Peripheral oxygen transport and utilization in rats following continued selective breeding for endurance running capacity

Higher mitochondrial oxidative capacity is the primary molecular differentiator in muscle of rats with high and low intrinsic cardiorespiratory fitness

OBJECTIVE

Cardiorespiratory fitness (CRF) is tightly linked with health and longevity and is implicated in metabolic flexibility and substrate metabolism. The high capacity runner (HCR) and low capacity runner (LCR) rat lines are a genetically heterogeneous rat model selected and bred for CRF that reflect CRF in humans by exhibiting differences in nutrient handling. This study aims to differentiate the intrinsic substrate preference of the HCR compared to LCR rats to better understand the intersection of mitochondrial respiration and intrinsic CRF.

METHODS

We performed bulk skeletal muscle RNA-Sequencing on male and female HCR and LCR rats and assessed the effect of rat line on mitochondrial gene expression pathways using the MitoCarta3.0 database. In a separate cohort of rats, mitochondria were isolated from skeletal and cardiac muscle and maximal oxidation rates were measured using an Oroboros O2k when provided either pyruvate or fatty acid substrates.

RESULTS

The expression of mitochondrial genes are significantly upregulated in HCR skeletal muscle in both male and female rats. In respirometry experiments, fatty acid oxidative capacities were greater in HCR compared to LCR, and male compared to female rats, as a function of both mitochondrial quality and mitochondrial density. This effect was greater in the skeletal muscle than in the heart. Pyruvate oxidation did not differ significantly between lines.

CONCLUSIONS

The capacity for increased fatty acid oxidation in the HCR rat is a result of selection for running capacity and is likely a key contributor to the healthy metabolic phenotype of individuals with high CRF.

Lactate thresholds and role of nitric oxide in male rats performing a test with forced swimming to exhaustion

The present study assessed a complex of biochemical parameters at the anaerobic threshold (AT) in untrained male Wistar rats with different times to exhaustion (Tex ) from swimming. The first group of rats was randomly divided into six subgroups and subjected to a swimming test to exhaustion without a load or with a load of 2%-10% of body weight (BW). In the first group, we established that for untrained rats, the load of 4% BW in the swimming to exhaustion test was optimal for endurance assessment in comparison with other loads. The second group of rats went through a preliminary test with swimming to exhaustion at 4% BW and was then divided into two subgroups: long swimming time (LST, Tex  > 240 min) and short swimming time (SST, Tex  < 90 min). All rats of the second group performed, for 6 days, an experimental training protocol: swimming for 20 min each day with weight increasing each day. We established that the AT was 3% BW in SST rats and 5% BW in LST rats. The AT shifted to the right on the lactate curve in LST rats. Also, at the AT in the LST rats, we found significantly lower levels of blood lactate, cortisol, and NO.

Intramuscular mitochondrial and lipid metabolic changes of rats after regular high-intensity interval training (HIIT) of different training periods

BACKGROUND

High-intensity Interval Training (HIIT) is a time-efficient form of exercise and has gained popularity in recent years. However, at molecular level, the understanding about the effects of HIIT is not comprehensive, and even less is elucidated about HIIT of different training duration cycles, although different durations always lead to different post-training consequences.

METHOD

In this study, by training SD rats using HIIT protocols lasting for different training duration cycles, we investigated the adaptive response of intramuscular triglyceride abundance as well as mitochondrial and lipid metabolic changes after HIIT training (2, 4, 6, 8, and 10 weeks). We selected 72 h after the last session of training as the time point of sacrifice.

RESULTS

The suppressed activation of the cAMP-PKA pathway indicates that skeletal muscle was in the recovery phase at this time point. Intramuscular triglyceride abundance was significantly elevated after 2, 4, and 10 weeks of HIIT. However, the lipid metabolism-related proteins inconsistently changed in a chaotic trend (see Table 1). The expression levels of PGC1-α and COX IV decreased after 2 and 4 weeks of training and raised after 6 and 8 weeks of training. The expression level of citrate synthase (CS) decreased after 2, 4, 8, and 10 weeks of training, and showed an upward trend after 6 weeks of training. While the activity of CS decreased after 2 and 8 weeks of training and showed an upward trend after 6 weeks of HIIT.

CONCLUSION

Given the abovementioned changing trends, we propose two speculations: (A) the damaged mitochondria oxidation capacity might be one of the causes of IMTG accumulation observed after 2 and 4 weeks of HIIT. This phase might be similar to the condition of type 2 diabetes. (B) after 6-week HIIT, mitochondria function and biogenesis might be improved and the IMTG contents declined to baseline. This might be explained as: mitochondrial enhancement increased the capacity of lipid oxidation and then offset the increase in IMTG achieved during the first 4 weeks. For HIIT Rat Modelling, if the aim is to observe HIIT-induced positive effects, caution should be exercised when considering 2 and 4 weeks of training under our HIIT frame. Also, implementing six-week training is at least effective for mitochondrial enhancement when using similar HIIT frame of this study.

Does Regular Physical Activity Mitigate the Age-Associated Decline in Pulmonary Function?

Whereas the negative effects of aging and smoking on pulmonary function are undisputed, the potential favorable effects of physical activity on the aging process of the otherwise healthy lung remain controversial. This question is of particular clinical relevance when reduced pulmonary function compromises aerobic exercise capacity (maximal oxygen consumption) and thus contributes to an increased risk of morbidity and mortality. Here, we discuss whether and when the aging-related decline in pulmonary function limits maximal oxygen consumption and whether, how, and to what extent regular physical activity can slow down this aging process and preserve pulmonary function and maximal oxygen consumption. Age-dependent effects of reduced pulmonary function (i.e., FEV₁, the volume that has been exhaled after the first second of forced expiration) on maximal oxygen consumption have been observed in several cross-sectional and longitudinal studies. Complex interactions between aging-related cellular and molecular processes affecting the lung, and structural and functional deterioration of the cardiovascular and respiratory systems account for the concomitant decline in pulmonary function and maximal oxygen consumption. Consequently, if long-term regular physical activity mitigates some of the aging-related decline in pulmonary function (i.e., FEV₁ decline), this could also prevent a steep fall in maximal oxygen consumption. In contrast to earlier research findings, recent large-scale longitudinal studies provide growing evidence for the beneficial effects of physical activity on FEV₁. Although further confirmation of those effects is required, these findings provide powerful arguments to start and/or maintain regular physical activity.

Low Intrinsic Aerobic Capacity Limits Recovery Response to Hindlimb Ischemia

Introduction: In this study, we determined the influence of intrinsic exercise capacity on the vascular adaptive responses to hind limb ischemia. High Capacity Running, HCR; Low Capacity Running, LCR, rats were used to assess intrinsic aerobic capacity effects on adaptive responses to ischemia.

Methods: Muscle samples from both ischemic and non-ischemic limb in both strains were compared, histologically for the muscle-capillary relationship, and functionally using microspheres to track blood flow and muscle stimulation to test fatigability. PCR was used to identify the differences in gene expression between the phenotypes following occlusive ischemia.

