Short- and long-term swimming exercise training increases myocardial insulin-like growth factor-I gene expression

Exercise-induced cardiac mitochondrial reorganization and enhancement in spontaneously hypertensive rats

The myocardium is a highly oxidative tissue in which mitochondria are essential to supply the energy required to maintain pump function. When pathological hypertrophy develops, energy consumption augments and jeopardizes mitochondrial capacity. We explored the cardiac consequences of chronic swimming training, focusing on the mitochondrial network, in spontaneously hypertensive rats (SHR). Male adult SHR were randomized to sedentary or trained (T: 8-week swimming protocol). Blood pressure and echocardiograms were recorded, and hearts were removed at the end of the training period to perform molecular, imaging, or isolated mitochondria studies. Swimming improved cardiac midventricular shortening and decreased the pathological hypertrophic marker atrial natriuretic peptide. Oxidative stress was reduced, and even more interesting, mitochondrial spatial distribution, dynamics, function, and ATP were significantly improved in the myocardium of T rats. In the signaling pathway triggered by training, we detected an increase in the phosphorylation level of both AKT and glycogen synthase kinase-3 β, key downstream targets of insulin-like growth factor 1 signaling that are crucially involved in mitochondria biogenesis and integrity. Aerobic exercise training emerges as an effective approach to improve pathological cardiac hypertrophy and bioenergetics in hypertension-induced cardiac hypertrophy.

Hormonal Response to Incremental and Continuous Exercise in Cyclists with Left Ventricle Hypertrophy

The aim of this study was to assess the effects of incremental and continuous exercise on the concentration of insulin-like growth factor-1 (IGF-1), growth hormone (GH), testosterone (T), and cortisol (C), as well as to investigate whether increased cardiac dimensions in cyclists were related to changes in these hormones and cardiac biomarkers. The study included 30 elite cyclists divided into two groups, i.e., athletes with left ventricle hypertrophy (a LVH group), and a control group (CG) without LVH. The study protocol included performance of a standard incremental exercise (IncEx) test to measure athletes’ maximum power (Pmax), maximum oxygen uptake (VO2max), and lactate threshold (LAT). The IncEx test results were then used to determine the intensity of the continuous exercise (ConEx) test which was performed after the 1-week washout period. Cyclists with LVH and without LVH did not differ in resting hormone concentrations and cardiac biomarkers levels. There was a significant effect of exercise on serum IGF-1 levels (p < 0.05) in the LVH group and a combined effect of the type of exercise and LVH on IGF-1 (p < 0.05). Cyclists with LVH demonstrated higher post exercise T levels recorded in response to exercise compared to the CG (p < 0.01). Significantly higher serum T levels were observed in response to ConEx compared to IncEx in the LVH group and the CG (p < 0.05 and p < 0.05, respectively). In the LVH group, a significant positive correlation between the post-exercise T/C ratio and left ventricular mass index was observed (r = 0.98, p < 0.01). There were no effects of heart hypertrophy on cardiac standard biomarkers. Incremental and continuous exercise caused a marked increase in steroid hormone concentrations and moderate strengthening of insulin growth factors effects. Regular incremental exercise seems to induce beneficial cardiac adaptations via significant increases in the concentration of anabolic factors compared to the same training mode yet with constant exercise intensity.

Na+/H+ exchanger and cardiac hypertrophy

Reactive cardiac hypertrophy (CH) is an increase in heart mass in response to hemodynamic overload. Exercise-induced CH emerges as an adaptive response with improved cardiac function, in contrast to pathological CH that represents a risk factor for cardiovascular health. The Na⁺/H⁺ exchanger (NHE-1) is a membrane transporter that not only regulates intracellular pH but also intracellular Na⁺ concentration. In the scenario of cardiovascular diseases, myocardial NHE-1 is activated by a variety of stimuli, such as neurohumoral factors and mechanical stress, leading to intracellular Na⁺ overload and activation of prohypertrophic cascades. NHE-1 hyperactivity is intimately linked to heart diseases, including ischemia-reperfusion injury, maladaptive CH and heart failure. In this review, we will present evidence to support that the NHE-1 hyperactivity constitutes a "switch on/off" for the pathological phenotype during CH development. We will also discuss some classical and novel strategies to avoid NHE-1 hyperactivity, and that are therefore worthwhile to improve cardiovascular health.

Cardiac adaptation to exercise training in health and disease

The heart is the primary pump that circulates blood through the entire cardiovascular system, serving many important functions in the body. Exercise training provides favorable anatomical and physiological changes that reduce the risk of heart disease and failure. Compared with pathological cardiac hypertrophy, exercise-induced physiological cardiac hypertrophy leads to an improvement in heart function. Exercise-induced cardiac remodeling is associated with gene regulatory mechanisms and cellular signaling pathways underlying cellular, molecular, and metabolic adaptations. Exercise training also promotes mitochondrial biogenesis and oxidative capacity leading to a decrease in cardiovascular disease. In this review, we summarized the exercise-induced adaptation in cardiac structure and function to understand cellular and molecular signaling pathways and mechanisms in preclinical and clinical trials.

