Regional differences in effects of exercise training on contractile and biochemical properties of rat cardiac myocytes

Fine tuning contractility: atrial sarcomere function in health and disease

The molecular mechanisms of sarcomere proteins underlie the contractile function of the heart. Although our understanding of the sarcomere has grown tremendously, the focus has been on ventricular sarcomere isoforms due to the critical role of the ventricle in health and disease. However, atrial-specific or -enriched myofilament protein isoforms, as well as isoforms that become expressed in disease, provide insight into ways this complex molecular machine is fine-tuned. Here, we explore how atrial-enriched sarcomere protein composition modulates contractile function to fulfill the physiological requirements of atrial function. We review how atrial dysfunction negatively affects the ventricle and the many cardiovascular diseases that have atrial dysfunction as a comorbidity. We also cover the pathophysiology of mutations in atrial-enriched contractile proteins and how they can cause primary atrial myopathies. Finally, we explore what is known about contractile function in various forms of atrial fibrillation. The differences in atrial function in health and disease underscore the importance of better studying atrial contractility, especially as therapeutics currently in development to modulate cardiac contractility may have different effects on atrial sarcomere function.

Effects of time-restricted exercise on activity rhythms and exercise-induced adaptations in the heart

Circadian rhythms play a crucial role in the regulation of various physiological processes, including cardiovascular function and metabolism. Exercise provokes numerous beneficial adaptations in heart, including physiological hypertrophy, and serves to shift circadian rhythms. This study investigated the impact of time-restricted exercise training on exercise-induced adaptations in the heart and locomotor activity rhythms. Male mice (n = 45) were allocated to perform voluntary, time-restricted exercise in the early active phase (EAP), late active phase (LAP), or remain sedentary (SED) for 6 weeks. Subsequently, mice were allowed 24-h ad libitum access to the running wheel to assess diurnal rhythms in locomotor activity. Heart weight and cross-sectional area were measured at sacrifice, and cardiac protein and gene expression levels were assessed for markers of mitochondrial abundance and circadian clock gene expression. Mice rapidly adapted to wheel running, with EAP mice exhibiting a significantly greater running distance compared to LAP mice. Time-restricted exercise induced a shift in voluntary wheel activity during the 24-h free access period, with the acrophase in activity being significantly earlier in EAP mice compared to LAP mice. Gene expression analysis revealed a higher expression of Per1 in LAP mice. EAP exercise elicited greater cardiac hypertrophy compared to LAP exercise. These findings suggest that the timing of exercise affects myocardial adaptations, with exercise in the early active phase inducing hypertrophy in the heart. Understanding the time-of-day dependent response to exercise in the heart may have implications for optimizing exercise interventions for cardiovascular health.

Sarco/endoplasmic reticulum calcium ATPase activity is unchanged despite increased myofilament calcium sensitivity in Zucker type 2 diabetic fatty rat heart

Systolic and diastolic dysfunction in diabetes have frequently been associated with abnormal calcium (Ca²⁺) regulation. However, there is emerging evidence that Ca²⁺ mishandling alone is insufficient to fully explain diabetic heart dysfunction, with focus shifting to the properties of the myofilament proteins. Our aim was to examine the effects of diabetes on myofilament Ca²⁺ sensitivity and Ca²⁺ handling in left ventricular tissues isolated from the same type 2 diabetic rat hearts. We measured the force-pCa relationship in skinned left ventricular cardiomyocytes isolated from 20-week-old type 2 diabetic and non-diabetic rats. Myofilament Ca²⁺ sensitivity was greater in the diabetic relative to non-diabetic cardiomyocytes, and this corresponded with lower phosphorylation of cardiac troponin I (cTnI) at ser23/24 in the diabetic left ventricular tissues. Protein expression of sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA), phosphorylation of phospholamban (PLB) at Ser16, and SERCA/PLB ratio were lower in the diabetic left ventricular tissues. However, the maximum SERCA Ca²⁺ uptake rate was not different between the diabetic and non-diabetic myocardium. Our data suggest that impaired contractility in the diabetic heart is not caused by SERCA Ca²⁺ mishandling. This study highlights the important role of the cardiac myofilament and provides new insight on the pathophysiology of diabetic heart dysfunction.

Animal exercise studies in cardiovascular research: Current knowledge and optimal design-A position paper of the Committee on Cardiac Rehabilitation, Chinese Medical Doctors' Association

Growing evidence has demonstrated exercise as an effective way to promote cardiovascular health and protect against cardiovascular diseases However, the underlying mechanisms of the beneficial effects of exercise have yet to be elucidated. Animal exercise studies are widely used to investigate the key mechanisms of exercise-induced cardiovascular protection. However, standardized procedures and well-established evaluation indicators for animal exercise models are needed to guide researchers in carrying out effective, high-quality animal studies using exercise to prevent and treat cardiovascular diseases. In our review, we present the commonly used animal exercise models in cardiovascular research and propose a set of standard procedures for exercise training, emphasizing the appropriate measurements and analysis in these chronic exercise models. We also provide recommendations for optimal design of animal exercise studies in cardiovascular research, including the choice of exercise models, control of exercise protocols, exercise at different stages of disease, and other considerations, such as age, sex, and genetic background. We hope that this position paper will promote basic research on exercise-induced cardiovascular protection and pave the way for successful translation of exercise studies from bench to bedside in the prevention and treatment of cardiovascular diseases.

