Rapid Electrical Stimulation Research Articles

Inhibition of late sodium current via PI3K/Akt signaling prevents cellular remodeling in tachypacing-induced HL-1 atrial myocytes

An aberrant late sodium current (INa,Late) caused by a mutation in the cardiac sodium channel (Nav1.5) has emerged as a contributor to electrical remodeling that causes susceptibility to atrial fibrillation (AF). Although downregulation of phosphoinositide 3-kinase (PI3K)/Akt signaling is associated with AF, the molecular mechanisms underlying the negative regulation of INa,Late in AF remain unclear, and potential therapeutic approaches are needed. In this work, we constructed a tachypacing-induced cellular model of AF by exposing HL-1 myocytes to rapid electrical stimulation (1.5 V/cm, 4 ms, 10 Hz) for 6 h. Then, we gathered data using confocal Ca2+ imaging, immunofluorescence, patch-clamp recordings, and immunoblots. The tachypacing cells displayed irregular Ca2+ release, delayed afterdepolarization, prolonged action potential duration, and reduced PI3K/Akt signaling compared with controls. Those detrimental effects were related to increased INa,Late and were significantly mediated by treatment with the INa,Late blocker ranolazine. Furthermore, decreased PI3K/Akt signaling via PI3K inhibition increased INa,Late and subsequent aberrant myocyte excitability, which were abolished by INa,Late inhibition, suggesting that PI3K/Akt signaling is responsible for regulating pathogenic INa,Late. These results indicate that PI3K/Akt signaling is critical for regulating INa,Late and electrical remodeling, supporting the use of PI3K/Akt-mediated INa,Late as a therapeutic target for AF.

The role of renal nerve stimulation in percutaneous renal denervation for hypertension: A mini-review.

Recent trials have demonstrated the efficacy and safety of percutaneous renal sympathetic denervation (RDN) for blood pressure (BP)‐lowering in patients with uncontrolled hypertension. Nevertheless, major challenges exist, such as the wide variation of BP‐lowering responses following RDN (from strong response to no response) and lack of feasible and reproducible peri‐procedural predictors for patient response. Both animal and human studies have demonstrated different patterns of BP responses following renal nerve stimulation (RNS), possibly related to varied regional proportions of sympathetic and parasympathetic nerve tissues along the renal arteries. Animal studies of RNS have shown that rapid electrical stimulation of the renal arteries caused renal artery vasoconstriction and increased norepinephrine secretion with a concomitant increase in BP, and the responses were attenuated after RDN. Moreover, selective RDN at sites with strong RNS‐induced BP increases led to a more efficient BP‐lowering effect. In human, when RNS was performed before and after RDN, blunted changes in RNS‐induced BP responses were noted after RDN. The systolic BP response induced by RNS before RDN and blunted systolic BP response to RNS after RDN, at the site with maximal RNS‐induced systolic BP response before RDN, both correlated with the 24‐h ambulatory BP reductions 3–12 months following RDN. In summary, RNS‐induced BP changes, before and after RDN, could be used to assess the immediate effect of RDN and predict BP reductions months following RDN. More comprehensive, large‐scale and long term trials are needed to verify these findings.

Effects of Sacubitril/Valsartan on the Expression of CaMKII/Cav1.2 in Atrial Fibrillation Stimulation Rabbit Model.

Overall, 18 rabbits were randomly divided into control, pacing (600 beats/min), and pacing+sac/val groups. The rabbits in the pacing+sac/val cohort received oral sac/val (10 mg/kg twice daily) across the 21-day investigation period. After three weeks, the atrial effective refractory period (AERP) and AF induction rate were compared. HL-1 cultures were exposed to fast pacing (24 h) with and without LBQ657 (active sacubitril form)/valsartan. Western blots were used for detecting Cav1.2 and CaMKII expression within atrial muscles of the rabbits and HL-1 cultures of AF model. In comparison to the sham cohort, the AF induction rate was markedly increased together with AERP reduction within pacing cohort. Such changes were markedly rescued through sac/val treatment in pacing+sac/val cohort. The proteomic expression profiles of CaMKII and Cav1.2 showed that the CaMKII expression was markedly upregulated, while Cav1.2 expression was downregulated in the pacing cohort. Importantly, these effects were absent in pacing+sac/val cohort. Results of this study show that sac/val treatment regulates the expression of CaMKII/Cav1.2 and could alter this pathway in atrial rapid electrical stimulation models. Therefore, this investigation could contribute to a novel strategy in AF therapeutics in clinical settings.

