Divergence in aerobic capacity and energy expenditure influence metabolic tissue mitochondrial protein synthesis rates in aged rats

Age-associated declines in aerobic capacity promote the development of various metabolic diseases. In rats selectively bred for high/low intrinsic aerobic capacity, greater aerobic capacity reduces susceptibility to metabolic disease while increasing longevity. However, little remains known how intrinsic aerobic capacity protects against metabolic disease, particularly with aging. Here, we tested the effects of aging and intrinsic aerobic capacity on systemic energy expenditure, metabolic flexibility and mitochondrial protein synthesis rates using 24-month-old low-capacity (LCR) or high-capacity runner (HCR) rats. Rats were fed low-fat diet (LFD) or high-fat diet (HFD) for eight weeks, with energy expenditure (EE) and metabolic flexibility assessed utilizing indirect calorimetry during a 48 h fast/re-feeding metabolic challenge. Deuterium oxide (D2O) labeling was used to assess mitochondrial protein fraction synthesis rates (FSR) over a 7-day period. HCR rats possessed greater EE during the metabolic challenge. Interestingly, HFD induced changes in respiratory exchange ratio (RER) in male and female rats, while HCR female rat RER was largely unaffected by diet. In addition, analysis of protein FSR in skeletal muscle, brain, and liver mitochondria showed tissue-specific adaptations between HCR and LCR rats. While brain and liver protein FSR were altered by aerobic capacity and diet, these effects were less apparent in skeletal muscle. Overall, we provide evidence that greater aerobic capacity promotes elevated EE in an aged state, while also regulating metabolic flexibility in a sex-dependent manner. Modulation of mitochondrial protein FSR by aerobic capacity is tissue-specific with aging, likely due to differential energetic requirements by each tissue.

Inherited physical capacity: Widening divergence from young to adult to old

Cardiorespiratory performance segregates into rat strains of inherited low- and high-capacity runners (LCRs and HCRs); during adulthood, this segregation remains stable, but widens in senescence and is followed by segregated function, health, and mortality. However, this segregation has not been investigated prior to adulthood. We, therefore, assessed cardiorespiratory performance and cardiac cell (cardiomyocyte) structure-function in 1- and 4-month-old LCRs and HCRs. Maximal oxygen uptake was 23% less in LCRs at 1-month compared to HCRs at 1-month, and 72% less at 4 months. Cardiomyocyte contractility was 37-56% decreased, and Ca2+ release was 34-62% decreased, in 1- and 4-month LCRs versus HCRs. This occurred because HCRs had improved contractility and Ca2+ release during maturation, whereas LCRs did not. In quiescent cardiomyocytes, LCRs displayed 180% and 297% more Ca2+ sparks and 91% and 38% more Ca2+ waves at 1 and 4 months versus HCRs. Cell sizes were not different between LCRs and HCRs, but LCRs showed reduced transverse-tubules versus HCRs, though no discrepant transverse-tubule generation occurred during maturation. In conclusion, LCRs show reduced scores for aerobic capacity and cardiomyocyte structure-function compared to HCRs and there is a widening divergence between LCRs and HCRs during juvenile to near-adult maturation.

Metformin treatment results in distinctive skeletal muscle mitochondrial remodeling in rats with different intrinsic aerobic capacities

The rationale for the use of metformin as a treatment to slow aging was largely based on data collected from metabolically unhealthy individuals. For healthspan extension metformin will also be used in periods of good health. To understand potential context specificity of metformin treatment on skeletal muscle, we used a rat model (HCR/LCR) with a divide in intrinsic aerobic capacity. Outcomes of metformin treatment differed based on baseline intrinsic mitochondrial function, oxidative capacity of the muscle (gastroc vs soleus), and the mitochondrial population (IMF vs SS). Metformin caused lower ADP-stimulated respiration in LCRs, with less of a change in HCRs. However, a washout of metformin resulted in an unexpected doubling of respiratory capacity in HCRs. These improvements in respiratory capacity were accompanied by mitochondrial remodeling that included increases in protein synthesis and changes in morphology. Our findings raise questions about whether the positive findings of metformin treatment are broadly applicable.

Exercise Training Differentially Affects Skeletal Muscle Mitochondria in Rats with Inherited High or Low Exercise Capacity

Exercise capacity has been related to morbidity and mortality. It consists of an inherited and an acquired part and is dependent on mitochondrial function. We assessed skeletal muscle mitochondrial function in rats with divergent inherited exercise capacity and analyzed the effect of exercise training. Female high (HCR)- and low (LCR)-capacity runners were trained with individually adapted high-intensity intervals or kept sedentary. Interfibrillar (IFM) and subsarcolemmal (SSM) mitochondria from gastrocnemius muscle were isolated and functionally assessed (age: 15 weeks). Sedentary HCR presented with higher exercise capacity than LCR paralleled by higher citrate synthase activity and IFM respiratory capacity in skeletal muscle of HCR. Exercise training increased exercise capacity in both HCR and LCR, but this was more pronounced in LCR. In addition, exercise increased skeletal muscle mitochondrial mass more in LCR. Instead, maximal respiratory capacity was increased following exercise in HCRs' IFM only. The results suggest that differences in skeletal muscle mitochondrial subpopulations are mainly inherited. Exercise training resulted in different mitochondrial adaptations and in higher trainability of LCR. HCR primarily increased skeletal muscle mitochondrial quality while LCR increased mitochondrial quantity in response to exercise training, suggesting that inherited aerobic exercise capacity differentially affects the mitochondrial response to exercise training.

Intrinsic running capacity associates with hippocampal electrophysiology and long-term potentiation in rats

Good aerobic and metabolic fitness associates with better cognitive performance and brain health. Conversely, poor metabolic health predisposes to neurodegenerative diseases. Our previous findings indicate that rats selectively bred for Low Capacity for Running (LCR) show less synaptic plasticity and more inflammation in the hippocampus and perform worse in tasks requiring flexible cognition than rats bred for High Capacity for Running (HCR). Here we aimed to determine whether hippocampal electrophysiological activity related to learning and memory would be impaired in LCR compared to HCR rats. We also studied whether an exercise intervention could even out the possible differences. We conducted in vivo recordings from the dorsal hippocampus under terminal urethane anesthesia in middle-aged sedentary males and female rats, and in females allowed to access running wheels for 6 weeks. Our results indicate stronger long-term potentiation (LTP) in the CA3-CA1 synapse in HCR than LCR rats, and in female than male rats. Compared to LCR rats, HCR rats had more dentate spikes and more gamma epochs, the occurrence of which also correlated positively with the magnitude of LTP. Voluntary running reduced the differences between female LCR and HCR rats. In conclusion, low innate fitness links to reduced hippocampal function and plasticity which can seems to improve with voluntary aerobic exercise even in middle age.

Parental cardiorespiratory fitness influences early life energetics and metabolic health

High cardiorespiratory fitness (CRF) is associated with a reduced risk of metabolic disease and is linked to superior mitochondrial respiratory function. This study investigated how intrinsic CRF affects bioenergetics and metabolic health in adulthood and early life. Adult rats selectively bred for low and high running capacity [low capacity runners (LCR) and high capacity runners (HCR), respectively] underwent metabolic phenotyping before mating. Weanlings were evaluated at 4-6 wk of age, and whole body energetics and behavior were assessed using metabolic cages. Mitochondrial respiratory function was assessed in permeabilized tissues through high-resolution respirometry. Proteomic signatures of adult and weanling tissues were determined using mass spectrometry. The adult HCR group exhibited lower body mass, improved glucose tolerance, and greater physical activity compared with the LCR group. The adult HCR group demonstrated higher mitochondrial respiratory capacities in the soleus and heart compared with the adult LCR group, which coincided with a greater abundance of proteins involved in lipid catabolism. HCR and LCR weanlings had similar body mass, but HCR weanlings displayed reduced adiposity. In addition, HCR weanlings exhibited better glucose tolerance and higher physical activity levels than LCR weanlings. Higher respiratory capacities were observed in the soleus, heart, and liver tissues of HCR weanlings compared with LCR weanlings, which were not owed to greater mitochondrial content. Proteomic analyses indicated a greater potential for lipid oxidation in the contractile muscles of HCR weanlings. In conclusion, offspring born to parents with high CRF possess an enhanced capacity for lipid catabolism and oxidative phosphorylation, thereby influencing metabolic health. These findings highlight that intrinsic CRF shapes the bioenergetic phenotype with implications for metabolic resilience in early life.NEW & NOTEWORTHY Inherited cardiorespiratory fitness (CRF) influences early life bioenergetics and metabolic health. Higher intrinsic CRF was associated with reduced adiposity and improved glucose tolerance in early life. This metabolic phenotype was accompanied by greater mitochondrial respiratory capacity in skeletal muscle, heart, and liver tissue. Proteomic profiling of these three tissues further revealed potential mechanisms linking inherited CRF to early life metabolism.