Chronic aerobic exercise associated to low-dose L-NAME improves contractility without changing calcium handling in rat cardiomyocytes

Nitric oxide (NO) inhibition by high-dose NG-nitro-L-arginine methyl ester (L-NAME) is associated with several detrimental effects on the cardiovascular system. However, low-dose L-NAME increases NO synthesis, which in turn induces physiological cardiovascular benefits, probably by activating a protective negative feedback mechanism. Aerobic exercise, likewise, improves several cardiovascular functions in healthy hearts, but its effects are not known when chronically associated with low-dose L-NAME. Thus, we tested whether the association between low-dose L-NAME administration and chronic aerobic exercise promotes beneficial effects to the cardiovascular system, evaluating the cardiac remodeling process. Male Wistar rats were randomly assigned to control (C), L-NAME (L), chronic aerobic exercise (Ex), and chronic aerobic exercise associated to L-NAME (ExL). Aerobic training was performed with progressive intensity for 12 weeks; L-NAME (1.5 mg·kg^-1·day^-1) was administered by orogastric gavage. Low-dose L-NAME alone did not change systolic blood pressure (SBP), but ExL significantly increased SBP at week 8 with normalization after 12 weeks. Furthermore, ExL promoted the elevation of left ventricle (LV) end-diastolic pressure without the presence of cardiac hypertrophy and fibrosis. Time to 50% shortening and relaxation were reduced in ExL, suggesting a cardiomyocyte contractile improvement. In addition, the time to 50% Ca²⁺ peak was increased without alterations in Ca²⁺ amplitude and time to 50% Ca²⁺ decay. In conclusion, the association of chronic aerobic exercise and low-dose L-NAME prevented cardiac pathological remodeling and induced cardiomyocyte contractile function improvement; however, it did not alter myocyte affinity and sensitivity to intracellular Ca²⁺ handling.

Left Ventricular Systolic Function Assessed by Speckle Tracking Echocardiography in Athletes with and without Left Ventricle Hypertrophy

The aim of this study was to evaluate selected parameters of strain and rotation of the left ventricle (the basal rotation (BR) index, the basal circumferential strain (BCS) index, and the global longitudinal strain (GLS) of the left ventricle) in male athletes with physiological cardiac hypertrophy (LVH group), and athletes (non-LVH group) and non-athletes without hypertrophy (control group, CG). They were evaluated using transthoracic echocardiography and speckle tracking echocardiography before and after an incremental exercise test. The LVH group demonstrated lower BR at rest than the non-LVH group (p < 0.05) and the CG (p < 0.05). Physical effort had no effect on BR, nor was this effect different between groups (p > 0.05). There was a combined influence of LVH and physical effort on BR (F = 5.70; p < 0.05) and BCS (F = 4.97; p < 0.05), but no significant differences in BCS and GLS at rest between the groups. A higher BCS and lower GLS after exercise in the LVH group were demonstrated in comparison with the CG (p < 0.05). Left ventricular basal rotation as well as longitudinal and circumferential strains showed less of a difference between rest and after physical effort in subjects with significant myocardial hypertrophy. In conclusion, the obtained results may suggest that echocardiographic assessment of basal rotation and circumferential strain of the left ventricular can be important in predicting cardiac disorders caused by physical effort in individuals with physiological and pathological heart hypertrophy.

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.

Electrocardiograms of mice selectively bred for high levels of voluntary exercise: Effects of short-term exercise training and the mini-muscle phenotype

