Morten Høydal

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 Ca2+ cycling and EC coupling. Methods and Results: Reduced contractility in cardiomyocytes isolated from sedentary db/db was associated with increased diastolic sarcoplasmic reticulum (SR)-Ca2+ leak, reduced synchrony of Ca2+ release, reduced transverse (T)-tubule density, and lower peak systolic and diastolic Ca2+ and caffeine-induced Ca2+ release. Additionally, the rate of SR Ca2+ ATPase–mediated Ca2+ uptake during diastole was reduced, whereas a faster recovery from caffeine-induced Ca2+ release indicated increased Na+/Ca2+-exchanger activity. The increased SR-Ca2+ leak was attributed to increased Ca2+-calmodulin–dependent protein kinase (CaMKII&dgr;) phosphorylation, supported by the normalization of SR-Ca2+ leak on inhibition of CaMKII&dgr; (AIP). Exercise training restored contractile function associated with restored SR Ca2+ release synchronicity, T-tubule density, twitch Ca2+ amplitude, SR Ca2+ ATPase and Na+/Ca2+-exchanger activities, and SR-Ca2+ leak. The latter was associated with reduced phosphorylation of cytosolic CaMKII&dgr;. Despite normal contractile function and Ca2+ 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-Ca2+ 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.

Running speed and maximal oxygen uptake in rats and mice: practical implications for exercise training.

Valid and reliable experimental models are essential to gain insight into the cellular and molecular mechanisms underlying the beneficial effects of exercise in prevention, treatment, and rehabilitation of lifestyle-related diseases. Studies with large changes, low variation, and reproducible training outcome require individualized training intensity, controlled by direct measurements of maximal oxygen uptake or heart rate. As this approach is expensive and time consuming, we discuss whether maximal treadmill running speed in a gradually increasing ramp protocol might be sufficient to control intensity without lossing accuracy. Combined data from six studies of rats and mice from our lab demonstrated a close correlation between running speed and oxygen uptake. This relationship changed towards a steeper linear slope after endurance training, indicating improved work economy, that is, less oxygen was consumed at fixed submaximal running speeds. Maximal oxygen uptake increased 40-70% after high-intensity aerobic interval training in mice and rats. The speed at which oxygen uptake reached a plateau, increased in parallel with the change in maximal oxygen uptake during the training period. Although this suggests that running speed can be used to assess training intensity throughout a training program, the problem is to determine the exact relative intensity related to maximal oxygen uptake from running speed alone. We therefore suggest that directly measured oxygen uptake should be used to assess exercise intensity and optimize endurance training in rats and mice. Running speed may serve as a supplement to ensure this intensity. Eur J Cardiovasc Prev Rehabil 14: 753-760

Intrinsic Aerobic Capacity Sets a Divide for Aging and Longevity

Rationale: Low aerobic exercise capacity is a powerful predictor of premature morbidity and mortality for healthy adults as well as those with cardiovascular disease. For aged populations, poor performance on treadmill or extended walking tests indicates closer proximity to future health declines. Together, these findings suggest a fundamental connection between aerobic capacity and longevity. Objectives: Through artificial selective breeding, we developed an animal model system to prospectively test the association between aerobic exercise capacity and survivability (aerobic hypothesis). Methods and Results: Laboratory rats of widely diverse genetic backgrounds (N:NIH stock) were selectively bred for low or high intrinsic (inborn) treadmill running capacity. Cohorts of male and female rats from generations 14, 15, and 17 of selection were followed for survivability and assessed for age-related declines in cardiovascular fitness including maximal oxygen uptake (VO2max), myocardial function, endurance performance, and change in body mass. Median lifespan for low exercise capacity rats was 28% to 45% shorter than high capacity rats (hazard ratio, 0.06; P<0.001). VO2max, measured across adulthood was a reliable predictor of lifespan (P<0.001). During progression from adult to old age, left ventricular myocardial and cardiomyocyte morphology, contractility, and intracellular Ca2+ handling in both systole and diastole, as well as mean blood pressure, were more compromised in rats bred for low aerobic capacity. Physical activity levels, energy expenditure (VO2), and lean body mass were all better sustained with age in rats bred for high aerobic capacity. Conclusions: These data obtained from a contrasting heterogeneous model system provide strong evidence that genetic segregation for aerobic exercise capacity can be linked with longevity and are useful for deeper mechanistic exploration of aging.

