Mechanical work and energetic analysis of eccentric cardiac remodeling in a volume overload heart failure in rats

Experimental models of cardiac physiology and pathology

Experimental models of cardiac disease play a key role in understanding the pathophysiology of the disease and developing new therapies. The features of the experimental models should reflect the clinical phenotype, which can have a wide spectrum of underlying mechanisms. We review characteristics of commonly used experimental models of cardiac physiology and pathophysiology in all translational steps including in vitro, small animal, and large animal models. Understanding their characteristics and relevance to clinical disease is the key for successful translation to effective therapies.

Methods for the Preparation of an Excised, Cross-Circulated Rat Heart

The Emax-Pressure-Volume Area (PVA)-VO₂ framework proposed by Dr. Suga for canine hearts has dramatically advanced the field of cardiac mechanical work and energetics, i.e., mechanoenergetics. He and his collaborators investigated mechanoenergetics in the left ventricle (LV) of excised, cross-circulated canine heart preparations. We instituted the excised cross-circulated rat whole heart preparations and found a curvilinear end-systolic pressure-volume relation (ESPVR) in the rat LV, in contrast to the linear ESPVR in canine, rabbit, and human LVs. Although Emax, the slope of the linear ESPVR, could be used as an index of LV contractility, it was not applicable for evaluating LV contractility in the rat LV. Thus, we proposed a new index of contractility, equivalent Emax (eEmax) in the rat LV. We also found a linear VO₂-PVA relationship in the rat LV. Here, we introduce the methods for the preparation of excised, cross-circulated rat whole hearts and the eEmax-PVA-VO₂ framework in the rat LV. Using this method, we can obtain accurate LV volume and myocardial O₂ consumption in real time for estimating cardiac mechanoenergetics, which is very challenging in in vivo experiments.

Importance of SERCA2a on early isolated diastolic dysfunction induced by supravalvular aortic stenosis in rats

Cardiac remodeling is defined as changes in shape and function of the heart in response to aggression (pressure overload). The sarcoplasmic reticulum calcium ATPase cardiac isoform 2a (SERCA2a) is a known factor that influences function. A wide spectrum of studies report a decrease in SERCA2a in heart failure, but none evaluate it's the role in early isolated diastolic dysfunction in supravalvular aortic stenosis (AoS). Our hypothesis was that SERCA2a participates in such dysfunction. Thirty-day-old male Wistar rats (60-80 g) were divided into AoS and Sham groups, which were submitted to surgery with or without aorta clipping, respectively. After 6 weeks, the animals were submitted to echocardiogram and functional analysis by isolated papillary muscle (IPM) in basal condition, hypoxia, and SERCA2a blockage with cyclopiazonic acid at calcium concentrations of 0.5, 1.5, and 2.5 mM. Western-blot analyses were used for SERCA2a and phospholamban detection. Data analysis was carried out with Student's t-test and ANOVA. AoS enhanced left atrium and E and A wave ratio, with preserved ejection fraction. Basal condition in IPM showed similar increases in developed tension (DT) and resting tension (RT) in AoS, and hypoxia was similar between groups. After cyclopiazonic acid blockage, final DT was equally decreased and RT was similar between groups, but the speed of relaxation was decreased in the AoS group. Western-blot was uniform in all evaluations. The hypothesis was confirmed, since functional parameters regarding SERCA2a were changed in the AoS group.

Differential patterns of replacement and reactive fibrosis in pressure and volume overload are related to the propensity for ischaemia and involve resistin