Genotype determining aerobic exercise capacity associates with behavioral plasticity in middle-aged rats

Good aerobic fitness associates positively with cognitive performance and brain health and conversely, low aerobic fitness predisposes to neurodegenerative diseases. To study how genotype together with exercise, started at older age, affects brain and behavior, we utilized rats that differ in inherited aerobic fitness. Rats bred for Low Capacity for Running (LCR) are shown to display less synaptic plasticity and more inflammation in the hippocampus and perform worse than rats bred for a High Capacity for Running (HCR) in tasks requiring flexible cognition. Here we used middle-aged (∼ 16 months) HCR and LCR rats to study how genotype and sex associate with anxiety and neural information filtering, termed sensory gating. Further, we assessed how inherited aerobic capacity associates with hippocampus-dependent learning, measured with contextual fear conditioning task. In females, we also investigated the effects of voluntary wheel running (5 weeks) on these characteristics. Our results indicate that independent of sex or voluntary running, HCR rats were more anxious in open-field tasks, exhibited lower sensory gating and learned more efficiently in contextual fear conditioning task than LCR rats. Voluntary running did not markedly affect innate behavior but slightly decreased the differences between female LCR and HCR rats in fear learning. In conclusion, inherited fitness seems to determine cognitive and behavioral traits independent of sex. Although the traits proved to be rather resistant to change at adult age, learning was slightly improved following exercise in LCR females, prone to obesity and poor fitness.

Reduced contextually induced muscle thermogenesis in rats with calorie restriction and lower aerobic fitness but not monogenic obesity

We have previously identified predator odor as a potent stimulus activating thermogenesis in skeletal muscle in rats. As this may prove relevant for energy balance and weight loss, the current study investigated whether skeletal muscle thermogenesis was altered with negative energy balance, obesity propensity seen in association with low intrinsic aerobic fitness, and monogenic obesity. First, weight loss subsequent to 3 wk of 50% calorie restriction suppressed the muscle thermogenic response to predator odor. Next, we compared rats bred based on artificial selection for intrinsic aerobic fitness - high- and low-capacity runners (HCR, LCR) - that display robust leanness and obesity propensity, respectively. Aerobically fit HCR showed enhanced predator odor-induced muscle thermogenesis relative to the less-fit LCR. This contrasted with the profound monogenic obesity displayed by rats homozygous for a loss of function mutation in Melanocortin 4 receptor (Mc4rK3a,4X/K314X rats), which showed no discernable deficit in thermogenesis. Taken together, these data imply that body size or obesity per se are not associated with deficient muscle thermogenesis. Rather, the physiological phenotype associated with polygenic obesity propensity may encompass pleiotropic mechanisms in the thermogenic pathway. Adaptive thermogenesis associated with weight loss also likely alters muscle thermogenic mechanisms.

Intrinsic cardiorespiratory fitness modulates clinical and molecular response to caloric restriction

OBJECTIVE

Caloric restriction (CR) is one extrinsic intervention that can improve metabolic health, and it shares many phenotypical parallels with intrinsic high cardiorespiratory fitness (CRF), including reduced adiposity, increased cardiometabolic health, and increased longevity. CRF is a highly heritable trait in humans and has been established in a genetic rat model selectively bred for high (HCR) and low (LCR) CRF, in which the HCR live longer and have reduced body weight compared to LCR. This study addresses whether the inherited high CRF phenotype occurs through similar mechanisms by which CR promotes health and longevity.

METHODS

We compared HCR and LCR male rats fed ad libitum (AL) or calorically restricted (CR) for multiple physiological, metabolic, and molecular traits, including running capacity at 2, 8, and 12 months; per-hour metabolic cage activity over daily cycles at 6 and 12 months; and plasma lipidomics, liver and muscle transcriptomics, and body composition after 12 months of treatment.

RESULTS

LCR-CR developed a physiological profile that mirrors the high-CRF phenotype in HCR-AL, including reduced adiposity and increased insulin sensitivity. HCR show higher spontaneous activity than LCR. Temporal modeling of hourly energy expenditure (EE) dynamics during the day, adjusted for body weight and hourly activity levels, suggest that CR has an EE-suppressing effect, and high-CRF has an EE-enhancing effect. Pathway analysis of gene transcripts indicates that HCR and LCR both show a response to CR that is similar in the muscle and different in the liver.

CONCLUSIONS

CR provides LCR a health-associated positive effect on physiological parameters that strongly resemble HCR. Analysis of whole-body EE and transcriptomics suggests that HCR and LCR show line-dependent responses to CR that may be accreditable to difference in genetic makeup. The results do not preclude the possibility that CRF and CR pathways may converge.

Altered skeletal muscle sarco-endoplasmic reticulum Ca2+-ATPase calcium transport efficiency after a thermogenic stimulus

Exposure to predator threat induces a rapid and robust increase in skeletal muscle thermogenesis in rats. The central nervous system relays threat information to skeletal muscle through activation of the sympathetic nervous system, but muscle mechanisms mediating this thermogenesis remain unidentified. Given the relevance of sarcolipin-mediated futile calcium cycling through the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pump to mammalian muscle nonshivering thermogenesis, we hypothesized that this plays a role in contextually induced muscle thermogenesis as well. This was assessed by measuring enzymatic activity of SERCA and sarcoplasmic reticulum Ca2+ transport, where the apparent coupling ratio (Ca2+ uptake rate divided by ATPase activity rate at a standard Ca2+ concentration) was predicted to decrease in association with muscle thermogenesis. Sprague-Dawley rats exposed to predator (ferret) odor (PO) showed a rapid decrease in the apparent coupling ratio in the soleus muscle, indicating SERCA uncoupling compared with control-odor-exposed rats. A rat model of high aerobic fitness and elevated muscle thermogenesis also demonstrated soleus muscle SERCA uncoupling relative to their obesity-prone, low-fitness counterparts. Both the high- and low-aerobic fitness rats showed soleus SERCA uncoupling with exposure to PO. Finally, no increase in sarcolipin expression in soleus muscle was detected with PO exposure. This dataset implicates muscle uncoupling of SERCA Ca2+ transport and ATP hydrolysis, likely through altered SERCA or sarcolipin function outside of translational regulation, as one contributor to the muscle thermogenesis provoked by exposure to predator threat. These data support the involvement of SERCA uncoupling in both muscle thermogenic induction and enhanced aerobic capacity.

