A comprehensive protocol combining in vivo and ex vivo electrophysiological experiments in an arrhythmogenic animal model.

Ventricular arrhythmias contribute significantly to cardiovascular mortality, with coronary artery disease as the predominant underlying cause. Understanding the mechanisms of arrhythmogenesis is essential to identify proarrhythmic factors and develop novel approaches for antiarrhythmic prophylaxis and treatment. Animal models are vital in basic research on cardiac arrhythmias, encompassing molecular, cellular, ex vivo whole heart, and in vivo models. Most studies use either in vivo protocols lacking important information on clinical relevance or exclusively ex vivo protocols, thereby missing the opportunity to explore underlying mechanisms. Consequently, interpretation may be difficult due to dissimilarities in animal models, interventions, and individual properties across animals. Moreover, proarrhythmic effects observed in vivo are often not replicated in corresponding ex vivo preparations during mechanistic studies. We have established a protocol to perform both an in vivo and ex vivo electrophysiological characterization in an arrhythmogenic rat model with heart failure following myocardial infarction. The same animal is followed throughout the experiment. In vivo methods involve intracardiac programmed electrical stimulation and external defibrillation to terminate sustained ventricular arrhythmia. Ex vivo methods conducted on the Langendorff-perfused heart include an electrophysiological study with optical mapping of regional action potentials, conduction velocities, and dispersion of electrophysiological properties. By exploring the retention of the in vivo proarrhythmic phenotype ex vivo, we aim to examine whether the subsequent ex vivo detailed measurements are relevant to in vivo pathological behavior. This protocol can enhance greater understanding of cardiac arrhythmias by providing a standardized, yet adaptable model for evaluating arrhythmogenicity or antiarrhythmic interventions in cardiac diseases.NEW & NOTEWORTHY Rodent models are widely used in arrhythmia research. However, most studies do not standardize clinically relevant in vivo and ex vivo techniques to support their conclusions. Here, we present a comprehensive electrophysiological protocol in an arrhythmogenic rat model, connecting in vivo and ex vivo programmed electrical stimulation with optical mapping. By establishing this protocol, we aim to facilitate the adoption of a standardized model for investigating arrhythmias, enhancing research rigor and comparability in this field.

Exercise training reveals micro-RNAs associated with improved cardiac function and electrophysiology in rats with heart failure after myocardial infarction.

AIMS: Endurance training improves aerobic fitness and cardiac function in individuals with heart failure. However, the underlying mechanisms are not well characterized. Exercise training could therefore act as a tool to discover novel targets for heart failure treatment. We aimed to associate changes in Ca2+ handling and electrophysiology with micro-RNA (miRNA) profile in exercise trained heart failure rats to establish which miRNAs induce heart failure-like effects in Ca2+ handling and electrophysiology. METHODS AND RESULTS: Post-myocardial infarction (MI) heart failure was induced in Sprague Dawley rats. Rats with MI were randomized to sedentary control (sed), moderate (mod)- or high-intensity (high) endurance training for 8 weeks. Exercise training improved cardiac function, Ca2+ handling and electrophysiology including reduced susceptibility to arrhythmia in an exercise intensity-dependent manner where high intensity gave a larger effect. Fifty-five miRNAs were significantly regulated (up or down) in MI-sed, of which 18 and 3 were changed towards Sham-sed in MI-high and MI-mod, respectively. Thereafter we experimentally altered expression of these "exercise-miRNAs" individually in human induced pluripotent stem cell-derived cardiomyocytes (hIPSC-CM) in the same direction as they were changed in MI. Of the "exercise-miRNAs", miR-214-3p prolonged AP duration, whereas miR-140 and miR-208a shortened AP duration. miR-497-5p prolonged Ca2+ release whereas miR-214-3p and miR-31a-5p prolonged Ca2+ decay. CONCLUSION: Using exercise training as a tool, we discovered that miR-214-3p, miR-497-5p, miR-31a-5p contribute to heart-failure like behaviour in Ca2+ handling and electrophysiology and could be potential treatment targets.

High Intensity Interval Training Ameliorates Mitochondrial Dysfunction in the Left Ventricle of Mice with Type 2 Diabetes.

