Intracellular calcium transients and arrhythmia in isolated heart cells

Avian cardiomyocyte architecture and what it reveals about the evolution of the vertebrate heart

Bird cardiomyocytes are long, thin and lack transverse (t)-tubules, which is akin to the cardiomyocyte morphology of ectothermic non-avian reptiles, who are typified by low maximum heart rates and low pressure development. However, birds can achieve greater contractile rates and developed pressures than mammals, whose wide cardiomyocytes contain a dense t-tubular network allowing for uniform excitation-contraction coupling and strong contractile force. To address this apparent paradox, this paper functionally links recent electrophysiological studies on bird cardiomyocytes with decades of ultrastructure measurements. It shows that it is the strong transsarcolemmal Ca2+ influx via the L-type Ca2+ current (ICaL) and the high gain of Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR), coupled with an internal SR Ca2+ release relay system, that facilitates the strong fast contractions in the long thin bird cardiomyocytes, without the need for t-tubules. The maintenance of an elongated myocyte morphology following the post-hatch transition from ectothermy to endothermy in birds is discussed in relation to cardiac load, myocyte ploidy, and cardiac regeneration potential in adult cardiomyocytes. Overall, the paper shows how little we know about cellular Ca2+ dynamics in the bird heart and suggests how increased research efforts in this area would provide vital information in our quest to understand the role of myocyte architecture in the evolution of the vertebrate heart. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'. Please see glossary at the end of the paper for definitions of specialized terms.

Deriving waveform parameters from calcium transients in human iPSC-derived cardiomyocytes to predict cardiac activity with machine learning

Human induced pluripotent stem cell-derived cardiomyocytes have been established to detect dynamic calcium transients by fast kinetic fluorescence assays that provide insights into specific aspects of clinical cardiac activity. However, the precise derivation and use of waveform parameters to predict cardiac activity merit deeper investigation. In this study, we derived, evaluated, and applied 38 waveform parameters in a novel Python framework, including (among others) peak frequency, peak amplitude, peak widths, and a novel parameter, shoulder-tail ratio. We then trained a random forest model to predict cardiac activity based on the 25 parameters selected by correlation analysis. The area under the curve (AUC) obtained for leave-one-compound-out cross-validation was 0.86, thereby replicating the predictions of conventional methods and outperforming fingerprint-based methods by a large margin. This work demonstrates that machine learning is able to automate the assessment of cardiovascular liability from waveform data, reducing any risk of user-to-user variability and bias.

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An open-source algorithm was developed to derive parameters from waveform data

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A machine learning model was trained to predict cardiac activity of compounds

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Three parameters for peak width, height, and shape were found to be most predictive

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The model can facilitate the assessment of cardiovascular liability

Full-length dystrophin deficiency leads to contractile and calcium transient defects in human engineered heart tissues

Cardiomyopathy is currently the leading cause of death for patients with Duchenne muscular dystrophy (DMD), a severe neuromuscular disorder affecting young boys. Animal models have provided insight into the mechanisms by which dystrophin protein deficiency causes cardiomyopathy, but there remains a need to develop human models of DMD to validate pathogenic mechanisms and identify therapeutic targets. Here, we have developed human engineered heart tissues (EHTs) from CRISPR-edited, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) expressing a truncated dystrophin protein lacking part of the actin-binding domain. The 3D EHT platform enables direct measurement of contractile force, simultaneous monitoring of Ca2+ transients, and assessment of myofibril structure. Dystrophin-mutant EHTs produced less contractile force as well as delayed kinetics of force generation and relaxation, as compared to isogenic controls. Contractile dysfunction was accompanied by reduced sarcomere length, increased resting cytosolic Ca2+ levels, delayed Ca2+ release and reuptake, and increased beat rate irregularity. Transcriptomic analysis revealed clear differences between dystrophin-deficient and control EHTs, including downregulation of genes related to Ca2+ homeostasis and extracellular matrix organization, and upregulation of genes related to regulation of membrane potential, cardiac muscle development, and heart contraction. These findings indicate that the EHT platform provides the cues necessary to expose the clinically-relevant, functional phenotype of force production as well as mechanistic insights into the role of Ca2+ handling and transcriptomic dysregulation in dystrophic cardiac function, ultimately providing a powerful platform for further studies in disease modeling and drug discovery.

Estrogen deficiency compromised the β2AR-Gs/Gi coupling: implications for arrhythmia and cardiac injury

Estrogen and β₂-adrenergic receptors (β₂AR) play important roles in the processes that protect the heart. Here, we investigated how ovariectomy influenced the β₂AR downstream pathways in the context of catecholaminergic stress. In vivo and in vitro stress models were developed in female Sprague-Dawley (SD) rats by epinephrine (Epi) treatments. The cardiac function was evaluated at in vivo and in vitro levels in terms of contraction, rhythm, and injury. We found that myocardial contractility was not significantly different between Sham and ovariectomized (OVX) group rats in the normal state. However, Epi pretreatment decreased the contractility and increased abnormal rhythms especially in OVX group, which were attributed to lack of estrogen. Inhibition of the β₂AR-Gi-PI3K/p38MAPK pathway with ICI118,551, PTX or LY294002 increased contractility and aggravated Epi-induced injury on cardiomyocytes, decreased p38MAPK phosphorylation, and only increased arrhythmia in Sham group. These results indicated that OVX exacerbated cardiac injury and abnormal rhythms through β₂AR-Gi-PI3K and β₂AR-Gi-p38MAPK pathways, respectively. In normal state, the levels of activated Gi were similar in both groups, but those of cAMP and activated Gs were higher in OVX group. Epi treatment increased activated Gi (especially in Sham group) and activated Gs and cAMP in Sham group but decreased it in OVX group. These results suggested that estrogen increased the Gi activity in normal and stress states and Gs activity in stress state. These results indicated that lack of estrogen impaired the β₂AR-Gs/Gi coupling during stress which compromised cardiac contractility and increased abnormal rhythms.

Calcium dynamics in cardiac excitatory and non-excitatory cells and the role of gap junction

Calcium ions aid in the generation of action potential in myocytes and are responsible for the excitation-contraction coupling of heart. The heart muscle has specialized patches of cells, called excitatory cells (EC) such as the Sino-atrial node cells capable of auto-generation of action potential and cells which receive signals from the excitatory cells, called non-excitatory cells (NEC) such as cells of the ventricular and auricular walls. In order to understand cardiac calcium homeostasis, it is, therefore, important to study the calcium dynamics taking into account both types of cardiac cells. Here we have developed a model to capture the calcium dynamics in excitatory and non-excitatory cells taking into consideration the gap junction mediated calcium ion transfer from excitatory cell to non-excitatory cell. Our study revealed that the gap junctional coupling between excitatory and non-excitatory cells plays important role in the calcium dynamics. It is observed that any reduction in the functioning of gap junction may result in abnormal calcium oscillations in NEC, even when the calcium dynamics is normal in EC cell. Sensitivity of gap junction is observed to be independent of the pacing rate and hence a careful monitoring is required to maintain normal cardiomyocyte condition. It also highlights that sarcoplasmic reticulum may not be always able to control the amount of cytoplasmic calcium under the condition of calcium overload.

Restoring calcium homeostasis in diabetic cardiomyocytes: an investigation through mathematical modelling

Regulated calcium flux from sarcoplasmic reticulum could be a possible therapeutic strategy in diabetic cardiomyocyte problem.

Bioactive nanoparticles improve calcium handling in failing cardiac myocytes

AIMS

To evaluate the ability of N-acetylglucosamine (GlcNAc) decorated nanoparticles and their cargo to modulate calcium handling in failing cardiac myocytes (CMs).