Changes in cardiac function that occur with exercise training have been studied in detail, but those accompanying evolved increases in the duration or intensity of physical activity are poorly understood. To address this gap, we studied electrocardiograms (ECGs) of mice from an artificial selection experiment in which four replicate lines are bred for high voluntary wheel running (HR) while four non-selected lines are maintained as controls (C). ECGs were recorded using an ECGenie (Mouse Specifics, Inc.) both before and after six days of wheel access (as used in the standard protocol to select breeders). We hypothesized that HR mice would show innate differences in ECG characteristics and that the response to training would be greater in HR mice relative to C mice because the former run more. After wheel access, in statistical analyses controlling for variation in body mass, all mice had lower heart rates, and mice from HR lines had longer PR intervals than C lines. Also after wheel access, male mice had increased heart rate variability, whereas females had decreased heart rate variability. With body mass as a covariate, six days of wheel access significantly increased ventricle mass in both HR and C males. Within the HR lines, a subset of mice known as mini-muscle individuals have a 50% reduction in hindlimb muscle mass and generally larger internal organs, including the heart ventricles. As compared with normal-muscled individuals, mini-muscle individuals had a longer QRS complex, both before and after wheel access. Some studies in other species of mammals have shown correlations between athletic performance and QRS duration. Correlations between wheel running and either heart rate or QRS duration (before wheel running) among the eight individual lines of the HR selection experiment or among 17 inbred mouse strains taken from the literature were not statistically significant. However, total revolutions and average speed were negatively correlated with PR duration among lines of the HR selection experiment for males, and duration of running was negatively correlated with PR duration among 17 inbred strains for females. We conclude that HR mice have enhanced trainability of cardiac function as compared with C mice (as indicated by their longer PR duration after wheel access), and that the mini-muscle phenotype causes cardiac changes that have been associated with increased athletic performance in previous studies of mammals.

Effects of regular exercise on ventricular myocyte biomechanics and KATP channel function

Exercise training is known to protect the heart from ischemia and improve function during exercise by reducing cardiomyocyte action potential duration (APD) and increasing contractility. The cellular mechanisms involve β-adrenergic regulation and the ATP-sensitive K+ (KATP) channel, but how each alters function of the left ventricle and sex specificity is unknown. To address this, female and male Sprague-Dawley rats were randomly assigned to wheel-running (TRN) or sedentary (SED) groups. After 6–8 wk of training, myocytes were isolated from the left ventricle and field stimulated at 1, 2, and 5 Hz. TRN significantly increased cardiomyocyte contractility, the kinetics of the Ca2+ transient, and responsiveness to the adrenergic receptor agonist isoproterenol (ISO), as reflected by an increased sarcomere shortening. Importantly, we demonstrated a TRN-induced upregulation of KATP channels, which was reflected by elevated content, current density, and the channel’s contribution to APD shortening at high activation rates and in the presence of the activator pinacidil. TRN induced increase in KATP current occurred throughout the left ventricle, but channel subunit content showed regional specificity with increases in Kir6.2 in the apex and SUR2A in base regions. In summary, TRN elevated cardiomyocyte cross-bridge kinetics, Ca2+ sensitivity, and the responsiveness of contractile function to β-adrenergic receptor stimulation in both sexes. Importantly, upregulation of the KATP channel accelerates repolarization and shortens APD during stress and exercise. These adaptations have clinical importance, as increased contractility and reduced APD would help protect cardiac output and reduce intracellular Ca2+ overload during stresses such as regional ischemia.

NEW & NOTEWORTHY Our results demonstrate that regular exercise significantly increased ventricular myocyte shortening and relaxation velocity and the rate of rise in intracellular Ca2+ transient and enhanced the response of biomechanics and Ca2+ reuptake to β-adrenergic stimulation. Importantly, exercise training upregulated the cardiomyocyte sarcolemma ATP-sensitive K+ channel across the left ventricle in both sexes, as reflected by elevated channel subunit content, current density, and the channel’s contribution to reduced action potential duration at high activation rates.

Force properties of skinned cardiac muscle following increasing volumes of aerobic exercise in rats

The positive effects of chronic endurance exercise training on health and performance have been well documented. These positive effects have been evaluated primarily at the structural level, and work has begun to evaluate mechanical adaptations of the myocardium. However, it remains poorly understood how the volume of exercise training affects cardiac adaptation. To gain some understanding, we subjected 3-mo-old Sprague-Dawley rats ( n = 23) to treadmill running for 11 wk at one of three exercise volumes (moderate, high, and extra high). Following training, hearts were excised and mechanical testing was completed on skinned trabecular fiber bundles. Performance on a maximal fitness test was dose dependent on training volume, where greater levels of training led to greater performance. No differences were observed between animals from any group for maximal active stress and passive stress at a sarcomere length of 2.2 µm. Heart mass and passive stress at sarcomere lengths beyond 2.4 µm increased in a dose-dependent manner for animals in the control and moderate- and high-duration groups. However, hearts from animals in the extra high-duration group presented with inhibited responses for heart mass and passive stress, despite performing greatest on a graded treadmill fitness test. These results suggest that heart mass and passive stress adapt in a dose-dependent manner, until exercise becomes excessive and adaptation is inhibited. Our findings are in agreement with the beneficial role exercise has in cardiac adaptation. However, excessive exercise comes with risks of maladaptation, which must be weighed against the desire to increase performance. NEW & NOTEWORTHY For the first time, we present findings on cardiac trabecular muscle passive stiffness and show the effect of excessive exercise on the heart. We demonstrated that heart mass increases with exercise until a maximum, after which greater exercise volume results in inhibited adaptation. At paraphysiological lengths, passive stiffness increases with exercise but to a lesser degree with excessive training. Despite greater performance on graded exercise tests, animals in the highest trained group exhibited possible maladaptation.

