Cardiac arrhythmia mechanisms in rats with heart failure induced by pulmonary hypertension

Pulmonary hypertension provokes right heart failure and arrhythmias. Better understanding of the mechanisms underlying these arrhythmias is needed to facilitate new therapeutic approaches for the hypertensive, failing right ventricle (RV). The aim of our study was to identify the mechanisms generating arrhythmias in a model of RV failure induced by pulmonary hypertension. Rats were injected with monocrotaline to induce either RV hypertrophy or failure or with saline (control). ECGs were measured in conscious, unrestrained animals by telemetry. In isolated hearts, electrical activity was measured by optical mapping and myofiber orientation by diffusion tensor-MRI. Sarcoplasmic reticular Ca(2+) handling was studied in single myocytes. Compared with control animals, the T-wave of the ECG was prolonged and in three of seven heart failure animals, prominent T-wave alternans occurred. Discordant action potential (AP) alternans occurred in isolated failing hearts and Ca(2+) transient alternans in failing myocytes. In failing hearts, AP duration and dispersion were increased; conduction velocity and AP restitution were steeper. The latter was intrinsic to failing single myocytes. Failing hearts had greater fiber angle disarray; this correlated with AP duration. Failing myocytes had reduced sarco(endo)plasmic reticular Ca(2+)-ATPase activity, increased sarcoplasmic reticular Ca(2+)-release fraction, and increased Ca(2+) spark leak. In hypertrophied hearts and myocytes, dysfunctional adaptation had begun, but alternans did not develop. We conclude that increased electrical and structural heterogeneity and dysfunctional sarcoplasmic reticular Ca(2+) handling increased the probability of alternans, a proarrhythmic predictor of sudden cardiac death. These mechanisms are potential therapeutic targets for the correction of arrhythmias in hypertensive, failing RVs.     

Steeper restitution slopes across right ventricular endocardium in patients with cardiomyopathy at high risk of ventricular arrhythmias

Steep action potential duration (APD) restitution slopes (>1) and spatial APD restitution heterogeneity provide the substrate for ventricular fibrillation in computational models and experimental studies. Their relationship to ventricular arrhythmia vulnerability in human cardiomyopathy has not been defined. Patients with cardiomyopathy [left ventricular (LV) ejection fraction <40%] and no history of ventricular arrhythmias underwent risk stratification with programmed electrical stimulation or T wave alternans (TWA). Low-risk patients (n = 10) had no inducible ventricular tachycardia (VT) or negative TWA, while high-risk patients (n = 8) had inducible VT or positive TWA. Activation recovery interval (ARI) restitution slopes were measured simultaneously from 10 right ventricular (RV) endocardial sites during an S1-S2 pacing protocol. ARI restitution slope heterogeneity was defined as the coefficient of variation of slopes. Mean ARI restitution slope was significantly steeper in the high-risk group compared with the low-risk group [1.16 (SD 0.31) vs. 0.59 (SD 0.19), P = 0.0002]. The proportion of endocardial recording sites with a slope >1 was significantly larger in the high-risk patients [47% (SD 35) vs. 13% (SD 21), P = 0.022]. Spatial heterogeneity of ARI restitution slopes was similar between the two groups [29% (SD 16) vs. 39% (SD 34), P = 0.48]. There was an inverse linear relationship between the ARI restitution slope and the minimum diastolic interval (P < 0.001). In cardiomyopathic patients at high risk of ventricular arrhythmias, ARI restitution slopes along the RV endocardium are steeper, but restitution slope heterogeneity is similar compared with those at low risk. Steeper ARI restitution slopes may increase the propensity for ventricular arrhythmias in patients with impaired left ventricular function.     

Characterization of right ventricular function after monocrotaline-induced pulmonary hypertension in the intact rat

