Metabolic Interventions to Prevent Hypertrophy-Induced Alterations in Contractile Properties In Vitro

(1) Background: The exact mechanism(s) underlying pathological changes in a heart in transition to hypertrophy and failure are not yet fully understood. However, alterations in cardiac energy metabolism seem to be an important contributor. We characterized an in vitro model of adrenergic stimulation-induced cardiac hypertrophy for studying metabolic, structural, and functional changes over time. Accordingly, we investigated whether metabolic interventions prevent cardiac structural and functional changes; (2) Methods: Primary rat cardiomyocytes were treated with phenylephrine (PE) for 16 h, 24 h, or 48 h, whereafter hypertrophic marker expression, protein synthesis rate, glucose uptake, and contractile function were assessed; (3) Results: 24 h PE treatment increased expression of hypertrophic markers, phosphorylation of hypertrophy-related signaling kinases, protein synthesis, and glucose uptake. Importantly, the increased glucose uptake preceded structural and functional changes, suggesting a causal role for metabolism in the onset of PE-induced hypertrophy. Indeed, PE treatment in the presence of a PAN-Akt inhibitor or of a GLUT4 inhibitor dipyridamole prevented PE-induced increases in cellular glucose uptake and ameliorated PE-induced contractile alterations; (4) Conclusions: Pharmacological interventions, forcing substrate metabolism away from glucose utilization, improved contractile properties in PE-treated cardiomyocytes, suggesting that targeting glucose uptake, independent from protein synthesis, forms a promising strategy to prevent hypertrophy and hypertrophy-induced cardiac dysfunction.

Cellular contribution to left and right atrial dysfunction in chronic arterial hypertension in pigs

Aims

Atrial contractile dysfunction contributes to worse prognosis in hypertensive heart disease (HHD), but the role of cardiomyocyte dysfunction in atrial remodelling in HHD is not well understood. We investigated and compared cellular mechanisms of left (LA) and right atrial (RA) contractile dysfunction in pigs with HHD.

Methods and results

In vivo electrophysiological and magnetic resonance imaging studies were performed in control and pigs treated with 11‐deoxycorticosterone acetate (DOCA)/high‐salt/glucose diet (12 weeks) to induce HHD. HHD leads to significant atrial remodelling and loss of contractile function in LA and a similar trend in RA (magnetic resonance imaging). Atrial remodelling was associated with a higher inducibility of atrial fibrillation but unrelated to changes in atrial refractory period or fibrosis (histology). Reduced atrial function in DOCA pigs was related to reduced contraction amplitude of isolated LA (already at baseline) and RA myocytes (at higher frequencies) due to reduced intracellular Ca release (Fura 2‐AM, field stimulation). However, Ca regulation differed in LA and RA cardiomyocytes: LA cardiomyocytes showed reduced sarcoplasmic reticulum (SR) [Ca], whereas in RA, SR [Ca] was unchanged and SR Ca²⁺‐ATPase activity was increased. Sodium–calcium exchanger (NCX) activity was not significantly altered. We used ORM‐10103 (3 μM), a specific NCX inhibitor to improve Ca availability in LA and RA cardiomyocytes from DOCA pigs. Partial inhibition of NCX increased Ca²⁺ transient amplitude and SR Ca in LA, but not RA cells.

Conclusions

In this large animal model of HHD, atrial remodelling in sinus rhythm in vivo was related to differential LA and RA cardiomyocyte dysfunction and Ca signalling. Selective acute inhibition of NCX improved Ca release in diseased LA cardiomyocytes, suggesting a potential therapeutic approach to improve atrial inotropy in HHD.

Augmenting Vacuolar H+-ATPase Function Prevents Cardiomyocytes from Lipid-Overload Induced Dysfunction

The diabetic heart is characterized by a shift in substrate utilization from glucose to lipids, which may ultimately lead to contractile dysfunction. This substrate shift is facilitated by increased translocation of lipid transporter CD36 (SR-B2) from endosomes to the sarcolemma resulting in increased lipid uptake. We previously showed that endosomal retention of CD36 is dependent on the proper functioning of vacuolar H⁺-ATPase (v-ATPase). Excess lipids trigger CD36 translocation through inhibition of v-ATPase function. Conversely, in yeast, glucose availability is known to enhance v-ATPase function, allowing us to hypothesize that glucose availability, via v-ATPase, may internalize CD36 and restore contractile function in lipid-overloaded cardiomyocytes. Increased glucose availability was achieved through (a) high glucose (25 mM) addition to the culture medium or (b) adenoviral overexpression of protein kinase-D1 (a kinase mediating GLUT4 translocation). In HL-1 cardiomyocytes, adult rat and human cardiomyocytes cultured under high-lipid conditions, each treatment stimulated v-ATPase re-assembly, endosomal acidification, endosomal CD36 retention and prevented myocellular lipid accumulation. Additionally, these treatments preserved insulin-stimulated GLUT4 translocation and glucose uptake as well as contractile force. The present findings reveal v-ATPase functions as a key regulator of cardiomyocyte substrate preference and as a novel potential treatment approach for the diabetic heart.

Arrhythmogenic Mechanisms in Heart Failure: Linking β-Adrenergic Stimulation, Stretch, and Calcium

Heart failure (HF) is associated with elevated sympathetic tone and mechanical load. Both systems activate signaling transduction pathways that increase cardiac output, but eventually become part of the disease process itself leading to further worsening of cardiac function. These alterations can adversely contribute to electrical instability, at least in part due to the modulation of Ca²⁺ handling at the level of the single cardiac myocyte. The major aim of this review is to provide a definitive overview of the links and cross talk between β-adrenergic stimulation, mechanical load, and arrhythmogenesis in the setting of HF. We will initially review the role of Ca²⁺ in the induction of both early and delayed afterdepolarizations, the role that β-adrenergic stimulation plays in the initiation of these and how the propensity for these may be altered in HF. We will then go onto reviewing the current data with regards to the link between mechanical load and afterdepolarizations, the associated mechano-sensitivity of the ryanodine receptor and other stretch activated channels that may be associated with HF-associated arrhythmias. Furthermore, we will discuss how alterations in local Ca²⁺ microdomains during the remodeling process associated the HF may contribute to the increased disposition for β-adrenergic or stretch induced arrhythmogenic triggers. Finally, the potential mechanisms linking β-adrenergic stimulation and mechanical stretch will be clarified, with the aim of finding common modalities of arrhythmogenesis that could be targeted by novel therapeutic agents in the setting of HF.

The Subcellular Distribution of Ryanodine Receptors and L-Type Ca2+ Channels Modulates Ca2+-Transient Properties and Spontaneous Ca2+-Release Events in Atrial Cardiomyocytes

Spontaneous Ca²⁺-release events (SCaEs) from the sarcoplasmic reticulum play crucial roles in the initiation of cardiac arrhythmias by promoting triggered activity. However, the subcellular determinants of these SCaEs remain incompletely understood. Structural differences between atrial and ventricular cardiomyocytes, e.g., regarding the density of T-tubular membrane invaginations, may influence cardiomyocyte Ca²⁺-handling and the distribution of cardiac ryanodine receptors (RyR2) has recently been shown to undergo remodeling in atrial fibrillation. These data suggest that the subcellular distribution of Ca²⁺-handling proteins influences proarrhythmic Ca²⁺-handling abnormalities. Here, we employ computational modeling to provide an in-depth analysis of the impact of variations in subcellular RyR2 and L-type Ca²⁺-channel distributions on Ca²⁺-transient properties and SCaEs in a human atrial cardiomyocyte model. We incorporate experimentally observed RyR2 expression patterns and various configurations of axial tubules in a previously published model of the human atrial cardiomyocyte. We identify an increased SCaE incidence for larger heterogeneity in RyR2 expression, in which SCaEs preferentially arise from regions of high local RyR2 expression. Furthermore, we show that the propagation of Ca²⁺ waves is modulated by the distance between RyR2 bands, as well as the presence of experimentally observed RyR2 clusters between bands near the lateral membranes. We also show that incorporation of axial tubules in various amounts and locations reduces Ca²⁺-transient time to peak. Furthermore, selective hyperphosphorylation of RyR2 around axial tubules increases the number of spontaneous waves. Finally, we present a novel model of the human atrial cardiomyocyte with physiological RyR2 and L-type Ca²⁺-channel distributions that reproduces experimentally observed Ca²⁺-handling properties. Taken together, these results significantly enhance our understanding of the structure-function relationship in cardiomyocytes, identifying that RyR2 and L-type Ca²⁺-channel distributions have a major impact on systolic Ca²⁺ transients and SCaEs.

