Do calcium waves propagate between cells and synchronize alternating calcium release in rat ventricular myocytes?

Calcium- and voltage-driven atrial alternans: Insight from [Ca]i and Vm asynchrony

Cardiac alternans is defined as beat-to-beat alternations in contraction strength, action potential duration (APD), and Ca transient (CaT) amplitude. Cardiac excitation-contraction coupling relies on the activity of two bidirectionally coupled excitable systems, membrane voltage (Vm ) and Ca release. Alternans has been classified as Vm - or Ca-driven, depending whether a disturbance of Vm or [Ca]i regulation drives the alternans. We determined the primary driver of pacing induced alternans in rabbit atrial myocytes, using combined patch clamp and fluorescence [Ca]i and Vm measurements. APD and CaT alternans are typically synchronized; however, uncoupling between APD and CaT regulation can lead to CaT alternans in the absence of APD alternans, and APD alternans can fail to precipitate CaT alternans, suggesting a considerable degree of independence of CaT and APD alternans. Using alternans AP voltage clamp protocols with extra APs showed that most frequently the pre-existing CaT alternans pattern prevailed after the extra-beat, indicating that alternans is Ca-driven. In electrically coupled cell pairs, dyssynchrony of APD and CaT alternans points to autonomous regulation of CaT alternans. Thus, with three novel experimental protocols, we collected evidence for Ca-driven alternans; however, the intimately intertwined regulation of Vm and [Ca]i precludes entirely independent development of CaT and APD alternans.

Lehnart S, Mongillo M. Multiphoton Imaging of Ca2+ Instability in Acute Myocardial Slices from a RyR2R2474S Murine Model of Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is a familial stress-induced arrhythmia syndrome, mostly caused by mutations in Ryanodine receptor 2 (RyR2), the sarcoplasmic reticulum (SR) Ca²⁺ release channel in cardiomyocytes. Pathogenetic mutations lead to gain of function in the channel, causing arrhythmias by promoting diastolic spontaneous Ca²⁺ release (SCR) from the SR and delayed afterdepolarizations. While the study of Ca²⁺ dynamics in single cells from murine CPVT models has increased our understanding of the disease pathogenesis, questions remain on the mechanisms triggering the lethal arrhythmias at tissue level. Here, we combined subcellular analysis of Ca²⁺ signals in isolated cardiomyocytes and in acute thick ventricular slices of RyR2^R2474S knock-in mice, electrically paced at different rates (1–5 Hz), to identify arrhythmogenic Ca²⁺ dynamics, from the sub- to the multicellular perspective. In both models, RyR2^R2474S cardiomyocytes had increased propensity to develop SCR upon adrenergic stimulation, which manifested, in the slices, with Ca²⁺ alternans and synchronous Ca²⁺ release events in neighboring cardiomyocytes. Analysis of Ca²⁺ dynamics in multiple cells in the tissue suggests that SCRs beget SCRs in contiguous cells, overcoming the protective electrotonic myocardial coupling, and potentially generating arrhythmia triggering foci. We suggest that intercellular interactions may underscore arrhythmic propensity in CPVT hearts with ‘leaky’ RyR2.

Cx43 hemichannel microdomain signaling at the intercalated disc enhances cardiac excitability

Cx43, a major cardiac connexin, forms precursor hemichannels that accrue at the intercalated disc to assemble as gap junctions. While gap junctions are crucial for electrical conduction in the heart, little is known about the potential roles of hemichannels. Recent evidence suggests that inhibiting Cx43 hemichannel opening with Gap19 has antiarrhythmic effects. Here, we used multiple electrophysiology, imaging, and super-resolution techniques to understand and define the conditions underlying Cx43 hemichannel activation in ventricular cardiomyocytes, their contribution to diastolic Ca2+ release from the sarcoplasmic reticulum, and their impact on electrical stability. We showed that Cx43 hemichannels were activated during diastolic Ca2+ release in single ventricular cardiomyocytes and cardiomyocyte cell pairs from mice and pigs. This activation involved Cx43 hemichannel Ca2+ entry and coupling to Ca2+ release microdomains at the intercalated disc, resulting in enhanced Ca2+ dynamics. Hemichannel opening furthermore contributed to delayed afterdepolarizations and triggered action potentials. In single cardiomyocytes, cardiomyocyte cell pairs, and arterially perfused tissue wedges from failing human hearts, increased hemichannel activity contributed to electrical instability compared with nonfailing rejected donor hearts. We conclude that microdomain coupling between Cx43 hemichannels and Ca2+ release is a potentially novel, targetable mechanism of cardiac arrhythmogenesis in heart failure.

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.

