Role of t-tubule remodeling on mechanisms of abnormal calcium release during heart failure development in canine ventricle.

The goal of this work was to investigate the role of t-tubule (TT) remodeling in abnormal Ca2+ cycling in ventricular myocytes of failing dog hearts. Heart failure (HF) was induced using rapid right ventricular pacing. Extensive changes in echocardiographic parameters, including left and right ventricular dilation and systolic dysfunction, diastolic dysfunction, elevated left ventricular filling pressures, and abnormal cardiac mechanics, indicated that severe HF developed. TT loss was extensive when measured as the density of total cell volume, derived from three-dimensional confocal image analysis, and significantly increased the distances in the cell interior to closest cell membrane. Changes in Ca2+ transients indicated increases in heterogeneity of Ca2+ release along the cell length. When critical properties of Ca2+ release variability were plotted as a function of TT organization, there was a complex, nonlinear relationship between impaired calcium release and decreasing TT organization below a certain threshold of TT organization leading to increased sensitivity in Ca2+ release below a TT density threshold of 1.5%. The loss of TTs was also associated with a greater incidence of triggered Ca2+ waves during rapid pacing. Finally, virtually all of these observations were replicated by acute detubulation by formamide treatment, indicating an important role of TT remodeling in impaired Ca2+ cycling. We conclude that TT remodeling itself is a major contributor to abnormal Ca2+ cycling in HF, reducing myocardial performance. The loss of TTs is also responsible for a greater incidence of triggered Ca2+ waves that may play a role in ventricular arrhythmias arising in HF.NEW & NOTEWORTHY Three-dimensional analysis of t-tubule density showed t-tubule disruption throughout the whole myocyte in failing dog ventricle. A double-linear relationship between Ca2+ release and t-tubule density displays a steeper slope at t-tubule densities below a threshold value (∼1.5%) above which there is little effect on Ca2+ release (T-tubule reserve). T-tubule loss increases incidence of triggered Ca2+ waves. Chemically induced t-tubule disruption suggests that t-tubule loss alone is a critical component of abnormal Ca2+ cycling in heart failure.

Phosphatidylinositol-4,5-Bisphosphate Binding to Amphiphysin-II Modulates T-Tubule Remodeling: Implications for Heart Failure.

BIN1 (amphyphysin-II) is a structural protein involved in T-tubule (TT) formation and phosphatidylinositol-4,5-bisphosphate (PIP2) is responsible for localization of BIN1 to sarcolemma. The goal of this study was to determine if PIP2-mediated targeting of BIN1 to sarcolemma is compromised during the development of heart failure (HF) and is responsible for TT remodeling. Immunohistochemistry showed co-localization of BIN1, Cav1.2, PIP2, and phospholipase-Cß1 (PLCß1) in TTs in normal rat and human ventricular myocytes. PIP2 levels were reduced in spontaneously hypertensive rats during HF progression compared to age-matched controls. A PIP Strip assay of two native mouse cardiac-specific isoforms of BIN1 including the longest (cardiac BIN1 #4) and shortest (cardiac BIN1 #1) isoforms as well human skeletal BIN1 showed that all bound PIP2. In addition, overexpression of all three BIN1 isoforms caused tubule formation in HL-1 cells. A triple-lysine motif in a short loop segment between two helices was mutated and replaced by negative charges which abolished tubule formation, suggesting a possible location for PIP2 interaction aside from known consensus binding sites. Pharmacological PIP2 depletion in rat ventricular myocytes caused TT loss and was associated with changes in Ca2+ release typically found in myocytes during HF, including a higher variability in release along the cell length and a slowing in rise time, time to peak, and decay time in treated myocytes. These results demonstrate that depletion of PIP2 can lead to TT disruption and suggest that PIP2 interaction with cardiac BIN1 is required for TT maintenance and function.

Attenuation of Oxidative Injury With Targeted Expression of NADPH Oxidase 2 Short Hairpin RNA Prevents Onset and Maintenance of Electrical Remodeling in the Canine Atrium: A Novel Gene Therapy Approach to Atrial Fibrillation.