How His bundle pacing prevents and reverses heart failure induced by right ventricular pacing.

Ideal heart performance demands vigorous systolic contractions and rapid diastolic relaxation. These sequential events are precisely timed and interdependent and require the rapid synchronous electrical stimulation provided by the His-Purkinje system. Right ventricular (RV) pacing creates slow asynchronous electrical stimulation that disrupts the timing of the cardiac cycle and results in left ventricular (LV) mechanical asynchrony. Long-term mechanical asynchrony produces LV dysfunction, remodeling, and clinical heart failure. His bundle pacing preserves synchronous electrical and mechanical LV function, prevents or reverses RV pacemaker-induced remodeling, and reduces heart failure.

Rapid Electrical Stimulation Increased Cardiac Apoptosis Through Disturbance of Calcium Homeostasis and Mitochondrial Dysfunction in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Background/Aims: Heart failure induced by tachycardia, the most common arrhythmia, is frequently observed in clinical practice. This study was designed to investigate the underlying mechanisms. Methods: Rapid electrical stimulation (RES) at a frequency of 3 Hz was applied on human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for 7 days, with 8 h/day and 24 h/day set to represent short-term and long-term tachycardia, respectively. Age-matched hiPSC-CMs without electrical stimulation or with slow electrical stimulation (1 Hz) were set as no electrical stimulation (NES) control or low-frequency electrical stimulation (LES) control. Following stimulation, JC-1 staining flow cytometry analysis was performed to examine mitochondrial conditions. Apoptosis in hiPSC-CMs was evaluated using Hoechst staining and Annexin V/propidium iodide (AV/PI) staining flow cytometry analysis. Calcium transients and L-type calcium currents were recorded to evaluate calcium homeostasis. Western blotting and qPCR were performed to evaluate the protein and mRNA expression levels of apoptosis-related genes and calcium homeostasis-regulated genes. Results: Compared to the controls, hiPSC-CMs following RES presented mitochondrial dysfunction and an increased apoptotic percentage. Amplitudes of calcium transients and L-type calcium currents were significantly decreased in hiPSC-CMs with RES. Molecular analysis demonstrated upregulated expression of Caspase3 and increased Bax/Bcl-2 ratio. Genes related to calcium re-sequence were downregulated, while phosphorylated Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) was significantly upregulated following RES. There was no significant difference between the NES control and LES control groups in these aspects. Inhibition of CaMKII with 1 µM KN93 partly reversed these adverse effects of RES. Conclusion: RES on hiPSC-CMs disturbed calcium homeostasis, which led to mitochondrial stress, promoted cell apoptosis and caused electrophysiological remodeling in a time-dependent manner. CaMKII played a central role in the damages induced by RES, pharmacological inhibition of CaMKII activity partly reversed the adverse effects of RES on both structural and electrophysiological properties of cells.

Activation of TGF-β1/α-SMA/Col I Profibrotic Pathway in Fibroblasts by Galectin-3 Contributes to Atrial Fibrosis in Experimental Models and Patients

