Chronic pressure overload cardiac hypertrophy and failure in guinea pigs: I. Regional hemodynamics and myocyte remodeling

Depressed myocardial cross-bridge cycling kinetics in a female guinea pig model of diastolic heart failure

Cardiac hypertrophy is associated with diastolic heart failure (DHF), a syndrome in which systolic function is preserved but cardiac filling dynamics are depressed. The molecular mechanisms underlying DHF and the potential role of altered cross-bridge cycling are poorly understood. Accordingly, chronic pressure overload was induced by surgically banding the thoracic ascending aorta (AOB) in ∼400 g female Dunkin Hartley guinea pigs (AOB); Sham-operated age-matched animals served as controls. Guinea pigs were chosen to avoid the confounding impacts of altered myosin heavy chain (MHC) isoform expression seen in other small rodent models. In vivo cardiac function was assessed by echocardiography; cardiac hypertrophy was confirmed by morphometric analysis. AOB resulted in left ventricle (LV) hypertrophy and compromised diastolic function with normal systolic function. Biochemical analysis revealed exclusive expression of β-MHC isoform in both sham control and AOB LVs. Myofilament function was assessed in skinned multicellular preparations, skinned single myocyte fragments, and single myofibrils prepared from frozen (liquid N2) LVs. The rates of force-dependent ATP consumption (tension-cost) and force redevelopment (Ktr), as well as myofibril relaxation time (Timelin) were significantly blunted in AOB, indicating reduced cross-bridge cycling kinetics. Maximum Ca2+ activated force development was significantly reduced in AOB myocytes, while no change in myofilament Ca2+ sensitivity was observed. Our results indicate blunted cross-bridge cycle in a β-MHC small animal DHF model. Reduced cross-bridge cycling kinetics may contribute, at least in part, to the development of DHF in larger mammals, including humans.

Radixin Relocalization and Nonmuscle α-Actinin Expression Are Features of Remodeling Cardiomyocytes in Adult Patients with Dilated Cardiomyopathy

Background

Pediatric patients show an impressive capacity of cardiac regeneration. In contrast, severely deteriorated adult hearts do usually not recover. Since cardiac remodeling—involving the expression of fetal genes—is regarded as an adaptation to stress, we compared hearts of adult patients suffering from dilated cardiomyopathy (DCM) with remodeling of cultured neonatal (NRC) as well as adult (ARC) rat cardiomyocytes and the developing postnatal myocardium.

Methods

NRC and ARC were stimulated with serum and cardiac morphogens derived from DCM hearts. Protein synthesis (PS) as well as protein accumulation (PA) was measured, and cell survival was determined under ischemic conditions. Fetal markers were investigated by Western blot. Biomarkers of remodeling were analyzed in controls, DCM, and 2- to 6-month-old children with tetralogy of Fallot as well as in neonatal and adult rats by immunofluorescence.

Results

In NRC, serum and morphogens strongly stimulated PS and PA and the reestablishment of cell-cell contacts (CCC). In ARC, both stimulants increased PS and CCC, but PA was only elevated after serum stimulation. In contrast to serum, morphogen treatment resulted in the expression of fetal genes in ARC as determined by nonmuscle α-actinin-1 and α-actinin-4 expression (NM-actinins) and was associated with increased survival under ischemia. NM-actinins were present in cardiomyocytes of DCM in a cross-striated pattern reminiscent of sarcomeres as well as in extensions of the area of the intercalated disc (ID). NM-actinins are expressed in NRC and in the developing heart. Radixin staining revealed remodeling of the area of the ID in DCM almost identical to stimulated cultured ARC.

Conclusions

Remodeling was similar in ARC and in cardiomyocytes of DCM suggesting evolutionary conserved mechanisms of regeneration. Despite activation of fetal genes, the atrophy of ARC indicates differences in their regenerative capacity from NRC. Cardiac-derived factors induced NM-actinin expression and increased survival of ischemic ARC while circulating molecules were less effective. Identification of these cardiac-derived factors and determination of their individual capacity to heal or damage are of particular importance for a biomarker-guided therapy in adult patients.

