ions (H(+), K(+), Na(+), Ca(++)) across the cardiomyocyte membrane, as well as the loss of intracellular ATP content. This results in changes in the electrical properties including shortening of the action potential, appearance of delayed afterpotentials, and a modified refractoriness of the cardiomyocyte. These changes may induce or support malignant cardiac arrhythmias. Supersensitivity of sympathetic denervated myocardium may further support the electrical instability of ischemic myocardium.Virtues of studies indicate that patients with coronary artery disease who develop complex arrhythmias during or after exercise bear a substantially increased risk for sudden cardiac death. Other studies report about arrhythmic stabilization and reduced mortality if patients with reversible myocardial ischemia receive complete revascularization. However, none of these studies is without methodological flaws. Due to the lack of methodologically sound studies in sufficiently large patient cohorts, the question whether complete coronary revascularisation improves the prognosis of patients with coronary artery disease and which strategy (medical, interventional, or surgical) warrants the best outcomes remains open.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Mechanisms of sudden cardiac death

Despite recent advances in preventing sudden cardiac death (SCD) due to cardiac arrhythmia, its incidence in the population at large has remained unacceptably high. Better understanding of the interaction among various functional, structural, and genetic factors underlying the susceptibility to, and initiation of, fatal arrhythmias is a major goal and will provide new tools for the prediction, prevention, and therapy of SCD. Here, we review the role of aberrant intracellular Ca handling, ionic imbalances associated with acute myocardial ischemia, neurohumoral changes, and genetic predisposition in the pathogenesis of SCD due to cardiac arrhythmia. Therapeutic measures to prevent SCD are also discussed.

In situ Ca2+ dynamics of Purkinje fibers and its interconnection with subjacent ventricular myocytes

Purkinje fibers play essential roles in impulse propagation to the ventricles, and their functional impairment can become arrhythmogenic. However, little is known about precise spatiotemporal pattern(s) of interconnection between Purkinje-fiber network and the underlying ventricular myocardium within the heart. To address this issue, we simultaneously visualized intracellular Ca(2+) dynamics at Purkinje fibers and subjacent ventricular myocytes in Langendorff-perfused rat hearts using multi-pinhole type, rapid-scanning confocal microscopy. Under recording of electrocardiogram at room temperature spatiotemporal changes in fluo3-fluorescence intensity were visualized on the subendocardial region of the right-ventricular septum. Staining of the heart with either fluo3, acetylthiocholine iodide (ATCHI), or di-4-ANEPPS revealed characteristic structures of Purkinje fibers. During sinus rhythm (about 60 bpm) or atrial pacing (up to 3 Hz) each Purkinje-fiber exhibited spatiotemporally synchronous Ca(2+) transients nearly simultaneously to ventricular excitation. Ca(2+) transients in individual fibers were still synchronized within the Purkinje-fiber network not only under high-K(+) (8 mM) perfusion-induced Purkinje-to-ventricular (P-V) conduction delay, but also under unidirectional, orthodromic P-V block produced by 10-mM K(+) perfusion. While spontaneous, asynchronous intracellular Ca(2+) waves were identified in injured fibers of Purkinje network locally, surrounding fibers still exhibited Ca(2+) transients synchronously to ventricular excitation. In summary, these results are the first demonstration of intracellular Ca(2+) dynamics in the Purkinje-fiber network in situ. The synchronous Ca(2+) transients, preserved even under P-V conduction disturbances or under emergence of Ca(2+) waves, imply a syncytial role of Purkinje fibers as a specialized conduction system, whereas unidirectional block at P-V junctions indicates a substrate for reentrant arrhythmias.

Experimental generation and computational modeling of intracellular pH gradients in cardiac myocytes

It is often assumed that pH(i) is spatially uniform within cells. A double-barreled microperfusion system was used to apply solutions of weak acid (acetic acid, CO(2)) or base (ammonia) to localized regions of an isolated ventricular myocyte (guinea pig). A stable, longitudinal pH(i) gradient (up to 1 pH(i) unit) was observed (using confocal imaging of SNARF-1 fluorescence). Changing the fractional exposure of the cell to weak acid/base altered the gradient, as did changing the concentration and type of weak acid/base applied. A diffusion-reaction computational model accurately simulated this behavior of pH(i). The model assumes that H(i)(+) movement occurs via diffusive shuttling on mobile buffers, with little free H(+) diffusion. The average diffusion constant for mobile buffer was estimated as 33 x 10(-7) cm(2)/s, consistent with an apparent H(i)(+) diffusion coefficient, D(H)(app), of 14.4 x 10(-7) cm(2)/s (at pH(i) 7.07), a value two orders of magnitude lower than for H(+) ions in water but similar to that estimated recently from local acid injection via a cell-attached glass micropipette. We conclude that, because H(i)(+) mobility is so low, an extracellular concentration gradient of permeant weak acid readily induces pH(i) nonuniformity. Similar concentration gradients for weak acid (e.g., CO(2)) occur across border zones during regional myocardial ischemia, raising the possibility of steep pH(i) gradients within the heart under some pathophysiological conditions.