Results: Prior to ligation, there were not significant differences between the phenotypes in the exhaustion time with high frequency pacing. Following ligation, LCR decreased significantly in the exhaustion time compare with HCRs (437 ± 47 vs. 824 ± 56, p < 0.001). The immediate decrease in flow was significantly more severe in LCRs than HCRs (52.5 vs. 37.8%, p < 0.001). VEGF, eNOS, and ANG2 (but not ANG1) gene expression were decreased in LCRs vs. HCRs before occlusion, and increased significantly in LCRs 14D after occlusion, but not in HCRs. LCR capillary density (CD) was significantly lower at all time points after occlusion (LCR 7D = 564.76 ± 40.5, LCR 14D = 507.48 ± 54.2, both p < 0.05 vs. HCR for respective time point). NCAF increased significantly in HCR and LCR in response to ischemia.

Summary: These results suggest that LCR confers increased risk for ischemic injury and is subject to delayed and less effective adaptive response to ischemic stress.

Physiological insight into the evolution of complex phenotypes: aerobic performance and the O2 transport pathway of vertebrates

Evolutionary physiology strives to understand how the function and integration of physiological systems influence the way in which organisms evolve. Studies of the O2 transport pathway - the integrated physiological system that transports O2 from the environment to mitochondria - are well suited to this endeavour. We consider the mechanistic underpinnings across the O2 pathway for the evolution of aerobic capacity, focusing on studies of artificial selection and naturally selected divergence among wild populations of mammals and fish. We show that evolved changes in aerobic capacity do not require concerted changes across the O2 pathway and can arise quickly from changes in one or a subset of pathway steps. Population divergence in aerobic capacity can be associated with the evolution of plasticity in response to environmental variation or activity. In some cases, initial evolutionary divergence of aerobic capacity arose exclusively from increased capacities for O2 diffusion and/or utilization in active O2-consuming tissues (muscle), which may often constitute first steps in adaptation. However, continued selection leading to greater divergence in aerobic capacity is often associated with increased capacities for circulatory and pulmonary O2 transport. Increases in tissue O2 diffusing capacity may augment the adaptive benefit of increasing circulatory O2 transport owing to their interactive influence on tissue O2 extraction. Theoretical modelling of the O2 pathway suggests that O2 pathway steps with a disproportionately large influence over aerobic capacity have been more likely to evolve, but more work is needed to appreciate the extent to which such physiological principles can predict evolutionary outcomes.

Brillouin microscopy for the evaluation of hair micromechanics and effect of bleaching

Brillouin microscopy is a new form of optical elastography and an emerging technique in mechanobiology and biomedical physics. It was applied here to map the viscoelastic properties of human hair and to determine the effect of bleaching on hair properties. For hair samples, longitudinal measurements (i.e. along the fibre axis) revealed peaks at 18.7 and 20.7 GHz at the location of the cuticle and cortex, respectively. For hair treated with a bleaching agent, the frequency shifts for the cuticle and cortex were 19.7 and 21.0 GHz, respectively, suggesting that bleaching increases the cuticle modulus and-to a minor extent-the cortex modulus. These results demonstrate the capability of Brillouin spectroscopy to address questions on micromechanical properties of hair and to validate the effect of applied treatments.

Rats genetically selected for low and high aerobic capacity exhibit altered soleus muscle myofilament functions

Aerobic exercise capacity is critical to bodily health. As a model to investigate the mechanisms that determine health and disease, we employed low (LCR) and high (HCR) capacity running rat models selectively bred to concentrate the genes responsible for divergent aerobic running capacity. To investigate the skeletal muscle contribution to this innate difference in running capacity we employed an approach combining examination of the myofilament protein composition and contractile properties of the fast fiber extensor digitorum longus (EDL) and slow fiber soleus (SOL) muscles from LCR and HCR rats. Intact muscle force experiments demonstrate that SOL, but not EDL, muscles from LCR rats exhibit a three times greater decrease in fatigued force. To investigate the mechanism of this increased fatigability in the LCR SOL muscle, we determined the myofilament protein composition and functional properties. Force-Ca²⁺ measurements demonstrate decreased Ca²⁺ sensitivity of single skinned SOL muscle fibers from LCR compared with that of HCR rats. Segregating SOL fibers into fast and slow types demonstrates that the decreased Ca²⁺ sensitivity in LCR SOL results from a specific decrease in slow-type SOL fiber Ca²⁺ sensitivity such that it was similar to that of fast-type fibers. These results identify that the altered myofilament contractile properties of LCR SOL slow-type fibers result in a fast muscle type Ca²⁺ sensitivity and the LCR muscle phenotype. Overall our findings demonstrate alterations of the myofilament proteins could contribute to fatigability of the SOL muscle and the decreased innate aerobic running performance of LCR compared with HCR rats.

Comparative Analysis of Skeletal Muscle Transcriptional Signatures Associated With Aerobic Exercise Capacity or Response to Training in Humans and Rats

Increasing exercise capacity promotes healthy aging and is strongly associated with lower mortality rates. In this study, we analyzed skeletal muscle transcriptomics coupled to exercise performance in humans and rats to dissect the inherent and response components of aerobic exercise capacity. Using rat models selected for intrinsic and acquired aerobic capacity, we determined that the high aerobic capacity muscle transcriptome is associated with pathways for tissue oxygenation and vascularization. Conversely, the low capacity muscle transcriptome indicated immune response and metabolic dysfunction. Low response to training was associated with an inflammatory signature and revealed a potential link to circadian rhythm. Next, we applied bioinformatics tools to predict potential secreted factors (myokines). The predicted secretome profile for exercise capacity highlighted circulatory factors involved in lipid metabolism and the exercise response secretome was associated with extracellular matrix remodelling. Lastly, we utilized human muscle mitochondrial respiration and transcriptomics data to explore molecular mediators of exercise capacity and response across species. Human transcriptome comparison highlighted epigenetic mechanisms linked to exercise capacity and the damage repair for response. Overall, our findings from this cross-species transcriptome analysis of exercise capacity and response establish a foundation for future studies on the mechanisms that link exercise and health.

Life Ascending: Mechanism and Process in Physiological Adaptation to High-Altitude Hypoxia

To cope with the reduced availability of O₂ at high altitude, air-breathing vertebrates have evolved myriad adjustments in the cardiorespiratory system to match tissue O₂ delivery with metabolic O₂ demand. We explain how changes at interacting steps of the O₂ transport pathway contribute to plastic and evolved changes in whole-animal aerobic performance under hypoxia. In vertebrates native to high altitude, enhancements of aerobic performance under hypoxia are attributable to a combination of environmentally induced and evolved changes in multiple steps of the pathway. Additionally, evidence suggests that many high-altitude natives have evolved mechanisms for attenuating maladaptive acclimatization responses to hypoxia, resulting in counter-gradient patterns of altitudinal variation for key physiological phenotypes. For traits that exhibit counteracting environmental and genetic effects, evolved changes in phenotype may be cryptic under field conditions and can only be revealed by rearing representatives of high-and low-altitude populations under standardized environmental conditions to control for plasticity.

Modelling the effects of age and sex on the resistive and viscoelastic components of the work of breathing during exercise

NEW FINDINGS

What is the central question of this study? What is the effect of age and sex on the resistive and viscoelastic components of work of breathing (W_b ) during exercise? What is the main finding and its importance? The resistive and viscoelastic components of W_b were higher in older adults, regardless of sex. The resistive, but not viscoelastic, component of W_b was higher in females than in males, regardless of age. These findings contribute to improving our understanding of the effects of ageing and sex on the mechanical ventilatory response to exercise.