[Changes in the diameter and length of hypertrophic cardiomyocytes in the dilated left ventricle of cardiac surgical patients]

AIM

to investigate changes in the diameter and length of hypertrophic cardiomyocytes (CMCs) in the dilated left ventricle (LV).

SUBJECTS AND METHODS

Light microscopy, morphometry, and statistical analysis were used to investigate the status of the contractile apparatus and changes in the length, diameter of CHC and diameter of CMC nuclei, by using intraoperative dilated LV biopsy samples from 31 patients with valvular disorders and dilated cardiomyopathy. Morphological findings were compared with the clinical parameters of the patients.

RESULTS

CMCs in the patients with the dilated LV were hypertrophic and were at different stages of restructuring with progressive myofibrillar loss (PML). In 81% of patients, the diameter of CMCs was not significantly changed as their zones of PML extended. The length of CMCs, which correlated with the enlarged LV cavity, was increased in 52% of patients during cell restructuring with PML. In 42% of patients, the CMC nuclear diameter increased during restructuring with PML, which appeared to be associated with CMC polyploidization; in some of these patients (19% of the total number of patients), the diameter of CMCs increased in parallel with the higher diameter of their nuclei.

CONCLUSION

The findings suggest that after completion of their transverse growth, hypertrophic CMCs are involved in a restructuring process with PML. The findings are consistent with the hypothesis that dilatation of the hypertrophied LV cavity is related to the preferential elongation of CMCs with an inadequate increase in their diameter. The results of the investigation may assume that the higher CMC diameter that brings to completion before the entry of the cells into the restructuring process with PML, resumes in the cells, the ploidy of which increases in the course of restructuring with PML, triggering an additional mechanism for raising the CMC diameter at this stage of myocardial hypertrophy. The results are indicative of different mechanisms for increasing the diameter and length of hypertrophic CMCs, since the diameter of CMCs directly correlates with that of their nuclei, and the length increases as the zones of PML extend in the CMCs.

A Comparison of Phenomenologic Growth Laws for Myocardial Hypertrophy

The heart grows in response to changes in hemodynamic loading during normal development and in response to valve disease, hypertension, and other pathologies. In general, a left ventricle subjected to increased afterload (pressure overloading) exhibits concentric growth characterized by thickening of individual myocytes and the heart wall, while one experiencing increased preload (volume overloading) exhibits eccentric growth characterized by lengthening of myocytes and dilation of the cavity. Predictive models of cardiac growth could be important tools in evaluating treatments, guiding clinical decision making, and designing novel therapies for a range of diseases. Thus, in the past 20 years there has been considerable effort to simulate growth within the left ventricle. While a number of published equations or systems of equations (often termed "growth laws") can capture some aspects of experimentally observed growth patterns, no direct comparisons of the various published models have been performed. Here we examine eight of these laws and compare them in a simple test-bed in which we imposed stretches measured during in vivo pressure and volume overload. Laws were compared based on their ability to predict experimentally measured patterns of growth in the myocardial fiber and radial directions as well as the ratio of fiber-to-radial growth. Three of the eight laws were able to reproduce most key aspects of growth following both pressure and volume overload. Although these three growth laws utilized different approaches to predict hypertrophy, they all employed multiple inputs that were weakly correlated during in vivo overload and therefore provided independent information about mechanics.

[Morphometric features of cardiomyocytes in the ventricular septum of patients with hypertrophic cardiomyopathy]

AIM

to determine the diameter and length of cardiomyocytes (CMC) in the ventricular septum (VS) of patients with hypertrophic cardiomyopathy (HCM) and to analyze correlations of a change in CMC sizes with the anatomical features of the heart in the patients.

MATERIAL AND METHODS

Longitudinal sections of intraoperative myocardial biopsy specimens taken from 23 patients aged 15-59 years with HCM were treated using immunohistochemical detection of connexin 43; the sizes of 50 CMCs were measured; a 4-point scale was used to assess the degree of myofibril loss (MFL) in these cells. The change in the diameter and length of the cells during their rearrangement as MFL gradually increased, as well as the correlations of CMC sizes with the anatomical parameters of the heart were analyzed.