We characterized hemodynamics and systolic and diastolic right ventricular (RV) function in relation to structural changes in the rat model of monocrotaline (MCT)-induced pulmonary hypertension. Rats were treated with MCT at 30 mg/kg body wt (MCT30, n = 15) and 80 mg/kg body wt (MCT80, n = 16) to induce compensated RV hypertrophy and RV failure, respectively. Saline-treated rats served as control (Cont, n = 13). After 4 wk, a pressure-conductance catheter was introduced into the RV to assess pressure-volume relations. Subsequently, rats were killed, hearts and lungs were rapidly dissected, and RV, left ventricle (LV), and interventricular septum (IVS) were weighed and analyzed histochemically. RV-to-(LV + IVS) weight ratio was 0.29 +/- 0.05 in Cont, 0.35 +/- 0.05 in MCT30, and 0.49 +/- 0.10 in MCT80 (P < 0.001 vs. Cont and MCT30) rats, confirming MCT-induced RV hypertrophy. RV ejection fraction was 49 +/- 6% in Cont, 40 +/- 12% in MCT30 (P < 0.05 vs. Cont), and 26 +/- 6% in MCT80 (P < 0.05 vs. Cont and MCT30) rats. In MCT30 rats, cardiac output was maintained, but RV volumes and filling pressures were significantly increased compared with Cont (all P < 0.05), indicating RV remodeling. In MCT80 rats, RV systolic pressure, volumes, and peak wall stress were further increased, and cardiac output was significantly decreased (all P < 0.05). However, RV end-systolic and end-diastolic stiffness were unchanged, consistent with the absence of interstitial fibrosis. MCT-induced pressure overload was associated with a dose-dependent development of RV hypertrophy. The most pronounced response to MCT was an overload-dependent increase of RV end-systolic and end-diastolic volumes, even under nonfailing conditions.     

慢性低氧对大鼠左右心室的功能及TRPC亚家族表达的影响

目的：探讨慢性低氧3周对大鼠左右心室的影响以及规范性瞬时感受器电位亚家族（TRPC）在慢性低氧诱导的右心室心肌肥厚中的表达。方法：将SD雄性大鼠48只随机分为对照组（CON组）和慢性低氧肺动脉高压模型组（CH组）（n=24）,CH组将大鼠置于连续的慢性低氧（10％±0．2％）环境饲养三周以诱导大鼠发生心肌肥厚。通过左、右心室插管法测定右心室内压（RVSP）、左心室内压（LVSP）、心率（HR）、平均体循环动脉压（mSAP）、左、右心室内压力最大上升速率（＋dp／dtmax）、最大下降速率（-dp／dkmax）、右心肥大指数（RVMI）、左心肥大指数（LVMI）;HE染色观察左、右心室心肌组织切片;通过SYBR Green荧光定量PCR法检测CON组、CH组大鼠的肥厚侧心室心肌组织编码TRPC 1／3／4／5／6／7的rnRNA表达;结合real-time RT-PCR结果对mRNA表达有显著变化的TRPC亚型通过免疫印迹法检测相应蛋白的表达。结果：与CON组相比：CH组的RVSP、RVMI、右心室±dp／dtmax显著增高（P〈0．01）,LVSP、左心室±dp／dmax无显著变化,LVMI显著降低（P〈0．01）;CH组右心室心肌细胞显著增粗（P〈0．01）,细胞内肌原纤维数量增多,心肌纤维排列紊乱,细胞核深染,形状不整;左心室心肌纤维无明显改变;CH组编码TRPCI的mRNA和蛋白显著增高（P〈0．05）,而编码其余TRPC亚型的mRNA无显著变化。结论：慢性低氧3周可特异性诱导sD大鼠产生右心室心肌肥厚,上调了编码右心室心肌细胞TRPCI通道蛋白的mRNA和蛋白的表达,TRPCI可能参与了心肌肥厚的发生发展。     

Preservation of diastolic function in monocrotaline-induced right ventricular hypertrophy in rats

During ischemic heart diseases and when heart failure progresses depletion of myocardial energy stores occurs. D-Ribose (R) has been shown to improve cardiac function and energy status after ischemia. Folic acid (FA) is an essential cofactor in the formation of adenine nucleotides. Therefore, we assessed whether chronic R-FA administration during the development of hypertrophy resulted in an improved cardiac function and energy status. In Wistar rats (n = 40) compensatory right ventricular (RV) hypertrophy was induced by monocrotaline (30 mg/kg; MCT), whereas saline served as control. Both groups received a daily oral dose of either 150 mg.kg(-1).day(-1) dextrose (placebo) or R-FA (150 and 40 mg.kg(-1).day(-1), respectively). In Langendorff-perfused hearts, RV and left ventricular (LV) pressure development and collagen content as well as total RV adenine nucleotides (TAN), creatine content, and RV and LV collagen content were determined. In the control group R-FA had no effect. In the MCT-placebo group, TAN and creatine content were reduced, RV and LV diastolic pressure-volume relations were steeper, RV systolic pressures were elevated, RV and LV collagen content was increased, and RV-LV diastolic interaction was altered compared with controls. In the MCT-R-FA group, TAN, RV and LV diastolic stiffness, RV and LV collagen content, and RV-LV diastolic interaction were normalized to the values in the control group while creatine content remained depressed and RV systolic function remained elevated. In conclusion, the depression of energy status in compensated hypertrophic myocardium observed was partly prevented by chronic R-FA administration and accompanied by a preservation of diastolic function and collagen deposition.     