MATERIALS & METHODS

Primary CMs isolated from normal and failing hearts were treated with GlcNAc nanoparticles in order to assess the ability of the nanoparticles and their cargo to correct dysfunctional calcium handling in failing myocytes.

RESULTS & CONCLUSION

GlcNAc particles reduced aberrant calcium release in failing CMs and restored sarcomere function. Additionally, encapsulation of a small calcium-modulating protein, S100A1, in GlcNAc nanoparticles also showed improved calcium regulation. Thus, the development of our bioactive nanoparticle allows for a 'two-hit' treatment, by which the cargo and also the nanoparticle itself can modulate intracellular protein activity.

Circulating Pneumolysin Is a Potent Inducer of Cardiac Injury during Pneumococcal Infection

Streptococcus pneumoniae accounts for more deaths worldwide than any other single pathogen through diverse disease manifestations including pneumonia, sepsis and meningitis. Life-threatening acute cardiac complications are more common in pneumococcal infection compared to other bacterial infections. Distinctively, these arise despite effective antibiotic therapy. Here, we describe a novel mechanism of myocardial injury, which is triggered and sustained by circulating pneumolysin (PLY). Using a mouse model of invasive pneumococcal disease (IPD), we demonstrate that wild type PLY-expressing pneumococci but not PLY-deficient mutants induced elevation of circulating cardiac troponins (cTns), well-recognized biomarkers of cardiac injury. Furthermore, elevated cTn levels linearly correlated with pneumococcal blood counts (r=0.688, p=0.001) and levels were significantly higher in non-surviving than in surviving mice. These cTn levels were significantly reduced by administration of PLY-sequestering liposomes. Intravenous injection of purified PLY, but not a non-pore forming mutant (PdB), induced substantial increase in cardiac troponins to suggest that the pore-forming activity of circulating PLY is essential for myocardial injury in vivo. Purified PLY and PLY-expressing pneumococci also caused myocardial inflammatory changes but apoptosis was not detected. Exposure of cultured cardiomyocytes to PLY-expressing pneumococci caused dose-dependent cardiomyocyte contractile dysfunction and death, which was exacerbated by further PLY release following antibiotic treatment. We found that high PLY doses induced extensive cardiomyocyte lysis, but more interestingly, sub-lytic PLY concentrations triggered profound calcium influx and overload with subsequent membrane depolarization and progressive reduction in intracellular calcium transient amplitude, a key determinant of contractile force. This was coupled to activation of signalling pathways commonly associated with cardiac dysfunction in clinical and experimental sepsis and ultimately resulted in depressed cardiomyocyte contractile performance along with rhythm disturbance. Our study proposes a detailed molecular mechanism of pneumococcal toxin-induced cardiac injury and highlights the major translational potential of targeting circulating PLY to protect against cardiac complications during pneumococcal infections.

Anti-arrhythmic effect of acupuncture pretreatment in the rats subjected to simulative global ischemia and reperfusion--involvement of intracellular Ca2+ and connexin 43

BACKGROUND

The previous study showed that the cardiac arrhythmias induced by myocardial ischemia and reperfusion were attenuated by the pretreatment of acupuncture; however, the related mechanism is not understood. The present study was therefore designed to determine whether intracellular Ca(2+) ([Ca(2+)]i) and connexin 43 (Cx43) are involved in the mediation of the anti-arrhythmic effect of electro-acupuncture (EA) pretreatment in the rats subjected to simulative global ischemia and reperfusion (SGIR).

METHODS

SGIR was made in the isolated heart by a low flow perfusion followed by a flow restoration. Four groups of animals are involved in the present study, including normal control group, SGIR group, EA group and EA plus 18 beta-glycyrrhetinic acid (EAG) group. For EA pretreatment, bilateral Neiguan acupoints (PC6) of the rats were stimulated for 30 min once a day in 3 consecutive days. Cx43 antagonist was given to the rats in EAG group 30 minutes before the EA pretreatment. The resting [Ca(2+)]i concentration, calcium oscillation, the contents of total Cx43 and non-phosphrylated Cx43 and arrhythmia score were compared among different groups.

RESULTS

In EA group, the arrhythmic score, the resting [Ca(2+)]i concentration and the number of [Ca(2+)]i oscillations were all significantly less than those in SGIR group (all P < 0.05), and interestingly, after EA pretreatment, the contents of nonphosphated Cx43 in the EA group were significantly lower than that in SGIR group respectively (P < 0.05). However, when the rats were treated with Cx43 antagonist prior to the EA pretreatment, the protection effects induced by EA pretreatment were reversed.

CONCLUSIONS

The results showed that EA pretreatment could produce anti-arrhythmic effect in the rats subjected to SGIR. The anti-arrhythmic effect of EA pretreatment may be due at least partially to the inhibition of SGIR-induced calcium overload and [Ca(2+)]i oscillations, reduction of non-phosphorylated Cx43 and the enhancement of the corresponding phosphorylated Cx43 in the cardiac cells.

Sudden cardiac arrest in takotsubo cardiomyopathy - a case study

We present a 59-year-old woman who was admitted to hospital after sudden cardiac arrest due to ventricular fibrillation. Finally takotsubo syndrome was diagnosed. In the acute phase of takotsubo syndrome life-threatening ventricular arrhythmias and significant hemodynamic disorders may occur due to strong adrenergic stimulation and myocardial ischemia. It has been proved that the occurrence of torsade de pointes tachycardia in the acute phase of takotsubo cardiomyopathy is associated with QT prolongation. There are no clear guidelines on pharmacological treatment and implantable cardioverter defibrillator implantation after a past takotsubo episode. Takotsubo cardiomyopathy has not been entirely explained as an etiological disease.

The role of dye affinity in optical measurements of Cai(2+) transients in cardiac muscle

Previous experiments in cultures of neonatal rat myocytes demonstrated that the shape of Cai(2+) transients measured using high-affinity Ca(2+)-sensitive dyes may be misrepresented. The purpose of this study was to examine the role of dye affinity in Cai(2+) measurements in intact adult cardiac tissue by comparing optical recordings obtained with high- and low-affinity dyes. Experiments were carried out in porcine left ventricular (LV) wedge preparations stained locally by intramural injection via microcapillaries (diameter = 150 μm) with a low-affinity Ca(2+)-sensitive dye Fluo-4FF or Fluo-2LA (nominal Kd, ~7-10 μmol/l), high-affinity dye Rhod-2 (Kd = 0.57 μmol/l), and Fluo-4 or Fluo-2MA (Kd, ~0.4 μmol/l); in addition, tissue was stained with transmembrane potential (Vm)-sensitive dye RH-237. Optical recordings of Vm and Cai(2+) were made using optical fibers (diameter = 325 μm) glued with the microcapillaries. The durations of Cai(2+) transients measured at 50% level of recovery (CaD50) using high-affinity Fluo-4/Fluo-2MA dyes were up to ~81% longer than those measured with low-affinity Fluo-4FF/Fluo-2LA at long pacing cycle lengths (CL). In Fluo-4/Fluo-2MA measurements at long CLs, Cai(2+) transients often (~50% of cases) exhibited slow upstroke rise and extended plateau. In Rhod-2 measurements, CaD50 was moderately longer (up to ~35%) than in Fluo-4FF recordings, but Cai(2+) transient shapes were similar. In all series of measurements, mean action potential duration values were not significantly different (P > 0.05). The delays between Vm and Cai(2+) upstrokes were comparable for low- and high-affinity dyes (P > 0.05). In conclusion, measurements of Cai(2+) transient in ventricular myocardium are strongly affected by the affinity of Ca(2+) dyes. The high-affinity dyes may overestimate the duration and alter the shape of Cai(2+) transients.

