Reduced susceptibility to ventricular tachyarrhythmias in rats selectively bred for high aerobic capacity

Influence of Intrinsic Aerobic Exercise Capacity and Sex on Cardiac Injury Following Acute Myocardial Ischemia and Reperfusion

Purpose: Previous reports have suggested that active exercise aside, intrinsic aerobic running capacity (Low = LCR, high = HCR) in otherwise sedentary animals may influence several cardiovascular health-related indicators. Relative to the HCR phenotype, the LCR phenotype is characterized by decreased endothelial reactivity, increased susceptibility to reperfusion-induced arrhythmias following short, non-infarction ischemia, and increased diet-induced insulin resistance. More broadly, the LCR phenotype has come to be characterized as a "disease prone" model, with the HCRs as "disease resistant." Whether these effects extend to injury outcomes in an overt infarction or whether the effects are gender specific is not known. This study was designed to determine whether HCR/LCR phenotypic differences would be evident in injury responses to acute myocardial ischemia-reperfusion injury (AIR), measured as infarct size and to determine whether sex differences in infarction size were preserved with phenotypic selection. Methods: Regional myocardial AIR was induced in vivo by either 15 or 30 min ligation of the left anterior descending coronary artery, followed by 2 h of reperfusion. Global ischemia was induced in isolated hearts ex vivo using a Langendorff perfusion system and cessation of perfusion for either 15 or 30 min followed by 2 h of reperfusion. Infarct size was determined using 2, 3, 5-triphenyltetrazolium chloride (TTC) staining, and normalized to area at risk in the regional model, or whole heart in the global model. Portions of the tissue were paraffin embedded for H&E staining and histology analysis. Results: Phenotype dependent differences in infarct size were seen with 15 min occlusion/2 h reperfusion (LCR > HCR, p < 0.05) in both regional and global models. In both models, longer occlusion times (30 min/2 h) produced significantly larger infarctions in both phenotypes, but phenotypic differences were no longer present (LCR vs. HCR, p = n.s.). Sex differences in infarct size were present in each phenotype (LCR male > LCR female, p < 0.05; HCR male > HCR female, p < 0.05 regardless of length of occlusion, or ischemia model. Conclusions: There is cardioprotection afforded by high intrinsic aerobic capacity, but it is not infinite/continuous, and may be overcome with sufficient injury burden. Phenotypic selection based on endurance running capacity preserved sex differences in response to both short and longer term coronary occlusive challenges. Outcomes could not be associated with differences in system characteristics such as circulating inflammatory mediators or autonomic nervous system influences, as similar phenotypic injury patterns were seen in vivo, and in isolated crystalloid perfused heart ex vivo.

Rats with elevated genetic risk for metabolic syndrome exhibit cognitive deficiencies when young

Metabolic syndrome (MetS) is a known risk factor for cognitive decline. Using polygenic rat models selectively bred for high and low intrinsic exercise capacity and simultaneously modelling as low and high innate risk factor for MetS respectively, we have previously shown that adult animals with lower exercise capacity/higher MetS risk perform poorly in tasks requiring flexible cognition. However, it is not known whether these deficits in cognition are present already at young age. Also, it is unclear whether the high risk genome is related also to lower-level cognition, such as sensory gating measured as prepulse inhibition. In this study, young and adult (5-8 weeks and ~9 months) rats selectively bred for 36 generations as High-Capacity Runners (HCR) or Low-Capacity Runners (LCR) were tested for behavior in an open field task, modulation of startle reflex, and spatial learning in a T-maze. HCR rats were more active in the open field than LCR rats independent of age. Responses to the startle stimulus habituated to the same extent in LCR compared to HCR rats when young, but as adults, stronger habituation was seen in the HCR animals. The prepulse inhibition of startle response was equally strong in young HCR and LCR animals but the effect was shorter lasting in HCR animals. In T-maze, adult HCR animals unexpectedly showed attenuated learning, but we interpret this finding to stem from differences in motivation rather than learning ability. Overall, in the LCR rats with the risk genome for poor aerobic fitness and MetS, indications of compromised cognitive function are present already at a young age.