BACKGROUND: Atrial fibrillation (AF) is the most common heart rhythm disorder in adults and a major cause of stroke. Unfortunately, current treatments of AF are suboptimal because they are not targeted to the molecular mechanisms underlying AF. Using a highly novel gene therapy approach in a canine, rapid atrial pacing model of AF, we demonstrate that NADPH oxidase 2 (NOX2) generated oxidative injury causes upregulation of a constitutively active form of acetylcholine-dependent K+ current (IKACh), called IKH; this is an important mechanism underlying not only the genesis, but also the perpetuation of electric remodeling in the intact, fibrillating atrium. METHODS: To understand the mechanism by which oxidative injury promotes the genesis and maintenance of AF, we performed targeted injection of NOX2 short hairpin RNA (followed by electroporation to facilitate gene delivery) in atria of healthy dogs followed by rapid atrial pacing. We used in vivo high-density electric mapping, isolation of atrial myocytes, whole-cell patch clamping, in vitro tachypacing of atrial myocytes, lucigenin chemiluminescence assay, immunoblotting, real-time polymerase chain reaction, immunohistochemistry, and Masson trichrome staining. RESULTS: First, we demonstrate that generation of oxidative injury in atrial myocytes is a frequency-dependent process, with rapid pacing in canine atrial myocytes inducing oxidative injury through the induction of NOX2 and the generation of mitochondrial reactive oxygen species. We show that oxidative injury likely contributes to electric remodeling in AF by upregulating IKACh by a mechanism involving frequency-dependent activation of PKCε (protein kinase C epsilon). The time to onset of nonsustained AF increased by >5-fold in NOX2 short hairpin RNA-treated dogs. Furthermore, animals treated with NOX2 short hairpin RNA did not develop sustained AF for up to 12 weeks. The electrophysiological mechanism underlying AF prevention was prolongation of atrial effective refractory periods, at least in part attributable to the attenuation of IKACh. Attenuated membrane translocation of PKCε appeared to be a likely molecular mechanism underlying this beneficial electrophysiological remodeling. CONCLUSIONS: NOX2 oxidative injury (1) underlies the onset, and the maintenance of electric remodeling in AF, as well, and (2) can be successfully prevented with a novel, gene-based approach. Future optimization of this approach may lead to a novel, mechanism-guided therapy for AF.

Triggered Ca2+ Waves Induce Depolarization of Maximum Diastolic Potential and Action Potential Prolongation in Dog Atrial Myocytes.

BACKGROUND: We have identified a novel form of abnormal Ca2+ wave activity in normal and failing dog atrial myocytes which occurs during the action potential (AP) and is absent during diastole. The goal of this study was to determine if triggered Ca2+ waves affect cellular electrophysiological properties. METHODS: Simultaneous recordings of intracellular Ca2+ and APs allowed measurements of maximum diastolic potential and AP duration during triggered calcium waves (TCWs) in isolated dog atrial myocytes. Computer simulations then explored electrophysiological behavior arising from TCWs at the tissue scale. RESULTS: At 3.3 to 5 Hz, TCWs occurred during the AP and often outlasted several AP cycles. Maximum diastolic potential was reduced, and AP duration was significantly prolonged during TCWs. All electrophysiological responses to TCWs were abolished by SEA0400 and ORM10103, indicating that Na-Ca exchange current caused depolarization. The time constant of recovery from inactivation of Ca2+ current was 40 to 70 ms in atrial myocytes (depending on holding potential) so this current could be responsible for AP activation during depolarization induced by TCWs. Modeling studies demonstrated that the characteristic properties of TCWs are potentially arrhythmogenic by promoting both conduction block and reentry arising from the depolarization induced by TCWs. CONCLUSIONS: Triggered Ca2+ waves activate inward NCX and dramatically reduce atrial maximum diastolic potential and prolong AP duration, establishing the substrate for reentry which could contribute to the initiation and maintenance of atrial arrhythmias.

Remodeling Promotes Proarrhythmic Disruption of Calcium Homeostasis in Failing Atrial Myocytes.