EFFECTS OF PHYSICAL TRAINING ON THE MYOCARDIUM OF FEMALE LDL KNOCKOUT OVARIECTOMIZED MICE

ABSTRACT Introduction: The emergence of coronary heart disease increases with menopause, physical inactivity and with dyslipidemia. It is known that physical training promotes the improvement of cardiovascular functions. Objective: The purpose of this study was to investigate the effects of aerobic physical training on the left ventricle in female LDL knockout ovariectomized mice. Methods: Thirty animals were divided into 6 groups (n=5), namely, sedentary non-ovariectomized control; sedentary ovariectomized control; trained ovariectomized control; sedentary non-ovariectomized; sedentary ovariectomized; and trained ovariectomized. We analyzed the mean nuclear volume parameters, the cross sectional area of the myocytes, the apparent density of the capillaries, interstitium, myocytes and collagen fibers. Results: The results show that for the density of the number of nuclei, the physical activity decreased to values close to the ovariectomized control group. Regarding the mean nuclear volume and the average area of myocytes, training and ovariectomy promoted the elevation of these values but hypercholesterolemia was lower. the volume density of myocytes, hypercholesterolemia showed an increase of these values as well as the training. There was no change in the volumetric density of the capillaries and the density of collagen fibers. The training caused the decrease in the density of the interstitial volume, and the hypercholesterolemia changed to a smaller one than the control group. Conclusion: We concluded that the moderate aerobic activity or the training time used in our study were not sufficient to generate significant alterations in the hypercholesterolemic group.

Altered Right Ventricular Mechanical Properties Are Afterload Dependent in a Rodent Model of Bronchopulmonary Dysplasia

Infants born premature are at increased risk for development of bronchopulmonary dysplasia (BPD), pulmonary hypertension (PH), and ultimately right ventricular (RV) dysfunction, which together carry a high risk of neonatal mortality. However, the role alveolar simplification and abnormal pulmonary microvascular development in BPD affects RV contractile properties is unknown. We used a rat model of BPD to examine the effect of hyperoxia-induced PH on RV contractile properties. We measured in vivo RV pressure as well as passive force, maximum Ca²⁺ activated force, calcium sensitivity of force (pCa₅₀) and rate of force redevelopment (k_tr) in RV skinned trabeculae isolated from hearts of 21-and 35-day old rats pre-exposed to 21% oxygen (normoxia) or 85% oxygen (hyperoxia) for 14 days after birth. Systolic and diastolic RV pressure were significantly higher at day 21 in hyperoxia exposed rats compared to normoxia control rats, but normalized by 35 days of age. Passive force, maximum Ca²⁺ activated force, and calcium sensitivity of force were elevated and cross-bridge cycling kinetics depressed in 21-day old hyperoxic trabeculae, whereas no differences between normoxic and hyperoxic trabeculae were seen at 35 days. Myofibrillar protein analysis revealed that 21-day old hyperoxic trabeculae had increased levels of beta-myosin heavy chain (β-MHC), atrial myosin light chain 1 (aMLC1; often referred to as essential light chain), and slow skeletal troponin I (ssTnI) compared to age matched normoxic trabeculae. On the other hand, 35-day old normoxic and hyperoxic trabeculae expressed similar level of α- and β-MHC, ventricular MLC1 and predominantly cTnI. These results suggest that neonatal exposure to hyperoxia increases RV afterload and affect both the steady state and dynamic contractile properties of the RV, likely as a result of hyperoxia-induced expression of β-MHC, delayed transition of slow skeletal TnI to cardiac TnI, and expression of atrial MLC1. These hyperoxia-induced changes in contractile properties are reversible and accompany the resolution of PH with further developmental age, underscoring the importance of reducing RV afterload to allow for normalization of RV function in both animal models and humans with BPD.

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.

Cardiac hypertrophy and IGF-1 response to testosterone propionate treatment in trained male rats

Objective

Several studies have suggested that testosterone exerts a growth-promoting effect in the heart. Limited data are available regarding interactions between possible endocrine/paracrine effects in response to exercise training. Therefore, we examined supraphysiological testosterone-induced heart hypertrophy and cardiac insulin-like growth factor (IGF)-1 content in sedentary and exercise-trained rats.

Design

Male Wistar rats (n=33) were randomly allocated to groups with a 6-week endurance training with or without testosterone, and sedentary animals with or without testosterone. The hormone (20 mg/250 g body weight was administrated once a week for six weeks. After six weeks the animals were anesthetized, euthanized and the heart was excised and weighed. The left ventricle was separated for biochemical analyses.

