Dissociation between cardiomyocyte function and remodeling with beta-adrenergic receptor blockade in isolated canine mitral regurgitation

An experimental model of chronic severe mitral regurgitation

Objective

To develop a minimally invasive, reproducible model of chronic severe mitral regurgitation (MR) that replicates the clinical phenotype of left atrial (LA) and left ventricular dilation and susceptibility to atrial fibrillation.

Methods

Under transesophageal echocardiographic guidance, chordae tendinae were avulsed using endovascular forceps until the ratio of regurgitant jet area to LA area was ≥70%. Animals survived for an average of 8.6 ± 1.6 months (standard deviation) and imaged with monthly transthoracic echocardiography (TTE). Animals underwent baseline and preterminal magnetic resonance imaging. Terminal studies included TTE, transesophageal echocardiography, and rapid atrial pacing to test inducibility of atrial tachyarrhythmias.

Results

Eight dogs underwent creation of severe MR and interval monitoring. Two were excluded-one died from acute heart failure, and the other had resolution of MR. Six dogs underwent the full experimental protocol; only one required medical management of clinical heart failure. MR remained severe over time, with a mean terminal regurgitant jet area to LA area of 71 ± 14% (standard deviation) and regurgitant fraction of 52 ± 11%. Mean LA volume increased over 130% (TTE: 163 ± 147%, P = .039; magnetic resonance imaging: 132 ± 54%, P = .011). Mean left ventricular end-diastolic volume increased by 38 ± 21% (P = .008). Inducible atrial tachyarrhythmias were seen in 4 of 6 animals at terminal surgery, and none at baseline.

Conclusions

Within the 6 dogs that successfully completed the full experimental protocol, this model replicated the clinical phenotype of severe MR, which led to marked structural and electrophysiologic cardiac remodeling. This model allowed for precise measurements at repeated time points and will facilitate future studies to elucidate the mechanisms of atrial and ventricular remodeling secondary to MR and the pathophysiology of valvular atrial fibrillation.

Mitochondrial haplotype modulates genome expression and mitochondrial structure/function in cardiomyocytes following volume overload

Mitochondrial DNA (mtDNA) haplotype regulates mitochondrial structure/function and reactive oxygen species in aortocaval fistula (ACF) in mice. Here, we unravel the mitochondrial haplotype effects on cardiomyocyte mitochondrial ultrastructure and transcriptome response to ACF in vivo. Phenotypic responses and quantitative transmission electron microscopy (TEM) and RNA sequence at 3 days were determined after sham surgery or ACF in vivo in cardiomyocytes from wild-type (WT) C57BL/6J (C57n:C57mt) and C3H/HeN (C3Hn:C3Hmt) and mitochondrial nuclear exchange mice (C57n:C3Hmt or C3Hn:C57mt). Quantitative TEM of cardiomyocyte mitochondria C3HWT hearts have more electron-dense compact mitochondrial cristae compared with C57WT. In response to ACF, mitochondrial area and cristae integrity are normal in C3HWT; however, there is mitochondrial swelling, cristae lysis, and disorganization in both C57WT and MNX hearts. Tissue analysis shows that C3HWT hearts have increased autophagy, antioxidant, and glucose fatty acid oxidation-related genes compared with C57WT. Comparative transcriptomic analysis of cardiomyocytes from ACF was dependent upon mtDNA haplotype. C57mtDNA haplotype was associated with increased inflammatory/protein synthesis pathways and downregulation of bioenergetic pathways, whereas C3HmtDNA showed upregulation of autophagy genes. In conclusion, ACF in vivo shows a protective response of C3Hmt haplotype that is in large part driven by mitochondrial nuclear genome interaction.NEW & NOTEWORTHY The results of this study support the effects of mtDNA haplotype on nuclear gene expression in cardiomyocytes. Currently, there is no acceptable therapy for volume overload due to mitral regurgitation. The findings of this study could suggest that mtDNA haplotype activates different pathways after ACF warrants further investigations on human population of heart disease from different ancestry backgrounds.

Management of Chronic Congestive Heart Failure Caused by Myxomatous Mitral Valve Disease in Dogs: A Narrative Review from 1970 to 2020

Simple Summary

Myxomatous mitral valve disease (MMVD) is the most common acquired cardiovascular disease in dogs. The progression of the disease and the increasing severity of valvular regurgitation cause a volume overload of the left heart, leading to left atrial and ventricular remodeling and congestive heart failure (CHF). The treatment of chronic CHF secondary to MMVD in dogs has not always been the same over time. In the last fifty years, the drugs utilized have considerably changed, as well as the therapeutic protocols. Some drugs have also changed their intended use. An analysis of the literature concerning the therapy of chronic heart failure in dogs affected by this widespread degenerative disease is not available; a synthesis of the published literature on this topic and a description of its current state of art are needed. To the authors’ knowledge, a review of this topic has never been published in veterinary medicine; therefore, the aim of this study is to overview the treatments of chronic CHF secondary to MMVD in dogs from 1970 to 2020 using the general framework of narrative reviews.