Both human and animal studies have shown mitochondrial and contractile dysfunction in hearts of type 2 diabetes mellitus (T2DM). Exercise training has shown positive effects on cardiac function, but its effect on the mitochondria have been insufficiently explored. The aim of this study was to assess the effect of exercise training on mitochondrial function in T2DM hearts. We divided T2DM mice (db/db) into a sedentary and an interval training group at 8 weeks of age and used heterozygote db/+ as controls. After 8 weeks of training, we evaluated mitochondrial structure and function, as well as the levels of mRNA and proteins involved in key metabolic processes from the left ventricle. db/db animals showed decreased oxidative phosphorylation capacity and fragmented mitochondria. Mitochondrial respiration showed a blunted response to Ca2+ along with reduced protein levels of the mitochondrial calcium uniporter. Exercise training ameliorated the reduced oxidative phosphorylation in complex (C) I + II, CII and CIV, but not CI or Ca2+ response. Mitochondrial fragmentation was partially restored. mRNA levels of isocitrate, succinate and oxoglutarate dehydrogenase were increased in db/db mice and normalized by exercise training. Exercise training induced an upregulation of two transcripts of peroxisome proliferator activated receptor gamma coactivator 1 alpha (PGC1α1 and PGC1α4) previously linked to endurance training adaptations and strength training adaptations, respectively. The T2DM heart showed mitochondrial dysfunction at multiple levels and exercise training ameliorated some, but not all mitochondrial dysfunctions.

Skeletal muscle metabolism in rats with low and high intrinsic aerobic capacity: Effect of aging and exercise training.

PURPOSE: Exercise training increases aerobic capacity and is beneficial for health, whereas low aerobic exercise capacity is a strong independent predictor of cardiovascular disease and premature death. The purpose of the present study was to determine the metabolic profiles in a rat model of inborn low versus high capacity runners (LCR/HCR) and to determine the effect of inborn aerobic capacity, aging, and exercise training on skeletal muscle metabolic profile. METHODS: LCR/HCR rats were randomized to high intensity low volume interval treadmill training twice a week or sedentary control for 3 or 11 months before they were sacrificed, at 9 and 18 months of age, respectively. Magnetic resonance spectra were acquired from soleus muscle extracts, and partial least square discriminative analysis was used to determine the differences in metabolic profile. RESULTS: Sedentary HCR rats had 54% and 30% higher VO2max compared to sedentary LCR rats at 9 months and 18 months, respectively. In HCR, exercise increased running speed significantly, and VO2max was higher at age of 9 months, compared to sedentary counterparts. In LCR, changes were small and did not reach the level of significance. The metabolic profile was significantly different in the LCR sedentary group compared to the HCR sedentary group at the age of 9 and 18 months, with higher glutamine and glutamate levels (9 months) and lower lactate level (18 months) in HCR. Irrespective of fitness level, aging was associated with increased soleus muscle concentrations of glycerophosphocholine and glucose. Interval training did not influence metabolic profiles in LCR or HCR rats at any age. CONCLUSION: Differences in inborn aerobic capacity gave the most marked contrasts in metabolic profile, there were also some changes with ageing. Low volume high intensity interval training twice a week had no detectable effect on metabolic profile.

Content of mitochondrial calcium uniporter (MCU) in cardiomyocytes is regulated by microRNA-1 in physiologic and pathologic hypertrophy.

The mitochondrial Ca2+ uniporter complex (MCUC) is a multimeric ion channel which, by tuning Ca2+ influx into the mitochondrial matrix, finely regulates metabolic energy production. In the heart, this dynamic control of mitochondrial Ca2+ uptake is fundamental for cardiomyocytes to adapt to either physiologic or pathologic stresses. Mitochondrial calcium uniporter (MCU), which is the core channel subunit of MCUC, has been shown to play a critical role in the response to ß-adrenoreceptor stimulation occurring during acute exercise. The molecular mechanisms underlying the regulation of MCU, in conditions requiring chronic increase in energy production, such as physiologic or pathologic cardiac growth, remain elusive. Here, we show that microRNA-1 (miR-1), a member of the muscle-specific microRNA (myomiR) family, is responsible for direct and selective targeting of MCU and inhibition of its translation, thereby affecting the capacity of the mitochondrial Ca2+ uptake machinery. Consistent with the role of miR-1 in heart development and cardiomyocyte hypertrophic remodeling, we additionally found that MCU levels are inversely related with the myomiR content, in murine and, remarkably, human hearts from both physiologic (i.e., postnatal development and exercise) and pathologic (i.e., pressure overload) myocardial hypertrophy. Interestingly, the persistent activation of ß-adrenoreceptors is likely one of the upstream repressors of miR-1 as treatment with ß-blockers in pressure-overloaded mouse hearts prevented its down-regulation and the consequent increase in MCU content. Altogether, these findings identify the miR-1/MCU axis as a factor in the dynamic adaptation of cardiac cells to hypertrophy.

