Caffeine-induced arrhythmias in murine hearts parallel changes in cellular Ca(2+) homeostasis

hESC derived cardiomyocyte biosensor to detect the different types of arrhythmogenic properties of drugs

In the present work, we introduce a new cell-based biosensor for detecting arrhythmias based on a novel utilization of the combination of the Atomic Force Microscope (AFM) lateral force measurement as a nanosensor with a dual 3D cardiomyocyte syncytium. Two spontaneously coupled clusters of cardiomyocytes form this. The syncytium's functional contraction behavior was assessed using video sequences analyzed with Musclemotion ImageJ/Fiji software, and immunocytochemistry evaluated phenotype composition. The application of caffeine solution induced arrhythmia as a model drug, and its spontaneous resolution was monitored by AFM lateral force recording and interpretation and calcium fluorescence imaging as a reference method describing non-synchronized contractions of cardiomyocytes. The phenotypic analysis revealed the syncytium as a functional contractile and conduction cardiac behavior model. Calcium fluorescence imaging was used to validate that AFM fully enabled to discriminate cardiac arrhythmias in this in vitro cellular model. The described novel 3D hESCs-based cellular biosensor is suitable to detect arrhythmic events on the level of cardiac contractile and conduction tissue cellular model. The resulting biosensor allows for screening of arrhythmogenic properties of tailored drugs enabling its use in precision medicine.

Caffeine: cardiorespiratory effects and tissue protection in animal models

The aim of this review is to analyze the cardiorespiratory and tissue-protective effects of caffeine in animal models. Peer-reviewed literature published between 1975 and 2021 was retrieved from CAB Abstracts, PubMed, ISI Web of Knowledge, and Scopus. Extracted data were analyzed to address the mechanism of action of caffeine on cardiorespiratory parameters (heart rate and rhythm), vasopressor effects, and some indices of respiratory function; we close this review by discussing the current debate on the research carried out on the effects of caffeine on tissue protection. Adenosine acts through specific receptors and is a negative inotropic and chronotropic agent. Blockage of its cardiac receptors can cause tachycardia (with arrhythmogenic potential) due to the intense activity of β1 receptors. In terms of tissue protection, caffeine inhibits hyperoxia-induced pulmonary inflammation by decreasing proinflammatory cytokine expression in animal models. The protection that caffeine provides to tissues is not limited to the CNS, as studies have demonstrated that it generates attenuation of inflammatory effects in pulmonary tissue. It inhibits the effects of some pro-inflammatory cytokines and prevents functional and structural changes.

Epac-induced ryanodine receptor type 2 activation inhibits sodium currents in atrial and ventricular murine cardiomyocytes

Acute RyR2 activation by exchange protein directly activated by cAMP (Epac) reversibly perturbs myocyte Ca²⁺ homeostasis, slows myocardial action potential conduction, and exerts pro‐arrhythmic effects. Loose patch‐clamp studies, preserving in vivo extracellular and intracellular conditions, investigated Na⁺ current in intact cardiomyocytes in murine atrial and ventricular preparations following Epac activation. Depolarising steps to varying test voltages activated typical voltage‐dependent Na⁺ currents. Plots of peak current against depolarisation from resting potential gave pretreatment maximum atrial and ventricular currents of −20.23 ± 1.48 (17) and −29.8 ± 2.4 (10) pA/μm² (mean ± SEM [n]). Challenge by 8‐CPT (1 μmol/L) reduced these currents to −11.21 ± 0.91 (12) (P < .004) and −19.3 ± 1.6 (11) pA/μm² (P < .04) respectively. Currents following further addition of the RyR2 inhibitor dantrolene (10 μmol/L) (−19.91 ± 2.84 (13) and −26.6 ± 1.7 (17)), and dantrolene whether alone (−19.53 ± 1.97 (8) and −27.6 ± 1.9 (14)) or combined with 8‐CPT (−19.93 ± 2.59 (12) and −29.9 ± 2.5(11)), were indistinguishable from pretreatment values (all P >> .05). Assessment of the inactivation that followed by applying subsequent steps to a fixed voltage 100 mV positive to resting potential gave concordant results. Half‐maximal inactivation voltages and steepness factors, and time constants for Na⁺ current recovery from inactivation in double‐pulse experiments, were similar through all the pharmacological conditions. Intracellular sharp microelectrode membrane potential recordings in intact Langendorff‐perfused preparations demonstrated concordant variations in maximum rates of atrial and ventricular action potential upstroke, (dV/dt)_max. We thus demonstrate an acute, reversible, Na⁺ channel inhibition offering a possible mechanism for previously reported pro‐arrhythmic slowing of AP propagation following modifications of Ca²⁺ homeostasis, complementing earlier findings from chronic alterations in Ca²⁺ homeostasis in genetically‐modified RyR2‐P2328S hearts.

Effects of ageing on pro-arrhythmic ventricular phenotypes in incrementally paced murine Pgc-1β -/- hearts

