Regional differences in myocyte size in normal rat heart

Promoting cardiomyocyte proliferation for myocardial regeneration in large mammals

From molecular and cellular perspectives, heart failure is caused by the loss of cardiomyocytes-the fundamental contractile units of the heart. Because mammalian cardiomyocytes exit the cell cycle shortly after birth, the cardiomyocyte damage induced by myocardial infarction (MI) typically leads to dilatation of the left ventricle (LV) and often progresses to heart failure. However, recent findings indicate that the hearts of neonatal pigs completely regenerated the cardiomyocytes that were lost to MI when the injury occurred on postnatal day 1 (P1). This recovery was accompanied by increases in the expression of markers for cell-cycle activity in cardiomyocytes. These results suggest that the repair process was driven by cardiomyocyte proliferation. This review summarizes findings from recent studies that found evidence of cardiomyocyte proliferation in 1) the uninjured hearts of newborn pigs on P1, 2) neonatal pig hearts after myocardial injury on P1, and 3) the hearts of pigs that underwent apical resection surgery (AR) on P1 followed by MI on postnatal day 28 (P28). Analyses of cardiomyocyte single-nucleus RNA sequencing data collected from the hearts of animals in these three experimental groups, their corresponding control groups, and fetal pigs suggested that although the check-point regulators and other molecules that direct cardiomyocyte cell-cycle progression and proliferation in fetal, newborn, and postnatal pigs were identical, the mechanisms that activated cardiomyocyte proliferation in response to injury may differ from those that regulate cardiomyocyte proliferation during development.

The prevalent I686T human variant and loss-of-function mutations in the cardiomyocyte-specific kinase gene TNNI3K cause adverse contractility and concentric remodeling in mice

TNNI3K expression worsens disease progression in several mouse heart pathology models. TNNI3K expression also reduces the number of diploid cardiomyocytes, which may be detrimental to adult heart regeneration. However, the gene is evolutionarily conserved, suggesting a beneficial function that has remained obscure. Here, we show that C57BL/6J-inbred Tnni3k mutant mice develop concentric remodeling, characterized by ventricular wall thickening and substantial reduction of cardiomyocyte aspect ratio. This pathology occurs in mice carrying a Tnni3k null allele, a K489R point mutation rendering the protein kinase-dead, or an allele corresponding to human I686T, the most common human non-synonymous TNNI3K variant, which is hypomorphic for kinase activity. Mutant mice develop these conditions in the absence of fibrosis or hypertension, implying a primary cardiomyocyte etiology. In culture, mutant cardiomyocytes were impaired in contractility and calcium dynamics and in protein kinase A signaling in response to isoproterenol, indicating diminished contractile reserve. These results demonstrate a beneficial function of TNNI3K in the adult heart that might explain its evolutionary conservation and imply that human TNNI3K variants, in particular the widespread I686T allele, may convey elevated risk for altered heart geometry and hypertrophy.

Intact myocardial preparations reveal intrinsic transmural heterogeneity in cardiac mechanics

Determining transmural mechanical properties in the heart provides a foundation to understand physiological and pathophysiological cardiac mechanics. Although work on mechanical characterisation has begun in isolated cells and permeabilised samples, the mechanical profile of living individual cardiac layers has not been examined. Myocardial slices are 300 μm-thin sections of heart tissue with preserved cellular stoichiometry, extracellular matrix, and structural architecture. This allows for cardiac mechanics assays in the context of an intact in vitro organotypic preparation. In slices obtained from the subendocardium, midmyocardium and subepicardium of rats, a distinct pattern in transmural contractility is found that is different from that observed in other models. Slices from the epicardium and midmyocardium had a higher active tension and passive tension than the endocardium upon stretch. Differences in total myocyte area coverage, and aspect ratio between layers underlined the functional readouts, while no differences were found in total sarcomeric protein and phosphoprotein between layers. Such intrinsic heterogeneity may orchestrate the normal pumping of the heart in the presence of transmural strain and sarcomere length gradients in the in vivo heart.

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The myocardial slice preparation is an intact cardiac model allowing the study of transmural properties.

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Mechanical behaviour is cardiac layer dependent.

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Structural differences in cardiomyocyte density, orientation, and aspect ratio may contribute to these findings.

The impact of signal-to-noise ratio, diffusion-weighted directions and image resolution in cardiac diffusion tensor imaging - insights from the ex-vivo rat heart

BACKGROUND

Cardiac diffusion tensor imaging (DTI) is limited by scan time and signal-to-noise (SNR) restrictions. This invariably leads to a trade-off between the number of averages, diffusion-weighted directions (ND), and image resolution. Systematic evaluation of these parameters is therefore important for adoption of cardiac DTI in clinical routine where time is a key constraint.

METHODS

High quality reference DTI data were acquired in five ex-vivo rat hearts. We then retrospectively set 2 ≤ SNR ≤ 97, 7 ≤ ND ≤ 61, varied the voxel volume by up to 192-fold and investigated the impact on the accuracy and precision of commonly derived parameters.

RESULTS

For maximal scan efficiency, the accuracy and precision of the mean diffusivity is optimised when SNR is maximised at the expense of ND. With typical parameter settings used clinically, we estimate that fractional anisotropy may be overestimated by up to 13% with an uncertainty of ±30%, while the precision of the sheetlet angles may be as poor as ±31°. Although the helix angle has better precision of ±14°, the transmural range of helix angles may be under-estimated by up to 30° in apical and basal slices, due to partial volume and tapering myocardial geometry.

CONCLUSIONS

These findings inform a baseline of understanding upon which further issues inherent to in-vivo cardiac DTI, such as motion, strain and perfusion, can be considered. Furthermore, the reported bias and reproducibility provides a context in which to assess cardiac DTI biomarkers.

A method to increase reproducibility in adult ventricular myocyte sizing and flow cytometry: Avoiding cell size bias in single cell preparations

RATIONALE

Flow cytometry (FCM) of ventricular myocytes (VMs) is an emerging technology in adult cardiac research that is challenged by the wide variety of VM shapes and sizes. Cellular variability and cytometer flow cell size can affect cytometer performance. These two factors of variance limit assay validity and reproducibility across laboratories. Washing and filtering of ventricular cells in suspension are routinely done to prevent cell clumping and minimize data variability without the appropriate standardization. We hypothesize that washing and filtering arbitrarily biases towards sampling smaller VMs than what actually exist in the adult heart.

OBJECTIVE

To determine the impact of washing and filtering on adult ventricular cells for cell sizing and FCM.

METHODS AND RESULTS

Left ventricular cardiac cells in single-cell suspension were harvested from New Zealand White rabbits and fixed prior to analysis. Each ventricular sample was aliquoted before washing or filtering through a 40, 70, 100 or 200μm mesh. The outcomes of the study are VM volume by Coulter Multisizer and light-scatter signatures by FCM. Data are presented as mean±SD. Myocyte volumes without washing or filtering (NF) served as the "gold standard" within the sample and ranged from 11,017 to 46,926μm3. Filtering each animal sample through a 200μm mesh caused no variation in the post-filtration volume (1.01+0.01 fold vs. NF, n = 4 rabbits, p = 0.999) with an intra-assay coefficient of variation (%CV) of <5% for all 4 samples. Filtering each sample through a 40, 70 or 100μm mesh invariably reduced the post-filtration volume by 41±10%, 9.0±0.8% and 8.8±0.8% respectively (n = 4 rabbits, p<0.0001), and increased the %CV (18% to 1.3%). The high light-scatter signature by FCM, a simple parameter for the identification of ventricular myocytes, was measured after washing and filtering. Washing discarded VMs and filtering cells through a 40 or 100μm mesh reduced larger VM by 46% or 11% respectively (n = 6 from 2 rabbits, p<0.001).