Arterial hypertension drives arrhythmia progression via specific structural remodeling in a porcine model of atrial fibrillation

BACKGROUND

Arterial hypertension (HT) contributes to progression of atrial fibrillation (AF) via unknown mechanisms.

OBJECTIVE

We aimed to characterize electrical and structural changes accounting for increased AF stability in a large animal model of rapid atrial pacing (RAP)-induced AF combined with desoxycorticosterone acetate (DOCA)-induced HT.

METHODS

Eighteen pigs were instrumented with right atrial endocardial pacemaker leads and custom-made pacemakers to induce AF by continuous RAP (600 beats/min). DOCA pellets were subcutaneously implanted in a subgroup of 9 animals (AF+HT group); the other 9 animals served as controls (AF group). Final experiments included electrophysiology studies, endocardial electroanatomic mapping, and high-density mapping with epicardial multielectrode arrays. In addition, 3-dimensional computational modeling was performed.

RESULTS

DOCA implantation led to secondary HT (median [interquartile range] aortic pressure 109.9 [100-137] mm Hg in AF+HT vs 82.2 [79-96] mm Hg in AF; P < .05), increased AF stability (55.6% vs 12.5% of animals with AF episodes lasting >1 hour; P < .05), concentric left ventricular hypertrophy, atrial dilatation (119 ± 31 cm² in AF+HT vs 78 ± 23 cm² in AF; P < .05), and fibrosis. Collagen accumulation in the AF+HT group was mainly found in non-intermyocyte areas (1.62 ± 0.38 cm³ in AF+HT vs 0.96 ± 0.3 cm³ in AF; P < .05). Left and right atrial effective refractory periods, action potential durations, endo- and epicardial conduction velocities, and measures of AF complexity were comparable between the 2 groups. A 3-dimensional computational model confirmed an increase in AF stability observed in the in vivo experiments associated with increased atrial size.

CONCLUSION

In this model of secondary HT, higher AF stability after 2 weeks of RAP is mainly driven by atrial dilatation.

A two dimensional electromechanical model of a cardiomyocyte to assess intra-cellular regional mechanical heterogeneities

Experimental studies on isolated cardiomyocytes from different animal species and human hearts have demonstrated that there are regional differences in the Ca2+ release, Ca2+ decay and sarcomere deformation. Local deformation heterogeneities can occur due to a combination of factors: regional/local differences in Ca2+ release and/or re-uptake, intra-cellular material properties, sarcomere proteins and distribution of the intracellular organelles. To investigate the possible causes of these heterogeneities, we developed a two-dimensional finite-element electromechanical model of a cardiomyocyte that takes into account the experimentally measured local deformation and cytosolic [Ca2+] to locally define the different variables of the constitutive equations describing the electro/mechanical behaviour of the cell. Then, the model was individualised to three different rat cardiac cells. The local [Ca2+] transients were used to define the [Ca2+]-dependent activation functions. The cell-specific local Young's moduli were estimated by solving an inverse problem, minimizing the error between the measured and simulated local deformations along the longitudinal axis of the cell. We found that heterogeneities in the deformation during contraction were determined mainly by the local elasticity rather than the local amount of Ca2+, while in the relaxation phase deformation was mainly influenced by Ca2+ re-uptake. Our electromechanical model was able to successfully estimate the local elasticity along the longitudinal direction in three different cells. In conclusion, our proposed model seems to be a good approximation to assess the heterogeneous intracellular mechanical properties to help in the understanding of the underlying mechanisms of cardiomyocyte dysfunction.

Mechanosensitivity of microdomain calcium signalling in the heart

In cardiac myocytes, calcium (Ca²⁺) signalling is tightly controlled in dedicated microdomains. At the dyad, i.e. the narrow cleft between t-tubules and junctional sarcoplasmic reticulum (SR), many signalling pathways combine to control Ca²⁺-induced Ca²⁺ release during contraction. Local Ca²⁺ gradients also exist in regions where SR and mitochondria are in close contact to regulate energetic demands. Loss of microdomain structures, or dysregulation of local Ca²⁺ fluxes in cardiac disease, is often associated with oxidative stress, contractile dysfunction and arrhythmias. Ca²⁺ signalling at these microdomains is highly mechanosensitive. Recent work has demonstrated that increasing mechanical load triggers rapid local Ca²⁺ releases that are not reflected by changes in global Ca²⁺. Key mechanisms involve rapid mechanotransduction with reactive oxygen species or nitric oxide as primary signalling molecules targeting SR or mitochondria microdomains depending on the nature of the mechanical stimulus. This review summarizes the most recent insights in rapid Ca²⁺ microdomain mechanosensitivity and re-evaluates its (patho)physiological significance in the context of historical data on the macroscopic role of Ca²⁺ in acute force adaptation and mechanically-induced arrhythmias. We distinguish between preload and afterload mediated effects on local Ca²⁺ release, and highlight differences between atrial and ventricular myocytes. Finally, we provide an outlook for further investigation in chronic models of abnormal mechanics (eg post-myocardial infarction, atrial fibrillation), to identify the clinical significance of disturbed Ca²⁺ mechanosensitivity for arrhythmogenesis.

Palmitate-Induced Vacuolar-Type H+-ATPase Inhibition Feeds Forward Into Insulin Resistance and Contractile Dysfunction

Dietary fat overconsumption leads to myocardial lipid accumulation through mechanisms that are incompletely resolved. Previously, we identified increased translocation of the fatty acid transporter CD36 from its endosomal storage compartment to the sarcolemma as the primary mechanism of excessive myocellular lipid import. Here, we show that increased CD36 translocation is caused by alkalinization of endosomes resulting from inhibition of proton pumping activity of vacuolar-type H⁺-ATPase (v-ATPase). Endosomal alkalinization was observed in hearts from rats fed a lard-based high-fat diet and in rodent and human cardiomyocytes upon palmitate overexposure, and appeared as an early lipid-induced event preceding the onset of insulin resistance. Either genetic or pharmacological inhibition of v-ATPase in cardiomyocytes exposed to low palmitate concentrations reduced insulin sensitivity and cardiomyocyte contractility, which was rescued by CD36 silencing. The mechanism of palmitate-induced v-ATPase inhibition involved its dissociation into two parts: the cytosolic V₁ and the integral membrane V₀ subcomplex. Interestingly, oleate also inhibits v-ATPase function, yielding triacylglycerol accumulation but not insulin resistance. In conclusion, lipid oversupply increases CD36-mediated lipid uptake that directly impairs v-ATPase function. This feeds forward to enhanced CD36 translocation and further increased lipid uptake. In the case of palmitate, its accelerated uptake ultimately precipitates into cardiac insulin resistance and contractile dysfunction.

Calcium release near L-type calcium channels promotes beat-to-beat variability in ventricular myocytes from the chronic AV block dog

Beat-to-beat variability of ventricular repolarization (BVR) has been proposed as a strong predictor of Torsades de Pointes (TdP). BVR is also observed at the myocyte level, and a number of studies have shown the importance of calcium handling in influencing this parameter. The chronic AV block (CAVB) dog is a model of TdP arrhythmia in cardiac hypertrophy, and myocytes from these animals show extensive remodeling, including of Ca(2+) handling. This remodeling process also leads to increased BVR. We aimed to determine the role that (local) Ca(2+) handling plays in BVR. In isolated LV myocytes an exponential relationship was observed between BVR magnitude and action potential duration (APD) at baseline. Inhibition of Ca(2+) release from sarcoplasmic reticulum (SR) with thapsigargin resulted in a reduction of [Ca(2+)]i, and of both BVR and APD. Increasing ICaL in the presence of thapsigargin restored APD but BVR remained low. In contrast, increasing ICaL with preserved Ca(2+) release increased both APD and BVR. Inhibition of Ca(2+) release with caffeine, as with thapsigargin, reduced BVR despite maintained APD. Simultaneous inhibition of Na(+)/Ca(2+) exchange and ICaL decreased APD and BVR to similar degrees, whilst increasing diastolic Ca(2+). Buffering of Ca(2+) transients with BAPTA reduced BVR for a given APD to a greater extent than buffering with EGTA, suggesting subsarcolemmal Ca(2+) transients modulated BVR to a larger extent than the cytosolic Ca(2+) transient. In conclusion, BVR in hypertrophied dog myocytes, at any APD, is strongly dependent on SR Ca(2+) release, which may act through modulation of the l-type Ca(2+) current in a subsarcolemmal microdomain.