Results

Testosterone-treated animals showed significantly higher cardiac IGF-1 content compared to untreated control and trained groups (p=0.01). The administration of supraphysiological testosterone significantly increased the heart weight to body weight ratio (HW/BW, p<0.01). A significant positive correlation was seen between IGF-1 levels and the HW/BW ratio (p=0.002; r=0.50) and between serum total testosterone levels and HW/BW (p=0.000; r=0.79).

Conclusions

The results demonstrate that increased cardiac IGF-1 content in response to higher serum testosterone might be responsible for heart hypertrophy observed in both sedentary and endurance-trained animals.

Structural, Contractile and Electrophysiological Adaptations of Cardiomyocytes to Chronic Exercise

Cardiac beneficial effects of chronic exercise is well admitted. These effects mainly studied at the organ and organism integrated levels find their origin in cardiomyocyte adaptation. This chapter try to highlight the main trends of the data related to the different parameters subject to such adaptations. This is addressed through cardiomyocytes size and structure, calcium and contractile properties, and finally electrophysiological alterations induced by training as they transpire from the literature. Despite the clarifications needed to decipher healthy cardiomyocyte remodeling, this overview clearly show that cardiac cell plasticity ensure the cardiac adaptation to exercise training and offers an interesting mean of action to counteract physiological disturbances induced by cardiac pathologies.

Cardiac tissue structure, properties, and performance: a materials science perspective

From an engineering perspective, many forms of heart disease can be thought of as a reduction in biomaterial performance, in which the biomaterial is the tissue comprising the ventricular wall. In materials science, the structure and properties of a material are recognized to be interconnected with performance. In addition, for most measurements of structure, properties, and performance, some processing is required. Here, we review the current state of knowledge regarding cardiac tissue structure, properties, and performance as well as the processing steps taken to acquire those measurements. Understanding the impact of these factors and their interactions may enhance our understanding of heart function and heart failure. We also review design considerations for cardiac tissue property and performance measurements because, to date, most data on cardiac tissue has been obtained under non-physiological loading conditions. Novel measurement systems that account for these design considerations may improve future experiments and lead to greater insight into cardiac tissue structure, properties, and ultimately performance.

Exercise training prior to myocardial infarction attenuates cardiac deterioration and cardiomyocyte dysfunction in rats

OBJECTIVES

The present study was performed to investigate 1) whether aerobic exercise training prior to myocardial infarction would prevent cardiac dysfunction and structural deterioration and 2) whether the potential cardiac benefits of aerobic exercise training would be associated with preserved morphological and contractile properties of cardiomyocytes in post-infarct remodeled myocardium.

METHODS

Male Wistar rats underwent an aerobic exercise training protocol for eight weeks. The rats were then assigned to sham surgery (SHAM), sedentary lifestyle and myocardial infarction or exercise training and myocardial infarction groups and were evaluated 15 days after the surgery. Left ventricular tissue was analyzed histologically, and the contractile function of isolated myocytes was measured. Student's t-test was used to analyze infarct size and ventricular wall thickness, and the other parameters were analyzed by the Kruskal-Wallis test followed by Dunn's test or a one-way analysis of variance followed by Tukey's test (p<0.05).

RESULTS

Myocardial infarctions in exercise-trained animals resulted in a smaller myocardial infarction extension, a thicker infarcted wall and less collagen accumulation as compared to myocardial infarctions in sedentary animals. Myocardial infarction-induced left ventricular dilation and cardiac dysfunction, as evaluated by +dP/dt and -dP/dt, were both prevented by previous aerobic exercise training. Moreover, aerobic exercise training preserved cardiac myocyte shortening, improved the maximum shortening and relengthening velocities in infarcted hearts and enhanced responsiveness to calcium.

CONCLUSION

Previous aerobic exercise training attenuated the cardiac dysfunction and structural deterioration promoted by myocardial infarction, and such benefits were associated with preserved cardiomyocyte morphological and contractile properties.

Moderate intensity, but not high intensity, treadmill exercise training alters power output properties in myocardium from aged rats

Aging is characterized by a progressive decline in cardiac function, but endurance exercise training has been shown to retard a number of deleterious effects of aging. However, underlying mechanisms by which exercise training improves age-related decrements in myocardial contractile function are not well understood. The purpose of this study was to determine the effects of exercise training on power output properties in permeablized (skinned) myocytes of old rats. Thirty-month-old rats were divided into sedentary control (C) and groups undergoing 11 weeks of treadmill exercise training at moderate intensity (MI) and at high intensity (HI). Peak power output normalized to maximal force was significantly increased in MI but not in HI compared to C with significant increases in atrial myosin light chain 1 in ventricle. These results suggest that MI exercise training is beneficial as a significant increase was seen in the ability of the myocardium to do work, but this effect was not seen with HI training.