CONCLUSION

Washing and filtering VM suspensions through meshes 100μm or less biases myocyte volumes to smaller sizes, excludes larger cells, and increases VM variability. These findings indicate that validity and reproducibility across laboratories can be compromised unless cell preparation is standardized. We propose no wash prior to fixation and a 200μm mesh for filtrations to provide a reproducible standard for VM studies using FCM.

Novel large-particle FACS purification of adult ventricular myocytes reveals accumulation of myosin and actin disproportionate to cell size and proteome in normal post-weaning development

RATIONALE

Quantifying cellular proteins in ventricular myocytes (MCs) is challenging due to tissue heterogeneity and the variety of cell sizes in the heart. In post-weaning cardiac ontogeny, rod-shaped MCs make up the majority of the cardiac mass while remaining a minority of cardiac cells in number. Current biochemical analyses of cardiac proteins do not correlate well the content of MC-specific proteins to cell type or size in normally developing tissue.

OBJECTIVE

To develop a new large-particle fluorescent-activated cell sorting (LP-FACS) strategy for the purification of adult rod-shaped MCs. This approach is developed to enable growth-scaled measurements per-cell of the MC proteome and sarcomeric proteins (i.e. myosin heavy chain (MyHC) and alpha-actin (α-actin)) content.

METHODS AND RESULTS

Individual cardiac cells were isolated from 21 to 94days old mice. An LP-FACS jet-in-air system with a 200-μm nozzle was defined for the first time to purify adult MCs. Cell-type specific immunophenotyping and sorting yielded ≥95% purity of adult MCs independently of cell morphology and size. This approach excluded other cell types and tissue contaminants from further analysis. MC proteome, MyHC and α-actin proteins were measured in linear biochemical assays normalized to cell numbers. Using the allometric coefficient α, we scaled the MC-specific rate of protein accumulation to growth post-weaning. MC-specific volumes (α=1.02) and global protein accumulation (α=0.94) were proportional (i.e. isometric) to body mass. In contrast, MyHC and α-actin accumulated at a much greater rate (i.e. hyperallometric) than body mass (α=1.79 and 2.19 respectively) and MC volumes (α=1.76 and 1.45 respectively).

CONCLUSION

Changes in MC proteome and cell volumes measured in LP-FACS purified MCs are proportional to body mass post-weaning. Oppositely, MyHC and α-actin are concentrated more rapidly than what would be expected from MC proteome accumulation, cell enlargement, or animal growth alone. LP-FACS provides a new standard for adult MC purification and an approach to scale the biochemical content of specific proteins or group of proteins per cell in enlarging MCs.

Monte Carlo Simulations of Diffusion Weighted MRI in Myocardium: Validation and Sensitivity Analysis

A model of cardiac microstructure and diffusion MRI is presented, and compared with experimental data from ex vivo rat hearts. The model includes a simplified representation of individual cells, with physiologically correct cell size and orientation, as well as intra- to extracellular volume ratio. Diffusion MRI is simulated using a Monte Carlo model and realistic MRI sequences. The results show good correspondence between the simulated and experimental MRI signals. Similar patterns are observed in the eigenvalues of the diffusion tensor, the mean diffusivity (MD), and the fractional anisotropy (FA). A sensitivity analysis shows that the diffusivity is the dominant influence on all three eigenvalues of the diffusion tensor, the MD, and the FA. The area and aspect ratio of the cell cross-section affect the secondary and tertiary eigenvalues, and hence the FA. Within biological norms, the cell length, volume fraction of cells, and rate of change of helix angle play a relatively small role in influencing tissue diffusion. Results suggest that the model could be used to improve understanding of the relationship between cardiac microstructure and diffusion MRI measurements, as well as in testing and refinement of cardiac diffusion MRI protocols.

Validation of diffusion tensor MRI measurements of cardiac microstructure with structure tensor synchrotron radiation imaging

BACKGROUND

Diffusion tensor imaging (DTI) is widely used to assess tissue microstructure non-invasively. Cardiac DTI enables inference of cell and sheetlet orientations, which are altered under pathological conditions. However, DTI is affected by many factors, therefore robust validation is critical. Existing histological validation is intrinsically flawed, since it requires further tissue processing leading to sample distortion, is routinely limited in field-of-view and requires reconstruction of three-dimensional volumes from two-dimensional images. In contrast, synchrotron radiation imaging (SRI) data enables imaging of the heart in 3D without further preparation following DTI. The objective of the study was to validate DTI measurements based on structure tensor analysis of SRI data.

METHODS

One isolated, fixed rat heart was imaged ex vivo with DTI and X-ray phase contrast SRI, and reconstructed at 100 μm and 3.6 μm isotropic resolution respectively. Structure tensors were determined from the SRI data and registered to the DTI data.

RESULTS

Excellent agreement in helix angles (HA) and transverse angles (TA) was observed between the DTI and structure tensor synchrotron radiation imaging (STSRI) data, where HADTI-STSRI = -1.4° ± 23.2° and TADTI-STSRI = -1.4° ± 35.0° (mean ± 1.96 standard deviation across all voxels in the left ventricle). STSRI confirmed that the primary eigenvector of the diffusion tensor corresponds with the cardiomyocyte long-axis across the whole myocardium.

CONCLUSIONS

We have used STSRI as a novel and high-resolution gold standard for the validation of DTI, allowing like-with-like comparison of three-dimensional tissue structures in the same intact heart free of distortion. This represents a critical step forward in independently verifying the structural basis and informing the interpretation of cardiac DTI data, thereby supporting the further development and adoption of DTI in structure-based electro-mechanical modelling and routine clinical applications.

Analysis of Microstructure of the Cardiac Conduction System Based on Three-Dimensional Confocal Microscopy

The specialised conducting tissues present in the ventricles are responsible for the fast distribution of the electrical impulse from the atrio-ventricular node to regions in the subendocardial myocardium. Characterisation of anatomical features of the specialised conducting tissues in the ventricles is highly challenging, in particular its most distal section, which is connected to the working myocardium via Purkinje-myocardial junctions. The goal of this work is to characterise the architecture of the distal section of the Purkinje network by differentiating Purkinje cells from surrounding tissue, performing a segmentation of Purkinje fibres at cellular scale, and mathematically describing its morphology and interconnections. Purkinje cells from rabbit hearts were visualised by confocal microscopy using wheat germ agglutinin labelling. A total of 16 3D stacks including labeled Purkinje cells were collected, and semi-automatically segmented. State-of-the-art graph metrics were applied to estimate regional and global features of the Purkinje network complexity. Two types of cell types, tubular and star-like, were characterised from 3D segmentations. The analysis of 3D imaging data confirms the previously suggested presence of two types of Purkinje-myocardium connections, a 2D interconnection sheet and a funnel one, in which the narrow side of a Purkinje fibre connect progressively to muscle fibres. The complex network analysis of interconnected Purkinje cells showed no small-world connectivity or assortativity properties. These results might help building more realistic computational PK systems at high resolution levels including different cell configurations and shapes. Better knowledge on the organisation of the network might help in understanding the effects that several treatments such as radio-frequency ablation might have when the PK system is disrupted locally.