Endogenous production of ghrelin and beneficial effects of its exogenous administration in monocrotaline-induced pulmonary hypertension

We investigated the endogenous production of ghrelin as well as cardiac and pulmonary vascular effects of its administration in a rat model of monocrotaline (MCT)-induced pulmonary hypertension (PH). Adult Wistar rats randomly received a subcutaneous injection of MCT (60 mg/kg) or an equal volume of vehicle. One week later, animals were randomly assigned to receive a subcutaneous injection of ghrelin (100 mug/kg bid for 2 wk) or saline. Four groups were analyzed: normal rats treated with ghrelin (n=7), normal rats injected with saline (n=7), MCT rats treated with ghrelin (n=9), and MCT rats injected with saline (n=9). At 22-25 days, right (RV) and left ventricular (LV) pressures were measured, heart and lungs were weighted, and samples were collected for histological and molecular analysis. Endogenous production of ghrelin was almost abolished in normal rats treated with ghrelin. In MCT-treated animals, pulmonary expression of ghrelin was preserved, and RV myocardial expression was increased more than 20 times. In these animals, exogenous administration of ghrelin attenuated PH, RV hypertrophy, wall thickening of peripheral pulmonary arteries, and RV diastolic disturbances and ameliorated LV dysfunction, without affecting its endogenous production. In conclusion, decreased tissular expression of ghrelin in healthy animals but not in PH animals suggests a negative feedback in the former that is lost in the latter. A selective increase of ghrelin mRNA levels in the RV of animals with PH might indicate distinct regulation of its cardiac expression. Finally, ghrelin administration attenuated MCT-induced PH, pulmonary vascular remodeling, and RV hypertrophy, indicating that it may modulate PH.     

Losartan attenuates phospholipase C isozyme gene expression in hypertrophied hearts due to volume overload

Because the left ventricular (LV) hypertrophy due to volume overload induced by arteriovenous (AV) shunt was associated with an increase in phospholipase C (PLC) isozyme mRNA levels, PLC is considered to be involved in the development of cardiac hypertrophy. Since the renin-angiotensin system (RAS) is activated in cardiac hypertrophy, the role of RAS in the stimulation of PLC isozyme gene expression in hypertrophied heart was investigated by inducing AV shunt in Sprague-Dawley rats. The animals were treated with or without losartan (20 mg/kg, daily) for 3 days as well as 1, 2 and 4 weeks, and atria, right ventricle (RV) and LV were used for analysis. The increased muscle mass as well as the mRNA levels for PLC beta1 and beta3 in atria and RV, unlike PLC beta3 gene expression in LV, at 3 days of AVshunt were attenuated by losartan. The increased gene expression for PLC beta1 at 2 weeks in atria, at 1 and 4 weeks in RV, and at 2 and 4 weeks in LV was also depressed by losartan treatment. Likewise, the elevated mRNA levels for PLC beta3 in RV at 1 week and in LVat 4 weeks of cardiac hypertrophy were decreased by losartan. On the other hand, the increased levels of mRNA for PLC gamma1 in RV and LV at 2 and 4 weeks of inducing hypertrophy, unlike in atria at 4 weeks were not attenuated by losartan treatment. While the increased mRNA level for PLC delta1 in LV was reduced by losartan, gene expression for PLC delta1 was unaltered in atria and decreased in RV at 3 days of inducing AV shunt. These results suggest that changes in PLC isozyme gene expression were chamber specific and time-dependent upon inducing cardiac hypertrophy due to AV shunt. Furthermore, partial attenuation of the increased gene expression for some of the PLC isozymes and no effect of losartan on others indicate that both RAS dependent and independent mechanisms may be involved in hypertrophied hearts due to volume overload.     