Persistent sodium current and Na+/H+ exchange contributes to the augmentation of the reverse Na+/Ca2+ exchange during hypoxia or acute ischemia in ventricular myocytes

The increases in persistent sodium currents (I (Na.P)) and Na(+)/H(+) exchange (NHE) causes intracellular Ca(2+) overload. The objective of this study was to determine the contribution of I (Na.P) and NHE on the hypoxia- or acute ischemia-induced increase in the reverse Na(+)/Ca(2+) exchange current (HIR- or AIR-I (NCX)). I (Na.P) and I (NCX) in rabbit ventricular myocytes were recorded during hypoxia or acute ischemia, combination of acidosis (pH values were 6.0 intracellularly and 6.8 extracellularly) and hypoxia, using whole-cell patch-clamp techniques. The results indicate that (1) under hypoxic condition, the augmentation of both HIR-I (NCX) and I (Na.P) was inhibited by TTX (2 to 8 μM) in a concentration-dependent manner. The inhibitions of I (Na,P) and HIR-I (NCX) reached maximum in the presence of either 4 μM TTX or 10 μM KR-32568 (a NHE inhibitor), respectively. The maximal inhibitions of HIR-I (NCX) by 4 μM TTX and 10 μM KR-32568 were 72.54% and 16.89%, respectively. (2) Administration of 2 μM TTX and 10 μM KR-32568 in either order in the same cells decreased HIR-I (NCX) by 64.83% and 16.94%, respectively. (3) I (Na.P) and the reverse I (NCX) were augmented during acute ischemia. TTX (4 μM) and KR-32568 (10 μM) reduced AIR-I (NCX) by 73.39% and 24.13%, respectively. (4) Under normoxic condition, veratridine (20 μM) significantly increased I (Na.P) and the reverse I (NCX), which was reversed by 4 μM TTX. In conclusion, during hypoxia or acute ischemia, both increased I (Na.P) and NHE contribute to the HIR- or AIR-I (NCX) with the former playing a major role comparing with the latter.

Diacylglycerol kinase ζ inhibits ventricular tachyarrhythmias in a mouse model of heart failure

BACKGROUND

Diacylglycerol kinase ζ (DGKζ) inhibited atrial tachyarrhythmias in a mouse model of heart failure (HF) in our study. However, whether DGKζ prevents the HF-induced ventricular tachyarrhythmia (VT) is unknown.

METHODS AND RESULTS

Effects of DGKζ on VT using transgenic mice with transient cardiac expression of activated G protein α(q) (Gα(q)-TG; model of HF) were elucidated and double transgenic mice with cardiac-specific overexpression of both DGKζ and the activated Gα(q) (Gα(q)/DGKζ-TG) were used. Premature ventricular contraction (PVC) and/or VT were frequently observed in Gα(q)-TG mice but not in Gα(q)/DGKζ-TG and wild-type (WT) mice (P<0.01). Protein expressions of canonical transient receptor potential (TRPC) channels 3 and 6 increased in Gα(q)-TG hearts compared with WT and Gα(q)/DGKζ-TG hearts. SK&F96365, a TRPC channel blocker, decreased the number of PVC and prevented VT in anesthetized Gα(q)-TG mice (P<0.05). 1-oleoyl-2-acyl-sn-glycerol (OAG), a diacylglycerol analogue, increased the number of PVC in isolated Gα(q)-TG hearts compared with WT hearts and induced VT in Gα(q)-TG hearts (P<0.01). SK&F96365 decreased the number of PVC and prevented VT in isolated Gα(q)-TG hearts (P<0.01) even in the presence of OAG. Early afterdepolarization (EAD)-induced triggered activity was frequently observed in single Gα(q)-TG ventricular myocytes. Moreover, SK&F96365 prevented the EAD.

CONCLUSIONS

These results demonstrated that DGKζ inhibited VT in a mouse model of HF and suggest that TRPC channels participate in VT induction in failing hearts.

Contribution of mechanical factors to arrhythmogenesis in calcium overloaded cardiomyocytes: model predictions and experiments

It is well-known that Ca²⁺ overload in cardiomyocytes may underlie arrhythmias. However, the possible contribution of mechanical factors to rhythm disturbances in Ca²⁺ overloaded myocytes has not been sufficiently investigated. We used a mathematical model of the electrical and mechanical activity of cardiomyocytes to reveal an essential role of the mechanisms of cardiac mechano-electric feedback in arrhythmogenesis in Ca²⁺ overloaded myocardium. In the model, the following mechanical factors increased Ca²⁺ overload in contracting cardiomyocytes and promoted rhythm disturbances: i) a decrease in the mechanical load for afterloaded contractions; and ii) a decrease in the initial length of sarcomeres for isometric twitches. In exact accordance with the model predictions, in experiments on papillary muscles from the right ventricle of guinea pigs with Ca²⁺ overloaded cardiomyocytes (using 0.5-1 μM of ouabain), we found that emergence of rhythm disturbances and extrasystoles depends on the mechanical conditions of muscle contraction.

K+ current changes account for the rate dependence of the action potential in the human atrial myocyte

Ongoing investigation of the electrophysiology and pathophysiology of the human atria requires an accurate representation of the membrane dynamics of the human atrial myocyte. However, existing models of the human atrial myocyte action potential do not accurately reproduce experimental observations with respect to the kinetics of key repolarizing currents or rate dependence of the action potential and fail to properly enforce charge conservation, an essential characteristic in any model of the cardiac membrane. In addition, recent advances in experimental methods have resulted in new data regarding the kinetics of repolarizing currents in the human atria. The goal of this study was to develop a new model of the human atrial action potential, based on the Nygren et al. model of the human atrial myocyte and newly available experimental data, that ensures an accurate representation of repolarization processes and reproduction of action potential rate dependence and enforces charge conservation. Specifically, the transient outward K(+) current (I(t)) and ultrarapid rectifier K(+) current (I(Kur)) were newly formulated. The inwardly recitifying K(+) current (I(K1)) was also reanalyzed and implemented appropriately. Simulations of the human atrial myocyte action potential with this new model demonstrated that early repolarization is dependent on the relative conductances of I(t) and I(Kur), whereas densities of both I(Kur) and I(K1) underlie later repolarization. In addition, this model reproduces experimental measurements of rate dependence of I(t), I(Kur), and action potential duration. This new model constitutes an improved representation of excitability and repolarization reserve in the human atrial myocyte and, therefore, provides a useful computational tool for future studies involving the human atrium in both health and disease.