ABSTRACT

Healthy ageing and biological sex each affect the work of breathing (W_b ) for a given minute ventilation ( V ̇ E ). Age-related structural changes to the respiratory system lead to an increase in both the resistive and viscoelastic components of W_b ; however, it is unclear whether healthy ageing differentially alters the mechanics of breathing in males and females. We analysed data from 22 older (60-80 years, n = 12 females) and 22 younger (20-30 years, n = 11 females) males and females that underwent an incremental cycle exercise test to exhaustion. V ̇ E and W_b were assessed at rest and throughout exercise. W_b - V ̇ E data for each participant were fitted to a non-linear equation (i.e. W_b  = a V ̇ E ³ + b V ̇ E ² ) that partitions W_b into resistive (i.e. a V ̇ E ³ ) and viscoelastic (i.e. b V ̇ E ² ) components. We then modelled the effects of healthy ageing and biological sex on each component of W_b . Overall, the model fit was excellent (r² : 0.99 ± 0.01). There was a significant main effect of age and sex on the resistive component of W_b (both P < 0.05), and a significant main effect of age (P < 0.001), but not sex (P = 0.309), on the viscoelastic component of W_b . No significant interactions between age and sex on a V ̇ E ³ or b V ̇ E ² were noted (both P > 0.05). Our findings indicate that during exercise: (i) the higher total W_b in females relative to males is due to a higher resistive, but not viscoelastic, component of W_b , and (ii) regardless of sex, the higher W_b in older adults relative to younger adults is due to higher resistive and viscoelastic components of W_b .

Metabolic and muscular factors limiting aerobic exercise in obese subjects

PURPOSE

The aim of the present study was to understand the role of central (cardiovascular O₂ delivery) and peripheral factors (muscle level) in limiting the maximal aerobic performance in obese (OB) subjects.

METHODS

Fifteen OB (mean age ± SD 25 ± 7 years; BMI 43 ± 7 kg/m²) and 13 lean sedentary subjects (CTRL, age 27 ± 7 years; BMI 22 ± 3 kg/m²) participated in this study. Oxygen uptake (VO₂), hearth rate (HR) and cardiac output (CO) were measured during cycle ergometer (CE) and knee extension (KE) incremental tests. Maximal voluntary contractions (MVCs) of knee extensor muscles were performed before and immediately after the two tests.

RESULTS

VO₂peak, HR peak and CO peak were significantly higher in CE than KE (+ 126%, + 33% and + 46%, respectively, p < 0.001), both in OB and CTRL subjects, without differences between the two subgroups. Maximal work rate was lower in OB than CTRL (191 ± 38 vs 226 ± 39 W, p < 0.05) in CE, while it was similar between the two subgroups in KE. Although CE and KE determined a reduction of MVC in both subgroups, MVC resulted less decreased after CE than KE exercises (- 14 vs - 32%, p < 0.001) in OB, while MVC decrements were similar after the two exercises in CTRL (- 26% vs - 30%, p > 0.05, for CE and KE, respectively).

CONCLUSIONS

The lower muscle fatigue observed in OB after CE compared to KE test suggests that central factors could be the most important limiting factor during cycling in OB.

Evolution of physiological performance capacities and environmental adaptation: insights from high-elevation deer mice (Peromyscus maniculatus)

Analysis of variation in whole-animal performance can shed light on causal connections between specific traits, integrated physiological capacities, and Darwinian fitness. Here, we review and synthesize information on naturally occurring variation in physiological performance capacities and how it relates to environmental adaptation in deer mice (Peromyscus maniculatus). We discuss how evolved changes in aerobic exercise capacity and thermogenic capacity have contributed to adaptation to high elevations. Comparative work on deer mice at high and low elevations has revealed evolved differences in aerobic performance capacities in hypoxia. Highland deer mice have consistently higher aerobic performance capacities under hypoxia relative to lowland natives, consistent with the idea that it is beneficial to have a higher maximal metabolic rate (as measured by the maximal rate of O₂ consumption, VO_2max) in an environment characterized by lower air temperatures and lower O₂ availability. Observed differences in aerobic performance capacities between highland and lowland deer mice stem from changes in numerous subordinate traits that alter the flux capacity of the O₂-transport system, the oxidative capacity of tissue mitochondria, and the relationship between O₂ consumption and ATP synthesis. Many such changes in physiological phenotype are associated with hypoxia-induced changes in gene expression. Research on natural variation in whole-animal performance forms a nexus between physiological ecology and evolutionary biology that requires insight into the natural history of the study species.

Systemic Oxygen Transport with Rest, Exercise, and Hypoxia: A Comparison of Humans, Rats, and Mice

The objective of this article is to compare and contrast the known characteristics of the systemic O₂ transport of humans, rats, and mice at rest and during exercise in normoxia and hypoxia. This analysis should help understand when rodent O₂ transport findings can-and cannot-be applied to human responses to similar conditions. The O₂ -transport system was analyzed as composed of four linked conductances: ventilation, alveolo-capillary diffusion, circulatory convection, and tissue capillary-cell diffusion. While the mechanisms of O₂ transport are similar in the three species, the quantitative differences are naturally large. There are abundant data on total O₂ consumption and on ventilatory and pulmonary diffusive conductances under resting conditions in the three species; however, there is much less available information on pulmonary gas exchange, circulatory O₂ convection, and tissue O₂ diffusion in mice. The scarcity of data largely derives from the difficulty of obtaining blood samples in these small animals and highlights the need for additional research in this area. In spite of the large quantitative differences in absolute and mass-specific O₂ flux, available evidence indicates that resting alveolar and arterial and venous blood PO₂ values under normoxia are similar in the three species. Additionally, at least in rats, alveolar and arterial blood PO₂ under hypoxia and exercise remain closer to the resting values than those observed in humans. This is achieved by a greater ventilatory response, coupled with a closer value of arterial to alveolar PO₂ , suggesting a greater efficacy of gas exchange in the rats. © 2018 American Physiological Society. Compr Physiol 8:1537-1573, 2018.

Intracellular oxygen tension limits muscle contraction-induced change in muscle oxygen consumption under hypoxic conditions during Hb-free perfusion

Under acute hypoxic conditions, the muscle oxygen uptake (mV˙O₂) during exercise is reduced by the restriction in oxygen-supplied volume to the mitochondria within the peripheral tissue. This suggests the existence of a factor restricting the mV˙O₂ under hypoxic conditions at the peripheral tissue level. Therefore, this study set out to test the hypothesis that the restriction in mV˙O₂ is regulated by the net decrease in intracellular oxygen tension equilibrated with myoglobin oxygen saturation (∆P_mbO₂) during muscle contraction under hypoxic conditions. The hindlimb of male Wistar rats (8 weeks old, n = 5) was perfused with hemoglobin-free Krebs-Henseleit buffer equilibrated with three different fractions of O₂ gas: 95.0%O₂, 71.3%O₂, and 47.5%O₂ The deoxygenated myoglobin (Mb) kinetics during muscle contraction were measured under each oxygen condition with a near-infrared spectroscopy. The ∆[deoxy-Mb] kinetics were converted to oxygen saturation of myoglobin (S_mbO₂), and the P_mbO₂ was then calculated based on the S_mbO₂ and the O₂ dissociation curve of the Mb. The S_mbO₂ and P_mbO₂ at rest decreased with the decrease in O₂ supply, and the muscle contraction caused a further decrease in S_mbO₂ and P_mbO₂ under all O₂ conditions. The net increase in mV˙O₂ from the muscle contraction (∆mV˙O₂) gradually decreased as the ∆P_mbO₂ decreased during muscle contraction. The results of this study suggest that ΔP_mbO₂ is a key determinant of the ΔmV˙O₂.