TRPC3 contributes to regulation of cardiac contractility and arrhythmogenesis by dynamic interaction with NCX1

Aim

TRPC3 is a non-selective cation channel, which forms a Ca²⁺ entry pathway involved in cardiac remodelling. Our aim was to analyse acute electrophysiological and contractile consequences of TRPC3 activation in the heart.

Methods and results

We used a murine model of cardiac TRPC3 overexpression and a novel TRPC3 agonist, GSK1702934A, to uncover (patho)physiological functions of TRPC3. GSK1702934A induced a transient, non-selective conductance and prolonged action potentials in TRPC3-overexpressing myocytes but lacked significant electrophysiological effects in wild-type myocytes. GSK1702934A transiently enhanced contractility and evoked arrhythmias in isolated Langendorff hearts from TRPC3-overexpressing but not wild-type mice. Interestingly, pro-arrhythmic effects outlasted TRPC3 current activation, were prevented by enhanced intracellular Ca²⁺ buffering, and suppressed by the NCX inhibitor 3′,4′-dichlorobenzamil hydrochloride. GSK1702934A substantially promoted NCX currents in TRPC3-overexpressing myocytes. The TRPC3-dependent electrophysiologic, pro-arrhythmic, and inotropic actions of GSK1702934A were mimicked by angiotensin II (AngII). Immunocytochemistry demonstrated colocalization of TRPC3 with NCX1 and disruption of local interaction upon channel activation by either GSK1702934A or AngII.

Conclusion

Cardiac TRPC3 mediates Ca²⁺ and Na⁺ entry in proximity of NCX1, thereby elevating cellular Ca²⁺ levels and contractility. Excessive activation of TRPC3 is associated with transient cellular Ca²⁺ overload, spatial uncoupling between TRPC3 and NCX1, and arrhythmogenesis. We propose TRPC3-NCX micro/nanodomain communication as determinant of cardiac contractility and susceptibility to arrhythmogenic stimuli.

Increased sarcolemmal Na(+)/H(+) exchange activity in hypertrophied myocytes from dogs with chronic atrioventricular block

Dogs with compensated biventricular hypertrophy due to chronic atrioventricular block (cAVB), are more susceptible to develop drug-induced Torsade-de-Pointes arrhythmias and sudden cardiac death. It has been suggested that the increased Na(+) influx in hypertrophied cAVB ventricular myocytes contribute to these lethal arrhythmias. The increased Na(+) influx was not mediated by Na(+) channels, in fact the Na(+) current proved reduced in cAVB myocytes. Here we tested the hypothesis that increased activity of the Na(+)/H(+) exchanger type 1 (NHE-1), commonly observed in hypertrophic hearts, causes the elevated Na(+) influx. Cardiac acid-base transport was studied with a pH-sensitive fluorescent dye in ventricular myocytes isolated from control and hypertrophied cAVB hearts; the H(+) equivalent flux through NHE-1, Na(+)-HCO(-) 3 cotransport (NBC), Cl(-)/OH(-) exchange (CHE), and Cl(-)/HCO(-) 3 exchange (AE) were determined and normalized per liter cell water and corrected for surface-to-volume ratio. In cAVB, sarcolemmal NHE-1 flux was increased by 65 ± 6.3% in the pH i interval 6.3-7.2 and NBC, AE, and CHE fluxes remained unchanged. Accordingly, at steady-state intracellular pH the total sarcolemmal Na(+) influx by NHE-1 + NBC increased from 8.5 ± 1.5 amol/μm(2)/min in normal myocytes to 15 ± 2.4 amol/μm(2)/min in hypertrophied cAVB myocytes. We conclude that compensated cardiac hypertrophy in cAVB dogs is accompanied with an increased sarcolemmal NHE-1 activity. This in conjunction with unchanged activity of the other acid-base transporters will raise the intracellular Na(+) in hypertrophied cAVB myocytes.

Selective modulation of coupled ryanodine receptors during microdomain activation of calcium/calmodulin-dependent kinase II in the dyadic cleft

RATIONALE

In ventricular myocytes of large mammals with low T-tubule density, a significant number of ryanodine receptors (RyRs) are not coupled to the sarcolemma; cardiac remodeling increases noncoupled RyRs.

OBJECTIVE

Our aim was to test the hypothesis that coupled and noncoupled RyRs have distinct microdomain-dependent modulation.

METHODS AND RESULTS

We studied single myocytes from pig left ventricle. The T-tubule network was analyzed in 3-dimension (3D) to measure distance to membrane of release sites. The rising phase of the Ca(2+) transient was correlated with proximity to the membrane (confocal imaging, whole-cell voltage-clamp, K5fluo-4 as Ca(2+) indicator). Ca(2+) sparks after stimulation were thus identified as resulting from coupled or noncoupled RyRs. We used high-frequency stimulation as a known activator of Ca(2+)/calmodulin-dependent kinase II. Spark frequency increased significantly more in coupled than in noncoupled RyRs. This specific modulation of coupled RyRs was abolished by the Ca(2+)/calmodulin-dependent kinase II blockers autocamtide-2-related inhibitory peptide and KN-93, but not by KN-92. Colocalization of Ca(2+)/calmodulin-dependent kinase II and RyR was not detectably different for coupled and noncoupled sites, but the F-actin disruptor cytochalasin D prevented the specific modulation of coupled RyRs. NADPH oxidase 2 inhibition by diphenyleneiodonium or apocynin, or global reactive oxygen species scavenging, also prevented coupled RyR modulation. During stimulated Ca(2+) transients, frequency-dependent increase of the rate of Ca(2+) rise was seen in coupled RyR regions only and abolished by autocamtide-2-related inhibitory peptide. After myocardial infarction, selective modulation of coupled RyR was lost.

CONCLUSIONS

Coupled RyRs have a distinct modulation by Ca(2+)/calmodulin-dependent kinase II and reactive oxygen species, dependent on an intact cytoskeleton and consistent with a local Ca(2+)/reactive oxygen species microdomain, and subject to modification with disease.

Intracellular dyssynchrony of diastolic cytosolic [Ca²⁺] decay in ventricular cardiomyocytes in cardiac remodeling and human heart failure

RATIONALE

Synchronized release of Ca²⁺ into the cytosol during each cardiac cycle determines cardiomyocyte contraction.

OBJECTIVE

We investigated synchrony of cytosolic [Ca²⁺] decay during diastole and the impact of cardiac remodeling.

METHODS AND RESULTS

Local cytosolic [Ca²⁺] transients (1-µm intervals) were recorded in murine, porcine, and human ventricular single cardiomyocytes. We identified intracellular regions of slow (slowCaR) and fast (fastCaR) [Ca²⁺] decay based on the local time constants of decay (TAUlocal). The SD of TAUlocal as a measure of dyssynchrony was not related to the amplitude or the timing of local Ca²⁺ release. Stimulation of sarcoplasmic reticulum Ca²⁺ ATPase with forskolin or istaroxime accelerated and its inhibition with cyclopiazonic acid slowed TAUlocal significantly more in slowCaR, thus altering the relationship between SD of TAUlocal and global [Ca²⁺] decay (TAUglobal). Na⁺/Ca²⁺ exchanger inhibitor SEA0400 prolonged TAUlocal similarly in slowCaR and fastCaR. FastCaR were associated with increased mitochondrial density and were more sensitive to the mitochondrial Ca²⁺ uniporter blocker Ru360. Variation in TAUlocal was higher in pig and human cardiomyocytes and higher with increased stimulation frequency (2 Hz). TAUlocal correlated with local sarcomere relengthening. In mice with myocardial hypertrophy after transverse aortic constriction, in pigs with chronic myocardial ischemia, and in end-stage human heart failure, variation in TAUlocal was increased and related to cardiomyocyte hypertrophy and increased mitochondrial density.