Rats with high aerobic capacity display enhanced transcriptional adaptability and upregulation of bile acid metabolism in response to an acute high-fat diet

Rats selectively bred for the high intrinsic aerobic capacity runner (HCR) or low aerobic capacity runner (LCR) show pronounced differences in susceptibility for high-fat/high sucrose (HFHS) diet-induced hepatic steatosis and insulin resistance, replicating the protective effect of high aerobic capacity in humans. We have previously shown multiple systemic differences in energy and substrate metabolism that impacts steatosis between HCR and LCR rats. This study aimed to investigate hepatic-specific mechanisms of action via changes in gene transcription. Livers of HCR rats had a greater number of genes that significantly changed in response to 3-day HFHS compared with LCR rats (171 vs. 75 genes: >1.5-fold, p < 0.05). HCR and LCR rats displayed numerous baseline differences in gene expression while on a low-fat control diet (CON). A 3-day HFHS diet resulted in greater expression of genes involved in the conversion of excess acetyl-CoA to cholesterol and bile acid (BA) synthesis compared with the CON diet in HCR, but not LCR rats. These results were associated with higher fecal BA loss and lower serum BA concentrations in HCR rats. Exercise studies in rats and mice also revealed higher hepatic expression of cholesterol and BA synthesis genes. Overall, these results suggest that high aerobic capacity and exercise are associated with upregulated BA synthesis paired with greater fecal excretion of cholesterol and BA, an effect that may play a role in protection against hepatic steatosis in rodents.

Biology: Motion is Function

In 1966 Francis Crick declared that: "The ultimate aim of the modern movement in biology is to explain all biology in terms of physics and chemistry." This motivated us to contemplate approaches that unify biology at a fundamental level. Exploration led us to consider the features of energy, entropy, and motion. Overall, it can be considered that motion of matter is the feature of life function. No motion. No function. In initial work we evaluated the hypothesis that the scope for biologic function is mediated mechanistically by a differential for energy transfer. Maximal treadmill running capacity served as a proxy for energy transfer. The span for capacity was estimated "biologically" by application of two-way artificial selection in rats for running capacity. Consistent with our "Energy Transfer Hypothesis" (ETH), low physical health and dysfunction segregated with low running capacity and high physical health and function segregated with high running capacity. The high energy yield of aerobic metabolism is also consonant with the ETH; that is, amongst the elements of the universe, oxygen is second only to fluorine in electronegativity. Although we deem these energy findings possibly correct, they are based on correlation and do not illuminate function via fundamental principles. For consideration of life, Entropy (2nd Law of thermodynamics) can be viewed as an open system that exchanges energy with the universe operating via nonequilibrium thermodynamics. The Principle of Maximal Entropy Production (MEP) states that: If a source of free energy is present, complex systems can intercept the free energy flow, and self-organize to enhance entropy production. The development of Benard convection cells in a water heat gradient demonstrate simplistic operation of MEP. A direct step forward would be to explain the mechanism of the obligatory motion of molecules for life function. Motion may be mediated by operation of "action at a distance" for molecules as considered by the Einstein-Podolsky-Rosen Paradox and confirmed by JS Bell. Magnetism, electricity, and gravity are also examples of action at a distance. We propose that some variant of "action at a distance" as directed by the property of Maximal Entropy Production (MEP) underwrites biologic motion.

Life-long exercise training and inherited aerobic endurance capacity produce converging gut microbiome signatures in rodents

High aerobic endurance capacity can be acquired by training and/or inherited. Aerobic exercise training (AET) and aging are linked to altered gut microbiome composition, but it is unknown if the environmental stress of exercise and host genetics that predispose for higher exercise capacity have similar effects on the gut microbiome during aging. We hypothesized that exercise training and host genetics would have conserved effects on the gut microbiome across different rodents. We studied young sedentary (Y-SED, 2-month-old) mice, old sedentary (O-SED, 26-month-old) mice, old mice with life-long AET (O-AET, 26-month-old), and aged rats selectively bred for high (HCR [High Capacity Runner], 21-month-old) and low (LCR [Low Capacity Runner], 21-month-old) aerobic capacity. Our results showed that O-SED mice had lower running capacity than Y-SED mice. The fecal microbiota of O-SED mice had a higher relative abundance of Lachnospiraceae, Ruminococcaceae, Turicibacteriaceae, and Allobaculum, but lower Bacteroidales, Alistipes, Akkermansia, and Anaeroplasma. O-AET mice had a higher running capacity than O-SED mice. O-AET mice had lower fecal levels of Lachnospiraceae, Turicibacteriaceae, and Allobaculum and higher Anaeroplasma than O-SED mice. Similar to O-AET mice, but despite no exercise training regime, aged HCR rats had lower Lachnospiraceae and Ruminococcaceae and expansion of certain Bacteroidales in the fecal microbiome compared to LCR rats. Our data show that environmental and genetic modifiers of high aerobic endurance capacity produce convergent gut microbiome signatures across different rodent species during aging. Therefore, we conclude that host genetics and life-long exercise influence the composition of the gut microbiome and can mitigate gut dysbiosis and functional decline during aging.

Age and Intrinsic Fitness Affect the Female Rotator Cuff Tendon Tissue

The risk of the development of tendon disorders or ruptures increases with age, but it is unclear whether intrinsic fitness during lifetime might also affect tendon properties. To investigate this, a contrasting rat model of high-capacity runners (HCR with high intrinsic fitness) and low-capacity runners (LCR with low intrinsic fitness) was employed. Histological and molecular changes in rotator cuff (RC) tendons from 10 weeks old (young; HCR-10 and LCR-10) and 100 weeks old (old; HCR-100 and LCR-100) female rats were investigated. Age-dependent changes of RC tendons observed in HCR and LCR were increase of weight, decrease of tenocytes and RNA content, reduction of the wavy pattern of collagen and elastic fibers, repressed expression of Col1a1, Eln, Postn, Tnmd, Tgfb3 and Egr1 and reduction of the Col1:Col3 and Col1:Eln ratio. The LCR rats showed less physical activity, increased body weight, signs of metabolic disease and a reduced life expectancy. Their RC tendons revealed increased weight (more than age-dependent) and enlargement of the tenocyte nuclei (consistent with degenerative tendons). Low intrinsic fitness led to repressed expression of a further nine genes (Col3a1, Fbn1, Dcn, Tnc, Scx, Mkx, Bmp1, Tgfb1, Esr1) as well as the rise of the Col1:Col3 and Col1:Eln ratios (related to the lesser expression of Col3a1 and Eln). The intrinsic fitness influences the female RC tendons at least as much as age. Lower intrinsic fitness accelerates aging of RC tendons and leads to further impairment; this could result in decreased healing potential and elasticity and increased stiffness.

Interactive effects of aging and aerobic capacity on energy metabolism-related metabolites of serum, skeletal muscle, and white adipose tissue

Aerobic capacity is a strong predictor of longevity. With aging, aerobic capacity decreases concomitantly with changes in whole body metabolism leading to increased disease risk. To address the role of aerobic capacity, aging, and their interaction on metabolism, we utilized rat models selectively bred for low and high intrinsic aerobic capacity (LCRs/HCRs) and compared the metabolomics of serum, muscle, and white adipose tissue (WAT) at two time points: Young rats were sacrificed at 9 months of age, and old rats were sacrificed at 21 months of age. Targeted and semi-quantitative metabolomics analysis was performed on the ultra-pressure liquid chromatography tandem mass spectrometry (UPLC-MS) platform. The effects of aerobic capacity, aging, and their interaction were studied via regression analysis. Our results showed that high aerobic capacity is associated with an accumulation of isovalerylcarnitine in muscle and serum at rest, which is likely due to more efficient leucine catabolism in muscle. With aging, several amino acids were downregulated in muscle, indicating more efficient amino acid metabolism, whereas in WAT less efficient amino acid metabolism and decreased mitochondrial β-oxidation were observed. Our results further revealed that high aerobic capacity and aging interactively affect lipid metabolism in muscle and WAT, possibly combating unfavorable aging-related changes in whole body metabolism. Our results highlight the significant role of WAT metabolism for healthy aging.