It is well known that heart failure (HF) typically coexists with atrial fibrillation (AF). However, until now, no clear mechanism has been established that relates HF to AF. In this study, we apply a multiscale computational framework to establish a mechanistic link between atrial myocyte structural remodeling in HF and AF. Using a spatially distributed model of calcium (Ca) signaling, we show that disruption of the spatial relationship between L-type Ca channels (LCCs) and ryanodine receptors results in markedly increased Ca content of the sarcoplasmic reticulum (SR). This increase in SR load is due to changes in the balance between Ca entry via LCCs and Ca extrusion due to the sodium-calcium exchanger after an altered spatial relationship between these signaling proteins. Next, we show that the increased SR load in atrial myocytes predisposes these cells to subcellular Ca waves that occur during the action potential (AP) and are triggered by LCC openings. These waves are common in atrial cells because of the absence of a well-developed t-tubule system in most of these cells. This distinct spatial architecture allows for the presence of a large pool of orphaned ryanodine receptors, which can fire and sustain Ca waves during the AP. Finally, we incorporate our atrial cell model in two-dimensional tissue simulations and demonstrate that triggered wave generation in cells leads to electrical waves in tissue that tend to fractionate to form wavelets of excitation. This fractionation is driven by the underlying stochasticity of subcellular Ca waves, which perturbs AP repolarization and consequently induces localized conduction block in tissue. We outline the mechanism for this effect and argue that it may explain the propensity for atrial arrhythmias in HF.

Region-specific parasympathetic nerve remodeling in the left atrium contributes to creation of a vulnerable substrate for atrial fibrillation.

Atrial fibrillation (AF) is the most common heart rhythm disorder and a major cause of stroke. Unfortunately, current therapies for AF are suboptimal, largely because the molecular mechanisms underlying AF are poorly understood. Since the autonomic nervous system is thought to increase vulnerability to AF, we used a rapid atrial pacing (RAP) canine model to investigate the anatomic and electrophysiological characteristics of autonomic remodeling in different regions of the left atrium. RAP led to marked hypertrophy of parent nerve bundles in the posterior left atrium (PLA), resulting in a global increase in parasympathetic and sympathetic innervation throughout the left atrium. Parasympathetic fibers were more heterogeneously distributed in the PLA when compared with other left atrial regions; this led to greater fractionation and disorganization of AF electrograms in the PLA. Computational modeling revealed that heterogeneously distributed parasympathetic activity exacerbates sympathetic substrate for wave break and reentry. We further discovered that levels of nerve growth factor (NGF) were greatest in the left atrial appendage (LAA), where AF was most organized. Preferential NGF release by the LAA - likely a direct function of frequency and regularity of atrial stimulation - may have important implications for creation of a vulnerable AF substrate.

Oxidative stress creates a unique, CaMKII-mediated substrate for atrial fibrillation in heart failure.

The precise mechanisms by which oxidative stress (OS) causes atrial fibrillation (AF) are not known. Since AF frequently originates in the posterior left atrium (PLA), we hypothesized that OS, via calmodulin-dependent protein kinase II (CaMKII) signaling, creates a fertile substrate in the PLA for triggered activity and reentry. In a canine heart failure (HF) model, OS generation and oxidized-CaMKII-induced (Ox-CaMKII-induced) RyR2 and Nav1.5 signaling were increased preferentially in the PLA (compared with left atrial appendage). Triggered Ca2+ waves (TCWs) in HF PLA myocytes were particularly sensitive to acute ROS inhibition. Computational modeling confirmed a direct relationship between OS/CaMKII signaling and TCW generation. CaMKII phosphorylated Nav1.5 (CaMKII-p-Nav1.5 [S571]) was located preferentially at the intercalated disc (ID), being nearly absent at the lateral membrane. Furthermore, a decrease in ankyrin-G (AnkG) in HF led to patchy dropout of CaMKII-p-Nav1.5 at the ID, causing its distribution to become spatially heterogeneous; this corresponded to preferential slowing and inhomogeneity of conduction noted in the HF PLA. Computational modeling illustrated how conduction slowing (e.g., due to increase in CaMKII-p-Nav1.5) interacts with fibrosis to cause reentry in the PLA. We conclude that OS via CaMKII leads to substrate for triggered activity and reentry in HF PLA by mechanisms independent of but complementary to fibrosis.

