Skip Navigation


Cardiovascular Research Advance Access first published online on August 14, 2008
This version [Corrected Proof] published online on September 12, 2008
Cardiovascular Research, doi:10.1093/cvr/cvn229
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Supplementary Data
Right arrow All Versions of this Article:
81/3/555    most recent
cvn229v3
cvn229v2
cvn229v1
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Aït Mou, Y.
Right arrow Articles by Cazorla, O.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Aït Mou, Y.
Right arrow Articles by Cazorla, O.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2008. For permissions please email: journals.permissions@oxfordjournals.org

Late exercise training improves non-uniformity of transmural myocardial function in rats with ischaemic heart failure

Younss Aït Mou1,2, Cyril Reboul3, Lucas Andre1,2, Alain Lacampagne1,2 and Olivier Cazorla1,2,*

1 INSERM U637, Physiopathologie Cardiovasculaire, CHU Arnaud de Villeneuve, 34295 Montpellier Cedex 5, Montpellier, France
2 Université Montpellier1, UFR de Médecine, F-34295 Montpellier, France
3 EA-4278, Université Avignon et des Pays de Vaucluse, UFR Sciences F-84000, Avignon, France

* Corresponding author. Tel: +33 467 41 52 42; fax: +33 467 41 52 42. E-mail address: olivier.cazorla{at}inserm.fr

Received 23 June 2008; revised 31 July 2008; accepted 8 August 2008

Time for primary review: 27 days


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 Funding
 References
 
Aims: The exercise-induced beneficial mechanisms after long-term myocardial infarction (MI) are incompletely understood. The present study evaluated the effect of treadmill exercise training (5 weeks), started at a late stage of heart failure (HF) (13 weeks post-MI), on rat left ventricle remodelling and dysfunction of the regional global and cellular contractile functions.

Methods and results: In vivo echocardiography confirmed that sub-endocardial (ENDO) layers contract more (+86%) and faster (+50%) than the sub-epicardial (EPI) layers. This gradient was lost in MI rats due to a predominant reduction in the ENDO layer contractility. Exercise partially restored the amplitude and velocity of ENDO contraction, resulting in a partial recovery of the pump function indexed by the aortic blood-flow velocity time integral. At the cellular level, MI impaired ENDO contractile properties by reducing cell shortening (10–7%), calcium transient, and myofilament Ca2+ sensitivity. These alterations were normalized by exercise. Sarcoplasmic reticulum Ca2+-ATPase (SERCA)2a expression and myosin light chain (MLC)-2 phosphorylation in ENDO cells were significantly reduced after MI and were restored by exercise. The EPI layer was only slightly reduced in vivo without cellular alterations.

Conclusion: This study shows that exercise performed at a late stage after MI restored a transmural non-uniformity of myocardium lost during HF. Recoveries of Ca2+ homeostasis and myofilament function of cardiomyocytes contribute to this beneficial effect.

KEYWORDS End-stage heart failure; Myocardial contractility; Contraction; Exercise; Myocardial infarction


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 Funding
 References
 
After myocardial infarction (MI), the heart is characterized by reduced contractility and impaired filling resulting from changes in cardiac structure (hypertrophy, dilatation), cell death, maladaptative remodelling of the extracellular matrix, abnormal energy metabolism, and cellular dysfunction. The underlying cellular mechanisms that have been suggested include altered calcium homeostasis,1 impaired myofilament calcium responsiveness,2–4 and reduced cross-bridge cycling rate.5 A hierarchy of cellular events during the development of heart failure (HF) has been proposed, starting with electrical remodelling and altered Ca2+ homeostasis during the early phase followed by myofilament dysfunction.6 Moreover, transmural myocardial contractile performance is non-uniform across the different layers of the left ventricular (LV) wall.7,8 This non-uniformity of cardiac function plays a fundamental role in cardiac mechanical work. Its loss has been previously reported to be a sensitive index to discriminate physiological from pathological LV remodelling.8 We previously reported that HF affects preferentially the contractile machinery of the sub-endocardial (ENDO) cells, leading to the loss of transmural heterogeneity.3

In clinic, pharmacological treatments are used to maintain the pump function of the failing heart as much as possible. However, non-pharmacological approaches may be attempted. For example, many clinical studies have shown that exercise training performed by the patients with HF is beneficial for heart performance and quality-of-life.9,10 In the normal heart of small11 and large12 animals, exercise training increases cellular contractile function such as cardiomyocyte shortening, Ca2+ dynamics and myocyte power-generating capacity, and myocardial perfusion capacity.13 There are also clinical evidences that exercise after MI has a beneficial effect on disease progression and survival.9,10 A recent study has shown that early exercise in MI mice had no effect on LV remodelling but attenuates global LV dysfunction, which can be essentially explained by the exercise-induced improvement of myofilament function.14 However, after a small MI, exercise has either no effect or improved LV function independently of the starting point of exercise (early or late after MI).14 For large infarct, exercise can have detrimental effects when performed at an early stage while beneficial effects on LV remodelling and function appear when exercise is started late after MI.15,16 In animal models, most of the studies tested the effect of exercise on LV global and/or cellular functions, 4 weeks after MI, when LV remodelling is still ongoing.17,18 Moreover, it has been shown in rat models that 3–5 weeks after MI exercise capacity is still high and close to Sham-operated animals while it decreased significantly from 10 weeks after MI.19 Thus, the cellular adaptations between early and late exercise after MI may be different. Moreover, part of the discrepancy on the effect of exercise on MI in the literature may be due to the use of swimming training,15 which is known to have different responses from those to treadmill running, complicated by factors such as the diving reflex, mental stress, and episodes of hypoxia associated with diving.