Mitochondrial health is enhanced in rats with higher vs. lower intrinsic exercise capacity and extended lifespan

The intrinsic aerobic capacity of an organism is thought to play a role in aging and longevity. Maximal respiratory rate capacity, a metabolic performance measure, is one of the best predictors of cardiovascular- and all-cause mortality. Rats selectively bred for high-(HCR) vs. low-(LCR) intrinsic running-endurance capacity have up to 31% longer lifespan. We found that positive changes in indices of mitochondrial health in cardiomyocytes (respiratory reserve, maximal respiratory capacity, resistance to mitochondrial permeability transition, autophagy/mitophagy, and higher lipids-over-glucose utilization) are uniformly associated with the extended longevity in HCR vs. LCR female rats. Cross-sectional heart metabolomics revealed pathways from lipid metabolism in the heart, which were significantly enriched by a select group of strain-dependent metabolites, consistent with enhanced lipids utilization by HCR cardiomyocytes. Heart–liver–serum metabolomics further revealed shunting of lipidic substrates between the liver and heart via serum during aging. Thus, mitochondrial health in cardiomyocytes is associated with extended longevity in rats with higher intrinsic exercise capacity and, probably, these findings can be translated to other populations as predictors of outcomes of health and survival.

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.

Mice in Bion-M 1 space mission: training and selection

After a 16-year hiatus, Russia has resumed its program of biomedical research in space, with the successful 30-day flight of the Bion-M 1 biosatellite (April 19-May 19, 2013). The principal species for biomedical research in this project was the mouse. This paper presents an overview of the scientific goals, the experimental design and the mouse training/selection program. The aim of mice experiments in the Bion-M 1 project was to elucidate cellular and molecular mechanisms, underlying the adaptation of key physiological systems to long-term exposure in microgravity. The studies with mice combined in vivo measurements, both in flight and post-flight (including continuous blood pressure measurement), with extensive in vitro studies carried out shortly after return of the mice and in the end of recovery study. Male C57/BL6 mice group housed in space habitats were flown aboard the Bion-M 1 biosatellite, or remained on ground in the control experiment that replicated environmental and housing conditions in the spacecraft. Vivarium control groups were used to account for housing effects and possible seasonal differences. Mice training included the co-adaptation in housing groups and mice adaptation to paste food diet. The measures taken to co-adapt aggressive male mice in housing groups and the peculiarities of "space" paste food are described. The training program for mice designated for in vivo studies was broader and included behavioral/functional test battery and continuous behavioral measurements in the home-cage. The results of the preliminary tests were used for the selection of homogenous groups. After the flight, mice were in good condition for biomedical studies and displayed signs of pronounced disadaptation to Earth's gravity. The outcomes of the training program for the mice welfare are discussed. We conclude that our training program was effective and that male mice can be successfully employed in space biomedical research.

Eszopiclone and dexmedetomidine depress ventilation in obese rats with features of metabolic syndrome

STUDY OBJECTIVES

Obesity alters the therapeutic window of sedative/hypnotic drugs and increases the probability of respiratory complications. The current experiments used an established rodent model of obesity to test the hypothesis that the sedative/hypnotic drugs eszopiclone and dexmedetomidine alter ventilation differentially in obese rats compared with lean/fit rats.

DESIGN

This study used a within-groups/between-groups experimental design.

SETTING

University of Michigan.

PARTICIPANTS

Experiments were conducted using lean/fit rats (n = 21) and obese rats (n = 21) that have features of metabolic syndrome.

INTERVENTIONS

Breathing was measured with whole-body plethysmography after systemic administration of vehicle (control), the nonbenzodiazepine, benzodiazepine site agonist eszopiclone, or the alpha-2 adrenergic receptor agonist dexmedetomidine.