The heart of an acrobatic bird

The courtship behavior of some species of birds can be energetically demanding, but it is unknown if cardiovascular specializations enable such behaviors. While performing a highly acrobatic courtship dance, heart rate in male golden-collared manakins increases briefly to 1300 beats per minute, among the highest heart rates recorded in any bird or mammal. We hypothesize that male manakins have enhanced cardiovascular capabilities to meet these demands on the heart. Using histological and molecular techniques, we examined manakin heart structure as well as expression of genes involved in Ca²⁺ handling, action potential duration, steroidal signaling and cardiac growth. These measures were also made on the hearts of zebra finches, a similar-sized bird with limited cardiovascular demands. Compared to the zebra finch, the manakin had a significantly thicker left ventricular (LV) muscle (cross-sectional thickness of the free LV wall and septum) with a smaller LV chamber. In addition, compared to zebra finches, manakin hearts had significantly greater gene expression of ryanodine receptors as well as androgen receptors. Testosterone (T) treatment of non-breeding manakins (with low T) increased gene expression of the Ca²⁺ pump SERCA. These observations suggest that hearts of breeding male manakins require specialized Ca²⁺ handling and androgens may facilitate manakin cardiovascular function.

Cardioprotective role of IGF-1 in the hypertrophied myocardium of the spontaneously hypertensive rats: A key effect on NHE-1 activity

AIM

Myocardial Na⁺/H⁺ exchanger-1 (NHE-1) hyperactivity and oxidative stress are interrelated phenomena playing pivotal roles in the development of pathological cardiac hypertrophy and heart failure. Exercise training is effective to convert pathological into physiological hypertrophy in the spontaneously hypertensive rats (SHR), and IGF-1-key humoral mediator of exercise training-inhibits myocardial NHE-1, at least in normotensive rats. Therefore, we hypothesize that IGF-1 by hampering NHE-1 hyperactivity and oxidative stress should exert a cardioprotective effect in the SHR.

METHODS

NHE-1 activity [proton efflux ( J H + ) mmol L^-1 min^-1], expression and phosphorylation; H₂O₂ production; superoxide dismutase (SOD) activity; contractility and calcium transients were measured in SHR hearts in the presence/absence of IGF-1.

RESULTS

IGF-1 significantly decreased NHE-1 activity ( J H + at pH_i 6.95: 1.39 ± 0.32, n = 9 vs C 3.27 ± 0.3, n = 20, P < .05); effect prevented by AG1024, an antagonist of IGF-1 receptor (2.7 ± 0.4, n = 7); by the PI3K inhibitor wortmannin (3.14 ± 0.41, n = 7); and the AKT inhibitor MK2206 (3.37 ± 0.43, n = 14). Moreover, IGF-1 exerted an antioxidant effect revealed by a significant reduction in H₂O₂ production accompanied by an increase in SOD activity. In addition, IGF-1 improved cardiomyocyte contractility as evidenced by an increase in sarcomere shortening and a decrease in the relaxation constant, underlined by an increase in the amplitude and rate of decay of the calcium transients.

CONCLUSION

IGF-1 exerts a cardioprotective role on the hypertrophied hearts of the SHR, in which the inhibition of NHE-1 hyperactivity, as well as the positive inotropic and antioxidant effects, emerges as key players.

Cardiovascular Adaptations Induced by Resistance Training in Animal Models

In the last 10 years the number of studies showing the benefits of resistance training (RT) to the cardiovascular system, have grown. In comparison to aerobic training, RT-induced favorable adaptations to the cardiovascular system have been ignored for many years, thus the mechanisms of the RT-induced cardiovascular adaptations are still uncovered. The lack of animal models with comparable protocols to the RT performed by humans hampers the knowledge. We have used squat-exercise model, which is widely used by many others laboratories. However, to a lesser extent, other models are also employed to investigate the cardiovascular adaptations. In the subsequent sections we will review the information regarding cardiac morphological adaptations, signaling pathway of the cardiac cell, cardiac function and the vascular adaptation induced by RT using this animal model developed by Tamaki et al. in 1992. Furthermore, we also describe cardiovascular findings observed using other animal models of RT.

Cardiac AT(1) receptor-dependent and IGF1 receptor-independent signaling is activated by a single bout of resistance exercise

AT(1) receptor (AT1R) blockade prevents physiological cardiac hypertrophy induced by resistance training. Also, our group showed that a single bout of resistance exercise (RE) activates the AKT/mTOR which was also inhibited by AT1R blocker. Here, we investigated whether IGF1-receptor (IGF1-R) and MAPKs were also activated after a single bout of RE. Wistar rats were divided into Sedentary (Sed), Sedentary treated with losartan (Sed+LOS), Exercise (EX), and Exercise treated with losartan (EX+LOS). Cardiac tissue was obtained 5 and 30 min after 4 sets of 12 repetitions of squat exercise (80 % 1RM). We demonstrated that a single bout of RE did not induce IGF1-R tyrosine phosphorylation. ERK1/2 and P38 phosphorylation levels were elevated in the EX 5min and EX 30min groups however, only ERK1/2 was inhibited by losartan treatment (AT1R blocker). Next, we showed that beta-arrestin-2 expression increased 28 % in trained animals compared to sedentary group. Altogether, our results demonstrate that AT1R, but not IGF1-R, may exert the hypertrophic cardiac stimulus RE-induced. Also, activation of AKT/mTOR and ERK1/2 pathways may occur through the beta-arrestin-dependent pathway.