Background/Aims: This study aimed to evaluate whether galectin-3 (Gal-3) contributes actively to atrial fibrosis both in patients and experimental atrial fibrillation (AF) models. Methods: Mouse HL-1 cardiomyocytes were subjected to rapid electrical stimulation (RES) to explore Gal-3 expression and secretion levels by western blotting (WB) and enzyme linked immunosorbent assay (ELISA). Neonatal rat cardiac fibroblasts were treated with conditioned culture medium and recombinant human Gal-3 to evaluate the activation of the transforming growth factor (TGF)-β1/α-smooth muscle actin (SMA)/collagen I (Col I) profibrotic pathway (WB) and fibroblast proliferation with a Cell Counting Kit-8 (CCK-8). Furthermore, in the rapid atrial pacing (RAP) rabbit AF model, atrial Gal-3 expression and its effects on the profibrotic pathway were evaluated (WB and Masson’s trichrome staining). Moreover, 44 consecutive patients who underwent single mitral valve repair/replacement were included, consisting of 28 patients with persistent AF (PeAF) and 16 with sinus rhythm (SR). Coronary sinus blood was also sampled to test circulating Gal-3 levels (ELISA), and atrial myocardium Gal-3 and its downstream TGF-β1/α-SMA pathway were also measured by WB and immunohistochemical staining. Results: Gal-3 expression in HL-1 cells and its secretion level in culture medium were greatly increased after 24 h RES. Treatment of neonatal rat cardiac fibroblasts with conditioned media collected from the RES group or recombinant human Gal-3 protein (10 and 30 µg/mL) for 72 h induced the activation of the TGF-β1/α-SMA/Col I profibrotic pathway. RAP increased Gal-3 levels and activated the TGF-β1/α-SMA/Col I pathway in rabbit left atria, while the Gal-3 inhibitor N-acetyllactosamine, injected at 4.5 mg/kg every 3 days, mitigated these adverse changes. Furthermore, Gal-3 levels in coronary sinus blood samples and myocardial Gal-3 expression levels were higher in the PeAF patients than in the SR patients, and higher level profibrotic pathway activation was also confirmed. Conclusions: Activation of Gal-3 expression in the atria can subsequently activate the TGF-β1/α-SMA/Col I pathway in cardiac fibroblasts, which may enhance atrial fibrosis.

Suppression of Oxidative Stress and Apoptosis in Electrically Stimulated Neonatal Rat Cardiomyocytes by Resveratrol and Underlying Mechanisms.

We explored the effects of resveratrol on oxidative stress in cardiomyocytes subjected to rapid electrical stimulation (RES) and also investigated the underlying mechanisms. Cultured ventricular myocytes of neonatal rat were subjected to RES at 4.0 Hz, with or without resveratrol, an NADPH oxidase inhibitor apocyanin (APO) or a Ca/calmodulin-dependent protein kinase II (CaMKII) inhibitor autocamtide-2-inhibitory peptide (AIP). Cell counts, to optimize resveratrol concentration, and angiotensin II content were evaluated. Reactive oxygen species (ROS), intracellular Ca in cardiomyocytes, and cardiomyocyte apoptosis were also assessed. Levels of methionine sulfoxide reductase A (MsrA), Nox, oxidative CaMKII (OX-CaMKII), and cleaved caspase-3 in cardiomyocytes were examined. Resveratrol treatment, as compared with APO and AIP, significantly decreased ROS levels, improved Ca amplitudes, and intracellular Ca transient decay rates, and inhibited cardiomyocyte apoptosis. Resveratrol also increased MsrA protein levels. In cardiomyocytes subjected to RES, after pretreatment with resveratrol or APO, protein levels of Nox4, Nox2, OX-CaMKII, and cleaved caspase-3 were decreased. In comparison, with AIP pretreatment, only Nox2, OX-CaMKII, and cleaved caspase-3 were decreased. However, in the presence of dimethyl sulfoxide, a competitive inhibitor of MsrA function, a decrease in cleaved caspase-3 did not occur. Resveratrol decreased ROS, partially through the inhibition of NADPH oxidase activity and upregulation of MsrA expression.