CONCLUSIONS

In cardiomyocytes, cytosolic [Ca²⁺] decay is regulated locally and related to local sarcomere relengthening. Dyssynchronous intracellular [Ca²⁺] decay in cardiac remodeling and end-stage heart failure suggests a novel mechanism of cellular contractile dysfunction.

Combined Na(+)/Ca(2+) exchanger and L-type calcium channel block as a potential strategy to suppress arrhythmias and maintain ventricular function

BACKGROUND

L-type calcium channel (LTCC) and Na(+)/Ca(2+) exchanger (NCX) have been implicated in repolarization-dependent arrhythmias, but also modulate calcium and contractility. Although LTCC inhibition is negative inotropic, NCX inhibition has the opposite effect. Combined block may, therefore, offer an advantage for hemodynamics and antiarrhythmic efficiency, particularly in diseased hearts. In a model of proarrhythmia, the dog with chronic atrioventricular block, we investigated whether combined inhibition of NCX and LTCC with SEA-0400 is effective against dofetilide-induced torsade de pointes arrhythmias (TdP), while maintaining calcium homeostasis and hemodynamics.

METHODS AND RESULTS

Left ventricular pressure (LVP) and ECG were monitored during infusion of SEA-0400 and verapamil in anesthetized dogs. Different doses were tested against dofetilide-induced TdP in chronic atrioventricular block dogs. In ventricular myocytes, effects of SEA-0400 were tested on action potentials, calcium transients, and early afterdepolarizations. In cardiomyocytes, SEA-0400 (1 μmol/L) blocked 66±3% of outward NCX, 50±2% of inward NCX, and 33±9% of LTCC current. SEA-0400 had no effect on systolic calcium, but slowed relaxation, despite action potential shortening, and increased diastolic calcium. SEA-0400 stabilized dofetilide-induced lability of repolarization and suppressed early afterdepolarizations. In vivo, SEA-0400 (0.4 and 0.8 mg/kg) had no effect on left ventricular pressure and suppressed dofetilide-induced TdPs dose dependently. Verapamil (0.3 mg/kg) also inhibited TdP, but caused a 15±8% drop of left ventricular pressure. A lower dose of verapamil without effects on left ventricular pressure (0.06 mg/kg) was not antiarrhythmic.

CONCLUSIONS

In chronic atrioventricular block dogs, SEA-0400 treatment is effective against TdP. Unlike specific inhibition of LTCC, combined NCX and LTCC inhibition has no negative effects on cardiac hemodynamics.

JTV519 (K201) reduces sarcoplasmic reticulum Ca²⁺ leak and improves diastolic function in vitro in murine and human non-failing myocardium

BACKGROUND AND PURPOSE

Ca²⁺ leak from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyR2s) contributes to cardiomyocyte dysfunction. RyR2 Ca²⁺ leak has been related to RyR2 phosphorylation. In these conditions, JTV519 (K201), a 1,4-benzothiazepine derivative and multi-channel blocker, stabilizes RyR2s and decrease SR Ca²⁺ leak. We investigated whether JTV519 stabilizes RyR2s without increasing RyR2 phosphorylation in mice and in non-failing human myocardium and explored underlying mechanisms.

EXPERIMENTAL APPROACH

SR Ca²⁺ leak was induced by ouabain in murine cardiomyocytes. [Ca²⁺]-transients, SR Ca²⁺ load and RyR2-mediated Ca²⁺ leak (sparks/waves) were quantified, with or without JTV519 (1 µmol·L⁻¹). Contribution of Ca²⁺ -/calmodulin-dependent kinase II (CaMKII) was assessed by KN-93 and Western blot (RyR2-Ser(2814) phosphorylation). Effects of JTV519 on contractile force were investigated in non-failing human ventricular trabeculae.

KEY RESULTS

Ouabain increased systolic and diastolic cytosolic [Ca²⁺](i) , SR [Ca²⁺], and SR Ca²⁺ leak (Ca²⁺ spark (SparkF) and Ca²⁺ wave frequency), independently of CaMKII and RyR-Ser(2814) phosphorylation. JTV519 decreased SparkF but also SR Ca²⁺ load. At matched SR [Ca²⁺], Ca²⁺ leak was significantly reduced by JTV519, but it had no effect on fractional Ca²⁺ release or Ca²⁺ wave propagation velocity. In human muscle, JTV519 was negatively inotropic at baseline but significantly enhanced ouabain-induced force and reduced its deleterious effects on diastolic function.

CONCLUSIONS AND IMPLICATIONS

JTV519 was effective in reducing SR Ca²⁺ leak by specifically regulating RyR2 opening at diastolic [Ca²⁺](i) in the absence of increased RyR2 phosphorylation at Ser(2814) , extending the potential use of JTV519 to conditions of acute cellular Ca²⁺ overload.

T-tubule remodelling and ryanodine receptor organization modulate sodium-calcium exchange

The Na(+)/Ca(2+) exchanger (NCX) is a key regulator of intracellular Ca(2+) in cardiac myocytes, predominantly contributing to Ca(2+) removal during the diastolic relaxation process but also modulating excitation-contraction coupling. NCX is preferentially located in the T-tubules and can be close to or within the dyad, where L-type Ca(2+) channels face ryanodine receptors (RyRs), the Ca(2+) release channels of the sarcoplasmic reticulum. However, especially in larger animals, not all RyRs are in dyads or adjacent to T-tubules, and a substantial fraction of Ca(2+) release from the sarcoplasmic reticulum thus occurs at distance from NCX. This chapter deals with the functional consequences of NCX location and how NCX can modulate diastolic and systolic Ca(2+) events. The loss of T-tubules and the effects on RyR function and NCX modulation are explored, as well as quantitative measurement of local Ca(2+) gradients at the level of the dyadic space.

Connexin mimetic peptides inhibit Cx43 hemichannel opening triggered by voltage and intracellular Ca2+ elevation

Connexin mimetic peptides (CxMPs), such as Gap26 and Gap27, are known as inhibitors of gap junction channels but evidence is accruing that these peptides also inhibit unapposed/non-junctional hemichannels (HCs) residing in the plasma membrane. We used voltage clamp studies to investigate the effect of Gap26/27 at the single channel level. Such an approach allows unequivocal identification of HC currents by their single channel conductance that is typically ~220 pS for Cx43. In HeLa cells stably transfected with Cx43 (HeLa-Cx43), Gap26/27 peptides inhibited Cx43 HC unitary currents over minutes and increased the voltage threshold for HC opening. By contrast, an elevation of intracellular calcium ([Ca(2+)](i)) to 200-500 nM potentiated the unitary HC current activity and lowered the voltage threshold for HC opening. Interestingly, Gap26/27 inhibited the Ca(2+)-potentiated HC currents and prevented lowering of the voltage threshold for HC opening. Experiments on isolated pig ventricular cardiomyocytes, which display strong endogenous Cx43 expression, demonstrated voltage-activated unitary currents with biophysical properties of Cx43 HCs that were inhibited by small interfering RNA targeting Cx43. As observed in HeLa-Cx43 cells, HC current activity in ventricular cardiomyocytes was potentiated by [Ca(2+)](i) elevation to 500 nM and was inhibited by Gap26/27. Our results indicate that under pathological conditions, when [Ca(2+)](i) is elevated, Cx43 HC opening is promoted in cardiomyocytes and CxMPs counteract this effect.

Relevance of calmodulin/CaMKII activation for arrhythmogenesis in the AV block dog

BACKGROUND

The calcium-dependent signaling molecules calcineurin and calcium/calmodulin-dependent protein kinase II (CaMKII) both have been linked to decompensated hypertrophy and arrhythmias. CaMKII is also believed to be involved in acute modulation of ion channels.