The present study evaluated, in a rat model, the effect of exercise training, started at a late stage of HF (13 weeks post-MI), on LV remodelling and function of the global and cellular contractile properties in various regions of the LV free wall. The results indicated that in vivo systolic function [fractional shortening (FS) and aortic velocity time integral (VTIAo)] was reduced in MI rats in association with a reduction of the amplitude and speed of contraction of the ENDO layer and to a lesser extent of the sub-epicardial (EPI) layer. These alterations led to a uniformity of the transmural myocardial function, which were partially reversed by exercise. Exercise improved part of the systolic parameters and slightly reduced LV dilatation. The in vivo beneficial effect of exercise was associated with a restoration of the ENDO cellular properties, by reversing the MI-induced abnormalities in Ca2+-handling function and proteins, phosphorylation status of contractile proteins, and myofilament function.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 Funding
 References
 
For a detailed description, see expanded Materials and Methods in Supplementary material online. Experiments complied with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publications No. 85–23, revised 1996) and with the approval of the French Ministry of Agriculture.

2.1 Animals and exercise training
MI was produced by permanent ligation of the left coronary artery in male Wistar rats as previously described.3 Thirteen weeks post-MI, rats were randomly assigned to the sedentary (MI) or 5 weeks treadmill exercised (MI-Ex, 40 min/day, 5 days/week, 16 m/min) groups. At the end of the training protocol, morphological and functional parameters were determined by echocardiography and compared with Sham-operated animals. Animals were then sacrificed for cellular investigations.

2.2 Echocardiography and non-invasive haemodynamics
Doppler-echocardiography was performed with a MyLab 30 (ESAOTE, Italy). Wall Thicknesses and Left Ventricular End Diameters (LVED) were obtained at the level of the papillary muscles. LV shortening fraction and end-systolic strain of the posterior wall measured as deformation from end-diastole to end systole [(PWTs – PWTd)/PWTd] * 100 were calculated. PWTs and PWTd are end-systolic and end-diastolic Posterior Wall Thickness, respectively.20

Ascending aortic blood flow was recorded, as previously described,21 via pulsed-wave Doppler permitting measurements of the VTIAo. LV ENDO and EPI posterior wall displacements were measured offline at the level of the papillary muscles. Tissue Doppler imaging was performed after each conventional echocardiography as previously described.22

2.3 Contractile properties in intact cardiomyocytes
Single LV cardiomyocytes were isolated by enzymatic dissociation from the remaining inner free wall (ENDO) and from the outer free wall (EPI) as previously described (n = 5 rats per group).23 Unloaded cell shortening and calcium concentration [Ca2+] (indo 1 dye) were studied using field stimulation (0.5 Hz, 22°C, 1.8 mM external Ca2+). Sarcomere length (SL) and fluorescence (405 and 480 nm) were simultaneously recorded (IonOptix system, Hilton, USA).

2.4 Force measurements in permeabilized cardiomyocytes
Isometric force was measured in single permeabilized cardiomyocytes at different [Ca2+], at 1.9 then 2.3 µm SL as described previously (n = 5 rats per group).3,23 Force was normalized by the cross-sectional area measured from the imaged cross-section. Force–pCa relationships were fitted to a Hill equation. To prevent degradation, all solutions contained protease inhibitors (PMSF, 0.5 mmol/L; leupeptin, 0.04 mmol/L; and E64, 0.01 mmol/L).

2.5 Myosin-heavy chain composition
Myosin-heavy chain (MyHC) isoforms were separated on a 6% SDS–PAGE and silver stained as previously described.24 β-MyHC content was expressed relative to the total amount of MyHC protein.

2.6 Western immunoblotting
Sarcoplasmic reticulum Ca2+-ATPase (SERCA)2a and phospholamban (PLB) were separated on a gradient SDS–PAGE (2–20%) and then blotted onto nitrocellulose membrane. Proteins were revealed with specific antibodies and were expressed relative to Calsequestrin content.

Troponin I (TnI) was separated by 15% SDS–PAGE and the protein kinase A phosphorylated form of cardiac TnI was normalized by the total TnI form as previously described.3 Myosin light chain 2 (MLC-2) phosphorylated and non-phosphorylated forms were separated by a 10% urea gel and were specifically detected with a cardiac MLC-2 antibody.3

Immunodetection was revealed with ECL Plus system.

2.7 Statistical analysis
Data were analysed using one-way or two-way ANOVA among groups. When significant interactions were found, a Bonferroni t-test was applied with adjusted P < 0.05 (SigmaStat 3.5). Data are presented as means ± SEM.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 Funding
 References
 
3.1 In vivo cardiac morphological and functional parameters
MI animals had large antero-lateral infarcts detected by visual inspection. In a preliminary longitudinal study (data not shown), we observed that in our model, most of the morphological and functional alterations occurred over 11–13 weeks. Minimal changes will occur after this period. Thirteen weeks after MI, LV diameters were increased in MI at the end-diastolic and -systolic phases without hypertrophy of the posterior wall thickness (Figure 1A). The posterior wall end-systolic strain during a cardiac cycle was largely reduced (–80%) in MI animals (Figure 1B). In Sham animals, the LV ENDO layer contracted faster (+50% of the section motion) than the EPI layers (Figure 1C). HF significantly decreased contraction velocities of both EPI and ENDO layers by 15 and 32%, respectively, homogenizing the transmural velocities of contraction towards the slowest values. The global cardiac systolic function was also altered as shown by the reduction in FS (Figure 1D). Since the calculation of FS, measured at the level of papillary muscle, is highly affected by the akinetic infarct zone, we measured the VTIAo as another index of systolic function. VTIAo was also decreased after MI (Figure 1D). Additional weeks after MI corresponding to the training duration had little impact on these parameters (see below).