A range of chronic clinical conditions accompany cardiomyocyte energetic dysfunction and constitute independent risk factors for cardiac arrhythmia. We investigated pro-arrhythmic and arrhythmic phenotypes in energetically deficient C57BL mice with genetic ablation of the mitochondrial promoter peroxisome proliferator-activated receptor-γ coactivator-1β (Pgc-1β), a known model of ventricular arrhythmia. Pro-arrhythmic and cellular action potential (AP) characteristics were compared in intact Langendorff-perfused hearts from young (12–16 week) and aged (> 52 week), wild-type (WT) and Pgc-1β^−/− mice. Simultaneous electrocardiographic and intracellular microelectrode recordings were made through successive trains of 100 regular stimuli at progressively incremented heart rates. Aged Pgc-1β^−/− hearts displayed an increased incidence of arrhythmia compared to other groups. Young and aged Pgc-1β^−/− hearts showed higher incidences of alternans in both AP activation (maximum AP upshoot velocity (dV/dt)_max and latency), recovery (action potential duration (APD₉₀) and resting membrane potential (RMP) characteristics compared to WT hearts. This was particularly apparent at lower pacing frequencies. These findings accompanied reduced (dV/dt)_max and increased AP latency values in the Pgc-1β^−/− hearts. APs observed prior to termination of the protocol showed lower (dV/dt)_max and longer AP latencies, but indistinguishable APD₉₀ and RMPs in arrhythmic compared to those in non-arrhythmic hearts. APD restitution analysis showed that Pgc-1β^−/− and WT hearts showed similar limiting gradients. However, Pgc-1β^−/− hearts had shortened plateau AP wavelengths, particularly in aged Pgc-1β^−/− hearts. Pgc-1β^−/− hearts therefore show pro-arrhythmic instabilities attributable to altered AP conduction and activation rather than recovery characteristics.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Caffeine: cognitive and physical performance enhancer or psychoactive drug?

Caffeine use is increasing worldwide. The underlying motivations are mainly concentration and memory enhancement and physical performance improvement. Coffee and caffeine-containing products affect the cardiovascular system, with their positive inotropic and chronotropic effects, and the central nervous system, with their locomotor activity stimulation and anxiogenic-like effects. Thus, it is of interest to examine whether these effects could be detrimental for health. Furthermore, caffeine abuse and dependence are becoming more and more common and can lead to caffeine intoxication, which puts individuals at risk for premature and unnatural death. The present review summarizes the main findings concerning caffeine’s mechanisms of action (focusing on adenosine antagonism, intracellular calcium mobilization, and phosphodiesterases inhibition), use, abuse, dependence, intoxication, and lethal effects. It also suggests that the concepts of toxic and lethal doses are relative, since doses below the toxic and/or lethal range may play a causal role in intoxication or death. This could be due to caffeine’s interaction with other substances or to the individuals' preexisting metabolism alterations or diseases.

Effect of Caffeine Chronically Consumed During Pregnancy on Adenosine A1 and A2A Receptors Signaling in Both Maternal and Fetal Heart from Wistar Rats

Background: Caffeine is the most widely consumed psychoactive substance in the world, even during pregnancy. Its stimulatory effects are mainly due to antagonism of adenosine actions by blocking adenosine A₁ and A_2A receptors. Previous studies have shown that caffeine can cross the placenta and therefore modulate these receptors not only in the fetal brain but also in the heart. Methods: In the present work, the effect of caffeine chronically consumed during pregnancy on A₁ and A_2A receptors in Wistar rat heart, from both mothers and their fetuses, were studied using radioligand binding, Western-blotting, and adenylyl cyclase activity assays, as well as reverse transcription polymerase chain reaction. Results: Caffeine did not significantly alter A₁R neither at protein nor at gene expression level in both the maternal and fetal heart. On the contrary, A_2AR significantly decreased in the maternal heart, although mRNA was not affected. Gi and Gs proteins were also preserved. Finally, A₁R-mediated inhibition of adenylyl cyclase activity did not change in the maternal heart, but A_2AR mediated stimulation of this enzymatic activity significantly decreased according to the detected loss of this receptor. Conclusions: Opposite to the downregulation and desensitization of the A₁R/AC pathway previously reported in the brain, these results show that this pathway is not affected in rat heart after caffeine exposure during pregnancy. In addition, A_2AR is downregulated and desensitized in the maternal heart, suggesting a differential modulation of these receptor-mediated pathways by caffeine.

Reversible neuronal and muscular toxicity of caffeine in developing vertebrates

This study utilizes zebrafish embryos to understand the cellular and molecular mechanisms of caffeine toxicity in developing vertebrate embryos. By using a high concentration of caffeine, we observed almost all the phenotypes that have been described in humans and/or in other animal models, including neural tube closure defect, jittery, touch insensitivity, and growth retardation as well as a drastic coiled body phenotype. Zebrafish embryos exposed to 5mM caffeine exhibited high frequent movement, 10 moves/min comparing with around 3 moves/min in control embryos, within half an hour post exposure (HPE). They later showed twitching, uncoordinated movement, and eventually severe body curvature by 6HPE. Exposure at later stages resulted in the same phenotypes but more posteriorly. Surprisingly, when caffeine was removed before 6HPE, the embryos were capable of recovering but still exhibited mild curvature and shorter bodies. Longer exposure caused irreversible body curvature and lethality. These results suggest that caffeine likely targets the neuro-muscular physiology in developing embryos. Immunohistochemistry revealed that the motorneurons in treated embryos developed shorter axons, abnormal branching, and excessive synaptic vesicles. Developing skeletal muscles also appeared smaller and lacked the well-defined boundaries seen in control embryos. Finally, caffeine increases the expression of genes involved in synaptic vesicle migration. In summary, our results provide molecular understanding of caffeine toxicity on developing vertebrate embryos.