OBJECTIVE

The purpose of this study was to determine the role of calcineurin and CaMKII in a dog model of compensated hypertrophy and a long QT phenotype.

METHODS

AV block was created in dogs to induce ventricular remodeling, including enhanced susceptibility to dofetilide-induced torsades de pointes arrhythmias. Dogs were treated with cyclosporin A for 3 weeks, which reduced calcineurin activity, as determined by mRNA expression levels of regulator of calcineurin 1 exon 4, but which was unable to prevent structural, contractile, or electrical remodeling and arrhythmias. Biopsies were taken before and at 2 or 9 weeks after AV block. Western blots were performed against phosphorylated and total CaMKII, phospholamban, Akt, and histone deacetylase 4 (HDAC4).

RESULTS

Chronic AV block showed an increase in Akt, CaMKII and phospholamban phosphorylation levels, but HDAC4 phosphorylation remained unaltered. Dofetilide induced torsades de pointes in vivo and early afterdepolarizations in cardiomyocytes, and increased [Ca(2+)](i) and CaMKII autophosphorylation. Both W-7 and KN-93 treatment counteracted this.

CONCLUSION

The calcineurin pathway seems not to be involved in long-term cardiac remodeling of the chronic AV block dog. Although CaMKII is chronically activated, this does not translate to HDAC4 phosphorylation. However, acute CaMKII overactivation is able to initiate arrhythmias based on triggered activity.

FKBP12.6 overexpression does not protect against remodelling after myocardial infarction

Reducing the open probability of the ryanodine receptor (RyR) has been proposed to have beneficial effects in heart failure. We investigated whether conditional FKBP12.6 overexpression at the time of myocardial infarction (MI) could improve cardiac remodelling and cell Ca(2+) handling. Wild-type (WT) mice and mice overexpressing FKBP12.6 (Tg) were studied on average 7.5 ± 0.2 weeks after MI and compared with sham-operated mice for in vivo, myocyte function and remodelling. At baseline, unloaded cell shortening in Tg was not different from WT. The [Ca(2+)](i) transient amplitude was similar, but sarcoplasmic reticulum (SR) Ca(2+) content was larger in Tg, suggesting reduced fractional release. Spontaneous spark frequency was similar despite the increased SR Ca(2+) content, consistent with a reduced RyR channel open probability in Tg. After MI, left ventricular dilatation and myocyte hypertrophy were present in both groups, but more pronounced in Tg. Cell shortening amplitude was unchanged with MI in WT, but increased with MI in Tg. The amplitude of the [Ca(2+)](i) transient was not affected by MI in either genotype, but time to peak was increased; this was most pronounced in Tg. The SR Ca(2+) content and Na(+)- Ca(2+) exchanger function were not affected by MI. Spontaneous spark frequency was increased significantly after MI in Tg, and larger than in WT (at 4 Hz, 2.6 ± 0.4 sparks (100 μm)(-1) s(-1) in Tg MI versus 1.6 ± 0.2 sparks (100 μm)(-1) s(-1) in WT MI; P < 0.05). We conclude that FKPB12.6 overexpression can effectively reduce RyR open probability with maintained cardiomyocyte contraction. However, this approach appears insufficient to prevent and reduce post-MI remodelling, indicating that additional pathways may need to be targeted.

Data-based theoretical identification of subcellular calcium compartments and estimation of calcium dynamics in cardiac myocytes

In cardiac cells, Ca(2+) release flux (J(rel)) via ryanodine receptors (RyRs) from the sarcoplasmic reticulum (SR) has a complex effect on the action potential (AP). Coupling between J(rel) and the AP occurs via L-type Ca(2+) channels (I(Ca)) and the Na(+)/Ca(2+) exchanger (I(NCX)). We used a combined experimental and modelling approach to study interactions between J(rel), I(Ca) and I(NCX) in porcine ventricular myocytes.We tested the hypothesis that during normal uniform J(rel), the interaction between these fluxes can be represented as occurring in two myoplasmic subcompartments for Ca(2+) distribution, one (T-space) associated with RyR and enclosed by the junctional portion of the SR membrane and corresponding T-tubular portion of the sarcolemma, the other (M-space) encompassing the rest of the myoplasm. I(Ca) and I(NCX) were partitioned into subpopulations in the T-space and M-space sarcolemma. We denoted free Ca(2+) concentrations in T-space and M-space Ca(t) and Ca(m), respectively. Experiments were designed to allow separate measurements of I(Ca) and I(NCX) as a function of J(rel). Inclusion of T-space in themodel allowed us to reproduce in silico the following important experimental results: (1) hysteresis of I(NCX) dependence on Ca(m); (2) delay between peak I(NCX) and peak Ca(m) during caffeine application protocol; (3) delay between I(NCX) and Ca(m) during Ca(2+)-induced-Ca(2+)-release; (4) rapid I(Ca) inactivation (within 2 ms) due to J(rel), with magnitude graded as a function of the SR Ca(2+) content; (5) time delay between I(Ca) inactivation due to J(rel) and Ca(m). Partition of 25% NCX in T-space and 75% in M-space provided the best fit to the experimental data. Measured Ca(m) and I(Ca) or I(NCX) were used as input to the model for estimating Ca(t). The actual model-computed Ca(t), obtained by simulating specific experimental protocols, was used as a gold standard for comparison. The model predicted peak Ca(t) in the range of 6–25 μM, with time to equilibrium of Ca(t) with Ca(m) of ~350 ms. These Ca(t) values are in the range of LCC and RyR sensitivity to Ca(2+). An increase of the SR Ca(2+) load increased the time to equilibrium. The I(Ca)-based estimation method was most accurate during the ascending phase of Ca(t). The I(NCX)-based method provided a good estimate for the descending phase of Ca(t). Thus, application of both methods in combination provides the best estimate of the entire Ca(t) time course.

Drug-Induced Torsade de Pointes Arrhythmias in the Chronic AV Block Dog Are Perpetuated by Focal Activity

Background—

The electrically remodeled canine heart after chronic AV block (CAVB) has a high susceptibility for drug-induced torsade de pointes (TdP) arrhythmias. Although focal mechanisms have been considered for initiation, there is still controversy about whether reentry is the dominant mechanism for perpetuation of TdP. In this animal model with known nonuniform prolongation of repolarization, the mechanism of perpetuation of TdP arrhythmia was explored.

Methods and Results—

Seventeen TdP-sensitive CAVB and 10 sinus rhythm (SR) dogs were studied. In 6 animals, 66 needle electrodes were evenly distributed transmurally to record 240 unipolar local electrograms simultaneously. Activation times and activation recovery intervals were determined before and during ibutilide-induced TdP. In 12 CAVB and 9 SR dogs, left ventricular (LV) and right ventricular (RV) epicardial electrograms were recorded with a 208-point multiterminal grid electrode allowing conduction velocity (CV) and ventricular effective refractory period (VERP) measurements. Biopsy specimens were processed for connexin43 (Cx43) expression and collagen content. Ventricular myocytes were isolated to determine sodium current ( I Na ) density and cell dimensions. Computer simulations were used to assess the effects of changes therein. In CAVB, VERP and ARI were increased, whereas CV was unaltered in LV. Transversal but not longitudinal CV was increased in RV. I Na was reduced by 37% in LV but unaltered in RV. LV and RV cell size were increased, but collagen and Cx43 content remained unchanged. Simulations showed increase in CV of RV as a consequence of increased cell size at normal I Na . Ibutilide increased ARI, ERP, and maximal transmural dispersion of ERP (45±25 to 120±65 ms; P <0.05). Twenty-eight of 47 episodes of self-terminating TdP (43±72 beats) were analyzed. The majority (>90%) of beats were focal; reentry was observed only occasionally.

Conclusions—

Focal activity is the dominant mechanism involved in perpetuation of ibutilide-induced TdP in CAVB dogs based on detailed 3D mapping. This conclusion is in line with unaltered conduction and documented increase in VERP.