MEASUREMENTS AND RESULTS

Data were analyzed using two-way analysis of variance and appropriate post hoc comparisons. At baseline, the obese/metabolic syndrome rats had increased respiratory rates (21.6%), lower tidal volumes/body weight (-24.1%), and no differences in minute ventilation compared to lean/fit rats. In the obese rats, respiratory rate was decreased by dexmedetomidine (-29%), but not eszopiclone. In the lean and the obese rats, eszopiclone decreased tidal volume (-12%). Both sedative/hypnotic drugs caused a greater decrease in minute ventilation in the obese (-26.3%) than lean (-18%) rats. Inspiratory flow rate (VT / TI) of the obese rats was decreased by dexmedetomidine (-10.6%) and eszopiclone (-18%). Duty cycle (TI / TTOT) in both rat lines was decreased by dexmedetomidine (-16.5%) but not by eszopiclone.

CONCLUSIONS

Dexmedetomidine, in contrast to eszopiclone, decreased minute ventilation in the obese/metabolic syndrome rats by depressing both duty cycle and inspiratory flow rate. The results show for the first time that the obese phenotype differentially modulates the respiratory effects of eszopiclone and dexmedetomidine. These differences in breathing are consistent with previously documented differences in sleep between lean/fit and obese rats. These findings also encourage future studies of obese/metabolic syndrome rats that quantify the effect of sedative/hypnotic drugs on respiratory mechanics as well as hypoxic and hypercapnic ventilatory responses. Continued findings of favorable homology between obese humans and rodents will support the interpretation that these obese rats offer a unique animal model for mechanistic studies.

Genetic analysis of a rat model of aerobic capacity and metabolic fitness

Aerobic capacity is a strong predictor of all-cause mortality and can influence many complex traits. To explore the biological basis underlying this connection, we developed via artificial selection two rat lines that diverge for intrinsic (i.e. inborn) aerobic capacity and differ in risk for complex disease traits. Here we conduct the first in-depth pedigree and molecular genetic analysis of these lines, the high capacity runners (HCR) and low capacity runners (LCR). Our results show that both HCR and LCR lines maintain considerable narrow-sense heritability (h²) for the running capacity phenotype over 28 generations (h² = 0.47 ± 0.02 and 0.43 ± 0.02, respectively). To minimize inbreeding, the lines were maintained by rotational mating. Pedigree records predict that the inbreeding coefficient increases at a rate of <1% per generation, ~37-38% slower than expected for random mating. Genome-wide 10K SNP genotype data for generations 5, 14, and 26 demonstrate substantial genomic evolution: between-line differentiation increased progressively, while within-line diversity deceased. Genome-wide average heterozygosity decreased at a rate of <1% per generation, consistent with pedigree-based predictions and confirming the effectiveness of rotational breeding. Linkage disequilibrium index r² decreases to 0.3 at ~3 Mb, suggesting that the resolution for mapping quantitative trait loci (QTL) can be as high as 2-3 cM. To establish a test population for QTL mapping, we conducted an HCR-LCR intercross. Running capacity of the F1 population (n=176) was intermediate of the HCR and LCR parentals (28 pairs); and the F2 population (n=645) showed a wider range of phenotypic distribution. Importantly, heritability in the F0-F2 pedigree remained high (h²~0.6). These results suggest that the HCR-LCR lines can serve as a valuable system for studying genomic evolution, and a powerful resource for mapping QTL for a host of characters relevant to human health.

Rats selectively bred for differences in aerobic capacity have similar hypertensive responses to chronic intermittent hypoxia