Abnormal Ca(2+) homeostasis, atrial arrhythmogenesis, and sinus node dysfunction in murine hearts modeling RyR2 modification

Ryanodine receptor type 2 (RyR2) mutations are implicated in catecholaminergic polymorphic ventricular tachycardia (CPVT) thought to result from altered myocyte Ca(2+) homeostasis reflecting inappropriate "leakiness" of RyR2-Ca(2+) release channels arising from increases in their basal activity, alterations in their phosphorylation, or defective interactions with other molecules or ions. The latter include calstabin, calsequestrin-2, Mg(2+), and extraluminal or intraluminal Ca(2+). Recent clinical studies additionally associate RyR2 abnormalities with atrial arrhythmias including atrial tachycardia (AT), fibrillation (AF), and standstill, and sinus node dysfunction (SND). Some RyR2 mutations associated with CPVT in mouse models also show such arrhythmias that similarly correlate with altered Ca(2+) homeostasis. Some examples show evidence for increased Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of RyR2. A homozygotic RyR2-P2328S variant demonstrates potential arrhythmic substrate resulting from reduced conduction velocity (CV) in addition to delayed afterdepolarizations (DADs) and ectopic action potential (AP) firing. Finally, one model with an increased RyR2 activity in the sino-atrial node (SAN) shows decreased automaticity in the presence of Ca(2+)-dependent decreases in I Ca, L and diastolic sarcoplasmic reticular (SR) Ca(2+) depletion.

High throughput measurement of Ca²⁺ dynamics for drug risk assessment in human stem cell-derived cardiomyocytes by kinetic image cytometry

Current methods to measure physiological properties of cardiomyocytes and predict fatal arrhythmias that can cause sudden death, such as Torsade de Pointes, lack either the automation and throughput needed for early-stage drug discovery and/or have poor predictive value. To increase throughput and predictive power of in vitro assays, we developed kinetic imaging cytometry (KIC) for automated cell-by-cell analyses via intracellular fluorescence Ca²⁺ indicators. The KIC instrument simultaneously records and analyzes intracellular calcium concentration [Ca²⁺](i) at 30-ms resolution from hundreds of individual cells/well of 96-well plates in seconds, providing kinetic details not previously possible with well averaging technologies such as plate readers. Analyses of human embryonic stem cell and induced pluripotent stem cell-derived cardiomyocytes revealed effects of known cardiotoxic and arrhythmogenic drugs on kinetic parameters of Ca²⁺ dynamics, suggesting that KIC will aid in the assessment of cardiotoxic risk and in the elucidation of pathogenic mechanisms of heart disease associated with drugs treatment and/or genetic background.

Effect of distinct sources of Ca2+ on cardiac hypertrophy in cardiomyocytes

It is believed that intracellular calcium (Ca2+) overload can cause the cardiac hypertrophy, but it is possible that the Ca2+ entering the cytoplasm through distinct pathways will induce various effects on cardiomyocytes. The aim of the present study is to explore the effect of different sources of Ca2+ on cardiomyocyte hypertrophy. The cardiomyocytes isolated from neonatal Sprague–Dawley rats were treated with three agents (ionomycin, caffeine and angiotensin II [Ang II]) that increased the intracellular Ca2+ concentration via different pathways. Treatments with ionomycin, caffeine and Ang II for 24 h caused a significant increase in resting [Ca2+]i by 108.0 ± 7.8%, 102.0 ± 6.9% and 59.8 ± 3.3%, respectively. Caffeine and Ang II increased the cell surface area of cardiomyocytes and the mRNA level of atrial natriuretic peptide, brain natriuretic peptide and β-myosin heavy chain, but ionomycin did not. Moreover, sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) activity and the amplitudes of the twitch [Ca2+]i transients were reduced in the caffeine-treated group and Ang II-treated group. Furthermore, cardiomyocyte hypertrophy induced by caffeine was inhibited by cyclosporin A (CsA) and KN93, whereas cardiomyocyte hypertrophy induced by Ang II was inhibited by KN93, but not CsA. Our results show that cardiomyocyte hypertrophy is associated with SERCA2a activity, contractile performance and signaling pathways of CaMKII and/or calcineurin, whereas the Ca2+ overload is not sufficient to cause the cardiomyocyte hypertrophy.

Methods in cardiomyocyte isolation, culture, and gene transfer

Since techniques for cardiomyocyte isolation were first developed 35 years ago, experiments on single myocytes have yielded great insight into their cellular and sub-cellular physiology. These studies have employed a broad range of techniques including electrophysiology, calcium imaging, cell mechanics, immunohistochemistry and protein biochemistry. More recently, techniques for cardiomyocyte culture have gained additional importance with the advent of gene transfer technology. While such studies require a high quality cardiomyocyte population, successful cell isolation and maintenance during culture remain challenging. In this review, we describe methods for the isolation of adult and neonatal ventricular myocytes from rat and mouse heart. This discussion outlines general principles for the beginner, but also provides detailed specific protocols and advice for common caveats. We additionally review methods for short-term myocyte culture, with particular attention given to the importance of substrate and media selection, and describe time-dependent alterations in myocyte physiology that should be anticipated. Gene transfer techniques for neonatal and adult cardiomyocytes are also reviewed, including methods for transfection (liposome, electroporation) and viral-based gene delivery.

Caffeine and cardiac arrhythmias: a review of the evidence

Limited data exist on the safety and physiologic effects of caffeine in patients with known arrhythmias. The studies presented suggest that in most patients with known or suspected arrhythmia, caffeine in moderate doses is well tolerated and there is therefore no reason to restrict ingestion of caffeine. A review of the literature is presented.

Acute atrial arrhythmogenesis in murine hearts following enhanced extracellular Ca(2+) entry depends on intracellular Ca(2+) stores

Aim

To investigate the effect of increases in extracellular Ca²⁺ entry produced by the L-type Ca²⁺ channel agonist FPL-64176 (FPL) upon acute atrial arrhythmogenesis in intact Langendorff-perfused mouse hearts and its dependence upon diastolic Ca²⁺ release from sarcoplasmic reticular Ca²⁺ stores.

Methods

Confocal microscope studies of Fluo-3 fluorescence in isolated atrial myocytes were performed in parallel with electrophysiological examination of Langendorff-perfused mouse hearts.