Figure 1
View larger version (22K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 1 Left ventricular morphological and functional characteristics 13 weeks post-myocardial infarction. End-diastolic and -systolic diameters and diastolic posterior wall thickness (A), end-systolic strain (B), systolic velocity of the left ventricular epicardial and endocardial layers (C), global systolic function (D) as indexed by fractional shortening (left), and aortic velocity time integral (right). *Adjusted P < 0.05 vs. corresponding Sham (n = 11 hearts/group).

 
3.2 Effect of exercise on in vivo cardiac function
We then investigated the effect of exercise training while most of the remodelling and dysfunction of the heart had already occurred. Eighteen weeks post-MI, hearts were hypertrophied as shown by the increase in the heart weight/body weight ratio between Sham and MI animals and further increased after exercise training (Table 1). Part of this exercise-induced hypertrophy may be explained by an increase of the posterior wall thickness although the difference did not reach significance (P = 0.069) (Table 1). The main effect of exercise on LV morphologies was a decrease of both end-systolic and -diastolic left ventricular diameters (Table 1). Eighteen weeks after MI, we observed a large decrease in the end-systolic strain (from 83 ± 4 to 24 ± 11% in Sham and MI, respectively) and in the absolute ENDO posterior wall displacement between diastole and systole (from 2.7 ± 0.2 to 1.3 ± 0.2 mm in Sham and MI, respectively). The values obtained were similar to the one measured 13 weeks post-MI confirming the stability of these indexes during this phase of the disease (Figure 2B). Exercise had a significant beneficial effect on the end-systolic strain (63 ± 7%) and on the ENDO posterior wall displacement (1.8 ± 0.4 mm). The section motion (Sm) was similarly decreased between 13 and 18 weeks after MI in both regions. Exercise restored Sm completely in EPI and only partially in the ENDO layers (Figure 2C). The gradient of velocity across the LV defined as the difference of velocity between ENDO and EPI, observed in normal conditions (1.66 ± 0.11 cm/s), disappeared almost completely in MI animals (0.28 ± 0.05 cm/s) and was partially restored in MI-Ex (0.76 ± 0.05 cm/s) (Figure 2C). Finally, VTIAo, used as an accurate index of LV stroke volume, decreased by 37% in MI animals and was partially restored by exercise in MI-EX remaining 14% lower than Sham (Figure 2D). Exercise had no impact on the mortality of the animals (Figure 2E).


View this table:
[in this window]
[in a new window]

  		Table 1 Cardiac morphological and functional parameters

 


Figure 2
View larger version (54K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 2 Left ventricular morphological and functional characteristics in myocardial infarction (MI) and MI-exercise rats. (A) Echocardiography images with an enlargement of the posterior wall. (B) End-systolic strain (left) and displacement of epicardial and endocardial layers during a cardiac cycle (right). (C) Systolic velocity (left) and gradient of velocity ENDO–EPI (right). (D) Aortic velocity time integral. (E) Survival curve following the beginning of exercise program. *Adjusted P < 0.05 (n = 12–17 hearts/group).

 
3.3 Effect of exercise on myocyte shortening and Ca2+ transient
The effect of exercise on the excitation–contraction coupling of long-term MI was tested in intact LV unloaded cardiomyocytes. For this purpose, SL shortening and intracellular calcium content were simultaneously measured on field stimulated cardiomyocytes. Unloaded contraction decreased significantly with HF only in ENDO cells and was restored by exercise (Figure 3). The duration of unloaded shortening (data not shown) and relaxation (Figure 3B) was not different between the various groups. However, the speed of contraction and relaxation was significantly reduced in ENDO MI cells and was restored by exercise (Figure 3C). The amplitude of Ca2+ transient decreased significantly only in ENDO MI cells and was restored by exercise (Figure 4B). In addition, the calcium transient decay in both EPI and ENDO MI myocytes was significantly slowed (increase in tau), reflecting an altered calcium reuptake. All parameters were normal in MI-Ex. The calcium transient decay in MI is known to be altered due to changes in SERCA2a expression or in the inhibition of SERCA2a/PLB activity. We found that SERCA2a expression was significantly decreased in MI animals, both in ENDO and EPI layers. PLB expression was unchanged. Levels of protein expression in MI-Ex were similar to Sham samples (Figure 4C).


Figure 3
View larger version (42K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 3 Sarcomere length (SL) shortening in isolated myocardial infarction (MI) and MI-exercise (MI-Ex) cardiomyocytes. (A) SL shortening examples of endocardial (left trace) and epicardial (right trace) Sham, MI, and MI-Ex myocytes. (B) Averaged data of SL shortening (in percentage on baseline) and time to 50% of relaxation (TR50). (C) Speeds of contraction (left) and relaxation (right) (n = 55–106 cells/5 hearts). *Adjusted P < 0.05.

 


Figure 4
View larger version (33K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 4 Calcium transient in isolated myocardial infarction (MI) and MI-exercise (MI-Ex) cardiomyocytes. (A) Ca2+ transient illustrations of endocardial (left trace) and epicardial (right trace) Sham, MI, and MI-Ex myocytes. (B) Average data of calcium transient amplitude and relaxation phase (tau). (n = 55–106 cells/5 hearts) (C) Immunoblot analysis of Ca2+-handling proteins (SERCA and PLB) normalized with calsequestrin content (n = 5 hearts). *Adjusted P < 0.05.