Pharmacological modulation of connexin-formed channels in cardiac pathophysiology

Coordinated electrical activity in the heart is supported by gap junction channels located at the intercalated discs of cardiomyocytes. Impaired gap junctional communication between neighbouring cardiomyocytes contributes to the development of re-entry arrhythmias after myocardial ischaemia. Current antiarrhythmic therapy is hampered by a lack of efficiency and side effects, creating the need for a new generation of drugs. In this review, we focus on compounds that increase gap junctional communication, thereby increasing the conduction velocity and decreasing the risk of arrhythmias. Some of these compounds also inhibit connexin 43 (Cx43) hemichannels, thereby limiting adenosine triphosphate loss and volume overload following ischaemia/reperfusion, thus potentially increasing the survival of cardiomyocytes. The compounds discussed in this review are: (i) antiarrythmic peptide (AAP), AAP10, ZP123; (ii) GAP-134; (iii) RXP-E; and (vi) the Cx mimetic peptides Gap 26 and Gap 27. None of these compounds have effects on Na(+) , Ca(2+) and K(+) channels, and therefore have no proarrhythmic activity associated with currently available antiarrhythmic drugs. GAP-134, RXP-E, Gap 26 and Gap 27 are pharmalogical agents with a favorable clinical safety profile, as already confirmed in phase I clinical trials for GAP-134. These agents show an excellent promise for treatment of arrhythmias in patients with ischaemic cardiomyopathy.

Microdomain [Ca²⁺] near ryanodine receptors as reported by L-type Ca²⁺ and Na+/Ca²⁺ exchange currents

During Ca²⁺ release from the sarcoplasmic reticulum triggered by Ca²⁺ influx through L-type Ca²⁺ channels (LTCCs), [Ca²⁺] near release sites ([Ca²⁺]nrs) temporarily exceeds global cytosolic [Ca²⁺]. [Ca²⁺]nrs can at present not be measured directly but the Na+/Ca2+ exchanger (NCX) near release sites and LTCCs also experience [Ca²⁺]nrs. We have tested the hypothesis that ICaL and INCX could be calibrated to report [Ca²⁺]nrs and would report different time course and values for local [Ca²⁺]. Experiments were performed in pig ventricular myocytes (whole-cell voltage-clamp, Fluo-3 to monitor global cytosolic [Ca²⁺], 37◦C). [Ca²⁺]nrs-dependent inactivation of ICaL during a step to +10 mV peaked around 10 ms. For INCX we computationally isolateda current fraction activated by [Ca²⁺]nrs; values were maximal at 10 ms into depolarization. The recovery of [Ca²⁺]nrs was comparable with both reporters (>90% within 50 ms). Calibration yielded maximal values for [Ca²⁺]nrs between 10 and 15 μmol l⁻¹ with both methods. When applied to a step to less positive potentials (-30 to -20 mV), the time course of [Ca²⁺]nrs was slower but peak values were not very different. In conclusion, both ICaL inactivation and INCX activation, using a subcomponent analysis, can be used to report dynamic changes of [Ca²⁺]nrs. Absolute values obtained by these different methods are within the same range, suggesting that they are reporting on a similar functional compartment near ryanodine receptors. Comparable [Ca²⁺]nrs at +10 mV and -20 mV suggests that, although the number of activated release sites differs at these potentials, local gradients at release sites can reach similar values.

Robust anti-arrhythmic efficacy of verapamil and flunarizine against dofetilide-induced TdP arrhythmias is based upon a shared and a different mode of action

BACKGROUND AND PURPOSE

The high predisposition to Torsade de Pointes (TdP) in dogs with chronic AV-block (CAVB) is well documented. The anti-arrhythmic efficacy and mode of action of Ca(2+) channel antagonists, flunarizine and verapamil against TdP were investigated.

EXPERIMENTAL APPROACH

Mongrel dogs with CAVB were selected based on the inducibility of TdP with dofetilide. The effects of flunarizine and verapamil were assessed after TdP and in different experiments to prevent dofetilide-induced TdP. Electrocardiogram and ventricular monophasic action potentials were recorded. Electrophysiological parameters and short-term variability of repolarization (STV) were determined. In vitro, flunarizine and verapamil were added to determine their effect on (i) dofetilide-induced early after depolarizations (EADs) in canine ventricular myocytes (VM); (ii) diastolic Ca(2+) sparks in RyR2(R4496+/+) mouse myocytes; and (iii) peak and late I(Na) in SCN5A-HEK 293 cells.

KEY RESULTS

Dofetilide increased STV prior to TdP and in VM prior to EADs. Both flunarizine and verapamil completely suppressed TdP and reversed STV to baseline values. Complete prevention of TdP was achieved with both drugs, accompanied by the prevention of an increase in STV. Suppression of EADs was confirmed after flunarizine. Only flunarizine blocked late I(Na). Ca(2+) sparks were reduced with verapamil.

CONCLUSIONS AND IMPLICATIONS

Robust anti-arrhythmic efficacy was seen with both Ca(2+) channel antagonists. Their divergent electrophysiological actions may be related to different additional effects of the two drugs.

Late na(+) current inhibition by ranolazine reduces torsades de pointes in the chronic atrioventricular block dog model

OBJECTIVES

This study investigated whether ranolazine reduces dofetilide-induced torsades de pointes (TdP) in a model of long QT syndrome with down-regulated K(+) currents due to hypertrophic remodeling in the dog with chronic atrioventricular block (cAVB).

BACKGROUND

Ranolazine inhibits the late Na(+) current (I(NaL)) and is effective against arrhythmias in long QT3 syndromes despite its blocking properties of the rapid component of delayed rectifying potassium current.

METHODS

Ranolazine was administered to cAVB dogs before or after TdP induction with dofetilide and electrophysiological parameters were determined including beat-to-beat variability of repolarization (BVR). In single ventricular myocytes, effects of ranolazine were studied on I(NaL), action potential duration, and dofetilide-induced BVR and early afterdepolarizations.

RESULTS

After dofetilide, ranolazine reduced the number of TdP episodes from 10 +/- 3 to 3 +/- 1 (p < 0.05) and partially reversed the increase of BVR with no abbreviation of the dofetilide-induced QT prolongation. Likewise, pre-treatment with ranolazine, or using lidocaine as a specific Na(+) channel blocker, attenuated TdP, but failed to prevent dofetilide-induced increases in QT, BVR, and ectopic activity. In cAVB myocytes, ranolazine suppressed dofetilide-induced early afterdepolarizations in 25% of cells at 5 micromol/l, in 75% at 10 micromol/l, and in 100% at 15 micromol/l. At 5 micromol/l, ranolazine blocked 26 +/- 3% of tetrodotoxin-sensitive I(NaL), and 49 +/- 3% at 15 micromol/l. Despite a 54% reduction of I(NaL) amplitude in cAVB compared with control cells, I(NaL) inhibition by 5 micromol/l tetrodotoxin equally shortened relative action potential duration and completely abolished dofetilide-induced early afterdepolarizations.

CONCLUSIONS

Despite down-regulation of I(NaL) in remodeled cAVB hearts, ranolazine is antiarrhythmic against drug-induced TdP. The antiarrhythmic effects are reflected in concomitant changes of BVR.

The anti-protozoal drug pentamidine blocks KIR2.x-mediated inward rectifier current by entering the cytoplasmic pore region of the channel

BACKGROUND AND PURPOSE

Pentamidine is a drug used in treatment of protozoal infections. Pentamidine treatment may cause sudden cardiac death by provoking cardiac arrhythmias associated with QTc prolongation and U-wave alterations. This proarrhythmic effect was linked to inhibition of hERG trafficking, but not to acute block of ion channels contributing to the action potential. Because the U-wave has been linked to the cardiac inward rectifier current (I(K1)), we examined the action and mechanism of pentamidine-mediated I(K1) block.

EXPERIMENTAL APPROACH

Patch clamp measurements of I(K1) were made on cultured adult canine ventricular cardiomyocytes, K(IR)2.1-HEK293 cells and K(IR)2.x inside-out patches. Pentamidine binding to cytoplasmic amino acid residues of K(IR)2.1 channels was studied by molecular modelling.