Milrinone inhibits hypoxia or hydrogen dioxide-induced persistent sodium current in ventricular myocytes

Much evidence indicates that increased persistent sodium current (I(Na.P)) is associated with cellular calcium overload and I(Na.P) is considered to be a potential target for therapeutic intervention in ischaemia and heart failure. By inhibiting type III phosphodiesterase, milrinone increases intracellular cyclic adenosine monophosphate (cAMP), with a positive inotropic effect. However, the effect of milrinone on increased I(Na.P) under pathological conditions remains unknown. Accordingly, we investigated the effect of milrinone on increased I(Na.P) induced by hypoxia or hydrogen dioxide in guinea pig ventricular myocytes. While milrinone (0.01 mM or 0.1mM) or cAMP (0.1 mM) decreased I(Na.P) respectively in control condition, application of 1 microM H-89, a selective cAMP-dependant protein kinase inhibitor, prevented the effect of 0.1mM milrinone in control condition. Milrinone (0.1 mM) reduced the increased I(Na.P) induced by hypoxia. Furthermore, 0.01 mM or 0.1mM milrinone reduced the enhanced I(Na.P) induced by 0.3 mM hydrogen peroxide. In addition, 0.01 mM or 0.1 mM milrinone shortened action potential duration at 90% repolarization (APD(90)). Bath application of 0.3 mM hydrogen dioxide markedly prolonged APD(90), while 2 microM tetrodotoxin (TTX) reversed the prolonged APD(90). In the other two groups, 0.01 mM or 0.1 mM milrinone shortened the prolonged APD(90) induced by 0.3 mM hydrogen peroxide, ultimately 2 microM TTX causing a further decurtation of APD(90). These findings demonstrate that milrinone inhibited I(Na.P) under normal condition, hypoxia or hydrogen dioxide-induced I(Na.P), and the APD(90) prolonged by hydrogen dioxide-induced I(Na.P) in ventricular myocytes, which is associated with the mechanism of milrinone increasing intracellular cAMP.

Antiarrhythmic effect of acupuncture pretreatment in rats subjected to simulative global ischemia and reperfusion--involvement of adenylate cyclase, protein kinase A, and L-type Ca2+ channel

Our previous study showed that electro-acupuncture (EA) pretreatment protects the heart from injury of ischemia. The present study explored further whether adenylate cyclase (AC), protein kinase A (PKA), and L-type Ca(2+) channel, the beta(1)-AR signaling components modulating intracellular Ca(2+) ([Ca(2+)](i)), are involved in the mediation of the antiarrhythmic effect of EA pretreatment in the rats from which the hearts were subsequently isolated and subjected to simulative global ischemia and reperfusion (SGIR). SGIR was performed by perfusing the isolated heart at a low flow followed by normal perfusion. Adult rats were randomized into four groups, namely, normal control (NC), SGIR, EA, and NC plus EA (NCEA) groups. The rats in the EA and NCEA groups were given EA pretreatment at bilateral Neiguan points (PC6) for 30 min once a day in 3 consecutive days before the hearts were isolated and perfused. The arrhythmia score and the response of [Ca(2+)](i) to the activators of AC, PKA, and L-type Ca(2+) channel in single ventricular myocyte isolated from the hearts subjected to SGIR were compared among the groups. The results showed that the arrhythmia score was significantly higher in the SGIR group as compared with the NC and NCEA groups. The SGIR-enhanced arrhythmia score was significantly attenuated in the EA group. More interesting, EA pretreatment also attenuated the SGIR-enhanced response of [Ca(2+)](i) to the activators of AC, PKA, and the L-type Ca(2+) channel in the myocytes isolated from the hearts subjected to SGIR. In conclusion, EA pretreatment can produce an antiarrhythmic effect in the rat of SGIR. AC, PKA and the L-type Ca(2+) channel are involved in the mediation of the antiarrhythmic effect of EA pretreatment.

Mathematical modeling of mechanically modulated rhythm disturbances in homogeneous and heterogeneous myocardium with attenuated activity of na+ -k+ pump

A mathematical model of the cardiomyocyte electromechanical function is used to study contribution of mechanical factors to rhythm disturbances in the case of the cardiomyocyte calcium overload. Particular attention is paid to the overload caused by diminished activity of the sodium-potassium pump. It is shown in the framework of the model, where mechano-calcium feedback is accounted for that myocardium mechanics may significantly enhance arrhythmogenicity of the calcium overload. Specifically, a role of cross-bridge attachment/detachment processes, a role of mechanical conditions of myocardium contractions (length, load), and a role of myocardium viscosity in the case of simulated calcium overload have been revealed. Underlying mechanisms are analyzed. Several approaches are designed in the model and compared to each other for recovery of the valid myocardium electrical and mechanical performance in the case of the partially suppressed sodium-potassium pump.

Involvement of transient receptor potential proteins in cardiac hypertrophy

Cardiac hypertrophy is an adaptive process that occurs in response to increased physical stress on the heart. Hypertrophy, which may be induced by hypertension among other factors, is characterized by an increase in left ventricular mass and an associated increase in force production capacity. However, as sustained cardiac hypertrophy may lead to heart failure and sudden death, an understanding of the molecular processes involved in both the onset and consequences of hypertrophy is of significant importance. Calcium is a key player in the process underlying the development of cardiac hypertrophy. Recently, several Transient Receptor Potential proteins (TRPs), including calcium-permeable and calcium-regulated ion channels, have been shown to be related to various aspects of cardiac hypertrophy. TRPs are implicated in the development of cardiac hypertrophy (TRPC1, TRPC3, TRPC6), the electrophysiological perturbations associated with hypertrophy (TRPM4) and the progression to heart failure (TRPC7). This review describes the major characteristics of cardiac hypertrophy and focuses on the roles of TRPs in the physiological processes underlying hypertrophy.

Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies

It has been six years since the first reported link between mutations in the cardiac ryanodine receptor Ca(2+) release channel (RyR2) and catecholaminergic polymorphic ventricular tachycardia (CPVT), a malignant stress-induced arrhythmia. In this time, rapid advances have been made in identifying new mutations, and in understanding how these mutations disrupt normal channel function to cause VT that frequently degenerates into ventricular fibrillation (VF) and sudden death. Functional characterisation of these RyR2 Ca(2+) channelopathies suggests that mutations alter the ability of RyR2 to sense its intracellular environment, and that channel modulation via covalent modification, Ca(2+)- and Mg(2+)-dependent regulation and structural feedback mechanisms are catastrophically disturbed. This review reconciles the current status of RyR2 mutation-linked etiopathology, the significance of mutational clustering within the RyR2 polypeptide and the mechanisms underlying channel dysfunction. We will also review new data that explores the link between abnormal Ca(2+) release and the resultant cardiac electrical instability in VT and VF, and how these recent developments impact on novel anti-arrhythmic therapies. Finally, we evaluate the concept that mechanistic differences between CPVT and other arrhythmogenic disorders may preclude a common therapeutic strategy to normalise RyR2 function in cardiac disease.

Dietary fish oil alters cardiomyocyte Ca2+ dynamics and antioxidant status

The n-3 polyunsaturated fatty acids (PUFAs) found in fish oil (FO) have been shown to protect against reperfusion arrhythmias, a manifestation of reperfusion injury, which is believed to be induced by the formation of reactive oxygen species (ROS) and intracellular calcium (Ca2+) overload. Adult rats fed a diet supplemented with 10% FO had a higher proportion of myocardial n-3 PUFAs and increased expression of antioxidant enzymes compared with the saturated fat (SF)-supplemented group. Addition of hydrogen peroxide (H2O2) to cardiomyocytes isolated from rats in the SF-supplemented group increased the proportions of cardiomyocytes contracting in an asynchronous manner, increased the rate of Ca2+ influx, and increased the diastolic and systolic [Ca2+]i compared with the FO group. H2O2 exposure increased the membrane fluidity of cardiomyocytes from the FO group. These results demonstrate that dietary FO supplementation is associated with a reduction in the susceptibility of myocytes to ROS-induced injury and this may be related to membrane incorporation of n-3 PUFAs, increased antioxidant defenses, changes in cardiomyocyte membrane fluidity, and the ability to prevent rises in cellular Ca2+ in response to ROS.