Pathological left ventricle (LV) hypertrophy (LVH) results in reactive and replacement fibrosis. Volume overload LVH (VOH) is less profibrotic than pressure overload LVH (POH). Studies attribute subendocardial fibrosis in POH to ischaemia, and reduced fibrosis in VOH to collagen degradation favouring dilatation. However, the mechanical origin of the relative lack of fibrosis in VOH is incompletely understood. We hypothesized that reduced ischaemia propensity in VOH compared to POH accounted for the reduced replacement fibrosis, along with reduced reactive fibrosis. Rats with POH (ascending aortic banding) evolved into either compensated-concentric POH (POH-CLVH) or dilated cardiomyopathy (POH-DCM); they were compared to VOH (aorta-caval fistula). We quantified LV fibrosis, structural and haemodynamic factors of ischaemia propensity, and the activation of profibrotic pathways. Fibrosis in POH-DCM was severe, subendocardial and subepicardial, in contrast with subendocardial fibrosis in POH-CLVH and nearly no fibrosis in VOH. The propensity for ischaemia was more important in POH versus VOH, explaining different patterns of replacement fibrosis. LV collagen synthesis and maturation, and matrix metalloproteinase-2 expression, were more important in POH. The angiotensin II-transforming growth-factor β axis was enhanced in POH, and connective tissue growth factor (CTGF) was overexpressed in all types of LVH. LV resistin expression was markedly elevated in POH, mildly elevated in VOH and independently reflected chronic ischaemic injury after myocardial infarction. In vitro, resistin is induced by angiotensin II and induces CTGF in cardiomyocytes. Based on these findings, we conclude that a reduced ischaemia propensity and attenuated upstream reactive fibrotic pathways account for the attenuated fibrosis in VOH versus POH.

Xanthine oxidase inhibition preserves left ventricular systolic but not diastolic function in cardiac volume overload

Xanthine oxidase (XO) is increased in human and rat left ventricular (LV) myocytes with volume overload (VO) of mitral regurgitation and aortocaval fistula (ACF). In the setting of increased ATP demand, XO-mediated ROS can decrease mitochondrial respiration and contractile function. Thus, we tested the hypothesis that XO inhibition improves cardiomyocyte bioenergetics and LV function in chronic ACF in the rat. Sprague-Dawley rats were randomized to either sham or ACF ± allopurinol (100 mg·kg(-1)·day(-1), n ≥7 rats/group). Echocardiography at 8 wk demonstrated a similar 37% increase in LV end-diastolic dimension (P < 0.001), a twofold increase in LV end-diastolic pressure/wall stress (P < 0.05), and a twofold increase in lung weight (P < 0.05) in treated and untreated ACF groups versus the sham group. LV ejection fraction, velocity of circumferential shortening, maximal systolic elastance, and contractile efficiency were significantly depressed in ACF and significantly improved in ACF + allopurinol rats, all of which occurred in the absence of changes in the maximum O2 consumption rate measured in isolated cardiomyocytes using the extracellular flux analyzer. However, the improvement in contractile function is not paralleled by any attenuation in LV dilatation, LV end-diastolic pressure/wall stress, and lung weight. In conclusion, allopurinol improves LV contractile function and efficiency possibly by diminishing the known XO-mediated ROS effects on myofilament Ca(2+) sensitivity. However, LV remodeling and diastolic properties are not improved, which may explain the failure of XO inhibition to improve symptoms and hospitalizations in patients with severe heart failure.

Novel aspects of ROS signalling in heart failure

Heart failure and many of the conditions that predispose to heart failure are associated with oxidative stress. This is considered to be important in the pathophysiology of the condition but clinical trials of antioxidant approaches to prevent cardiovascular morbidity and mortality have been unsuccessful. Part of the reason for this may be the failure to appreciate the complexity of the effects of reactive oxygen species. At one extreme, excessive oxidative stress damages membranes, proteins and DNA but lower levels of reactive oxygen species may exert much more subtle and specific regulatory effects (termed redox signalling), even on physiological signalling pathways. In this article, we review our current understanding of the roles of such redox signalling pathways in the pathophysiology of heart failure, including effects on cardiomyocyte hypertrophy signalling, excitation-contraction coupling, arrhythmia, cell viability and energetics. Reactive oxygen species generated by NADPH oxidase proteins appear to be especially important in redox signalling. The delineation of specific redox-sensitive pathways and mechanisms that contribute to different components of the failing heart phenotype may facilitate the development of newer targeted therapies as opposed to the failed general antioxidant approaches of the past.