Early changes in rat hearts with developing pulmonary arterial hypertension can be detected with three-dimensional electrocardiography

The study aim was to assess three-dimensional electrocardiogram (ECG) changes during development of pulmonary arterial hypertension (PAH). PAH was induced in male Wistar rats (n = 23) using monocrotaline (MCT; 40 mg/kg sc). Untreated healthy rats served as controls (n = 5). ECGs were recorded with an orthogonal three-lead system on days 0, 14, and 25 and analyzed with dedicated computer software. In addition, left ventricular (LV)-to-right ventricular (RV) fractional shortening ratio was determined using echocardiography. Invasively measured RV systolic pressure was 49 (SD 10) mmHg on day 14 and 64 (SD 10) mmHg on day 25 vs. 25 (SD 2) mmHg in controls (both P < 0.001). Baseline ECGs of controls and MCT rats were similar, and ECGs of controls did not change over time. In MCT rats, ECG changes were already present on day 14 but more explicit on day 25: increased RV electromotive forces decreased mean QRS-vector magnitude and changed QRS-axis orientation. Important changes in action potential duration distribution and repolarization sequence were reflected by a decreased spatial ventricular gradient magnitude and increased QRS-T spatial angle. On day 25, LV-to-RV fractional shortening ratio was increased, and RV hypertrophy was found, but not on day 14. In conclusion, developing PAH is characterized by early ECG changes preceding RV hypertrophy, whereas severe PAH is marked by profound ECG changes associated with anatomical and functional changes in the RV. Three-dimensional ECG analysis appears to be very sensitive to early changes in RV afterload.     

Muscle RING Finger-1 Promotes a Maladaptive Phenotype in Chronic Hypoxia-Induced Right Ventricular Remodeling

Exposure to chronic hypoxia (CH) induces elevated pulmonary artery pressure/resistance, leading to an eventual maladaptive right ventricular hypertrophy (RVH). Muscle RING finger-1 (MuRF1) is a muscle-specific ubiquitin ligase that mediates myocyte atrophy and has been shown to play a role in left ventricular hypertrophy and altered cardiac bioenergetics in pressure overloaded hearts. However, little is known about the contribution of MuRF1 impacting RVH in the setting of CH. Therefore, we hypothesized that MuRF1 deletion would enhance RVH compared to their wild-type littermates, while cardiac-specific overexpression would reduce hypertrophy following CH-induced pulmonary hypertension. We assessed right ventricular systolic pressure (RVSP), right ventricle to left ventricle plus septal weight ratio (RV/LV+S) and hematocrit (Hct) following a 3-wk isobaric CH exposure. Additionally, we conducted dual-isotope SPECT/CT imaging with cardiac function agent ²⁰¹Tl-chloride and cell death agent ^99mTc-annexin V. Predictably, CH induced pulmonary hypertension, measured by increased RVSP, RV/LV+S and Hct in WT mice compared to normoxic WT mice. Normoxic WT and MuRF1-null mice exhibited no significant differences in RVSP, RV/LV+S or Hct. CH-induced increases in RVSP were also similar between WT and MuRF1-null mice; however, RV/LV+S and Hct were significantly elevated in CH-exposed MuRF1-null mice compared to WT. In cardiac-specific MuRF1 overexpressing mice, RV/LV+S increased significantly due to CH exposure, even greater than in WT mice. This remodeling appeared eccentric, maladaptive and led to reduced systemic perfusion. In conclusion, these results are consistent with an atrophic role for MuRF1 regulating the magnitude of right ventricular hypertrophy following CH-induction of pulmonary hypertension.     

钙调神经磷酸酶信号通路参与前列腺素F2α诱导的心肌肥厚

利用野百合碱（monocrotaline,MCT）诱导大鼠右心室肥厚模型和培养乳鼠心肌细胞,研究前列腺素F2α（prostaglandin F2α,PGF2α）在心肌肥厚中的作用及钙调神经磷酸酶（calcineurin,CaN）信号通路征其中的作用。在雄性Sprague-Dawley大鼠中,用MCT（60mg／kg）单次i．p．诱导右心室肥厚,同时用塞来旨布（20mg／kg）预防／治疗给药2周。用病理检测、电镜观察等方法观察心肌肥厚时组织病理改变;EIA试剂盒检测心肌组织PGF2α含量;RT-PCR检测心房钠尿肽（atrial natriuretic peptide,ANP）和CaNmRNA的表达;用蛋白免疫印迹法检测CaN及其下游因了NFAT3和GATA4蛋门质的表达。以心肌细胞直径、蛋白含量和ANP mRNA表达的变化为0．1μmol／L PGF2α诱导心肌细胞肥大的指标。以CaN mRNA表达作为该信号通路的主要指标,并观察CaN抑制剂环孢素A对PGF2α所致心肌细胞肥人和CaN mRNA表达的影响。结果显示：MCT注射2周（M2W组）,右心室肥厚指数（RVHI）、右心室／体重比及肺重／体重比分别增加了47％、53％和118％;注射后4周（M4W组）增加了64％、94％及156％。电镜观察发现右心室组织损伤。同时,右心室组织PGF2α含量在M2W和M4W组分别增加了44％和51％,与RVHI、ANP和CaN的mRNA表达,及CaN／NFAT3／GATA4的蛋白质表达均呈正相关。环氧酶抑制剂塞来昔布预防和治疗给药均明显改善MCT诱导的组织病理学改变。在高体细胞培养中,PGF2α（0．1μmol／L）明显使心肌细胞增大,蛋白质含量增加,ANP和CaN mRNA表达增强：同时,CaN抑制剂环孢素A明显抑制PGF2α诱导的心肌细胞肥大和CaN mRNA表达。上述结果提示：心肌组织局部PGF2α参与了MCT诱导的心肌肥厚过程,CaN信号通路是其细胞内信号转导通路之一。     