Rapamycin increases grip strength and attenuates age-related decline in maximal running distance in old low capacity runner rats

Rapamycin is known to extend lifespan. We conducted a randomized placebo-controlled study of enteric rapamycin-treatment to evaluate its effect on physical function in old low capacity runner (LCR) rats, a rat model selected from diverse genetic background for low intrinsic aerobic exercise capacity without genomic manipulation and characterized by increased complex disease risks and aging phenotypes. The study was performed in 12 male and 16 female LCR rats aged 16-22 months at baseline. The treatment group was fed with rapamycin-containing diet pellets at approximately 2.24mg/kg body weight per day and the placebo group with the same diet without rapamycin for six months. Observation was extended for additional 2 months. Physical function measurements include grip strength measured as maximum tensile force using a rat grip strength meter and maximum running distance (MRD) using rat physical treadmill test. The results showed that rapamycin improved grip strength by 13% (p=.036) and 60% (p<.001) from its baseline in female and male rats, respectively. Rapamycin attenuated MRD decline by 66% (p<.001) and 46% (p=.319) in females and males, respectively. These findings provide initial evidence for beneficial effect of rapamycin on physical functioning in an aging rat model of high disease risks with significant implication in humans.

Selection-, age-, and exercise-dependence of skeletal muscle gene expression patterns in a rat model of metabolic fitness

Intrinsic aerobic exercise capacity can influence many complex traits including obesity and aging. To study this connection we established two rat lines by divergent selection of untrained aerobic capacity. After 32 generations the high capacity runners (HCR) and low capacity runners (LCR) differed in endurance running distance and body fat, blood glucose, other health indicators, and natural life span. To understand the interplay among genetic differences, chronological age, and acute exercise we performed microarray-based gene expression analyses in skeletal muscle with a 2×2×2 design to simultaneously compare HCR and LCR, old and young animals, and rest and exhaustion. Transcripts for mitochondrial function are expressed higher in HCRs than LCRs at both rest and exhaustion and for both age groups. Expression of cell adhesion and extracellular matrix genes tend to decrease with age. This and other age effects are more prominent in LCRs than HCRs, suggesting that HCRs have a slower aging process and this may be partly due to their better metabolic health. Strenuous exercise mainly affects transcription regulation and cellular response. The effects of any one factor often depend on the other two. For example, there are ∼140 and ∼110 line-exercise "interacting" genes for old and young animals, respectively. Many genes highlighted in our study are consistent with prior reports, but many others are novel. The gene- and pathway-level statistics for the main effects, either overall or stratified, and for all possible interactions, represent a rich reference dataset for understanding the interdependence among lines, aging, and exercise.

Hypoxia-inducible factor-1 modulates the expression of vascular endothelial growth factor and endothelial nitric oxide synthase induced by eccentric exercise

The present study investigated the effects of acute and chronic eccentric exercise on the hypoxia-inducible factor (HIF)-1α activation response and the concomitant modulation of vascular endothelial growth factor (VEGF) and endothelial nitric oxide synthase (eNOS) expression in rat skeletal muscle. Twenty-four male Wistar rats were randomly assigned to three experimental groups: rested control group, acutely exercised group after an intermittent downhill protocol for 90 min, and acutely exercise group with a previous eccentric training of 8 wk. HIF-1α activation, VEGF and eNOS gene expression, protein content, and promoter activation were assessed in vastus lateralis muscle biopsies. Acute eccentric exercise induced a marked activation of HIF-1α and resulted in increased VEGF and eNOS mRNA level and protein concentration. The binding of HIF-1α to the VEGF and eNOS promoters, measured by a chromatin immunoprecipitation assay, was undetectable in rested rats, whereas it was evident in acutely exercised animals. Acute exercise also increased myeloperoxidase, toll-like receptor-4, tumor necrosis factor-α, and interleukin-1β protein content, suggesting a contribution of proinflammatory stimuli to HIF-1α activation and VEGF overexpression. All of these effects were partially abolished by training. Moreover, training resulted in an increased capillary density. In summary, our findings indicate that eccentric exercise prompts an HIF-1α response in untrained skeletal muscle that contributes to the upregulation of VEGF and eNOS gene expression and is attenuated after an eccentric training program.

Quantitative genomics of voluntary exercise in mice: transcriptional analysis and mapping of expression QTL in muscle

Motivation and ability both underlie voluntary exercise, each with a potentially unique genetic architecture. Muscle structure and function are one of many morphological and physiological systems acting to simultaneously determine exercise ability. We generated a large (n = 815) advanced intercross line of mice (G4) derived from a line selectively bred for increased wheel running (high runner) and the C57BL/6J inbred strain. We previously mapped quantitative trait loci (QTL) contributing to voluntary exercise, body composition, and changes in body composition as a result of exercise. Using brain tissue in a subset of the G4 (n = 244), we have also previously reported expression QTL (eQTL) colocalizing with the QTL for the higher-level phenotypes. Here, we examined the transcriptional landscape of hind limb muscle tissue via global mRNA expression profiles. Correlations revealed an ∼1,168% increase in significant relationships between muscle transcript expression levels and the same exercise and body composition phenotypes examined previously in the brain. The exercise trait most often significantly correlated with gene expression in the brain was running duration while in the muscle it was maximum running speed. This difference may indicate that time spent engaging in exercise behavior may be more influenced by central (neurobiological) mechanisms, while intensity of exercise may be largely controlled by peripheral mechanisms. Additionally, we used subsets of cis-acting eQTL, colocalizing with QTL, to identify candidate genes based on both positional and functional evidence. We discuss three plausible candidate genes (Insig2, Prcp, Sparc) and their potential regulatory role.

Striatal enkephalinergic differences in rats selectively bred for intrinsic running capacity

Rats selectively bred for high- and low-capacity for running on a treadmill (HCR; LCR) also differ in wheel-running behavior, but whether wheel-running can be explained by intrinsic or adaptive brain mechanisms is not as yet understood. It is established that motivation of locomotory behavior is driven by dopaminergic transmission in mesolimbic and mesostriatal systems. However, whether voluntary wheel running is associated with enkephalinergic activity in the ventral striatum is not known.

MATERIALS AND METHODS

40 male (20 HCR and 20 LCR) and 40 female (20 HCR and 20 LCR) rats were randomly assigned to 3 weeks of activity wheel exposure or sedentary conditions without wheel access. After 3 weeks of activity-wheel running, rats were decapitated and brains were extracted. Coronal sections were analyzed utilizing in situ hybridization histochemistry for enkephalin (ENK) mRNA in the ventral striatum.

RESULTS

HCR rats expressed less ENK than LCR rats in the nucleus accumbens among females (p<0.01) and in the olfactory tubercle among both females (p<0.05) and males (p<0.05). There was no effect of wheel running on ENK mRNA expression.

CONCLUSION

Line differences in ENK expression in the olfactory tubercle, and possibly the nucleus accumbens, partly explain divergent wheel-running behavior. The lower striatal ENK in the HCR line is consistent with enhanced dopaminergic tone, which may explain the increased motivation for wheel running observed in the HCR line.