Role of thyroid hormones in ventricular remodeling

Cardiac remodeling includes alterations in molecular, cellular, and interstitial systems contributing to changes in size, shape, and function of the heart. This may be the result of injury, alterations in hemodynamic load, neurohormonal effects, electrical abnormalities, metabolic changes, etc. Thyroid hormones (THs) serve as master regulators for diverse remodeling processes of the cardiovascular system-from the prenatal period to death. THs promote a beneficial cardiomyocyte shape and improve contractility, relaxation, and survival via reversal of molecular remodeling. THs reduce fibrosis by decreasing interstitial collagen and reduce the incidence and duration of arrhythmias via remodeling ion channel expression and function. THs restore metabolic function and also improve blood flow both by direct effects on the vessel architecture and decreasing atherosclerosis. Optimal levels of THs both in the circulation and in cardiac tissues are critical for normal homeostasis. This review highlights TH-based remodeling and clinically translatable strategies for diverse cardiovascular disorders.

Spin echo versus stimulated echo diffusion tensor imaging of the in vivo human heart

Purpose

To compare signal‐to‐noise ratio (SNR) efficiency and diffusion tensor metrics of cardiac diffusion tensor mapping using acceleration‐compensated spin‐echo (SE) and stimulated echo acquisition mode (STEAM) imaging.

Methods

Diffusion weighted SE and STEAM sequences were implemented on a clinical 1.5 Tesla MR system. The SNR efficiency of SE and STEAM was measured (b = 50–450 s/mm²) in isotropic agar, anisotropic diffusion phantoms and the in vivo human heart. Diffusion tensor analysis was performed on mean diffusivity, fractional anisotropy, helix and transverse angles.

Results

In the isotropic phantom, the ratio of SNR efficiency for SE versus STEAM, SNR_t(SE/STEAM), was 2.84 ± 0.08 for all tested b‐values. In the anisotropic diffusion phantom the ratio decreased from 2.75 ± 0.05 to 2.20 ± 0.13 with increasing b‐value, similar to the in vivo decrease from 2.91 ± 0.43 to 2.30 ± 0.30. Diffusion tensor analysis revealed reduced deviation of helix angles from a linear transmural model and reduced transverse angle standard deviation for SE compared with STEAM. Mean diffusivity and fractional anisotropy were measured to be statistically different (P < 0.001) between SE and STEAM.

Conclusion

Cardiac DTI using motion‐compensated SE yields a 2.3–2.9× increase in SNR efficiency relative to STEAM and improved accuracy of tensor metrics. The SE method hence presents an attractive alternative to STEAM based approaches. Magn Reson Med 76:862–872, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Control of crosslinking for tailoring collagen-based scaffolds stability and mechanics

We provide evidence to show that the standard reactant concentrations used in tissue engineering to cross-link collagen-based scaffolds are up to 100 times higher than required for mechanical integrity in service, and stability against degradation in an aqueous environment. We demonstrate this with a detailed and systematic study by comparing scaffolds made from (a) collagen from two different suppliers, (b) gelatin (a partially denatured collagen) and (c) 50% collagen-50% gelatin mixtures. The materials were processed, using lyophilisation, to produce homogeneous, highly porous scaffolds with isotropic architectures and pore diameters ranging from 130 to 260 μm. Scaffolds were cross-linked using a carbodiimide treatment, to establish the effect of the variations in crosslinking conditions (down to very low concentrations) on the morphology, swelling, degradation and mechanical properties of the scaffolds. Carbodiimide concentration of 11.5mg/ml was defined as the standard (100%) and was progressively diluted down to 0.1%. It was found that 10-fold reduction in the carbodiimide content led to the significant increase (almost 4-fold) in the amount of free amine groups (primarily on collagen lysine residues) without compromising mechanics and stability in water of all resultant scaffolds. The importance of this finding is that, by reducing cross-linking, the corresponding cell-reactive carboxylate anions (collagen glutamate or aspartate residues) that are essential for integrin-mediated binding remain intact. Indeed, a 10-fold reduction in carbodiimide crosslinking resulted in near native-like cell attachment to collagen scaffolds. We have demonstrated that controlling the degree of cross-linking, and hence retaining native scaffold chemistry, offers a major step forward in the biological performance of collagen- and gelatin-based tissue engineering scaffolds.

STATEMENT OF SIGNIFICANCE

This work developed collagen and gelatine-based scaffolds with structural, material and biological properties suitable for use in myocardial tissue regeneration. The novelty and significance of this research consist in elucidating the effect of the composition, origin of collagen and crosslinking concentration on the scaffold physical and cell-binding characteristics. We demonstrate that the standard carbodiimide concentrations used to crosslink collagenous scaffolds are up to 100 times higher than required for mechanical integrity in service, and stability against dissolution. The importance of this finding is that, by reducing crosslinking, the corresponding cell-reactive carboxylate anions (essential for integrin-mediated binding) remain intact and the native scaffold chemistry is retained. This offers a major step forward in the biological performance of tissue engineered scaffolds.

Systemic, but not cardiomyocyte-specific, deletion of the natriuretic peptide receptor guanylyl cyclase A increases cardiomyocyte number in neonatal mice

Guanylyl cyclase A (GC-A), the receptor for atrial and B-type natriuretic peptides, is implicated in the regulation of blood pressure and cardiac growth. We used design-based stereological methods to examine the effect of GC-A inactivation on cardiomyocyte volume, number and subcellular composition in postnatal mice at day P2. In mice with global, systemic GC-A deletion, the cardiomyocyte number was significantly increased, demonstrating that hyperplasia is the main cause for the increase in ventricle weight in these early postnatal animals. In contrast, conditional, cardiomyocyte-restricted inactivation of GC-A had no significant effect on ventricle weight or cardiomyocyte number. The mean volume of cardiomyocytes and the myocyte-related volumes of the four major cell organelles (myofibrils, mitochondria, nuclei and sarcoplasm) were similar between genotypes. Taken together, systemic GC-A deficiency induces cardiac enlargement based on a higher number of normally composed and sized cardiomyocytes early after birth, whereas cardiomyocyte-specific GC-A abrogation is not sufficient to induce cardiac enlargement and has no effect on number, size and composition of cardiomyocytes. We conclude that postnatal cardiac hyperplasia in mice with global GC-A inactivation is provoked by systemic alterations, e.g., arterial hypertension. Direct GC-A-mediated effects in cardiomyocytes seem not to be involved in the regulation of myocyte proliferation at this early stage.

How to improve the overall quality of cardiac morphometric data

By the mid-1990s, experts realized that drugs leading to improved ventricular remodeling were doing something remarkable in cardiac patients. The "age of cardiac remodeling" had begun. This created an experimental need for high-quality assessment of changes in cardiac tissue composition, including myocyte shape, myocardial fibrosis/collagen, and vascular remodeling. Many working in the field today have little or no training related to recognition of fixation artifacts or common errors associated with quantitative morphology. Unfortunately, such skills had become somewhat of a lost art during the ages of cardiac physiology in the mid-20th century and molecular biology, gaining prominence by the mid-1970s. Consequently, cardiac remodeling studies today are often seriously flawed to the point where data are not reproducible and subsequent researchers may be chasing the molecular basis of a nonexistent or erroneous phenotype. The current unacceptably high incidence of irreproducible data is a serious waste of time and resources as recently noted in comments by the National Institutes of Health director. The goal of this "how to" article is to share some lessons I have learned during nearly 40 years of assessing morphological changes in the heart. It is possible for any laboratory to routinely publish highly reproducible morphological data that stand the test of time and contribute to our fundamental knowledge of cardiac remodeling and the molecular mechanisms that drive it.