Myocardial dysfunction and neurohumoral activation without remodeling in left ventricle of monocrotaline-induced pulmonary hypertensive rats

In monocrotaline (MCT)-induced pulmonary hypertension (PH), only the right ventricle (RV) endures overload, but both ventricles are exposed to enhanced neuroendocrine stimulation. To assess whether in long-standing PH the left ventricular (LV) myocardium molecular/contractile phenotype can be disturbed, we evaluated myocardial function, histology, and gene expression of autocrine/paracrine systems in rats with severe PH 6 wk after subcutaneous injection of 60 mg/kg MCT. The overloaded RV underwent myocardial hypertrophy (P < 0.001) and fibrosis (P = 0.014) as well as increased expression of angiotensin-converting enzyme (ACE) (8-fold; P < 0.001), endothelin-1 (ET-1) (6-fold; P < 0.001), and type B natriuretic peptide (BNP) (15-fold; P < 0.001). Despite the similar upregulation of ET-1 (8-fold; P < 0.001) and overexpression of ACE (4-fold; P < 0.001) without BNP elevation, the nonoverloaded LV myocardium was neither hypertrophic nor fibrotic. LV indexes of contractility (P < 0.001) and relaxation (P = 0.03) were abnormal, however, and LV muscle strips from MCT-treated compared with sham rats presented negative (P = 0.003) force-frequency relationships (FFR). Despite higher ET-1 production, BQ-123 (ET(A) antagonist) did not alter LV MCT-treated muscle strip contractility distinctly (P = 0.005) from the negative inotropic effect exerted on shams. Chronic daily therapy with 250 mg/kg bosentan (dual endothelin receptor antagonist) after MCT injection not only attenuated RV hypertrophy and local neuroendocrine activation but also completely reverted FFR of LV muscle strips to positive values. In conclusion, the LV myocardium is altered in advanced MCT-induced PH, undergoing neuroendocrine activation and contractile dysfunction in the absence of hypertrophy or fibrosis. Neuroendocrine mediators, particularly ET-1, may participate in this functional deterioration.     

Nicorandil prevents right ventricular remodeling by inhibiting apoptosis and lowering pressure overload in rats with pulmonary arterial hypertension

Background

Most of the deaths among patients with severe pulmonary arterial hypertension (PAH) are caused by progressive right ventricular (RV) pathological remodeling, dysfunction, and failure. Nicorandil can inhibit the development of PAH by reducing pulmonary artery pressure and RV hypertrophy. However, whether nicorandil can inhibit apoptosis in RV cardiomyocytes and prevent RV remodeling has been unclear.

Methodology/Principal Findings

RV remodeling was induced in rats by intraperitoneal injection of monocrotaline (MCT). RV systolic pressure (RVSP) was measured at the end of each week after MCT injection. Blood samples were drawn for brain natriuretic peptide (BNP) ELISA analysis. The hearts were excised for histopathological, ultrastructural, immunohistochemical, and Western blotting analyses. The MCT-injected rats exhibited greater mortality and less weight gain and showed significantly increased RVSP and RV hypertrophy during the second week. These worsened during the third week. MCT injection for three weeks caused pathological RV remodeling, characterized by hypertrophy, fibrosis, dysfunction, and RV mitochondrial impairment, as indicated by increased levels of apoptosis. Nicorandil improved survival, weight gain, and RV function, ameliorated RV pressure overload, and prevented maladaptive RV remodeling in PAH rats. Nicorandil also reduced the number of apoptotic cardiomyocytes, with a concomitant increase in Bcl-2/Bax ratio. 5-hydroxydecanoate (5-HD) reversed these beneficial effects of nicorandil in MCT-injected rats.