Heat acclimation and exercise training interact when combined in an overriding and trade-off manner: physiologic-genomic linkage

Combined heat acclimation (AC) and exercise training (EX) enhance exercise performance in the heat while meeting thermoregulatory demands. We tested the hypothesis that different stress-specific adaptations evoked by each stressor individually trigger similar cardiac alterations, but when combined, overriding/trade-off interactions take place. We used echocardiography, isolated cardiomyocyte imaging and cDNA microarray techniques to assay in situ cardiac performance, excitation-contraction (EC) coupling features, and transcriptional programs associated with cardiac contractility. Rat groups studied were controls (sedentary 24°C); AC (sedentary, 34°C, 1 mo); normothermic EX (treadmill at 24°C, 1 mo); and heat-acclimated, exercise-trained (EXAC; treadmill at 34°C, 1 mo). Prolonged heat exposure decreased heart rate and contractile velocity and increased end ventricular diastolic diameter. Compared with controls, AC/EXAC cardiomyocytes demonstrated lower l-type Ca(2+) current (I(CaL)) amplitude, higher Ca(2+) transient (Ca(2+)T), and a greater Ca(2+)T-to-I(CaL) ratio; EX alone enhanced I(CaL) and Ca(2+)T, whereas aerobic training in general induced cardiac hypertrophy and action potential elongation in EX/EXAC animals. At the genomic level, the transcriptome profile indicated that the interaction between AC and EX yields an EXAC-specific molecular program. Genes affected by chronic heat were linked with the EC coupling cascade, whereas aerobic training upregulated genes involved with Ca(2+) turnover via an adrenergic/metabolic-driven positive inotropic response. In the EXAC cardiac phenotype, the impact of chronic heat overrides that of EX on EC coupling components and heart rate, whereas EX regulates cardiac morphometry. We suggest that concerted adjustments induced by AC and EX lead to enhanced metabolic and mechanical performance of the EXAC heart.

Transmural variation in myosin heavy chain isoform expression modulates the timing of myocardial force generation in porcine left ventricle

Recent studies have shown that the sequence and timing of mechanical activation of myocardium vary across the ventricular wall. However, the contributions of variable expression of myofilament protein isoforms in mediating the timing of myocardial activation in ventricular systole are not well understood. To assess the functional consequences of transmural differences in myofilament protein expression, we studied the dynamic mechanical properties of multicellular skinned preparations isolated from the sub-endocardial and sub-epicardial regions of the porcine ventricular midwall. Compared to endocardial fibres, epicardial fibres exhibited significantly faster rates of stretch activation and force redevelopment (k(tr)), although the amount of force produced at a given [Ca2+] was not significantly different. Consistent with these results, SDS-PAGE analysis revealed significantly elevated expression of alpha myosin heavy chain (MHC) isoform in epicardial fibres (13 +/- 1%) versus endocardial fibres (3 +/- 1%). Linear regression analysis revealed that the apparent rates of delayed force development and force decay following stretch correlated with MHC isoform expression (r2 = 0.80 and r2 = 0.73, respectively, P < 0.05). No differences in the relative abundance or phosphorylation status of other myofilament proteins were detected. These data show that transmural differences in MHC isoform expression contribute to regional differences in dynamic mechanical function of porcine left ventricles, which in turn modulate the timing of force generation across the ventricular wall and work production during systole.

Alteration in left ventricular normal and shear strains evaluated by 2D-strain echocardiography in the athlete's heart

The contraction of cardiomyocytes induces a systolic increase in left ventricular (LV) normal (radial, circumferential and longitudinal) and shear strains, whose functional consequences have not been evaluated, so far, in athletes. We used 2D ultrasound speckle tracking imaging (STI) to evaluate LV regional strain in high-level cyclists compared to sedentary controls. Sixteen male elite cyclists and 23 sedentary controls underwent conventional, tissue Doppler, and STI echocardiography at rest. We assessed LV long and short axis normal strains and shear strains. We evaluated circumferential-longitudinal shear strain from LV torsion, and circumferential-radial shear strain from the difference between subendocardial and subepicardial torsion. Apical radial strain (42.7 +/- 10.5% versus 52.2 +/- 14.3%, P < 0.05) and LV torsion (6.0 +/- 1.8 deg versus 9.2 +/- 3.2 deg, P < 0.01) were lower in cyclists than in controls, respectively. Rotations and torsion were higher in the subendocardial than in the subepicardial region in sedentary controls, but not in cyclists. Haemodynamic and tissue Doppler based indexes of global LV diastolic and systolic functions were not different between cyclists and controls. Athlete's heart is associated with specific LV adaptation including lower apical strain and lower myocardial shear strains, with no change in global LV diastolic and systolic function. These mechanical alterations could improve the cardiovascular adjustments to exercise by increasing the radial strain and torsional (and thus untwisting) response to exercise, a key element of diastolic filling and thus of cardiac performance in athletes.

