Left ventricular (LV) remodeling after myocardial infarction (MI) is one of the most common causes of heart failure (HF), and correlates directly with ventricular dysfunction and increased morbidity and mortality.1 HF is characterized by cardiomyocyte death or hypertrophy, collagen accumulation and molecular changes, such as re-expression of fetal genes including skeletal α-actin and beta-myosin heavy chain (β-MHC) in the heart.2 The use of non-pharmacological therapies is known to improve cardiac function, quality of life and survival rates in patients with post-infarction HF.3,4

One such approach is mesenchymal stem cell (MSC) therapy. This therapy is associated with reduced myocardial infarct size, reduced LV wall thinning, antiapoptotic effects and attenuation of ventricular remodeling by antifibrotic action. Moreover, MSC therapy improves myocardial perfusion and preserves systolic and diastolic performance as well as cardiac electrical viability and impulse propagation.5–8 Aerobic exercise training has also been shown to attenuate HF symptoms and to improve systolic and diastolic function and cellular contraction, as well as presenting a pattern of cardiac gene expression that is distinct from that of pathological cardiac adaptation.9–11

The effects on infarcted hearts of combining MSC therapy and aerobic exercise training, however, are not yet clear.12 Recently, our group showed that early exercise training and MSC therapy proved beneficial to LV morphology and function and to myocyte contractility in infarcted rats, although a beneficial effect of combining these therapies was not seen.13 Nevertheless, adaptations of the LV extracellular matrix and fetal gene re-expression to this combination of therapies have not been elucidated. Thus, the aim of this study was to test whether aerobic exercise training enhances the benefits of stem cell therapy on remodeling of the extracellular matrix and fetal gene expression in the left ventricle of infarcted rats.

The general characteristics and exercise capacity of experimental rats are shown in Table 1. The exercise training applied increased total distance run and reduced resting heart rate in infarcted rats by itself and in combination with MSC, compared to sedentary controls. However, no effect of MCS therapy by itself was found. In addition, both MSC therapy and exercise training improved LV function in infarcted rats, increasing ejection fraction (EF) and fractional shortening (FS), and this effect was maintained when the therapies were combined. Regarding infarct size, MSC therapy and exercise training, both alone and combined, reduced infarct size, compared to the SC group; MSC therapy was more effective.

Body mass, heart mass and volume and morphometric data are presented in Table 2. Body mass, heart mass and the ratio between them, and heart volume were not affected by the therapies either alone or in combination. However, body mass tended to be lower in exercised animals (p=0.07). LV dimensions and posterior wall thickness in diastole did not differ between groups. LV posterior wall thickness in systole was greater in exercised animals than in the other groups.

LV collagen content was positively altered by both therapies, both combined and in isolation (Table 3). Collagen content was reduced in the LV regions assessed in SCMSC (transition [∼40%]; remote [∼25%]) and in EX (transition [∼25%]; remote [∼25%]) compared to SC. The reduction in the transition region was also seen in EXMSC (combined effect). However, in the remote region, collagen content was further reduced in EXMSC (∼24%).

Data on fetal gene expression in the left ventricle are presented in Figure 1. Although no statistically significant effect of MSC therapy on skeletal α-actin expression was found, the exercise training regime used here reduced it considerably (∼50%). Moreover, the two therapies combined reduced it further (∼30%). Concerning α- and β-MHC expression, although MSC therapy had no effect in isolation, aerobic exercise training considerably increased the α/β-MHC ratio (∼200%). This benefit was maintained when these therapies were combined.

Figure 1.

Gene expression in the remote area of the left ventricular infarct scar of infarcted rats. Data are means ± standard error of the mean of six animals in each group. α-MHC: alpha-myosin heavy chain; β-MHC: beta-myosin heavy chain; EX: exercised; EXMSC: exercised treated with stem cells; SC: sedentary control; SCMSC: sedentary treated with stem cells. a: p<0.05 vs. SC; b: p<0.05 vs. SCMSC.

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Male Wistar rats with moderate-size infarcts (∼30%) had impaired LV function (i.e. reduced FS and EF in the SC group), mainly due to the loss of contractile tissue and subsequent reparative fibrosis (leading to increased collagen content), which together result in reduced cardiac compliance.2 In this setting, the left ventricle of infarcted animals also presented enhanced expression of skeletal α-actin and a high α/β-MHC ratio. Despite this, these animals responded positively to both the non-pharmacological therapies used here. Although there is disagreement concerning the efficacy of early and late interventions after MI using such therapeutic approaches,11,17,18 our results provide evidence of the benefits of early intervention.

Concerning the effects of MSC therapy in isolation, it improved LV function in infarcted rats, as shown by increased FE and FS. This adaptation may be due to enhanced impulse propagation and electrical conductivity in response to better preservation of cardiomyocytes and extracellular matrix in the infarcted heart.6 Moreover, MSC therapy also improves excitation-contraction coupling in infarcted hearts by modifying the Ca2+ transient and SERCA2a activity, resulting in better contractile function and preventing arrhythmias.5,13

Our results show that MSC therapy also reduced collagen content in the transition and remote regions of the infarct scar in the left ventricle of infarcted hearts. These findings are in agreement with other studies.7,8,19,20 MSCs are thought to act by releasing growth factors and cytokines that improve myocyte survival and cardiac remodeling. In addition, cell death by apoptosis and necrosis is reduced by MSC therapy, while angiogenesis and extracellular matrix remodeling are improved.21,22 Suggested mechanisms behind the reduction in collagen content include changes in matrix metalloproteinase expression and collagenase activity or other enzymatic pathways,8,22 and attenuation of collagen type I and III expression by inhibition of the proliferation of cardiac fibroblasts, thus exerting antifibrotic effects.11,20

Despite these benefits of MCS therapy on cardiac parameters, it did not result in improvement in the exercise capacity of infarcted rats. This indicates that the effect of MSC therapy on cardiac muscle was not sufficient to impact on exercise performance. It is also possible that our exercise test outcome (i.e. distance run) is more dependent on adaptations in peripheral and skeletal muscle than on those in cardiac muscle. Moreover, MSC therapy did not affect LV dimensions or posterior wall thickness.