Conclusions/Significance

Nicorandil inhibits PAH-induced RV remodeling in rats not only by reducing RV pressure overload but also by inhibiting apoptosis in cardiomyocytes through the activation of mitochondrial ATP-sensitive K⁺ (mitoK_ATP) channels. The use of a mitoK_ATP channel opener such as nicorandil for PAH-associated RV remodeling and dysfunction may represent a new therapeutic strategy for the amelioration of RV remodeling during the early stages of PAH.     

Chronic treatment with probucol effectively inhibits progression of pulmonary hypertension in rats

It has been reported that probucol is a lipid-lowering agent having a strong antioxidative effect and inhibitory action on vascular smooth muscle cell proliferation. In this work, we studied the effect of treatment with a 1% probucol diet on pulmonary hypertension induced by monocrotaline (MCT) in rats. Rats were fed a control or 1% probucol-supplemented diet for 7 days, then given a single subcutaneous injection of 60 mg/kg MCT or saline, and continuously fed the same diet for 20 days, respectively. MCT caused an increase in right ventricular systolic pressure (RVSP), an indicator of pulmonary hypertension, and central venous pressure (CVP) on day 20. In rats receiving a diet containing 1% probucol, RVSP was significantly lower than that in rats treated with control diet, and CVP remained essentially at the basal level. On day 20, MCT also caused an increase in the ratio of right ventricular (RV) to body weight (BW), compared to the control value, indicating the development of RV hypertrophy in MCT rats. RV hypertrophy was significantly inhibited in 1% probucol-treated rats. These findings suggest that chronic treatment with probucol effectively inhibits the progression of pulmonary hypertension in rats.     

Differences in distribution of fibrosis in the ventricles underlie dominant arrhythmia vulnerability of the right ventricle in senescent mice

Mutations that are supposed to affect right (RV) and left ventricular (LV) electrophysiology equally, often reveal dominant conduction slowing and arrhythmia vulnerability in RV. In this study we investigated the mechanism of dominant arrhythmia vulnerability of RV in senescent mice. We performed epicardial ventricular activation mapping on adult and senescent Langendorff perfused hearts. Longitudinal and transversal conduction velocity, as well as arrhythmia inducibility were determined. Subsequently, hearts were processed for immunohisto-chemistry and Picro Sirius Red staining. Senescent mice revealed decreased conduction velocity, increased aniso-tropic ratio and reduced excitation wavelength in RV, but not in LV. Arrhythmias were mainly induced in RV of senescent hearts. No arrhythmias were induced in adult hearts. Immunohistochemistry revealed that the amount of Connexin 43 and cardiac sodium channel Nav1 .5 were equally decreased, and that collagen content was equally increased in senescent RV and LV. However, patches of replacement fibrosis were found throughout the RV wall, but only in the sub-endocardium and mid-myocardium of LV. The study shows that the dominant arrhythmia vulnerability in RV of senescent mice is caused by the distribution of replacement fibrosis which involves the entire RV but only part of the LV. (Neth Heart J 2008; 16:356-8.)     

Impaired distensibility of the left ventricle after stiffening of the right ventricle.

Acute and chronic alterations of right ventricular (RV) wall properties can change left ventricular (LV) performance. We investigated whether and how stiffening of the RV free wall alters LV diastolic distensibility. We used cross-circulated isolated hearts, in which the LV and RV were independently controllable. Stiffness of the RV free wall was altered by intramuscular injections of glutaraldehyde into the RV free wall after right coronary artery ligation. We measured circumferential and longitudinal regional lengths in the septum and LV free wall. During data acquisition, RV volume was held constant. After the RV free wall was stiffened by glutaraldehyde, the LV diastolic pressure-volume relation shifted upward and became steeper. Importantly, stiffening of the RV free wall increased the diastolic regional area in the septum and LV free wall under constant LV volume. The augmented regional dimensions may result in enhanced regional tension under constant LV volume and may be related to the observed increase in LV diastolic intracavitary pressure. The impaired LV diastolic distensibility by stiffening of the RV free wall may be at least partly explained by myocardial stretch, probably due to LV deformation.     