Exposure to chronic intermittent hypoxia (CIH) is an animal model that mimics the repetitive bouts of hypoxemia experienced by humans with sleep apnea. Rats exposed to CIH develop hypertension that depends on the activation of sympathetic nerve activity (SNA). Since obesity and metabolic syndrome have been linked to neurogenic hypertension and sleep apnea, and because sleep apnea can adversely affect aerobic exercise capacity, we tested the hypothesis that rats bred for selection of low aerobic capacity running (LCR) would have a greater hypertensive response to CIH than rats bred for high aerobic capacity running (HCR). Blockade of ganglionic transmission was performed to compare the contribution of SNA to the maintenance of resting mean arterial pressure (MAP). Next, hypertensive responses to 7 days of CIH were compared across LCR and HCR rats (14-16 mo old). Finally, the contribution of the hypothalamic paraventricular nucleus (PVN) to the maintenance of SNA and hypertension after CIH was determined and compared across groups. Although LCR rats were less active and had greater body weights than HCR rats, resting MAP, the contribution of ongoing SNA to the maintenance of MAP, and hypertensive responses to CIH were similar between groups. Contrary to our hypothesis, chemical inhibition of the PVN with muscimol (1 mmol/100 nl) caused a larger fall of MAP in HCR rats than in LCR rats. We conclude that LCR rats do not have resting hypertension or an exaggerated hypertensive response to CIH. Interestingly, the maintenance of CIH hypertension in LCR rats compared with HCR rats appears less reliant on ongoing PVN neuronal activity.

A rat model system to study complex disease risks, fitness, aging, and longevity

The association between low exercise capacity and all-cause morbidity and mortality is statistically strong yet mechanistically unresolved. By connecting clinical observation with a theoretical base, we developed a working hypothesis that variation in capacity for oxygen metabolism is the central mechanistic determinant between disease and health (aerobic hypothesis). As an unbiased test, we show that two-way artificial selective breeding of rats for low and high intrinsic endurance exercise capacity also produces rats that differ for numerous disease risks, including the metabolic syndrome, cardiovascular complications, premature aging, and reduced longevity. This contrasting animal model system may prove to be translationally superior relative to more widely used simplistic models for understanding geriatric biology and medicine.

Myocardial ischemia, reperfusion, and infarction in chronically instrumented, intact, conscious, and unrestrained mice

In the United States alone, the National Heart, Lung, and Blood Institute (NHLBI) has invested several hundred million dollars in pursuit of myocardial infarct-sparing therapies. However, due largely to methodological limitations, this investment has not produced any notable clinical application or cardioprotective therapy. Among the major methodological limitations is the reliance on animal models that do not mimic the clinical situation. In this context, the limited use of conscious animal models is of major concern. In fact, whenever possible, studies of cardiovascular physiology and pathophysiology should be conducted in conscious, complex models to avoid the complications associated with the use of anesthesia and surgical trauma. The mouse has significant advantages over other experimental models for the investigation of infarct-sparing therapies. The mouse is inexpensive, has a high throughput, and presents the ability of one to create genetically modified models. However, successful infarct-sparing therapies in anesthetized mice or isolated mouse hearts may not be successful in more complex models, including conscious mice. Accordingly, a conscious mouse model of myocardial ischemia and reperfusion has the potential to be of major importance for advancing the concepts and methods that drive the development of infarct-sparing therapies. Therefore, we describe, for the first time, the use of an intact, conscious, and unrestrained mouse model of myocardial ischemia-reperfusion and infarction. The conscious mouse model permits occlusion and reperfusion of the left anterior descending coronary artery in an intact, complex model free of the confounding influences of anesthetics and surgical trauma. This methodology may be adopted for advancing the concepts and ideas that drive cardiovascular research.

Proteomic analysis reveals perturbed energy metabolism and elevated oxidative stress in hearts of rats with inborn low aerobic capacity

Selection on running capacity has created rat phenotypes of high-capacity runners (HCRs) that have enhanced cardiac function and low-capacity runners (LCRs) that exhibit risk factors of metabolic syndrome. We analysed hearts of HCRs and LCRs from generation 22 of selection using DIGE and identified proteins from MS database searches. The running capacity of HCRs was six-fold greater than LCRs. DIGE resolved 957 spots and proteins were unambiguously identified in 369 spots. Protein expression profiling detected 67 statistically significant (p<0.05; false discovery rate <10%, calculated using q-values) differences between HCRs and LCRs. Hearts of HCR rats exhibited robust increases in the abundance of each enzyme of the β-oxidation pathway. In contrast, LCR hearts were characterised by the modulation of enzymes associated with ketone body or amino acid metabolism. LCRs also exhibited enhanced expression of antioxidant enzymes such as catalase and greater phosphorylation of α B-crystallin at serine 59, which is a common point of convergence in cardiac stress signalling. Thus, proteomic analysis revealed selection on low running capacity is associated with perturbations in cardiac energy metabolism and provided the first evidence that the LCR cardiac proteome is exposed to greater oxidative stress.