T cell costimulation blockade blunts pressure overload-induced heart failure.

Heart failure (HF) is a leading cause of mortality. Inflammation is implicated in HF, yet clinical trials targeting pro-inflammatory cytokines in HF were unsuccessful, possibly due to redundant functions of individual cytokines. Searching for better cardiac inflammation targets, here we link T cells with HF development in a mouse model of pathological cardiac hypertrophy and in human HF patients. T cell costimulation blockade, through FDA-approved rheumatoid arthritis drug abatacept, leads to highly significant delay in progression and decreased severity of cardiac dysfunction in the mouse HF model. The therapeutic effect occurs via inhibition of activation and cardiac infiltration of T cells and macrophages, leading to reduced cardiomyocyte death. Abatacept treatment also induces production of anti-inflammatory cytokine interleukin-10 (IL-10). IL-10-deficient mice are refractive to treatment, while protection could be rescued by transfer of IL-10-sufficient B cells. These results suggest that T cell costimulation blockade might be therapeutically exploited to treat HF.

Exercise training reverses myocardial dysfunction induced by CaMKIIδC overexpression by restoring Ca2+ homeostasis.

Several conditions of heart disease, including heart failure and diabetic cardiomyopathy, are associated with upregulation of cytosolic Ca(2+)/calmodulin-dependent protein kinase II (CaMKIIδC) activity. In the heart, CaMKIIδC isoform targets several proteins involved in intracellular Ca(2+) homeostasis. We hypothesized that high-intensity endurance training activates mechanisms that enable a rescue of dysfunctional cardiomyocyte Ca(2+) handling and thereby ameliorate cardiac dysfunction despite continuous and chronic elevated levels of CaMKIIδC CaMKIIδC transgenic (TG) and wild-type (WT) mice performed aerobic interval exercise training over 6 wk. Cardiac function was measured by echocardiography in vivo, and cardiomyocyte shortening and intracellular Ca(2+) handling were measured in vitro. TG mice had reduced global cardiac function, cardiomyocyte shortening (47% reduced compared with WT, P < 0.01), and impaired Ca(2+) homeostasis. Despite no change in the chronic elevated levels of CaMKIIδC, exercise improved global cardiac function, restored cardiomyocyte shortening, and reestablished Ca(2+) homeostasis to values not different from WT. The key features to explain restored Ca(2+) homeostasis after exercise training were increased L-type Ca(2+) current density and flux by 79 and 85%, respectively (P < 0.01), increased sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) function by 50% (P < 0.01), and reduced diastolic SR Ca(2+) leak by 73% (P < 0.01), compared with sedentary TG mice. In conclusion, exercise training improves global cardiac function as well as cardiomyocyte function in the presence of a maintained high CaMKII activity. The main mechanisms of exercise-induced improvements in TG CaMKIIδC mice are mediated via increased L-type Ca(2+) channel currents and improved SR Ca(2+) handling by restoration of SERCA2a function in addition to reduced diastolic SR Ca(2+) leak.

Chronic CaMKII inhibition blunts the cardiac contractile response to exercise training.

Activation of the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays a critical role modulating cardiac function in both health and disease. Here, we determined the effect of chronic CaMKII inhibition during an exercise training program in healthy mice. CaMKII was inhibited by KN-93 injections. Mice were randomized to the following groups: sham sedentary, sham exercise, KN-93 sedentary, and KN-93 exercise. Cardiorespiratory function was evaluated by ergospirometry during treadmill running, echocardiography, and cardiomyocyte fractional shortening and calcium handling. The results revealed that KN-93 alone had no effect on exercise capacity or fractional shortening. In sham animals, exercise training increased maximal oxygen uptake by 8% (p < 0.05) compared to a 22% (p < 0.05) increase after exercise in KN-93 treated mice (group difference p < 0.01). In contrast, in vivo fractional shortening evaluated by echocardiography improved after exercise in sham animals only: from 25 to 32% (p < 0.02). In inactive mice, KN-93 reduced rates of diastolic cardiomyocyte re-lengthening (by 25%, p < 0.05) as well as Ca(2+) transient decay (by 16%, p < 0.05), whereas no such effect was observed after exercise training. KN-93 blunted exercise training response on cardiomyocyte fractional shortening (63% sham vs. 18% KN-93; p < 0.01 and p < 0.05, respectively). These effects could not be solely explained by the Ca(2+) transient amplitude, as KN-93 reduced it by 20% (p < 0.05) and response to exercise training was equal (64% sham and 47% KN-93; both p < 0.01). We concluded that chronic CaMKII inhibition increased time to 50% re-lengthening which were recovered by exercise training, but paradoxically led to a greater increase in maximal oxygen uptake compared to sham mice. Thus, the effect of chronic CaMKII inhibition is multifaceted and of a complex nature.