Molecular Mechanisms Underlying Cardiac Adaptation to Exercise

Exercise elicits coordinated multi-organ responses including skeletal muscle, vasculature, heart, and lung. In the short term, the output of the heart increases to meet the demand of strenuous exercise. Long-term exercise instigates remodeling of the heart including growth and adaptive molecular and cellular re-programming. Signaling pathways such as the insulin-like growth factor 1/PI3K/Akt pathway mediate many of these responses. Exercise-induced, or physiologic, cardiac growth contrasts with growth elicited by pathological stimuli such as hypertension. Comparing the molecular and cellular underpinnings of physiologic and pathologic cardiac growth has unveiled phenotype-specific signaling pathways and transcriptional regulatory programs. Studies suggest that exercise pathways likely antagonize pathological pathways, and exercise training is often recommended for patients with chronic stable heart failure or following myocardial infarction. Herein, we summarize the current understanding of the structural and functional cardiac responses to exercise as well as signaling pathways and downstream effector molecules responsible for these adaptations.

Physiological cardiac hypertrophy: critical role of AKT in the prevention of NHE-1 hyperactivity

BACKGROUND

The involvement of NHE-1 hyperactivity, critical for pathological cardiac hypertrophy (CH), in physiological CH has not been elucidated yet. Stimulation of NHE-1 increases intracellular Na(+) and Ca(2+) favouring calcineurin activation. Since myocardial stretch, an activator of NHE-1, is common to both types of CH, we speculate that NHE-1 hyperactivity may also happen in physiological CH. However, calcineurin activation is characteristic only for pathological hypertrophy. We hypothesize that an inhibitory AKT-dependent mechanism prevents NHE-1 hyperactivity in the setup of physiological CH.

METHODS

Physiological CH was induced in rats by swimming (90 min/day, 12 weeks) or in cultured isolated cardiomyocytes with IGF-1 (10 nmol/L).

RESULTS

Training induced eccentric CH development (left ventricular weight/tibial length: 22.0±0.3 vs. 24.3±0.7 mg/mm; myocyte cross sectional area: 100±3.2 vs. 117±4.1 %; sedentary (Sed) and swim-trained (Swim) respectively; p<0.05] with decreased myocardial stiffness and collagen deposition [1.7±0.05 % (Sed) vs. 1.4±0.09 % (Swim); p<0.05]. Increased phosphorylation of AKT, ERK1/2, p90(RSK) and NHE-1 at the consensus site for ERK1/2-p90(RSK) were detected in the hypertrophied hearts (P-AKT: 134±10 vs. 100±5; P-ERK1/2: 164±17 vs. 100±18; P-p90(RSK): 160±18 vs. 100±9; P-NHE-1 134±10 vs. 100±10; % in Swim vs. Sed respectively; p<0.05). No significant changes were detected neither in calcineurin activation [calcineurin Aβ 100±10 (Sed) vs. 96±12 (Swim)], nor NFAT nuclear translocation [100±3.11 (Sed) vs. 95±9.81 % (Swim)] nor NHE-1 expression [100±8.5 (Sed) vs. 95±6.7 % (Swim)]. Interestingly, the inhibitory phosphorylation of the NHE-1 consensus site for AKT was increased in the hypertrophied myocardium (151.6±19.4 (Swim) vs. 100±9.5 % (Sed); p<0.05). In isolated cardiomyocytes 24 hours IGF-1 increased cell area (114±1.3 %; p<0.05) and protein/DNA content (115±3.9 %, p<0.05), effects not abolished by NHE-1 inhibition with cariporide (114±3 and 117±4.4 %, respectively). IGF-1 significantly decreased NHE-1 activity during pHi recovery from sustained intracellular acidosis (JH+ at pHi 6.8: 4.08±0.74 and 9.09±1.21 mmol/L/min, IGF-1 vs. control; p<0.05), and abolished myocardial slow force response, the mechanical counterpart of stretch-induced NHE-1 activation.

CONCLUSIONS

NHE-1 hyperactivity seems not to be involved in physiological CH development, contrary to what characterizes pathological CH. We propose that AKT, through an inhibitory phosphorylation of the NHE-1, prevents its stretch-induced activation. This posttranslational modification emerges as an adaptive mechanism that avoids NHE-1 hyperactivity preserving its housekeeping functioning.