The R21C Mutation in Cardiac Troponin I Imposes Differences in Contractile Force Generation between the Left and Right Ventricles of Knock-In Mice

We investigated the effect of the hypertrophic cardiomyopathy-linked R21C (arginine to cysteine) mutation in human cardiac troponin I (cTnI) on the contractile properties and myofilament protein phosphorylation in papillary muscle preparations from left (LV) and right (RV) ventricles of homozygous R21C(+/+) knock-in mice. The maximal steady-state force was significantly reduced in skinned papillary muscle strips from the LV compared to RV, with the latter displaying the level of force observed in LV or RV from wild-type (WT) mice. There were no differences in the Ca(2+) sensitivity between the RV and LV of R21C(+/+) mice; however, the Ca(2+) sensitivity of force was higher in RV-R21C(+/+) compared with RV-WT and lower in LV- R21C(+/+) compared with LV-WT. We also observed partial loss of Ca(2+) regulation at low [Ca(2+)]. In addition, R21C(+/+)-KI hearts showed no Ser23/24-cTnI phosphorylation compared to LV or RV of WT mice. However, phosphorylation of the myosin regulatory light chain (RLC) was significantly higher in the RV versus LV of R21C(+/+) mice and versus LV and RV of WT mice. The difference in RLC phosphorylation between the ventricles of R21C(+/+) mice likely contributes to observed differences in contractile force and the lower tension monitored in the LV of HCM mice.

Effect of transverse aortic constriction on cardiac structure, function and gene expression in pregnant rats

Background

There is an increased risk of heart failure and pulmonary edema in pregnancies complicated by hypertensive disorders. However, in a previous study we found that pregnancy protects against fibrosis and preserves angiogenesis in a rat model of angiotensin II induced cardiac hypertrophy. In this study we test the hypothesis that pregnancy protects against negative effects of increased afterload.

Methods

Pregnant (gestational day 5.5–8.5) and non-pregnant Wistar rats were randomized to transverse aortic constriction (TAC) or sham surgery. After 14.2±0.14 days echocardiography was performed. Aortic blood pressure and left ventricular (LV) pressure-volume loops were obtained using a conductance catheter. LV collagen content and cardiomyocyte circumference were measured. Myocardial gene expression was assessed by real-time polymerase chain reaction.

Results

Heart weight was increased by TAC (p<0.001) but not by pregnancy. Cardiac myocyte circumference was larger in pregnant compared to non-pregnant rats independent of TAC (p = 0.01), however TAC per se did not affect this parameter. Collagen content in LV myocardium was not affected by pregnancy or TAC. TAC increased stroke work more in pregnant rats (34.1±2.4 vs 17.5±2.4 mmHg/mL, p<0.001) than in non-pregnant (28.2±1.7 vs 20.9±1.5 mmHg/mL, p = 0.06). However, it did not lead to overt heart failure in any group. In pregnant rats, α-MHC gene expression was reduced by TAC. Increased in the expression of β-MHC gene was higher in pregnant (5-fold) compared to non-pregnant rats (2-fold) after TAC (p = 0.001). Nine out of the 19 genes related to cardiac remodeling were affected by pregnancy independent of TAC.

Conclusions

This study did not support the hypothesis that pregnancy is cardioprotective against the negative effects of increased afterload. Some differences in cardiac structure, function and gene expression between pregnant and non-pregnant rats following TAC indicated that afterload increase is less tolerated in pregnancy.

Improvement of left ventricular remodeling after myocardial infarction with eight weeks L-thyroxine treatment in rats

Background

Left ventricular (LV) remodeling following large transmural myocardial infarction (MI) remains a pivotal clinical issue despite the advance of medical treatment over the past few decades. Identification of new medications to improve the remodeling process and prevent progression to heart failure after MI is critical. Thyroid hormones (THs) have been shown to improve LV function and remodeling in animals post-MI and in the human setting. However, changes in underlying cellular remodeling resulting from TH treatment are not clear.

Methods

MI was produced in adult female Sprague–Dawley rats by ligation of the left descending coronary artery. L-thyroxine (T4) pellet (3.3 mg, 60 days sustained release) was used to treat MI rats for 8 weeks. Isolated myocyte shape, arterioles, and collagen deposition in the non-infarcted area were measured at terminal study.

Results

T4 treatment improved LV ±dp/dt, normalized TAU, and increased myocyte cross-sectional area without further increasing myocyte length in MI rats. T4 treatment increased the total LV tissue area by 34%, increased the non-infarcted tissue area by 41%, and increased the thickness of non-infarcted area by 36% in MI rats. However, myocyte volume accounted for only ~1/3 of the increase in myocyte mass in the non-infarct area, indicating the presence of more myocytes with treatment. T4 treatment tended to increase the total length of smaller arterioles (5 to 15 μm) proportional to LV weight increase and also decreased collagen deposition in the LV non-infarcted area. A tendency for increased metalloproteinase-2 (MMP-2) expression and tissue inhibitor of metalloproteinases (TIMPs) -1 to −4 expression was also observed in T4 treated MI rats.

Conclusions

These results suggest that long-term T4 treatment after MI has beneficial effects on myocyte, arteriolar, and collagen matrix remodeling in the non-infarcted area. Most importantly, results suggest improved survival of myocytes in the peri-infarct area.

Regulation of gene expression with thyroid hormone in rats with myocardial infarction

Introduction

The expression of hundreds of genes is altered in response to left ventricular (LV) remodeling following large transmural myocardial infarction (MI). Thyroid hormone (TH) improves LV remodeling and cardiac performance after MI. However, the molecular basis is unknown.

Methods

MI was produced by ligation of the left anterior descending coronary artery in female SD rats. Rats were divided into the following groups: (1) Sham MI, (2) MI, and (3) MI+T4 treatment (T4 pellet 3.3 mg, 60 days release, implanted subcutaneously immediately following MI). Four weeks after surgery, total RNA was isolated from LV non-infarcted areas for microarray analysis using the Illumina RatRef-12 Expression BeadChip Platform.

Results

Signals were detected in 13,188 genes (out of 22,523), of which the expression of 154 genes were decreased and the expression of 200 genes were increased in MI rats compared with Sham MI rats (false discovery rate (FDR) <0.05). Compared to MI rats, T4 treatment decreased expression of 27 genes and increased expression of 28 genes. In particular, 6 genes down-regulated by MI and 12 genes up-regulated by MI were reversed by T4. Most of the 55 genes altered by T4 treatment are in the category of molecular function under binding (24) and biological processes which includes immune system process (9), multi-organism process (5) and biological regulation (19) nonexclusively.

Conclusions

These results suggest that altered expression of genes for molecular function and biological process may be involved in the beneficial effects of thyroid hormone treatment following MI in rats.