Pulmonary edema post-cardioversion: a potential calcium signalling problem

The present report describes an unusual case of pulmonary edema after adenosine cardioversion of a supraventricular tachycardia. Despite a structurally normal heart, a 52-year-old woman presented with pulmonary edema on two separate occasions, having had her atrioventricular nodal re-entrant tachycardia terminated with 12 mg of intravenous adenosine. A third similar episode of tachycardia that was terminated with verapamil was not complicated by pulmonary edema.

The kinetics of spontaneous calcium oscillations and arrhythmogenesis in the in vivo heart during ischemia/reperfusion

BACKGROUND

The correlation between spontaneous calcium oscillations (S-CaOs) and arrhythmogenesis has been investigated in a number of theoretical and experimental in vitro models. There is an obvious lack of studies that directly investigate how the kinetics of S-CaOs correlates with a specific arrhythmia in the in vivo heart.

OBJECTIVES

The purpose of the study is to investigate the correlation between the kinetics of S-CaOs and arrhythmogenesis in the intact heart using an experimental model of ischemia/reperfusion (I/R).

METHODS

Perfused Langendorff guinea pig (GP) hearts were subjected to global I/R (10-15 minutes/10-15 minutes). The heart was stained with a voltage-sensitive dye (RH237) and loaded with a Ca2+ indicator (Rhod-2 AM). Membrane voltage (Vm) and intracellular calcium transient (Ca(i)T) were simultaneously recorded with an optical mapping system of two 16 x 16 photodiode arrays. S-CaOs were considered to arise from a localized focal site within the mapped surface when these preceded the associated membrane depolarizations by 2-15 ms.

RESULTS

In 135 episodes of ventricular arrhythmias from 28 different GP experiments, 23 were linked to S-CaOs that were considered to arise from or close to the mapped epicardial window. Self-limited or sustained S-CaOs had a cycle length of 130-430 ms and could trigger propagated ventricular depolarizations. Self-limited S-CaOs that followed the basic beat action potential (AP)/Ca(i)T closely resembled phase 3 early afterdepolarizations. Fast S-CaOs could remain confined to a localized site (concealed) or exhibit varying conduction patterns. This could manifest as (1) an isolated premature beat (PB), bigeminal, or trigeminal rhythm; (2) ventricular tachycardia (VT) when a regular 2:1 conduction from the focal site develops; or (3) ventricular fibrillation (VF) when a complex conduction pattern results in wave break and reentrant excitation.

CONCLUSIONS

The study examined, for the first time in the intact heart, the correlation between the kinetics of focal S-CaOs during I/R and arrhythmogenesis. S-CaOs may remain concealed or manifest as PBs, VT, or VF. A "benign looking" PB during I/R may represent "the tip of the iceberg" of an underlying potentially serious arrhythmic mechanism.

Synchronous progression of calcium transient-dependent beating and sarcomere destruction in apoptotic adult cardiomyocytes

During early apoptosis, adult cardiomyocytes show unusual beating, suggesting possible participation of abnormal Ca(2+) transients in initiation of apoptotic processes in this cell type. Simultaneously with the beating, these cells show dynamic structural alteration resulting from cytoskeletal disintegration that is quite rapid. Because of the specialized structure and extensive cytoskeleton of cardiomyocytes, we hypothesized that its degradation in so short a time would require a particularly efficient mechanism. To better understand this mechanism, we used serial video microscopy to observe beta-adrenergic stimulation-induced apoptosis in isolated adult rat cardiomyocytes while simultaneously recording intracellular Ca(2+) concentration and cell length. Trains of Ca(2+) transients and corresponding rhythmic contractions and relaxations (beating) were observed in apoptotic cells. Frequencies of Ca(2+) transients and beating gradually increased with time and were accompanied by cellular shrinkage. As the cells shrank, amplitudes of Ca(2+) transients declined and diastolic intracellular Ca(2+) concentration increased until the transients were lost. Beating and progression of apoptosis were significantly inhibited by antagonists against the L-type Ca(2+) channel (nifedipine), ryanodine receptor (ryanodine), inositol 1,4,5-trisphosphate receptor (heparin), sarco(endo)plasmic Ca(2+)-ATPase (thapsigargin), and Na(+)/Ca(2+) exchanger (KB-R7943). Electron-microscopic examination of beating cardiomyocytes revealed progressive breakdown of Z disks. Immunohistochemical analysis and Western blot confirmed that disappearance of Z disk constituent proteins (alpha-actinin, desmin, and tropomyosin) preceded degradation of other cytoskeletal proteins. It thus appears that, in adult cardiomyocyte apoptosis, Ca(2+) transients mediate apoptotic beating and efficient sarcomere destruction initiated by Z disk breakdown.

Activation of the Ca(2+)-activated nonselective cation channel by diacylglycerol analogues in rat cardiomyocytes

INTRODUCTION

Cardiac hypertrophy is associated with changes in electrophysiologic properties due to ionic channel modifications and increases in protein kinase C (PKC) activity and diacylglycerol (DAG) content. These changes may contribute to an increased propensity for arrhythmia. Similar electrophysiologic modifications have been reported in adult rat cardiomyocytes undergoing dedifferentiation in primary culture.

METHODS AND RESULTS

Single-channel measurements on such cells identified the appearance of a Ca(2+)-activated nonselective cation channel (NSC(Ca)) during the dedifferentiation process. The current study investigated the sensitivity of this channel to PKC and DAG analogues. In the cell-attached configuration, channel conductance was 20.2 pS under physiologic conditions. Perfusion with the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG, 0.1 mM) or the PKC activator phorbol 12-myristate 13-acetate (PMA, 0.5 microM) increased the channel normalized open probability (nPo), whereas in the presence of the PKC inhibitor calphostin C (1 microM), only OAG retained this effect. In the inside-out configuration, perfusion of both DAG analogues OAG (0.1 mM) and 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG, 10 microM) on the inside of the membrane increased nPo. These results indicate that DAG regulates the NSC(Ca) channel via both the PKC pathway and by a direct interaction.

CONCLUSION

DAG content, PKC activity, and channel expression increased during hypertrophy. This indicates that the NSC(Ca) channel exhibits high activity in this condition and, therefore, is a candidate for the genesis of arrhythmias in ventricular cardiomyocytes. In addition, regulation of the channel by DAG and PKC contributes to current understanding of the physiologic role of this channel, which shares properties with the cloned TRPM4b channel.

Postfibrillatory Enhancement of Left Atrial Contractility After Short Paroxysms of Atrial Fibrillation

Implantable cardioverter defibrillators and pacemakers detect an increasing number of silent episodes of AF. In a porcine model, the study evaluated the contractility of the left atrial appendage (LAA) during AF paroxysms as they may occur in patients. Peak outflow velocity of the LA and mean outflow velocity of the LAA (LAA-V(outmean)) (n = 17) were measured before, during, and after induction of self-terminating AF. LAA-V(outmean) was also measured during incremental pacing from different atrial sites using epicardial Doppler probes (n = 6) and during continuous recordings (n = 5) of 40 minutes of pacing maintained AF. Compared to baseline sinus rhythm, LAA-V(outmean) increased during short AF episodes (41 +/- 3 vs 35 +/- 2 cm/s, P < 0.05). After termination of the AF episodes, LAA-V(outmean) further increased (69 +/- 15 cm/s, P < 0.001 vs baseline). This "postfibrillatory enhancement" maintained after repeated induction of short AF paroxysms. During prolonged AF episodes lasting 40 minutes, an initial hypercontractility (44 +/- 2 vs 38 +/- cm/s, P < 0.01) was followed by a hypocontractility after 20 minutes (29 +/- 12 P < 0.05 vs SR) and a postfibrillatory enhancement after cessation of AF (56 +/- 12 vs 27 +/- 9 cm/s at 40 minutes AF, P < 0.001). L-type Ca channel blockade abolished the initial hypercontractility during AF and the postfibrillatory enhancement. Repetitive AF paroxysms up to 2 minutes did not decrease left atrial contractility. During maintained AF up to 40 minutes an initial hypercontractility and a consecutive hypocontractility, which is overcompensated by a postfibrillatory enhancement of atrial inotropy after cessation of AF, are present. The observed phenomenon seems to be related to an increased Ca(2+) influx through the L-type Ca(2+) channel.

