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.

ATP-binding cassette B10 regulates early steps of heme synthesis

RATIONALE

Heme plays a critical role in gas exchange, mitochondrial energy production, and antioxidant defense in cardiovascular system. The mitochondrial transporter ATP-binding cassette (ABC) B10 has been suggested to export heme out of the mitochondria and is required for normal hemoglobinization of erythropoietic cells and protection against ischemia-reperfusion injury in the heart; however, its primary function has not been established.

OBJECTIVE

The aim of this study was to identify the function of ABCB10 in heme synthesis in cardiac cells.

METHODS AND RESULTS

Knockdown of ABCB10 in cardiac myoblasts significantly reduced heme levels and the activities of heme-containing proteins, whereas supplementation with δ-aminolevulinic acid reversed these defects. Overexpression of mitochondrial δ-aminolevulinic acid synthase 2, the rate-limiting enzyme upstream of δ-aminolevulinic acid export, failed to restore heme levels in cells with ABCB10 downregulation. ABCB10 and heme levels were increased by hypoxia, and reversal of ABCB10 upregulation caused oxidative stress and cell death. Furthermore, ABCB10 knockdown in neonatal rat cardiomyocytes resulted in a significant delay of calcium removal from the cytoplasm, suggesting a relaxation defect. Finally, ABCB10 expression and heme levels were altered in failing human hearts and mice with ischemic cardiomyopathy.

CONCLUSIONS

ABCB10 plays a critical role in heme synthesis pathway by facilitating δ-aminolevulinic acid production or export from the mitochondria. In contrast to previous reports, we show that ABCB10 is not a heme exporter and instead is required for the early mitochondrial steps of heme biosynthesis.

Contribution of fibrosis and the autonomic nervous system to atrial fibrillation electrograms in heart failure

BACKGROUND

Fibrotic and autonomic remodeling in heart failure (HF) increase vulnerability to atrial fibrillation (AF). Because AF electrograms (EGMs) are thought to reflect the underlying structural substrate, we sought to (1) determine the differences in AF EGMs in normal versus HF atria and (2) assess how fibrosis and nerve-rich fat contribute to AF EGM characteristics in HF.

METHODS AND RESULTS

AF was induced in 20 normal dogs by vagal stimulation and in 21 HF dogs (subjected to 3 weeks of rapid ventricular pacing at 240 beats per minute). AF EGMs were analyzed for dominant frequency (DF), organization index, fractionation intervals (FIs), and Shannon entropy. In 8 HF dogs, AF EGM correlation with underlying fibrosis/fat/nerves was assessed. In HF compared with normal dogs, DF was lower and organization index/FI/Shannon entropy were greater. DF/FI were more heterogeneous in HF. Percentage fat was greater, and fibrosis and fat were more heterogeneously distributed in the posterior left atrium than in the left atrial appendage. DF/organization index correlated closely with %fibrosis. Heterogeneity of DF/FI correlated with the heterogeneity of fibrosis. Autonomic blockade caused a greater change in DF/FI/Shannon entropy in the posterior left atrium than left atrial appendage, with the decrease in Shannon entropy correlating with %fat.

CONCLUSIONS

The amount and distribution of fibrosis in the HF atrium seems to contribute to slowing and increased organization of AF EGMs, whereas the nerve-rich fat in the HF posterior left atrium is positively correlated with AF EGM entropy. By allowing for improved detection of regions of dense fibrosis and high autonomic nerve density in the HF atrium, these findings may help enhance the precision and success of substrate-guided ablation for AF.

Targeted nonviral gene-based inhibition of Gα(i/o)-mediated vagal signaling in the posterior left atrium decreases vagal-induced atrial fibrillation

BACKGROUND

Pharmacologic and ablative therapies for atrial fibrillation (AF) have suboptimal efficacy. Newer gene-based approaches that target specific mechanisms underlying AF are likely to be more efficacious in treating AF. Parasympathetic signaling appears to be an important contributor to AF substrate.

OBJECTIVE

The purpose of this study was to develop a nonviral gene-based strategy to selectively inhibit vagal signaling in the left atrium and thereby suppress vagal-induced AF.