Interventricular differences in myofilament function in experimental congestive heart failure

This study was conducted to identify molecular mechanisms which explain interventricular differences in myofilament function in experimental congestive heart failure (CHF). CHF was induced in rats by chronic aortic banding or myocardial infarction for 32-36 weeks. Right and left ventricular (RV, LV) myocytes were mechanically isolated, triton-skinned, and attached to a force transducer and motor arm. Myofilament force-[Ca(2+)] relations assessed maximal Ca(2+)-saturated force (F (max)) and the [Ca(2+)] at 50% of F (max) (EC(50)). Myofilament protein phosphorylation was determined via ProQ diamond phospho-staining. Protein kinase C (PKC)-α expression/activation and site-specific phosphorylation of cardiac troponin I (cTnI) and cardiac troponin T (cTnT) were measured via immunoblotting. Relative to controls, failing RV myocytes displayed a ~45% decrease in F (max) with no change in EC(50), whereas failing LV myocytes displayed a ~45% decrease in F (max) and ~50% increase in EC(50). Failing LV myofilaments were less Ca(2+)-sensitive (37% increase in EC(50)) than failing RV myofilaments. Expression and activation of PKC-α was increased twofold in failing RV myocardium and relative to the RV, PKC-α was twofold higher in the failing LV, while PKC-β expression was unchanged by CHF. PKC-α-dependent phosphorylation and PP1-mediated dephosphorylation of failing RV myofilaments increased EC(50) and increased F (max), respectively. Phosphorylation of cTnI and cTnT was greater in failing LV myofilaments than in failing RV myofilaments. RV myofilament function is depressed in experimental CHF in association with increased PKC-α signaling and myofilament protein phosphorylation. Furthermore, myofilament dysfunction is greater in the LV compared to the RV due in part to increased PKC-α activation and phosphorylation of cTnI and cTnT.

Three-dimensional modeling and quantitative analysis of gap junction distributions in cardiac tissue

Gap junctions play a fundamental role in intercellular communication in cardiac tissue. Various types of heart disease including hypertrophy and ischemia are associated with alterations of the spatial arrangement of gap junctions. Previous studies applied two-dimensional optical and electron-microscopy to visualize gap junction arrangements. In normal cardiomyocytes, gap junctions were primarily found at cell ends, but can be found also in more central regions. In this study, we extended these approaches toward three-dimensional reconstruction of gap junction distributions based on high-resolution scanning confocal microscopy and image processing. We developed methods for quantitative characterization of gap junction distributions based on analysis of intensity profiles along the principal axes of myocytes. The analyses characterized gap junction polarization at cell ends and higher-order statistical image moments of intensity profiles. The methodology was tested in rat ventricular myocardium. Our analysis yielded novel quantitative data on gap junction distributions. In particular, the analysis demonstrated that the distributions exhibit significant variability with respect to polarization, skewness, and kurtosis. We suggest that this methodology provides a quantitative alternative to current approaches based on visual inspection, with applications in particular in characterization of engineered and diseased myocardium. Furthermore, we propose that these data provide improved input for computational modeling of cardiac conduction.

β-myosin heavy chain is induced by pressure overload in a minor subpopulation of smaller mouse cardiac myocytes

RATIONALE

Induction of the fetal hypertrophic marker gene β-myosin heavy chain (β-MyHC) is a signature feature of pressure overload hypertrophy in rodents. β-MyHC is assumed present in all or most enlarged myocytes.

OBJECTIVE

To quantify the number and size of myocytes expressing endogenous β-MyHC by a flow cytometry approach.

METHODS AND RESULTS

Myocytes were isolated from the left ventricle of male C57BL/6J mice after transverse aortic constriction (TAC), and the fraction of cells expressing endogenous β-MyHC was quantified by flow cytometry on 10,000 to 20,000 myocytes with use of a validated β-MyHC antibody. Side scatter by flow cytometry in the same cells was validated as an index of myocyte size. β-MyHC-positive myocytes constituted 3 ± 1% of myocytes in control hearts (n=12), increasing to 25 ± 10% at 3 days to 6 weeks after TAC (n=24, P<0.01). β-MyHC-positive myocytes did not enlarge with TAC and were smaller at all times than myocytes without β-MyHC (≈70% as large, P<0.001). β-MyHC-positive myocytes arose by addition of β-MyHC to α-MyHC and had more total MyHC after TAC than did the hypertrophied myocytes that had α-MyHC only. Myocytes positive for β-MyHC were found in discrete regions of the left ventricle in 3 patterns: perivascular, in areas with fibrosis, and in apparently normal myocardium.

CONCLUSIONS

β-MyHC protein is induced by pressure overload in a minor subpopulation of smaller cardiac myocytes. The hypertrophied myocytes after TAC have α-MyHC only. These data challenge the current paradigm of the fetal hypertrophic gene program and identify a new subpopulation of smaller working ventricular myocytes with more myosin.

Histologically measured cardiomyocyte hypertrophy correlates with body height as strongly as with body mass index

Cardiac myocytes are presumed to enlarge with left ventricular hypertrophy (LVH). This study correlates histologically measured myocytes with lean and fat body mass. Cases of LVH without coronary heart disease and normal controls came from forensic autopsies. The cross-sectional widths of myocytes in H&E-stained paraffin sections followed log normal distributions almost to perfection in all 104 specimens, with constant coefficient of variation across the full range of ventricular weight, as expected if myocytes of all sizes contribute proportionately to hypertrophy. Myocyte sizes increased with height. By regression analysis, height^2.7 as a proxy for lean body mass and body mass index (BMI) as a proxy for fat body mass, exerted equal effects in the multiple correlation with myocyte volume, and the equation rejected race and sex. In summary, myocyte sizes, as indexes of LVH, suggest that lean and fat body mass may contribute equally.

Stereological and histological analysis of the developing rat heart

We studied with quantitative and microscopical methods the heart of rats divided into five age groups: embryos at the age of 11 days, fetuses at the age of 16 days and 20 days and also heart samples of 3-day-old pups and young adults (5 weeks of age) were used (n = 10 samples in each group). At the end of the study; heart samples were obtained from all animals. Stereological estimations were performed on heart volume, volume of heart lumen (ventricles and atria), volume of myocardium, numerical density of the myocyte nuclei and mean nuclear diameter of myocytes. Volumetric values and numerical data were estimated via Cavalieri method and physical dissector, respectively. In this study, histological examination was performed at light and electron microscopic levels. The numerical density of the myocyte nuclei increased from fetuses to young adults. Differences between embryos and fetuses, between fetuses and 3-day-old pups, and between 3-day-old pups and young adults were statistically significant. These results indicate that myogenesis continued in the rat myocardium during prenatal life and after birth.

Are ECG premature complexes induced by ultrasonic cavitation electrophysiological responses to irreversible cardiomyocyte injury?

The objective of this study was to explore the relationship between premature complexes (PCs) in the electrocardiogram (ECG) and lethal injury of cardiomyocytes induced by ultrasound exposure of the heart with contrast-agent gas bodies in the circulation. Anesthetized rats were exposed in a heated water bath to 1.55 MHz focused ultrasound with bursts triggered at end systole during contrast agent infusion. PCs were detected in ECG recordings and cardiomyocyte necrosis was scored by identifying Evans blue-stained cells in multiple frozen sections. With 0.1 μL/kg/min infusion of contrast agent for 5 min, both effects increased strongly for 2-ms bursts with increasing peak rarefactional pressure amplitude >1 MPa. At 8 MPa, statistically significant effects were found even for no agent infusion relative to sham tests. For 2-ms bursts at 2 MPa, the highly significant bioeffects seen for 10-, 1- and 0.1-μL/kg/min infusion became marginally significant for 0.01 μL/kg/min, which indicated a lower probability of cavitation nucleation. Burst duration variation from 0.2-20 ms produced no substantial trends in the results. Overall, the two effects were well correlated (r(2) = 0.88). The PCs occurring during contrast-enhanced ultrasound therefore appear to be electrophysiological responses to irreversible cardiomyocyte injury induced by ultrasonic cavitation.