Synchronization of Triggered Waves in Atrial Tissue.

When an atrial cell is paced rapidly, calcium (Ca) waves can form on the cell boundary and propagate to the cell interior. These waves are referred to as "triggered waves" because they are initiated by Ca influx from the L-type Ca channel and occur during the action potential. However, the consequences of triggered waves in atrial tissue are not known. Here, we develop a phenomenological model of Ca cycling in atrial myocytes that accounts for the formation of triggered waves. Using this model, we show that a fundamental requirement for triggered waves to induce abnormal electrical activity in tissue is that these waves must be synchronized over large populations of cells. This is partly because triggered waves induce a long action potential duration (APD) followed by a short APD. Thus, if these events are not synchronized between cells, then they will on average cancel and have minimal effects on the APD in tissue. Using our computational model, we identify two distinct mechanisms for triggered wave synchronization. The first relies on cycle length (CL) variability, which can prolong the CL at a given beat. In cardiac tissue, we show that CL prolongation leads to a substantial amplification of APD because of the synchronization of triggered waves. A second synchronization mechanism applies in a parameter regime in which the cell exhibits stochastic alternans in which a triggered wave fires, on average, only every other beat. In this scenario, we identify a slow synchronization mechanism that relies on the bidirectional feedback between the APD in tissue and triggered wave initiation. On large cables, this synchronization mechanism leads to spatially discordant APD alternans with spatial variations on a scale of hundreds of cells. We argue that these spatial patterns can potentially serve as an arrhythmogenic substrate for the initiation of atrial fibrillation.

Interventricular differences in sodium current and its potential role in Brugada syndrome.

Brugada syndrome (BrS) is an inherited disease associated with ST elevation in the right precordial leads, polymorphic ventricular tachycardia (PVT), and sudden cardiac death in adults. Mutations in the cardiac sodium channel account for a large fraction of BrS cases. BrS manifests in the right ventricle (RV), which led us to examine the biophysical and molecular properties of sodium channel in myocytes isolated from the left (LV) and right ventricle. Patch clamp was used to record sodium current (INa ) in single canine RV and LV epicardial (epi) and endocardial (endo) myocytes. Action potentials were recorded from multicellular preparations and single cells. mRNA and proteins were determined using quantitative RT-PCR and Western blot. Although LV wedge preparations were thicker than RV wedges, transmural ECG recordings showed no difference in the width of the QRS complex or transmural conduction time. Action potential characteristics showed RV epi and endo had a lower Vmax compared with LV epi and endo cells. Peak INa density was significantly lower in epi and endo RV cells compared with epi and endo LV cells. Recovery from inactivation of INa in RV cells was slightly faster and half maximal steady-state inactivation was more positive. ß2 and ß4 mRNA was detected at very low levels in both ventricles, which was confirmed at the protein level. Our observations demonstrate that Vmax and Na+ current are smaller in RV, presumably due to differential Nav 1.5/ß subunit expression. These results provide a potential mechanism for the right ventricular manifestation of BrS.

T-tubule remodeling and increased heterogeneity of calcium release during the progression to heart failure in intact rat ventricle.

A highly organized transverse-tubule (TT) system is essential to normal Ca2+ cycling and cardiac function. We explored the relationship between the progressive disruption of TTs and resulting Ca2+ cycling during the development of heart failure (HF). Confocal imaging was used to measure Ca2+ transients and 2-D z-stack images in left ventricular epicardial myocytes of intact hearts from spontaneously hypertensive rats (SHR) and Wistar-Kyoto control rats. TT organization was measured as the organizational index (OI) derived from a fast Fourier transform of TT organization. We found little decrease in the synchrony of Ca2+ release with TT loss until TT remodeling was severe, suggesting a TT "reserve" characterized by a wide range of TT remodeling with little effect on synchrony of release but beyond which variability in release shows an accelerating sensitivity to TT loss. To explain this observation, we applied a computational model of spatially distributed Ca2+ signaling units to investigate the relationship between OI and excitation-contraction coupling. Our model showed that release heterogeneity exhibits a nonlinear relationship on both the spatial distribution of release units and the separation between L-type Ca2+ channels and ryanodine receptors. Our results demonstrate a unique relationship between the synchrony of Ca2+ release and TT organization in myocytes of intact rat ventricle that may contribute to both the compensated and decompensated phases of heart failure.