Interval and continuous exercise training produce similar increases in skeletal muscle and left ventricle microvascular density in rats

Interval training (IT), consisting of alternated periods of high and low intensity exercise, has been proposed as a strategy to induce more marked biological adaptations than continuous exercise training (CT). The purpose of this study was to assess the effects of IT and CT with equivalent total energy expenditure on capillary skeletal and cardiac muscles in rats. Wistar rats ran on a treadmill for 30 min per day with no slope (0%), 4 times/week for 13 weeks. CT has constant load of 70% max; IT has cycles of 90% max for 1 min followed by 1 min at 50% max. CT and IT increased endurance and muscle oxidative capacity and attenuated body weight gain to a similar extent (P > 0.05). In addition, CT and IT similarly increased functional capillary density of skeletal muscle (CT: 30.6 ± 11.7%; IT: 28.7 ± 11.9%) and the capillary-to-fiber ratio in skeletal muscle (CT: 28.7 ± 14.4%; IT: 40.1 ± 17.2%) and in the left ventricle (CT: 57.3 ± 53.1%; IT: 54.3 ± 40.5%). In conclusion, at equivalent total work volumes, interval exercise training induced similar functional and structural alterations in the microcirculation of skeletal muscle and myocardium in healthy rats compared to continuous exercise training.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Temporal response of positive and negative regulators in response to acute and chronic exercise training in mice

Angiogenesis is controlled by a balance between positive and negative angiogenic factors, but temporal protein expression of many key angiogenic regulators in response to exercise are still poorly defined. In C57BL/6 mice, we evaluated the temporal protein expression of several pro-angiogenic and anti-angiogenic factors in response to (1) a single acute bout of exercise and (2) chronic exercise training resulting from 3, 5, 7, 14 and 28 days of voluntary wheel running. Following acute exercise, protein levels of vascular endothelial growth factor-A (VEGF), endostatin and nucleolin were increased at 2-4 h (P < 0.05), whereas matrix metalloproteinase (MMP)-2 was elevated within a 12-24 h window (P < 0.05). Training increased muscle capillarity 11%, 15% and 22% starting with 7, 14 and 28 days of training, respectively (P < 0.01). Basal VEGF and MMP-2 were increased by 31% and 22%, respectively, compared to controls (P < 0.05) after 7 days (7d) training, but decreased to back to baseline after 14d training. After 28d training VEGF fell 49% below baseline control (P < 0.01). Basal muscle expression of thrombospondin 1 (TSP-1) was ∼900% greater in 14d- and 28d-trained mice compared to either 5d- and 7d-trained mice (P < 0.05), and tended to increase by ∼180-258% compared to basal control levels (P < 0.10). The acute responsiveness of VEGF to exercise in untrained mice (i.e. 161% increase, P < 0.001) was lost with capillary adaptation occurring after 7, 14 and 28d training. Taken together, these data support the notion that skeletal muscle angiogenesis is controlled by a balance between positive and negative mitogens, and reveals a complex, highly-coordinated, temporal scheme whereby these factors can differentially influence capillary growth in response to acute versus chronic exercise.

Explorations in statistics: the analysis of ratios and normalized data

Learning about statistics is a lot like learning about science: the learning is more meaningful if you can actively explore. This ninth installment of Explorations in Statistics explores the analysis of ratios and normalized-or standardized-data. As researchers, we compute a ratio-a numerator divided by a denominator-to compute a proportion for some biological response or to derive some standardized variable. In each situation, we want to control for differences in the denominator when the thing we really care about is the numerator. But there is peril lurking in a ratio: only if the relationship between numerator and denominator is a straight line through the origin will the ratio be meaningful. If not, the ratio will misrepresent the true relationship between numerator and denominator. In contrast, regression techniques-these include analysis of covariance-are versatile: they can accommodate an analysis of the relationship between numerator and denominator when a ratio is useless.

(-)-Epicatechin is associated with increased angiogenic and mitochondrial signalling in the hindlimb of rats selectively bred for innate low running capacity

Alternative approaches to reduce congenital muscle dysfunction are needed in cases where the ability to exercise is limited. (-)-Epicatechin is found in cocoa and may stimulate capillarity and mitochondrial proliferation in skeletal muscle. A total of 21 male rats bred for LCR (low running capacity) from generation 28 were randomized into three groups: vehicle for 30 days (control); (-)-epicatechin for 30 days; and (-)-epicatechin for 30 days followed by 15 days without (-)-epicatechin. Groups 2 and 3 received 1.0 mg of (-)-epicatechin/kg of body mass twice daily, whereas water was given to the control group. The plantaris muscle was harvested for protein and morphometric analyses. In addition, in vitro experiments were conducted to examine the role of (-)-epicatechin on mitochondrial respiratory kinetics at different incubation periods. Treatment for 30 days with (-)-epicatechin increased capillarity (P<0.001) and was associated with increases in protein expression of VEGF (vascular endothelial growth factor)-A with a concomitant decrease in TSP-1 (thrombospondin-1) and its receptor, which remained after 15 days of (-)-epicatechin cessation. Analyses of the p38 MAPK (mitogen-activated protein kinase) signalling pathway indicated an associated increase in phosphorylation of MKK3/6 (MAPK kinase 3/6) and p38 and increased protein expression of MEF2A (myocyte enhancer factor 2A). In addition, we observed significant increases in protein expression of PGC-1α (peroxisome-proliferator-activated receptor γ co-activator 1α), PGC-1β, Tfam and cristae abundance. Interestingly, these increases associated with (-)-epicatechin treatment remained after 15 days of cessation. Lastly, in vitro experiments indicated that acute exposure of LCR muscle to (-)-epicatechin incubation was not sufficient to increase mitochondrial respiration. The results suggest that increases in skeletal muscle capillarity and mitochondrial biogenesis are associated with 30 days of (-)-epicatechin treatment and sustained for 15 days following cessation of treatment. Clinically, the use of this natural compound may have potential application in populations that experience muscle fatigue and are unable to perform endurance exercise.

Vascularity of myocardium and gastrocnemius muscle in rats selectively bred for endurance running capacity

We tested the hypothesis that changes in the arteriolar branching architecture contributed to increased running capacity of rats subjected to two-way artificial selection for intrinsic aerobic endurance treadmill running capacity resulting in strains of low-capacity and high-capacity endurance rats. Hearts and gastrocnemius muscles were harvested from each strain, and the microvasculature's branching geometry measured from micro-CT images. The vascular branching geometry of the hearts and skeletal muscle from the high capacity was indistinguishable from low-capacity rats. Our hypothesis was not supported. Neither remodeling nor an increase in arteriolar microvasculature branching appears to play a role in the enhanced performance of the high capacity rats. We are led to speculate that endothelial tolerance for shear stress and/or increased coupling of myocardial muscle fiber metabolic-to-contractile function is increased in the high-capacity runner strain to the effect of allowing either higher flow rate per unit volume of muscle or more efficient use of oxygen and nutrients in the high-capacity endurance rats.

