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ARTICLE Exercise training induced myosin heavy chain isoform alteration in the infarcted heart Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by San Francisco (UCSF) on 04/26/15 For personal use only.

Wenhan Wan, Xiaohua Xu, Weiyan Zhao, Michael A. Garza, and John Q. Zhang

Abstract: The myosin heavy chain isoform MHC-␣ has 3-fold higher ATPase activity than MHC-␤. After myocardial infarction (MI), MHC-␣ expression is profoundly downregulated and MHC-␤ expression is reciprocally upregulated. This shift, which is attributed to low thyroid hormone (TH), contributes to myocardial systolic dysfunction. We investigated the effect of post-MI exercise training on MHC isoforms, TH, and cardiac function. MI was surgically induced in 7-week-old rats by ligation of the coronary artery. The survivors were assigned to 3 groups (n = 10/group): Sham (no MI, no exercise), MISed (MI, no exercise), and MIEx (MI, exercise). Treadmill exercise training began 1 week post-MI and lasted for 8 weeks. Echocardiogram measurements were taken on the day prior to initiation of exercise training and at the end of exercise training. Tissue and blood samples were collected at the end of the experiment. MHC isoform gene and protein expression and TH were measured. Our results illustrated that MHC-␣ gene expression was higher and MHC-␤ gene expression was lower in the MIEx group than in the MISed group. Resting serum TH concentrations (T3 and T4) were similar between the 2 MI groups. The MIEx group had higher fractional shortening than the MISed group. In conclusion, post-MI exercise training beneficially altered MHC isoforms and improved cardiac function without changing TH. Key words: myocardial infarction, exercise training, myosin heavy chain. Résumé : L’isoforme ␣ de la chaîne lourde de la myosine (« MHC-␣ ») présente trois fois plus d’activité ATPase que MHC-␤. À la suite d’un infarctus du myocarde (« MI »), on observe une très grande régulation a` la baisse de l’expression de MHC-␣ et, inversement, une régulation a` la hausse de MHC-␤. Ce phénomène contribue a` la dysfonction systolique du myocarde qui est attribuable a` une baisse des hormones thyroïdiennes (« TH »). On examine l’effet de l’entraînement physique a` la suite d’un MI sur les isoformes de MHC, les TH et la fonction cardiaque. On induit un MI chirurgicalement chez des rats âgés de 7 semaines par la ligature de l’artère coronaire. On répartit les survivants dans 3 groupes (n = 10/groupe) : Simulation (sans MI ni exercice), MISed (MI sans exercice) et MIEx (MI avec exercice). L’entraînement sur tapis roulant commence 1 semaine après le MI et dure 8 semaines. On effectue des échocardiogrammes (Echo) le jour précédant le début de l’entraînement physique et a` la fin de cet entraînement. On prélève des échantillons de sang et de tissus a` la fin de l’expérimentation. On évalue les TH et l’expression des gènes et des protéines des isoformes de MHC. Les résultats révèlent une plus grande expression du gène de MHC-␣ et une moins grande expression de MHC- ␤ dans le groupe MIEx comparativement au groupe MISed. On observe des concentrations similaires de TH (T3 et T4) dans les deux groupes présentant un MI. Le groupe MIEx présente une plus grande fraction de raccourcissement que le groupe MISed. En conclusion, l’entraînement physique post-MI modifie avantageusement les isoformes de MHC et améliorent la fonction cardiaque sans modifier les TH. [Traduit par la Rédaction] Mots-clés : infarctus du myocarde, entraînement physique, chaîne lourde de myosine.