Proteomic responses of skeletal and cardiac muscle to exercise

Regular exercise is effective in the prevention of chronic diseases and confers a lower risk of death in individuals displaying risk factors such as hypertension and dyslipidemia. Thus, knowledge of the molecular responses to exercise provides a valuable contrast for interpreting investigations of disease and can highlight novel therapeutic targets. While exercise is an everyday experience and can be conceptualized in simple terms, it is also a complex physiological phenomenon and investigation of exercise responses requires sophisticated analytical techniques and careful standardization of the exercise stimulus. Proteomic investigation of exercise is in its infancy but the ability to link changes in function with comprehensive changes in protein expression and post-translational modification holds great promise for advancing physiology. This article highlights recent pioneering work investigating the effects of exercise in skeletal and cardiac muscle that has uncovered novel mechanisms underlying the benefits of physical activity.

Cardiac spinal deafferentation reduces the susceptibility to sustained ventricular tachycardia in conscious rats

The response to myocardial ischemia is complex and involves the cardio-cardiac sympathetic reflex. Specifically, cardiac spinal (sympathetic) afferents are excited by ischemic metabolites and elicit an excitatory sympathetic reflex, which plays a major role in the genesis of ventricular arrhythmias. For example, brief myocardial ischemia leads to ATP release, which activates cardiac spinal afferents through stimulation of P2 receptors. Clinical work with patients and preclinical work with animals document that disruption of this reflex protects against ischemia-induced ventricular arrhythmias. However, the role of afferent signals in the initiation of sustained ventricular tachycardia has not been investigated. Therefore, we tested the hypothesis that cardiac spinal deafferentation reduces the susceptibility to sustained ventricular tachycardia in adult (12-15 wk of age), conscious, male Sprague-Dawley rats. To test this hypothesis, the susceptibility to ventricular tachyarrhythmias produced by occlusion of the left main coronary artery was determined in two groups of conscious rats: 1) deafferentation (bilateral excision of the T1-T5 dorsal root ganglia) and 2) control (sham deafferentation). The ventricular arrhythmia threshold (VAT) was defined as the time from coronary occlusion to sustained ventricular tachycardia resulting in a reduction in arterial pressure. Results document a significantly higher VAT in the deafferentation group (7.0 ± 0.7 min) relative to control (4.3 ± 0.3 min) rats. The decreased susceptibility to tachyarrhythmias with deafferentation was associated with a reduced cardiac metabolic demand (lower rate-pressure product and ST segment elevation) during ischemia.

Risk-assessment and coping strategies segregate with divergent intrinsic aerobic capacity in rats

Metabolic function is integrally related to an individual's susceptibility to, and progression of, disease. Selective breeding for intrinsic treadmill running in rats has produced distinct lines of high- or low-capacity runners (HCR and LCR, respectively) that exhibit numerous physiological differences. To date, the role of intrinsic aerobic capacity on behavior and stress response in these rats has not been addressed and was the focus of these studies. HCR and LCR rats did not differ in their locomotor response to novelty or behavior in the light/dark box. In contrast, immobility in the forced swim test was higher in LCR rats compared with HCR rats, regardless of desipramine treatment. Although both HCR and LCR rats responded to cat odor with decreased exploration and increased risk assessment, HCR rats showed greater contextual conditioning to cat odor. HCR rats exhibited higher expression of corticotropin-releasing hormone in the central nucleus of the amygdala, as well as heavier adrenal and thymus weight. Corticosterone was comparable among HCR and LCR rats at light/dark transitions, and in response to unavoidable cat odor. HCR rats, however, exhibited a greater corticosterone response following the light/dark box. These experiments show that the LCR phenotype associates with decreased risk assessment in response to salient danger signals and passive coping. In contrast, HCR rats show a more naturalistic strategy in that they employ active coping and a more vigilant and cautious response to environmental novelty and salient danger signals. Within this context, we propose that intrinsic aerobic capacity is a central feature mechanistically linking complex metabolic disease and behavior.