Results

Atrial myocytes stimulated at 1 Hz and exposed to FPL (0.1 μm) initially showed (<10 min) frequent, often multiple, diastolic peaks following the evoked Ca²⁺ transients whose amplitudes remained close to control values. With continued pacing (>10 min) this reverted to a regular pattern of evoked transients with increased amplitudes but in which diastolic peaks were absent. Higher FPL concentrations (1.0 μm) produced sustained and irregular patterns of cytosolic Ca²⁺ activity, independent of pacing. Nifedipine (0.5 μm), and caffeine (1.0 mm) and cyclopiazonic acid (CPA) (0.15 μm) pre-treatments respectively produced immediate and gradual reductions in the F/F₀ peaks. Such nifedipine and caffeine, or CPA pre-treatments, abolished, or reduced, the effects of 0.1 and 1.0 μm FPL on cytosolic Ca²⁺ signals. FPL (1.0 μm) increased the incidence of atrial tachycardia and fibrillation in intact Langendorff-perfused hearts without altering atrial effective refractory periods. These effects were inhibited by nifedipine and caffeine, and reduced by CPA.

Conclusion

Enhanced extracellular Ca²⁺ entry exerts acute atrial arrhythmogenic effects that is nevertheless dependent upon diastolic Ca²⁺ release. These findings complement reports that associate established, chronic, atrial arrhythmogenesis with decreased overall inward Ca²⁺ current.

Altered cytosolic Ca2+ dynamics in cultured Guinea pig cardiomyocytes as an in vitro model to identify potential cardiotoxicants

Altered intracellular calcium (Ca(i)(2+)) handling by cardiomyocytes has been implicated in drug-induced cardiomyopathy and arrhythmogenesis. To explore whether such alterations predict cardiotoxicity, Ca(i)(2+) imaging was conducted in cultured, spontaneously contracting Guinea pig cardiomyocytes to characterize the effects of 13 cardiotoxicants and 2 safe drugs. All cardiotoxicants perturbed Ca(i)(2+) at therapeutically relevant concentrations. The cytotoxic chemotherapeutics doxorubicin and epirubicin, known to cause cardiomyopathy, preferentially reduced Ca(i)(2+) transient amplitude and sarcoplasmic reticulum (SR) Ca(2+) content, whereas Torsade de Pointes (TdP) inducers and potent hERG channel blockers (amiodarone, cisapride, dofetilide, E-4031 and terfenadine) predominately suppressed diastolic Ca(i)(2+) and contraction rate, and prolonged Ca(i)(2+) transient duration. The molecularly targeted cancer therapeutics, sunitinib and imatinib, exhibited profound effects on Ca(i)(2+), combining effects of cytotoxic chemotherapeutics, TdP inducers and potent hERG channel blockers. TdP inducers lacking direct hERG inhibition, ouabain and pentamidine, significantly elevated Ca(i)(2+) transient amplitude and SR Ca(2+) content while aconitine primarily accelerated automaticity and elevated diastolic Ca(i)(2+) similar to ouabain. Finally, amoxicillin and aspirin did not exert any significant effects on Ca(i)(2+) at concentrations up to 100 microM. These results suggest that detecting altered Ca(i)(2+) handling in cultured cardiomyocytes may be used as an in vitro model for early cardiac drug safety assessment.

Calmodulin kinase II-mediated sarcoplasmic reticulum Ca2+ leak promotes atrial fibrillation in mice

A trial fibrillation (AF), the most common human cardiac arrhythmia, is associated with abnormal intracellular Ca2+ handling. Diastolic Ca2+ release from the sarcoplasmic reticulum via "leaky" ryanodine receptors (RyR2s) is hypothesized to contribute to arrhythmogenesis in AF, but the molecular mechanisms are incompletely understood. Here, we have shown that mice with a genetic gain-of-function defect in Ryr2 (which we termed Ryr2R176Q/+ mice) did not exhibit spontaneous AF but that rapid atrial pacing unmasked an increased vulnerability to AF in these mice compared with wild-type mice. Rapid atrial pacing resulted in increased Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of RyR2, while both pharmacologic and genetic inhibition of CaMKII prevented AF inducibility in Ryr2R176Q/+ mice. This result suggests that AF requires both an arrhythmogenic substrate (e.g., RyR2 mutation) and enhanced CaMKII activity. Increased CaMKII phosphorylation of RyR2 was observed in atrial biopsies from mice with atrial enlargement and spontaneous AF, goats with lone AF, and patients with chronic AF. Genetic inhibition of CaMKII phosphorylation of RyR2 in Ryr2S2814A knockin mice reduced AF inducibility in a vagotonic AF model. Together, these findings suggest that increased RyR2-dependent Ca2+ leakage due to enhanced CaMKII activity is an important downstream effect of CaMKII in individuals susceptible to AF induction.

Pancreatic protease activation by alcohol metabolite depends on Ca2+ release via acid store IP3 receptors

Toxic alcohol effects on pancreatic acinar cells, causing the often fatal human disease acute pancreatitis, are principally mediated by fatty acid ethyl esters (non-oxidative products of alcohol and fatty acids), emptying internal stores of Ca(2+). This excessive Ca(2+) liberation induces Ca(2+)-dependent necrosis due to intracellular trypsin activation. Our aim was to identify the specific source of the Ca(2+) release linked to the fatal intracellular protease activation. In 2-photon permeabilized mouse pancreatic acinar cells, we monitored changes in the Ca(2+) concentration in the thapsigargin-sensitive endoplasmic reticulum (ER) as well as in a bafilomycin-sensitive acid compartment, localized exclusively in the apical granular pole. We also assessed trypsin activity in the apical granular region. Palmitoleic acid ethyl ester (POAEE) elicited Ca(2+) release from both the ER as well as the acid pool, but trypsin activation depended predominantly on Ca(2+) release from the acid pool, that was mainly mediated by functional inositol 1,4,5- trisphosphate receptors (IP(3)Rs) of types 2 and 3. POAEE evoked very little Ca(2+) release and trypsin activation when IP(3)Rs of both types 2 and 3 were knocked out. Antibodies against IP(3)Rs of types 2 and 3, but not type 1, markedly inhibited POAEE-elicited Ca(2+) release and trypsin activation. We conclude that Ca(2+) release through IP(3)Rs of types 2 and 3 in the acid granular Ca(2+) store induces intracellular protease activation, and propose that this is a critical process in the initiation of alcohol-related acute pancreatitis.