Introduction Myosin heavy chain (MHC) acts as a chemical-mechanical transducer in muscle fibers by using energy from ATP to drive the sliding of myofilaments (Nadal-Ginard and Mahdavi 1989). The isoform MHC-␣ elicits 2- to 3-fold higher actin-activated ATPase activity and actin filament sliding velocity than the isoform MHC-␤ (Herron and McDonald 2002; Krenz and Robbins 2004). Thyroid hormone (TH) has profound effects on the cardiovascular system and is known to critically regulate the expression of MHC isoforms in the myocardium (Klein and Ojamaa 2001); in fact, in the absence of TH, the MHC-␣ gene is not transcribed (Nadal-Ginard and Mahdavi 1989). Triiodothyronine (T3), the active cellular form of TH, mediates its actions upon binding to TH receptors (TRs) (Brent 1994; Dillmann 1990). Clinical studies have revealed that circulating and cardiac T3 levels are significantly reduced in pa-

tients with acute uncomplicated myocardial infarction (MI) (Pantos et al. 2005); similarly, decreased serum concentrations of TH have been observed in patients with chronic heart failure (CHF) and may contribute to impaired cardiac functioning (Hamilton et al. 1990). In experimental post-MI rat models, following the decrease of serum T3, significant downregulation of MHC-␣ and concomitant upregulation of MHC-␤ were observed in the non-infarcted left ventricular (LV) myocardium, along with changes in TH receptor isoforms at the mRNA level (Kinugawa et al. 2001a; Pantos et al. 2005, 2007). These changes, in addition to other MI-induced alterations in cardiac phenotype, are thought to further contribute to the progressive nature of LV systolic dysfunction and have been associated with poor prognosis (Nadal-Ginard and Mahdavi 1989; Yue et al. 1998; Krenz and Robbins 2004; Rafalski et al. 2007). Previous studies by our group and others have demonstrated that exercise training positively influences cardiac function and

Received 24 June 2013. Accepted 28 August 2013. W. Wan, W. Zhao, M.A. Garza, and J.Q. Zhang. Laboratory of Cardiovascular Research, University of Texas at San Antonio, 1 UTSA Circle, San Antonio, TX 78249, USA. X. Xu. Department of Veterinary Biosciences, Ohio State University, 1900 Coffey Road, Columbus, OH 43210, USA. Corresponding author: John Q. Zhang (e-mail: [email protected]). Appl. Physiol. Nutr. Metab. 39: 226–232 (2014) dx.doi.org/10.1139/apnm-2013-0268

Published at www.nrcresearchpress.com/apnm on 5 September 2013.

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attenuates myocardial remodeling in rats with MI or congestive heart failure (Jugdutt et al. 1988; Giannuzzi et al. 1997; Alhaddad et al. 1998; Sysa-Shah et al. 2012). In addition, Ojamaa et al. (2000) demonstrated that T3 treatment after acute MI in the rat significantly improved cardiac function and normalized a number of MI-induced changes in gene expression. Likewise, studies with experimental models of LV dysfunction and preliminary clinical investigation of patients with CHF have shown that the TH analog 3,5-diiodothyropropionic acid elicits improvements in both systolic and diastolic LV function, accompanied by an increase in cardiac output and improved lipid profile (Morkin et al. 2002). To date, however, few studies have examined the effects of post-MI exercise training on TH-mediated transcription and its impact on cardiac function. Therefore, the purpose of this study was to investigate exercise-induced changes in MHC isoform gene and protein expression and serum T3 and their effects on cardiac function in post-MI rats. We hypothesized that post-MI exercise training would upregulate MHC-␣ and concomitantly downregulate MHC-␤, but that the changes would not be attributable to resting TH.