METHODS

In eight dogs, plasmid DNA vectors (minigenes) expressing Gα(i) C-terminal peptide (Gα(i)ctp) was injected in the posterior left atrium either alone or in combination with minigene expressing Gα(o)ctp, followed by electroporation. In five control dogs, minigene expressing scrambled peptide (Gα(R)ctp) was injected. Vagal- and carbachol-induced left atrial effective refractory periods (ERPs), AF inducibility, and Gα(i/o)ctp expression were assessed 3 days following minigene delivery.

RESULTS

Vagal stimulation- and carbachol-induced effective refractory period shortening and AF inducibility were significantly attenuated in atria receiving a Gα(i2)ctp-expressing minigene and were nearly eliminated in atria receiving both Gα(i2)ctp- and Gα(o1)ctp-expressing minigenes.

CONCLUSION

Inhibition of both G(i) and G(o) proteins is necessary to abrogate vagal-induced AF in the left atrium and can be achieved via constitutive expression of Gα(i/o)ctps expressed by nonviral plasmid vectors delivered to the posterior left atrium.

Arrhythmia triggers in heart failure: the smoking gun of [Ca2+]i dysregulation

Among the most serious problems associated with heart failure is the increased likelihood of life-threatening arrhythmias. Both triggered and reentrant arrhythmias in heart failure may arise as a result of aberrant intracellular Ca cycling. This article presents some new ideas, based on recent studies, about how altered Ca cycling in heart failure might serve as the cellular basis for arrhythmogenesis.

Autonomic remodeling in the left atrium and pulmonary veins in heart failure: creation of a dynamic substrate for atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is commonly associated with congestive heart failure (CHF). The autonomic nervous system is involved in the pathogenesis of both AF and CHF. We examined the role of autonomic remodeling in contributing to AF substrate in CHF.

METHODS AND RESULTS

Electrophysiological mapping was performed in the pulmonary veins and left atrium in 38 rapid ventricular-paced dogs (CHF group) and 39 control dogs under the following conditions: vagal stimulation, isoproterenol infusion, β-adrenergic blockade, acetylcholinesterase (AChE) inhibition (physostigmine), parasympathetic blockade, and double autonomic blockade. Explanted atria were examined for nerve density/distribution, muscarinic receptor and β-adrenergic receptor densities, and AChE activity. In CHF dogs, there was an increase in nerve bundle size, parasympathetic fibers/bundle, and density of sympathetic fibrils and cardiac ganglia, all preferentially in the posterior left atrium/pulmonary veins. Sympathetic hyperinnervation was accompanied by increases in β(1)-adrenergic receptor R density and in sympathetic effect on effective refractory periods and activation direction. β-Adrenergic blockade slowed AF dominant frequency. Parasympathetic remodeling was more complex, resulting in increased AChE activity, unchanged muscarinic receptor density, unchanged parasympathetic effect on activation direction and decreased effect of vagal stimulation on effective refractory period (restored by AChE inhibition). Parasympathetic blockade markedly decreased AF duration.

CONCLUSIONS

In this heart failure model, autonomic and electrophysiological remodeling occurs, involving the posterior left atrium and pulmonary veins. Despite synaptic compensation, parasympathetic hyperinnervation contributes significantly to AF maintenance. Parasympathetic and/or sympathetic signaling may be possible therapeutic targets for AF in CHF.

A mathematical model of spontaneous calcium release in cardiac myocytes

In cardiac myocytes, calcium (Ca) can be released from the sarcoplasmic reticulum independently of Ca influx from voltage-dependent membrane channels. This efflux of Ca, referred to as spontaneous Ca release (SCR), is due to Ryanodine receptor fluctuations, which can induce spontaneous Ca sparks, which propagate to form Ca waves. This release of Ca can then induce delayed after-depolarizations (DADs), which can lead to arrhythmogenic-triggered activity in the heart. However, despite its importance, to date there is no mathematical model of SCR that accounts for experimentally observed features of subcellular Ca. In this article, we present an experimentally based model of SCR that reproduces the timing distribution of spontaneous Ca sparks and key features of the propagation of Ca waves emanating from these spontaneous sparks. We have coupled this model to an ionic model for the rabbit ventricular action potential to simulate SCR within several thousand cells in cardiac tissue. We implement this model to study the formation of an ectopic beat on a cable of cells that exhibit SCR-induced DADs.