Insulin receptor substrates are essential for the bioenergetic and hypertrophic response of the heart to exercise training

Insulin and insulin-like growth factor 1 (IGF-1) receptor signaling pathways differentially modulate cardiac growth under resting conditions and following exercise training. These effects are mediated by insulin receptor substrate 1 (IRS1) and IRS2, which also differentially regulate resting cardiac mass. To determine the role of IRS isoforms in mediating the hypertrophic and metabolic adaptations of the heart to exercise training, we subjected mice with cardiomyocyte-specific deletion of either IRS1 (CIRS1 knockout [CIRS1KO] mice) or IRS2 (CIRS2KO mice) to swim training. CIRS1KO hearts were reduced in size under basal conditions, whereas CIRS2KO hearts exhibited hypertrophy. Following exercise swim training in CIRS1KO and CIRS2KO hearts, the hypertrophic response was equivalently attenuated, phosphoinositol 3-kinase (PI3K) activation was blunted, and prohypertrophic signaling intermediates, such as Akt and glycogen synthase kinase 3β (GSK3β), were dephosphorylated potentially on the basis of reduced Janus kinase-mediated inhibition of protein phosphatase 2a (PP2A). Exercise training increased peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) protein content, mitochondrial capacity, fatty acid oxidation, and glycogen synthesis in wild-type (WT) controls but not in IRS1- and IRS2-deficient hearts. PGC-1α protein content remained unchanged in CIRS1KO but decreased in CIRS2KO hearts. These results indicate that although IRS isoforms play divergent roles in the developmental regulation of cardiac size, these isoforms exhibit nonredundant roles in mediating the hypertrophic and metabolic response of the heart to exercise.

IGF-I receptor 275124A>C (rs1464430) polymorphism and athletic performance

OBJECTIVES

To examine the prevalence of the Insulin-Like Growth Factor-I receptor (IGF-IR) 275124A>C polymorphism, known to be associated with exercise-related cardiac hypertrophy, among elite endurance and power athletes.

DESIGN

One hundred and fifty-nine athletes (118 men and 41 women, age: 35.9±12.2 yrs) participated in the study.

METHODS

We hypothesized that presence of the A allele will be significantly more common among endurance athletes (n=77) compared to power athletes (n=82) and non-physically active controls (n=68). Athletes within each group were further divided according to their individual best performance into elite athletes (those who had represented the country in international track-and-field or triathlon competitions or in the Olympic Games) and national-level athletes.

RESULTS

The prevalence of the AA genotype was significantly higher (p<0.05) in the endurance athletes group (49%) compared to the power athletes group (33%), but did not differ from the control group (46%). There was no significant difference in the prevalence of the AA genotype between elite and national level endurance athletes (44% versus 52%, respectively). In contrast, among power athletes, the prevalence of the AA genotype was significantly lower in the elite compared to national level athletes (17% versus 42%, respectively; p<0.05).

CONCLUSIONS

The results of the present study may suggest that the IGF-IR AA polymorphism is beneficial for endurance-type sports, but is not associated with elite endurance performance. In contrast, the presence of the AA genotype may be a disadvantage in power sports. All together the results of the present study suggest that IGF-IR polymorphism may differentiate between the two edges of the endurance-power athletic performance spectrum.

Pregnancy as a cardiac stress model

Cardiac hypertrophy occurs during pregnancy as a consequence of both volume overload and hormonal changes. Both pregnancy- and exercise-induced cardiac hypertrophy are generally thought to be similar and physiological. Despite the fact that there are shared transcriptional responses in both forms of cardiac adaptation, pregnancy results in a distinct signature of gene expression in the heart. In some cases, however, pregnancy can induce adverse cardiac events in previously healthy women without any known cardiovascular disease. Peripartum cardiomyopathy is the leading cause of non-obstetric mortality during pregnancy. To understand how pregnancy can cause heart disease, it is first important to understand cardiac adaptation during normal pregnancy. This review provides an overview of the cardiac consequences of pregnancy, including haemodynamic, functional, structural, and morphological adaptations, as well as molecular phenotypes. In addition, this review describes the signalling pathways responsible for pregnancy-induced cardiac hypertrophy and angiogenesis. We also compare and contrast cardiac adaptation in response to disease, exercise, and pregnancy. The comparisons of these settings of cardiac hypertrophy provide insight into pregnancy-associated cardiac adaptation.

High molecular mass proteomics analyses of left ventricle from rats subjected to differential swimming training

BACKGROUND

Regular exercises are commonly described as an important factor in health improvement, being directly related to contractile force development in cardiac cells.In order to evaluate the links between swimming exercise intensity and cardiac adaptation by using high molecular mass proteomics, isogenic Wistar rats were divided into four groups: one control (CG) and three training groups (TG's), with low, moderate and high intensity of exercises.In order to evaluate the links between swimming exercise intensity and cardiac adaptation by using high molecular mass proteomics, isogenic Wistar rats were divided into four groups: one control (CG) and three training groups (TG's), with low, moderate and high intensity of exercises.

RESULTS

Findings here reported demonstrated clear morphologic alterations, significant cellular injury and increased energy supplies at high exercise intensities. α-MyHC, as well proteins associated with mitochondrial oxidative metabolism were shown to be improved. α-MyHC expression increase 1.2 fold in high intensity training group when compared with control group. α-MyHC was also evaluated by real-time PCR showing a clear expression correlation with protein synthesis data increase in 8.48 fold in high intensity training group. Other myofibrillar protein, troponin , appear only in high intensity group, corroborating the cellular injury data. High molecular masses proteins such as MRS2 and NADH dehydrogenase, involved in metabolic pathways also demonstrate increase expression, respectily 1.5 and 1.3 fold, in response to high intensity exercise.