Materials and methods Animal preparation Seven-week-old (185–200 g) male Sprague–Dawley rats (Harlan, Indianapolis, Ind.) were treated in accordance with the Guide for the Care and Use of Laboratory Animals (ILAR 2011), and study protocols were approved by the Institutional Animal Care and Use Committee of the University of Texas at San Antonio. To ensure the rats were accustomed to running, they were familiarized with a rodent treadmill at 10 to 16 m/min, 5 min/day for 1 week prior to surgery. MI was surgically induced by ligation of the left anterior descending coronary artery as described previously (Xu et al. 2008a). One week after surgery, the surviving rats were matched with cardiac function (fractional shortening) determined by echocardiographic measurement and randomly assigned to 1 of 3 experimental groups (n = 10/group): a sham-operated control (Sham), a sedentary group with MI (MISed), and an exercised group with MI (MIEx). The MIEx group started exercising 1 week post-MI using a motorized rodent treadmill, while the Sham and MISed groups remained sedentary throughout the entire experiment. To allow gradual adaptation to exercise stress, training was initiated at 10 m/min, 5° incline for 10 min per session. The speed and duration were gradually increased to 16 m/min and 50 min per session (including a 5 min warm-up at 10 m/min) and then maintained throughout the experiment. The exercise intensity was moderate, about 55% V˙O2max (Lawler et al. 1993). The exercise training was performed 5 days per week for 8 weeks. The determination of treadmill speed and exercise duration was based on previous studies (Veras-Silva et al. 1997; Carlson and Winder 1999; Xu et al. 2008b). The exercise regimen was well tolerated by rats with MI. There were no mortalities during the 8 weeks of exercise training. Blood and tissue collection Forty-eight hours after the last exercise session, the rats were anesthetized and blood collection was performed via cardiac puncture. The hearts were quickly harvested and rinsed in cold saline. The myocardial tissue of the non-infarcted left ventricle was collected and immediately frozen in isopentane with dry ice. Tissues and serum were stored at −80 °C until use. Echocardiographic measurements Transthoracic Doppler echocardiographic measurements were performed the day before the initiation of post-MI exercise training and after 8 weeks of exercise training using an echocardiographic system equipped with a 10 MHz transducer (SonoHeart Elite, SonoSite, Bothell, Wash., USA). A 2-dimensional short-axis view of the left ventricle was obtained at the level of the papillary

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muscle to record M-mode tracings. The LV anterior wall thickness, LV end-diastolic dimensions (LVEDd), and LV end-systolic dimensions (LVESd) were measured over 3 consecutive cardiac cycles. LV fractional shortening was calculated as (LVEDd – LVESd)/LVEDd. Infarct size determination Sections of the heart (6 ␮m thick) were cut and stained with Masson’s trichrome. Infarct size was calculated by dividing the sum of the planimetered endocardial and epicardial circumferences of the infarcted area by the sum of the total epicardial and endocardial circumferences of the left ventricle (Xu et al. 2008b). Total epicardial and endocardial lengths occupied by the infarct as identified by Masson’s trichrome staining were measured using Image-Pro Plus (Media Cybernetics, Silver Spring, Md., USA). Total RNA isolation Total RNA was isolated using TRIzol Reagent (Invitrogen, Carlsbad, Calif., USA) according to the manufacturer’s protocol. Briefly, the non-infarcted LV tissue sample was ground, and the resulting powder was resuspended in 1 mL of TRIzol Reagent. The suspension was then homogenized and incubated for 5 min at room temperature. The homogenate was extracted with 0.2 mL of chloroform, and after centrifugation (12 000g, 15 min, 4 °C) the aqueous phase was mixed with 0.5 mL of isopropyl alcohol. The resulting pellet was washed with 1 mL of 75% ethanol and resuspended in 50 ␮L of RNase-free water. Total RNA samples were stored at −80 °C until use. Real-time polymerase chain reaction (PCR) All RNA samples were treated with DNase, and 1 ␮g of total RNA from each sample was reverse-transcribed with oligo(dT) primers and MMLV reverse transcriptase (Promega, Madison, Wisc., USA). Quantification of cardiac gene expression was determined by realtime PCR. The relative expression of MHC-␣ and MHC-␤ mRNA was normalized to the amount of ␤-actin in the same cDNA sample by using the standard curve method. Analysis of each specific mRNA was conducted in triplicate. The primers and probes (Table 1) used in this study were Assay-on-Demand gene expression products (Applied Biosystems, Foster City, Calif., USA). Because of proprietary issues and the policy of Applied Biosystems, the exact primer sequences used for the real-time PCR experiments are not provided but can be requested from the company based on the information in Table 1. Electrophoretic separation analysis for measurement of MHC isoforms in heart Electrophoretic separation analysis was performed by the method described by Warren and Greaser (2003) with minor modification. MHC isoforms were assessed using N-N=-diallyltartardiamide crosslinked acrylamide gels. Homogenates of non-infarcted LV tissue were heated (3 min at 100 °C) and loaded onto polyacrylamide gels. The electrophoresis gel was run in an SE600 Hoefer gel system (Hoefer, San Francisco, Calif., USA) at 16 mA constant current for 4.5 h with constant cooling at 8 °C. After electrophoresis, gels were stained with a Bio-Rad Silver Stain Plus Kit and bands were scanned to evaluate the isoforms of MHC. Serum T3 and T4 quantitative radioimmunoassay The serum concentrations of T3 and T4 were measured by using an RIA kit (DiaSorin, Stillwater, Minn., USA) following the manufacturer’s protocol. The concentrations of T3 and T4 in the serum sample were determined from a standard calibration curve and expressed as ng/mL and ␮g/dL, respectively. Statistical analysis Values are expressed as means ± SE. The group means were compared with one-way analysis of variance (ANOVA), and Student– Newman–Keuls post hoc comparisons were performed when Published by NRC Research Press