RESULTS

VS CMCs from the patients with HCM were hypertrophic and were in the early stages of rearrangement accompanied by MFL. During this rearrangement, CMCs in some patients grew in length and, less frequently, diameter. The average diameter of CMCs was directly correlated with VS thickness. The average length of the cells in the CMC population with a considerable degree of MFL also directly correlated with VS thickness.

CONCLUSION

The findings could suggest that the factor raising VS thickness in HCM may be an increase in not only diameter of CMCs, but also in length of CMCs, which had impaired orientation in the VS - which are oriented perpendicular to their normal tangential position. The presence of such CMCs in the VS myocardium in patients with HCM can be discussed due to the typical large number of myocardial areas with the impaired parallel arrangement of CMCs.

Atrial remodeling is directly related to end-diastolic left ventricular pressure in a mouse model of ventricular pressure overload

Background

Atrial fibrillation (AF) is often preceded by underlying cardiac diseases causing ventricular pressure overload.

Objective

It was our aim to investigate the progression of atrial remodeling in a small animal model of ventricular pressure overload and its association with induction of AF.

Methods

Male mice were subjected to transverse aortic constriction (TAC) or sham operation. After four or eight weeks, echocardiographic measurements and hemodynamic measurements were made and AF induction was tested. The hearts were either fixed in formalin or ventricles and atria were separated, weighed and snap-frozen for RNA analysis.

Results

Four weeks of pressure overload induced ventricular hypertrophy and minor changes in the atria. After eight weeks a significant reduction in left ventricular function occurred, associated with significant atrial remodeling including increased atrial weight, a trend towards an increased left atrial cell diameter, atrial dilatation and increased expression of markers of hypertrophy and inflammation. Histologically, no fibrosis was found in the left atrium. But atrial gene expression related to fibrosis was increased. Minor changes related to electrical remodeling were observed. AF inducibility was not different between the groups. Left ventricular end diastolic pressures were increased and correlated with the severity of atrial remodeling but not with AF induction.

Conclusion

Permanent ventricular pressure overload by TAC induced atrial remodeling, including hypertrophy, dilatation and inflammation. The extent of atrial remodeling was directly related to LVEDP and not duration of TAC per se.

Gender differences in non-ischemic myocardial remodeling: are they due to estrogen modulation of cardiac mast cells and/or membrane type 1 matrix metalloproteinase

This review is focused on gender differences in cardiac remodeling secondary to sustained increases in cardiac volume (VO) and generated pressure (PO). Estrogen has been shown to favorably alter the course of VO-induced remodeling. That is, the VO-induced increased extracellular matrix proteolytic activity and mast cell degranulation responsible for the adverse cardiac remodeling in males and ovariectomized rodents do not occur in intact premenopausal females. While less is known regarding the mechanisms responsible for female cardioprotection in PO-induced stress, gender differences in remodeling have been reported indicating the ability of premenopausal females to adequately compensate. In view of the fact that, in male mice with PO, mast cells have been shown to play a role in the adverse remodeling suggests favorable estrogen modification of mast cell phenotype may also be responsible for cardioprotection in females with PO. Thus, while evidence is accumulating regarding premenopausal females being cardioprotected, there remains the need for in-depth studies to identify critical downstream molecular targets that are under the regulation of estrogen and relevant to cardiac remodeling. Such studies would result in the development of therapy which provides cardioprotection while avoiding the adverse effects of systemic estrogen delivery.

Model-specific selection of molecular targets for heart failure gene therapy

Heart failure (HF) is a complex multifaceted problem of abnormal ventricular function and structure. In recent years, new information has been accumulated allowing for a more detailed understanding of the cellular and molecular alterations that are the underpinnings of diverse causes of HF, including myocardial ischemia, pressure-overload, volume-overload or intrinsic cardiomyopathy. Modern pharmacological approaches to treat HF have had a significant impact on the course of the disease, although they do not reverse the underlying pathological state of the heart. Therefore gene-based therapy holds a great potential as a targeted treatment for cardiovascular diseases. Here, we survey the relative therapeutic efficacy of genetic modulation of β-adrenergic receptor signaling, Ca(2+) handling proteins and angiogenesis in the most common extrinsic models of HF.