Abstract

The treatment of chronic congestive heart failure (CHF), secondary to myxomatous mitral valve disease (MMVD) in dogs, has considerably changed in the last fifty years. An analysis of the literature concerning the therapy of chronic CHF in dogs affected by MMVD is not available, and it is needed. Narrative reviews (NRs) are aimed at identifying and summarizing what has been previously published, avoiding duplications, and seeking new study areas that have not yet been addressed. The most accessible open-access databases, PubMed, Embase, and Google Scholar, were chosen, and the searching time frame was set in five decades, from 1970 to 2020. The 384 selected studies were classified into categories depending on the aim of the study, the population target, the pathogenesis of MMVD (natural/induced), and the resulting CHF. Over the years, the types of studies have increased considerably in veterinary medicine. In particular, there have been 43 (24.29%) clinical trials, 41 (23.16%) randomized controlled trials, 10 (5.65%) cross-over trials, 40 (22.60%) reviews, 5 (2.82%) comparative studies, 17 (9.60%) case-control studies, 2 (1.13%) cohort studies, 2 (1.13%) experimental studies, 2 (1.13%) questionnaires, 6 (3.40%) case-reports, 7 (3.95%) retrospective studies, and 2 (1.13%) guidelines. The experimental studies on dogs with an induced form of the disease were less numerous (49–27.68%) than the studies on dogs affected by spontaneous MMVD (128–72.32%). The therapy of chronic CHF in dogs has considerably changed in the last fifty years: in the last century, some of the currently prescribed drugs did not exist yet, while others had different indications.

Mechanism of Eccentric Cardiomyocyte Hypertrophy Secondary to Severe Mitral Regurgitation

BACKGROUND

Primary valvular heart disease is a prevalent cause of morbidity and mortality in both industrialized and developing countries. Although the primary consequence of valvular heart disease is myocardial dysfunction, treatment of valvular heart diseases centers around valve repair or replacement rather than prevention or reversal of myocardial dysfunction. This is particularly evident in primary mitral regurgitation (MR), which invariably results in eccentric hypertrophy and left ventricular (LV) failure in the absence of timely valve repair or replacement. The mechanism of LV dysfunction in primary severe MR is entirely unknown.

METHODS

Here, we developed the first mouse model of severe MR. Valvular damage was achieved by severing the mitral valve leaflets and chords with iridectomy scissors, and MR was confirmed by echocardiography. Serial echocardiography was performed to follow up LV morphology and systolic function. Analysis of cardiac tissues was subsequently performed to evaluate valve deformation, cardiomyocyte morphology, LV fibrosis, and cell death. Finally, dysregulated pathways were assessed by RNA-sequencing analysis and immunofluorescence.

RESULTS

In the ensuing 15 weeks after the induction of MR, gradual LV dilatation and dysfunction occurred, resulting in severe systolic dysfunction. Further analysis revealed that severe MR resulted in a marked increase in cardiac mass and increased cardiomyocyte length but not width, with electron microscopic evidence of sarcomere disarray and the development of sarcomere disruption. From a mechanistic standpoint, severe MR resulted in activation of multiple components of both the mammalian target of rapamycin and calcineurin pathways. Inhibition of mammalian target of rapamycin signaling preserved sarcomeric structure and prevented LV remodeling and systolic dysfunction. Immunohistochemical analysis uncovered a differential pattern of expression of the cell polarity regulator Crb2 (crumbs homolog 2) along the longitudinal axis of cardiomyocytes and close to the intercalated disks in the MR hearts. Electron microscopy images demonstrated a significant increase in polysome localization in close proximity to the intercalated disks and some areas along the longitudinal axis in the MR hearts.

CONCLUSIONS

These results indicate that LV dysfunction in response to severe MR is a form of maladaptive eccentric cardiomyocyte hypertrophy and outline the link between cell polarity regulation and spatial localization protein synthesis as a pathway for directional cardiomyocyte growth.

Reduced Left Atrial Emptying Fraction and Chymase Activation in Pathophysiology of Primary Mitral Regurgitation

Increasing left atrial (LA) size predicts outcomes in patients with isolated mitral regurgitation (MR). Chymase is plentiful in the human heart and affects extracellular matrix remodeling. Chymase activation correlates to LA fibrosis, LA enlargement, and a decreased total LA emptying fraction in addition to having a potential intracellular role in mediating myofibrillar breakdown in LA myocytes. Because of the unreliability of the left ventricular ejection fraction in predicting outcomes in MR, LA size and the total LA emptying fraction may be more suitable indicators for timing of surgical intervention.