Estimating the probabilities of rare arrhythmic events in multiscale computational models of cardiac cells and tissue

Ectopic heartbeats can trigger reentrant arrhythmias, leading to ventricular fibrillation and sudden cardiac death. Such events have been attributed to perturbed Ca2+ handling in cardiac myocytes leading to spontaneous Ca2+ release and delayed afterdepolarizations (DADs). However, the ways in which perturbation of specific molecular mechanisms alters the probability of ectopic beats is not understood. We present a multiscale model of cardiac tissue incorporating a biophysically detailed three-dimensional model of the ventricular myocyte. This model reproduces realistic Ca2+ waves and DADs driven by stochastic Ca2+ release channel (RyR) gating and is used to study mechanisms of DAD variability. In agreement with previous experimental and modeling studies, key factors influencing the distribution of DAD amplitude and timing include cytosolic and sarcoplasmic reticulum Ca2+ concentrations, inwardly rectifying potassium current (IK1) density, and gap junction conductance. The cardiac tissue model is used to investigate how random RyR gating gives rise to probabilistic triggered activity in a one-dimensional myocyte tissue model. A novel spatial-average filtering method for estimating the probability of extreme (i.e. rare, high-amplitude) stochastic events from a limited set of spontaneous Ca2+ release profiles is presented. These events occur when randomly organized clusters of cells exhibit synchronized, high amplitude Ca2+ release flux. It is shown how reduced IK1 density and gap junction coupling, as observed in heart failure, increase the probability of extreme DADs by multiple orders of magnitude. This method enables prediction of arrhythmia likelihood and its modulation by alterations of other cellular mechanisms.

Cardiotoxic Effects of Short-Term Doxorubicin Administration: Involvement of Connexin 43 in Calcium Impairment

The use of Doxorubicin (DOXO), a potent antineoplastic agent, is limited by the development of cardiotoxicity. DOXO-induced cardiotoxicity is multifactorial, although alterations in calcium homeostasis, seem to be involved. Since even the Connexin43 (Cx43) plays a pivotal role in these two phenomena, in this study we have analyzed the effects of DOXO on Cx43 expression and localization. Damage caused by anthracyclines on cardiomyocytes is immediate after each injection, in the present study we used a short-term model of DOXO-induced cardiomyopathy. C57BL/6j female mice were randomly divided in groups and injected with DOXO (2 or 10 mg/kg i.p.) for 1–3 or 7 days once every other day. Cardiac function was assessed by Echocardiography. Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCAII) and phospholamban (PLB) expression were assessed by Western blot analysis, intracellular [Ca²⁺] were detected spectrofluorometrically by means of Fura-2 pentakis (acetoxymethyl) ester (FURA-2AM), and Cx43 and pCx43 expression and localization was analyzed by Western blot and confirmed by immunofluorescence analysis. DOXO induces impairment in Ca²⁺ homeostasis, already evident after a single administration, and affects Cx43 expression and localization. Our data suggest that DOXO-induced alterations in Ca²⁺ homeostasis causes in the cells the induction of compensatory mechanisms until a certain threshold, above which cardiac injury is triggered.

What is a Ca(2+) wave? Is it like an Electrical Wave?

Arrhythmia subcellular mechanisms are constantly being explored. Recent knowledge has shown that travelling Ca(2+) waves in cardiac cells are critical for delayed afterdepolarisations and in some cases, early afterdepolarisations. In this review, we comment on the properties of cardiac Ca(2+) waves and abnormal Ca(2+) releases in terms of properties used to describe electrical waves; propagation, excitability and refractoriness.

Variations in local calcium signaling in adjacent cardiac myocytes of the intact mouse heart detected with two-dimensional confocal microscopy

Dyssynchronous local Ca release within individual cardiac myocytes has been linked to cellular contractile dysfunction. Differences in Ca kinetics in adjacent cells may also provide a substrate for inefficient contraction and arrhythmias. In a new approach we quantify variation in local Ca transients between adjacent myocytes in the whole heart. Langendorff-perfused mouse hearts were loaded with Fluo-8 AM to detect Ca and Di-4-ANEPPS to visualize cell membranes. A spinning disc confocal microscope with a fast camera allowed us to record Ca signals within an area of 465 μm by 315 μm with an acquisition speed of 55 fps. Images from multiple transients recorded at steady state were registered to their time point in the cardiac cycle to restore averaged local Ca transients with a higher temporal resolution. Local Ca transients within and between adjacent myocytes were compared with regard to amplitude, time to peak and decay at steady state stimulation (250 ms cycle length). Image registration from multiple sequential Ca transients allowed reconstruction of high temporal resolution (2.4 ± 1.3 ms) local CaT in 2D image sets (N = 4 hearts, n = 8 regions). During steady state stimulation, spatial Ca gradients were homogeneous within cells in both directions and independent of distance between measured points. Variation in CaT amplitudes was similar across the short and the long side of neighboring cells. Variations in TAU and TTP were similar in both directions. Isoproterenol enhanced the CaT but not the overall pattern of spatial heterogeneities. Here we detected and analyzed local Ca signals in intact mouse hearts with high temporal and spatial resolution, taking into account 2D arrangement of the cells. We observed significant differences in the variation of CaT amplitude along the long and short axis of cardiac myocytes. Variations of Ca signals between neighboring cells may contribute to the substrate of cardiac remodeling.