Ryanodine receptor-mediated arrhythmias and sudden cardiac death

The cardiac ryanodine receptor-Ca²⁺ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca²⁺ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.

Pharmacological changes in cellular Ca2+ homeostasis parallel initiation of atrial arrhythmogenesis in murine Langendorff-perfused hearts

1. Intracellular Ca(2+) overload has been associated with established atrial arrhythmogenesis. The present experiments went on to correlate acute initiation of atrial arrhythmogenesis in Langendorff-perfused mouse hearts with changes in Ca(2+) homeostasis in isolated atrial myocytes following pharmacological procedures that modified the storage or release of sarcoplasmic reticular (SR) Ca(2+) or inhibited entry of extracellular Ca(2+). 2. Caffeine (1 mmol/L) elicited diastolic Ca(2+) waves in regularly stimulated atrial myocytes immediately following addition. This was followed by a decline in the amplitude of the evoked transients and the disappearance of such diastolic events, suggesting partial SR Ca(2+) depletion. 3. Cyclopiazonic acid (CPA; 0.15 micromol/L) produced more gradual reductions in evoked Ca(2+) transients and abolished diastolic Ca(2+) events produced by the further addition of caffeine. 4. Nifedipine (0.5 micromol/L) produced immediate reductions in evoked Ca(2+) transients. Further addition of caffeine produced an immediate increase followed by a decline in the amplitude of the evoked Ca(2+) transients, without eliciting diastolic Ca(2+) events. 5. These findings correlated with changes in spontaneous and provoked atrial arrhythmogenecity in mouse isolated Langendorf-perfused hearts. Thus, caffeine was pro-arrhythmogenic immediately following but not > 5 min after application and both CPA and nifedipine pretreatment inhibited such arrhythmogenesis. 6. Together, these findings relate acute atrial arrhythmogenesis in intact hearts to diastolic Ca(2+) events in atrial myocytes that, in turn, depend upon a finite SR Ca(2+) store and diastolic Ca(2+) release following Ca(2+)-induced Ca(2+) release initiated by the entry of extracellular Ca(2+).

Arrhythmogenic actions of the Ca2+ channel agonist FPL-64716 in Langendorff-perfused murine hearts

The experiments explored the extent to which alterations in L-type Ca(2+) channel-mediated Ca(2+) entry triggers Ca(2+)-mediated arrhythmogenesis in Langendorff-perfused murine hearts through use of the specific L-type Ca(2+) channel modulator FPL-64716 (FPL). Introduction of FPL (1 microm) resulted in a gradual development (>10 min) of diastolic electrical events and alternans in spontaneously beating hearts from which monophasic action potentials were recorded. In regularly paced hearts, they additionally led to non-sustained and sustained ventricular tachycardia (nsVT and sVT). Programmed electrical stimulation (PES) resulted in nsVT and sVT after 5-10 and >10 min perfusion, respectively. Pretreatments with nifedipine, diltiazem and cyclopiazonic acid abolished arrhythmogenic tendency induced by subsequent introduction of FPL, consistent with its dependence upon both extracellular Ca(2+) entry and the degree of filling of the sarcoplasmic reticular Ca(2+) store. Values for action potential duration at 90% repolarization when any of these agents were applied to FPL-treated hearts became indistinguishable from those shown by untreated control hearts, in contrast to earlier reports of their altering in long QT syndrome type 3 and hypokalaemic murine models for re-entrant arrhythmogenesis. These arrhythmic effects instead correlated with alterations in Ca(2+) homeostasis at the single-cell level found in investigations of the effects of both FPL and the same agents in regularly stimulated fluo-3 loaded myocytes. These findings are compatible with a prolonged extracellular Ca(2+) entry that potentially results in an intracellular Ca(2+) overload and produces the cardiac arrhythmogenecity following addition of FPL.

Caffeine induces Ca2+ release by reducing the threshold for luminal Ca2+ activation of the ryanodine receptor

Caffeine has long been used as a pharmacological probe for studying RyR (ryanodine receptor)-mediated Ca(2+) release and cardiac arrhythmias. However, the precise mechanism by which caffeine activates RyRs is elusive. In the present study, we investigated the effects of caffeine on spontaneous Ca(2+) release and on the response of single RyR2 (cardiac RyR) channels to luminal or cytosolic Ca(2+). We found that HEK-293 cells (human embryonic kidney cells) expressing RyR2 displayed partial or 'quantal' Ca(2+) release in response to repetitive additions of submaximal concentrations of caffeine. This quantal Ca(2+) release was abolished by ryanodine. Monitoring of endoplasmic reticulum luminal Ca(2+) revealed that caffeine reduced the luminal Ca(2+) threshold at which spontaneous Ca(2+) release occurs. Interestingly, spontaneous Ca(2+) release in the form of Ca(2+) oscillations persisted in the presence of 10 mM caffeine, and was diminished by ryanodine, demonstrating that unlike ryanodine, caffeine, even at high concentrations, does not hold the channel open. At the single-channel level, caffeine markedly reduced the threshold for luminal Ca(2+) activation, but had little effect on the threshold for cytosolic Ca(2+) activation, indicating that the major action of caffeine is to reduce the luminal, but not the cytosolic, Ca(2+) activation threshold. Furthermore, as with caffeine, the clinically relevant, pro-arrhythmic methylxanthines aminophylline and theophylline potentiated luminal Ca(2+) activation of RyR2, and increased the propensity for spontaneous Ca(2+) release, mimicking the effects of disease-linked RyR2 mutations. Collectively, our results demonstrate that caffeine triggers Ca(2+) release by reducing the threshold for luminal Ca(2+) activation of RyR2, and suggest that disease-linked RyR2 mutations and RyR2-interacting pro-arrhythmic agents may share the same arrhythmogenic mechanism.