Assessment of contractility in intact ventricular cardiomyocytes using the dimensionless 'Frank-Starling Gain' index

This paper briefly recapitulates the Frank–Starling law of the heart, reviews approaches to establishing diastolic and systolic force–length behaviour in intact isolated cardiomyocytes, and introduces a dimensionless index called ‘Frank–Starling Gain’, calculated as the ratio of slopes of end-systolic and end-diastolic force–length relations. The benefits and limitations of this index are illustrated on the example of regional differences in Guinea pig intact ventricular cardiomyocyte mechanics. Potential applicability of the Frank–Starling Gain for the comparison of cell contractility changes upon stretch will be discussed in the context of intra- and inter-individual variability of cardiomyocyte properties.

Pathophysiological basis of right ventricular remodeling

The pathophysiology of right ventricular (RV) remodeling is a complex process and may include unique elements not observed in left ventricular (LV) remodeling. The RV also has a relatively irregular geometry not accounted for in LV analyses. RV remodeling includes basic changes in geometry, wall thickness, and ventricular pressure-volume relationships. Also, myocyte dimensions and number increase, and myocardial extracellular matrix and biochemical milieu are modified. Remodeling has been associated with such diseases as pulmonary hypertension, lung transplant, LV pathology, Chagas' disease, and arrhythmogenic right ventricular cardiomyopathy. Disease progression may lead to further RV changes, including hypertrophy, dilatation, and subsequently to variable alterations in RV hemodynamic status. The multiple methods to assess RV hypertrophy include cine magnetic resonance imaging and 3-D echocardiography. Each technique offers different precision in evaluating RV dimensions and functional performance characteristics. Strategies to prevent RV remodeling include pharmacological agents, such as vasodilators and angiotensin-converting enzyme inhibitors, as well as more invasive interventions, such as ventricular assist devices.

Cardiac-specific haploinsufficiency of beta-catenin attenuates cardiac hypertrophy but enhances fetal gene expression in response to aortic constriction

In addition to its role in cell adhesion, beta-catenin is an important signaling molecule in the Wnt/Wingless signaling pathway. Recent studies have indicated that beta-catenin is stabilized by hypertrophic stimuli and may regulate cardiac hypertrophic responses. To explore the role and requirement of beta-catenin in cardiac development and hypertrophy, we deleted the beta-catenin gene specifically in cardiac myocytes by crossing loxP-floxed beta-catenin mice with transgenic mice expressing a Cre recombinase under the control of the alpha-myosin heavy chain promoter. No homozygous beta-catenin-deleted mice were born alive and died before embryonic day 14.5, indicating significant and irreplaceable roles of beta-catenin in embryonic heart development. Heterozygous beta-catenin-deleted mice, however, demonstrated no structural and functional abnormality. The response of heterozygous beta-catenin-deleted mice to transverse aortic constriction, however, was significantly attenuated with decreased heart weight and heart weight/body weight ratio compared to controls with intact beta-catenin genes. Hemodynamic analysis revealed that there was no difference in cardiac function between wild-type and heterozygous beta-catenin-deleted mice. On the other hand, the expression of fetal genes, beta-myosin heavy chain, atrial and brain natriuretic peptides was significantly higher in heterozygous beta-catenin-deleted mice when compared to wild-type beta-catenin mice. These results suggest that the cytoplasmic level of beta-catenin modulates hypertrophic response and fetal gene reprogramming after pressure overload.

Cardiocyte cytoskeleton in hypertrophied myocardium

The Frank-Starling mechanism, by which load directly regulates muscle length and thus performance is the means by which the mechanics and energetics of cardiac muscle are regulated on a beat-to-beat basis. When this short-term compensation for increased load is insufficient, the long-term compensation of cardiac hypertrophy ensues. The simplest and most direct mechanism for load regulation of cardiac mass would obtain if an analog of the short-term Frank-Starling mechanism of functional regulation operated in the long-term time domain of mass regulation; that is, if heart muscle were able to directly transduce increased load into growth. It is now clear that load does indeed serve as a direct regulator of cardiac mass in the adult. Cardiac hypertrophy, at the levels of intact animal, isolated tissue, and cultured cells, is a direct response of the adult mammalian cardiocyte to increased load, modified by but without the requisite involvement of factors external to the cell. The extent to which such hypertrophy is compensatory is critically dependent on the type of hemodynamic overload that serves as the hypertrophic stimulus. Thus, cardiac hypertrophy is not intrinsically maladaptive; rather, it is the nature of the inducing load rather than hypertrophy itself that is responsible for the frequent deterioration of initially compensatory hypertrophy into the congestive heart failure state. As one example reviewed here of this load specificity of maladaptation, increased microtubule network density is a persistent feature of severely pressure overloaded, hypertrophied and failing myocardium which imposes a viscous load on active myofilaments during contraction.