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Table 1. Primers and probes used for real-time polymerase chain reaction. Gene

Assay ID*

Reference sequence (GenBank)

␤-actin MHC-␣ MHC-␤

Rn00667869_m1 Rn00568304_m1 Rn00568328_m1

NM_031144.2 NM_017239.1 NM_017240.1

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*Gene Expression Assays product ID of Applied Biosystems (Foster City, Calif., USA).

(AWDT), at week 1 or week 9 post-MI (Table 3). As shown in Table 3, the posterior wall thickness in systole (PWST) and in diastole (PWDT) was similar between MI groups. Nevertheless, exercise training significantly reduced LV end-systolic dimension compared with the sedentary infarcted group (LVESd, 9.5 vs. 10.2 mm, P < 0.05). Furthermore, exercise training significantly preserved fractional shortening compared with the sedentary MI rats (16.2% vs. 11.3%, P < 0.05). These data reveal that exercise training appreciably preserved cardiac function after MI.

Discussion Table 2. Experimental groups and their physical characteristics. Group

Sham (n = 10)

MISed (n = 10)

MIEx (n = 10)

Infarct size (%) BW (g) HW (g) HW (g)/BW (kg) × 1000

O 383.4±7.49 1.28±0.09* 3.35±0.06*

41.2±2.94 389.8±8.29 1.54±0.24 3.94±0.15

40.1±1.37 393.6±4.88 1.52±0.18 3.85±0.13

Note: Values are expressed as means ± SE. BW, body weight at sacrifice; HW, heart weight; *, P < 0.05 compared with MISed and MIEx. MI, myocardial infarction; Ex, exercise; Sed, sedentary.

significant F ratios were obtained. A P value less than 0.05 was considered statistically significant.

Results General characteristics and post-infarction survival Table 2 summarizes the general characteristics of the experimental groups. The exercise training regimen was well tolerated by rats of the MIEx group. MI was associated with ⬃50% mortality during the first 48 h following ligation. No deaths occurred during the 8 week experimental period. Infarct sizes were comparable between MISed (41.2% ± 2.94%, n = 10) and MIEx (40.1% ± 1.37, n = 10). There was no significant difference in body weight among the experimental groups (P < 0.05). The ratio of heart weight to body weight was significantly higher in the MISed and MIEx groups than in the Sham group (P < 0.05). Serum T3 and T4 Figure 1 illustrates resting serum TH levels among experimental groups. Our results indicate that resting serum triiodothyronine (T3) and thyroxine (T4) concentrations were similar between the MIEx and MISed groups (P > 0.05). Both MI groups had lower levels of T3 and T4 than the Sham group (P < 0.01). These results indicate that 8 weeks of post-MI exercise training did not significantly alter circulating levels of resting T3 and T4. MHC gene and protein expression in the non-infarcted left ventricular myocardium To determine whether training altered MHC mRNA levels, realtime PCR was performed to quantify cardiac gene expression among groups. MHC-␣ mRNA expression (Fig. 2) in the noninfarcted LV myocardium was 1.38-fold higher in the MIEx group than in the MISed group (P < 0.02). MHC-␤ mRNA levels (Fig. 3) were 1.84-fold lower in the MIEx group than in the MISed group (P < 0.02). Thus, the results show that exercise training increased MHC-␣ gene expression and decreased MHC-␤ gene expression. Consistent with mRNA levels, the electrophoresis data (Fig. 4) showed that the MHC-␣ to MHC-␤ protein ratio was higher in the MIEx group than in the MISed group (2.53 ± 0.28 vs. 1.74 ± 0.09, P < 0.02). However, when compared with the Sham group, both MI groups had lower MHC-␣ to MHC-␤ ratios (P < 0.05), indicating that exercise training reverses cardiac MHC ␣ to ␤ isoform shifts after MI at both gene and protein levels. Echocardiographic data Figure 5 shows the typical M-mode images of all groups. There was no significant difference in the anterior wall thickness between the MI groups, either in systole (AWST) or in diastole