Physiologic and molecular characterization of a murine model of right ventricular volume overload

Pulmonary insufficiency (PI) is a common long-term sequel after repair of tetralogy of Fallot, causing progressive right ventricular (RV) dilation and failure. We describe the physiologic and molecular characteristics of the first murine model of RV volume overload. PI was created by entrapping the pulmonary valve leaflets with sutures. Imaging, catheterization, and exercise testing were performed at 1, 3, and 6 mo and compared with sham controls. RNA from the RV free wall was hybridized to Agilent whole genome oligonucleotide microarrays. Volume overload resulted in RV enlargement, decreased RV outflow tract shortening fraction at 1 mo followed by normalization at 3 and 6 mo (39 ± 2, 44 ± 2, and 41 ± 2 vs. 46 ± 3% in sham), early reversal of early and late diastolic filling velocities (E/A ratio) followed by pseudonormalization (0.87 ± 0.08, 0.82 ± 0.08, and 0.96 ± 0.08 vs. 1.04 ± 0.03; P < 0.05), elevated end-diastolic pressure (7.6 ± 0.7, 6.9 ± 0.8, and 7 ± 0.5 vs. 2.7 ± 0.2 mmHg; P < 0.05), and decreased exercise duration (26 ± 0.4, 26 ± 1, and 22 ± 1.3 vs. 30 ± 1.1 min; P < 0.05). Subendocardial RV fibrosis was evident by 1 mo. At 1 mo, 372 genes were significantly downregulated. Mitochondrial pathways and G protein-coupled receptor signaling were the most represented categories. At 3 mo, 434 genes were upregulated and 307 downregulated. While many of the same pathways continued to be downregulated, TNF-α, transforming growth factor-β(1) (TGF-β(1)), p53-signaling, and extracellular matrix (ECM) remodeling transitioned from down- to upregulated. We describe a novel murine model of chronic RV volume overload recapitulating aspects of the clinical disease with gene expression changes suggesting early mitochondrial bioenergetic dysfunction, enhanced TGF-β signaling, ECM remodeling, and apoptosis.

Myocardial twitch duration and the dependence of oxygen consumption on pressure-volume area: experiments and modelling

We tested the proposition that linear length dependence of twitch duration underlies the well-characterised linear dependence of oxygen consumption (V(O(2)) ) on pressure–volume area (PVA) in the heart. By way of experimental simplification, we reduced the problem from three dimensions to one by substituting cardiac trabeculae for the classically investigated whole-heart. This allowed adoption of stress–length area (SLA) as a surrogate for PVA, and heat as a proxy for V(O(2)) . Heat and stress (force per cross-sectional area), at a range of muscle lengths and at both 1 mM and 2 mM [Ca(2+)](o), were recorded from continuously superfused rat right-ventricular trabeculae undergoing fixed-end contractions. The heat–SLA relations of trabeculae (reported here, for the first time) are linear. Twitch duration increases monotonically (but not strictly linearly) with muscle length. We probed the cellular mechanisms of this phenomenon by determining: (i) the length dependence of the duration of the Ca(2+) transient, (ii) the length dependence of the rate of force redevelopment following a length impulse (an index of Ca(2+) binding to troponin-C), (iii) the effect on the simulated time course of the twitch of progressive deletion of length and Ca(2+)-dependent mechanisms of crossbridge cooperativity, using a detailed mathematical model of the crossbridge cycle, and (iv) the conditions required to achieve these multiple length dependencies, using a greatly simplified model of twitch mechano-energetics. From the results of these four independent investigations, we infer that the linearity of the heat–SLA relation (and, by analogy, the V(O(2))–PVA relation) is remarkably robust in the face of departures from linearity of length-dependent twitch duration.

Stroke volume-to-wall stress ratio as a load-adjusted and stiffness-adjusted indicator of ventricular systolic performance in chronic loading

Load-adjusted measures of left ventricle (LV) systolic performance are limited by dependence on LV stiffness and afterload. To our knowledge, no stiffness-adjusted and afterload-adjusted indicator was tested in models of pressure (POH) and volume overload hypertrophy (VOH). We hypothesized that wall stress reflects changes in loading, incorporating chamber stiffness and afterload; therefore, stroke volume-to-wall stress ratio more accurately reflects systolic performance. We used rat models of POH (ascending aortic banding) and VOH (aorto-cava shunt). Animals underwent echocardiography and pressure-volume analysis at baseline and dobutamine challenge. We achieved extreme bidirectional alterations in LV systolic performance, end-systolic elastance (Ees), passive stiffness, and arterial elastance (Ea). In POH with LV dilatation and failure, some load-independent indicators of systolic performance remained elevated compared with controls, while some others failed to decrease with wide variability. In VOH, most, but not all indicators, including LV ejection fraction, were significantly reduced compared with controls, despite hyperdynamic circulation, lack of heart failure, and preserved contractile reserve. We related systolic performance to Ees adjusted for Ea and LV passive stiffness in multivariate models. Calculated residual Ees was not reduced in POH with heart failure and was reduced in VOH, while it positively correlated to dobutamine dose. Conversely, stroke volume-to-wall stress ratio was normal in compensated POH, markedly decreased in POH with heart failure, and, in contrast with LV ejection fraction, normal in VOH. Our results support stroke volume-to-wall stress ratio as a load-adjusted and stiffness-adjusted indicator of systolic function in models of POH and VOH.