Disrupted sleep and delayed recovery from chronic peripheral neuropathy are distinct phenotypes in a rat model of metabolic syndrome

BACKGROUND

Sleep apnea, hypertension, atherosclerosis, and obesity are features of metabolic syndrome associated with decreased restorative sleep and increased pain. These traits are relevant for anesthesiology because they confer increased risks of a negative anesthetic outcome. This study tested the one-tailed hypothesis that rats bred for low intrinsic aerobic capacity have enhanced nociception and disordered sleep.

METHODS

Rats were developed from a breeding strategy that selected for low aerobic capacity runners (LCR) and high aerobic capacity runners (HCR). Four phenotypes were quantified. Rats underwent von Frey sensory testing (n = 12), thermal nociceptive testing (n = 12), electrographic recordings of sleep and wakefulness (n = 16), and thermal nociceptive testing (n = 14) before and for 6 weeks after a unilateral chronic neuropathy of the sciatic nerve.

RESULTS

Paw withdrawal latency to a thermal nociceptive stimulus was significantly (P < 0.01) lower in LCR than HCR rats. There were also significant differences in sleep, with LCR rats spending significantly (P < 0.01) more time awake (18%) and less time in nonrapid eye movement sleep (-19%) than HCR rats. Nonrapid eye movement sleep episodes were of shorter duration (-34%) in LCR than HCR rats. Rapid eye movement sleep of LCR rats was significantly more fragmented than rapid eye movement sleep of HCR rats. LCR rats required 2 weeks longer than HCR rats to recover from peripheral neuropathy.

CONCLUSIONS

Rodents with low aerobic capacity exhibit features homologous to human metabolic syndrome. This rodent model offers a novel tool for characterizing the mechanisms through which low aerobic function and obesity might confer increased risks for anesthesia.

Partial hindlimb occlusion reduced the susceptibility to sustained ventricular tachycardia in conscious rats

Remote conditioning induced by ischemia in distant organs protects the heart from ischemia/reperfusion injury; however, its effect on ischemia-induced ventricular arrhythmias is unknown. Therefore, we tested the hypothesis that partial hindlimb occlusion during coronary artery occlusion increases the ventricular arrhythmia threshold (VAT) induced by coronary artery occlusion. Rats (n = 7) were instrumented with a radio-telemetry device for recording arterial pressure, electrocardiogram (ECG), and body temperature. A Doppler ultrasonic flow probe and vascular occluder were placed around the terminal aorta. Finally, a snare was placed around the left main coronary artery. The VAT was determined without and, on an alternate day, during partial hindlimb occlusion (remote conditioning) in conscious rats. Without remote conditioning, the VAT was 4.56 + 0.15 minutes. Importantly, remote conditioning significantly increased the VAT (6.29 + 0.49 minutes), suggesting that ischemia in a distant organ may delay the development of ischemia-induced ventricular arrhythmias.

Cardiac autonomic neural remodeling and susceptibility to sudden cardiac death: effect of endurance exercise training