Aerobic interval training increases peak oxygen uptake more than usual care exercise training in myocardial infarction patients: a randomized controlled study.

OBJECTIVE: Exercise capacity strongly predicts survival and aerobic interval training (AIT) increases peak oxygen uptake effectively in cardiac patients. Usual care in Norway provides exercise training at the hospitals following myocardial infarction (MI), but the effect and actual intensity of these rehabilitation programmes are unknown. DESIGN: Randomized controlled trial. SETTING: Hospital cardiac rehabilitation. SUBJECTS: One hundred and seven patients, recruited two to 12 weeks after MI, were randomized to usual care rehabilitation or treadmill AIT. INTERVENTIONS: Usual care aerobic group exercise training or treadmill AIT as 4 × 4 minutes intervals at 85-95% of peak heart rate. Twice weekly exercise training for 12 weeks. MAIN MEASURES: The primary outcome measure was peak oxygen uptake. Secondary outcome measures were endothelial function, blood markers of cardiovascular disease, quality of life, resting heart rate, and heart rate recovery. RESULTS: Eighty-nine patients (74 men, 15 women, 57.4 ± 9.5 years) completed the programme. Peak oxygen uptake increased more (P = 0.002) after AIT (from 31.6 ± 5.8 to 36.2 ± 8.6 mL·kg(-1)·min(-1), P < 0.001) than after usual care rehabilitation (from 32.2 ± 6.7 to 34.7 ± 7.9 mL·kg(-1)·min(-1), P < 0.001). The AIT group exercised with significantly higher intensity in the intervals compared to the highest intensity in the usual care group (87.3 ± 3.9% versus 78.7 ± 7.2% of peak heart rate, respectively, P < 0.001). Both programmes increased endothelial function, serum adiponectin, and quality of life, and reduced serum ferritin and resting heart rate. High-density lipoprotein cholesterol increased only after AIT. CONCLUSIONS: AIT increased peak oxygen uptake more than the usual care rehabilitation provided to MI patients by Norwegian hospitals.

Time course of endothelial adaptation after acute and chronic exercise in patients with metabolic syndrome.

Clustering of cardiovascular risk factors may lead to endothelial dysfunction. Physical exercise is an important factor in prevention and treatment of endothelial dysfunction. We wanted to determine the time course of adaptation to a single bout of exercise at either high or moderate intensity upon endothelial function both before and after a 16-week fitness program in patients with metabolic syndrome. Twenty-eight patients with metabolic syndrome participated in the study and were randomized and stratified (according to age and sex) into an aerobic interval exercise training group (AIT, n = 11), a continuously moderate-intensity exercise training group (CME, n = 8) or to a control group (n = 9). Flow-mediated dilatation (FMD) was determined at baseline, immediately, 24, 48, and 72 hours after 1 bout of exercise and repeated after 16 weeks of exercise. In the untrained state, FMD improved from 5 to 11% (p = 0.003) immediately after a single bout of aerobic interval training (AIT), an effect lasting 72 hours postexercise. In comparison, continuous moderate exercise (CME) improved FMD immediately after a single bout of exercise from 5 to 8% (p = 0.02), an effect lasting 24 hours postexercise (group difference, p < 0.001). In the trained state, a single bout of AIT resulted in a 2% (p = 0.007) acute increase of FMD lasting 48 hours postexercise. The CME increased FMD by 3% (p < 0.01), an effect lasting 24 hours postexercise (group difference p = 0.0012). Blood glucose level decreased after 1 single bout of AIT in the untrained state (p < 0.05), and the effect lasted at least 72 hours postexercise (p < 0.01). Acute CME decreased blood glucose with normalization of the values 24 hours postexercise (p < 0.01). A single bout of exercise in the trained state reduced fasting blood glucose by 10% (p < 0.05) after both AIT and CME. Exercise training, especially high intensity, thus appears to be highly beneficial in reducing blood glucose and improving endothelial function.

Interval training normalizes cardiomyocyte function, diastolic Ca2+ control, and SR Ca2+ release synchronicity in a mouse model of diabetic cardiomyopathy.