Inhomogeneity in the response to mechanical stimulation: cardiac muscle function and gene expression

Mechanical stimulation has important consequences for myocardial function. However, this stimulation and the response to it, is not uniform. The right ventricle is thinner walled and operates at lower pressure than the left ventricle. Within the ventricles, differences in the orientation of myocardial fibres exist. These differences produce inhomogeneity in the stress and strain between and across the ventricles. Possibly as a result of these variations in mechanical stimulation, there are well characterised inhomogeneities in gene expression and protein function within the ventricular myocardium, for example in the transient outward K+ current and its associated Kv channels. Perhaps not surprisingly, it is becoming apparent that gradients of expression and function exist for proteins that are intimately involved in the response to mechanical stimulation in the heart, for example in the left ventricle of the rat there is a transmural gradient in mRNA and current density of the mechanosensitive two-pore domain K+ channel TREK-1 (ENDO>EPI). In healthy hearts it is assumed that these gradients are important for normal function and therefore that their disruption in diseased myocardium is involved in the dysfunction that occurs.

Microphthalmia transcription factor isoforms in mast cells and the heart

The microphthalmia transcription factor (Mitf) is critical for the survival and differentiation of a variety of cell types. While on the transcript level it has been noted that melanocytes and cardiomyocytes express specific Mitf isoforms, mast cells express several isoforms, mainly Mitf-H and Mitf-MC, whose function has not been thoroughly investigated. We found that in mast cells the expression of the specific Mitf isoforms is dependent on physiological stimuli that cause a major shifting of promoter usage and internal splicing. For example, activation of the c-kit signaling pathway almost totally abolished one of the main splice isoforms. Since cardiomyocytes express only the Mitf-H isoform, they were an ideal system to determine this isoform's physiological role. We identified that the expression of myosin light-chain 1a (MLC-1a) is regulated by Mitf-H. Interestingly, the transactivation of MLC-1a by Mitf-H in cardiomyocytes is decreased by overexpression of the splice form with exon 6a. In conclusion, we found that there is physiological switching of Mitf isoforms and that the promoter context and the cell context have a combined influence on gene expression programs.

Early Adaptations to Training

Chronic exercise induces adaptations that increase the functional capacity of the cardiovascular system. Aside from ventricular growth, these adaptations include a shift in the MHC isoenzyme pattern to enhance ventricular contractility. It is unclear whether adaptations by the contractile elements are an early event and specific to exercise, or whether they progress as a function of cardiac growth. Examining early adaptations to training is also important because it is during this period when the greatest imbalance between increased demand and functional capacity exists, and it is likely that the mechanisms responsible for propagating changes in the myocardial phenotype are most active.

PURPOSE

To determine whether changes in left ventricular (LV) contractile elements are an early adaptation to chronic exercise.

METHODS

Rats were randomly assigned to sedentary control or exercise training groups for 1 or 10 wk of training. After training, the LV was analyzed for protein by Western blot or mRNA by Northern and real-time QRT-PCR analysis.

RESULTS

Plantaris cytochrome oxidase activity was significantly (P < 0.05) increased by 1 wk (+28%) or 10 wk (+32%) of training. Training significantly increased LV myofibrillar alpha-MHC protein and alpha-MHC-mRNA after both training periods. No changes in myofibrillar beta-MHC protein or beta-MHC-mRNA were observed. After 1 wk of training, LV skeletal alpha-actin-mRNA was significantly increased, whereas no changes were found for ANF, glyceraldehyde dehydrogenase, or cytochrome oxidase IV. Gel mobility shift analysis determined that YY1 DNA binding was significantly decreased in LV extracts from trained animals, although no change in YY1-mRNA expression was observed.

CONCLUSIONS

Increased myofibrillar alpha-MHC protein and alpha-MHC-mRNA expression are early events in the adaptation to chronic exercise and occur before significant cardiac growth. These adaptations enhance myocardial contractility and permit increases in maximal cardiac output during heavy exercise.

Exercise training increases myocardial inotropic response in food restricted rats

This study evaluated the effects of exercise training on myocardial function and ultrastructure of rats submitted to different levels of food restriction (FR). Male Wistar-Kyoto rats, 60 days old, were submitted to free access to food, light FR (20%), severe FR (50%) and/or to swimming training (one hour per day with 5% of load, five days per week for 90 days). Myocardial function was evaluated by left ventricular papillary muscle under basal condition (calcium 1.25 mM), and after extracellular calcium elevation to 5.2 mM and isoproterenol (1 microM) addition. The ultrastructure of the myocardium was examined in the papillary muscle. The training effectiveness was verified by improvement of myocardial metabolic enzyme activities. Both 20% and 50% food restriction protocols presented minor body and ventricular weights gain. The 20%-FR, in sedentary or trained rats, did not alter myocardial function or ultrastructure. The 50%-FR, in sedentary rats, caused myocardial dysfunction under basal condition, decreased response to inotropic stimulation, and promoted myocardial ultrastructural damage. The 50%-FR, in exercised rats, increased myocardial dysfunction under basal condition but increased response to inotropic stimulation although there was myocardial ultrastructural damage. In conclusion, the exercise training in severe restriction caused marked myocardial dysfunction at basal condition but increased myocardial response to inotropic stimulation.