Proteomic and transcriptomic analysis of heart failure due to volume overload in a rat aorto-caval fistula model provides support for new potential therapeutic targets - monoamine oxidase A and transglutaminase 2

Background

Chronic hemodynamic overloading leads to heart failure (HF) due to incompletely understood mechanisms. To gain deeper insight into the molecular pathophysiology of volume overload-induced HF and to identify potential markers and targets for novel therapies, we performed proteomic and mRNA expression analysis comparing myocardium from Wistar rats with HF induced by a chronic aorto-caval fistula (ACF) and sham-operated rats harvested at the advanced, decompensated stage of HF.

Methods

We analyzed control and failing myocardium employing iTRAQ labeling, two-dimensional peptide separation combining peptide IEF and nano-HPLC with MALDI-MS/MS. For the transcriptomic analysis we employed Illumina RatRef-12v1 Expression BeadChip.

Results

In the proteomic analysis we identified 2030 myocardial proteins, of which 66 proteins were differentially expressed. The mRNA expression analysis identified 851 differentially expressed mRNAs.

Conclusions

The differentially expressed proteins confirm a switch in the substrate preference from fatty acids to other sources in the failing heart. Failing hearts showed downregulation of the major calcium transporters SERCA2 and ryanodine receptor 2 and altered expression of creatine kinases. Decreased expression of two NADPH producing proteins suggests a decreased redox reserve. Overexpression of annexins supports their possible potential as HF biomarkers. Most importantly, among the most up-regulated proteins in ACF hearts were monoamine oxidase A and transglutaminase 2 that are both potential attractive targets of low molecular weight inhibitors in future HF therapy.

Effect of metformin therapy on cardiac function and survival in a volume-overload model of heart failure in rats

Advanced HF (heart failure) is associated with altered substrate metabolism. Whether modification of substrate use improves the course of HF remains unknown. The antihyperglycaemic drug MET (metformin) affects substrate metabolism, and its use might be associated with improved outcome in diabetic HF. The aim of the present study was to examine whether MET would improve cardiac function and survival also in non-diabetic HF. Volume-overload HF was induced in male Wistar rats by creating ACF (aortocaval fistula). Animals were randomized to placebo/MET (300 mg·kg(-1) of body weight·day(-1), 0.5% in food) groups and underwent assessment of metabolism, cardiovascular and mitochondrial functions (n=6-12/group) in advanced HF stage (week 21). A separate cohort served for survival analysis (n=10-90/group). The ACF group had marked cardiac hypertrophy, increased LVEDP (left ventricular end-diastolic pressure) and lung weight confirming decompensated HF, increased circulating NEFAs (non-esterified 'free' fatty acids), intra-abdominal fat depletion, lower glycogen synthesis in the skeletal muscle (diaphragm), lower myocardial triacylglycerol (triglyceride) content and attenuated myocardial (14)C-glucose and (14)C-palmitate oxidation, but preserved mitochondrial respiratory function, glucose tolerance and insulin sensitivity. MET therapy normalized serum NEFAs, decreased myocardial glucose oxidation, increased myocardial palmitate oxidation, but it had no effect on myocardial gene expression, AMPK (AMP-activated protein kinase) signalling, ATP level, mitochondrial respiration, cardiac morphology, function and long-term survival, despite reaching therapeutic serum levels (2.2±0.7 μg/ml). In conclusion, MET-induced enhancement of myocardial fatty acid oxidation had a neutral effect on cardiac function and survival. Recently reported cardioprotective effects of MET may not be universal to all forms of HF and may require AMPK activation or ATP depletion. No increase in mortality on MET supports its safe use in diabetic HF.