RATIONALE: In the present study we explored the mechanisms behind excitation-contraction (EC) coupling defects in cardiomyocytes from mice with type-2 diabetes (db/db). OBJECTIVE: We determined whether 13 weeks of aerobic interval training could restore cardiomyocyte Ca(2+) cycling and EC coupling. METHODS AND RESULTS: Reduced contractility in cardiomyocytes isolated from sedentary db/db was associated with increased diastolic sarcoplasmic reticulum (SR)-Ca(2+) leak, reduced synchrony of Ca(2+) release, reduced transverse (T)-tubule density, and lower peak systolic and diastolic Ca(2+) and caffeine-induced Ca(2+) release. Additionally, the rate of SR Ca(2+) ATPase-mediated Ca(2+) uptake during diastole was reduced, whereas a faster recovery from caffeine-induced Ca(2+) release indicated increased Na(+)/Ca(2+)-exchanger activity. The increased SR-Ca(2+) leak was attributed to increased Ca(2+)-calmodulin-dependent protein kinase (CaMKIIdelta) phosphorylation, supported by the normalization of SR-Ca(2+) leak on inhibition of CaMKIIdelta (AIP). Exercise training restored contractile function associated with restored SR Ca(2+) release synchronicity, T-tubule density, twitch Ca(2+) amplitude, SR Ca(2+) ATPase and Na(+)/Ca(2+)-exchanger activities, and SR-Ca(2+) leak. The latter was associated with reduced phosphorylation of cytosolic CaMKIIdelta. Despite normal contractile function and Ca(2+) handling after the training period, phospholamban was hyperphosphorylated at Serine-16. Protein kinase A inhibition (H-89) in cardiomyocytes from the exercised db/db group abolished the differences in SR-Ca(2+) load when compared with the sedentary db/db mice. EC coupling changes were observed without changes in serum insulin or glucose levels, suggesting that the exercise training-induced effects are not via normalization of the diabetic condition. CONCLUSIONS: These data demonstrate that aerobic interval training almost completely restored the contractile function of the diabetic cardiomyocyte to levels close to sedentary wild type.

Endothelial dysfunction induced by post-prandial lipemia: complete protection afforded by high-intensity aerobic interval exercise.

OBJECTIVES: This study was designed to study the effect of exercise and a high-fat meal (HFM) on endothelial function. BACKGROUND: Post-prandial lipemia and exercise oppose each other in terms of cardiovascular risk; however, the mechanism of their interaction is not well understood. METHODS: Endothelial function was assessed by brachial artery flow-mediated dilation (FMD) in 8 healthy men before and after an HFM preceded (16 to 18 h) by rest, a single bout of continuous moderate-intensity exercise (CME), and high-intensity interval exercise (HIIE). RESULTS: Before the HFM, initial brachial artery diameters were similar in all trials (0.43 +/- 0.04 cm), but after the HFM, basal diameter decreased only in the control (0.39 +/- 0.03 cm) and CME (0.38 +/- 0.04 cm) trials. Before the HFM, FMD/shear was improved by a single bout of CME (+20%, p < 0.01) and HIIE (+45%, p < 0.01; group differences, p < 0.01), with no effect in the control trial. After the HFM (30, 120, and 240 min), FMD decayed to a lesser extent with CME, but in a similar fashion to the control trial. In contrast, FMD in the HIIE trial remained elevated following the exercise despite a clear meal-induced lipemia. Although there were no correlations between vascular function and food-induced markers of cardiovascular risk, antioxidant status was strongly correlated with FMD (r = 0.9, p < 0.001). CONCLUSIONS: These findings reveal a clinically relevant protective effect of acute exercise on the vasculature that is clearly exercise intensity dependent and tightly related to exercise-induced antioxidant capacity. (Endothelial Dysfunction Induced by Postprandial Lipemia; NCT00660491).

Transcriptional changes in blood after aerobic interval training in patients with the metabolic syndrome.