Triggered intracellular calcium waves in dog and human left atrial myocytes from normal and failing hearts.

AIMS: Abnormal intracellular Ca2+ cycling contributes to triggered activity and arrhythmias in the heart. We investigated the properties and underlying mechanisms for systolic triggered Ca2+ waves in left atria from normal and failing dog hearts. METHODS AND RESULTS: Intracellular Ca2+ cycling was studied using confocal microscopy during rapid pacing of atrial myocytes (36 °C) isolated from normal and failing canine hearts (ventricular tachypacing model). In normal atrial myocytes (NAMs), Ca2+ waves developed during rapid pacing at rates ≥ 3.3 Hz and immediately disappeared upon cessation of pacing despite high sarcoplasmic reticulum (SR) load. In heart failure atrial myocytes (HFAMs), triggered Ca2+ waves (TCWs) developed at a higher incidence at slower rates. Because of their timing, TCW development relies upon action potential (AP)-evoked Ca2+ entry. The distribution of Ca2+ wave latencies indicated two populations of waves, with early events representing TCWs and late events representing conventional spontaneous Ca2+ waves. Latency analysis also demonstrated that TCWs arise after junctional Ca2+ release has occurred and spread to non-junctional (cell core) SR. TCWs also occurred in intact dog atrium and in myocytes from humans and pigs. ß-adrenergic stimulation increased Ca2+ release and abolished TCWs in NAMs but was ineffective in HFAMs making this a potentially effective adaptive mechanism in normals but potentially arrhythmogenic in HF. Block of Ca-calmodulin kinase II also abolished TCWs, suggesting a role in TCW formation. Pharmacological manoeuvres that increased Ca2+ release suppressed TCWs as did interventions that decreased Ca2+ release but these also severely reduced excitation-contraction coupling. CONCLUSION: TCWs develop during the atrial AP and thus could affect AP duration, producing repolarization gradients and creating a substrate for reentry, particularly in HF where they develop at slower rates and a higher incidence. TCWs may represent a mechanism for the initiation of atrial fibrillation particularly in HF.

Mechanism for Triggered Waves in Atrial Myocytes.

Excitation-contraction coupling in atrial cells is mediated by calcium (Ca) signaling between L-type Ca channels and Ryanodine receptors that occurs mainly at the cell boundary. This unique architecture dictates essential aspects of Ca signaling under both normal and diseased conditions. In this study we apply laser scanning confocal microscopy, along with an experimentally based computational model, to understand the Ca cycling dynamics of an atrial cell subjected to rapid pacing. Our main finding is that when an atrial cell is paced under Ca overload conditions, Ca waves can then nucleate on the cell boundary and propagate to the cell interior. These propagating Ca waves are referred to as "triggered waves" because they are initiated by L-type Ca channel openings during the action potential. These excitations are distinct from spontaneous Ca waves originating from random fluctuations of Ryanodine receptor channels, and which occur after much longer waiting times. Furthermore, we argue that the onset of these triggered waves is a highly nonlinear function of the sarcoplasmic reticulum Ca load. This strong nonlinearity leads to aperiodic response of Ca at rapid pacing rates that is caused by the complex interplay between paced Ca release and triggered waves. We argue further that this feature of atrial cells leads to dynamic instabilities that may underlie atrial arrhythmias. These studies will serve as a starting point to explore the nonlinear dynamics of atrial cells and will yield insights into the trigger and maintenance of atrial fibrillation.

Biophysical and molecular comparison of sodium current in cells isolated from canine atria and pulmonary vein.