Metabolic characterization of volume overload heart failure due to aorto-caval fistula in rats

Metabolic interactions between adipose tissue and the heart may play an active role in progression of heart failure (HF). The aim of the study was to examine changes in myocardial and adipose tissue metabolism and gene expression in a rat HF model induced by chronic volume overload. HF was induced by volume overload from aorto-caval fistula (ACF) in 3-month-old male Wistar rats and animals were studied in the phase of decompensated HF (22nd week). HF rats showed marked eccentric cardiac hypertrophy, pulmonary congestion, increased LV end-diastolic pressure, and intraabdominal fat depletion. HF rats had preserved glucose tolerance, but increased circulating free fatty acids (FFA) and attenuated insulin response during oral glucose challenge. Isolated organ studies showed preserved responsiveness of adipose tissue lipolysis and lipogenesis to epinephrine and insulin in ACF. The heart of HF animals had markedly reduced triglyceride content (almost to half of controls), attenuated anti-oxidative reserve (GSH/GSSG), upregulated HF markers (ANP, periostin, thrombospondin-4), specific signaling pathways (Wnt, TGF-β), and downregulated enzymes of mitochondrial fatty acid oxidation, citric acid cycle, and respiratory chain. Adipose tissue transcription profiling showed upregulated receptor for gastric inhibitory polypeptide. In conclusion, ACF-induced HF model displays several deregulations of systemic metabolism. Despite elevation of systemic FFAs, myocardial triglycerides are low and insulin levels are attenuated, arguing against a role of lipotoxicity or insulin resistance in this model. Attenuated postprandial insulin response and relative lack of its antilipolytic effects may facilitate intraabdominal fat depletion observed in ACF-HF animals.

Cardiac gene therapy

Heart failure is a chronic progressive disorder in which frequent and recurrent hospitalizations are associated with high mortality and morbidity. The incidence and the prevalence of this disease will increase with the increase in the number of the aging population of the United States. Understanding the molecular pathology and pathophysiology of this disease will uncover novel targets and therapies that can restore the function or attenuate the damage of malfunctioning cardiomyocytes by gene therapy that becomes an interesting and a promising field for the treatment of heart failure as well as other diseases in the future. Of equal importance are developing vectors and delivery methods that can efficiently transduce most of the cardiomyocytes that can offer a long-term expression and that can escape the host immune response. Recombinant adeno-associated virus vectors have the potential to become a promising novel therapeutic vehicles for molecular medicine in the future.

Left ventricular function of isoproterenol-induced hypertrophied rat hearts perfused with blood: mechanical work and energetics

We investigated left ventricular (LV) mechanical work and energetics in the cross-circulated (blood-perfused) isoproterenol [Iso 1.2 mg x kg(-1).day(-1) for 3 days (Iso3) or 7 days (Iso7)]-induced hypertrophied rat heart preparation under isovolumic contraction-relaxation. We evaluated pressure-time curves per beat, end-systolic pressure-volume and end-diastolic pressure-volume relations, and myocardial O(2) consumption per beat (Vo(2))-systolic pressure-volume area (PVA; a total mechanical energy per beat) linear relations at 240 beats/min, because Iso-induced hypertrophied hearts failed to completely relax at 300 beats/min. The LV relaxation rate at 240 beats/min in Iso-induced hypertrophied hearts was significantly slower than that in control hearts [saline 24 microl/day for 3 and 7 days (Sa)] with unchanged contraction rate. The Vo(2)-intercepts (composed of basal metabolism and Ca(2+) cycling energy consumption in excitation-contraction coupling) of Vo(2)-PVA linear relations were unchanged associated with their unchanged slopes in Sa, Iso3, and Iso7 groups. The oxygen costs of LV contractility were also unchanged in all three groups. The amounts of expression of sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban (PLB), phosphorylated-Ser(16) PLB, phospholemman, and Na(+)-K(+)-ATPase are significantly decreased in Iso3 and Iso7 groups, although the amount of expression of NCX1 is unchanged in all three groups. Furthermore, the marked collagen production (types I and III) was observed in Iso3 and Iso7 groups. These results suggested the possibility that lowering the heart rate was beneficial to improve mechanical work and energetics in isoproterenol-induced hypertrophied rat hearts, although LV relaxation rate was slower than in normal hearts.