ATRIAL FIBRILLATION INDUCES A MITOCHONDRIAL STRESS RESPONSE WHICH FACILITATES SUSTAINED RAPID CONDUCTION

Atrial Fibrillation (AF) is the most common sustained cardiac arrhythmia. A progressive disorder, AF begins with brief paroxysms, becoming increasingly persistent and resistant to therapies, ultimately becoming permanent. At the cellular level, fibrillation quickly results in functional changes in human atrium which permit the ultra-rapid conduction of multiple re-entrant waves and/or rotors which previously could not have been maintained. Ultra-rapid conduction results in ultra-rapid calcium flux and increased oxidative stress which demands a sustained increase in mitochondrial respiration for cell survival. We hypothesized that sustained high frequency stimulation results in a mitochondrial stress response which subsequently enables sustained fibrillatory conduction. We performed rapid (5 Hz) electrical field stimulation (EFS) experiments for 24 hours at 37oC, “fibrillatory stress”, or 1 Hz “control”, in cultured HL-1 cells. These atrial cells maintain an atrial phenotype, beating spontaneously in vitro at 0.5- 1 Hz. HL-1 cells have previously been shown by us, and others, to demonstrate electrophysiological remodeling and altered gene transcription in response to fibrillatory stress similar to those observed in human atrial myocytes isolated from patients suffering from persistent AF. Electron and confocal microscopy were used to perform morphometric analysis and to record calcium release events, oxidative stress and mitochondrial potential (Ψm). Fibrillatory stress resulted in enlarged hyper-fused mitochondria, increased expression of mitofusin-2 and co-localization of mitochondria with the sarcoplasmic reticulum (n= 50, p< 0.001 for all comparisons). Following fibrillatory stress, Ψm became synchronized with calcium release events and prevented detection of both mitochondrial and cytoplasmic oxidative stress upon subsequent rapid activation (n= 50, p< 0.001 for all comparisons) [Figure 1]. These observations were reproducible when fibrillatory stress was performed in the presence of reducing agents or the L-type calcium antagonist Verapamil. Sustained fibrillatory stress in our cell model resulted in a mitochondrial stress response which was associated with enhanced structural and functional co-ordination with intracellular calcium handling. These changes appeared to facilitate subsequent rapid activation without inducing oxidative stress. This “mitochondrial remodeling” process did not appear to be mediated by oxidative stress or sarcolemmal L-type calcium channels. These data suggest that the mitochondrial stress response may represent a novel mechanism to prevent AF progression. Much further work is required to determine whether the mitochondrial stress response is truly arrhythmogenic.

Functional and Pharmacological Analysis of Cardiomyocytes Differentiated from Human Peripheral Blood Mononuclear-Derived Pluripotent Stem Cells

SummaryAdvances in induced pluripotent stem cell (iPSC) technology have set the stage for routine derivation of patient- and disease-specific human iPSC-cardiomyocyte (CM) models for preclinical drug screening and personalized medicine approaches. Peripheral blood mononuclear cells (PBMCs) are an advantageous source of somatic cells because they are easily obtained and readily amenable to transduction. Here, we report that the electrophysiological properties and pharmacological responses of PBMC-derived iPSC CM are generally similar to those of iPSC CM derived from other somatic cells, using patch-clamp, calcium transient, and multielectrode array (MEA) analyses. Distinct iPSC lines derived from a single patient display similar electrophysiological features and pharmacological responses. Finally, we demonstrate that human iPSC CMs undergo acute changes in calcium-handling properties and gene expression in response to rapid electrical stimulation, laying the foundation for an in-vitro-tachypacing model system for the study of human tachyarrhythmias.

Rapid electrical stimulation causes alterations in cardiac intercellular junction proteins of cardiomyocytes

The intercellular junctions contain two complexes, adhesion junctions (AJ) and connexin (Cx) gap junctions (GJs). GJs provide the pathway for intercellular current flow. AJs mediate normal mechanical coupling and play an important role in the stability of GJs. We investigated the effects of rapid electrical stimulation (RES) on cardiac intercellular junctions, especially β-catenin and Cx43 alterations. We also studied the effects of ANG II receptor blockade on intercellular junction remodeling. Neonatal rats were euthanized by decapitation, and cardiomyocytes were prepared, cultured, and subjected to RES. We used real-time PCR, western blot analysis, and immunohistochemical methods. Conduction properties were examined by an extracellular potential mapping system. Cx43 protein expression in cardiomyocytes was significantly increased after 60 min. β-Catenin expression in the total cell fraction was significantly increased after 30 min. The expression level of β-catenin in the nucleus, which functions as a T cell factor/lymphocyte enhancer binding factor transcriptional activator of Cx43 with its degradation regulated by glycogen synthase kinase-3β, was dramatically increased after 10 min. Conduction velocity was increased significantly by RES for 60 min. Olmesartan prevented most these effects of RES. We showed an increase of phosphorylated glycogen synthase kinase-3β, which is phosphorylated by activated MAPKs and inhibits β-catenin degradation, was attenuated by olmesartan. The changes in β-catenin precede Cx43 GJ remodeling and might play an important role in the formation and stability of GJs. Olmesartan might be a new upstream arrhythmia therapy by modulating intercellular junction remodeling through the β-catenin signaling pathway.