Stretch-induced regional mechanoelectric dispersion and arrhythmia in the right ventricle of anesthetized lambs

Regional mechanical and electrophysiological changes accompany most ventricular arrhythmias and, it has been suggested, by mechanoelectric feedback. We hypothesized that an intervention producing regional mechanical dispersion was associated with regional, proarrhythmic electrical dispersion and studied the regional mechanoelectric feedback in the right ventricle (RV) of anesthetized lambs. Ten lambs were deeply anesthetized, and their hearts were exposed. Three tripodal devices, each incorporating three monophasic action potential electrodes and an integrated strain-gauge system, were placed on the RV apex outflow and inflow regions. Measurements were made before, during, and after 10-s pulmonary arterial occlusion. Pulmonary arterial occlusion increased RV pressure and overall regional segment length. Length excursion became out of phase with RV pressure beats immediately after occlusion, and the strain patterns were different in the three regions at the peak of occlusion. The occlusion resulted in different alterations in regional monophasic action potential morphology, including reduction in monophasic action potential amplitude and duration by different amounts and early afterdepolarizations that were unevenly distributed in the monophasic action potential recordings. This was associated with dispersion of repolarization and recovery time. The combination of electromechanical events precipitated a variety of arrhythmias. Acute RV distension is proarrhythmic, possibly through a causal relationship among mechanically induced afterdepolarizations, dispersion (heterogeneity) of mechanical strain, and dispersion of electrical recovery. The relationship among the different wall motions, the dispersion of repolarization, and arrhythmia underscored mechanoelectric feedback as an important part of arrhythmogenesis in pulmonary embolism and commotio cordis.     

Nonlinearity between action potential alternans and restitution, which both predict ventricular arrhythmic properties in Scn5a+/- and wild-type murine hearts

Electrocardiographic QT- and T-wave alternans, presaging ventricular arrhythmia, reflects compromised adaptation of action potential (AP) duration (APD) to altered heart rate, classically attributed to incomplete Na(v)1.5 channel recovery prior to subsequent stimulation. The restitution hypothesis suggests a function whose slope directly relates to APD alternans magnitude, predicting a critical instability condition, potentially generating arrhythmia. The present experiments directly test for such correlations among arrhythmia, APD alternans and restitution. Mice haploinsufficient in the Scn5a, cardiac Na(+) channel gene (Scn5a(+/-)), previously used to replicate Brugada syndrome, were used, owing to their established arrhythmic properties increased by flecainide and decreased by quinidine, particularly in right ventricular (RV) epicardium. Monophasic APs, obtained during pacing with progressively decrementing cycle lengths, were systematically compared at RV and left ventricular epicardial and endocardial recording sites in Langendorff-perfused Scn5a(+/-) and wild-type hearts before and following flecainide (10 μM) or quinidine (5 μM) application. The extent of alternans was assessed using a novel algorithm. Scn5a(+/-) hearts showed greater frequencies of arrhythmic endpoints with increased incidences of ventricular tachycardia, diminished by quinidine, and earlier onsets of ventricular fibrillation, particularly following flecainide challenge. These features correlated directly with increased refractory periods, specifically in the RV, and abnormal restitution and alternans properties in the RV epicardium. The latter variables were related by a unique, continuous higher-order function, rather than a linear relationship with an unstable threshold. These findings demonstrate a specific relationship between alternans and restitution, as well as confirming their capacity to predict arrhythmia, but implicate mechanisms additional to the voltage feedback suggested in the restitution hypothesis.     

Altered expression of connexin43 contributes to the arrhythmogenic substrate during the development of heart failure in cardiomyopathic hamster

Heart failure is known to predispose to life-threatening ventricular tachyarrhythmias even before compromising the systemic circulation, but the underlying mechanism is not well understood. The aim of this study was to clarify the connexin43 (Cx43) gap junction remodeling and its potential role in the pathogenesis of arrhythmias during the development of heart failure. We investigated stage-dependent changes in Cx43 expression in UM-X7.1 cardiomyopathic hamster hearts and associated alterations in the electrophysiological properties using a high-resolution optical mapping system. UM-X7.1 hamsters developed left ventricular (LV) hypertrophy by ages 6 approximately 10 wk and showed a moderate reduction in LV contractility at age 20 wk. Appreciable interstitial fibrosis was recognized at these stages. LV mRNA and protein levels of Cx43 in UM-X7.1 were unaffected at age 10 wk but significantly reduced at 20 wk. The expression level of Ser255-phosphorylated Cx43 in UM-X7.1 at age 20 wk was significantly greater than that in control golden hamsters at the same age. In UM-X7.1 at age 10 wk, almost normal LV conduction was preserved, whereas the dispersion of action potential duration was significantly increased. UM-X7.1 at age 20 wk showed significant reduction of cardiac space constant, significant decrease in conduction velocity, marked distortion of activation fronts, and pronounced increase in action potential duration dispersion. Programmed stimulation resulted in sustained ventricular tachycardia or fibrillation in UM-X7.1. LV activation during polymorphic ventricular tachycardia was characterized by multiple phase singularities or wavebreaks. During the development of heart failure in the cardiomyopathic hamster, alterations of Cx43 expression and phosphorylation in concert with interstitial fibrosis may create serious arrhythmogenic substrate through an inhibition of cell-to-cell coupling.     