CONCLUSIONS

High intensity exercise demonstrated an increase expression in some high molecular masses myofibrilar proteins, α-MyHC and troponin. Furthermore this intensity also lead a significant increase of other high molecular masses proteins such as MRS2 and NADH dehydrogenase in comparison to low and moderate intensities. However, high intensity exercise also represented a significant degree of cellular injury, when compared with the individuals submitted to low and moderate intensities.

Effect of N-2-mercaptopropionyl glycine on exercise-induced cardiac adaptations

The purpose of this study was to test the hypothesis that exercise-induced cardiac adaptations would be attenuated by the free radical scavenger N-2-mercaptopropionyl glycine (MPG). Male Sprague-Dawley rats were divided into four groups ( n = 9–13 per group) for 3–4 wk: sedentary (S), S+MPG (100 mg/kg ip daily), exercised on a treadmill (E) (60 min/day, 5 days/wk, at a speed of 20 m/min up a 6° grade in a 6°C room), or E+MPG given 10 min prior to exercise. Additional rats ( n = 55) were used to determine acute exercise effects on myocardial redox state [nonprotein nonglutathione sulfhydryls (NPNGSH)] and PI3K/Akt signaling pathway activation. Compared with S, NPNGSH levels were 48% lower in E ( P < 0.05) and unchanged in E+MPG ( P > 0.05). MPG also attenuated exercise-induced activation of the signaling proteins Akt and S6. Hearts from the 4-wk groups were weighed, and cardiac function was evaluated using an isolated perfused working heart preparation. Similar increases ( P < 0.05) in both exercised groups were observed for heart weight and heart weight-to-body weight ratio. Cardiac function improved in E vs. S, as indicated by greater ( P < 0.05) external work performed (cardiac output × systolic pressure) and efficiency of external work (work/V̇o2). MPG prevented these exercise-induced functional improvements. Skeletal muscle mitochondria content increased to similar levels in E and E+MPG. This study provides evidence that free radicals do not play an essential role in the development of exercise-induced cardiac hypertrophy; however, they appear to be involved in functional cardiac adaptations, which may be mediated through the PI3K/Akt pathway.

Activation of mitochondrial energy metabolism protects against cardiac failure

Cardiac failure is the most prevalent cause of death at higher age, and is commonly associated with impaired energy homeostasis in the heart. Mitochondrial metabolism appears critical to sustain cardiac function to counteract aging. In this study, we generated mice transgenically over-expressing the mitochondrial protein frataxin, which promotes mitochondrial energy conversion by controlling iron-sulfur-cluster biogenesis and hereby mitochondrial electron flux. Hearts of transgenic mice displayed increased mitochondrial energy metabolism and induced stress defense mechanisms, while overall oxidative stress was decreased. Following standardized exposure to doxorubicin to induce experimental cardiomyopathy, cardiac function and survival was significantly improved in the transgenic mice. The insulin/IGF-1 signaling cascade is an important pathway that regulates survival following cytotoxic stress through the downstream targets protein kinase B, Akt, and glycogen synthase kinase 3. Activation of this cascade is markedly inhibited in the hearts of wild-type mice following induction of cardiomyopathy. By contrast, transgenic overexpression of frataxin rescues impaired insulin/IGF-1 signaling and provides a mechanism to explain enhanced cardiac stress resistance in transgenic mice. Taken together, these findings suggest that increased mitochondrial metabolism elicits an adaptive response due to mildly increased oxidative stress as a consequence of increased oxidative energy conversion, previously named mitohormesis. This in turn activates protective mechanisms which counteract cardiotoxic stress and promote survival in states of experimental cardiomyopathy. Thus, induction of mitochondrial metabolism may be considered part of a generally protective mechanism to prevent cardiomyopathy and cardiac failure.

Molecular distinction between physiological and pathological cardiac hypertrophy: experimental findings and therapeutic strategies

Cardiac hypertrophy can be defined as an increase in heart mass. Pathological cardiac hypertrophy (heart growth that occurs in settings of disease, e.g. hypertension) is a key risk factor for heart failure. Pathological hypertrophy is associated with increased interstitial fibrosis, cell death and cardiac dysfunction. In contrast, physiological cardiac hypertrophy (heart growth that occurs in response to chronic exercise training, i.e. the 'athlete's heart') is reversible and is characterized by normal cardiac morphology (i.e. no fibrosis or apoptosis) and normal or enhanced cardiac function. Given that there are clear functional, structural, metabolic and molecular differences between pathological and physiological hypertrophy, a key question in cardiovascular medicine is whether mechanisms responsible for enhancing function of the athlete's heart can be exploited to benefit patients with pathological hypertrophy and heart failure. This review summarizes key experimental findings that have contributed to our understanding of pathological and physiological heart growth. In particular, we focus on signaling pathways that play a causal role in the development of pathological and physiological hypertrophy. We discuss molecular mechanisms associated with features of cardiac hypertrophy, including protein synthesis, sarcomeric organization, fibrosis, cell death and energy metabolism and provide a summary of profiling studies that have examined genes, microRNAs and proteins that are differentially expressed in models of pathological and physiological hypertrophy. How gender and sex hormones affect cardiac hypertrophy is also discussed. Finally, we explore how knowledge of molecular mechanisms underlying pathological and physiological hypertrophy may influence therapeutic strategies for the treatment of cardiovascular disease and heart failure.