 
3.4 Force development in single permeabilized myocytes
Force development of intact myocytes depends on the amount of calcium released by the sarcoplasmic reticulum and the myofilament Ca2+ sensitivity. Thus, we measured the myofilament Ca2+ sensitivity (pCa50) at short length (1.9 µm SL) and at long length (2.3 µm SL). Passive force and maximal isometric tension measured at both SL were similar between Sham, MI, and MI-Ex (Table 2). Neither pathology nor exercise had any effect on myofilament Ca2+ sensitivity at short SL (Figure 5 and Table 2). Stretching the cells to 2.3 µm SL induced a leftward shift of tension in all conditions reflecting an increase in myofilament Ca2+ sensitivity. However, pCa50 was significantly lower in the ENDO cells isolated from MI rats. Exercise restored Ca2+ sensitivity at 2.3 µm SL (Figure 5A). The difference between pCa50 at the long and short SL ({Delta}pCa50), used as an index of the length-dependent activation of contractile machinery, was significantly smaller in ENDO MI cells compared with the Sham and MI-Ex cells. EPI myocytes were affected neither by the pathology nor by the exercise at both lengths (Figure 5B).


View this table:
[in this window]
[in a new window]

  		Table 2 Regional mechanical properties of cardiomyocytes isolated from sham, myocardial infarcted (MI), and MI-exercised (MI-Ex) rats

 


Figure 5
View larger version (28K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 5 Effect of myocardial infarction (MI) and MI-exercise (MI-Ex) on myofilament properties in skinned cardiomyocytes: tension (normalized to maximal tension)-pCa curves of ENDO (A) and EPI (B) myocytes, measured at 1.9 (solid line, solid symbols) and at 2.3 µm (dashed line, open symbols) sarcomere length in Sham (square) MI (circle) and MI-Ex (triangle). (A and B, right) Myofilament Ca2+ sensitivity (pCa for half maximal activation) and the stretch-induced Ca2+ sensitization ({Delta}pCa50 difference in pCa50 obtained at 2.3 and 1.9 µm SL). (C) Correlation between {Delta}pCa50 and passive tension across the ventricle. Linear regressions were calculated on a scatter plot, including all individual data in Sham (solid line, square; y = 0.15+0.005x, r = 0.51 P = 0.007); MI (dash-dot line, solid circle, y = 0.18–0.005x, r = –0.34 P = 0.16); and MI-Ex (dashed line, open circle, y = 0.15+0.005x, r = 0.34 P = 0.15). *P < 0.05.

 
We have previously shown that {Delta}pCa50 is closely related to passive tension rather than to SL.3 The differences in passive and active properties across the left free wall described a positive relationship in which ENDO cells develop more passive tension after a stretch to 2.3 µm SL associated with higher stretch-induced myofilament Ca2+ sensitivity than did EPI cells (Figure 5C). This correlation disappeared in MI rats, mostly due to a decrease in {Delta}pCa50 of ENDO cells. Interestingly, values obtained in MI-Ex myocytes were similar to those found in Sham rats.

3.5 Contractile protein isoforms and phosphorylation
Rat heart expresses two isoforms of MyHC ({alpha}- or β-MyHC). Variation in {alpha}/β-MyHC expression influences cardiac function. In our conditions, β-MyHC content increased after MI in both ENDO and EPI cells. Exercise had no effect on β-MyHC content in either ENDO or EPI cells of MI rats (Figure 6A).


Figure 6
View larger version (32K):
[in this window]
[in a new window]
[Download PowerPoint slide]
  		Figure 6 Effect of myocardial infarction and exercise on sarcomeric proteins: SDS–PAGE for {alpha} and β-MyHC isoforms; β-MyHC content was expressed in percentage of total MyHC in ENDO and EPI (A). Immunoblots with an anti-cardiac TnI and anti-cTnI phosphorylated by PKA (B), and with an anti-cardiac MLC-2 (C), on ENDO (left) and EPI (right) strips, in relax state (R, open bar) and after stretch (S, gray bar). *Adjusted P < 0.05.

 
TnI and MLC-2 phosphorylations are known to shift the tension-pCa curves in cardiac muscle. Western blot analysis was performed on non-stretched and stretched skinned muscle strips dissected from the ENDO layer or the EPI layer. Phosphorylation of TnI on the PKA sites was similar between regions, before and after stretch, in Sham, MI and MI-Ex animals (Figure 6B). We have previously shown that the linear relationship between pCa50 and passive tension was associated with changes in the phosphorylation level of MLC-2.3 As previously reported, stretch increased by {approx}10%, the amount of phosphorylated MLC-2 in the ENDO myocardium of Sham animals and not in MI animals (Figure 6C). Exercise restored the stretch-induced increase in MLC-2 phosphorylation in MI animals. MLC-2 phosphorylation was not affected by pathology, stretch, and exercise in EPI myocardium.


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
Funding
 References
 
The present study tested whether a remodelled end-stage failing heart following MI could benefit from 5 weeks of endurance exercise training. To this end, we evaluated the effects of exercise on LV remodelling, regional in vivo function, cardiomyocyte contractility, Ca2+ handling, and myofilament Ca2+ sensitivity. The main findings were that1 normal myocardial function is non-uniform due to heterogeneous cellular properties across the wall2 transmural non-uniformity of myocardial function was lost in MI rats due to ENDO cellular dysfunction3 exercise recovered a transmural heterogeneity by improving LV function mostly in ENDO layer, in relation with a normalization of MI-induced dysfunctions of both Ca2+ handling and myofilaments Ca2+ sensitivity in ENDO cardiomyocytes.