KEY RESULTS

Pentamidine application (24 h) decreased I(K1) in cultured canine cardiomyocytes and K(IR)2.1-HEK293 cells under whole cell clamp conditions. Pentamidine inhibited I(K1) in K(IR)2.1-HEK293 cells 10 min after application. When applied to the cytoplasmic side under inside-out patch clamp conditions, pentamidine block of I(K1) was acute (IC(50)= 0.17 microM). Molecular modelling predicted pentamidine-channel interactions in the cytoplasmic pore region of K(IR)2.1 at amino acids E224, D259 and E299. Mutation of these conserved residues to alanine reduced pentamidine block of I(K1). Block was independent of the presence of spermine. K(IR)2.2, and K(IR)2.3 based I(K1) was also sensitive to pentamidine blockade.

CONCLUSIONS AND IMPLICATIONS

Pentamidine inhibits cardiac I(K1) by interacting with three negatively charged amino acids in the cytoplasmic pore region. Our findings may provide new insights for development of specific I(K1) blocking compounds.

Crosstalk between L-type Ca2+ channels and the sarcoplasmic reticulum: alterations during cardiac remodelling

In the cardiac dyad, sarcolemmal L-type Ca(2+) channels (LCCs) and sarcoplasmic reticulum (SR) Ca(2+) release channels (RyR) are structurally in close proximity. This organization provides for an efficient functional coupling, tuning SR Ca(2+) release for optimal contraction of the myocyte. Given that LCC are regulated by the prevailing [Ca(2+)], this structural organization is the setting for feedback mechanisms and crosstalk. A defective coupling of Ca(2+) influx via LCC to activation of RyR has been implicated in reduced SR Ca(2+) release in heart failure. Both functional changes in LCC properties and structural re-organization of LCC in T-tubules could be involved. LCC are regulated by cytosolic Ca(2+), and crosstalk with SR Ca(2+) handling occurs on a long-term basis, i.e. during steady-state changes in heart rate, on an intermediate-term basis, i.e. on a beat-to-beat basis during sudden rate changes, and on a very short- or immediate-term basis, i.e. during a single heartbeat. We review the properties and consequences of these different feedback mechanisms and the changes in heart failure and cardiac hypertrophy that have thus far been studied.

Dominant arrhythmia vulnerability of the right ventricle in senescent mice

BACKGROUND

Several cardiac disorders affect the right ventricle (RV) and left ventricle (LV) equally, but nevertheless, RV vulnerability to conduction slowing and arrhythmias exceeds that of the LV.

OBJECTIVE

This study sought to assess the mechanism of dominant RV arrhythmia vulnerability in senescent mice as a model of general reduced myocardial integrity.

METHODS

Epicardial ventricular activation mapping was performed on senescent (22 months) and adult (3 months) Langendorff perfused mouse hearts. Arrhythmia inducibility was tested by programmed stimulation. Conduction velocity longitudinal and transversal (CVT) to fiber orientation, conduction heterogeneity, and effective refractory period were determined. Subsequently, hearts were processed for immunohistochemistry, Western blotting, and Sirius red staining.

RESULTS

In senescent RV, but not LV, CVT was reduced and wavelength decreased, whereas anisotropic ratio and conduction heterogeneity increased. Arrhythmias, based on anisotropic reentry, were induced in 55% of senescent hearts only and predominantly in RV. In senescent mice, Connexin 43 (Cx43) and Cardiac Sodium Channel (Nav1.5) were decreased and interstitial fibrosis increased comparably in RV and LV. However, in senescent mice, heterogeneously distributed patches of replacement fibrosis were present throughout the entire RV myocardium, but only in midendocardium and subendocardium of LV. Cx43 expression in these areas was disrupted.

CONCLUSION

Widespread presence of replacement fibrosis in senescent RV compared with LV, combined with Cx43 and Nav1.5 disruption, potentiate shorter wavelength, conduction slowing, and conduction heterogeneity in RV, resulting in greater vulnerability of senescent RV to arrhythmias.

Na/Ca exchange and cardiac ventricular arrhythmias

Ventricular arrhythmias are a major cause of death in cardiovascular disease. Ca2+ removal from the cell by the electrogenic Na/Ca exchanger is essential for the Ca2+ flux balance during excitation-contraction coupling but also contributes to the electrical events. "Classic" views on the exchanger in arrhythmias include its well-recognized role as depolarizing current underlying delayed afterdepolarizations (DADs) during spontaneous Ca2+ release and the alterations in expression in certain forms of cardiac hypertrophy and heart failure. "Novel" views relate to more subtle roles for the exchanger in arrhythmias. Na/Ca exchange function in disease could be modulated indirectly, through phosphorylation or anchoring proteins. Ongoing studies relate Na/Ca exchange to variability in action potential duration (APD) and early afterdepolarizations (EADs) in a dog model of cardiac hypertrophy and arrhythmias. Further research on drugs that target Na/Ca exchange will have to carefully examine the effects on Ca2+ balance.

Window Ca2+ current and its modulation by Ca2+ release in hypertrophied cardiac myocytes from dogs with chronic atrioventricular block

Torsades de pointes (TdP) ventricular tachycardia typically occurs in the setting of early afterdepolarizations; it contributes to arrhythmias and sudden death in congenital and acquired heart disease. Window L-type Ca2+ current (ICaL) has a central role in the arrhythmogenesis and may be particularly important under beta-adrenergic stimulation. We studied the properties of ICaL in myocytes from the dog with chronic atrioventricular block (cAVB) that has cardiac hypertrophy and an increased susceptibility to TdP. Peak ICaL densities at baseline (K+ - and Na+ -free solutions, 10 mmol l(-1) [EGTA]pip) in cAVB were comparable to control, but inactivation was shifted to the right, resulting in a larger window current area in cAVB. Under beta-adrenergic stimulation, the window current area was increased and shifted to the left, but less so in cAVB (maximum at -27 mV, versus -32 mV in control). ICaL during a step to -35 mV showed a transient reduction immediately after the peak. Test steps to 0 mV, simultaneous recording of [Ca2+]i and manipulation of sarcoplasmic reticulum (SR) Ca2+ release showed that this resulted from inhibition and fast recovery of ICaL with SR Ca2+ release. The extent of this dynamic modulation was larger in cAVB than in control (23 +/- 2% of the initially available current, versus 13 +/- 3%; P<0.05). Early afterdepolarizations (EADs) in cAVB myocytes under beta-adrenergic stimulation typically occurred in the window current voltage range and after decline of [Ca2+]i. In conclusion, in cAVB, the larger window current, its rightward shift and enhanced dynamic modulation by SR Ca2+ release may contribute to an increased incidence of EADs in cAVB under beta-adrenergic stimulation.

Temporal patterns of electrical remodeling in canine ventricular hypertrophy: Focus on IKs downregulation and blunted β-adrenergic activation

OBJECTIVES

Electrical remodeling in cardiac hypertrophy often involves the downregulation of K+ currents, including beta-adrenergic (beta-A)-sensitive IKs. Temporal patterns of ion-channel downregulation are poorly resolved. In dogs with complete atrioventricular block (AVB), we examined (1) the time course of molecular alterations underlying IKs downregulation from acute to chronic hypertrophy; and (2) concomitant changing responses of repolarization to beta-adrenergic receptor (beta-AR) stimulation.

METHODS AND RESULTS

Serial left-ventricular (LV) biopsies were collected from anesthetized dogs during sinus rhythm (SR; control) and at 3, 7 and 30 days of AVB. KCNQ1 mRNA and protein decreased within 3 days (protein expression 58 +/- 10% of control), remaining low thereafter. beta1-AR mRNA and protein decreased more gradually to 53 +/- 8% at 7 days. In chronic-AVB LV myocytes, IKs -tail density was reduced: 1.4 +/- 0.3 pA/pF versus 2.6 +/- 0.4 pA/pF in controls. beta-A enhancement of IKs was reduced. Isoproterenol shortened action-potential duration in control cells, while causing heterogeneous repolarization responses in chronic AVB. beta-A early afterdepolarizations were induced in 4 of 13 chronic-AVB cells, but not in controls. In intact conscious dogs, isoproterenol shortened QTc at SR (by -8 +/- 3% from 295 ms), left it unaltered at 3 days AVB (+1 +/- 3% from 325 ms) and prolonged QTc at 30 days (+6 +/- 3% from 365 ms).