Physiological consequences of the P2328S mutation in the ryanodine receptor (RyR2) gene in genetically modified murine hearts

Aim

To explore the physiological consequences of the ryanodine receptor (RyR2)-P2328S mutation associated with catecholaminergic polymorphic ventricular tachycardia (CPVT).

Methods

We generated heterozygotic (RyR2^p/s) and homozygotic (RyR2^s/s) transgenic mice and studied Ca²⁺ signals from regularly stimulated, Fluo-3-loaded, cardiac myocytes. Results were compared with monophasic action potentials (MAPs) in Langendorff-perfused hearts under both regular and programmed electrical stimulation (PES).

Results

Evoked Ca²⁺ transients from wild-type (WT), heterozygote (RyR2^p/s) and homozygote (RyR2^s/s) myocytes had indistinguishable peak amplitudes with RyR2^s/s showing subsidiary events. Adding 100 nm isoproterenol produced both ectopic peaks and subsidiary events in WT but not RyR2^p/s and ectopic peaks and reduced amplitudes of evoked peaks in RyR2^s/s. Regularly stimulated WT, RyR2^p/s and RyR2^s/s hearts showed indistinguishable MAP durations and refractory periods. RyR2^p/s hearts showed non-sustained ventricular tachycardias (nsVTs) only with PES. Both nsVTs and sustained VTs (sVTs) occurred with regular stimuli and PES with isoproterenol treatment. RyR2^s/s hearts showed higher incidences of nsVTs before but mainly sVTs after introduction of isoproterenol with both regular stimuli and PES, particularly at higher pacing frequencies. Additionally, intrinsically beating RyR2^s/s showed extrasystolic events often followed by spontaneous sVT.

Conclusion

The RyR2-P2328S mutation results in marked alterations in cellular Ca²⁺ homeostasis and arrhythmogenic properties resembling CPVT with greater effects in the homozygote than the heterozygote demonstrating an important gene dosage effect.

Epac activation, altered calcium homeostasis and ventricular arrhythmogenesis in the murine heart

The recently described exchange protein directly activated by cAMP (Epac) has been implicated in distinct protein kinase A-independent cellular signalling pathways. We investigated the role of Epac activation in adrenergically mediated ventricular arrhythmogenesis. In contrast to observations in control conditions (n = 20), monophasic action potentials recorded in 2 of 10 intrinsically beating and 5 of 20 extrinsically paced Langendorff-perfused wild-type murine hearts perfused with the Epac activator 8-pCPT-2'-O-Me-cAMP (8-CPT, 1 microM) showed spontaneous triggered activity. Three of 20 such extrinsically paced hearts showed spontaneous ventricular tachycardia (VT). Programmed electrical stimulation provoked VT in 10 of 20 similarly treated hearts (P < 0.001; n = 20). However, there were no statistically significant accompanying changes (P > 0.05) in left ventricular epicardial (40.7 +/- 1.2 versus 44.0 +/- 1.7 ms; n = 10) or endocardial action potential durations (APD(90); 51.8 +/- 2.3 versus 51.9 +/- 2.2 ms; n = 10), transmural (DeltaAPD(90)) (11.1 +/- 2.6 versus 7.9 +/- 2.8 ms; n = 10) or apico-basal repolarisation gradients, ventricular effective refractory periods (29.1 +/- 1.7 versus 31.2 +/- 2.4 ms in control and 8-CPT-treated hearts, respectively; n = 10) and APD(90) restitution characteristics. Nevertheless, fluorescence imaging of cytosolic Ca(2+) levels demonstrated abnormal Ca(2+) homeostasis in paced and resting isolated ventricular myocytes. Epac activation using isoproterenol in the presence of H-89 was also arrhythmogenic and similarly altered cellular Ca(2+) homeostasis. Epac-dependent effects were reduced by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibition with 1 microM KN-93. These findings associate VT in an intact cardiac preparation with altered cellular Ca(2+) homeostasis and Epac activation for the first time, in the absence of altered repolarisation gradients previously implicated in reentrant arrhythmias through a mechanism dependent on CaMKII activity.

Caffeine-activated large-conductance plasma membrane cation channels in cardiac myocytes: characteristics and significance

Caffeine-activated, large-conductance, nonselective cation channels (LCCs) have been found in the plasma membrane of isolated cardiac myocytes in several species. However, little is known about the effects of opening these channels. To examine such effects and to further understand the caffeine-activation mechanism, we carried out studies using whole-cell patch-clamp techniques with freshly isolated cardiac myocytes from rats and mice. Unlike previous studies, thapsigargin was used so that both the effect of opening LCCs and the action of caffeine were independent of Ca2+ release from intracellular stores. These Ca2+-permeable LCCs were found in a majority of the cells from atria and ventricles, with a conductance of ∼370 pS in rat atria. Caffeine and all its direct metabolic products (theophylline, theobromine, and paraxanthine) activated the channel, while isocaffeine did not. Although they share some similarities with ryanodine receptors (RyRs, the openings of which give rise to Ca2+ sparks), LCCs also showed some different characteristics. With simultaneous Ca2+ imaging and current recording, the localized fluorescence increase due to Ca2+ entry through a single opening of an LCC (SCCaFT) was detected. When membrane potential, instead of current, was recorded, SCCaFT-like fluorescence transients (indicating single LCC openings) were found to accompany membrane depolarizations. To our knowledge, this is the first report directly linking membrane potential changes to a single opening of an ion channel. Moreover, these events in cardiac cells suggest a possible additional mechanism by which caffeine and theophylline contribute to the generation of cardiac arrhythmias.