Reduction in hexokinase II levels results in decreased cardiac function and altered remodeling after ischemia/reperfusion injury

RATIONALE

Cardiomyocytes switch substrate utilization from fatty acid to glucose under ischemic conditions; however, it is unknown how perturbations in glycolytic enzymes affect cardiac response to ischemia/reperfusion (I/R). Hexokinase (HK)II is a HK isoform that is expressed in the heart and can bind to the mitochondrial outer membrane.

OBJECTIVE

We sought to define how HKII and its binding to mitochondria play a role in cardiac response and remodeling after I/R.

METHODS AND RESULTS

We first showed that HKII levels and its binding to mitochondria are reduced 2 days after I/R. We then subjected the hearts of wild-type and heterozygote HKII knockout (HKII(+/)⁻) mice to I/R by coronary ligation. At baseline, HKII(+/)⁻ mice have normal cardiac function; however, they display lower systolic function after I/R compared to wild-type animals. The mechanism appears to be through an increase in cardiomyocyte death and fibrosis and a reduction in angiogenesis; the latter is through a decrease in hypoxia-inducible factor-dependent pathway signaling in cardiomyocytes. HKII mitochondrial binding is also critical for cardiomyocyte survival, because its displacement in tissue culture with a synthetic peptide increases cell death. Our results also suggest that HKII may be important for the remodeling of the viable cardiac tissue because its modulation in vitro alters cellular energy levels, O₂ consumption, and contractility.

CONCLUSIONS

These results suggest that reduction in HKII levels causes altered remodeling of the heart in I/R by increasing cell death and fibrosis and reducing angiogenesis and that mitochondrial binding is needed for protection of cardiomyocytes.

Early development of intracellular calcium cycling defects in intact hearts of spontaneously hypertensive rats

Defects in excitation-contraction coupling have been reported in failing hearts, but little is known about the relationship between these defects and the development of heart failure (HF). We compared the early changes in intracellular Ca(2+) cycling to those that underlie overt pump dysfunction and arrhythmogenesis found later in HF. Laser-scanning confocal microscopy was used to measure Ca(2+) transients in myocytes of intact hearts in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) at different ages. Early compensatory mechanisms include a positive inotropic effect in SHRs at 7.5-9 mo compared with 6 mo. Ca(2+) transient duration increased at 9 mo in SHRs, indicating changes in Ca(2+) reuptake during decompensation. Cell-to-cell variability in Ca(2+) transient duration increased at 7.5 mo, decreased at 9 mo, and increased again at 22 mo (overt HF), indicating extensive intercellular variability in Ca(2+) transient kinetics during disease progression. Vulnerability to intercellular concordant Ca(2+) alternans increased at 9-22 mo in SHRs and was mirrored by a slowing in Ca(2+) transient restitution, suggesting that repolarization alternans and the resulting repolarization gradients might promote reentrant arrhythmias early in disease development. Intercellular discordant and subcellular Ca(2+) alternans increased as early as 7.5 mo in SHRs and may also promote arrhythmias during the compensated phase. The incidence of spontaneous and triggered Ca(2+) waves was increased in SHRs at all ages, suggesting a higher likelihood of triggered arrhythmias in SHRs compared with WKY rats well before HF develops. Thus serious and progressive defects in Ca(2+) cycling develop in SHRs long before symptoms of HF occur. Defective Ca(2+) cycling develops early and affects a small number of myocytes, and this number grows with age and causes the transition from asymptomatic to overt HF. These defects may also underlie the progressive susceptibility to Ca(2+) alternans and Ca(2+) wave activity, thus increasing the propensity for arrhythmogenesis in HF.

Variability in timing of spontaneous calcium release in the intact rat heart is determined by the time course of sarcoplasmic reticulum calcium load

BACKGROUND

Abnormalities in intracellular calcium (Ca) cycling during Ca overload can cause triggered activity because spontaneous calcium release (SCR) activates sufficient Ca-sensitive inward currents to induce delayed afterdepolarizations (DADs). However, little is known about the mechanisms relating SCR and triggered activity on the tissue scale.