The collar of the pulmonary vein (PV) is the focal point for the initiation of atrial arrhythmias, but the mechanisms underlying how PV cells differ from neighboring left atrial tissue are unclear. We examined the biophysical and molecular properties of INa in cells isolated from the canine pulmonary sleeve and compared the properties to left atrial tissue. PV and left atrial myocytes were isolated and patch clamp techniques were used to record INa. Action potential recordings from either tissue type were made using high-resistance electrodes. mRNA was determined using quantitative RT-PCR and proteins were determined by Western blot. Analysis of the action potential characteristics showed that PV tissue had a lower Vmax compared with left atrial tissue. Fast INa showed that current density was slightly lower in PV cells compared with LA cells (-96 ± 18.7 pA/pF vs. -120 ± 6.7 pA/pF, respectively, p < 0.05). The recovery from inactivation of INa in PV cells was slightly slower but no marked difference in steady-state inactivation was noted. Analysis of late INa during a 225-ms pulse showed that late INa was significantly smaller in PV cells compared to LA cells at all measured time points into the pulse. These results suggest PV cells have lower density of both peak and late INa. Molecular analysis of Nav1.5 and the four beta subunits showed lower levels of Nav1.5 as well as Navß1 subunits, confirming the biophysical findings. These data show that a lower density of INa may lead to depression of excitability and predispose the PV collar to re-entrant circuits under pathophysiological conditions.

Regional distribution of T-tubule density in left and right atria in dogs.

BACKGROUND: The peculiarities of transverse tubule (T-tubule) morphology and distribution in the atrium-and how they contribute to excitation-contraction coupling-are just beginning to be understood. OBJECTIVES: The objectives of this study were to determine T-tubule density in the intact, live right and left atria in a large animal and to determine intraregional differences in T-tubule organization within each atrium. METHODS: Using confocal microscopy, T-tubules were imaged in both atria in intact, Langendorf-perfused normal dog hearts loaded with di-4-ANEPPS. T-tubules were imaged in large populations of myocytes from the endocardial surface of each atrium. Computerized data analysis was performed using a new MatLab (Mathworks, Natick, MA) routine, AutoTT. RESULTS: There was a large percentage of myocytes that had no T-tubules in both atria with a higher percentage in the right atrium (25.1%) than in the left atrium (12.5%) (P < .02). The density of transverse and longitudinal T-tubule elements was low in cells that did contain T-tubules, but there were no significant differences in density between the left atrial appendage, the pulmonary vein-posterior left atrium, the right atrial appendage, and the right atrial free wall. In contrast, there were significant differences in sarcomere spacing and cell width between different regions of the atria. CONCLUSION: There is a sparse T-tubule network in atrial myocytes throughout both dog atria, with significant numbers of myocytes in both atria-the right atrium more so than the left atrium-having no T-tubules at all. These regional differences in T-tubule distribution, along with differences in cell width and sarcomere spacing, may have implications for the emergence of substrate for atrial fibrillation.

Constitutive Expression of a Dominant-Negative TGF-ß Type II Receptor in the Posterior Left Atrium Leads to Beneficial Remodeling of Atrial Fibrillation Substrate.

RATIONALE: Fibrosis is an important structural contributor to formation of atrial fibrillation (AF) substrate in heart failure. Transforming growth factor-ß (TGF-ß) signaling is thought to be intricately involved in creation of atrial fibrosis. OBJECTIVE: We hypothesized that gene-based expression of dominant-negative type II TGF-ß receptor (TGF-ß-RII-DN) in the posterior left atrium in a canine heart failure model will sufficiently attenuate fibrosis-induced changes in atrial conduction and restitution to decrease AF. Because AF electrograms are thought to reflect AF substrate, we further hypothesized that TGF-ß-RII-DN would lead to increased fractionation and decreased organization of AF electrograms. METHODS AND RESULTS: Twenty-one dogs underwent injection+electroporation in the posterior left atrium of plasmid expressing a dominant-negative TGF-ß type II receptor (pUBc-TGFß-DN-RII; n=9) or control vector (pUBc-LacZ; n=12), followed by 3 to 4 weeks of right ventricular tachypacing (240 bpm). Compared with controls, dogs treated with pUBC-TGFß-DN-RII demonstrated an attenuated increase in conduction inhomogeneity, flattening of restitution slope and decreased duration of induced AF, with AF electrograms being more fractionated and less organized in pUBc-TGFß-DN-RII versus pUBc-LacZ dogs. Tissue analysis revealed a significant decrease in replacement/interstitial fibrosis, p-SMAD2/3 and p-ERK1/2. CONCLUSIONS: Targeted gene-based reduction of TGF-ß signaling in the posterior left atrium-with resulting decrease in replacement fibrosis-led to beneficial remodeling of both conduction and restitution characteristics of the posterior left atrium, translating into a decrease in AF and increased complexity of AF electrograms. In addition to providing mechanistic insights, this data may have important diagnostic and therapeutic implications for AF.