Greater adenosine A(2A) receptor densities in cardiac and skeletal muscle in endurance-trained men: a [11C]TMSX PET study

We examined the densities of adenosine A(2A) receptors in cardiac and skeletal muscles between untrained and endurance-trained subjects using positron emission tomography (PET) and [7-methyl-11C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ([11C]TMSX), a newly developed radioligand for mapping adenosine A(2A) receptors. Five untrained and five endurance-trained subjects participated in this study. The density of adenosine A(2A) receptors was evaluated as the distribution volume of [11C]TMSX in cardiac and triceps brachii muscles in the resting state using PET. The distribution volume of [11C]TMSX in the myocardium was significantly greater than in the triceps brachii muscle in both groups. Further, distribution volumes [11C]TMSX in the trained subjects were significantly grater than those in untrained subjects (myocardium, 3.6+/-0.3 vs. 3.1+/-0.4 ml g(-1); triceps brachii muscle, 1.7+/-0.3 vs. 1.2+/-0.2 ml g(-1), respectively). These results indicate that the densities of adenosine A(2A) receptors in the cardiac and skeletal muscles are greater in the endurance-trained men than in the untrained men.

Intra-uterine growth restriction and the programming of left ventricular remodelling in female rats

Epidemiological studies link intra-uterine growth restriction (IUGR) with increased incidence of hypertension and cardiac disease in adulthood. Our rat model of IUGR supports this contention and provides evidence for the programming of susceptibility for hypertension in all offspring. Moreover, in the female offspring only, gross anatomical changes (cardiac ventricle to body ratios) and increased left cardiac ventricular atrial natriuretic peptide (ANP) mRNA levels provide evidence for programming of cardiac disease in this gender. The aim of the current study was to measure changes in cardiac tissue that support remodelling that could be implicated in the initiation of hypertrophy. Adult female rats from our IUGR model and age- and sex-matched controls were killed at 12 weeks of age. Left cardiac ventricles were removed and used for monitoring changes in several key genes, Na+,K+-ATPase beta1 protein expression, cardiomyocyte morphology and contractility as well as citrate synthase and aconitase activities. When compared to controls, female offspring of our IUGR rat model exhibit higher expression (mRNA) of ANP and the atrial isoform of the myosin light chain, lower levels of Na+,K+-ATPase beta1 protein, increased cardiomyocyte depth and volume, increased sarcomere length, diminished cardiomyocyte contractility and lower aconitase activity. Female offspring of our IUGR rat model exhibit changes as adults that are consistent with the onset of cardiac remodelling. The decrease in aconitase activity suggests that oxidative stress may be implicated in this response.

Adaptation of cardiac myocyte contractile properties to exercise training

Recent work suggests that chronic exercise induces alterations in the contractile properties of cardiac myocytes. These alterations include increased sensitivity to activation by Ca, changes in the force-length relationship, and increased power output. A recently observed shift in expression of myosin light chain 1 subunit isoforms induced by training may provide a molecular mechanism for these contractile alterations.

Expression profiling identifies dysregulation of myosin heavy chains IIb and IIx during limb immobilization in the soleus muscles of old rats

Aged individuals suffer from multiple dysfunctions during skeletal muscle atrophy. The purpose of this study was to determine differential changes in gene expression in atrophied soleus muscle induced by hindlimb immobilization in young (3-4 months) and old (30-31 months) rats. The hypothesis was that differentially expressed mRNAs with age-atrophy interactions would reveal candidates that induce loss of function responses in aged animals. Each muscle was applied to an independent set of Affymetrix micoarrays, with 385 differentially expressed mRNAs with atrophy and 354 age-atrophy interactions detected by two-factor ANOVA (alpha of 0.05 with a Bonferroni adjustment). Functional trends were observed for 23 and 15 probe sets involved in electron transport and the extracellular matrix, respectively, decreasing more in the young than in the old. Other functional categories with atrophy in both ages included chaperones, glutathione-S-transferases, the tricarboxylic acid cycle, reductions in Z-line-associated proteins and increases in probe sets for protein degradation. Surprisingly, myosin heavy chain IIb and IIx mRNAs were suppressed in the atrophied soleus muscle of old rats as opposed to the large increases in the young animals (16- and 25-fold, respectively, with microarrays, and 61- and 68-fold, respectively, with real-time PCR). No significant changes were observed in myosin heavy chain IIb and IIx mRNA with micoarrays in the atrophied soleus muscles of old rats, but they were found to increase six- and fivefold, respectively, with real-time PCR. Therefore, deficiencies in pre-translational signals that normally upregulate myosin heavy chain IIb and IIx mRNAs during atrophy may exist in the soleus muscle of old animals.