Cardiac ß -catenin Alterations Play an Important Role in Gap Junction Remodeling of Cardiomyocytes Exposed to Rapid Electrical Stimulation

The intercalated disc (ID) contains two complexes, adhesion junction (AJ) and connexin (Cx) gap junction (GJ). GJs provide the pathway for intercellular current flow, and AJ mediates normal mechanical coupling and plays an important role in the stability of GJs.

Alterations in cardiac beta-catenin precede connexin gap junction remodeling in cardiomyocytes exposed to rapid electrical stimulation

The Intercalated Disc (ID) contains two complexes, Adhesion Junction (AJ) and Connexin (Cx) Gap Junction (GJ). GJs provide the pathway for intercellular current flow, enabling coordinated action potential propagation and contraction. AJ mediates normal mechanical coupling between cardiomyocytes and plays an important role in the stability of GJ. Purpose: The purpose of this study was to study ID remodeling, especially β-catenin, which is one of the component proteins of AJ, and Cx43 alterations caused by Rapid Electrical Stimulation (RES) in cardiomyocytes. In addition, we investigated the effects of olmesartan, Angiotensin II receptor blockade on ID remodeling. Methods: Cultured neonatal rat ventricular myocytes were subjected to RES at 3.0 Hz for up to 60 min. We used real-time PCR, western blotting, and immunohistochemical methods to investigate expressions. Conduction properties of the cultured ventricular myocytes were examined by extracellular potential mapping by using a specifically designed multi-electrode array system. Results: Cx43 protein expression in cardiomyocytes was significantly increased (x 1.5 vs 0 min. p<0.01) after 60 min RES. Beta-catenin expression in total cell fraction was also significantly increased (x 1.5 vs 0 min. p<0.05) after 30 min. Interestingly, the expression level of β-catenin at nucleus, which functions as T Cell Factor/Lymphocyte Enhancer binding Factor (LEF/TCF) transcriptional activator of Cx43 and its degradation is modulated by glycogen synthase kinase 3β(GSK3β), was dramatically increased (x 1.4 vs 0 min. p<0.05) after 10 min. Conduction velocity was increased significantly by RES for 60min (25% increase). Treatment of cardiomyocytes with olmesartan prevented most these effects of RES. Moreover, we showed that an increase of phospholylated GSK3β, which is phosphorylated by activated mitogen-activated protein kinases due to RES and inhibits β-catenin degradations, was attenuated by olmesartan. Conclusions: The changes in AJ protein β-catenin precede Cx43 GJ remodeling and might play an important role in the formation and stability of GJs during RES exposure to cardiomyocytes. Olmesartan might be a new upstream arrhythmia therapy by modulating both AJ and GJ remodeling through β-catenin signaling pathway.

High Voltage, Not Age and Blood Pressure, of Stimulus Exaggerates Inducibility of Atrial Fibrillation by Rapid Electrical Stimulation in Hypertensive Rats

High Voltage, Not Age and Blood Pressure, of Stimulus Exaggerates Inducibility of Atrial Fibrillation by Rapid Electrical Stimulation in Hypertensive Rats

Changes of β-catenin Protein Precede Gap Junction Remodeling in Cardiomyocyte Caused by Rapid Electrical Stimulation

Changes of β-catenin Protein Precede Gap Junction Remodeling in Cardiomyocyte Caused by Rapid Electrical Stimulation

The regulation and packaging of synaptic vesicles as related to recruitment within glutamatergic synapses