Searching for "order" in atrial fibrillation using electrogram morphology recurrence plots.

BACKGROUND: Bipolar electrograms recorded during atrial fibrillation (AF) can have an appearance of chaotic/random behavior. The aim of this study was to use a novel electrogram morphology recurrence (EMR) analysis to quantify the level of order in the morphology patterns in AF. METHODS: Rapid atrial pacing was performed in seven dogs at 600bpm for 3 weeks leading to sustained AF. Open chest high density electrical recordings were made in multiple atrial sites. EMR plots of bipolar electrograms at each site were created by cross-correlating morphologies of each detected activations with morphologies of every other activation. The following features of the EMR plots were quantified: recurrence rate (RR), determinism (DET), laminarity (LAM), average diagonal line length (L), trapping time (TT), divergence (DIV), and Shannon׳s entropy (ENTR). For each recording site, these measures were calculated for the normal sequence of morphologies and also after random shuffling of the electrogram orders. RESULTS: Electrograms recordings from a total of 3961 sites had average cycle lengths of 104±22ms resulting in an average of 100±19 activations detected per 10-s recording and an average RR of 0.38±0.28 (range 0.02-1.00). Shuffling the order of the activation morphologies resulted in significant decreases in DET, LAM, L, TT, and ENTR and significant increases in DIV. CONCLUSIONS: EMR plots of AF electrograms show varying rates of recurrence with patterns that suggest an underlying deterministic structure to the activation sequences. A better understanding of AF dynamics could lead to improved methods in mapping and treating AF.

Preservation of cardiac function by prolonged action potentials in mice deficient of KChIP2.

Inherited ion channelopathies and electrical remodeling in heart disease alter the cardiac action potential with important consequences for excitation-contraction coupling. Potassium channel-interacting protein 2 (KChIP2) is reduced in heart failure and interacts under physiological conditions with both Kv4 to conduct the fast-recovering transient outward K(+) current (Ito,f) and with CaV1.2 to mediate the inward L-type Ca(2+) current (ICa,L). Anesthetized KChIP2(-/-) mice have normal cardiac contraction despite the lower ICa,L, and we hypothesized that the delayed repolarization could contribute to the preservation of contractile function. Detailed analysis of current kinetics shows that only ICa,L density is reduced, and immunoblots demonstrate unaltered CaV1.2 and CaVß2 protein levels. Computer modeling suggests that delayed repolarization would prolong the period of Ca(2+) entry into the cell, thereby augmenting Ca(2+)-induced Ca(2+) release. Ca(2+) transients in disaggregated KChIP2(-/-) cardiomyocytes are indeed comparable to wild-type transients, corroborating the preserved contractile function and suggesting that the compensatory mechanism lies in the Ca(2+)-induced Ca(2+) release event. We next functionally probed dyad structure, ryanodine receptor Ca(2+) sensitivity, and sarcoplasmic reticulum Ca(2+) load and found that increased temporal synchronicity of the Ca(2+) release in KChIP2(-/-) cardiomyocytes may reflect improved dyad structure aiding the compensatory mechanisms in preserving cardiac contractile force. Thus the bimodal effect of KChIP2 on Ito,f and ICa,L constitutes an important regulatory effect of KChIP2 on cardiac contractility, and we conclude that delayed repolarization and improved dyad structure function together to preserve cardiac contraction in KChIP2(-/-) mice.

Ultrastructural and cellular basis for the development of abnormal myocardial mechanics during the transition from hypertension to heart failure.