Dynamic Changes in the Molecular Signature of Adverse Left Ventricular Remodeling in Patients With Compensated and Decompensated Chronic Primary Mitral Regurgitation

BACKGROUND

There is no proven medical therapy that attenuates adverse left ventricular remodeling in patients with chronic primary mitral regurgitation (CPMR). Identification of molecular pathways important in the progression of left ventricular remodeling in patients with CPMR may lead to development of new therapeutic strategies.

METHODS AND RESULTS

We performed baseline echocardiographic, cardiac catheterization, and serum NT-pro-BNP analysis in patients with severe CPMR awaiting mitral valve surgery and stratified the study population into compensated or decompensated CPMR. We obtained left ventricular endomyocardial biopsies (n=12) for mRNA expression analysis, and compared baseline transcript levels of 109 genes important in volume-overload left ventricular remodeling with levels in normal hearts (n=5) and between patients with compensated (n=6) versus decompensated (n=6) CPMR. Patients were then randomized to treatment with and without carvedilol and followed until the time of surgery (mean follow-up 8.3 months) when repeat endomyocardial biopsies were obtained to correlate transcriptional dynamics with indices of adverse remodeling. CPMR was associated with increased NPPA expression levels (21.6-fold, P=0.004), decreased transcripts of genes important in cell survival, and enrichment of extracellular matrix genes. Decompensated CPMR was associated with downregulation of SERCA2 (0.77-fold, P=0.009) and mitochondrial gene expression levels and upregulation of genes related to inflammation, the extracellular matrix, and apoptosis, which were refractory to carvedilol therapy.

CONCLUSIONS

Transition to decompensated CPMR is associated with calcium dysregulation, increased expression of inflammatory, extracellular matrix and apoptotic genes, and downregulation of genes important in bioenergetics. These changes are not attenuated by carvedilol therapy and highlight the need for development of specific combinatorial therapies, targeting myocardial inflammation and apoptosis, together with urgent surgical or percutaneous valve interventions.

Left ventricular remodelling in chronic primary mitral regurgitation: implications for medical therapy

Surgical repair or replacement of the mitral valve is currently the only recommended therapy for severe primary mitral regurgitation. The chronic elevation of wall stress caused by the resulting volume overload leads to structural remodelling of the muscular, vascular and extracellular matrix components of the myocardium. These changes are initially compensatory but in the long term have detrimental effects, which ultimately result in heart failure. Understanding the changes that occur in the myocardium due to volume overload at the molecular and cellular level may lead to medical interventions, which potentially could delay or prevent the adverse left ventricular remodelling associated with primary mitral regurgitation. The pathophysiological changes involved in left ventricular remodelling in response to chronic primary mitral regurgitation and the evidence for potential medical therapy, in particular beta-adrenergic blockers, are the focus of this review.

Impact of Beta-Blockade on Cardiac Events in Patients with Chronic Severe Nonischemic Mitral Regurgitation

OBJECTIVES

The aim of this study was to examine the impact of beta-blockade on cardiac events among patients with initially asymptomatic chronic severe nonischemic mitral valve regurgitation (MR).

METHODS

Data from 52 consecutive patients in our prospective natural history study of isolated chronic severe nonischemic MR were assessed post hoc over 19 years to examine the relation of chronic beta-blockade use to subsequent cardiac events (death or indications for mitral valve surgery, MVS). At entry, all patients were free of surgical indications; 9 received beta-blockers. Cardiac event rate differences were analyzed by Kaplan-Meier log rank comparison.

RESULTS

During follow-up, cardiac events included sudden death (1), heart failure (8), atrial fibrillation (6), left ventricular dimensions at systole ≥4.5 cm (11), left ventricular ejection fraction <60% (6), right ventricular ejection fraction <35% (2), and a combination of cardiac events (7). The cardiac event risk was 4-fold higher among patients receiving beta-blockers (average annual risk = 60.6%) versus those not receiving beta-blockers (average annual risk = 15.2%; p = 0.001). These effects remained statistically significant (p = 0.005) when analysis was adjusted for other baseline covariates.

CONCLUSIONS

Beta-blockade appears to confer an increased risk of sudden cardiac death or indications for MVS among patients with chronic severe nonischemic MR. Randomized trials are needed to confirm these findings.

Disruption of desmin-mitochondrial architecture in patients with regurgitant mitral valves and preserved ventricular function

OBJECTIVE

Recent studies have demonstrated improved outcomes in patients receiving early surgery for degenerative mitral regurgitation (MR) rather than adhering to conventional guidelines for surgical intervention. However, studies providing a mechanistic basis for these findings are limited.

METHODS

Left ventricular (LV) myocardium from 22 patients undergoing mitral valve repair for American Heart Association class I indications was evaluated for desmin, the voltage-dependent anion channel, α-B-crystallin, and α, β-unsaturated aldehyde 4-hydroxynonenal by fluorescence microscopy. The same was evaluated in 6 normal control LV autopsy specimens. Cardiomyocyte ultrastructure was examined by transmission electron microscopy. Magnetic resonance imaging with tissue tagging was performed in 55 normal subjects and 22 MR patients before and 6 months after mitral valve repair.