Brain activation patterns at exhaustion in rats that differ in inherent exercise capacity

In order to further understand the genetic basis for variation in inherent (untrained) exercise capacity, we examined the brains of 32 male rats selectively bred for high or low running capacity (HCR and LCR, respectively). The aim was to characterize the activation patterns of brain regions potentially involved in differences in inherent running capacity between HCR and LCR. Using quantitative in situ hybridization techniques, we measured messenger ribonuclease (mRNA) levels of c-Fos, a marker of neuronal activation, in the brains of HCR and LCR rats after a single bout of acute treadmill running (7.5-15 minutes, 15° slope, 10 m/min) or after treadmill running to exhaustion (15-51 minutes, 15° slope, initial velocity 10 m/min). During verification of trait differences, HCR rats ran six times farther and three times longer prior to exhaustion than LCR rats. Running to exhaustion significantly increased c-Fos mRNA activation of several brain areas in HCR, but LCR failed to show significant elevations of c-Fos mRNA at exhaustion in the majority of areas examined compared to acutely run controls. Results from these studies suggest that there are differences in central c-Fos mRNA expression, and potential brain activation patterns, between HCR and LCR rats during treadmill running to exhaustion and these differences could be involved in the variation in inherent running capacity between lines.

Do mice bred selectively for high locomotor activity have a greater reliance on lipids to power submaximal aerobic exercise?

Patterns of fuel use during locomotion are determined by exercise intensity and duration, and are remarkably similar across many mammalian taxa. However, as lipids have a high yield of ATP per mole and are stored in large quantities, their use should be favored in endurance-adapted animals. To examine the capacity for alteration or differential regulation of fuel-use patterns, we studied two lines of mice that had been selectively bred for high voluntary wheel running (HR), including one characterized by small hindlimb muscles (HR(mini)) and one without this phenotype (HR(normal)), as well as a nonselected control line. We evaluated: 1) maximal aerobic capacity (Vo(2 max)); 2) whole body fuel use during exercise by indirect calorimetry; 3) cardiac properties; and 4) many factors involved in regulating lipid use. HR mice achieved an increased Vo(2 max) compared with control mice, potentially in part due to HR cardiac capacities for metabolic fuel oxidation and the larger relative heart size of HR(mini) mice. HR mice also exhibited enhanced whole body lipid oxidation rates at 66% Vo(2 max), but HR(mini), HR(normal), and control mice did not differ in the proportional mix of fuels sustaining exercise (% total Vo(2)). However, HR(mini) gastrocnemius muscle had elevated fatty acid translocase (FAT/CD36) sarcolemmal protein and cellular mRNA, fatty acid binding protein (H-FABP) cytosolic protein, peroxisome proliferator-activated receptor (PPAR) α mRNA, and mass-specific activities of citrate synthase, β-hydroxyacyl-CoA dehydrogenase, and hexokinase. Therefore, high-running mouse lines had whole body fuel oxidation rates commensurate with maximal aerobic capacity, despite notable differences in skeletal muscle metabolic phenotypes.

Mechanisms underlying parallel reductions in aerobic capacity in non-migratory threespine stickleback (Gasterosteus aculeatus) populations

Non-migratory, stream-resident populations of threespine stickleback, Gasterosteus aculeatus, have a lower maximum oxygen consumption (ṀO2,max) than ancestral migratory marine populations. Here, we examined laboratory-bred stream-resident and marine crosses from two locations (West and Bonsall Creeks) to determine which steps in the oxygen transport and utilization cascade evolved in conjunction with, and thus have the potential to contribute to, these differences in ṀO2,max. We found that West Creek stream-resident fish have larger muscle fibres (not measured in Bonsall fish), Bonsall Creek stream-resident fish have smaller ventricles, and both stream-resident populations have evolved smaller pectoral adductor and abductor muscles. However, many steps of the oxygen cascade did not evolve in stream-resident populations (gill surface area, hematocrit, mean cellular hemoglobin content and the activities of mitochondrial enzymes per gram ventricle and pectoral muscle), arguing against symmorphosis. We also studied F1 hybrids to determine which traits in the oxygen cascade have a genetic architecture similar to that of ṀO2,max. In West Creek, ṀO2,max, abductor and adductor size all showed dominance of marine alleles, whereas in Bonsall Creek, ṀO2,max and ventricle mass showed dominance of stream-resident alleles. We also found genetically based differences among marine populations in hematocrit, ventricle mass, pectoral muscle mass and pectoral muscle pyruvate kinase activity. Overall, reductions in pectoral muscle mass evolved in conjunction with reductions in ṀO2,max in both stream-resident populations, but the specific steps in the oxygen cascade that have a genetic basis similar to that of ṀO2,max, and are thus predicted to have the largest impact on ṀO2,max, differ among populations.

Selective breeding for endurance running capacity affects cognitive but not motor learning in rats

The ability to utilize oxygen has been shown to affect a wide variety of physiological factors often considered beneficial for survival. As the ability to learn can be seen as one of the core factors of survival in mammals, we studied whether selective breeding for endurance running, an indication of aerobic capacity, also has an effect on learning. Rats selectively bred over 23 generations for their ability to perform forced treadmill running were trained in an appetitively motivated discrimination-reversal classical conditioning task, an alternating T-maze task followed by a rule change (from a shift-win to stay-win rule) and motor learning task. In the discrimination-reversal and T-maze tasks, the high-capacity runner (HCR) rats outperformed the low-capacity runner (LCR) rats, most notably in the phases requiring flexible cognition. In the Rotarod (motor-learning) task, the HCR animals were overall more agile but learned at a similar rate with the LCR group as a function of training. We conclude that the intrinsic ability to utilize oxygen is associated especially with tasks requiring plasticity of the brain structures implicated in flexible cognition.

Changes in mRNA levels for brain-derived neurotrophic factor after wheel running in rats selectively bred for high- and low-aerobic capacity

We evaluated levels of exercise-induced brain-derived neurotrophic factor (BDNF) messenger RNA (mRNA) within the hippocampal formation in rats selectively bred for 1) high intrinsic (i.e., untrained) aerobic capacity (High Capacity Runners, HCR), 2) low intrinsic aerobic capacity (Low Capacity Runners, LCR), and 3) unselected Sprague-Dawley (SD) rats with or without free access to running wheels for 3 weeks. The specific aim of the study was to determine whether a dose-response relationship exists between cumulative running distance and levels of BDNF mRNA. No additional treatments or behavioral manipulations were used. HCR, LCR, and SD rats were grouped by strain and randomly assigned to sedentary or activity (voluntary access to activity wheel) conditions. Animals were killed after 21 days of exposure to the assigned conditions. Daily running distances (mean ± standard deviation meters/day) during week three were: HCR (4726 ± 3220), SD (2293 ± 3461), LCR (672 ± 323). Regardless of strain, levels of BDNF mRNA in CA1 were elevated in wheel runners compared to sedentary rats and this difference persisted after adjustment for age (p=0.040). BDNF mRNA was not affected by intrinsic aerobic capacity and was not related to total running distance. The results support that BDNF mRNA expression is increased by unlimited access to activity wheel running for 3 weeks but is not dependent upon accumulated running distance.