Post-myocardial infarction left ventricular myocyte remodeling: are there gender differences in rats?

BACKGROUND

Previous studies have shown gender differences in left ventricular remodeling after myocardial infarction. Results are varied, however, and reliable, comprehensive data for changes in cardiac myocyte shape are not available.

METHODS

Young adult female and male Sprague-Dawley rats were used in this study and randomly assigned to the myocardial infarction and sham myocardial infarction groups. Myocardial infarction was produced by ligation of the left descending coronary artery. Four weeks after surgery, left ventricular echocardiography and hemodynamics were performed before isolating myocytes for size determination.

RESULTS

In general, left ventricular functional changes after myocardial infarction were comparable. Females developed slightly, but significantly, more left ventricular hypertrophy than males, and this was reflected by the relative increases in left ventricular myocyte volume. In both males and females, however, myocyte hypertrophy was due exclusively to lengthening of myocytes with no change in myocyte cross-sectional area.

CONCLUSIONS

This study demonstrates that post-myocardial infarction changes in LV function and myocyte remodeling are remarkably similar in young adult male and female rats.

Models of cardiac tissue electrophysiology: progress, challenges and open questions

Models of cardiac tissue electrophysiology are an important component of the Cardiac Physiome Project, which is an international effort to build biophysically based multi-scale mathematical models of the heart. Models of tissue electrophysiology can provide a bridge between electrophysiological cell models at smaller scales, and tissue mechanics, metabolism and blood flow at larger scales. This paper is a critical review of cardiac tissue electrophysiology models, focussing on the micro-structure of cardiac tissue, generic behaviours of action potential propagation, different models of cardiac tissue electrophysiology, the choice of parameter values and tissue geometry, emergent properties in tissue models, numerical techniques and computational issues. We propose a tentative list of information that could be included in published descriptions of tissue electrophysiology models, and used to support interpretation and evaluation of simulation results. We conclude with a discussion of challenges and open questions.

Histopathological changes of the heart after neonatal dexamethasone treatment: studies in 4-, 8-, and 50-week-old rats

Dexamethasone (Dex), for prevention of chronic lung disease in preterm infants, showed potential negative long-term effects. Studies regarding long-term cardiovascular effects are lacking. We investigated possible histopathological myocardial changes after neonatal Dex in the young and adult rat heart. Rats were treated with Dex on d 1, 2, and 3 (0.5, 0.3, and 0.1 mg/kg) of life. Control-pups received saline. At 4, 8, and 50 wk after birth rats were killed and anatomic data collected. Heart tissue was stained with hematoxylin and eosin, Cadherin-periodic acid schiff, and sirius red for cardiomyocyte morphometry and collagen determination. Presence of macrophages and mast cells was analyzed. Cardiomyocyte length of the Dex-treated rats was increased in all three age groups, whereas ventricular weight was reduced. Cardiomyocyte volumes were increased at 50 wk indicating cellular hypertrophy. Collagen content gradually increased with age and was 62% higher in Dex rats at 50 wk. Macrophage focus score and mast cell count were also higher. Neonatal Dex affects normal heart growth resulting in cellular hypertrophy and increased collagen deposition in the adult rat heart. Because previous studies in rats showed premature death, suggesting cardiac failure, cardiovascular follow-up of preterm infants treated with glucocorticoids should be considered.

Mesenchymal stem cells promote cardiomyocyte hypertrophy in vitro through hypoxia-induced paracrine mechanisms

1. Mesenchymal stem cell (MSC) therapy for myocardial infarction has received increased attention since transplanted MSC were shown to improve cardiac function by transdifferentiating into cardiomyocytes and endothelial cells. However, recent studies have demonstrated that other mechanisms may contribute to the improvement in cardiac function observed after transplantation of MSC. The paracrine effect of MSC on cardiomyocyte is not clear. Thus, in the present study, we investigated the paracrine effect of MSC on the growth of neonatal rat cardiomyocytes in vitro. 2. Samples of MSC-conditioned medium (MSC-CM) were collected after rat MSC had been cultured under conditions of hypoxia and serum deprivation for 0, 3, 6, 9 or 24 h. Cardiomyocytes were then stimulated with the MSC-CM for 48 h. Then, the protein content, cell area, [(3)H]-leucine incorporation and atrial natriuretic factor-luciferase (ANF-Luc) expression of cardiomyocytes were measured. 3. The data showed that MSC-CM collected at different time points had different effects and that MSC-CM collected at 6 h significantly promoted cardiomyocyte hypertrophy by increasing total protein content, cell area, [(3)H]-leucine incorporation and ANF gene expression. 4. In conclusion, MSC-CM promoted cardiomyocyte hypertrophy in a paracrine manner. The results provide a better understanding of the mechanisms underlying the improvement in heart function after MSC transplantation.

Isometric contraction induces rapid myocyte remodeling in cultured rat right ventricular papillary muscles

The hypothesis that elevated systolic stress induces myocyte thickening has been difficult to test directly. We tested this hypothesis in working rat right ventricular papillary muscles using a recently developed technique for long-term muscle culture. Muscles were cultured for 36 h either isometrically at different levels of systolic stress or at physiological amounts and rates of shortening. Isometric contraction induced rapid increases in myocyte diameter regardless of the level of systolic stress, whereas control myocyte dimensions were maintained if physiological amounts and rates of systolic shortening were imposed. Myocyte thickening was accompanied by a significant decrease in cell length and number of sarcomeres in series along the long axis of the myocyte, suggesting that thickening may have occurred in part by rearrangement of existing sarcomeres. We conclude that the pattern of systolic shortening and/or diastolic lengthening regulates myocyte shape in working rat right ventricular papillary muscles, whereas systolic stress plays little or no role.

Regional localisation of left ventricular sheet structure: integration with current models of cardiac fibre, sheet and band structure

The architecture of the heart remains controversial despite extensive effort and recent advances in imaging techniques. Several opposing and non-mutually compatible models have been proposed to explain cardiac structure, and these models, although limited, have advanced the study and understanding of heart structure, function and development. We describe key areas of similarity and difference, highlight areas of contention and point to the important limitations of these models. Recent research in animal models on the nature, geometry and interaction of cardiac sheet structure allows unification of some seemingly conflicting features of the structural models. Intriguingly, evidence points to significant inter-individual structural variability (within constrained limits) in the canine, leading to the concept of a continuum (or distribution) of cardiac structures. This variability in heart structure partly explains the ongoing debate on myocardial architecture. These developments are used to construct an integrated description of cardiac structure unifying features of fibre, sheet and band architecture that provides a basis for (i) explaining cardiac electromechanics, (ii) computational simulations of cardiac physiology and (iii) designing interventions.