Sudden cardiac death resulting from ventricular tachyarrhythmias remains the leading cause of death in industrially developed countries, accounting for between 300,000 and 500,000 deaths each year in the United States. Yet, despite the enormity of this problem, both the identification of factors contributing to ventricular fibrillation as well as the development of safe and effective antiarrhythmic agents remain elusive. Subnormal cardiac parasympathetic regulation coupled with an elevated cardiac sympathetic activation may allow for the formation of malignant ventricular arrhythmias. In particular, myocardial infarction can reduce cardiac parasympathetic regulation and alter beta-adrenoceptor subtype expression enhancing beta(2)-adrenoceptor sensitivity that can lead to intracellular calcium dysregulation and arrhythmias. As such, myocardial infarction can induce a remodeling of cardiac autonomic regulation that may be required to maintain cardiac pump function. If alterations in cardiac autonomic regulation play an important role in the genesis of life-threatening arrhythmias, then one would predict that interventions designed to either augment parasympathetic activity and/or reduce cardiac adrenergic activity would also protect against ventricular fibrillation. Recently, studies using a canine model of sudden death demonstrate that endurance exercise training (treadmill running) enhanced cardiac parasympathetic regulation (increased heart rate variability), restored a more normal beta-adrenoceptor balance (i.e., reduced beta(2)-adrenoceptor sensitivity and expression), and protected against ventricular fibrillation induced by acute myocardial ischemia. Thus exercise training may reverse the autonomic neural remodeling induced by myocardial infarction and thereby enhance the electrical stability of the heart in individuals shown to be at an increased risk for sudden cardiac death.

Artificial selection for whole animal low intrinsic aerobic capacity co-segregates with hypoxia-induced cardiac pump failure

Oxygen metabolism is a strong predictor of the general health and fitness of an organism. In this study, we hypothesized that a divergence in intrinsic aerobic fitness would co-segregate with susceptibility for cardiovascular dysfunction. To test this hypothesis, cardiac function was assessed in rats specifically selected over nineteen generations for their low (LCR) and high (HCR) intrinsic aerobic running capacity. As an integrative marker of native aerobic capacity, run time to exhaustion between LCR and HCR rats had markedly diverged by 436% at generation nineteen of artificial selection. In vivo assessment of baseline cardiac function by echocardiography and catheter-based conductance micromanometry showed no marked difference in cardiac performance. However, when challenged by exposure to acute hypoxia, cardiac pump failure occurred significantly earlier in LCR rats compared to HCR animals. Acute cardiac decompensation in LCR rats was exclusively due to the development of intractable irregular ventricular contractions. Analysis of isolated cardiac myocytes showed significantly slower sarcomeric relaxation and delayed kinetics of calcium cycling in LCR myocytes compared to HCR myocytes. This study also revealed that artificial selection for low native aerobic capacity is a novel pathologic stimulus that results in myosin heavy chain isoform switching in the heart as shown by increased levels of beta-MHC in LCR rats. Together, these results provide evidence that alterations in sub-cellular calcium handling and MHC isoform composition are associated with susceptibility to compensatory cardiac remodeling and hypoxia induced pump failure in animals with low intrinsic aerobic capacity.

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 β-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.

Sex differences to myocardial ischemia and beta-adrenergic receptor blockade in conscious rats

We recently documented sex differences in the susceptibility to reperfusion-induced sustained ventricular tachycardia and beta-adrenergic receptor blockade in conscious rats. However, the effect of sex on ischemia-induced ventricular arrhythmias and beta-adrenergic receptor blockade is under-investigated. Therefore, we tested the hypothesis that gonadal hormones influence the ventricular arrhythmia threshold (VAT) induced by coronary artery occlusion as well as the response to beta-adrenergic receptor blockade. The VAT was defined as the time from coronary occlusion to sustained ventricular tachycardia resulting in a reduction in arterial pressure. Male and female intact and gonadectomized (GnX) rats were instrumented with a radiotelemetry device for recording arterial pressure, temperature, and ECG, as well as a Doppler ultrasonic flow probe to measure cardiac output and a snare around the left main coronary artery. The VAT was determined in conscious rats by pulling on the snare. The VAT was significantly longer in intact females (5.56 +/- 0.19) vs. intact males (4.31 +/- 0.14 min). This sex difference was abolished by GnX. Specifically, GnX decreased the VAT in females (4.55 +/- 0.22) and increased the VAT in males (5.14 +/- 0.30 min). Thus male sex hormones increase and female sex hormones decrease the susceptibility to ischemia-induced sustained ventricular tachycardia. beta-Adrenergic receptor blockade increased the VAT in intact males and GnX females only. Thus gonadal hormones influence the response to beta-adrenergic receptor blockade. Uncovering major differences between males and females in the pathophysiology of the cardiovascular system may result in sex-specific optimization of patient treatments.