Structural basis of ventricular remodeling: role of the myocyte

Structural remodeling plays a major role in the progression of various heart diseases to congestive heart failure (CHF). Major contributors to this remodeling process in the heart include alterations in myocyte shape, myocyte number, and extracellular matrix. However, it is unclear as to which of these changes is most critical in the development of CHF, and this may vary by etiology. Myocyte shape alterations largely underlie the increase in chamber diameter/wall thickness characteristic of CHF. This review mainly focuses on the role of myocyte shape in ventricular remodeling. Several signaling molecules have been implicated in this process. As we learn more about the components of myocardial remodeling, new strategies to combat the progression of heart disease should arise.

Altered connexin43 expression produces arrhythmia substrate in heart failure

Recently, we found that repolarization heterogeneities between subepicardial and midmyocardial cells can form a substrate for reentrant ventricular arrhythmias in failing myocardium. We hypothesized that the mechanism responsible for maintaining transmural action potential duration heterogeneities in heart failure is related to intercellular uncoupling from downregulation of cardiac gap junction protein connexin43 (Cx43). With the use of the canine model of pacing-induced heart failure, left ventricles were sectioned to expose the transmural surface (n = 5). To determine whether heterogeneous Cx43 expression influenced electrophysiological function, high-resolution transmural optical mapping of the arterially perfused canine wedge preparation was used to measure conduction velocity (theta(TM)), effective transmural space constant (lambda(TM)), and transmural gradients of action potential duration (APD). Absolute Cx43 expression in failing myocardium, quantified by confocal immunofluorescence, was uniformly reduced (by 40 +/- 3%, P < 0.01) compared with control. Relative Cx43 expression was heterogeneously distributed and lower (by 32 +/- 18%, P < 0.05) in the subepicardium compared with deeper layers. Reduced Cx43 expression in heart failure was associated with significant reductions in intercellular coupling between transmural muscle layers, as evidenced by reduced theta(TM) (by 18.9 +/- 4.9%) and lambda(TM) (by 17.2 +/- 1.4%; P < 0.01) compared with control. Heterogeneous transmural distribution of Cx43 in failing myocardium was associated with lower subepicardial theta(TM) (by 12 +/- 10%) and lambda(TM) (by 13 +/- 7%), compared with deeper transmural layers (P < 0.05). APD dispersion was greatest in failing myocardium, and the largest transmural APD gradients were consistently found in regions exhibiting lowest relative Cx43 expression. These data demonstrate that reduced Cx43 expression produces uncoupling between transmural muscle layers leading to slowed conduction and marked dispersion of repolarization between epicardial and deeper myocardial layers. Therefore, Cx43 expression patterns can potentially contribute to an arrhythmic substrate in failing myocardium.

Beta-adrenergic activation initiates chamber dilatation in concentric hypertrophy