Interaction of myocardial insulin receptor and IGF receptor signaling in exercise-induced cardiac hypertrophy

Insulin-like growth factor-1 (IGF-1) signaling has recently been implicated in the development of cardiac hypertrophy after long-term endurance training, via mechanisms that may involve energetic stress. Given the potential overlap of insulin and IGF-1 signaling we sought to determine if both signaling pathways could contribute to exercise-induced cardiac hypertrophy following shorter-term exercise training. Studies were performed in mice with cardiac-specific IGF-1 receptor (IGF1R) knockout (CIGFRKO), mice with cardiac-specific insulin receptor (IR) knockout (CIRKO), CIGFRKO mice that lacked one IR allele in cardiomyocytes (IGFR-/-IR+/-), and CIRKO mice that lacked one IGF1R allele in cardiomyocytes (IGFR+/-IR-/-). Intravenous administration of IGF-1 or 75 hours of swimming over 4 weeks increased IGF1R tyrosine phosphorylation in the heart in control and CIRKO mice but not in CIGFRKO mice. Intriguingly, IR tyrosine phosphorylation in the heart was also increased following IGF-1 administration or exercise training in control and CIGFRKO mice but not in CIRKO mice. The extent of cardiac hypertrophy following exercise training in CIGFRKO and CIRKO mice was comparable to that in control mice. In contrast, exercise-induced cardiac hypertrophy was significantly attenuated in IGFR-/-IR+/- and IGFR+/-IR-/- mice. Thus, IGF-1 and exercise activates both IGF1R and IR in the heart, and IGF1R- and IR-mediated signals may serve redundant roles in the hypertrophic responses of the heart to exercise training.

The Impact of Pre-Dialytic Endurance Training on Nutritional Status and Quality of Life in Stable Hemodialysis Patients (Sawada Study)

BACKGROUND

Protein-energy malnutrition and decreased quality of life (QOL) are common in hemodialysis (HD) patients. Although several studies have proved that regular exercise has beneficial effects, few control studies have shown the effects of exercise training on the nutritional status and QOL in HD patients.

METHODS

Fifty-five HD patients were recruited, and 22 of them were trained to exercise on an ergometer prior to dialysis three times a week for one year. Serum albumin levels, creatinine generation rate (CGR), and the Short Form 36 were assessed as outcome measures.

RESULTS

The serum albumin levels and CGR increased in the training group compared with baseline. The QOL scores also increased in half of the physical health and mental health dimensions in the training group.

CONCLUSION

These observations suggest that low-dose, long-term pre-dialytic endurance training might reverse the poor clinical outcome by improving the nutritional status and QOL in HD patients.

Effects of short-term treadmill exercise training or growth hormone supplementation on diastolic function and exercise tolerance in old rats

Whether the lusitropic potential of short-term exercise in aged rats is linked to an augmentation in the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis and an alteration in the cardiac renin angiotensin system (RAS) is unknown. Old (28-month-old) male, Fischer 344xBrown Norway rats were randomized to 4 weeks of GH supplementation (300 microg subcutaneous, twice daily) or 4 weeks of treadmill running, or were used as sedentary controls. Six-month-old rats, sedentary or exercised, were used as young controls. Training improved exercise capacity in old animals. Exercise and GH attenuated age-related declines in myocardial relaxation despite an exercise-induced suppression of IGF-1. The regulatory protein, sarcoplasmic Ca2+ adenosine triphosphatase (SERCA2), increased with exercise but not GH. Among aged rats, the cardiac RAS was not altered by training or GH. Thus, the signaling pathway underlying the lusitropic benefit of short-term habitual exercise in the aged rat may be distinct from GH-mediated benefits and independent of the cardiac RAS.

Insulin-like growth factor I receptor signaling is required for exercise-induced cardiac hypertrophy

The receptors for IGF-I (IGF-IR) and insulin (IR) have been implicated in physiological cardiac growth, but it is unknown whether IGF-IR or IR signaling are critically required. We generated mice with cardiomyocyte-specific knockout of IGF-IR (CIGF1RKO) and compared them with cardiomyocyte-specific insulin receptor knockout (CIRKO) mice in response to 5 wk exercise swim training. Cardiac development was normal in CIGF1RKO mice, but the hypertrophic response to exercise was prevented. In contrast, despite reduced baseline heart size, the hypertrophic response of CIRKO hearts to exercise was preserved. Exercise increased IGF-IR content in control and CIRKO hearts. Akt phosphorylation increased in exercise-trained control and CIRKO hearts and, surprisingly, in CIGF1RKO hearts as well. In exercise-trained control and CIRKO mice, expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) and glycogen content were both increased but were unchanged in trained CIGF1RKO mice. Activation of AMP-activated protein kinase (AMPK) and its downstream target eukaryotic elongation factor-2 was increased in exercise-trained CIGF1RKO but not in CIRKO or control hearts. In cultured neonatal rat cardiomyocytes, activation of AMPK with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) prevented IGF-I/insulin-induced cardiomyocyte hypertrophy. These studies identify an essential role for IGF-IR in mediating physiological cardiomyocyte hypertrophy. IGF-IR deficiency promotes energetic stress in response to exercise, thereby activating AMPK, which leads to phosphorylation of eukaryotic elongation factor-2. These signaling events antagonize Akt signaling, which although necessary for mediating physiological cardiac hypertrophy, is insufficient to promote cardiac hypertrophy in the absence of myocardial IGF-I signaling.