Effects of preterm birth and fetal growth retardation on cardiovascular risk factors in young adulthood

BACKGROUND The association between low birth weight (LBW) and increased risk of obesity, hypertension and cardiovascular disease later in life is well documented in epidemiological studies. However, clinical follow-up studies of LBW populations have only partly supported this. AIMS Evaluate associations between LBW and body fat, blood pressure (BP), lung and endothelial function, and maximal oxygen uptake (VO(2max)) in 18 year old young adults. SUBJECTS Thirty-seven subjects born prematurely with birth weight <1501 g (VLBW group), 47 born at term with low weight (<10th centile) for gestational age (SGA group) and 63 controls with normal birth weight participated in the study. OUTCOME MEASURES Anthropometric measurements, BP, endothelial function, lung function and VO(2max) were recorded. RESULTS Both LBW groups were shorter, lighter, had smaller head circumference and higher subscapular-to-triceps skinfold-ratio than controls. Systolic and mean arterial BP was higher in the VLBW compared with the control group, whereas there were no differences between the groups in endothelial function. The VLBW group had reduced dynamic lung volumes lower carbon monoxide transfer factor and lower VO(2max) compared with controls. In particular young adults born VLBW who were also growth retarded in utero had higher indices of central body fat, higher BP and lower VO(2max). CONCLUSION We found that very preterm birth, but not growth retardation at term, was associated with higher BP and a less favourable fat distribution. In particular, the young adults born VLBW who were also growth retarded in utero had less favourable outcomes.

Remote Ischemic Preconditioning Preserves Mitochondrial Function and Influences Myocardial MicroRNA Expression in Atrial Myocardium During Coronary Bypass Surgery

Rationale: Remote ischemic preconditioning (RIPC) has been suggested to induce cardioprotection during cardiac surgery. Maintaining proper atrial function is imperative in preventing arrhythmia and thrombus formation. Mitochondria have been proposed as key targets in conveying RIPC mechanisms and effects. MicroRNA (miR) is emerging as an important regulator of mitochondrial function, arrhythmia, and protection from ischemia and reperfusion. Objective: This study aimed to evaluate the effect of RIPC on mitochondrial respiration and miR expression in human atrial tissue. Methods and Results: Sixty patients undergoing coronary artery bypass graft surgery were randomized to RIPC (n=30) or control (n=30). RIPC was performed preoperatively by inflating a blood pressure cuff on the upper arm to 200 mm Hg for 3×5 minutes, with 5 minutes reperfusion intervals. Biopsies were obtained from the right atrial appendage before and after aortic cross-clamping. Mitochondrial respiration was measured in situ and miR assessed by commercial miR array and quantitative reverse transcription polymerase chain reaction. Postoperative atrial fibrillation occurrence was monitored by biotelemetry. Maximal mitochondrial respiration was preserved throughout surgery after RIPC but significantly reduced (−28%; P<0.05) after aortic cross-clamping in control. Incidence of postoperative atrial fibrillation was lower after RIPC versus control (14% versus 50%; P<0.01). Myocardial expression of miR-133a and miR-133b increased after aortic cross-clamping in both RIPC and control, whereas miR-1 was upregulated in control only. MiR-338-3p expression was higher in RIPC versus control after aortic cross-clamping. Conclusions: RIPC preserves mitochondrial respiration and prevents upregulation of miR-1 in the right atrium during coronary artery bypass graft. Clinical Trial Registration: URL: http://www.clinicaltrials.gov. Unique identifier: NCT01308138