Aerobic metabolism underlies complexity and capacity

The evolution of biological complexity beyond single-celled organisms was linked temporally with the development of an oxygen atmosphere. Functionally, this linkage can be attributed to oxygen ranking high in both abundance and electronegativity amongst the stable elements of the universe. That is, reduction of oxygen provides for close to the largest possible transfer of energy for each electron transfer reaction. This suggests the general hypothesis that the steep thermodynamic gradient of an oxygen environment was permissive for the development of multicellular complexity. A corollary of this hypothesis is that aerobic metabolism underwrites complex biological function mechanistically at all levels of organization. The strong contemporary functional association of aerobic metabolism with both physical capacity and health is presumably a product of the integral role of oxygen in our evolutionary history. Here we provide arguments from thermodynamics, evolution, metabolic network analysis, clinical observations and animal models that are in accord with the centrality of oxygen in biology.

Creatine and creatine kinase in health and disease--a bright future ahead?

Many links are reported or suspected between the functioning of creatine, phosphocreatine, the creatine kinase isoenzymes or the creatine biosynthesis enzymes on one hand, and health or disease on the other hand. The aim of the present book was to outline our current understanding on many of these links. In this chapter, we summarize the main messages and conclusions presented in this book. In addition, we refer to a number of recent publications that highlight the pleiotropy in physiological functions of creatine and creatine kinase, and which suggest that numerous discoveries on new functions of this system are still ahead of us. Finally, we present our views on the most promising future avenues of research to deepen our knowledge on creatine and creatine kinase. In particular, we elaborate on how state-of-the-art high-throughput analytical ("omics") technologies and systems biology approaches may be used successfully to unravel the complex network of interdependent physiological functions related to creatine and creatine kinase.

Evolution, atmospheric oxygen, and complex disease

If evolution is an accurate statement of our biology, then disease must be tightly associated with its patterns. We considered selection for more optimal capacity for energy transfer as the most general pattern of evolution. From this, we propose that the etiology of complex disease is linked tightly to the evolutionary transition to cellular complexity that was afforded by the steep thermodynamic gradient of an oxygen atmosphere. In accord with this thesis, clinical studies reveal a strong statistical link between low aerobic capacity and all-cause mortality. In addition, large-scale unbiased network analyses demonstrate the pivotal role of oxygen metabolism in cellular function. The demonstration that multiple disease risks segregated during two-way artificial selection for low and high aerobic capacity in rats provides a remote test of these possible connections between evolution, oxygen metabolism, and complex disease. Even more broadly, an atmosphere with oxygen may be uniquely essential for development of complex life anywhere because oxygen is stable as a diatomic gas, is easily transported, and has a high electronegativity for participation in energy transfer via redox reactions.

Electroacupuncture decreases the susceptibility to ventricular tachycardia in conscious rats by reducing cardiac metabolic demand

Reperfusion after a brief period of cardiac ischemia can lead to potentially lethal arrhythmias. Clinical observations and experimental work with animals suggest that acupuncture may have therapeutic effects for individuals with coronary heart disease, certain arrhythmias, and myocardial ischemia. Therefore, we tested the hypothesis that electroacupuncture reduces the susceptibility to ischemia-reperfusion-mediated ventricular tachyarrhythmias. To test this hypothesis, we measured the susceptibility to ventricular tachyarrhythmias produced by 3 min of occlusion and reperfusion of the left main coronary artery in conscious rats under two experimental conditions: 1) control and 2) with electroacupuncture. Acupuncture was simulated by electrically stimulating the median nerves, corresponding to the Jianshi-Neiguan [pericardial meridian (P) 5-6] acupoints. Results document a significantly lower incidence of ventricular tachyarrhythmias with electroacupuncture (2 of 8, 25%) relative to control (14 of 14, 100%) rats. The decreased susceptibility to tachyarrhythmias with electroacupuncture was associated with a reduced cardiac metabolic demand (lower rate-pressure product and ST-segment elevation) during ischemia.