4.1 Regional pathophysiology of myocardial infarction-induced left ventricular dysfunction
It is now established that cardiac function is non-uniform across the wall due to larger and faster contraction of the ENDO layer as compared with the EPI tissue one.7 A recent study using transmural bead markers under biplane cineradiography showed that the onset of myofibre shortening occurred earlier in endocardium than epicardium, whereas the onset of myofibre relaxation occurred earlier in epicardium than endocardium.25 These differences in fibre strains may be explained by telediastolic and telesystolic transmural differences in wall stress, increasing towards the endocardium.26 Our data showed that the higher ENDO fibre shortening during systole was likely due to higher basal cardiomyocyte contractility and a higher stretch-induced increase in myofilament Ca2+ sensitivity.

As previously reported,27,28 the remodelling of MI heart was characterized by LV dilatation, hypertrophy, and dysfunction. The cardiac pump dysfunction was characterized here by a reduction in LV FS and other indexes, such as the VTIAo, and the amplitudes and velocities of contraction of the myocardial layers. We also observed that the pathology did not affect the heart uniformly. Following MI, wall contractility decreased mostly in the ENDO layer with a higher decrease in the amplitude of displacement (–52 vs. –28%, ENDO vs. EPI) and in the velocity of contraction (–36 vs. –13%, ENDO vs. EPI). These changes resulted in a homogenization in regional myocardial function and the complete loss of transmural non-uniformity.

4.2 Beneficial effects of exercise training on MI cardiac function
In our MI model, most parameters of the LV remodelling and in vivo dysfunctions were achieved 13 weeks after MI and were stable 18 weeks after MI (Figures 1 and 2). We thus assessed whether exercise could improve myocardial function and restore the transmural heterogeneity in a remodelled MI heart. Studies in humans with large MI reported that exercise had either no29 or a beneficial30 effect on ejection fraction and LV volumes. Previous studies in animal models reported that exercise had no effect on LV function parameters such as LV dP/dTmax or FS irrespective of whether exercise was started early or late after MI, despite an improvement in cell function.15,17 Neither was FS in MI improved by exercise in our study. However, other indexes of cardiac function such as aortic blood flow (VTIAo) and regional amplitude and velocities of contraction were improved after exercise. In several clinical studies, alterations in wall motion (including wall thickening) measured by echocardiography were shown to be good predictors of subsequent cardiac events of morbidity and mortality31,32 and interventions that halt, slow or reverse these ventricular dysfunctions should markedly improve clinical outcomes.33 The higher exercise-induced improvement of amplitude and velocity of contraction in ENDO allowed partially to recovering a transmural non-uniformity. This may in turn contribute in the exercise-induced beneficial effect on cardiac function.

In view of the concern that late exercise may aggravate LV remodelling after a large MI and life expectancy, we investigated LV remodelling with or without exercise. Five weeks of exercise decreased LV end-diastolic and -systolic diameters, and did not aggravate LV remodelling. Our exercise protocol was a moderate endurance protocol, confirmed by the fact that Sham animals exercised at the same intensity did not show any change in any parameters investigated (data not shown). These observations are in agreement with a recent study reporting that 8 weeks of moderate exercise in mice with MI cardiomyopathy14 and hypertrophic cardiomyopathy34 reversed collagen content with little effect on cardiac hypertrophy. In our study, exercise did not induce a higher mortality consistent with previous studies with human patients.16

4.3 Exercise-induced effects on contractile cellular properties
The mechanisms for LV dysfunction after MI remain incompletely understood but have been proposed to be a consequence of cellular alterations and extracellular matrix remodelling.1 Recent studies have shown that early exercise in mice after MI had no effect on LV remodelling but slightly attenuates global LV dysfunction mostly by improving myofilament function without Ca2+ signalling.14 However, similar protocols performed in female Sprague–Dawley rat model showed a recovery of contraction function, Ca2+ handling and some indexes of improved myofilament Ca2+ sensitivity following early exercise after MI.17

In our study, cell shortening decreased after MI exclusively in ENDO cells, most probably due to both reduced Ca2+ release and decreased myofilament function. Exercise fully restored the ENDO cellular properties without affecting the EPI cells. The recovery of calcium transient after exercise was associated with changes in SERCA2a17 and Na+/Ca2+ exchanger17 expression suggesting a recovery of the loading conditions of the sarcoplasmic reticulum. Further studies are needed to determine whether the exercise-induced improvement in systolic Ca2+ levels are due to changes in Ca2+ release properties through the ryanodine receptors.

Myofilament Ca2+ sensitivity was depressed at end-stage HF in the present study. Previous studies in single skinned myocytes have reported either increases35 or decreases3,4 in myofilament Ca2+ sensitivity both in human and experimental HF. The reasons for these different findings are not entirely clear. One explanation for these different findings may relate to the different experimental preparations that were studied (multicellular vs. single myocyte skinned myocardium). Another explanation might be the level of neurohormonal stimulation present at the time of tissue preparation that will affect the balance between kinases and phosphatases. Indeed, most of the studies reported alterations of the level of phosphorylation of sarcomeric regulatory proteins that correlated with the changes in myofilament Ca2+ sensitivity. Finally, the origin of HF (ischaemic, pressure overload) and the stage of HF may also affect the results.6

HF in small rodents is associated with a shift in isomyosin synthesis from predominantly {alpha}-myosin towards β-myosin.36 This shift observed during the LV remodelling has been proposed to improve myocardial work efficiency by generating cross-bridge force with a higher economy of energy consumption and thus maintaining contractility.6 Similar shift can be observed in exercised animals.24 In our study, β-MyHC content was increased by HF but was not affected by exercise. Thus, the beneficial effect of exercise on MI myocardial function is independent on myosin expression.