In the present study, we have demonstrated that exercise training after MI improves post-MI cardiac function and LV remodeling by altering the expression of genes and proteins that regulate cardiomyocyte ATPase activity and actin filament sliding velocity. These data provide further insights into the mechanisms underlying the reduction in morbidity and mortality produced by exercise training in patients with MI. There are 3 major findings in the current study. First, exercise training significantly increased cardiac expression of MHC-␣ and attenuated expression of MHC-␤ at both gene and protein levels 9 weeks after MI. Second, resting serum TH levels did not contribute to exercise-induced shifts in MHC isoforms. Third, cardiac function and exercise capacity improved independently of resting serum TH levels. Various models of heart failure have consistently demonstrated that MHC-␣ expression is downregulated, whereas MHC-␤ expression is upregulated, resulting in ratios of protein isoforms that resemble those normally found in the fetal heart (Simpson et al. 1989). This MHC isoform shift is thought to be a regulatory response necessary to preserve myocardial work efficiency under pathological conditions, but it also impairs cardiac contractile system ATPase activity, thus contributing to both the development and progression of myocardial systolic dysfunction and cardiomyopathy (Hashimoto et al. 2004; Krenz and Robbins 2004). Our results demonstrated that MHC-␣ gene expression was 1.38fold higher (Fig. 2) and MHC-␤ gene expression was 1.84-fold lower (Fig. 3) in the MIEx group compared with the MISed group; furthermore, protein electrophoresis data illustrated that the MHC-␣ to MHC-␤ isoform ratio was significantly higher in the MIEx group than in the MISed group (2.53 ± 0.28 vs. 1.74 ± 0.09, Fig. 4). Similarly, a previous study reported that mild-intensity post-MI exercise training induced a significant reduction of MHC-␤ at gene and protein levels in both the anterior and posterior LV walls when compared with a sedentary MI group (Hashimoto et al. 2004). Likewise, it has also been reported that exercise training significantly increases MHC-␣ expression while repressing the ␤ isoform at gene and protein levels in both Sham and MI rats (Hashimoto et al. 2004; Fernandes et al. 2011). This shift in MHC isoforms is a compensatory response to the imbalance between increased glycolytic demand and functional capacity during exercise, and is likely responsible for increased myocardial contractility and the preservation of cardiac function after MI (Rafalski et al. 2007). T3, the active cellular metabolite of TH, has profound effects on cardiac contractile function as well as cardiovascular hemodynamics, including blood pressure and systemic vascular resistance, and regulates expression of specific cardiomyocyte genes (Brent 1994). These T3-reponsive cardiac genes are important determinants of cardiac contractility and include myosin heavy chains, phospholamban, and sarcoplasmic reticulum calcium-activated ATPase (Brent 1994; Klein and Ojamaa 2001). Studies conducted both in vitro and in vivo have shown that T3 upregulates the expression of the MHC-␣ isoform in cardiomyocytes while downregulating the ␤ isoform (Nadal-Ginard and Mahdavi 1989; Beck-Peccoz and Chatterjee 1994; Ojamaa et al. 2000; Rafalski et al. 2007). In fact, in the absence of T3, the MHC-␣ gene is not transcribed (Nadal-Ginard and Mahdavi 1989). After MI, circulating and cardiac levels of T3 are significantly decreased despite the presence of normal serum T4, a Published by NRC Research Press

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Fig. 1. Resting serum levels of T3 and T4 (ng/mL) in Sham rats and rats with myocardial infarction (MI). Blood samples were taken 9 weeks after MI. Values are means ± SE. T3 and T4 levels were similar between MI groups but were significantly lower than Sham levels.