Apoptosis signal-regulating kinase 1/p38 signaling pathway negatively regulates physiological hypertrophy

BACKGROUND

Mechanical stress on the heart can lead to crucially different outcomes. Physiological stimuli such as exercise cause adaptive cardiac hypertrophy, characterized by a normal cardiac structure and normal or enhanced cardiac function. Pathological stimuli such as hypertension and aortic valvular stenosis cause maladaptive cardiac remodeling and ultimately heart failure. Apoptosis signal-regulating kinase 1 (ASK1) is known to be involved in pathological cardiac remodeling, but it has not been determined whether ASK1 pathways coordinate the signaling cascade leading to physiological type cardiac growth.

METHODS AND RESULTS

To evaluate the role of ASK1 in the physiological form of cardiac growth, mice lacking ASK1 (ASK1-/-) were exercised by swimming for 4 weeks. ASK1-/- mice showed exaggerated growth of the heart accompanied by typical characteristics of physiological hypertrophy. Their swimming-induced activation of Akt, a key molecule in the signaling cascade of physiological hypertrophy, increased more than that seen in wild-type controls. The activation of p38, a downstream kinase of ASK1, was suppressed selectively in the swimming-exercised ASK1-/- mice. Furthermore, the inhibition of ASK1 or p38 activity enhanced insulin-like growth factor 1-induced protein synthesis in rat neonatal ventricular cardiomyocytes, and the treatment with a specific inhibitor of p38 resulted in enhancement of Akt activation and suppression of protein phosphatase 2A activation. The cardiac-specific p38alpha-deficient mice developed an exacerbated form of cardiac hypertrophy in response to swimming exercise.

CONCLUSIONS

These results indicate that the ASK1/p38 signaling pathway negatively regulates physiological hypertrophy.

Early gene expression profiles during intraoperative myocardial ischemia-reperfusion in cardiac surgery

OBJECTIVE

The effects of cold cardioplegic arrest and reperfusion on human ventricular gene expression are unknown. We tested the hypothesis that intraoperative ischemia-reperfusion under conditions of blood cardioplegic arrest would induce a unique myocardial genomic profile indicative of a cardioprotective response.

METHODS

Right ventricular samples were serially acquired during surgical repair of ventricular septal defect.

RESULTS

Expression profiling revealed 3 patterns of gene expression: (1) increased expression above control levels within 1 hour of cardioplegic arrest, with further amplification during early reperfusion; (2) increased expression limited to the reperfusion phase; and (3) reduced expression during reperfusion. Functional annotation and network mapping of differentially expressed genes indicated activation of multiple signaling pathways regulated by phosphatidylinositide 3'-OH kinase convergent on cellular growth and reparative programs. Also observed was increased expression of genes regulating hemoglobin synthesis, suggesting a novel cardioprotective pathway evoked during ischemia-reperfusion.

CONCLUSION

Reversible myocardial ischemia-reperfusion during cardiac surgery is associated with an immediate genomic response that predicts a net cardioprotective phenotype.

Essential role of insulin and insulin-like growth factor 1 receptor signaling in cardiac development and function

Cardiovascular disease is the leading cause of death in people with type 2 diabetes and is linked to insulin resistance even in the absence of diabetes. Here we show that mice with combined deficiency of the insulin receptor and insulin-like growth factor 1 (IGF-1) receptor in cardiac and skeletal muscle develop early-onset dilated cardiomyopathy and die from heart failure within the first month of life despite having a normal glucose homeostasis. Mice lacking the insulin receptor show impaired cardiac performance at 6 months, and mice lacking the insulin receptor plus one Igf1r allele have slightly increased mortality. By contrast, mice lacking the IGF-1 receptor or the IGF-1 receptor plus one Ir allele appear normal. Morphological characterization and oligonucleotide array analysis of gene expression demonstrate that prior to development of these physiological defects, mice with combined deficiency of both insulin and IGF-1 receptors have a coordinated down-regulation of genes encoding components of the electron transport chain and mitochondrial fatty acid beta-oxidation pathways and altered expression of contractile proteins. Thus, while neither the insulin receptor nor IGF-1 receptor in muscle is critical for glucose homeostasis during the first month of life, signaling from these receptors, particularly the insulin receptor, is required for normal cardiac metabolism and function.

Effects of gradual coronary artery occlusion and exercise training on gene expression in swine heart

Gradual occlusion (O) of the swine left circumflex coronary artery (LCX) with an ameroid occluder results in complete O within 3 weeks, collateral vessel development, and compensatory hypertrophy. The purpose of this investigation was to determine the independent and combined effects of O and exercise training (E) on gene expression in the swine heart. Adult Yucatan miniature swine were assigned to one of the following groups (n=6-9/group): sedentary control (S), exercise-trained (E), sedentary swine subjected to LCX occlusion (SO), and exercise-trained swine with LCX occlusion (EO). Exercise consisted of progressive treadmill running conducted 5 d/wk for 16 weeks. Gene expression was studied in myocardium isolated from the collateral-dependent left ventricle free wall (LV) and the collateral-independent septum (SEP) by RNA blotting. E and O each stimulated cardiac hypertrophy independently (p<0.001) with no interaction. O but not E increased atrial natriuretic factor expression in the LV, but not in the SEP. E decreased the expression of beta-myosin heavy chain in the LV, but not in the SEP. E retarded the expression of collagen III mRNA in SEP; but not in the LV. Exercise training and coronary artery occlusion each stimulate cardiac hypertrophy independently and induce different patterns of gene expression.