CONCLUSIONS

Profound decrease of KCNQ1 occurs within days after AVB induction and is followed by a more gradual decrease of beta1-AR expression. Downregulation and blunted beta-A activation of IKs contribute to the loss of beta-A-induced shortening of ventricular repolarization, favoring proarrhythmia. Provocation testing with isoproterenol identifies repolarization instability based on acquired channelopathy.

Increased phospholamban phosphorylation limits the force-frequency response in the MLP-/- mouse with heart failure

Reduced Ca(2+) release from the sarcoplasmic reticulum (SR) and a negative force-frequency relation characterize end-stage human heart failure. The MLP(-/-) mouse with dilated cardiomyopathy is used as a model to explore novel therapeutic interventions but the alterations in Ca(2+) handling in MLP(-/-) remain incompletely understood. We studied [Ca(2+)](i) in left ventricular myocytes from MLP(-/-) and WT mice (3-4 months old; whole-cell voltage clamp, 30 degrees C). At 1 Hz stimulation, the amplitude of [Ca(2+)](i) transients was similar. However, in contrast to WT, at higher frequencies the [Ca(2+)](i) transient amplitude declined in MLP(-/-) and there was no increase in SR Ca(2+) content. Unexpectedly, the decline of [Ca(2+)](i) was faster in MLP(-/-) than in WT (at 1 Hz, tau of 80 +/- 9 vs. 174 +/- 29 ms, P < 0.001) and the frequency-dependent acceleration of the decline was abolished suggesting an enhanced basal SERCA activity. Indeed, the Ca(2+) affinity of SR Ca(2+) uptake in homogenates was higher in MLP(-/-), with the maximal uptake rate similar to WT. Phosphorylation of phospholamban in MLP(-/-) was increased (2.3-fold at Ser(16) and 2.9-fold at the Thr(17) site, P < 0.001) with similar SERCA and total phospholamban protein levels. On increasing stimulation frequency to 4 Hz, WT, but not MLP(-/-), myocytes had a net gain of Ca(2+), suggesting inadequate Ca(2+) sequestration in MLP(-/-). In conclusion, increased baseline phosphorylation of phospholamban in MLP(-/-) leads to a reduced reserve for frequency-dependent increase of Ca(2+) release. This represents a novel paradigm for altered Ca(2+) handling in heart failure, underscoring the importance of phosphorylation pathways.

Ca2+ uptake by the sarcoplasmic reticulum in ventricular myocytes of the SERCA2b/b mouse is impaired at higher Ca2+ loads only

SERCA2a is the cardiac-specific isoform of Ca2+-ATPase of the sarcoplasmic reticulum (SR). A reduction of SERCA2a has been implicated in the contractile dysfunction of heart failure, and partial knockout of the SERCA2 gene (Atp2a2+/- mice) reiterated many of the features of heart failure. Yet, mice with a mutation of Atp2a2, resulting in full suppression of the SERCA2a isoform and expression of the SERCA2b isoform only (SERCA2b/b), showed only moderate functional impairment, despite a reduction by 40% of the SERCA2 protein levels. We examined in more detail the Ca2+ handling in isolated cardiac myocytes from SERCA2b/b. At 0.25 Hz stimulation, the amplitude of the [Ca2+]i transients, SR Ca2+ content, diastolic [Ca2+]i, and density of ICaL were comparable between WT and SERCA2b/b. However, the decline of [Ca2+]i was slower (t1/2 154+/-7 versus 131+/-5 ms; P<0.05). Reducing the amplitude of the [Ca2+]i transient (eg, SR depletion), removed the differences in [Ca2+]i decline. In contrast, increasing the Ca2+ load revealed pronounced reduction of SR Ca2+ uptake at high [Ca2+]i. There was no increase in Na+-Ca2+ exchange protein or function. Theoretical modeling indicated that in the SERCA2b/b mouse, the higher Ca2+ affinity of SERCA2b partially compensates for the 40% reduction of SERCA expression. The lack of SR depletion in the SERCA2b/b may also be related to the absence of upregulation of Na+-Ca2+ exchange. We conclude that for SERCA isoforms with increased affinity for Ca2+, a reduced expression level is better tolerated as Ca2+ uptake and storage are impaired only at higher Ca2+ loads.

Spatial and temporal inhomogeneities during Ca2+ release from the sarcoplasmic reticulum in pig ventricular myocytes

The [Ca2+]i transient of ventricular myocytes during normal excitation-contraction coupling is the summation of primary Ca2+ release events, which originate at the junction of the sarcoplasmic reticulum (SR) and the T-tubular system. Studies in small mammals have shown a high density of release sites, but little is known of larger mammals. We have studied the spatial distribution of SR Ca2+ release in pig ventricular myocytes using a confocal microscopy. In 69 of 107 cells, large inhomogeneities of Ca2+ release were observed along the longitudinal scan line. Areas where the increase of [Ca2+]i was delayed (time to 50% of peak F/F0 [where F indicates fluorescence intensity, and F0 indicates F at rest] was 26+/-1 ms in delayed areas versus 11+/-2 ms in early areas) and smaller (peak F/F0 was 2.27+/-0.10 for delayed areas versus 2.69+/-0.13 for early areas; n=13 cells, P<0.05) could be up to 26 microm wide. The sum of all delayed areas could make up to 55% of the line scan. The spatial pattern was constant during steady-state stimulation and was not altered by enhancing Ca2+ channel opening or SR Ca2+ content (Bay K8644, isoproterenol). Imaging of sarcolemmal membranes revealed several areas devoid of T tubules, but SR Ca2+ release channels were homogeneously distributed. In contrast, compared with pig myocytes, mouse myocytes had a very dense T-tubular network, no large inhomogeneities of release, and a faster rate of rise of [Ca2+]i. In conclusion, in pig ventricular myocytes, areas of delayed release are related to regional absence of T tubules but not ryanodine receptors. This lower number of functional couplons contributes to a slower overall rate of rise of [Ca2+]i.

Mechanisms underlying the frequency dependence of contraction and [Ca(2+)](i) transients in mouse ventricular myocytes

In most mammalian species force of contraction of cardiac muscle increases with increasing rate of stimulation, i.e. a positive force-frequency relationship. In single mouse ventricular cells, both positive and negative relationships have been described and little is known about the underlying mechanisms. We studied enzymatically isolated single ventricular mouse myocytes, at 30 degrees C. During field stimulation, amplitude of unloaded cell shortening increased with increasing frequency of stimulation (0.04 +/- 0.01 Delta L/L(0) at 1 Hz to 0.07 +/- 0.01 Delta L/L(0) at 4 Hz, n = 12, P < 0.05). During whole cell voltage clamp with 50 microM [K5-fluo-3](pip), both peak and baseline [Ca(2+)](i) increased at higher stimulation frequencies, but the net Delta[Ca(2+)](i) increased only modestly from 1.59 +/- 0.08 Delta F/F(0) at 1 Hz, to 1.71 +/- 0.11 Delta F/F(0) at 4 Hz (n = 17, P < 0.05). When a 1 s pause was interposed during stimulation at 2 and 4 Hz, [Ca(2+)](i) transients were significantly larger (at 4 Hz, peak F/F(0) increased by 78 +/- 2 %, n = 5). SR Ca(2+) content assessed during caffeine application, significantly increased from 91 +/- 24 micromol l(-1) at 1 Hz to 173 +/- 20 micromol l(-1) at 4 Hz (n = 5, P < 0.05). Peak I(Ca,L) decreased at higher frequencies (by 28 +/- 6 % at 2 Hz, and 45 +/- 8 % at 4 Hz), due to slow recovery from inactivation. This loss of I(Ca,L) resulted in reduced fractional release. Thus, in mouse ventricular myocytes the [Ca(2+)](i)-frequency response depends on a balance between the increase in SR content and the loss of trigger I(Ca,L). Small changes in this balance may contribute to variability in frequency-dependent behaviour. In addition, there may be a regulation of the contractile response downstream of [Ca(2+)](i).