RESULTS

LV end-diastolic volume was 1.5-fold (P < .0001) higher and LV mass-to-volume ratio was lower in MR (P = .004) hearts versus normal hearts and showed improvement 6 months after mitral valve surgery. However, LV ejection fraction decreased from 65% ± 7% to 52% ± 9% (P < .0001) and LV circumferential (P < .0001) and longitudinal strain decreased significantly below normal values (P = .002) after surgery. Hearts with MR had a 53% decrease in desmin (P < .0001) and a 2.6-fold increase in desmin aggregates (P < .0001) versus normal, along with substantial, intense perinuclear staining of α, β-unsaturated aldehyde 4-hydroxynonenal in areas of mitochondrial breakdown and clustering. Transmission electron microscopy demonstrated numerous electron-dense deposits, myofibrillar loss, Z-disc abnormalities, and extensive granulofilamentous debris identified as desmin-positive by immunogold transmission electron microscopy.

CONCLUSIONS

Despite well-preserved preoperative LV ejection fraction, severe oxidative stress and disruption of cardiomyocyte desmin-mitochondrial sarcomeric architecture may explain postoperative LV functional decline and further supports the move toward earlier surgical intervention.

Pathologic Manifestations on Surgical Biopsy and Their Correlation with Clinical Indices in Dogs with Degenerative Mitral Valve Disease

BACKGROUND

Evaluation of myocardial function is clinically challenging in dogs with degenerative mitral valve disease (DMVD). Although myocardial dysfunction is caused by pathologic degeneration, histopathologic progression is poorly understood.

OBJECTIVES

To characterize myocardial and pulmonary pathologic changes according to severity in dogs with naturally occurring DMVD, and to investigate whether or not pathologic degeneration is reflected by traditional clinical indices.

ANIMALS

One hundred and seventeen dogs with naturally occurring DMVD.

METHODS

Prospective observational study. Biopsied left atrium (LA), left ventricle (LV), and lung were evaluated histologically, and an attempt was made to correlate pathologic findings with clinical indices.

RESULTS

Severe myocardial changes were observed in all International Small Animal Cardiac Health Council classes. In the lung, heart failure cell levels were significantly increased in class III patients (P < .0001). In a paired comparison, the LA showed significantly more severe degeneration than the LV, including myocardial fatty replacement, immune cell infiltration, and interstitial fibrosis (P < .0001). In contrast, myocardial cells were more hypertrophied in the LV than in the LA (P < .0001). Left ventricular end-diastolic dimension (LVEDd) was associated with fatty replacement (P = .033, R(2) = 0.584) and myocardial vacuolization (P = .003, R(2) = 0.588) in the LA.

CONCLUSIONS AND CLINICAL IMPORTANCE

In DMVD, although severe pathologic changes may be evident even in early stages, there may be pathologic discrepancy between the LA and the LV. Myocardial degeneration may be reflected by clinical indices such as LVEDd and EF.

The multiple mechanistic faces of a pure volume overload: implications for therapy

: Mitral regurgitation and other conditions marked by a pure isolated volume overload (VO) of the heart result in a progressive form of eccentric left ventricular remodeling and dysfunction. As opposed to the more extensively studied pressure overload, there are no approved medical therapies because an understanding of the underlying pathological mechanisms at work in VO is lacking. Over the past 20 years, our laboratory has identified multiple key biological functions involved in the pathological remodeling in VO. Specifically, we have noted perturbed matrix homeostasis, detrimental adrenergic signaling, increased intracellular reactive oxygen species and an intense inflammatory response that implicates mast cells and their product chymase, which seems to cause extensive remodeling both inside and outside the cardiomyocyte. How these multiple pathways intersect over the course of VO and their response to various single and combined interventions are now the subject of intense investigation.

Chronic β1-adrenergic blockade enhances myocardial β3-adrenergic coupling with nitric oxide-cGMP signaling in a canine model of chronic volume overload: new insight into mechanisms of cardiac benefit with selective β1-blocker therapy