Fig. 2. Exercise-induced MHC-␣ isoform gene expression in postinfarcted rat heart. Data are expressed as ratios of target genes to ␤-actin relative to Sham. MI, myocardial infarction; Ex, exercise; Sed, sedentary. Values are means ± SE.

Fig. 4. Electrophoresis results and ratio of MHC-␣ to MHC-␤. Values are means ± SE. The ratio of MHC-␣ to MHC-␤ in MIEx is higher than in MISed (P = 0.03). The ratio in Sham is higher than that in both MIEx and MISed (P < 0.02). MI, myocardial infarction; Ex, exercise; Sed, sedentary.

Fig. 3. Exercise-induced MHC-␤ isoform gene expression in postinfarcted rat heart. Data are expressed as ratios of target genes to ␤-actin relative to Sham. Values are means ± SE.

common characteristic of low T3 syndrome (Franklyn et al. 1984; Chopra 1997; De Groot 1999). The finding that T3 treatment after acute MI significantly improves cardiac function and normalizes some, but not all, of the changes in gene expression is additional evidence for a hypothyroid state (Izumo et al. 1987). It has also been demonstrated that physiological hypertrophy caused by exercise is characterized by hyperthyroid-like changes in TH-responsive genes (Rupp and Wahl 1990). In line with previous data, our results indicated that resting T3 levels (ng/mL) were similar between the MISed and MIEx groups (0.58 ± 0.36 vs. 0.45 ± 0.03, Fig. 1) but higher in the Sham group (0.69 ± 0.03, Fig. 1); likewise, T4 levels were similar between MI groups yet markedly higher in the Sham group (Fig. 1). Similarly, previous studies have reported low levels of circulating TH in Published by NRC Research Press

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Fig. 5. M-mode images of echocardiograms in the left ventricle. These are typical M-mode images for each group. LVEDd, left ventricular enddiastolic dimension; LVESd, left ventricular end-systolic dimension; MI, myocardial infarction; Ex, exercise; Sed, sedentary.

Table 3. Doppler echocardiographic assessment of left ventricular geometry and function. 1 wk post-MI

Heart rate (bpm) AWDT (mm) AWST (mm) PWDT (mm) PWST (mm) LVEDd (mm) LVESd (mm) FS (%)

9 wk post-MI

Sham

MISed

MIEx

Sham

MISed

MIEx

350±6 1.52±0.09* 2.66±0.1* 2.01±0.10 2.68±0.11 6.65±0.15* 3.75±0.17* 43.78±1.47*

339±10 0.50±0.02 0.67±0.05 1.78±0.09 2.53±0.14 9.03±0.22 7.56±0.29 16.45±1.73

334±8 0.51±0.02 0.71±0.01 1.88±0.07 2.54±0.15 8.99±0.15 7.41±0.20 17.59±1.42

321±7 1.61±0.1* 2.95±0.13* 2.08±0.1 3.05±0.14 7.45±0.19* 4.29±0.18* 42.75±1.41*

328±10 0.61±0.05 0.71±0.07 2.37±0.15 2.80±0.13 11.49±0.23 10.18±0.27 11.27±1.12

310±9 0.64±0.04 0.81±0.05 2.16±0.11 2.75±0.09 11.37±0.17 9.50±0.20† 16.2±1.28†

Note: MI, myocardial infarction; Ex, exercise; Sed, sedentary; AWDT, anterior wall diastolic thickness; AWST, anterior wall systolic thickness; PWDT, posterior wall diastolic thickness; PWST, posterior wall systolic thickness; LVEDd, left ventricular end-diastolic dimension; LVESd, left ventricular end-systolic dimension; FS, left ventricular fractional shortening; *, P < 0.001 compared with MI groups; †, P < 0.05 compared with MISed group.