Alternative Splicing of Ryanodine Receptors Modulates Cardiomyocyte Ca 2+ Signaling and Susceptibility to Apoptosis

Ca 2+ release via type 2 ryanodine receptors (RyR2) regulates cardiac function. Molecular cloning of human RyR2 identified 2 alternatively spliced variants, comprising 30- and 24-bp sequence insertions; yet their role in shaping cardiomyocyte Ca 2+ signaling and cell phenotype is unknown. We profiled the developmental regulation and the tissue and species specificity of these variants and showed that their recombinant expression in HL-1 cardiomyocytes profoundly modulated nuclear and cytoplasmic Ca 2+ release. All splice variants localized to the sarcoplasmic reticulum, perinuclear Golgi apparatus, and to finger-like invaginations of the nuclear envelope (nucleoplasmic reticulum). Strikingly, the 24-bp splice insertion that was present at low levels in embryonic and adult hearts was essential for targeting RyR2 to an intranuclear Golgi apparatus and promoted the intracellular segregation of this variant. The amplitude variability of nuclear and cytoplasmic Ca 2+ fluxes were reduced in nonstimulated cardiomyocytes expressing both 30- and 24-bp splice variants and were associated with lower basal levels of apoptosis. Expression of RyR2 containing the 24-bp insertion also suppressed intracellular Ca 2+ fluxes following prolonged caffeine exposure (1 mmol/L, 16 hours) that protected cells from apoptosis. The antiapoptotic effects of this variant were linked to increased levels of Bcl-2 phosphorylation. In contrast, RyR2 containing the 30-bp insertion, which was abundant in human embryonic heart but was decreased during cardiac development, did not protect cardiomyocytes from caffeine-evoked apoptosis. Thus, we provide the first evidence that RyR2 splice variants exquisitely modulate intracellular Ca 2+ signaling and are key determinants of cardiomyocyte apoptotic susceptibility.

Effects of flecainide and quinidine on arrhythmogenic properties of Scn5a+/- murine hearts modelling the Brugada syndrome

Brugada syndrome (BrS) is associated with a loss of Na+ channel function and an increased incidence of rapid polymorphic ventricular tachycardia (VT) and sudden cardiac death. A programmed electrical stimulation (PES) technique assessed arrhythmic tendency in Langendorff-perfused wild-type (WT) and genetically modified (Scn5a+/-) 'loss-of-function' murine hearts in the presence and absence of flecainide and quinidine, and the extent to which Scn5a+/- hearts model the human BrS. Extra-stimuli (S2), applied to the right ventricular epicardium, followed trains of pacing stimuli (S1) at progressively reduced S1-S2 intervals. These triggered VT in 16 out of 29 untreated Scn5a+/- and zero out of 31 WT hearts. VT occurred in 11 out of 16 (10 microM) flecainide-treated WT and nine out of the 13 initially non-arrhythmogenic Scn5a+/- hearts treated with (1.0 microM) flecainide. Quinidine (10 microM) prevented VT in six out of six flecainide-treated WT and 13 out of the 16 arrhythmogenic Scn5a+/- hearts in parallel with its clinical effects. Paced electrogram fractionation analysis demonstrated increased electrogram durations, expressed as electrogram duration (EGD) ratios, with shortening S1-S2 intervals in arrhythmogenic Scn5a+/- hearts, and prolonged ventricular effective refractory periods (VERPs) in non-arrhythmogenic Scn5a+/- hearts. Flecainide increased EGD ratios in WT (at 10 microM) and non-arrhythmogenic Scn5a+/- hearts (at 1.0 microM), whereas quinidine (10 microM) reduced EGD ratios and prolonged VERPs in WT and arrhythmogenic Scn5a+/- hearts. However, epicardial and endocardial monophasic action potential recordings consistently demonstrated positive gradients of repolarization in WT, arrhythmogenic and non-arrhythmogenic Scn5a+/- hearts under all pharmacological conditions. Together, these findings demonstrate proarrhythmic effects of flecainide in WT and Scn5a+/- murine hearts that recapitulate its clinical effects. They further attribute the arrhythmogenic phenomena observed here to re-entrant substrates resulting from delayed epicardial activation despite an absence of transmural heterogeneities of repolarization, in sharp contrast to recent characterizations in 'gain-of-function' Scn5a+/Delta murine hearts modelling the long-QT(3) syndrome.

Ryanodine receptor dysfunction and triggered activity in the heart

Arrhythmogenesis has been increasingly linked to cardiac ryanodine receptor (RyR) dysfunction. However, the mechanistic relationship between abnormal RyR function and arrhythmogenesis in the heart is not clear. We hypothesize that, under abnormal RyR conditions, triggered activity will be caused by spontaneous calcium release (SCR) events that depend on transmural heterogeneities of calcium handling. We performed high-resolution optical mapping of intracellular calcium and transmembrane potential in the canine left ventricular wedge preparation (n = 28). Rapid pacing was used to initiate triggered activity under normal and abnormal RyR conditions induced by FKBP12.6 dissociation and beta-adrenergic stimulation (20-150 microM rapamycin, 0.2 microM isoproterenol). Under abnormal RyR conditions, almost all preparations experienced SCRs and triggered activity, in contrast to control, rapamycin, or isoproterenol conditions alone. Furthermore, under abnormal RyR conditions, complex arrhythmias (monomorphic and polymorphic tachycardia) were commonly observed. After washout of rapamycin and isoproterenol, no triggered activity was observed. Surprisingly, triggered activity and SCRs occurred preferentially near the epicardium but not the endocardium (P < 0.01). Interestingly, the occurrence of triggered activity and SCR events could not be explained by cytoplasmic calcium levels, but rather by fast calcium reuptake kinetics. These data suggest that, under abnormal RyR conditions, triggered activity is caused by multiple SCR events that depend on the faster calcium reuptake kinetics near the epicardium. Furthermore, multiple regions of SCR may be a mechanism for multifocal arrhythmias associated with RyR dysfunction.