Genetically determined exercise capacity affects systemic glucose response to insulin in rats

INTRODUCTION

Aerobic exercise capacity is inversely related to morbidity and mortality as well as to insulin resistance. However, exercising in patients has led to conflicting results, presumably because aerobic exercise capacity consists of intrinsic (genetically determined) and extrinsic (environmentally determined) parts. The contribution of both parts to insulin sensitivity is also not clear. We investigated sedentary and exercised (aerobic interval training) high (HCR) and low capacity runners (LCR) differing in their genetically determined aerobic exercise capacity to determine the contribution of both parts to insulin sensitivity.

METHODS AND RESULTS

LCR and HCR differed in their untrained exercise capacity and body weight. Sedentary LCR displayed a diabetic phenotype with higher random glucose, lower glucose infusion rate during hyperinsulinemic euglycemic clamping than HCR. Echocardiography showed equal morphological and functional parameters and no change with exercise. Four weeks of exercise caused significant improvements in aerobic exercise capacity, which was more pronounced in LCR. However, with respect to glucose use, exercise affected HCR only. In these animals, exercise increased 2-deoxyglucose uptake in gastrocnemius (+58.5 %, p= 0.1) and in epididymal fat (+106 %; p<0.05). Citrate synthase activity also increased in these tissues (gastrocnemius 69 % epididymal fat 63 %).

CONCLUSION

In our model of HCR and LCR, genetic predisposition for low exercise capacity is associated with impaired insulin sensitivity and impedes exercise-induced improvements in insulin response. Our results suggest that genetic predisposition for low aerobic exercise capacity impairs insulin response, which may not be overcome by exercise.

Intrinsic Aerobic Capacity Affects Hippocampal pAkt and HSP72 Response to an Acute High Fat Diet and Heat Treatment in Rats

BACKGROUND

Aerobic capacity is associated with metabolic, cardiovascular, and neurological health. Low-capacity runner (LCR) rats display low aerobic capacity, metabolic dysfuction, and spatial memory deficits. A heat treatment (HT) can improve metabolic dysfunction in LCR peripheral organs after high fat diet (HFD). Little is known about metabolic changes in the brains of these rats following HT.

OBJECTIVE

Our objective was to examine the extent to which high or low aerobic capacity impacts Akt (a protein marker of metabolism) and heat shock protein 72 (HSP72, a marker of heat shock response) after HFD and HT in hippocampus.

METHODS

We measured phosphorylated Akt (pAkt) in the striatum and hippocampus, and HSP72 in the hippocampus, of HFD-fed and chow-fed LCR and high-capacity runner (HCR) rats with and without HT.

RESULTS

pAkt was lower in the hippocampus of chow-fed LCR than HCR rats. HFD resulted in greater pAkt in LCR but not HCR rats, but HT resulted in lower pAkt in the LCR HFD group. HSP72 was greater in both HCR and LCR rat hippocampus after HT. The HFD blunted this effect in LCR compared to HCR hippocampus.

CONCLUSION

The abnormal phosphorylation of Akt and diminished HSP response in the hippocampus of young adult LCR rats might indicate early vulnerability to metabolic challenges in this key brain region associated with learning and memory.

Differential weight loss with intermittent fasting or daily calorie restriction in low- and high-fitness phenotypes

NEW FINDINGS

What is the central question of this study? How does intrinsic aerobic capacity impact weight loss with 50% daily caloric restriction and alternate-day fasting? What is the main finding and its importance? Intermittent fasting is effective for weight loss in rats with low fitness, which highlights the importance of how intermittent fasting interacts with aerobic fitness.

ABSTRACT

Recent interest has focused on the benefits of time-restricted feeding strategies, including intermittent fasting, for weight loss. It is not yet known whether intermittent fasting is more effective than daily caloric restriction at stimulating weight loss and how each is subject to individual differences. Here, rat models of leanness and obesity, artificially selected for intrinsically high (HCR) and low (LCR) aerobic capacity, were subjected to intermittent fasting and 50% calorie restrictive diets in two separate experiments using male rats. The lean, high-fitness HCR and obesity-prone, low-fitness LCR rats underwent 50% caloric restriction while body weight and composition were monitored. The low-fitness LCR rats were better able to retain lean mass than the high-fitness HCR rats, without significantly different proportional loss of weight or fat. In a separate experiment using intermittent fasting in male HCR and LCR rats, alternate-day fasting induced significantly greater loss of weight and fat mass in LCR compared with HCR rats, although the HCR rats had a more marked reduction in ad libitum daily food intake. Altogether, this suggests that intermittent fasting is an effective weight-loss strategy for those with low intrinsic aerobic fitness; however, direct comparison of caloric restriction and intermittent fasting is warranted to determine any differential effects on energy expenditure in lean and obesity-prone phenotypes.

A collagen extraction and deuterium oxide stable isotope tracer method for the quantification of bone collagen synthesis rates in vivo

The development of safe and practical strategies to prevent weakening of bone tissue is vital, yet attempts to achieve this have been hindered by a lack of understanding of the short-term (days-weeks) physiology of bone collagen turnover. To address this, we have developed a method to quantify bone collagen synthesis in vivo, using deuterium oxide (D₂ O) tracer incorporation techniques combined with gas chromatography pyrolysis isotope-ratio mass spectrometry (GC-pyrolysis-IRMS). Forty-six male and female rats from a selectively bred model ingested D₂ O for 3 weeks. Femur diaphyses (FEM), tibia proximal (T-PRO), and distal (T-DIS) epiphyses-metaphyses and tibia mid-shaft diaphyses (T-MID) were obtained from all rats after necropsy. After demineralisation, collagen proteins were isolated and hydrolysed and collagen fractional synthetic rates (FSRs) determined by incorporation of deuterium into protein-bound alanine via GC-pyrolysis-IRMS. The collagen FSR for the FEM (0.131 ± 0.078%/day; 95% CI [0.106-0.156]) was greater than the FSR at T-MID (0.055 ± 0.049%/day; 95% CI [0.040-0.070]; p < 0.001). The T-PRO site had the highest FSR (0.203 ± 0.123%/day; 95% CI [0.166-0.241]) and T-DIS the lowest (0.027 ± 0.015%/day; 95% CI [0.022-0.031]). The three tibial sites exhibited different FSRs (p < 0.001). Herein, we have developed a sensitive method to quantify in vivo bone collagen synthesis and identified site-specific rates of synthesis, which could be applicable to studies of human bone collagen turnover.

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.

Rat models for low and high adaptive response to exercise differ for stress-related memory and anxiety

Physical exercise and fitness may serve as resilience factors to stress exposure. However, the extreme range in human exercise performance suggests that genetic variation for exercise capacity could be a confounding feature to understanding the connection between exercise and stress exposure. To test this idea, we use laboratory rat models selectively bred for a low and high gain in aerobic running capacity in response to training to examine whether an inherent capacity to respond to physical exercise reflects how stress changes neurobiological functioning and regulates fear-associated memory processing. Utilization of this contrasting rat model system of low and high responders has the potential to guide the interpretation of the reported association with exercise involvement and the reduction of stress-induced anxiety disorders. Our data show that aerobic fitness may be linked to the ability to regulate fear-associated memories. We also show that acquired exercise capacity may play a key role in regulating responses to an acute stressor. Exercise sensitivity plays a significant role in the activation of the plasticity-associated molecule extracellular signal-regulated kinase, changes in stress hormone activity, and anatomical modifications to the noradrenergic locus coeruleus. These data identify a unique operational mechanism that may serve as translational targets for lessening symptoms of stress and anxiety.

Low responders to endurance training exhibit impaired hypertrophy and divergent biological process responses in rat skeletal muscle

NEW FINDINGS

What is the central question of this study? The extent to which genetics determines adaptation to endurance versus resistance exercise is unclear. Previously, a divergent selective breeding rat model showed that genetic factors play a major role in the response to aerobic training. Here, we asked: do genetic factors that underpin poor adaptation to endurance training affect adaptation to functional overload? What is the main finding and its importance? Our data show that heritable factors in low responders to endurance training generated differential gene expression that was associated with impaired skeletal muscle hypertrophy. A maladaptive genotype to endurance exercise appears to dysregulate biological processes responsible for mediating exercise adaptation, irrespective of the mode of contraction stimulus.