BACKGROUND: Regular physical activity has beneficial effects on the metabolic syndrome. Eleven metabolic syndrome patients performing 16 weeks of aerobic interval training, significantly reduced their risk of cardiovascular disease, in terms of improved VO2max, endothelial function, blood pressure, insulin signaling, and plasma lipid composition. The knowledge on underlying mechanism of exercise-induced improvements is sparse, and a broad spectrum of methods is needed to gain more insight. DESIGN: The aim was, for the first time, to determine whether transcriptional changes occur in blood cells of metabolic syndrome patients after participating in an exercise program. METHODS: Blood was collected in PAXgene and EDTA tubes before and after 16 weeks of exercise. RNA was extracted and run on microarrays. RESULTS: Eleven biological processes and molecular functions were upregulated after exercise, whereas seven were downregulated. Blood clotting, cell adhesion, and steroid metabolism were among the downregulated processes, whereas steroid hormone-mediated signaling was upregulated. Downregulated protein levels of arginase 1 and von Willebrand factor confirmed microarray results. CONCLUSION: Increased transcription of genes involved in steroid hormone-mediated signaling, decreased levels of arginase 1, and reduced transcription of genes involved in cell adhesion, and blood clotting are likely to be involved in exercise-induced improvements of endothelial function, and improved cardiovascular risk profile of metabolic syndrome patients. These findings have provided new insights on exercise-induced improvement of cardiovascular health.

Aerobic interval training reduces cardiovascular risk factors more than a multitreatment approach in overweight adolescents.

The aim of the present study was to compare the effects of a multidisciplinary approach (MTG) and aerobic interval training (AIT) on cardiovascular risk factors in overweight adolescents. A total of 62 overweight and obese adolescents from Trøndelag County in Norway, referred to medical treatment at St Olav's Hospital, Trondheim, Norway, were invited to participate. Of these, 54 adolescents (age, 14.0 +/- 0.3 years) were randomized to either AIT (4 x 4 min intervals at 90% of maximal heart rate, each interval separated by 3 min at 70%, twice a week for 3 months) or to MTG (exercise, dietary and psychological advice, twice a month for 12 months). Follow-up testing occurred at 3 and 12 months. VO(2max) (maximal oxygen uptake) increased more after AIT compared with MTG, both at 3 months (11 compared with 0%; P<0.01) and 12 months (12 compared with -1%; P<0.01). AIT enhanced endothelial function compared with MTG at both 3 months (absolute change, 5.1 compared with 3.9%; P<0.01) and 12 months (absolute change, 6.3 compared with 1.0%; P<0.01). AIT was favourable compared with MTG in reducing BMI (body mass index), percentage of fat, MAP (mean arterial blood pressure) and increasing peak oxygen pulse. In addition, AIT induced a more favourable regulation of blood glucose and insulin compared with MTG. In conclusion, the novel findings of the present proof-of-concept study was that 3 months of twice weekly high-intensity exercise sessions reduced several known cardiovascular risk factors in obese adolescents more than that observed after a multitreatment strategy, which was initiated as hospital treatment. Follow-up at 12 months confirmed that AIT improved or maintained these risk factors to a better degree than MTG.

Aerobic interval training versus continuous moderate exercise as a treatment for the metabolic syndrome: a pilot study.

BACKGROUND: Individuals with the metabolic syndrome are 3 times more likely to die of heart disease than healthy counterparts. Exercise training reduces several of the symptoms of the syndrome, but the exercise intensity that yields the maximal beneficial adaptations is in dispute. We compared moderate and high exercise intensity with regard to variables associated with cardiovascular function and prognosis in patients with the metabolic syndrome. METHODS AND RESULTS: Thirty-two metabolic syndrome patients (age, 52.3+/-3.7 years; maximal oxygen uptake [o(2)max], 34 mL x kg(-1) x min(-1)) were randomized to equal volumes of either moderate continuous moderate exercise (CME; 70% of highest measured heart rate [Hfmax]) or aerobic interval training (AIT; 90% of Hfmax) 3 times a week for 16 weeks or to a control group. o(2)max increased more after AIT than CME (35% versus 16%; P<0.01) and was associated with removal of more risk factors that constitute the metabolic syndrome (number of factors: AIT, 5.9 before versus 4.0 after; P<0.01; CME, 5.7 before versus 5.0 after; group difference, P<0.05). AIT was superior to CME in enhancing endothelial function (9% versus 5%; P<0.001), insulin signaling in fat and skeletal muscle, skeletal muscle biogenesis, and excitation-contraction coupling and in reducing blood glucose and lipogenesis in adipose tissue. The 2 exercise programs were equally effective at lowering mean arterial blood pressure and reducing body weight (-2.3 and -3.6 kg in AIT and CME, respectively) and fat. CONCLUSIONS: Exercise intensity was an important factor for improving aerobic capacity and reversing the risk factors of the metabolic syndrome. These findings may have important implications for exercise training in rehabilitation programs and future studies.