METHODS AND RESULTS

Laser scanning confocal microscopy was used to measure the spatiotemporal properties of SCR within large myocyte populations in intact rat heart. Computer simulations were used to predict how these properties of SCR determine DAD magnitude. We measured the average and standard deviation of the latency distribution of SCR within a large population of myocytes in intact tissue. We found that as external [Ca] is increased, and with faster pacing rates, the average and SD of the latency distribution decreases substantially. This result demonstrates that the timing of SCR occurs with less variability as the sarcoplasmic reticulum (SR) Ca load is increased, causing more sites to release Ca within each cell. We then applied a mathematical model of subcellular Ca cycling to show that a decrease in SCR variability leads to a higher DAD amplitude and is dictated by the rate of SR Ca refilling following an action potential.

CONCLUSIONS

Our results demonstrate that the variability of the timing of SCR in a population of cells in tissue decreases with SR load and is dictated by the time course of the SR Ca content.

Embryonic stem cells overexpressing Pitx2c engraft in infarcted myocardium and improve cardiac function

This study investigated the effects on cardiomyocyte differentiation of embryonic stem cells by the overexpression of the transcription factor, Pitx2c, and examined the effects of transplantation of these differentiated cells on cardiac function in a mouse model of myocardial infarction. Pitx2c overexpressing embryonic stem cells were characterized for cardiac differentiation by immunocytochemistry, RNA analysis, and electrophysiology. Differentiated cells were transplanted by directed injection into the infarcted murine myocardium and functional measurements of blood pressure, contractility, and relaxation were performed. Histochemistry and FISH analysis performed on these mice confirmed the engraftment and cardiac nature of the transplanted cells. Pitx2c overexpressing embryonic stem cells robustly differentiated into spontaneously contracting cells which acquired cardiac protein markers and exhibited action potentials resembling that of cardiomyocytes. These cells could also be synchronized to an external pacemaker. Significant improvements (P < 0.01) in blood pressure (56%), contractility (57%), and relaxation (59%) were observed in infarcted mice with transplants of these differentiated cells but not in mice which were transplanted with control cells. The Pitx2c overexpressing cells secrete paracrine factors which when adsorbed onto a heparinated gel and injected into the infarcted myocardium produce a comparable and significant (P < 0.01) functional recovery. Pitx2c overexpression is a valuable method for producing cardiomyocytes from embryonic stem cells, and transplantation of these cardiomyocytes into infracted myocardium restores cardiac function through multiple mechanisms.

Ranolazine antagonizes the effects of increased late sodium current on intracellular calcium cycling in rat isolated intact heart

Pathological conditions, including ischemia and heart failure, are associated with altered sodium channel function and increased late sodium current (I(Na,L)), leading to prolonged action potential duration, increased intracellular sodium and calcium concentrations, and arrhythmias. We used anemone toxin (ATX)-II to study the effects of increasing I(Na,L) on intracellular calcium cycling in rat isolated hearts. Cardiac contraction was abolished using paralytic agents. Ranolazine (RAN) was used to inhibit late I(Na). Hearts were loaded with fluo-4-acetoxymethyl ester, and myocyte intracellular calcium transients (CaTs) were measured using laser scanning confocal microscopy. ATX (1 nM) prolonged CaT duration at 50% recovery in hearts paced at a basal rate of 2 Hz and increased the sensitivity of the heart to the development of calcium alternans caused by fast pacing. ATX increased the time required for recovery of CaT amplitude following a previous beat, and ATX induced spontaneous calcium release waves during rapid pacing of the heart. ATX prolonged the duration of repolarization from the initiation of the activation to terminal repolarization in the pseudo-electrocardiogram. All actions of ATX were both reversed and prevented by subsequent or prior exposure, respectively, of hearts to RAN (10 microM). Most importantly, the increased vulnerability of the heart to the development of calcium alternans during rapid pacing was reversed or prevented by 10 microM RAN. These results suggest that enhancement of I(Na,L) alters calcium cycling. Reduction by RAN of I(Na,L)-induced dysregulation of calcium cycling could contribute to the antiarrhythmic actions of this agent in both reentrant and triggered arrhythmias.