Regarding aerobic exercise, rats subjected to exercise training 24 hours after MI induction exhibited improved physical capacity and reduced resting heart rate, and tended to have lower body mass. These parameters are established benefits of moderate-intensity aerobic exercise in infarcted animals.11,23,24 In line with these benefits, exercise training also increased FS and EF in infarcted rats. The main cellular mechanisms by which exercise improves cardiac function are increases in myocyte contractility, expression of Ca2+ regulatory proteins and/or activity, and sensitivity of myofilaments to Ca2+.9,13,25

The exercised rats also exhibited reduced LV collagen content. Aerobic exercise training begun one week after infarction has been shown to attenuate the action of tissue inhibitor of matrix metalloproteinase 1, which increases matrix metalloproteinase activity in infarcted rats.11 Thus, proteolytic activity may have increased, resulting in increased collagen degradation in our exercised animals. This adaptation is in line with the enhanced EF and FS detected in these rats, and reflects the improved cardiac complacency typical of endurance-trained hearts,18 although no effect of exercise on LV dimensions or posterior wall thickness in diastole was observed.

Moreover, our results showed that exercise training reduced the expression of skeletal α-actin and increased the α/β-MHC ratio in the left ventricle of infarcted animals. This result is contrary to that observed in infarcted rat hearts10 that exhibited elevated skeletal α-actin expression and reduced α/β-MHC ratio. Since the β-MHC isoform has fivefold lower ATPase activity than α-MHC, leading to decreases in contraction and relaxation velocities and poor energy utilization,26 the altered proportions of MHC isoforms (i.e. increased α/β MHC ratio) observed in our exercised animals helps to explain the enhanced cardiac function found in these rats. The exercise training used here also positively affected the genetic program of cardiac development after MI induction, as it reduced skeletal α-actin expression, a marker of pathological cardiac hypertrophy. Furthermore, we observed that exercise training increased LV posterior wall thickness in systole. These findings indicate that the exercise regime applied counteracted the deleterious fetal gene re-expression induced by MI and reinforces the idea that aerobic exercise training promotes beneficial gene adaptations that may be responsible, at least in part, for the improved cardiac function and exercise capacity in infarcted rats.

Our main interest here was in the effectiveness of aerobic exercise training in enhancing the effects of early MSC therapy. When early MSC therapy and aerobic exercise were combined (i.e. in the EXMSC group), some additional benefits were seen. For instance, in animals in that group, benefit was seen in the LV extracellular matrix, as the collagen content in the remote region of the infarct scar showed additional reduction (∼24%). Moreover, the expression of fetal genes was also positively affected. While the benefits of exercise training on α- and β-MHC expression were maintained, the expression of skeletal α-actin showed a further decrease (∼30%). In line with this effect, these animals had enhanced exercise capacity (∼30%) and LV posterior wall thickness in systole returned to control levels, though it was not reflected in additional gains in cardiac function or infarct size, as the benefits of the therapy in isolation remained the same or even decreased. Some synergy in the association of exercise training and cell transplantation in infarcted rats has been observed by others12 in a model of late interventions, but using different exercise regimes, stem cells and cell transplantation technique. It therefore appears that aerobic exercise can enhance the action of MSC therapy.

In our model, the MSCs injected into the tail vein were detected only in the lungs of the infarcted animals and not at the site of injury. This outcome has been reported elsewhere.27 Even so, we detected beneficial changes induced by MSC therapy in the infarcted hearts. Studies have shown that MSCs are not mere producers and mobile replacements, but secrete various factors, such as transforming growth factor-beta, prostaglandin E2, interleukin-10, programmed cell death ligand 1 and hepatocyte growth factor, that inhibit the immune response mediated by B and T lymphocytes, as well as natural killer cells, thus helping to reduce the inflammatory response caused by MI.28–30 Their immunomodulatory effect appears to be exerted locally in resident immune cells at the injury site as well as systemically.31 In addition, early transplantation after the ischemic event provides trophic support by stimulating angiogenesis and mitosis in specific tissue progenitor cells, and by inhibiting ischemia-induced apoptosis.32

In conclusion, aerobic exercise training appears to enhance the benefits of stem cell therapy on remodeling of the extracellular matrix and fetal gene re-expression in rats with moderate infarction. Together, these benefits improved the exercise capacity of infarcted rats.

This study was funded by CNPq (grant 473041/2012-0) and FAPEMIG (grant APQ01941-12). JS Freitas held a master's scholarship from CAPES. AJ Natali and EM de Oliveira are CNPq fellows.

This article is part of the doctoral thesis by Juliana Silveira de Freitas at the Universidade Federal de Viçosa.

The authors have no conflicts of interest to declare.