patients after acute MI, as well as in end-stage human heart failure (Franklyn et al. 1984; d’Amati et al. 2001); however, there are no previous data on the effect of chronic post-MI exercise training on resting TH levels. The mechanism of decreasing TH levels after MI is not fully understood, although a plausible candidate is the regulation of cardiac thyroid hormone receptors (TRs) and their influence on T3 sensitivity in the myocardium, which could explain unchanged resting serum TH levels. Studies in the aging rat and in human end-stage heart failure reveal that alterations in myocardial TR levels correlate with changes in TH target gene transcription (Long et al. 1999; Kinugawa et al. 2001a). In a subsequent study, Kinugawa et al. (2001b) established a direct relationship between TR levels and transcription of TH-responsive genes in both pathological and physiological forms of cardiac hypertrophy in rats. In a pathological model of pressure overload, all TRs were downregulated, consistent with the expression of a hypothyroid-like

molecular phenotype; conversely, physiological hypertrophy induced by 10 weeks of wheel running resulted in the upregulation of TR␤1, where TH-responsive transcription was enhanced (Kinugawa et al. 2001). Interestingly, studies with experimental models of LV dysfunction and preliminary clinical investigation of patients with CHF have shown that treatment with the TH analog 3,5-diiodothyropropionic acid elicits improvements in both systolic and diastolic LV function, accompanied by an increase in cardiac output and improved lipid profile (Morkin et al. 2002). Therefore, it is conceivable that the MHC isoform switches observed in our study were mediated by increased TR expression rather than circulating TH. It remains unclear whether exercise combined with TH treatment would potentiate results. Although exercise training improves cardiac performance and may be beneficial in the management of some forms of cardiovascular disease, the use of post-MI exercise in the treatment regimen has attracted much attention, as results from both human and Published by NRC Research Press

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animal studies are conflicting. A human study by Judgutt et al. (1988) showed significant deterioration in both global and regional LV function after 12 weeks of post-MI training starting 15 weeks post-MI. The authors indicated that exercise might be injurious in patients with an extensive transmural infarct, blaming exercise-induced increases in wall stress for the LV dilation (Jugdutt et al. 1988). Aggravated LV enlargement was also reported after exercise training was initiated 3 weeks post-MI, and it was speculated that adverse remodeling stemmed from early exercise training (Kubo et al. 2004). In contrast, previous studies by our group and others have consistently demonstrated that post-MI exercise provides a variety of therapeutic benefits, such as improved exercise capacity without LV dilation, and preserved cardiac functioning compared with control groups (Hochman and Healy 1986; Shephard and Balady 1999; Brown et al. 2003; Wan et al. 2007; Xu et al. 2008a, 2008b). In the current study, we observed LV dilation in MI rats at both 1 week and 9 weeks post-MI (Fig. 5). MIEx rats and their sedentary counterparts possessed similar degrees of LV dilation (Table 3). Compared with the Sham group, LVEDd and LVESd were significantly higher in the MI groups at 9 weeks (Table 3, P < 0.05). These results are in agreement with those of Hochman and Healy (1986) suggesting that exercise training is not responsible for the LV dilation. Furthermore, Doppler echocardiographic assessment of LV geometry and function revealed that fractional shortening values were significantly preserved in the MIEx group compared with the MISed group (16.2% vs. 11.3%, respectively). These data suggest that post-MI exercise training prevents deterioration of cardiac function. In conclusion, our present study demonstrated that post-MI exercise training upregulated MHC-␣ and downregulated MHC-␤. Exercise training beneficially altered the cardiac MHC-␣ to MHC-␤ ratio at both gene and protein levels. These changes, in turn, contributed to improvement in cardiac function. However, the exercise training did not normalize MI-induced resting T3 and T4 levels. Thus, we postulate that the exercise training-induced changes in MHC isoforms may be caused by changes in TRs other than TH.

Acknowledgement This study was supported in part by a grant from the National Heart, Lung, and Blood Institute (RO1-HL074273).

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Exercise training induced myosin heavy chain isoform alteration in the infarcted heart.

The myosin heavy chain isoform MHC-α has 3-fold higher ATPase activity than MHC-β. After myocardial infarction (MI), MHC-α expression is profoundly do...
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