Aerobic capacity-dependent differences in cardiac gene expression

Aerobic capacity is a strong predictor of cardiovascular mortality. To determine the relationship between inborn aerobic capacity and cardiac gene expression we examined genome-wide gene expression in hearts of rats artificially selected for high and low running capacity (HCR and LCR, respectively) over 16 generations. The artificial selection of LCR caused accumulation of risk factors of cardiovascular disease similar to the metabolic syndrome seen in human, whereas HCR had markedly better cardiac function. We also studied alterations in gene expression in response to exercise training in these animals. Left ventricle gene expression of both sedentary and exercise-trained HCR and LCR was characterized by microarray and gene ontology analysis. Out of 28,000 screened genes, 1,540 were differentially expressed between sedentary HCR and LCR. Only one gene was found differentially expressed by exercise training, but this gene had unknown name and function. Sedentary HCR expressed higher amounts of genes involved in lipid metabolism, whereas sedentary LCR expressed higher amounts of the genes involved in glucose metabolism. This suggests a switch in cardiac energy substrate utilization from normal mitochondrial fatty acid beta-oxidation in HCR to carbohydrate metabolism in LCR, an event that often occurs in diseased hearts. LCR were also associated with pathological growth signaling and cellular stress. Hypoxic conditions seemed to be a common source for several of these observations, triggering hypoxia-induced alterations of transcription. In conclusion, inborn high vs. low aerobic capacity was associated with differences in cardiac energy substrate, growth signaling, and cellular stress.

The effect of exercise training on transverse tubules in normal, remodeled, and reverse remodeled hearts

The response of transverse (T)‐tubules to exercise training in health and disease remains unclear. Therefore, we studied the effect of exercise training on the density and spacing of left ventricle cardiomyocyte T‐tubules in normal and remodeled hearts that associate with detubulation, by confocal laser scanning microscopy. First, exercise training in normal rats increased cardiomyocyte volume by 16% (P < 0.01), with preserved T‐tubule density. Thus, the T‐tubules adapted to the physiologic hypertrophy. Next, we studied T‐tubules in a rat model of metabolic syndrome with pressure overload‐induced concentric left ventricle hypertrophy, evidenced by 15% (P < 0.01) increased cardiomyocyte size. These rats had only 85% (P < 0.01) of the T‐tubule density of control rats. Exercise training further increased cardiomyocyte volume by 8% (P < 0.01); half to that in control rats, but the T‐tubule density remained unchanged. Finally, post‐myocardial infarction heart failure induced severe cardiac pathology, with a 70% (P < 0.01) increased cardiomyocyte volume that included both eccentric and concentric hypertrophy and 55% (P < 0.01) reduced T‐tubule density. Exercise training reversed 50% (P < 0.01) of the pathologic hypertrophy, whereas the T‐tubule density increased by 40% (P < 0.05) compared to sedentary heart failure, but remained at 60% of normal hearts (P < 0.01). Physiologic hypertrophy associated with conserved T‐tubule spacing (∼1.8–1.9 µm), whereas in pathologic hypertrophy, T‐tubules appeared disorganized without regular spacing. In conclusion, cardiomyocytes maintain the relative T‐tubule density during physiologic hypertrophy and after mild concentric pathologic hypertrophy, whereas after severe pathologic remodeling with a substantial loss of T‐tubules; exercise training reverses the remodeling and partly corrects the T‐tubule density. J. Cell. Physiol. 226: 2235–2243, 2011.

Nitric oxide synthase type-1 modulates cardiomyocyte contractility and calcium handling: association with low intrinsic aerobic capacity.

Background The neuronal isoform of nitric oxide synthase (NOS-1) may be an important regulator of cardiac contractility by modifying calcium release and uptake from sarcoplasmic reticulum. Our working hypothesis was that NOS-1 modulates cardiomyocyte contractility more markedly in rat lines with low versus high congenital aerobic fitness. Methods and results Rats performed high-intensity interval treadmill running 5 days per week over 8 weeks; age-matched sedentary rats served as controls. At baseline before the training program, aerobic fitness measured as maximal oxygen uptake was 30% higher, and cardiomyocyte contractility measured as fractional shortening 42% higher in high than in low congenital aerobic fitness rats. Training markedly increased aerobic fitness as well as cardiomyocyte contractility, relaxation and corresponding changes in calcium transient in both lines. Selective inhibition of NOS-1 increased cardiomyocyte contractility (12–43%) and calcium transient amplitude (10–28%), prolonged time to 50% relengthening (13–52%) and time to 50% calcium decay (17–35%), in all groups. Interestingly, NOS-1-inhibition abolished the difference in systolic events between low and high congenital aerobic fitness whereas no such findings occurred in diastolic parameters. Conclusion NOS-1-derived nitric oxide production is a modulator of cardiomyocyte contractile performance and calcium handling in rats. It accounts for some of the difference between rats with low versus high congenital aerobic fitness, whereas it contributes little during adaptation to exercise training.