Atrial stunning: basics and clinical considerations

Conversion of atrial fibrillation and flutter to sinus rhythm results in a transient mechanical dysfunction of atrium and atrial appendage, termed atrial stunning. Atrial stunning has been reported with all modes of conversion of atrial fibrillation and flutter to sinus rhythm including both transthoracic and low energy internal electrical, pharmacological, and spontaneous cardioversion, and conversion by overdrive pacing and by radiofrequency ablation. Atrial stunning is a function of the underlying arrhythmia becoming apparent at the restoration of sinus rhythm, not the function of the mode of conversion, and does not develop after the unsuccessful attempts of cardioversion or the delivery of electric current to the heart during rhythms other than atrial fibrillation or flutter. Tachycardia-induced atrial cardiomyopathy, cytosolic calcium accumulation, and atrial hibernation are the suggested mechanisms of atrial stunning. Atrial stunning is at maximum immediately after cardioversion and improves progressively with a complete resolution within a few minutes to 4-6 weeks depending on the duration of the preceding atrial fibrillation, atrial size, and structural heart disease. Atrial stunning causes postcardioversion thromboembolism despite restoration of sinus rhythm. Duration of anticoagulation therapy after successful cardioversion should depend on the duration of atrial stunning. Lack of improvement in cardiac output and functional recovery of patients immediately after cardioversion is attributed to the atrial stunning. Verapamil, acetylstrophenathidine, isoproterenol, and dofetilide have been reported to protect from atrial stunning in animal and small human studies. Right atrium stunning is less marked and improves earlier than that of left atrium, resulting in a differential atrial stunning explaining the rare occurrence of pulmonary edema after cardioversion.

Delayed sodium channel inactivation mimics long QT syndrome 3

DPI 201-106 delays sodium channel inactivation. Acute administration of DPI 201-106 prolonged the QT interval, provoked spontaneous torsades de pointes in one patient, and facilitated stimulation-induced polymorphic ventricular tachyarrhythmias in two patients. Similar to the observations in animal studies, delaying sodium channel inactivation is a new form of the acquired long QT syndrome, mimicking long QT syndrome type 3.

Atrial stunning: determinants and cellular mechanisms

BACKGROUND

Atrial stunning is a transient depression of atrial and atrial-appendage mechanical function after successful cardioversion of atrial fibrillation compared with its precardioversion state.

METHOD

Atrial stunning associated with different methods of cardioversion of atrial fibrillation and the determinants and cellular mechanisms of atrial stunning were elaborated by thoroughly examining the studies on the subject identified through a comprehensive literature search.

RESULTS AND CONCLUSION

Atrial stunning has been reported with all methods of cardioversion of atrial fibrillation, including transthoracic electrical, low-energy internal electrical, pharmacological, and spontaneous. It is a function of the underlying atrial fibrillation becoming apparent at the restoration of sinus rhythm, regardless of the method used for conversion. Unsuccessful cardioversion does not result in atrial stunning. The duration of the preceding atrial fibrillation, atrial size, and underlying structural heart disease are the determinants of atrial stunning. A shorter duration of atrial fibrillation and smaller atrial diameters are associated with a relatively less severe stunning, lasting for a shorter duration. Atrial stunning after cardioversion of atrial fibrillation of <1 week usually resolves within 24 hours, and atrial stunning after cardioversion of chronic atrial fibrillation usually resolves within 4 weeks. Tachycardia-induced atrial cardiomyopathy, atrial cytosolic calcium alterations with down-regulation of the L-type Ca2+ channels and up-regulation of the Na+/Ca2+ exchanger, atrial hibernation with myocyte dedifferentiation and myolysis, and atrial fibrosis are the suggested mechanisms underlying atrial stunning. Atrial stunning determines the risk of postcardioversion thrombus formation in atria and atrial appendages, the duration of postcardioversion anticoagulation therapy, the recovery of the atrial contribution to the ventricular function, and the functional recovery of the patients after successful cardioversion of atrial fibrillation.

Effect of low-dose isoproterenol infusion on left atrial appendage function soon after cardioversion of chronic atrial tachyarrhythmias

BACKGROUND

Cardioversion of chronic atrial fibrillation or atrial flutter to sinus rhythm is often associated with transient atrial mechanical dysfunction, i.e. 'atrial stunning', which may increase the risk of subsequent thromboembolic events. We hypothesized that, because of its positive inotropic action, a low-dose isoproterenol infusion might improve postcardioversion atrial mechanical function.

METHOD

Eighteen patients (15 male, three female; 12 atrial fibrillation, six atrial flutter; mean age 65+/-10 years) exhibiting atrial postcardioversion stunning were included in the study. Isoproterenol was infused for 10 min at a dose sufficient to increase the heart rate by about 10%. Using transesophageal echocardiography, both the left atrial appendage emptying/filling flow velocity and function (fractional area change) were examined at baseline, before isoproterenol (immediately after cardioversion) and after isoproterenol.

RESULTS

With infusion of 0.005-0.008 microg/kg/min isoproterenol, heart rate increased by 11.1+/-2.9%, and left atrial appendage emptying velocity, which was diminished following cardioversion, increased significantly (P<0.001) (baseline, before and after isoproterenol: 41.1+/-18.0, 20.3+/-8.5 and 27.3+/-9.6 cm/s, respectively). No major complications were associated with isoproterenol infusion.

CONCLUSIONS

Short-term infusion of low-dose isoproterenol improved atrial function after cardioversion of chronic atrial fibrillation and atrial flutter.

Involvement of the cardiac ryanodine receptor/calcium release channel in catecholaminergic polymorphic ventricular tachycardia

The cardiac ryanodine receptor (RyR2), the major calcium release channel on the sarcoplasmic reticulum (SR) in cardiomyocytes, has recently been shown to be involved in at least two forms of sudden cardiac death (SCD): (1) Catecholaminergic polymorphic ventricular tachycardia (CPVT) or familial polymorphic VT (FPVT); and (2) Arrhythmogenic right ventricular dysplasia type 2 (ARVD2). Eleven RyR2 missense mutations have been linked to these diseases. All eleven RyR2 mutations cluster into 3 regions of RyR2 that are homologous to the three malignant hyperthermia (MH)/central core disease (CCD) mutation regions of the skeletal muscle ryanodine receptor/calcium release channel RyR1. MH/CCD RyR1 mutations have been shown to alter calcium-induced calcium release. Sympathetic nervous system stimulation leads to phosphorylation of RyR2 by protein kinase A (PKA). PKA phosphorylation of RyR2 activates the channel. In conditions associated with high rates of SCD such as heart failure RyR2 is PKA hyperphosphorylated resulting in "leaky" channels. SR calcium leak during diastole can generate "delayed after depolarizations" that can trigger fatal cardiac arrhythmias (e.g., VT). We propose that RyR2 mutations linked to genetic forms of catecholaminergic-induced SCD may alter the regulation of the channel resulting in increased SR calcium leak during sympathetic stimulation.