The level of MLC-2 phosphorylation affects myofilament Ca2+ sensitivity.37 Increasing MLC-2 phosphorylation by incubating myofilaments with exogenous MLC Kinase (MLCK) increases myofilament Ca2+ sensitivity.38–40 A decrease in MLC-2 phosphorylation has been described in failing human41 and animal3 hearts and has been associated with the observed decrease in myofilament Ca2+ sensitivity; similar results were obtained in ENDO MI myocytes of the present study. Thus, the increase in MLC-2 phosphorylation in the ENDO tissue exclusively observed in the present study after exercise may by itself explain the improved myofilament Ca2+ sensitivity. The phosphorylation level of MLC-2 depends on the balance of activities between the MLCK and a protein phosphatase (PP).40 MLCK activity is modulated by PKA and PKC phosphorylations, and Ca2+–calmodulin interaction offering potential regulators for the exercise-induced effect on MLC-2 phosphorylation.40,42 The normalization of Ca2+ transient after exercise in MI may contribute to increasing MLCK activity and thus MLC-2 phosphorylation. MLC-2 dephosphorylation is classically attributed to PP1. Studies suggest that PP1 activity and expression are increased in end-stage chronic heart failure (CHF) in dogs43 but PP1 expression was found unchanged in rats with CHF.44 In our study, alteration in PP1 activity after MI and/or exercise could account for the changes in MLC-2 phosphorylation and the lack of effect on TnI between MI and MI-Ex. However, more experiments are required to explore the kinase/phosphatase alteration after MI and exercise.

4.4 Limitations of the study
From the present study, we cannot draw any conclusions as to the effect of this protocol on life expectancy, since the animals were sacrificed at the end of the exercise protocol. In particular, future studies should determine how the improvement of the global and regional cardiac function and the normalization of the cellular dysfunctions post-MI at the end of the training could affect the occurrence of sudden cardiac death because of arrhythmic events. It would have been also interesting to obtain intraventricular pressure measurements that would have given more in vivo functional index, allowing to further explore the effect of exercise training on diastolic (dys)function in our model.

The present study indicates that exercise training started late after a large MI improved regional LV function and molecular phenotype, without any adverse effects on LV remodelling and survival during the protocol. The beneficial effects of exercise on cellular function have been proposed to restore β1-adrenergic signaling.14 However, there is no sign here of such improvement that should affect the phosphorylation status of various proteins (TnI, PLB, titin-based passive tension). Future studies should be aimed at investigating whether this difference is due to the fact that exercise was performed in organisms in which the β-adrenergic signalling has been already extensively stimulated, suggesting other signalling pathways for exercise-induced beneficial effects. This is of particular interest since most patients with HF are treated with β-blockers. Another positive aspect of this study is that exercise preferentially restored the contractile properties of the tissue altered by the pathology (ENDO), allowing physiological cardiac contractile heterogeneity to be restored. Because of its relatively low cost, high availability and ease of use, exercise training is an intervention that could be accessible to most patients with HF.


   Funding
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 Funding
References
 
This work was supported by the ‘Association Française contre les Myopathies' and the Leducq Foundation. O.C. and A.L. are established investigators of CNRS.


   Acknowledgements
 
Thanks are due to Patrice Bideaux, Guillermo Salazar and Christine Bernard-Crozier for technical assistance.

Conflict of interest: none declared.


   Notes
 
This is a new version of this article as the first author's name was incorrect in the running header.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 Funding
 References
 