Effects of oxygen availability on maximum aerobic performance in Mus musculus selected for basal metabolic rate or aerobic capacity

Maximum aerobic metabolism cannot increase indefinitely in response to demands for ATP production and, therefore, must be constrained by one (or many) of the steps of the oxygen transport and utilization pathways. To elucidate those constraints we compared peak metabolic rate elicited by running (V(.)(O₂,run)) in hypoxia (14% O₂), normoxia (21% O₂) and hyperoxia (30% O₂) of laboratory mice divergently selected for low and high basal metabolic rate (L-BMR and H-BMR, respectively), mice selected for maximum metabolic rate elicited by swimming (V(.)(O₂,swim)) and mice from unselected lines. In all line types (V(.)(O₂,run)) was lowest in hypoxia, intermediate in normoxia and highest in hyperoxia, which suggests a 'central' limitation of oxygen uptake or delivery instead of a limit set by cellular oxidative capacity. However, the existence of a common central limitation is not in agreement with our earlier studies showing that selection on high V(.)(O₂,swim) (in contrast to selection on high BMR) resulted in considerably higher oxygen consumption during cold exposure in a He-O₂ atmosphere than V(.)(O₂,run). Likewise, between-line-type differences in heart mass and blood parameters are inconsistent with the notion of central limitation. Although responses of V(.)(O₂,run) to hypoxia were similar across different selection regimens, the selection lines showed contrasting responses under hyperoxic conditions. V(.)(O₂,run) in the H-BMR line type was highest, suggesting that selection on high BMR led to increased cellular oxidative capacity. Overall, between-line-type differences in the effect of the oxygen partial pressure on V(.)(O₂,run) and in the components of O₂ flux pathways are incompatible with the notion of symmorphosis. Our results suggest that constraints on V(.)(O₂,max) are context dependent and determined by interactions between the central and peripheral organs and tissues involved in O₂ delivery.

The biological control of voluntary exercise, spontaneous physical activity and daily energy expenditure in relation to obesity: human and rodent perspectives

Mammals expend energy in many ways, including basic cellular maintenance and repair, digestion, thermoregulation, locomotion, growth and reproduction. These processes can vary tremendously among species and individuals, potentially leading to large variation in daily energy expenditure (DEE). Locomotor energy costs can be substantial for large-bodied species and those with high-activity lifestyles. For humans in industrialized societies, locomotion necessary for daily activities is often relatively low, so it has been presumed that activity energy expenditure and DEE are lower than in our ancestors. Whether this is true and has contributed to a rise in obesity is controversial. In humans, much attention has centered on spontaneous physical activity (SPA) or non-exercise activity thermogenesis (NEAT), the latter sometimes defined so broadly as to include all energy expended due to activity, exclusive of volitional exercise. Given that most people in Western societies engage in little voluntary exercise, increasing NEAT may be an effective way to maintain DEE and combat overweight and obesity. One way to promote NEAT is to decrease the amount of time spent on sedentary behaviours (e.g. watching television). The effects of voluntary exercise on other components of physical activity are highly variable in humans, partly as a function of age, and have rarely been studied in rodents. However, most rodent studies indicate that food consumption increases in the presence of wheels; therefore, other aspects of physical activity are not reduced enough to compensate for the energetic cost of wheel running. Most rodent studies also show negative effects of wheel access on body fat, especially in males. Sedentary behaviours per se have not been studied in rodents in relation to obesity. Several lines of evidence demonstrate the important role of dopamine, in addition to other neural signaling networks (e.g. the endocannabinoid system), in the control of voluntary exercise. A largely separate literature points to a key role for orexins in SPA and NEAT. Brain reward centers are involved in both types of physical activities and eating behaviours, likely leading to complex interactions. Moreover, voluntary exercise and, possibly, eating can be addictive. A growing body of research considers the relationships between personality traits and physical activity, appetite, obesity and other aspects of physical and mental health. Future studies should explore the neurobiology, endocrinology and genetics of physical activity and sedentary behaviour by examining key brain areas, neurotransmitters and hormones involved in motivation, reward and/or the regulation of energy balance.

Lower oxidative DNA damage despite greater ROS production in muscles from rats selectively bred for high running capacity

Artificial selection in rat has yielded high-capacity runners (HCR) and low-capacity runners (LCR) that differ in intrinsic (untrained) aerobic exercise ability and metabolic disease risk. To gain insight into how oxygen metabolism may have been affected by selection, we compared mitochondrial function, oxidative DNA damage (8-dihydroxy-guanosine; 8dOHG), and antioxidant enzyme activities in soleus muscle (Sol) and gastrocnemius muscle (Gas) of adult and aged LCR vs. HCR rats. In Sol of adult HCR rats, maximal ADP-stimulated respiration was 37% greater, whereas in Gas of adult HCR rats, there was a 23% greater complex IV-driven respiratory capacity and 54% greater leak as a fraction of electron transport capacity (suggesting looser mitochondrial coupling) vs. LCR rats. H(2)O(2) emission per gram of muscle was 24-26% greater for both muscles in adult HCR rats vs. LCR, although H(2)O(2) emission in Gas was 17% lower in HCR, after normalizing for citrate synthase activity (marker of mitochondrial content). Despite greater H(2)O(2) emission, 8dOHG levels were 62-78% lower in HCR rats due to 62-96% higher superoxide dismutase activity in both muscles and 47% higher catalase activity in Sol muscle in adult HCR rats, with no evidence for higher 8 oxoguanine glycosylase (OGG1; DNA repair enzyme) protein expression. We conclude that genetic segregation for high running capacity has generated a molecular network of cellular adaptations, facilitating a superior response to oxidative stress.

Locus coeruleus galanin expression is enhanced after exercise in rats selectively bred for high capacity for aerobic activity

The neuropeptide galanin extensively coexists with norepinephrine in locus coeruleus (LC) neurons. Previous research in this laboratory has demonstrated that unlimited access to activity wheels in the home cage increases mRNA for galanin (GAL) in the LC, and that GAL mediates some of the beneficial effects of exercise on brain function. To assess whether capacity for aerobic exercise modulates this upregulation in galanin mRNA, three heterogeneous rat models were tested: rats selectively bred for (1) high intrinsic (untrained) aerobic capacity (High Capacity Runners, HCR) and (2) low intrinsic aerobic capacity (Low Capacity Runners, LCR) and (3) unselected Sprague-Dawley (SD) rats with and without free access to running wheels for 3 weeks. Following this exercise protocol, mRNA for tyrosine hydroxylase (TH) and GAL was measured in the LC. The wheel running distances between the three models were significantly different, and age contributed as a significant covariate. Both selection and wheel access condition significantly affected GAL mRNA expression, but not TH mRNA expression. GAL was elevated in exercising HCR and SD rats compared to sedentary rats while LCR rats did not differ between conditions. Overall running distance significantly correlated with GAL mRNA expression, but not with TH mRNA expression. No strain differences in GAL or TH gene expression were observed in sedentary rats. Thus, intrinsic aerobic running capacity influences GAL gene expression in the LC only insofar as actual running behavior is concerned; aerobic capacity does not influence GAL expression in addition to changes associated with running.

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.