Dynamic process of apoptosis in adult rat cardiomyocytes analyzed using 48-hour videomicroscopy and electron microscopy: beating and rate are associated with the apoptotic process

Dynamic process of apoptosis has not been elucidated in adult rat cardiomyocytes. Soluble Fas ligand (0.1 microg/ml) in the presence of actinomycin D (0.05 microg/ml) induced apoptosis in cultured adult rat cardiomyocytes, as documented by activated caspase-3, DNA fragmentation, and apoptotic ultrastructure. In the present model, we observed 60 adult cardiomyocytes with a normal rod shape under a real-time videomicroscope continuously for 48 hours. Seventeen cells (28%) were unchanged and 7 cells (12%) showed oncosis (so-called necrosis) in which no beating was evident. In the remaining 36 cells (apoptosis, 60%), a slow beating (17 +/- 3/min) was initiated 16 +/- 1 hours later. Approximately 1 hour later, the rod cells showed long-axial shortening as bone- or club-like, or square-shaped, accompanied with faster beating rates (35 +/- 7/min). In 29 cells (type A1 and A2), marked shrinkage occurred; the cellular shape became almost completely round with a smooth surface and the beating ceased 3.0 +/- 0.4 hours later. Then, smooth budding appeared 0.6 +/- 0.2 hours later. Apoptotic bodies were found in 8 cells 10 +/- 4 hours later (type A1, 13%) but not in 21 cells (type A2, 35%). In the other 7 cells (type A3, 12%), the cell surface became rough 8 +/- 3 hours later and the beating ceased. Maximal beating rate was greatest in type A1 (72 +/- 26/min) and greater in type A2 (29 +/- 5/min) than in type A3 (10 +/- 2/min). Electron microscopy confirmed apoptotic ultrastructure even in the cardiomyocytes with bone-, club-like, or square shapes, suggesting that type A3 as well as A1 and A2 is also under apoptotic process. A caspase inhibitor, zVAD.fmk, blocked beating, apoptotic morphology, and DNA fragmentation, indicating these depended on caspase activation. In the caspase-dependent apoptotic process of cultured adult cardiomyocytes, beating and the following deformity of the cellular edges were the initial signs and the rate of beating was related with the subsequent three different processes of apoptosis.

Localized injury in cardiomyocyte network: a new experimental model of ischemia-reperfusion arrhythmias

We developed a new experimental approach to study the effects of local injury in a multicellular preparation and tested the ability of the method to induce reperfusion arrhythmias in cardiomyocyte monolayers. A small region of injury was created using geometrically defined flows of control and ischemia-like solutions. Calcium transients were acquired simultaneously from injured, control, and border zone cells using fluo 4. Superfusion with the injury solution rapidly diminished the amplitude of calcium transients within the injury zone, followed by cessation of cell beating. Reperfusion caused an immediate tachyarrhythmic response in approximately 17% of experiments, with a wave front propagating from a single cell or small cell cluster within the former injury zone. Inclusion of a gap junction uncoupler (1 mM heptanol) in the injury solution narrowed the functional border and sharply increased the number of ectopic foci and the incidence of reperfusion arrhythmias. The model holds a potential to reveal both micro- and macroscopic features of propagation, conduction, and cell coupling in the normal and diseased myocardium and to serve as a new tool to test antiarrhythmic protocols in vitro.

Ryanodine receptors/calcium release channels in heart failure and sudden cardiac death

Calcium (Ca2+) ions are second messengers in signaling pathways in all types of cells. They regulate muscle contraction, electrical signals which determine the cardiac rhythm and cell growth pathways in the heart. In the past decade cDNA cloning has provided clues as to the molecular structure of the intracellular Ca2+ release channels (ryanodine receptors, RyR, and inositol 1,4,5-trisphosphate receptors, IP3R) on the sarcoplasmic and endoplasmic reticulum (SR/ER) and an understanding of how these molecules regulate Ca2+ homeostasis in the heart is beginning to emerge. The intracellular Ca2+ release channels form a distinct class of ion channels distinguished by their structure, size, and function. Both RyRs and IP3Rs have gigantic cytoplasmic domains that serve as scaffolds for modulatory proteins that regulate the channel pore located in the carboxy terminal 10% of the channel sequence. The channels are tetramers comprised of four RyR or IP3R subunits. RyR2 is required for excitation-contraction (EC) coupling in the heart. Using co-sedimentation and co-immunoprecipitation we have defined a macromolecular complex comprised of RyR2, FKBP12.6, PKA, the protein phosphatases PP1 and PP2A, and an anchoring protein mAKAP. We have shown that protein kinase A (PKA) phosphorylation of RyR2 dissociates FKBP12.6 and regulates the channel open probability (P(o)). In failing human hearts RyR2 is PKA hyperphosphorylated resulting in defective channel function due to increased sensitivity to Ca2+-induced activation.

Preischemic and postischemic treatment with a new Na+/H+-exchange inhibitor, FR183998, shows cardioprotective effects in rats with cardiac ischemia and reperfusion

This study describes the pharmacologic profile of a new Na+/H(+)-exchange inhibitor, FR183998, in anesthetized rats. FR183998 had a potent inhibitory effect on Na+/H+ exchange of rat lymphocytes with median inhibitory (IC50) value of 0.3 nM. Treatment with FR183998 (0.01-0.32 mg/kg, i.v.) reduced or completely abolished ventricular fibrillation and mortality induced by 5-min ischemia followed by reperfusion, when it was administered not only 5 min before ischemia but also 1 min before reperfusion. Myocardial infarct size induced by 30-min ischemia and 60-min reperfusion was reduced significantly in a dose-dependent manner by FR183998 (0.1-1.0 mg/kg, i.v.) when the drug was administered preischemically or at an early phase of ischemia. The ventricular tachycardia and the ventricular fibrillation observed during the ischemic period also were suppressed significantly. These results indicate that FR183998 has a strong inhibitory effect on Na+/H+ exchange and suggest that treatment with FR183998 either before or immediately after the onset of ischemia can prevent the occurrence of arrhythmias and myocardial cell necrosis in situations of ischemia and reperfusion.

Anti-arrhythmic effect of kappa-opioid receptor stimulation in the perfused rat heart: involvement of a cAMP-dependent pathway

During myocardial ischaemia the beta-adrenoceptor is activated, which contributes, at least partly, to cardiac arrhythmias via inducing [Ca2+]i oscillations. Since beta-adrenoceptor is negatively modulated by the kappa-opioid receptor in the heart, the present study attempted to determine if kappa-opioid receptor stimulation modulates the arrhythmogenic action of beta-adrenoceptor stimulation and to delineate the underlying mechanism. The effect of U50,488H, a selective kappa-opioid agonist, on arrhythmias in the isolated perfused rat heart subjected to low flow and 10(-6)mol/l norepinephrine (NE) were determined. Low flow induced arrhythmias, which were potentiated by NE, but not by 10(-6)mol/l U50,488H. The arrhythmia-potentiating effect of NE was antagonized by 10(-6)mol/l propranolol, a beta-adrenoceptor antagonist. U50,488H at 10(-6)mol/l also abolished the potentiation in arrhythmias by NE without affecting the arrhythmias induced by low flow. The anti-arrhythmic action of the kappa-opioid receptor agonist was abolished by 10(-6)mol/l nor-binaltorphimine, a selective kappa-opioid receptor antagonist, but not by 10(-7)mol/l calphostin C, an inhibitor of protein kinase C. Similarly, kappa-opioid receptor stimulation with U50,488H also abolished the NE-induced [Ca2+]i oscillations which are believed to cause cardiac arrhythmias, in ventricular myocytes. To determine whether the inhibitory actions of U50,488H against the effects of beta-adrenoceptor stimulation was via a cAMP-dependent or a cAMP-independent pathway, we determined the effects of U50,488H on NE-enhanced cAMP production and [Ca2+]i oscillations induced by either forskolin, an activator of adenylate cyclase, or Bay K-8644, a selective L-type Ca2+ channel agonist, in the ventricular myocytes. We found that U50,488H abolished the effect of forskolin, but did not alter the effect of Bay K-8644, on [Ca2+]i oscillations in the ventricular myocyte. In addition, U50, 488H also attenuated significantly the NE-induced elevation in cAMP in the heart. The observations suggest that kappa-opioid receptor stimulation abolishes the effect of beta-adrenoceptor stimulation on arrhythmias and [Ca2+]i oscillation via a cAMP-dependent pathway. The finding may be useful for the prevention and treatment of ischaemic heart diseases.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Change of atrial refractory period after short duration of rapid atrial pacing: regional differences and possible mechanisms