  1. Houser SR, Margulies KB. Is depressed myocyte contractility centrally involved in heart failure? Circ Res (2003) 92:350–358.[Abstract/Free Full Text]
  2. Hajjar RJ, Gwathmey JK, Briggs GM, Morgan JP. Differential effect of DPI 201–106 on the sensitivity of the myofilaments to Ca2+ in intact and skinned trabeculae from control and myopathic human hearts. J Clin Invest (1988) 82:1578–1584.[Web of Science][Medline]
  3. Cazorla O, Szilagyi S, Le Guennec JY, Vassort G, Lacampagne A. Transmural stretch-dependent regulation of contractile properties in rat heart and its alteration after myocardial infarction. FASEB J (2005) 19:88–90.[Abstract/Free Full Text]
  4. Belin RJ, Sumandea MP, Kobayashi T, Walker LA, Rundell VL, Urboniene D, et al. Left ventricular myofilament dysfunction in rat experimental hypertrophy and congestive heart failure. Am J Physiol (2006) 291:H2344–H2353.[Web of Science]
  5. Ruf T, Schulte-Baukloh H, Ludemann J, Posival H, Beyersdorf F, Just H, et al. Alterations of cross-bridge kinetics in human atrial and ventricular myocardium. Cardiovasc Res (1998) 40:580–590.[Abstract/Free Full Text]
  6. Daniels MC, Naya T, Rundell VL, de Tombe PP. Development of contractile dysfunction in rat heart failure: hierarchy of cellular events. Am J Physiol (2007) 293:R284–R292.[Web of Science]
  7. Bogaert J, Rademakers FE. Regional nonuniformity of normal adult human left ventricle. Am J Physiol (2001) 280:H610–H620.[Web of Science]
  8. Derumeaux G, Mulder P, Richard V, Chagraoui A, Nafeh C, Bauer F, et al. Tissue Doppler imaging differentiates physiological from pathological pressure-overload left ventricular hypertrophy in rats. Circulation (2002) 105:1602–1608.[Abstract/Free Full Text]
  9. Casillas JM, Gremeaux V, Damak S, Feki A, Perennou D. Exercise training for patients with cardiovascular disease. Ann Readapt Med Phys (2007) 50:403–418. 386–402.[Medline]
  10. Joshi SB. Exercise training in the management of cardiac failure and ischaemic heart disease. Heart Lung Circ (2007) 16:S83–S87.[Medline]
  11. Palmer BM, Thayer AM, Snyder SM, Moore RL. Shortening and Ca2+ dynamics of left ventricular myocytes isolated from exercise-trained rats. J Appl Physiol (1998) 85:2159–2168.[Abstract/Free Full Text]
  12. Hinken AC, Korte FS, McDonald KS. Porcine cardiac myocyte power output is increased after chronic exercise training. J Appl Physiol (2006) 101:40–46.[Abstract/Free Full Text]
  13. Cohen MV. Coronary and collateral blood flows during exercise and myocardial vascular adaptations to training. Exerc Sport Sci Rev (1983) 11:55–98.[Medline]
  14. de Waard MC, van der Velden J, Bito V, Ozdemir S, Biesmans L, Boontje NM, et al. Early exercise training normalizes myofilament function and attenuates left ventricular pump dysfunction in mice with a large myocardial infarction. Circ Res (2007) 100:1079–1088.[Abstract/Free Full Text]
  15. Gaudron P, Hu K, Schamberger R, Budin M, Walter B, Ertl G. Effect of endurance training early or late after coronary artery occlusion on left ventricular remodeling, hemodynamics, and survival in rats with chronic transmural myocardial infarction. Circulation (1994) 89:402–412.[Abstract/Free Full Text]
  16. Kubo N, Ohmura N, Nakada I, Yasu T, Katsuki T, Fujii M, et al. Exercise at ventilatory threshold aggravates left ventricular remodeling in patients with extensive anterior acute myocardial infarction. Am Heart J (2004) 147:113–120.[CrossRef][Web of Science][Medline]
  17. Wisloff U, Loennechen JP, Currie S, Smith GL, Ellingsen O. Aerobic exercise reduces cardiomyocyte hypertrophy and increases contractility, Ca2+ sensitivity and SERCA-2 in rat after myocardial infarction. Cardiovasc Res (2002) 54:162–174.[Abstract/Free Full Text]
  18. Krenz M, Robbins J. Impact of beta-myosin heavy chain expression on cardiac function during stress. JACC (2004) 44:2390–2397.[Abstract/Free Full Text]
  19. Trueblood NA, Inscore PR, Brenner D, Lugassy D, Apstein CS, Sawyer DB, et al. Biphasic temporal pattern in exercise capacity after myocardial infarction in the rat: relationship to left ventricular remodeling. Am J Physiol (2005) 288:H244–H249.[Web of Science]
  20. Turschner O, D’Hooge J, Dommke C, Claus P, Verbeken E, De Scheerder I, et al. The sequential changes in myocardial thickness and thickening which occur during acute transmural infarction, infarct reperfusion and the resultant expression of reperfusion injury. Eur Heart J (2004) 25:794–803.[Abstract/Free Full Text]
  21. Reboul C, Tanguy S, Juan JM, Dauzat M, Obert P. Cardiac remodeling and functional adaptations consecutive to altitude training in rats: implications for sea level aerobic performance. J Appl Physiol (2005) 98:83–92.[Abstract/Free Full Text]
  22. Borenstein N, Bruneval P, Behr L, Laborde F, Montarras D, Daures JP, et al. An ovine model of chronic heart failure: echocardiographic and tissue Doppler imaging characterization. J Card Surg (2006) 21:50–56.[CrossRef][Web of Science][Medline]
  23. Cazorla O, Lacampagne A, Fauconnier J, Vassort G. SR33805, a Ca2+ antagonist with length-dependent Ca2+ -sensitizing properties in cardiac myocytes. Br J Pharmacol (2003) 139:99–108.[CrossRef][Web of Science][Medline]
  24. Cazorla O, Ait Mou Y, Goret L, Vassort G, Dauzat M, Lacampagne A, et al. Effects of high-altitude exercise training on contractile function of rat skinned cardiomyocyte. Cardiovasc Res (2006) 71:652–660.[Abstract/Free Full Text]
  25. Ashikaga H, Coppola BA, Hopenfeld B, Leifer ES, McVeigh ER, Omens JH. Transmural dispersion of myofiber mechanics: implications for electrical heterogeneity in vivo. JACC (2007) 49:909–916.[Abstract/Free Full Text]