Effects of Excessive Long-Term Exercise on Cardiac Function and Myocyte Remodeling in Hypertensive Heart Failure Rats

The long-term effects of exercise on cardiac function and myocyte remodeling in hypertension/progression of heart failure are poorly understood. We investigated whether exercise can attenuate pathological remodeling under hypertensive conditions. Fifteen female Spontaneously Hypertensive Heart Failure rats and 10 control rats were housed with running wheels beginning at 6 months of age. At 22 months of age, heart function of the trained rats was compared with heart function of age-matched sedentary hypertensive and control rats. Heart function was measured using echocardiography and left ventricular catheterization. Cardiac myocytes were isolated to measure cellular dimensions. Fetal gene expression was determined using Western blots. Exercise did not significantly impact myocyte remodeling or ventricular function in control animals. Sedentary hypertensive rats had significant chamber dilatation and cardiac hypertrophy. In exercised hypertensive rats, however, exercise time was excessive and resulted in a 21% increase in left ventricular diastolic dimension (P<0.001), a 24% increase in heart to body weight ratio (P<0.05), a 27% increase in left ventricular myocyte volume (P<0.01), a 13% reduction in ejection fraction (P<0.001), and a 22% reduction in fractional shortening (P<0.01) compared with sedentary hypertensive rats. Exercise resulted in greater fibrosis and did not prevent activation of the fetal gene program in hypertensive rats. We conclude that excessive exercise, in the untreated hypertensive state can have deleterious effects on cardiac remodeling and may actually accelerate the progression to heart failure.

Rapamycin prevents thyroid hormone-induced cardiac hypertrophy

Thyroid hormones (THs) have many effects on the cardiovascular system including cardiac hypertrophy. Although THs induce cardiac hypertrophy, the mechanism through which they exert this effect is unknown. We previously found that THs activate signaling related to increased protein synthesis [mammalian target of rapamycin (mTOR) and p70 S6 kinase] in the heart. It is unknown whether this activation contributes to TH-induced hypertrophy or whether it is merely incidental. In this study, we used rapamycin to inhibit mTOR function in mice and neonatal cardiomyocyte cultures treated with THs to test whether mTOR/S6 kinase signaling is involved in TH-mediated cardiac hypertrophy. C57 mice were treated with T4 for 3 d, 1 wk, 2 wk, or 1 month with either placebo, T4 (50 microg/100 g body weight.d), rapamycin (200 microg/100 g body weight.d) or T4/rapamycin by sc slow-release pellets. At the end of the treatment period, hemodynamics and physical data were collected and hearts were frozen for Western blot analysis or myocytes were isolated. The effects of T3 and rapamycin were also investigated using neonatal cardiomyocytes. THs activated specific components of the AKT signaling pathway in vivo and in vitro. THs induced cardiac hypertrophy, which was completely inhibited by rapamycin. Our results suggest that TH-induced hypertrophy is mediated by AKT/mTOR/S6 kinase signaling, which is important in the regulation of protein synthesis, a hallmark of cardiac hypertrophy.

A modular instrument for exploring the mechanics of cardiac myocytes

The cardiac ventricular myocyte is a key experimental system for exploring the mechanical properties of the diseased and healthy heart. Millions of primary myocytes, which remain viable for 4–6 h, can be readily isolated from animal models. However, currently available instrumentation allows the mechanical properties of only a few physically loaded myocytes to be explored within 4–6 h. Here we describe a modular and inexpensive prototype instrument that could form the basis of an array of devices for probing the mechanical properties of single mammalian myocytes in parallel. This device would greatly increase the throughput of scientific experimentation and could be applied as a high-content screening instrument in the pharmaceutical industry. The instrument module consists of two independently controlled Lorentz force actuators-force transducers in the form of 0.025 × 1 × 5 mm stainless steel cantilevers with 0.5 m/N compliance and 360-Hz resonant frequency. Optical position sensors focused on each cantilever provide position and force resolution of <1 nm/√Hz and <2 nN/√Hz, respectively. The motor structure can produce peak displacements and forces of ±200 μm and ±400 μN, respectively. Custom Visual Basic.Net software provides data acquisition, signal processing, and digital control of cantilever position. The functionality of the instrument was demonstrated by implementation of novel methodologies for loading and attaching healthy mammalian ventricular myocytes to the force sensor and actuator and use of stochastic system identification techniques to measure their passive dynamic stiffness at various sarcomere lengths.

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.

A method to collect isolated myocytes and whole tissue from the same heart

A technique for isolation of cardiac myocytes and collection of whole heart tissue from individual hearts of adult rats is described in this study. After excision of the apical half of the left ventricle (LV) and cauterization of the cut edge, aortas were cannulated and high-quality isolated cardiac myocytes were collected after collagenase perfusion of the basal portion. Myocyte dimensions from the basal portion of cauterized and noncauterized hearts from matching rats were identical. Additionally, myocyte dimensions from the basal and apical halves of the LV were compared with the use of whole heart-isolated myocyte preps. No regional differences between basal and apical LV myocyte size were found. Therefore, this cauterization method can be used to collect isolated myocytes from the basal half and whole heart tissue from the apical half, with each half being representative of the other with respect to myocyte dimensions.

Thyroid hormone analog, diiodothyropropionic acid (DITPA), exerts beneficial effects on chamber and cellular remodeling in cardiomyopathic hamstersThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

Diiodothyropropionic acid (DITPA) is a thyroid hormone analog that is currently in phase II clinical trials. However, there have not been any studies to comprehensively analyze its effect on myocyte morphology. In addition, long-term studies with DITPA have not been done. This study compares the effects of DITPA with L-thyroxine (T4) on chamber remodeling, cardiac function, cellular morphology, cardiac blood flow, and protein expression. Normal and cardiomyopathic hamsters were treated with T4 or DITPA for 2 months. At the end of the treatment, echos, hemodynamics, coronary blood flow, cell morphology, and protein expression data were collected. Both T4 and DITPA treatment reduced chamber diameter during diastole, suggesting attenuated chamber dilatation in cardiomyopathic hamsters. Wall thickness also tended to increase, which was supported by cell morphology data in which DITPA significantly increased cross-sectional growth of myocytes specifically in the minor dimension, which is oriented transmurally. T4 and DITPA also increased myocardial blood flow both at baseline and after maximal dilation. This suggests there was increased angiogenesis or reduced loss of arterioles. Both T4 and DITPA had beneficial effects on chamber remodeling, which was most likely due to beneficial changes in cell shape and improved vascular supply.

Myocyte Morphological Characteristics Differ Between the Phases of Pulmonary Hypertension-Induced Ventricular Hypertrophy and Failure

Pulmonary hypertensive model rats were prepared by treating them with monochrotaline (MCT). Using these model rats, we examined myocyte remodeling in the right ventricle in response to increased right ventricular pressure. Male Sprague-Dawley rats were divided into 2 groups. Group M received MCT and group C received physiological saline. The 2 groups were examined at weeks 2, 5, and 7 after MCT or saline injection, respectively. At week 2, a significant difference in cell form was not observed in either group. At week 5, cell volume and myocyte cross-sectional area (CSA) of the right ventricle in group M were significantly greater than those in group C. At week 7, cell volume, CSA, and cell length of the right ventricle in group M were all significantly greater than those in group C. These results suggest that pulmonary hypertension causes hypertrophy, accompanying the enlargement of CSA in the right ventricle, and that cells lengthen in the phase of right ventricular failure. These results are similar to the changes observed in left ventricular myocytes due to overload pressure. Both right and left ventricular myocytes may share a common mechanism for myocyte remodeling as an adaptive and maladaptive response to increased ventricular pressure.