It is unknown whether there are regional differences in the change of atrial effective refractory period (ERP) after a short duration of rapid atrial pacing. Furthermore, the effects of calcium channel and potassium channel on this phenomenon have not been extensively investigated. In opened-chest dogs, the endocardial monophasic action potential duration at 90% repolarization (APD90) from the right atrial appendage, and ERP from seven atrial sites were measured before and after rapid atrial pacing at 800 beats/min for 30 minutes. Both atrial ERP and APD90 significantly shortened after rapid atrial pacing. The postpacing atrial ERP and APD90 shortening persisted for 119 +/- 3 and 123 +/- 4 seconds after cessation of pacing, respectively. There was no significant difference in the magnitude or recovery course of atrial ERP shortening after pacing among the seven atrial sites. Pretreatment with nicorandil and d-sotalol had no effects on the magnitude or recovery course of atrial ERP shortening after pacing. However, the degree of ERP and APD90 shortening after pacing was significantly attenuated in the verapamil and ryanodine groups; furthermore, the recovery of ERP and APD90 after cessation of pacing was faster in the two groups. In conclusion, shortening of atrial ERP induced by short-duration rapid atrial pacing was uniform in both atria. Both the adenosine triphosphatase (ATP) dependent potassium current and rapid component of the delayed rectifier did not significantly influence this phenomenon, but both the verapamil and ryanodine could significantly attenuate the degree of atrial ERP and APD90 shortening.

Effects of mibefradil, a novel calcium channel blocking agent with T-type activity, in acute experimental myocardial ischemia: maintenance of ventricular fibrillation threshold without inotropic compromise

OBJECTIVES

We tested whether mibefradil, a selective T-type calcium channel blocking agent, could differentially inhibit experimental ventricular arrhythmogenesis more than contractility during acute regional ischemia and reperfusion compared with that during L-channel blockade by verapamil.

BACKGROUND

T-type calcium channels are found in nodal and conduction tissue and in vascular smooth muscle, but in much lower density in contractile myocardium. The potential role of mibefradil in ventricular arrhythmogenesis remains unclear.

METHODS

Mibefradil (Ro 40-5967, 1 mg/kg body weight intravenously [i.v.]) was given as a bolus 30 min before anterior descending coronary artery ligation, followed by 2 mg/kg per h i.v. during 20 min of ischemia and 25 min of reperfusion in open chest pigs. In a second group, mibefradil was given in a dose twice as high. A third group received verapamil (0.3 mg/kg i.v.), followed by an infusion of 0.6 mg/kg per h.

RESULTS

During the ischemic period, the low (clinically relevant) dose of mibefradil prevented the fall of the ventricular fibrillation threshold, without depressing the maximal rate of pressure development of the left ventricle (LVmax dP/dt). This low dose increased left ventricular blood flow, whereas peripheral arterial pressure remained unchanged. The higher dose of both mibefradil and verapamil was antiarrhythmic during ischemia, at the cost of depressed contractile activity. During reperfusion, only the higher dose of mibefradil and verapamil was antiarrhythmic but both depressed contractile activity.

CONCLUSIONS

Mibefradil is antiarrhythmic, without inotropic compromise. Speculatively, both T-type and L-type calcium channel blockade are involved in these effects.

A superfusion system to study border zones in confluent cultures of neonatal rat heart cells

We present a new experimental method to study intracellular ion regulation in cultured cardiomyocytes at a border zone separating two different and distinct environments. Our system uses a dual-flow superfusion chamber to produce two different but adjacent environments over a monolayer of cardiomyocytes. Fluorescent microscopy of fluorescein showed that the transition between the two environments was nearly linear and was 220-320 micron wide depending on fluid viscosity and velocity. We superfused cultured monolayers on one side with a solution at pH 6.5 and on the other side with a solution at pH 7.4. We observed a sharply demarcated difference in intracellular pH (pHi) between the two halves of the cell monolayer as measured with the fluorescent pHi indicator carboxy-seminaphthorhodafluor-1. The demarcation of pHi corresponded well with the demarcation of the border measured with fluorescein. We conclude that our superfusion system will facilitate the study of intercellular communication and interactions across boundaries of cardiac tissue where different ionic or metabolic conditions are present, for example, between ischemic and nonischemic myocardium.

Different mechanisms for [Ca2+]i oscillations induced by carbachol and high concentrations of [Ca2+]o in the rat ventricular myocyte

1. The purpose of the present study was to explore the different mechanisms of [Ca2+]i oscillations induced by high concentrations of either carbachol (CCh) or extracellular Ca2+ ([Ca2+]o). First, we compared the oscillations induced by CCh at concentrations of 100-300 micromol/L and [Ca2+]o (5 mmol/L) in the single rat ventricular myocyte. Second, we studied CCh- and [Ca2+]o-induced [Ca2+]i oscillations following either interference with the production of inositol trisphosphate (IP3), reductions in cytosolic Ca2+ ([Ca2+]i), inhibition of Ca2+ influx and Na+-Ca2+ exchange or depletion of Ca2+ from its intracellular store. 2. The [Ca2+]i oscillations induced by CCh were frequent and were superimposed on [Ca2+]i transients in electrically stimulated cells, whereas those induced by high [Ca2+]o were occasional and occurred in quiescent cells and between [Ca2+]i transients in electrically stimulated cells. In both cases, [Ca2+]i oscillations were preceded by an increase in resting levels of [Ca2+]i. 3. Carbachol-induced [Ca2+]i oscillations were accompanied by an increase in amplitude and prolongation of the time of decline to 80% of the peak of the [Ca2+]i transient, while high [Ca2+]o-induced [Ca2+]i oscillations were the opposite. 4. A reduction of [Ca2+]o to 0.1 mmol/L and treatment with Ni2+ or ryanodine or 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid AM (BAPTA-AM) abolished the [Ca2+]i oscillations induced by both CCh and high [Ca2+]o. 5. The calcium channel blockers verapamil and nifedipine and inhibitors of phospholipase C (neomycin and U-73122) abolished the [Ca2+]i oscillations induced by CCh; Li+ accelerated the onset of the [Ca2+]i oscillations induced by CCh. 6. These observations suggest that the mechanisms responsible for the [Ca2+]i oscillations induced by CCh and high [Ca2+]o are different from each other. Other than an increase in extracellular Ca2+ influx as a mechanism common for both CCh- and high [Ca2+]o-induced [Ca2+]i oscillations, the CCh-induced [Ca2+]i oscillations involve influx of Ca2+ via L-type Ca2+ channels, Na+-Ca2+ exchange, mobilization of intracellular Ca2+ and IP3 production.