  26. Buchi M, Hess OM, Murakami T, Krayenbuehl HP. Left ventricular wall stress distribution in chronic pressure and volume overload: effect of normal and depressed contractility on regional stress-velocity relations. Basic Res Cardiol (1990) 85:367–383.[CrossRef][Web of Science][Medline]
  27. Litwin SE, Katz SE, Morgan JP, Douglas PS. Serial echocardiographic assessment of left ventricular geometry and function after large myocardial infarction in the rat. Circulation (1994) 89:345–354.[Abstract/Free Full Text]
  28. Konstam MA, Udelson JE, Sharpe N. Prevention and reversal of left ventricular remodeling: Summation. J Card Fail (2002) 8:S506–S511.[CrossRef][Web of Science][Medline]
  29. Sullivan MJ, Higginbotham MB, Cobb FR. Exercise training in patients with severe left ventricular dysfunction. Hemodynamic and metabolic effects. Circulation (1988) 78:506–515.[Abstract/Free Full Text]
  30. Giannuzzi P, Temporelli PL, Corra U, Gattone M, Giordano A, Tavazzi L. Attenuation of unfavorable remodeling by exercise training in postinfarction patients with left ventricular dysfunction: results of the Exercise in Left Ventricular Dysfunction (ELVD) trial. Circulation (1997) 96:1790–1797.[Abstract/Free Full Text]
  31. Carluccio E, Tommasi S, Bentivoglio M, Buccolieri M, Prosciutti L, Corea L. Usefulness of the severity and extent of wall motion abnormalities as prognostic markers of an adverse outcome after a first myocardial infarction treated with thrombolytic therapy. Am J Cardiol (2000) 85:411–415.[CrossRef][Web of Science][Medline]
  32. Moller JE, Egstrup K, Kober L, Poulsen SH, Nyvad O, Torp-Pedersen C. Prognostic importance of systolic and diastolic function after acute myocardial infarction. Am Heart J (2003) 145:147–153.[CrossRef][Web of Science][Medline]
  33. Haykowsky MJ, Liang Y, Pechter D, Jones LW, McAlister FA, Clark AM. A meta-analysis of the effect of exercise training on left ventricular remodeling in heart failure patients: the benefit depends on the type of training performed. JACC (2007) 49:2329–2336.[Abstract/Free Full Text]
  34. Konhilas JP, Watson PA, Maass A, Boucek DM, Horn T, Stauffer BL, et al. Exercise can prevent and reverse the severity of hypertrophic cardiomyopathy. Circ Res (2006) 98:540–548.[Abstract/Free Full Text]
  35. van der Velden J, Papp Z, Zaremba R, Boontje NM, de Jong JW, Owen VJ, et al. Increased Ca2+-sensitivity of the contractile apparatus in end-stage human heart failure results from altered phosphorylation of contractile proteins. Cardiovasc Res (2003) 57:37–47.[Abstract/Free Full Text]
  36. Swynghedauw B. Developmental and functional adaptation of contractile proteins in cardiac and skeletal muscles. Physiol Rev (1986) 66:710–771.[Abstract/Free Full Text]
  37. Morano I. Tuning the human heart molecular motors by myosin light chains. J Mol Med (1999) 77:544–555.[CrossRef][Web of Science][Medline]
  38. Clement O, Puceat M, Walsh MP, Vassort G. Protein kinase C enhances myosin light-chain kinase effects on force development and ATPase activity in rat single skinned cardiac cells. Biochem J (1992) 285:311–317.[Web of Science][Medline]
  39. Olsson MC, Patel JR, Fitzsimons DP, Walker JW, Moss RL. Basal myosin light chain phosphorylation is a determinant of Ca2+ sensitivity of force and activation dependence of the kinetics of myocardial force development. Am J Physiol (2004) 287:H2712–H2718.[Web of Science]
  40. Rajashree R, Blunt BC, Hofmann PA. Modulation of myosin phosphatase targeting subunit and protein phosphatase 1 in the heart. Am J Physiol (2005) 289:H1736–H1743.[Web of Science]
  41. van der Velden J, Papp Z, Boontje NM, Zaremba R, de Jong JW, Janssen PML, et al. The effect of myosin light chain 2 dephosphorylation on Ca2+-sensitivity of force is enhanced in failing human hearts. Cardiovasc Res (2003) 57:505–514.[Abstract/Free Full Text]
  42. Grimm M, Mahnecke N, Soja F, El-Armouche A, Haas P, Treede H, et al. The MLCK-mediated alpha1-adrenergic inotropic effect in atrial myocardium is negatively modulated by PKCepsilon signaling. Br J Pharmacol (2006) 148:991–1000.[CrossRef][Web of Science][Medline]
  43. Gupta RC, Mishra S, Rastogi S, Imai M, Habib O, Sabbah HN. Cardiac SR-coupled PP1 activity and expression are increased and inhibitor 1 protein expression is decreased in failing hearts. Am J Physiol (2003) 285:H2373–H2381.[Web of Science]
  44. Belin RJ, Sumandea MP, Allen EJ, Schoenfelt K, Wang H, Solaro RJ, et al. Augmented protein kinase C-alpha-induced myofilament protein phosphorylation contributes to myofilament dysfunction in experimental congestive heart failure. Circ Res (2007) 101:195–204.[Abstract/Free Full Text]

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?


This article has been cited by other articles:


Home page
 Am. J. Physiol. Heart Circ. Physiol.Home page
Q. Jiao, Y. Bai, T. Akaike, H. Takeshima, Y. Ishikawa, and S. Minamisawa
Sarcalumenin is essential for maintaining cardiac function during endurance exercise training
Am J Physiol Heart Circ Physiol, August 1, 2009; 297(2): H576 - H582.
[Abstract] [Full Text] [PDF]

Home page
 Circ Heart FailHome page
D. J. Duncker, N. M. Boontje, D. Merkus, A. Versteilen, J. Krysiak, G. Mearini, A. El-Armouche, V. J. de Beer, J. M.J. Lamers, L. Carrier, et al.
Prevention of Myofilament Dysfunction by {beta}-Blocker Therapy in Postinfarct Remodeling
Circ Heart Fail, May 1, 2009; 2(3): 233 - 242.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow Supplementary Data
Right arrow All Versions of this Article:
81/3/555    most recent
cvn229v3
cvn229v2
cvn229v1
Right arrow E-letters: Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when E-letters are posted
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Aït Mou, Y.
Right arrow Articles by Cazorla, O.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Aït Mou, Y.
Right arrow Articles by Cazorla, O.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?