Myocyte vascular endothelial growth factor is required for exercise-induced skeletal muscle angiogenesis

We have previously shown, using a Cre-LoxP strategy, that vascular endothelial growth factor (VEGF) is required for the development and maintenance of skeletal muscle capillarity in sedentary adult mice. To determine whether VEGF expression is required for skeletal muscle capillary adaptation to exercise training, gastrocnemius muscle capillarity was measured in myocyte-specific VEGF gene-deleted (mVEGF(-/-)) and wild-type (WT) littermate mice following 6 wk of treadmill running (1 h/day, 5 days/wk) at the same running speed. The effect of training on metabolic enzyme activity levels and whole body running performance was also evaluated in mVEGF(-/-) and WT mice. Posttraining capillary density was significantly increased by 59% (P < 0.05) in the deep muscle region of the gastrocnemius in WT mice but did not change in mVEGF(-/-) mice. Maximal running speed and time to exhaustion during submaximal running increased by 20 and 13% (P < 0.05), respectively, in WT mice after training but were unchanged in mVEGF(-/-) mice. Training led to increases in skeletal muscle citrate synthase (CS) and phosphofructokinase (PFK) activities in both WT and mVEGF(-/-) mice (P < 0.05), whereas β-hydroxyacyl-CoA dehydrogenase (β-HAD) activity was increased only in WT mice. These data demonstrate that skeletal muscle capillary adaptation to physical training does not occur in the absence of myocyte-expressed VEGF. However, skeletal muscle metabolic adaptation to exercise training takes place independent of myocyte VEGF expression.

Gene expression centroids that link with low intrinsic aerobic exercise capacity and complex disease risk

A strong link exists between low aerobic exercise capacity and complex metabolic diseases. To probe this linkage, we utilized rat models of low and high intrinsic aerobic endurance running capacity that differ also in the risk for metabolic syndrome. We investigated in skeletal muscle gene-phenotype relationships that connect aerobic endurance capacity with metabolic disease risk factors. The study compared 12 high capacity runners (HCRs) and 12 low capacity runners (LCRs) from generation 18 of selection that differed by 615% for maximal treadmill endurance running capacity. On average, LCRs were heavier and had increased blood glucose, insulin, and triglycerides compared with HCRs. HCRs were higher for resting metabolic rate, voluntary activity, serum high density lipoproteins, muscle capillarity, and mitochondrial area. Bioinformatic analysis of skeletal muscle gene expression data revealed that many genes up-regulated in HCRs were related to oxidative energy metabolism. Seven mean mRNA expression centroids, including oxidative phosphorylation and fatty acid metabolism, correlated significantly with several exercise capacity and disease risk phenotypes. These expression-phenotype correlations, together with diminished skeletal muscle capillarity and mitochondrial area in LCR rats, support the general hypothesis that an inherited intrinsic aerobic capacity can underlie disease risks.—Kivelä, R., Silvennoinen, M., Lehti, M., Rinnankoski-Tuikka, R., Purhonen, T., Ketola, T., Pullinen, K., Vuento, M., Mutanen, N., Sartor, M. A., Reunanen, H., Koch, L. G., Britton, S. L., Kainulainen, H. Gene expression centroids that link with low intrinsic aerobic exercise capacity and complex disease risk.

Research in the exercise sciences: where we are and where do we go from here--Part II

This decadal perspective summarizes novel, insightful observations achieved in exercise science. The topics span genomics and gene function, epigenetics, cell signaling, epidemiological phenomena, and other important areas. A future strategy is presented along two parallel, integrated paths involving the following: 1) a continuance of genomic discovery and gene function, and 2) classical biochemical/physiological approaches toward solving biological- and health/disease-related phenomena.

Detraining losses of skeletal muscle capillarization are associated with vascular endothelial growth factor protein expression in rats

The purposes of this study were as follows: (1) to examine basal vascular endothelial growth factor (VEGF) protein concentrations following 10 weeks of endurance training and after 7 days of detraining; and (2) to examine the acute VEGF protein response to a single 1 h exercise work bout in trained and detrained animals in relationship to changes in capillary indices following training and detraining. Thirty-three Sprague-Dawley rats were randomized into the following six groups: (1) control-basal; (2) control-acute exercise; (3) trained-basal; (4) trained-acute exercise; (5) detrained-basal; and (6) detrained-acute exercise. Groups 3-6 performed endurance training on a rodent treadmill three times per week for 10 weeks. Following the training intervention, rats in groups 5 and 6 remained cage confined (i.e. detrained) for 7 days. As expected, training increased soleus and plantaris muscle capillarity and attenuated the VEGF response to acute exercise. Seven days of detraining, however, resulted in a regression of capillary contacts and individual capillary-to-fibre ratio in the plantaris and soleus muscles compared with the trained group (P < 0.05). Restoration of the VEGF protein response to acute exercise was evident in both muscles, but only statistically significant in the plantaris muscle (P < 0.05). This is the first study to demonstrate the temporal relationship between VEGF protein expression and skeletal muscle capillarity within the first week of detraining. The findings of the present investigation are consistent with the hypothesis that reduced capillarity impairs oxygen availability to the working muscles. The results indicated that training-induced angiogenic remodelling was reversible following 1 week of detraining and may be modulated by VEGF.

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.

Control entropy identifies differential changes in complexity of walking and running gait patterns with increasing speed in highly trained runners

Regularity statistics have been previously applied to walking gait measures in the hope of gaining insight into the complexity of gait under different conditions and in different populations. Traditional regularity statistics are subject to the requirement of stationarity, a limitation for examining changes in complexity under dynamic conditions such as exhaustive exercise. Using a novel measure, control entropy (CE), applied to triaxial continuous accelerometry, we report changes in complexity of walking and running during increasing speeds up to exhaustion in highly trained runners. We further apply Karhunen-Loeve analysis in a new and novel way to the patterns of CE responses in each of the three axes to identify dominant modes of CE responses in the vertical, mediolateral, and anterior/posterior planes. The differential CE responses observed between the different axes in this select population provide insight into the constraints of walking and running in those who may have optimized locomotion. Future comparisons between athletes, healthy untrained, and clinical populations using this approach may help elucidate differences between optimized and diseased locomotor control.

Continued artificial selection for running endurance in rats is associated with improved lung function

Previous studies found that selection for endurance running in untrained rats produced distinct high (HCR) and low (LCR) capacity runners. Furthermore, despite weighing 14% less, 7th generation HCR rats achieved the same absolute maximal oxygen consumption (Vo(2max)) as LCR due to muscle adaptations that improved oxygen extraction and use. However, there were no differences in cardiopulmonary function after seven generations of selection. If selection for increased endurance capacity continued, we hypothesized that due to the serial nature of oxygen delivery enhanced cardiopulmonary function would be required. In the present study, generation 15 rats selected for high and low endurance running capacity showed differences in pulmonary function. HCR, now 25% lighter than LCR, reached a 12% higher absolute Vo(2max) than LCR, P < 0.05 (49% higher Vo(2max)/kg). Despite the 25% difference in body size, both lung volume (at 20 cmH(2)O airway pressure) and exercise diffusing capacity were similar in HCR and LCR. Lung volume of LCR lay on published mammalian allometrical relationships while that of HCR lay above that line. Alveolar ventilation at Vo(2max) was 30% higher, P < 0.05 (78% higher, per kg), arterial Pco(2) was 4.5 mmHg (17%) lower, P < 0.05, while total pulmonary vascular resistance was (insignificantly) 5% lower (30% lower, per kg) in HCR. The smaller mass of HCR animals was due mostly to a smaller body frame rather than to a lower fat mass. These findings show that by generation 15, lung size in smaller HCR rats is not reduced in concert with their smaller body size, but has remained similar to that of LCR, supporting the hypothesis that continued selection for increased endurance capacity requires relatively larger lungs, supporting greater ventilation, gas exchange, and pulmonary vascular conductance.