It is uncertain whether chronic beta-adrenoreceptor (beta-AR)-activation in hypertension could initiate the progression from compensated left ventricular (LV) hypertrophy to pump dysfunction. It is also uncertain if this effect is through adverse LV remodeling (chamber dilatation with wall thinning and pump dysfunction) or intrinsic myocardial contractile dysfunction. We evaluated the effect of 5 months of isoprenaline (0.02 mg x kg(-1) x d(-1)) on hemodynamics, LV wall thickness, cavity size, and interstitial characteristics in spontaneously hypertensive rats (SHR) with compensated LV hypertrophy. In the absence of myocyte necrosis, changes in volume preload, pressure afterload, and heart rate or decreases in baseline systolic myocardial elastance (load independent measure of intrinsic myocardial contractility), ISO produced a right shift in LV diastolic pressure-volume (P-V) relations (chamber dilatation), a decrease in LV wall thickness despite a further increase in LV weight in SHR, LV pump dysfunction (right shift in LV systolic P-V relations), and deleterious interstitial remodeling (increments in total and noncrosslinked myocardial collagen concentrations). The isoprenaline-induced LV geometric, chamber performance, and interstitial changes were similar to alterations noted during decompensation in older SHR. In summary, in the absence of tissue necrosis and baseline intrinsic myocardial contractile dysfunction, chronic beta-AR activation induces interstitial and chamber remodeling and, hence, pump dysfunction. These data suggest that chronic sympathetic activation initiates the progression from compensated concentric LV hypertrophy in hypertension to cardiac dysfunction primarily through deleterious cardiac remodeling rather than intrinsic myocardial contractile dysfunction.

Early anti-remodeling effect of labetalol in the congestive heart failure model induced by aortic constriction in the guinea pig

We investigated the effects of the beta1-beta2-alpha1-blocker, labetalol, in the congestive heart failure (CHF) model induced by aortic constriction in the guinea pig. One hundred days after aortic constriction, 52 animals were given either placebo, labetalol 2 mg/kg/d, or labetalol 20 mg/kg/d for 60 days. Eighteen sham-operated animals were used as controls. Investigations were performed at the end of the treatment period. Compared with sham-operated animals, banded animals receiving placebo showed signs of overt CHF with cardiac, systemic and regional (mesenteric and femoral) hemodynamic dysfunction, and pulmonary and hepatic congestion. An increase in whole heart, atria, and left and right ventricle weights associated with left ventricular cavity enlargement and left and right ventricular wall thickening indicated a remodeling process. Compared with placebo, labetalol did not significantly modify cardiac, systemic, or regional hemodynamic variables but significantly decreased pulmonary and hepatic congestion. Labetalol significantly reduced left ventricular cavity area (-10 and -20% after 2 and 20 mg/kg, respectively) and left ventricular (-4 and -16%) and right ventricular (-4 and -19%) wall thickness. In conclusion, labetalol induced partial regression of cardiac remodeling before hemodynamic improvement. This early anti-remodeling effect could play a role in the favorable effects observed with beta1-beta2-alpha1-blockers in humans.

Attenuation of cardiac failure, dilatation, damage, and detrimental interstitial remodeling without regression of hypertrophy in hypertensive rats

Whether left ventricular (LV) hypertrophy is important in the development of LV failure associated with advanced myocardial damage and detrimental chamber and interstitial remodeling in hypertension has not been established. We examined the effect of an antihypertensive agent without the ability to regress LV hypertrophy on the development of LV changes in spontaneously hypertensive rats (SHR). Hydralazine given to SHR from 5.2 to 26 months of age returned systolic blood pressure to Wistar Kyoto (WKY) control values but failed to prevent the increase in LV mass noted in SHR (at 26 months of age: WKY, 0.99+/-0.02 g; untreated SHR, 1.40+/-0.02 g; treated SHR, 1.36+/-0.02 g; P<0.001 in SHR versus WKY). In comparison to both 16-month-old SHR and age-matched WKY, 26-month-old untreated SHR developed signs consistent with heart failure, LV dilatation (an increased LV internal radius), an eccentric LV geometry, advanced myocyte necrosis, an increase in myocardial collagen solubility (an index of decreases in myocardial collagen cross-linking), and marked increases in myocardial total, type III, and non-cross-linked myocardial collagen concentrations. Despite the inability of hydralazine to regress LV hypertrophy, treated SHR did not develop signs of heart failure, myocyte necrosis, decreases in myocardial collagen cross-linking, or increases in myocardial total, type III, and non-cross-linked collagen at 26 months of age. Moreover, treatment attenuated the development of LV dilatation and an eccentric LV geometry. In conclusion, antihypertensive therapy that does not attenuate LV hypertrophy but achieves normal blood pressure in SHR, is able to hinder the development of heart failure associated with advanced myocardial damage and detrimental chamber and interstitial remodeling.