The reserve pool (RP) and readily releasable pool (RRP) of synaptic vesicles within presynaptic nerve terminals, at crayfish and larval Drosophila neuromuscular junctions (NMJs), were examined for physiological differentiation into distinctly separate functional groups. These NMJs are glutamatergic and produce graded excitatory postsynaptic potentials (EPSPs). The packaging of glutamate was perturbed by blocking the vesicular glutamate transporter (VGlut) with bafilomycin A1. Various frequencies of motor nerve stimulation, exposure time, and concentration of bafilomycin A1 were examined. The low-output tonic opener NMJs in crayfish exposed to 4μM bafilomycin A1 and 20-Hz continuous stimulation decreased the EPSP amplitude to 50% in ∼30min with controls lasting 3h. After activity and bafilomycin A1-induced synaptic depression, the EPSPs were rapidly revitalized by serotonin (5-HT, 1μM) in the crayfish preparations. The 5-HT action can be blocked with a PLC inhibitor. We postulate 5-HT recruits unused vesicles from the RP. The perception is the RRP is selectively activated during rapid electrical stimulation (20Hz) sparing the RP. When stimulation frequency is high (40Hz) the RP is recruited to the RRP and dampens subsequent recruitment with 5-HT. The higher output synapses of the larval Drosophila NMJ when stimulated at 1Hz or 5Hz and exposed to 4μM of bafilomycin A1 showed a depression rate of 50% within ∼10min with controls lasting ∼40min. After low frequency depression and/or exposure to bafilomycin A1 a burst of higher frequency (10Hz) can recruit vesicles from the RP to the RRP.

Direct action of angiotensin II on the conduction through papillary muscle preparations of rat heart immediately after reoxygenation

We investigated the direct action of angiotensin II (Ang II) on myocardial conduction and transmembrane action potential immediately after reoxygenation. MethodAfter superfusion in a simulated ischemic solution, ventricular papillary muscle preparations of rat heart were washed with oxygenated Tyrode solution containing Ang II, Ang II plus CV-11974 (an AT1 receptor blocker), or Ang II plus 5-hydroxydecanoic acid (mito-KATP blocker), under rapid electrical stimulation (RES) for 60s. ResultsIn the control experiments, the incidence of conduction delay and block was the highest within the first 10s, and subsequently, 1:1 conduction was established after 40s. Ang II significantly enhanced the 2:1 conduction block during the later phase of RES (40–60s after reoxygenation). This effect of Ang II was abolished by either CV-11974 (P<0.001) or 5-HD (P<0.001). ConclusionRES-induced conduction delay and block immediately after reoxygenation were accelerated by Ang II, which could be relevant to the maintenance of reperfusion arrhythmias. The mito-KATP channel may participate in the mechanism underlying this phenomenon.

Ca 2+ disorder caused by rapid electrical field stimulation can be modulated by CaMKIIδ expression in primary rat atrial myocytes

Abnormalities in intracellular Ca 2+ handing are believed to contribute to arrhythmogenesis during atrial fibrillation (AF). Ca 2+/calmodulin-dependent protein kinaseII δ (CaMKIIδ) overexpression was detected in atrial myocytes from patients and animal models with persistent AF. In the present study, we found that rapid electrical field stimulation applied to primary atrial myocytes altered the CaMKIIδ activity, not expression level, resulting in Ca 2+ disorder. By lentivirus mediated delivery of CaMKIIδ gene or siRNA into atrial myocytes, cells with different CaMKIIδ expression were generated. Changes of CaMKIIδ expression altered the sarcoplasmic reticulum (SR) Ca 2+ release and L-type Ca 2+ channels current ( I Ca) in both steady and electrical stimulating state. These results revealed the important role of CaMKIIδ in Ca 2+ disorder caused by electrical field stimulation. It also provided a potential method to improve Ca 2+ disorder in AF by modulating CaMKIIδ expression level.

Tachycardia of atrial myocytes induces collagen expression in atrial fibroblasts through transforming growth factor 1