ABSTRACT

Divergent skeletal muscle phenotypes result from chronic resistance-type versus endurance-type contraction, reflecting the principle of training specificity. Our aim was to determine whether there is a common set of genetic factors that influence skeletal muscle adaptation to divergent contractile stimuli. Female rats were obtained from a genetically heterogeneous rat population and were selectively bred from high responders to endurance training (HRT) or low responders to endurance training (LRT; n = 6/group; generation 19). Both groups underwent 14 days of synergist ablation to induce functional overload of the plantaris muscle before comparison to non-overloaded controls of the same phenotype. RNA sequencing was performed to identify Gene Ontology biological processes with differential (LRT vs. HRT) gene set enrichment. We found that running distance, determined in advance of synergist ablation, increased in response to aerobic training in HRT but not LRT (65 ± 26 vs. -6 ± 18%, mean ± SD, P < 0.0001). The hypertrophy response to functional overload was attenuated in LRT versus HRT (20.1 ± 5.6 vs. 41.6 ± 16.1%, P = 0.015). Between-group differences were observed in the magnitude of response of 96 upregulated and 101 downregulated pathways. A further 27 pathways showed contrasting upregulation or downregulation in LRT versus HRT in response to functional overload. In conclusion, low responders to aerobic endurance training were also low responders for compensatory hypertrophy, and attenuated hypertrophy was associated with differential gene set regulation. Our findings suggest that genetic factors that underpin aerobic training maladaptation might also dysregulate the transcriptional regulation of biological processes that contribute to adaptation to mechanical overload.

Myokine Responses to Exercise in a Rat Model of Low/High Adaptive Potential

INTRODUCTION

Assuming myokines underlie some of the health benefits of exercise, we hypothesised that 'high responder trainer' (HRT) rats would exhibit distinct myokine profiles to 'low responder trainers' (LRT), reflecting distinct health and adaptive traits.

METHODS

Blood was collected from LRT and HRT (N=8) rats at baseline (BL), immediately (0h), 1h, and 3h after running; repeated after 3-wks training. Myokines were analysed by ELISA (i.e. BDNF/Fractalkine/SPARC/Irisin/FGF21/Musclin/IL-6).

RESULTS

At baseline, Musclin (LRT: 84 ± 24 vs HRT: 26 ± 3 pg/ml, P=0.05) and FGF21 (LRT: 133 ± 34 vs HRT: 63.5 ± 13 pg/ml, P=0.08) were higher in LRT than HRT. Training increased Musclin in HRT (26 ± 3 to 54 ± 9 pg/ml, P<0.05) and decreased FGF21 in LRT (133 ± 34 to 60 ± 28 pg/ml, P<0.05). Training increased SPARC (LRT: 0.8 ± 0.1 to 2.1 ± 0.6 ng/ml, P<0.05; HRT: 0.7 ± 0.06 to 1.8 ± 0.3 ng/ml, P=0.06) and Irisin (LRT 0.62 ± 0.1 to 2.6 ± 0.4 ng/ml, P<0.01; HRT 0.53 ± 0.1 to 2.8 ± 0.7 ng/ml, P<0.01) while decreasing BDNF (LRT: 2747 ± 293 to 1081 ± 330 pg/ml, P<0.01; HRT: 1976 ± 328 to 797 ± 160 pg/ml, P<0.05). Acute exercise response of Musclin (AUC) was higher in LRT vs HRT (306 ± 74 vs. 88 ± 12 pg/ml×3h^-1, P<0.01) and elevated in HRT after training (221 ± 31 pg/ml×3h^-1, P<0.01). Training elevated SPARC (LRT: 2.4 ± 0.1 to 7.7 ± 1.3 ng/ml×3h^-1, P<0.05; HRT: 2.5 ± 0.13 to 11.2 ± 2.2 ng/ml×3h^-1, P<0.001) and Irisin (LRT: 1.34 ± 0.3 to 9.6 ± 1.7 ng/ml×3h^-1, P<0.001; HRT: 1.5 ± 0.5 to 12.1 ± 1.9 ng/ml×3h^-1, P<0.0001).

CONCLUSION

Exercise training alters how myokines are secreted in response to acute exercise. Myokine responses were not robustly linked to adaptive potential in aerobic capacity, making them an unlikely regulator of adaptive traits.

Enhanced weight and fat loss from long-term intermittent fasting in obesity-prone, low-fitness rats

BACKGROUND

Intermittent fasting (IF) strategies have emerged as viable alternatives to traditional calorie-restricted diets. A key predictor of metabolic health and response to diet is cardiometabolic fitness, including intrinsic aerobic capacity. In a contrasting rat model of aerobic capacity-high- and low-capacity runners (HCR, LCR)-we found that the lean and physically active HCR were also more responsive to a standard calorie-restricted diet. Here, we assessed the ability of IF to induce weight loss on a background of high and low aerobic fitness accompanied by different levels of daily physical activity.

METHODS

Female HCR and LCR (8 per line) were subjected to IF (alternate-day fasting) for 14 weeks. Outcomes included changes in body weight, fat and lean mass, daily physical activity, and food and water intake. After initial measurements, IF was continued, and measurements were repeated after one year of IF.

RESULTS

All rats lost weight with IF, and LCR lost significantly more weight than HCR. This difference was primarily due to differential fat loss; loss of lean mass, on the other hand, was similar between HCR and LCR. Total food intake decreased with IF, and LCR showed lower intake than HCR only during the first 5 weeks of IF. Physical activity was suppressed by long-term IF. Physical activity increased on fed days compared to fasted days, and this pattern was more pronounced in HCR. The differential effects of IF in HCR and LCR persisted after one year of IF, with IF preventing the marked weight gain seen in ad libitum fed LCR during this time.

CONCLUSION

Weight and fat loss from IF was more pronounced in obesity-prone, low-aerobic capacity LCR, despite the low activity levels seen in these rats. The possibility that aerobic capacity modulates response to IF in human participants remains unexplored.

Differences in intrinsic aerobic capacity alters sensitivity to ischemia-reperfusion injury but not cardioprotective capacity by ischemic preconditioning in rats

INTRODUCTION

Aerobic capacity is a strong predictor of cardiovascular mortality. Whether aerobic capacity influences myocardial ischemia and reperfusion (IR) injury is unknown.

PURPOSE

To investigate the impact of intrinsic differences in aerobic capacity and the cardioprotective potential on IR injury.

METHODS

We studied hearts from rats developed by selective breeding for high (HCR) or low (LCR) capacity for treadmill running. The rats were randomized to: (1) control, (2) local ischemic preconditioning (IPC) or (3) remote ischemic preconditioning (RIC) followed by 30 minutes of ischemia and 120 minutes of reperfusion in an isolated perfused heart model. The primary endpoint was infarct size. Secondary endpoints included uptake of labelled glucose, content of selected mitochondrial proteins in skeletal and cardiac muscle, and activation of AMP-activated kinase (AMPK).

RESULTS

At baseline, running distance was 203±7 m in LCR vs 1905±51 m in HCR rats (p<0.01). Infarct size was significantly lower in LCR than in HCR controls (49±5% vs 68±5%, p = 0.04). IPC reduced infarct size by 47% in LCR (p<0.01) and by 31% in HCR rats (p = 0.01). RIC did not modulate infarct size (LCR: 52±5, p>0.99; HCR: 69±6%, p>0.99, respectively). Phosphorylaion of AMPK did not differ between LCR and HCR controls. IPC did not modulate cardiac phosphorylation of AMPK. Glucose uptake during reperfusion was similar in LCR and HCR rats. IPC increased glucose uptake during reperfusion in LCR animals (p = 0.02). Mitochondrial protein content in skeletal muscle was lower in LCR than in HCR (0.77±0.10 arbitrary units (AU) vs 1.09±0.07 AU, p = 0.02), but not in cardiac muscle.