Nitroxyl improves cellular heart function by directly enhancing cardiac sarcoplasmic reticulum Ca2+ cycling

Heart failure remains a leading cause of morbidity and mortality worldwide. Although depressed pump function is common, development of effective therapies to stimulate contraction has proven difficult. This is thought to be attributable to their frequent reliance on cAMP stimulation to increase activator Ca(2+). A potential alternative is nitroxyl (HNO), the 1-electron reduction product of nitric oxide (NO) that improves contraction and relaxation in normal and failing hearts in vivo. The mechanism for myocyte effects remains unknown. Here, we show that this activity results from a direct interaction of HNO with the sarcoplasmic reticulum Ca(2+) pump and the ryanodine receptor 2, leading to increased Ca(2+) uptake and release from the sarcoplasmic reticulum. HNO increases the open probability of isolated ryanodine-sensitive Ca(2+)-release channels and accelerates Ca(2+) reuptake into isolated sarcoplasmic reticulum by stimulating ATP-dependent Ca(2+) transport. Contraction improves with no net rise in diastolic calcium. These changes are not induced by NO, are fully reversible by addition of reducing agents (redox sensitive), and independent of both cAMP/protein kinase A and cGMP/protein kinase G signaling. Rather, the data support HNO/thiolate interactions that enhance the activity of intracellular Ca(2+) cycling proteins. These findings suggest HNO donors are attractive candidates for the pharmacological treatment of heart failure.

Effects of flecainide and quinidine on arrhythmogenic properties of Scn5a+/Delta murine hearts modelling long QT syndrome 3

Long QT3 (LQT3) syndrome is associated with incomplete Na+ channel inactivation, abnormal repolarization kinetics and prolonged cardiac action potential duration (APD). Electrophysiological effects of flecainide and quinidine were compared in Langendorff-perfused wild-type (WT), and genetically modified (Scn5a+/Delta) murine hearts modelling LQT3. Extra stimuli (S2) following trains of pacing stimuli (S1) applied to the right ventricular epicardium triggered ventricular tachycardia (VT) in 16 out of 28 untreated Scn5a+/Delta and zero out of 12 WT hearts. Paced electrogram fractionation analysis then demonstrated increased electrogram durations (EGD), expressed as EGD ratios, in arrhythmogenic Scn5a+/Delta hearts, and prolonged ventricular effective refractory periods in initially non-arrhythmogenic Scn5a+/Delta hearts. Nevertheless, comparisons of epicardial and endocardial monophasic action potential recordings demonstrated negative transmural repolarization gradients in both groups, giving DeltaAPD(90) values at 90% repolarization of -20.88 +/- 1.93 ms (n = 11) and -16.91 +/- 1.43 ms (n = 23), respectively. Flecainide prevented initiation of VT in 13 out of 16 arrhythmogenic Scn5a+/Delta hearts, reducing EGD ratio and restoring DeltaAPD90 to + 7.55 +/- 2.24 ms (n = 9) (P < 0.05). VT occurred in four out of eight non-arrhythmogenic Scn5a+/Delta hearts in the presence of quinidine, which increased EGD ratio but left DeltaAPD90 unchanged. In contrast (P < 0.05), WT hearts had positive DeltaAPD90 values (+ 11.72 +/- 2.17 ms) (n = 20). Flecainide then increased arrhythmic tendency and EGD ratio but conserved DeltaAPD90; reduced EGD ratios and unaltered DeltaAPD90 values accompanied the lower arrhythmogenicity associated with quinidine treatment. In addition to the changes in EGD ratio shown by WT hearts, these findings attribute arrhythmogenesis and its modification by flecainide and quinidine to alterations in DeltaAPD90 in Scn5a+/Delta hearts. This is consistent with a hypothesis in which incomplete Na+ channel inactivation in Scn5a+/Delta hearts generates functional substrates dependent on altered refractoriness that cause abnormalities in activation and conduction of subsequent cardiac impulses. Any spatial heterogeneities between the epicardial and endocardial layers would thus cause fragmentation of the activation wavefront and contribute to electrogram spreading.

Chronic intake of caffeine during gestation down regulates metabotropic glutamate receptors in maternal and fetal rat heart

Caffeine is the most widely consumed substance in the world which antagonizes adenosine effects. Adenosine acting through A(1) receptors inhibits glutamate release which binds to metabotropic glutamate receptors (mGluRs). Recently, we have shown that maternal caffeine intake during gestation causes down-regulation of A(1) and metabotropic glutamate receptors in the brain of both rat mothers and fetuses. In the present work we provide evidence that caffeine also affects receptors in hearts, causing a decrease in mGluRs from both maternal and fetal hearts. A decrease in G(q/11) and PLC beta(1) proteins level was also observed in both tissues. However, phospholipase C activity was only affected in fetal heart, being significantly decreased. These results suggest an in vivo cross-talk mechanism between adenosine and glutamate receptors in peripheral tissues. Therefore, special attention should be paid to caffeine ingestion during gestation.