We investigated the molecular mechanism of rapid-depolarization-induced atrial fibrosis. We used a direct atrial myocyte-fibroblast contact co-culture and a fibroblast-specific transforming growth factor β1 (TGF-β1), connective tissue growth factor (CTGF) and procollagen type I α-1 (COL1A1) luciferase reporter system to investigate the possible molecular mechanism of rapid-depolarization-induced atrial fibrosis. Mouse atrial fibroblasts were first transfected with promoter-luciferase reporters, and then co-cultured with HL-1 atrial myocytes. Rapid depolarization of atrial myocytes by rapid electrical field stimulation induced increased TGF-β1, CTGF and COL1A1 promoter activities in the co-cultured atrial fibroblasts (2.4 ± 0.3-fold increase, P= 0.008 for TGF-β1; 2.9 ± 0.4-fold increase, P< 0.001 for CTGF; and 2.1 ± 0.2-fold increase, P= 0.008 for COL1A1). Rapid depolarization of atrial myocytes increased paracrine secretion of angiotensin II (Ang II) and reactive oxygen species in the co-culture medium. Rapid electrical field stimulation-induced ROS generation in atrial myocytes was attenuated by the membrane NADPH oxidase inhibitor, apocynin. Atrial myocyte-induced expression of TGF-β1, CTGF and COL1A1 in atrial fibroblasts was attenuated by co-treatment with the Ang II receptor blocker, losartan, and apocynin. Atrial myocyte-induced COL1A1 expression in atrial fibroblasts was attenuated by anti-TGF-β1 antibody and RNA interference knockdown of the TGF-β1 receptor. We first demonstrated that tachycardia of atrial myocytes induced paracrine secretion of Ang II and reactive oxygen species, which in turn induced expression of CTGF and procollagen in co-cultured atrial fibroblasts through increasing TGF-β1 expression. The results may imply that use of an AngII receptor blocker, in combination with an anti-oxidant, blocks rapid-depolarization-induced atrial fibrosis.

Spatiotemporal intracellular calcium dynamics during cardiac alternans

Cellular calcium transient alternans are beat-to-beat alternations in the peak cytosolic calcium concentration exhibited by cardiac cells during rapid electrical stimulation or under pathological conditions. Calcium transient alternans promote action potential duration alternans, which have been linked to the onset of life-threatening ventricular arrhythmias. Here we use a recently developed physiologically detailed mathematical model of ventricular myocytes to investigate both stochastic and deterministic aspects of intracellular calcium dynamics during alternans. The model combines a spatially distributed description of intracellular calcium cycling, where a large number of calcium release units are spatially distributed throughout the cell, with a full set of ionic membrane currents. The results demonstrate that ion channel stochasticity at the level of single calcium release units can influence the whole-cell alternans dynamics by causing phase reversals over many beats during fixed frequency pacing close to the alternans bifurcation. They also demonstrate the existence of a wide range of dynamical states. Depending on the sign and magnitude of calcium-voltage coupling, calcium alternans can be spatially synchronized or desynchronized, in or out of phase with action potential duration alternans, and the node separating out-of-phase regions of calcium alternans can be expelled from or trapped inside the cell. This range of states is found to be larger than previously anticipated by including a robust global attractor where calcium alternans can be spatially synchronized but out of phase with action potential duration alternans. The results are explained by a combined theoretical analysis of alternans stability and node motion using general iterative maps of the beat-to-beat dynamics and amplitude equations.

Direct action of an angiotensin II receptor blocker on angiotensin II-induced left atrial conduction delay in spontaneously hypertensive rats

In this study, the inhibitory action of angiotensin II (Ang II) receptor blocker (ARB) on Ang II-induced conduction blocking in the left atrium was studied. Transmembrane action potentials and effective refractory period (ERP) were recorded from the left atrial trabeculae of spontaneously hypertensive rats (SHR(Izm)s) that had been treated with or without olmesartan medoxomil (AT1 receptor blocker, 3.0 mg kg(-1)) for 8 weeks before the recording. During rapid electrical stimulation (RES), changes in activation time (AT) and frequencies of the conduction block were studied under perfusion of Ang II (10(-8) M) or Ang II plus ARB (10(-7) M). Interstitial fibrosis in the left atrium was quantitatively assessed using histological sections. Although action potential parameters and ERP did not differ between the two groups when preparations were driven with a basic cycle length, an Ang II-induced conduction delay was observed during RES, and this was significantly suppressed in SHR(Izm)s treated with ARB (P<0.05). The Ang II-induced conduction block was also significantly reduced with perfusion of ARB (P<0.05). The interstitial fibrous area was significantly larger in untreated SHR(Izm)s than in treated animals (P<0.01). This antagonizing action of AT1 blockade might contribute to its non-antiarrhythmic therapeutic effect on atrial fibrillation because of its indirect and direct actions on the structural and functional remodeling of the left atrium.