CONCLUSION

Aerobic capacity is associated with altered myocardial sensitivity to IR injury, but the cardioprotective effect of IPC is not. Glucose uptake, AMPK activation immediately prior to ischemia and basal mitochondrial protein content in the heart seem to be of minor importance as underlying mechanisms for the cardioprotective effects.

Aerobic capacity modulates adaptive thermogenesis: Contribution of non-resting energy expenditure

Decreases in energy stores requires negative energy balance where caloric expenditure exceeds energy intake, which can induce adaptive thermogenesis-the reduction of energy expenditure (EE) beyond that accounted for by the weight lost. Adaptive thermogenesis varies between individuals. The component of total daily EE responsible for the interindividual variation in adaptive thermogenesis was investigated in this study, using a rat model that differs in obesity propensity and physical activity. Total daily EE and physical activity were examined before and after 21 days of 50% calorie restriction in male and female rats with lean and obesity-prone phenotypes-rats selectively bred for high and low intrinsic aerobic capacity (HCR and LCR, respectively). Calorie restriction significantly decreased EE more than was predicted by loss of weight and lean mass, demonstrating adaptive thermogenesis. Within sex, HCR and LCR did not significantly differ in resting EE. However, the calorie restriction-induced suppression in non-resting EE, which includes activity EE, was significantly greater in HCR than in LCR; this phenotypic difference was significant for both male and female rats. Calorie restriction also significantly suppressed physical activity levels more in HCR than LCR. When VO2max was assessed in male rats, calorie restriction significantly decreased O2 consumption without significantly affecting running performance (running time, distance), indicating increased energy efficiency. Percent weight loss did not significantly differ between groups. Altogether, these results suggest that individual differences in calorie restriction-induced adaptive thermogenesis may be accounted for by variation in aerobic capacity. Moreover, it is likely that activity EE, not resting or basal metabolism, may explain or predict the variation in individuals' adaptive thermogenesis.

Rats Genetically Selected for High Aerobic Exercise Capacity Have Elevated Plasma Bilirubin by Upregulation of Hepatic Biliverdin Reductase-A (BVRA) and Suppression of UGT1A1

Exercise in humans and animals increases plasma bilirubin levels, but the mechanism by which this occurs is unknown. In the present study, we utilized rats genetically selected for high capacity running (HCR) and low capacity running (LCR) to determine pathways in the liver that aerobic exercise modifies to control plasma bilirubin. The HCR rats, compared to the LCR, exhibited significantly higher levels of plasma bilirubin and the hepatic enzyme that produces it, biliverdin reductase-A (BVRA). The HCR also had reduced expression of the glucuronyl hepatic enzyme UGT1A1, which lowers plasma bilirubin. Recently, bilirubin has been shown to activate the peroxisome proliferator-activated receptor-α (PPARα), a ligand-induced transcription factor, and the higher bilirubin HCR rats had significantly increased PPARα-target genes Fgf21, Abcd3, and Gys2. These are known to promote liver function and glycogen storage, which we found by Periodic acid–Schiff (PAS) staining that hepatic glycogen content was higher in the HCR versus the LCR. Our results demonstrate that exercise stimulates pathways that raise plasma bilirubin through alterations in hepatic enzymes involved in bilirubin synthesis and metabolism, improving liver function, and glycogen content. These mechanisms may explain the beneficial effects of exercise on plasma bilirubin levels and health in humans.

Hyperglycaemia is associated with impaired muscle signalling and aerobic adaptation to exercise

Increased aerobic exercise capacity, as a result of exercise training, has important health benefits. However, some individuals are resistant to improvements in exercise capacity, probably due to undetermined genetic and environmental factors. Here, we show that exercise-induced improvements in aerobic capacity are blunted and aerobic remodelling of skeletal muscle is impaired in several animal models associated with chronic hyperglycaemia. Our data point to chronic hyperglycaemia as a potential negative regulator of aerobic adaptation, in part, via glucose-mediated modifications of the extracellular matrix, impaired vascularization and aberrant mechanical signalling in muscle. We also observe low exercise capacity and enhanced c-Jun N-terminal kinase activation in response to exercise in humans with impaired glucose tolerance. Our work indicates that current shifts in dietary and metabolic health, associated with increasing incidence of hyperglycaemia, might impair muscular and organismal adaptations to exercise training, including aerobic capacity as one of its key health outcomes.

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.

Striated muscle-specific serine/threonine-protein kinase beta segregates with high versus low responsiveness to endurance exercise training

Bidirectional selection for either high or low responsiveness to endurance running has created divergent rat phenotypes of high-response trainers (HRT) and low-response trainers (LRT). We conducted proteome profiling of HRT and LRT gastrocnemius of 10 female rats (body weight 279 ± 35 g; n = 5 LRT and n = 5 HRT) from generation 8 of selection. Differential analysis of soluble proteins from gastrocnemius was conducted by label-free quantitation. Genetic association studies were conducted in 384 Russian international-level athletes (age 23.8 ± 3.4 yr; 202 men and 182 women) stratified to endurance or power disciplines. Proteomic analysis encompassed 1,024 proteins, 76 of which exhibited statistically significant (P < 0.05, false discovery rate <1%) differences between HRT and LRT muscle. There was significant enrichment of enzymes involved in glycolysis/gluconeogenesis in LRT muscle but no enrichment of gene ontology phrases in HRT muscle. Striated muscle-specific serine/threonine-protein kinase-beta (SPEG-β) exhibited the greatest difference in abundance and was 2.64-fold greater (P = 0.0014) in HRT muscle. Coimmunoprecipitation identified 24 potential binding partners of SPEG-β in HRT muscle. The frequency of the G variant of the rs7564856 polymorphism that increases SPEG gene expression was significantly greater (32.9 vs. 23.8%; OR = 1.6, P = 0.009) in international-level endurance athletes (n = 258) compared with power athletes (n = 126) and was significantly associated (β = 8.345, P = 0.0048) with a greater proportion of slow-twitch fibers in vastus lateralis of female endurance athletes. Coimmunoprecipitation of SPEG-β in HRT muscle discovered putative interacting proteins that link with previously reported differences in transforming growth factor-β signaling in exercised muscle.

Denervation and senescence markers data from old rats with intrinsic differences in responsiveness to aerobic training

The data described below is related to the manuscript “Late life maintenance and enhancement of functional exercise capacity in low and high responding rats after low intensity treadmill training” [1]. Rodents exhibit age-related declines in skeletal muscle function that is associated with muscle denervation and cellular senescence. Exercise training is a proven method to delay or even reverse some aging phenotypes, thus improving healthspan in the elderly. The beneficial effects of exercise to preserve muscle may be reliant on an individual's innate ability to adapt to aerobic training. To examine this question, we assessed aged rats that were selectively bred to be either minimally or highly responsive to aerobic exercise training. We specifically asked whether mild treadmill training initiated late in life would be beneficial to preserve muscle function in high response and low response trainer rats. We examined gene expression data on markers of denervation and senescence. We also evaluated measures of aerobic training and neuromuscular muscle function through work capacity, contractile properties, and endplate fragmentation for further analysis of the aging phenotype in older rodents.