The β1-adrenergic antagonist metoprolol improves cardiac function in animals and patients with chronic heart failure, isolated mitral regurgitation (MR), and ischemic heart disease, though the molecular mechanisms remain incompletely understood. Metoprolol has been reported to upregulate cardiac expression of β3-adrenergic receptors (β3AR) in animal models. Myocardial β3AR signaling via neuronal nitric oxide synthase (nNOS) activation has recently emerged as a cardioprotective pathway. We tested whether chronic β1-adrenergic blockade with metoprolol enhances myocardial β3AR coupling with nitric oxide-stimulated cyclic guanosine monophosphate (β3AR/NO-cGMP) signaling in the MR-induced, volume-overloaded heart. We compared the expression, distribution, and inducible activation of β3AR/NO-cGMP signaling proteins within myocardial membrane microdomains in dogs (canines) with surgically induced MR, those also treated with metoprolol succinate (MR+βB), and unoperated controls. β3AR mRNA transcripts, normalized to housekeeping gene RPLP1, increased 4.4 × 10(3)- and 3.2 × 10(2)-fold in MR and MR+βB hearts, respectively, compared to Control. Cardiac β3AR expression was increased 1.4- and nearly twofold in MR and MR+βB, respectively, compared to Control. β3AR was detected within caveolae-enriched lipid rafts (Cav3(+)LR) and heavy density, non-lipid raft membrane (NLR) across all groups. However, in vitro selective β3AR stimulation with BRL37344 (BRL) triggered cGMP production within only NLR of MR+βB. BRL induced Ser (1412) phosphorylation of nNOS within NLR of MR+βB, but not Control or MR, consistent with detection of NLR-specific β3AR/NO-cGMP coupling. Treatment with metoprolol prevented MR-associated oxidation of NO biosensor soluble guanylyl cyclase (sGC) within NLR. Metoprolol therapy also prevented MR-induced relocalization of sGCβ1 subunit away from caveolae, suggesting preserved NO-sGC-cGMP signaling, albeit without coupling to β3AR, within MR+βB caveolae. Chronic β1-blockade is associated with myocardial β3AR/NO-cGMP coupling in a microdomain-specific fashion. Our canine study suggests that microdomain-targeted enhancement of myocardial β3AR/NO-cGMP signaling may explain, in part, β1-adrenergic antagonist-mediated preservation of cardiac function in the volume-overloaded heart.

Chronic vagus nerve stimulation improves left ventricular function in a canine model of chronic mitral regurgitation

BACKGROUND

Autonomic dysfunction, characterized by sympathetic activation and vagal withdrawal, contributes to the progression of heart failure (HF). We hypothesized that chronic vagus nerve stimulation (VNS) could prevent left ventricular (LV) remodeling and dysfunction in a canine HF model induced by chronic mitral regurgitation (MR).

METHODS AND RESULTS

After the MR inducing procedure, 12 survived canines were randomly divided into the control (n = 6) and the VNS (n = 6) groups. At month 2, a VNS stimulator system was implanted in all canines. From month 3 to month 6, VNS therapy was applied in the VNS group but not in the control group. At month 6, compared with the control group, the canines in VNS group had significantly higher cardiac output (2.3 ± 0.3 versus 2.9 ± 0.4 L/min, P < 0.05, LV forward stroke volume (20.1 ± 3.7 versus 24.8 ± 3.9 ml, P < 0.05), and end-systolic stiffness constant (2.2 ± 0.3 versus 2.7 ± 0.3, P < 0.05). NT-proBNP and C-reactive protein were decreased significantly in the VNS group. However, no statistical difference was found in LV ejection fraction, LV end-diastolic dimension, LV end-diastolic volume, myocyte cross-sectional area, or collagen volume fraction between two groups.

CONCLUSIONS

Chronic VNS therapy may ameliorate MR-induced LV contractile dysfunction and improve the expression of biomarkers, but has less effect in improving LV chamber remodeling.

Associations between N-terminal procollagen type III, fibrosis and echocardiographic indices in dogs that died due to myxomatous mitral valve disease

OBJECTIVES

To evaluate associations between N-terminal procollagen type III (PIIINP), a serum biomarker of collagen biosynthesis, and myocardial fibrosis in dogs with naturally-occurring myxomatous mitral valve disease (MMVD).

ANIMALS

Twenty-two dogs with echocardiographically-confirmed MMVD were prospectively recruited from a hospital population. All died as a result of MMVD and their hearts were available for post mortem examination.

METHODS

Echocardiographic measurements and serum PIIINP concentrations were obtained from all dogs prior to death or euthanasia. Serum PIIINP concentrations (μg/mL) were measured using a validated commercially available radioimmunoassay. Myocardial tissue samples were collected post mortem and myocardial fibrosis was scored. The average fibrosis score for all cardiac sites in the heart was designated the global fibrosis score (GFS). The average fibrosis score for all papillary muscle sites was designated the papillary fibrosis score (PFS). Univariate and multivariate linear regression analyses were used separately to evaluate associations between GFS and PFS, respectively, and PIIINP and echocardiographic variables.

RESULTS

Left ventricular end-diastolic diameter normalized for body weight (LVEDDN) and PIIINP were weakly independently positively associated with both GFS and PFS. LVEDDN and PIIINP were weakly negatively correlated.

CONCLUSIONS

Both LVEDDN and serum PIIINP increase with increasing fibrosis score, although these relationships were not strong enough to be clinically useful. Although LVEDDN and PIIINP were positively correlated with fibrosis, PIIINP decreased with increasing LVEDDN, suggesting a complex interplay between fibrosis and remodeling in MMVD.