Treatment of subclinical hypothyroidism reverses ischemia and prevents myocyte loss and progressive LV dysfunction in hamsters with dilated cardiomyopathy

Growing evidence suggests that thyroid dysfunction may contribute to progression of cardiac disease to heart failure. We investigated the effects of a therapeutic dose of thyroid hormones (TH) on cardiomyopathic (CM) hamsters from 4 to 6 mo of age. CM hamsters had subclinical hypothyroidism (normal thyroxine, elevated TSH). Left ventricular (LV) function was determined by echocardiography and hemodynamics. Whole tissue pathology and isolated myocyte size and number were assessed. TH treatment prevented the decline in heart rate and rate of LV pressure increase and improved LV ejection fraction. The percentage of fibrosis/necrosis in untreated 4-mo-old CM (4CM; 15.5 ± 2.2%) and 6-mo-old CM (6CM; 21.5 ± 2.4%) hamsters was pronounced and was reversed in treated CM (TCM; 11.9 ± 0.9%) hamsters. Total ventricular myocyte number was the same between 4- and 6-mo-old controls but was reduced by 30% in 4CM and 43% in 6CM hamsters. TH treatment completely prevented further loss of myocytes in TCM hamsters. Compared with age-matched controls, resting and maximum coronary blood flow was impaired in 4CM and 6CM hamsters. Blood flow was completely normalized by TH treatment. We conclude that TH treatment of CM hamsters with subclinical hypothyroidism normalized impaired coronary blood flow, which prevented the decline in LV function and loss of myocytes.

The total length of myocytes and capillaries, and total number of myocyte nuclei in the rat heart are time-dependently increased by growth hormone

Growth hormone (GH) can increase size and dimensions of rat hearts. The aim was to study how GH administration influences the growth of cardiac myocytes and capillaries in relation to time. Three-month-old female rats were divided into 10 groups (n=3), and injected with either GH (5mg/kg/day) or vehicle for 5, 10, 20, 40, or 80 days. From the left ventricle (LV) histological sections were made and stereological methods applied. Linear regression showed that GH time-dependently increased: LV volume (r=0.96, P<0.001), total volume of myocytes (r=0.96, P<0.001) and capillaries (r=0.64, P<0.05), total length of myocytes (r=0.90, P<0.001) and capillaries (r=0.78, P<0.001), and total number of myocyte nuclei (r=0.85, P<0.001). In conclusion, during 80 days of GH treatment the total volume and length of myocytes and capillaries, and total number of myocyte nuclei increased in a linear way. The results indicate that GH is a potent mediator of myocardial growth.

Effects of induced hyperthyroidism in normal and cardiomyopathic hamsters

Thyroid hormones (TH) enhance cardiac function and reverse gene changes typical of pathological hypertrophy. However, reports in humans, but not animals, indicate that excess TH can cause heart failure. Also, the effects of TH on normal and cardiomyopathic hearts are likely to be different. The goal of this study was to characterize the effects of prolonged hyperthyroidism on cardiac function, chamber and cellular remodeling, and protein expression in both normal and cardiomyopathic hearts. Hyperthyroidism was induced in 3-mo-old normal BIO F1B and dilated cardiomyopathic BIO TO2 hamsters. After TH treatment for 10 days and 2 mo, hemodynamics, echos, myocyte length, histology, and protein expression were assessed. After 10 days and 2 mo, there were no differences between TO2-treated (Tx) and TO2-untreated (Untx) hamsters in chamber diameters or left ventricular function. After 2 mo of treatment, however, F1B-Tx showed evidence of dilated heart failure vs. F1B-Untx. Chamber diameters were increased, and ejection fraction and positive and negative changes in pressure over time were reduced. In F1B-Tx and TO2-Tx hamsters, beta-myosin isoform expression was reduced, whereas alpha-myosin increased significantly in F1B-Tx only. In TO2-Tx hamsters, the percent of viable myocardium was increased, and percent fibronecrosis was reduced vs. TO2-Untx. Myocyte length increased with TH treatment in both hamster strains. We conclude that 1) excess TH can induce heart failure in normal animals as observed in humans, 2) reversal of myosin heavy chain expression does not necessarily improve heart function, and 3) excess TH altered cellular remodeling but did not adversely affect chamber function or dimensions in TO2 hamsters.

Link Between SCN5A Mutation and the Brugada Syndrome ECG Phenotype Simulation Study

BACKGROUND

The specific changes in the gating kinetics of the sodium current (I(Na)) responsible for its phenotype have remained to be elucidated. In the present study the effect of changes in the gating kinetics of I(Na) on early repolarization (ER) and initiation of phase 2 reentry (P2R) were evaluated in a theoretical epicardial ventricular fiber model.

METHODS AND RESULTS

Miyoshi-I(CaL) was incorporated into the modified Luo-Rudy dynamic (LRd) model. Dispersion at Ito-density was set within a theoretical fiber composed of serially arranged epicardial cells with gap junctions. The following changes in I(Na) kinetics were made: (1) a-10 mV shift in steady-state inactivation, (2) a+10 mV shift in steady-state activation curve, (3) a small inactivation time constant (DEC); P2R and ER were observed. A conduction disturbance within the fiber was simulated and only when the I(Na)-density was decreased did DEC, especially, show a marked increase in the likelihood of causing ER and P2R. Conduction disturbance significantly increased the likelihood causing ER or P2R.

CONCLUSIONS

In this one-dimension model with Ito-density dispersion, DEC-I(Na) precipitates I(Na)-blocker inducible ER. This suggests that the characteristic ST-segment elevation in the Brugada syndrome with SCN5A mutation can be interpreted in part by DEC-I(Na). Concomitant conduction disturbance may be required to cause P2R at physiological Ito density.

Thyroid hormones induce unique and potentially beneficial changes in cardiac myocyte shape in hypertensive rats near heart failure

We examined the effects of thyroid hormones (THs) on left ventricular (LV) function and myocyte remodeling in rats with spontaneously hypertensive heart failure (SHHF). SHHF rats were treated with three different TH doses from 20-21 mo of age. In terminal experiments, LV function (as determined by echocardiography and catheterization) and isolated myocyte shape were examined in SHHF rat groups and age-matched Wistar-Furth control animals. Compared with Wistar-Furth rats, the ratio of alpha- to beta-myosin was reduced in untreated SHHF rats. The alpha-to-beta-myosin ratio increased in all TH groups, which suggests a reversal of the fetal gene program. Low-dose TH produced no changes in LV myocyte size or function, but high-dose TH produced signs of hyperthyroidism (e.g., increased heart weight, tachycardia). The chamber diameter-to-wall thickness ratio declined with increasing dose due to reduced chamber diameter and increased wall thickness. This resulted in a 38% reduction in LV systolic wall stress in the middle- and high-dose groups despite sustained hypertension. Isolated myocyte data indicated that chamber remodeling and reduced wall stress were due to a unique alteration in myocyte transverse shape (e.g., reduced major diameter and increased minor diameter). Based on our present understanding of ventricular remodeling and wall stress, we believe these changes are likely beneficial. Results suggest that TH may be an important regulator of myocyte transverse shape in heart disease.