Vertical selection for nuclear and mitochondrial genomes shapes gut microbiota and modifies risks for complex diseases

Here we postulate that the heritability of complex disease traits previously ascribed solely to the inheritance of the nuclear and mitochondrial genomes is broadened to encompass a third component of the holobiome, the microbiome. To test this, we expanded on the selectively bred low capacity runner/high capacity runner (LCR/HCR) rat exercise model system into four distinct rat holobiont model frameworks including matched and mismatched host nuclear and mitochondrial genomes. Vertical selection of varying nuclear and mitochondrial genomes resulted in differential acquisition of the microbiome within each of these holobiont models. Polygenic disease risk of these novel models were assessed and subsequently correlated with patterns of acquisition and contributions of their microbiomes in controlled laboratory settings. Nuclear-mitochondrial-microbiotal interactions were not for exercise as a reporter of health, but significantly noted for increased adiposity, increased blood pressure, compromised cardiac function, and loss of long-term memory as reporters of disease susceptibility. These findings provide evidence for coselection of the microbiome with nuclear and mitochondrial genomes as an important feature impacting the heritability of complex diseases.

Late life maintenance and enhancement of functional exercise capacity in low and high responding rats after low intensity treadmill training

Intrinsic exercise capacity is predictive of both lifespan and healthspan but whether adaptive exercise capacity influences the benefits achieved from aerobic training implemented later in life is not known.

AIM

To determine if exercise late in life provides any functional improvements or underlying beneficial biochemical adaptations in rats bred to have a high response to training (HRT rats) or little to no response to training (LRT rats).

METHODS

Adult (11 months) and old (22 months) female LRT and HRT rats either remained sedentary (SED) or were exercised (EXER) on a treadmill 2-3 times/week at 60% of their initial maximum running speed and distance for 4 months. At 26 months of age, exercise capacity was re-evaluated and extensor digitorum longus, gastrocnemius (GTN), and tibialis anterior (TA) muscles were excised for histological and biochemical analysis.

RESULTS

Both SED-HRT and SED-LRT rats showed decreased exercise capacity from 22 to 26 months, but with 4 months of treadmill training, EXER-HRT rats displayed a 50% improvement in exercise capacity while EXER-LRT rats maintained pre-training levels. Protein levels of antioxidant enzymes PRDX3, CuZnSOD, and PRXV were 6-fold greater in TA muscles of aged HRT rats compared to LRT rats. PGC-1α protein levels were ~2-fold greater in GTN and TA muscles of aged HRT than in LRT rats and TFAM protein was similarly elevated in GTN muscles of aged HRT rats compared with LRT rats. BNIP3 protein levels were 5-fold greater in TA muscles of aged HRT than in LRT rats while PINK1 protein content was reduced by 78% in GTN muscles of aged HRT rats compared with LRT rats.

CONCLUSION

HRT rats retained the ability to improve exercise capacity into late life and that ability was associated with inherent and adaptive changes in antioxidant enzyme levels and markers of and mitochondrial quality related to healthspan benefits in aging. Moreover, low intensity exercise prevented the age-associated decline in functional exercise capacity in LRT rats.

Intrinsic High Aerobic Capacity in Male Rats Protects Against Diet-Induced Insulin Resistance

Low aerobic capacity increases the risk for insulin resistance but the mechanisms are unknown. In this study, we tested susceptibility to acute (3-day) high-fat, high-sucrose diet (HFD)-induced insulin resistance in male rats selectively bred for divergent intrinsic aerobic capacity, that is, high-capacity running (HCR) and low-capacity running (LCR) rats. We employed hyperinsulinemic-euglycemic clamps, tracers, and transcriptome sequencing of skeletal muscle to test whether divergence in aerobic capacity impacted insulin resistance through systemic and tissue-specific metabolic adaptations. An HFD evoked decreased insulin sensitivity and insulin signaling in muscle and liver in LCR rats, whereas HCR rats were protected. An HFD led to increased glucose transport in skeletal muscle (twofold) of HCR rats while increasing glucose transport into adipose depots of the LCR rats (twofold). Skeletal muscle transcriptome revealed robust differences in the gene profile of HCR vs LCR on low-fat diet and HFD conditions, including robust differences in specific genes involved in lipid metabolism, adipogenesis, and differentiation. HCR transcriptional adaptations to an acute HFD were more robust than for LCR and included genes driving mitochondrial energy metabolism. In conclusion, intrinsic aerobic capacity robustly impacts systemic and skeletal muscle adaptations to HFD-induced alterations in insulin resistance, an effect that is likely driven by baseline differences in oxidative capacity, gene expression profile, and transcriptional adaptations to an HFD.

Abstract P4-13-05: Evolving treatment patterns in hormone receptor-positive, HER2-negative metastatic breast cancer

Background: The last several years have seen the approval of multiple targeted agents for use alone or combined with standard endocrine therapies (ET) in the 1st, 2nd, and 3rd line hormone receptor positive (HR+), HER2 negative (HER2-) metastatic breast cancer (MBC) settings. Due to these new approvals, standards of care for treatment are evolving.

Methods: Prescribing preferences (PPrefs) of 592 U.S.-based medical oncologists were studied using a validated, proprietary, case-based market research tool (Challenging Cases®). Data were acquired using blinded, audience-response iPad technology at 8 live and virtual events during 2016-2018.

Two core hypothetical cases were presented: 1st line MBC and recurrent (REC) MBC. For each core case the following variables were introduced: time from completion of adjuvant (adj) therapy (Tx) to REC disease, site of metastases [visceral (VIS) vs non-visceral (N-VIS)], and age for the 1st line case, and type of metastases and line of Tx for the recurrent case. Rx choices for which there are published phase 3 data were offered, as well as a category for “other.”

Results:

Table 1.Preferred Rx for recurrent disease during or after 5 yrs of adjuvant non-steroidal aromatase inhibitor (NSAI)*ScenarioExemestane (EXE) + Everolimus (EVE)NSAI + CDK inhibitor (CDKi)Fulvestrant (FUL) + CDKiChemotherapy (CT)Endocrine therapy (ET)OtherREC 18 months (mos) into adj AI Tx, VIS) and N-VIS mets, Age 635%25%63%3%4%0%REC post 5 yrs adj AI and 2 yr treatment free interval (TFI), N-VIS mets, Age 674%49%34%1%12%1%REC post 5 yrs adj AI and 2 yr TFI, N-VIS mets, Age 853%34%19%0%42%1%* May not equal 100% due to rounding.

Table 2.Preferred Rx for 2nd or 3rd recurrence*ScenarioEXE + EVENSAI + CDKiFUL + CDKiCTETAbemaciclibOtherREC post 5 yrs adj AI and 2 yr TFI, N-VIS mets, age 67 --&gt; 12 mos 1st line Tx --&gt; N-VIS mets31%12%38%6%12%1%1%REC post 5 yrs adj AI and 2 yr TFI, N-VIS mets, age 67 --&gt; 12 mos 1st line NSAI + palbociclib Tx --&gt; N-VIS mets38%1%33%9%15%0%3%REC 12 mos into adj AI, age 61,N-VIS mets --&gt;12 mos Tx 1st line --&gt;N-VIS mets21%9%53%9%6%0%1%REC post 5 yrs adj AI and 2 yr TFI, N-VIS mets, age 67 --&gt;12 mos 1st line Tx, N-VIS mets --&gt; 6 mos 2nd line Tx --&gt; VIS and N-VIS mets23%1%9%59%5%2%1%* May not equal 100% due to rounding.

In the setting of a patient (pt), age 63 with early REC on an adj AI, the most preferred strategy was FUL + a CDKi. In a pt, age 67, who completed 5 years of an adj AI with a TFI of 2 years, the most common preference was NSAI + a CDKi. In an older pt (age 85) who completed 5 years of an adj AI and TFI of 2 years, the most common choice of TX was single agent ET. For a pt with N-VIS mets treated in the 2nd line setting, FUL + a CDKi was the most preferred strategy. In a 2nd line pt with REC at 12 mos on adj Tx and after 12 mos of 1st line Tx, FUL + a CDKi and EXE + EVE were the most preferred therapies. In a 3rd line pt, with VIS mets CT was the preferred choice.

Conclusion: Treatment patterns in HR+ HER2- MBC are evolving with the approval of several new agents and emerging data. Age, time to relapse, line of Tx, and type of mets may be some of the key factors that determine PPrefs in HR+, HER2- MBC.