Increased sarcolipin expression and adrenergic drive in humans with preserved left ventricular ejection fraction and chronic isolated mitral regurgitation

BACKGROUND

There is currently no therapy proven to attenuate left ventricular (LV) dilatation and dysfunction in volume overload induced by isolated mitral regurgitation (MR). To better understand molecular signatures underlying isolated MR, we performed LV gene expression analyses and overlaid regulated genes into ingenuity pathway analysis in patients with isolated MR.

METHODS AND RESULTS

Gene arrays from LV tissue of 35 patients, taken at the time of surgical repair for isolated MR, were compared with 13 normal controls. Cine-MRI was performed in 31 patients before surgery to measure LV function and volume from serial short-axis summation. LV end-diastolic volume was 2-fold (P=0.005) higher in MR patients than in normal controls, and LV ejection fraction was 64±7% (50%-79%) in MR patients. Ingenuity pathway analysis identified significant activation of pathways involved in β-adrenergic, cAMP, and G-protein-coupled signaling, whereas there was downregulation of pathways associated with complement activation and acute phase response. SERCA2a and phospholamban protein were unchanged in MR versus control left ventricles. However, mRNA and protein levels of the sarcoplasmic reticulum Ca2+ ATPase (SERCA) regulatory protein sarcolipin, which is predominantly expressed in normal atria, were increased 12- and 6-fold, respectively. Immunofluorescence analysis confirmed the absence of sarcolipin in normal left ventricles and its marked upregulation in MR left ventricles.

CONCLUSIONS

These results demonstrate alterations in multiple pathways associated with β-adrenergic signaling and sarcolipin in the left ventricles of patients with isolated MR and LV ejection fraction>50%, suggesting a beneficial role for β-adrenergic blockade in isolated MR.

Volume overload induces differential spatiotemporal regulation of myocardial soluble guanylyl cyclase in eccentric hypertrophy and heart failure

Nitric oxide activation of soluble guanylyl cyclase (sGC) blunts the cardiac stress response, including cardiomyocyte hypertrophy. In the concentric hypertrophied heart, oxidation and re-localization of myocardial sGC diminish cyclase activity, thus aggravating depressed nitric oxide-cyclic guanosine monophosphate (NO-cGMP) signaling in the pressure-overloaded failing heart. Here, we hypothesized that volume-overload differentially disrupts myocardial sGC activity during early compensated and late decompensated stages of eccentric hypertrophy. To this end, we studied the expression, redox state, subcellular localization, and activity of sGC in the left ventricle of dogs subjected to chordal rupture-induced mitral regurgitation (MR). Unoperated dogs were used as Controls. Animals were studied at 4weeks and 12months post chordal rupture, corresponding with early (4wkMR) and late stages (12moMR) of eccentric hypertrophy. We found that the sGC heterodimer subunits relocalized away from caveolae-enriched lipid raft microdomains at different stages; sGCβ1 at 4wkMR, followed by sGCα1 at 12moMR. Moreover, expression of both sGC subunits fell at 12moMR. Using the heme-dependent NO donor DEA/NO and NO-/heme-independent sGC activator BAY 60-2770, we determined the redox state and inducible activity of sGC in the myocardium, within caveolae and non-lipid raft microdomains. sGC was oxidized in non-lipid raft microdomains at 4wkMR and 12moMR. While overall DEA/NO-responsiveness remained intact in MR hearts, DEA/NO responsiveness of sGC in non-lipid raft microdomains was depressed at 12moMR. Caveolae-localization protected sGC against oxidation. Further studies revealed that these modifications of sGC were also reflected in caveolae-localized cGMP-dependent protein kinase (PKG) and MAPK signaling. In MR hearts, PKG-mediated phosphorylation of vasodilator-stimulated phosphoprotein (VASP) disappeared from caveolae whereas caveolae-localization of phosphorylated ERK5 increased. These findings show that differential oxidation, re-localization, and expression of sGC subunits distinguish eccentric from concentric hypertrophy as well as compensated from decompensated heart failure.

Effects of early, late, and long-term nonselective β-blockade on left ventricular remodeling, function, and survival in chronic organic mitral regurgitation

BACKGROUND

Mitral regurgitation (MR) produces sympathetic nervous system activation which is detrimental in other causes of heart failure. However, whether β-blockade is beneficial in MR has not been determined.