Although the development of abnormal myocardial mechanics represents a key step during the transition from hypertension to overt heart failure (HF), the underlying ultrastructural and cellular basis of abnormal myocardial mechanics remains unclear. We therefore investigated how changes in transverse (T)-tubule organization and the resulting altered intracellular Ca(2+) cycling in large cell populations underlie the development of abnormal myocardial mechanics in a model of chronic hypertension. Hearts from spontaneously hypertensive rats (SHRs; n = 72) were studied at different ages and stages of hypertensive heart disease and early HF and were compared with age-matched control (Wistar-Kyoto) rats (n = 34). Echocardiography, including tissue Doppler and speckle-tracking analysis, was performed just before euthanization, after which T-tubule organization and Ca(2+) transients were studied using confocal microscopy. In SHRs, abnormalities in myocardial mechanics occurred early in response to hypertension, before the development of overt systolic dysfunction and HF. Reduced longitudinal, circumferential, and radial strain as well as reduced tissue Doppler early diastolic tissue velocities occurred in concert with T-tubule disorganization and impaired Ca(2+) cycling, all of which preceded the development of cardiac fibrosis. The time to peak of intracellular Ca(2+) transients was slowed due to T-tubule disruption, providing a link between declining cell ultrastructure and abnormal myocardial mechanics. In conclusion, subclinical abnormalities in myocardial mechanics occur early in response to hypertension and coincide with the development of T-tubule disorganization and impaired intracellular Ca(2+) cycling. These changes occur before the development of significant cardiac fibrosis and precede the development of overt cardiac dysfunction and HF.

Inhibition of the late sodium current slows t-tubule disruption during the progression of hypertensive heart disease in the rat.

The treatment of heart failure (HF) is challenging and morbidity and mortality are high. The goal of this study was to determine if inhibition of the late Na(+) current with ranolazine during early hypertensive heart disease might slow or stop disease progression. Spontaneously hypertensive rats (aged 7 mo) were subjected to echocardiographic study and then fed either control chow (CON) or chow containing 0.5% ranolazine (RAN) for 3 mo. Animals were then restudied, and each heart was removed for measurements of t-tubule organization and Ca(2+) transients using confocal microscopy of the intact heart. RAN halted left ventricular hypertrophy as determined from both echocardiographic and cell dimension (length but not width) measurements. RAN reduced the number of myocytes with t-tubule disruption and the proportion of myocytes with defects in intracellular Ca(2+) cycling. RAN also prevented the slowing of the rate of restitution of Ca(2+) release and the increased vulnerability to rate-induced Ca(2+) alternans. Differences between CON- and RAN-treated animals were not a result of different expression levels of voltage-dependent Ca(2+) channel 1.2, sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a, ryanodine receptor type 2, Na(+)/Ca(2+) exchanger-1, or voltage-gated Na(+) channel 1.5. Furthermore, myocytes with defective Ca(2+) transients in CON rats showed improved Ca(2+) cycling immediately upon acute exposure to RAN. Increased late Na(+) current likely plays a role in the progression of cardiac hypertrophy, a key pathological step in the development of HF. Early, chronic inhibition of this current slows both hypertrophy and development of ultrastructural and physiological defects associated with the progression to HF.

Can triggered arrhythmias arise from propagation of calcium waves between cardiac myocytes?

Intracellular Ca2+ overload can induce regenerative Ca2+ waves that activate inward current in cardiac myocytes, allowing the cell membrane to achieve threshold. The result is a triggered extrasystole that can, under the right conditions, lead to sustained triggered arrhythmias. In this review, we consider the issue of whether or not Ca2+ waves can travel between neighboring myocytes and if this intercellular Ca2+ diffusion can involve enough cells over a short enough period of time to actually induce triggered activity in the heart. This review is not intended to serve as an exhaustive review of the literature summarizing Ca2+ flux through cardiac gap junctions or of how Ca2+ waves move from cell to cell. Rather, it summarizes many of the pertinent experimental studies and considers their results in the theoretical context of whether or not the intercellular propagation of Ca2+ overload can contribute to triggered beats and arrhythmias in the intact heart.