Akt1 Is Required for Physiological Cardiac Growth

Background— Postnatal growth of the heart chiefly involves nonproliferative cardiomyocyte enlargement. Cardiac hypertrophy exists in a “physiological” form that is an adaptive response to long-term exercise training and as a “pathological” form that often is a maladaptive response to provocative stimuli such as hypertension and aortic valvular stenosis. A signaling cascade that includes the protein kinase Akt regulates the growth and survival of many cell types, but the precise role of Akt1 in either form of cardiac hypertrophy is unknown.

Methods and Results— To evaluate the role of Akt1 in physiological cardiac growth, akt1 −/− adult murine cardiac myocytes (AMCMs) were treated with IGF-1, and akt1 −/− mice were subjected to exercise training. akt1 −/− AMCMs were resistant to insulin-like growth factor-1–stimulated protein synthesis. The akt1 −/− mice were found to be resistant to swimming training–induced cardiac hypertrophy. To evaluate the role of Akt in pathological cardiac growth, akt1 −/− AMCMs were treated with endothelin-1, and akt1 −/− mice were subjected to pressure overload by transverse aortic constriction. Surprisingly, akt1 −/− AMCMs were sensitized to endothelin-1–induced protein synthesis, and akt1 −/− mice developed an exacerbated form of cardiac hypertrophy in response to transverse aortic constriction.

Conclusions— These results establish Akt1 as a pivotal regulatory switch that promotes physiological cardiac hypertrophy while antagonizing pathological hypertrophy.

Loaded wheel running and muscle adaptation in the mouse

Voluntary cage wheel exercise has been used extensively to determine the physiological adaptation of cardiac and skeletal muscle in mice. In this study, we tested the effect of different loading conditions on voluntary cage wheel performance and muscle adaptation. Male C57Bl/6 mice were exposed to a cage wheel with no-resistance (NR), low-resistance (LR), or high-resistance (HR) loads for 7 wk. Power output was elevated (3-fold) under increased loading (LR and HR) conditions compared with unloaded (NR) exercise training. Only unloaded (NR) exercise induced an increase in heart mass, whereas only loaded (LR and HR) exercise training induced an increase in skeletal (soleus) muscle mass. Moreover, unloaded and loaded exercise training had a differential impact on the cross-sectional area of muscle fibers, depending on the type of myosin heavy chain expressed by each fiber. The biochemical adaptation of the heart was characterized by a decrease in genes associated with pathological (but not physiological) cardiac hypertrophy and a decrease in calcineurin expression in all exercise groups. In addition, transcriptional activity of myocyte enhancer factor-2 (MEF-2) was significantly decreased in the hearts of the LR group as determined by a MEF-2-dependent transgene driving the expression of beta-galactosidase. Phosphorylation of glycogen synthase kinase-3beta, protein kinase B (Akt), and p70 S6 kinase was increased only in the hearts of the NR group, consistent with the significant increase in cardiac mass. In conclusion, unloaded and loaded cage wheel exercise have a differential impact on cage wheel performance and muscle (cardiac and skeletal) adaptation.

Sex modifies exercise and cardiac adaptation in mice

How an individual's sex and genetic background modify cardiac adaptation to increased workload is a topic of great interest. We systematically evaluated morphological and physiological cardiac adaptation in response to voluntary and forced exercise. We found that sex/gender is a dominant factor in exercise performance (in two exercise paradigms and two mouse strains) and that females of one of these strains have greater capacity to increase their cardiac mass in response to similar amounts of exercise. To explore the biochemical mechanisms for these differences, we examined signaling pathways previously implicated in cardiac hypertrophy. Ca2+/calmodulin-dependent protein kinase (CaMK) activity was significantly greater in males compared with females and increased after voluntary cage-wheel exposure in both sexes, but the proportional increase in CaMK activity was twofold higher in females compared with males. Phosphorylation of glycogen synthase kinase-3beta (GSK-3beta) was evident after 7 days of cage-wheel exposure in both sexes and remained elevated in females only by 21 days of exercise. Despite moderate increases in myocyte enhancer factor-2 (a downstream effector of CaMK) transcriptional activity and phosphorylation of Akt with exercise, there were no sex differences. Mitogen-activated protein kinase signaling components (p38 mitogen-activated protein kinase and extracellular regulated kinase 1/2) were not different between male and female mice and were not affected by exercise. We conclude that females have increased exercise capacity and increased hypertrophic response to exercise. We have also identified sex-specific differences in hypertrophic signaling within the cardiac myocyte that may contribute to sexual dimorphism in exercise and cardiac adaptation to exercise.