Gene expression profiling of skeletal muscle in exercise-trained and sedentary rats with inborn high and low VO2max

The relationship between inborn maximal oxygen uptake (VO(2max)) and skeletal muscle gene expression is unknown. Since low VO(2max) is a strong predictor of cardiovascular mortality, genes related to low VO(2max) might also be involved in cardiovascular disease. To establish the relationship between inborn VO(2max) and gene expression, we performed microarray analysis of the soleus muscle of rats artificially selected for high- and low running capacity (HCR and LCR, respectively). In LCR, a low VO(2max) was accompanied by aggregation of cardiovascular risk factors similar to the metabolic syndrome. Although sedentary HCR were able to maintain a 120% higher running speed at VO(2max) than sedentary LCR, only three transcripts were differentially expressed (FDR <or=0.05) between the groups. Sedentary LCR expressed high levels of a transcript with strong homology to human leucyl-transfer RNA synthetase, of whose overexpression has been associated with a mutation linked to mitochondrial dysfunction. Moreover, we studied exercise-induced alterations in soleus gene expression, since accumulating evidence indicates that long-term endurance training has beneficial effects on the metabolic syndrome. In terms of gene expression, the response to exercise training was more pronounced in HCR than LCR. HCR upregulated several genes associated with lipid metabolism and fatty acid elongation, whereas LCR upregulated only one transcript after exercise training. The results indicate only minor differences in soleus muscle gene expression between sedentary HCR and LCR. However, the inborn level of fitness seems to influence the transcriptional adaption to exercise, as more genes were upregulated after exercise training in HCR than LCR.

Remote ischemic preconditioning preserves mitochondrial function and activates pro-survival protein kinase Akt in the left ventricle during cardiac surgery: A randomized trial

BACKGROUND Understanding the intracellular mechanisms induced by remote ischemic preconditioning (RIPC) in the human left ventricle opens new possibilities for development of pharmacological cardioprotection against ischemia and reperfusion injury. In this study we investigated the effects of RIPC on mitochondrial function, activation of pro-survival protein kinase Akt and microRNA expression in left ventricular biopsies from patients undergoing coronary artery bypass surgery (CABG). METHODS Sixty patients were randomized to control (n=30) or RIPC (n=30). A blood pressure cuff was applied to the arm of all patients preoperatively. The cuff remained deflated in control group, whereas RIPC was performed by 3 cycles of cuff inflation to 200 mm Hg for 5 min, separated by 5 min deflation intervals. Left ventricular biopsies were obtained before and 15 min after aortic declamping. The primary outcome was mitochondrial respiration measured in situ. Secondary outcomes were activation of protein kinase Akt, assessed by western immunoblotting, and expression of microRNAs assessed by array and real-time polymerase chain reaction. RESULTS Mitochondrial respiration was preserved during surgery in patients receiving RIPC (+0.2 μmol O2/min/g, p=0.69), and reduced by 15% in controls (-1.5 μmol O2/min/g, p=0.02). Furthermore, RIPC activated protein kinase Akt before aortic clamping (difference from control +43.3%, p=0.04), followed by increased phosphorylation of Akt substrates at reperfusion (+26.8%, p<0.01). No differences were observed in microRNA expression. CONCLUSIONS RIPC preserves mitochondrial function and activates pro-survival protein kinase Akt in left ventricle of patients undergoing CABG. Modulation of mitochondrial function and Akt activation should be further explored as cardioprotective drug targets. CLINICAL TRIAL REGISTRATION http://www.clinicaltrials.gov, unique identifier: NCT01308138.