METHODS AND RESULTS

Eighty-seven rats with significant organic MR were randomized to the β-blockade group (n=43) or the control group (n=44). Carvedilol was started in week 2 post MR induction and given for 23 to 35 weeks in the β-blockade group. Echocardiography was performed at baseline and at weeks 2, 6, 12, 24, 30, and 36 after MR induction. After 23 weeks of β-blockade, heart rates were significantly reduced by carvedilol (308 ± 25 versus 351 ± 31 beats per minute; P<0.001). Left ventricular end-diastolic (2.2 ± 0.7 versus 1.59 ± 0.6 mL; P<0.001), end-systolic volumes (0.72 ± 0.42 versus 0.40 ± 0.19 mL; P<0.001), and mass index (2.40 ± 0.55 versus 2.06 ± 0.62 g/kg; P<0.001) were significantly higher, and left ventricular fraction shortening (33 ± 7% versus 38 ± 7%; P<0.001) and ejection fraction (69 ± 11% versus 75 ± 7%; P<0.001) were significantly lower in the β-blockade group than in the control group. Systolic blood pressure was lower in the β-blockade group than in the control group (114 ± 10 versus 93 ± 12 mm Hg; P<0.005). Survival probability was significantly lower in the early β-blockade group than in the control group (88% versus 96%; P=0.03).

CONCLUSIONS

Early and long-term nonselective β-blockade was associated with adverse left ventricular remodeling, systolic dysfunction, and a reduction in survival in the experimental rat model of organic MR.

Importance of three-dimensional geometric analysis in the assessment of the athlete's heart

How the left ventricle remodels in response to a high-volume stimulus is important in evaluating the endurance athlete's heart. Marathoners and patients with isolated, moderate chronic compensated mitral regurgitation (MR) represent physiologic and pathologic forms of eccentric left ventricular (LV) remodeling in response to intermittent and chronic volume overload, respectively. Thus, in this study, magnetic resonance imaging with tissue tagging and 3-dimensional data analysis at rest were performed in 19 marathoners (mean age 39 ± 10 years, 47% women), 17 patients with isolated MR without coronary artery disease or medical therapy (mean age 46 ± 5 years, 53% women), and 24 controls (mean age 45 ± 8 years, 50% women). Marathoners and patients with MR had approximately 35% greater LV end-diastolic volume indexes, approximately 50% greater end-systolic volume indexes, and approximately 34% greater LV stroke volume indexes (p <0.0001) compared to controls. However, marathoners' hearts had increased long-axis length, while those of patients with MR did not differ from the hearts of controls. The hearts of patients with MR had greater LV global and apex sphericity compared to those of marathoners and controls (p <0.0001). Marathoners had normal LV mass/volume ratios and wall thicknesses, whereas these were significantly decreased in the MR group. In marathoners, the baseline LV work rate was similar to that in controls and higher in patients with MR compared to controls. In conclusion, marathoners' hearts achieve elevated stroke volume at rest with adherence to an elliptical shape defined by 3-dimensional geometry and mass/volume ratio. Thus, a comprehensive evaluation of LV geometry and mass/volume ratio may be important in the evaluation of the athlete's heart.

A randomized controlled phase IIb trial of beta(1)-receptor blockade for chronic degenerative mitral regurgitation

OBJECTIVES

The purpose of the study was to evaluate the effect of long-term β(1)-aderergic receptor (AR) blockade on left ventricular (LV) remodeling and function in patients with chronic, isolated, degenerative mitral regurgitation (MR).

BACKGROUND

Isolated MR currently has no proven therapy that attenuates LV remodeling or preserves systolic function.

METHODS

Thirty-eight asymptomatic subjects with moderate to severe, isolated MR were randomized either to placebo or β(1)-AR blockade (Toprol-XL, AstraZeneca, London, United Kingdom) for 2 years. Magnetic resonance imaging with tissue tagging and 3-dimensional analysis was performed at baseline and at 6-month intervals for 2 years. Rate of progression analysis was performed for endpoint variables for primary outcomes: LV end-diastolic volume/body surface area, LV ejection fraction, LV end-diastolic (ED) mass/ED volume ratio, LV ED 3-dimensional radius/wall thickness; LV end-systolic volume/body surface area, LV longitudinal strain rate, and LV early diastolic filling rate.

RESULTS

Baseline LV magnetic resonance imaging or demographic variables did not differ between the 2 groups. Significant treatment effects were found on LV ejection fraction (p = 0.006) and LV early diastolic filling rate (p = 0.001), which decreased over time in untreated patients on an intention-to-treat analysis and remained significant after sensitivity analysis. There were no significant treatment effects found on LV ED or LV end-systolic volumes, LV ED mass/LV ED volume or LV ED 3-dimensional radius/wall thickness, or LV longitudinal strain rate. Over 2 years, 6 patients treated in the placebo group and 2 patients in the β(1)-AR blockade group required mitral valve surgery (p = 0.23).

CONCLUSIONS

β(1)-AR blockade improves LV function over a 2-year follow-up in isolated MR and provides the impetus for a large-scale clinical trial with clinical outcomes. (Molecular Mechanisms of Volume Overload-Aim 1 [SCCOR in Cardiac Dysfunction and Disease]; NCT01052428).