Bone marrow mesenchymal stem cells protected post‐infarcted myocardium against arrhythmias via reversing potassium channels remodelling

Bone marrow mesenchymal stem cells (BMSCs) emerge as a promising approach for treating heart diseases. However, the effects of BMSCs‐based therapy on cardiac electrophysiology disorders after myocardial infarction were largely unclear. This study was aimed to investigate whether BMSCs transplantation prevents cardiac arrhythmias and reverses potassium channels remodelling in post‐infarcted hearts. Myocardial infarction was established in male SD rats, and BMSCs were then intramyocardially transplanted into the infarcted hearts after 3 days. Cardiac electrophysiological properties in the border zone were evaluated by western blotting and whole‐cell patch clamp technique after 2 weeks. We found that BMSCs transplantation ameliorated the increased heart weight index and the impaired LV function. The survival of infarcted rats was also improved after BMSCs transplantation. Importantly, electrical stimulation‐induced arrhythmias were less observed in BMSCs‐transplanted infarcted rats compared with rats without BMSCs treatment. Furthermore, BMSCs transplantation effectively inhibited the prolongation of action potential duration and the reduction of transient and sustained outward potassium currents in ventricular myocytes in post‐infarcted rats. Consistently, BMSCs‐transplanted infarcted hearts exhibited the increased expression of K_V4.2, K_V4.3, K_V1.5 and K_V2.1 proteins when compared to infarcted hearts. Moreover, intracellular free calcium level, calcineurin and nuclear NFATc3 protein expression were shown to be increased in infarcted hearts, which was inhibited by BMSCs transplantation. Collectively, BMSCs transplantation prevented ventricular arrhythmias by reversing cardiac potassium channels remodelling in post‐infarcted hearts.     

Na+ channel distribution and electrophysiological heterogeneities in guinea pig ventricular wall

We sought to explore the distribution pattern of Na(+) channels across ventricular wall, and to determine its functional correlates, in the guinea pig heart. Voltage-dependent Na(+) channel (Na(v)) protein expression levels were measured in transmural samples of ventricular tissue by Western blotting. Isolated, perfused heart preparations were used to record monophasic action potentials and volume-conducted ECG, and to measure effective refractory periods (ERPs) and pacing thresholds, in order to assess excitability, electrical restitution kinetics, and susceptibility to stimulation-evoked tachyarrhythmias at epicardial and endocardial stimulation sites. In both ventricular chambers, Na(v) protein expression was higher at endocardium than epicardium, with midmyocardial layers showing intermediate expression levels. Endocardial stimulation sites showed higher excitability, as evidenced by lower pacing thresholds during regular stimulation and downward displacement of the strength-interval curve reconstructed after extrasystolic stimulation compared with epicardium. ERP restitution assessed over a wide range of pacing rates showed greater maximal slope and faster kinetics at endocardial than epicardial stimulation sites. Flecainide, a Na(+) channel blocker, reduced the maximal ERP restitution slope, slowed restitution kinetics, and eliminated epicardial-to-endocardial difference in dynamics of electrical restitution. Greater excitability and steeper electrical restitution have been associated with greater arrhythmic susceptibility of endocardium than epicardium, as assessed by measuring ventricular fibrillation threshold, inducibility of tachyarrhythmias by rapid cardiac pacing, and the magnitude of stimulation-evoked repolarization alternans. In conclusion, higher Na(+) channel expression levels may contribute to greater excitability, steeper electrical restitution slopes and faster restitution kinetics, and greater susceptibility to stimulation-evoked tachyarrhythmias at endocardium than epicardium in the guinea pig heart.