Pharmacologic management of myxomatous mitral valve disease in dogs

Myxomatous mitral valve disease (MMVD) causing mitral regurgitation is the most important disease of the heart in small animal cardiovascular medicine. Because MMVD is an example of a chronic disease that progresses from mild to severe over years, treatment strategies change with the stage of the disease. In this review the treatment options are compared and contrasted as they are discussed relative to the recently published ACVIM consensus statement regarding the treatment of MMVD. Results from clinical trials and evidence-based medicine are likely to provide significant improvements in the management of MMVD in the coming decades.

Left ventricular remodeling in preclinical experimental mitral regurgitation of dogs

Dogs with experimental mitral regurgitation (MR) provide insights into the left ventricular remodeling in preclinical MR. The early preclinical left ventricular (LV) changes after mitral regurgitation represent progressive dysfunctional remodeling, in that no compensatory response returns the functional stroke volume (SV) to normal even as total SV increases. The gradual disease progression leads to mitral annulus stretch and enlargement of the regurgitant orifice, further increasing the regurgitant volume. Remodeling with loss of collagen weave and extracellular matrix (ECM) is accompanied by stretching and hypertrophy of the cross-sectional area and length of the cardiomyocyte. Isolated ventricular cardiomyocytes demonstrate dysfunction based on decreased cell shortening and reduced intracellular calcium transients before chamber enlargement or decreases in contractility in the whole heart can be clinically appreciated. The genetic response to increased end-diastolic pressure is down-regulation of genes associated with support of the collagen and ECM and up-regulation of genes associated with matrix remodeling. Experiments have not demonstrated any beneficial effects on remodeling from treatments that decrease afterload via blocking the renin-angiotensin system (RAS). Beta-1 receptor blockade and chymase inhibition have altered the progression of the LV remodeling and have supported cardiomyocyte function. The geometry of the LV during the remodeling provides insight into the importance of regional differences in responses to wall stress.

Survival and echocardiographic data in dogs with congestive heart failure caused by mitral valve disease and treated by multiple drugs: a retrospective study of 21 cases

This retrospective study reports the survival time [onset of congestive heart failure (CHF) to death from any cause] of 21 dogs with mitral regurgitation (MR) and CHF treated with a combination of furosemide, angiotensin-converting enzyme inhibitor (ACEI, benazepril, or enalapril), pimobendan, spironolactone, and amlodipine. Baseline echocardiographic data: end-systolic and end-diastolic volume indices (ESVI and EDVI), left atrium to aorta ratio (LA/Ao), and regurgitant fraction (RF) are reported. Median survival time (MST) was 430 d. Initial dosage of furosemide (P = 0.0081) and LA/Ao (P = 0.042) were negatively associated with survival. Baseline echocardiographic indices (mean ± standard deviation) were 40.24 ± 16.76 for ESVI, 161.48 ± 44.49 mL/m(2) for EDVI, 2.11 ± 0.75 for LA/Ao, and 64.71 ± 16.85% for RF. Combining furosemide, ACEI, pimobendan, spironolactone, and amlodipine may result in long survival times in dogs with MR and CHF. Severity of MR at onset of CHF is at least moderate.

Chymase inhibition prevents fibronectin and myofibrillar loss and improves cardiomyocyte function and LV torsion angle in dogs with isolated mitral regurgitation

BACKGROUND

The left ventricular (LV) dilatation of isolated mitral regurgitation (MR) is associated with an increase in chymase and a decrease in interstitial collagen and extracellular matrix. In addition to profibrotic effects, chymase has significant antifibrotic actions because it activates matrix metalloproteinases and kallikrein and degrades fibronectin. Thus, we hypothesize that chymase inhibitor (CI) will attenuate extracellular matrix loss and LV remodeling in MR.

METHODS AND RESULTS

We studied dogs with 4 months of untreated MR (MR; n=9) or MR treated with CI (MR+CI; n=8). Cine MRI demonstrated a >40% increase in LV end-diastolic volume in both groups, consistent with a failure of CI to improve a 25% decrease in interstitial collagen in MR. However, LV cardiomyocyte fractional shortening was decreased in MR versus normal dogs (3.71±0.24% versus 4.81±0.31%; P<0.05) and normalized in MR+CI dogs (4.85±0.44%). MRI with tissue tagging demonstrated an increase in LV torsion angle in MR+CI versus MR dogs. CI normalized the significant decrease in fibronectin and FAK phosphorylation and prevented cardiomyocyte myofibrillar degeneration in MR dogs. In addition, total titin and its stiffer isoform were increased in the LV epicardium and paralleled the changes in fibronectin and FAK phosphorylation in MR+CI dogs.

CONCLUSIONS

These results suggest that chymase disrupts cell surface-fibronectin connections and FAK phosphorylation that can adversely affect cardiomyocyte myofibrillar structure and function. The greater effect of CI on epicardial versus endocardial titin and noncollagen cell surface proteins may be responsible for the increase in torsion angle in chronic MR.