Citation Format: Lankford ML, Britton SL, Buettner AD, Scharf M, Tiscione B, Willey JP, Mahtani RL. Evolving treatment patterns in hormone receptor-positive, HER2-negative metastatic breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P4-13-05.

Rat Models of Exercise for the Study of Complex Disease

Variation in exercise capacity is a translationally powerful indicator for overall health and disease. Here we review the basic methods used for development of theoretically based and hypothesis-driven rat models that divide for both exercise capacity and numerous complex disease risks This rat model system was made by selectively breeding genetically heterogeneous rat populations for low and high performance on a speed ramped treadmill running test.

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

Purpose

Exercise training increases aerobic capacity and is beneficial for health, whereas low aerobic exercise capacity is a strong independent predictor of cardiovascular disease and premature death. The purpose of the present study was to determine the metabolic profiles in a rat model of inborn low versus high capacity runners (LCR/HCR) and to determine the effect of inborn aerobic capacity, aging, and exercise training on skeletal muscle metabolic profile.

Methods

LCR/HCR rats were randomized to high intensity low volume interval treadmill training twice a week or sedentary control for 3 or 11 months before they were sacrificed, at 9 and 18 months of age, respectively. Magnetic resonance spectra were acquired from soleus muscle extracts, and partial least square discriminative analysis was used to determine the differences in metabolic profile.

Results

Sedentary HCR rats had 54% and 30% higher VO_2max compared to sedentary LCR rats at 9 months and 18 months, respectively. In HCR, exercise increased running speed significantly, and VO_2max was higher at age of 9 months, compared to sedentary counterparts. In LCR, changes were small and did not reach the level of significance. The metabolic profile was significantly different in the LCR sedentary group compared to the HCR sedentary group at the age of 9 and 18 months, with higher glutamine and glutamate levels (9 months) and lower lactate level (18 months) in HCR. Irrespective of fitness level, aging was associated with increased soleus muscle concentrations of glycerophosphocholine and glucose. Interval training did not influence metabolic profiles in LCR or HCR rats at any age.

Conclusion

Differences in inborn aerobic capacity gave the most marked contrasts in metabolic profile, there were also some changes with ageing. Low volume high intensity interval training twice a week had no detectable effect on metabolic profile.

Are There Limitations to Exercise Benefits in Peripheral Arterial Disease?

Substantial evidence exists indicating that inactivity contributes to the progression of chronic disease, and conversely, that regular physical activity can both prevent the onset of disease as well as delay the progression of existing disease. To that end "exercise as medicine" has been advocated in the broad context as general medical care, but also in the specific context as a therapeutic, to be considered in much the same way as other drugs. As there are non-responders to many medications, there also are non-responders to exercise; individual who participate but do not demonstrate appreciable improvement/benefit. In some settings, the stress induced by exercise may aggravate an underlying condition, rather than attenuate chronic disease. As personalized medicine evolves with ready access to genetic information, so too will the incorporation of exercise in the context of those individual genetics. The focus of this brief review is to distinguish between the inherent capacity to perform, as compared to adaptive response to active exercise training in relation to cardiovascular health and peripheral arterial disease.

Physiological adaptations to resistance training in rats selectively bred for low and high response to aerobic exercise training

NEW FINDINGS

What is the central question of this study? Can phenotypic traits associated with low response to one mode of training be extrapolated to other exercise-inducible phenotypes? The present study investigated whether rats that are low responders to endurance training are also low responders to resistance training. What is the main finding and its importance? After resistance training, rats that are high responders to aerobic exercise training improved more in maximal strength compared with low-responder rats. However, the greater gain in strength in high-responder rats was not accompanied by muscle hypertrophy, suggesting that the responses observed could be mainly neural in origin.

ABSTRACT

The purpose of this study was to determine whether rats selectively bred for low and high response to aerobic exercise training co-segregate for differences in muscle adaptations to ladder-climbing resistance training. Five high-responder (HRT) and five low-responder (LRT) rats completed the resistance training, while six HRT and six LRT rats served as sedentary control animals. Before and after the 6 week intervention, body composition was determined by dual energy X-ray absorptiometry. Before tissue harvesting, the right triceps surae muscles were loaded by electrical stimulation. Muscle fibre cross-sectional areas, nuclei per cell, phosphorylation status of selected signalling proteins of mTOR and Smad pathways, and muscle protein, DNA and RNA concentrations were determined for the right gastrocnemius muscle. The daily protein synthesis rate was determined by the deuterium oxide method from the left quadriceps femoris muscle. Tissue weights of fore- and hindlimb muscles were measured. In response to resistance training, maximal carrying capacity was greater in HRT (∼3.3 times body mass) than LRT (∼2.5 times body mass), indicating greater improvements of strength in HRT. However, muscle hypertrophy that could be related to greater strength gains in HRT was not observed. Furthermore, noteworthy changes within the experimental groups or differences between groups were not observed in the present measures. The lack of hypertrophic muscular adaptations despite considerable increases in muscular strength suggest that adaptations to the present ladder-climbing training in HRT and LRT rats were largely induced by neural adaptations.

Soy protein improves tibial whole-bone and tissue-level biomechanical properties in ovariectomized and ovary-intact, low-fit female rats

BACKGROUND

Osteoporosis and related fractures, decreased physical activity, and metabolic dysfunction are serious health concerns for postmenopausal women. Soy protein might counter the negative effects of menopause on bone and metabolic health due to the additive or synergistic effects of its bioactive components.

OBJECTIVE

To evaluate the effects of ovariectomy (OVX) and a soy-protein diet (SOY) on bone outcomes in female, low-capacity running (LCR) rats selectively bred for low aerobic fitness as a model of menopause.

METHODS

At 27 weeks of age, LCR rats (N = 40) underwent OVX or sham (SHAM) surgery and were randomized to one of two isocaloric and isonitrogenous plant-protein-based dietary treatments: 1) soy-protein (SOY; soybean meal); or, 2) control (CON, corn-gluten meal), resulting in four treatment groups. During the 30-week dietary intervention, animals were provided ad libitum access to food and water; body weight and food intake were measured weekly. At completion of the 30-week intervention, body composition was measured using EchoMRI; animals were fasted overnight, euthanized, and blood and hindlimbs collected. Plasma markers of bone formation (osteocalcin, OC; N-terminal propeptide of type I procollagen, P1NP) and resorption (tartrate-resistant acid phosphatase, TRAP5b; C-terminal telopeptide of type I collagen, CTx) were measured using ELISA. Tibial trabecular microarchitecture and cortical geometry were evaluated using μCT; and torsional loading to failure was used to assess cortical biomechanical properties. Advanced glycation end-product (AGE) content of the femur was measured using a fluorimetric assay, and was expressed relative to collagen content measured by a colorimetric OH-proline assay. Two-factor ANOVA or ANOVCA was used to test for significant main and interactive effects of ovarian status (OV STAT: OVX vs. SHAM) and DIET (SOY vs. CON); final body weight was included as a covariate for body-weight-dependent cortical geometry and biomechanical properties.

RESULTS

OVX had significantly greater CTx than SHAM; SOY did not affect bone turnover markers. OVX adversely affected trabecular microarchitecture as evidenced by reduced BV/TV, trabecular thickness (Tb.Th), trabecular number (Tb.N), and connectivity density (Conn.D), and by increased trabecular separation (Tb.Sp) and structural model index (SMI). SOY increased BV/TV only in ovary-intact animals. There was no effect of OVX or SOY on tibial cortical geometry. In SHAM and OVX rats, SOY significantly improved whole-bone strength and stiffness; SOY also increased tissue-level stiffness and tended to increase tissue-level strength (p = 0.067). There was no effect of OVX or SOY on AGE content.

CONCLUSION

Soy protein improved cortical bone biomechanical properties in female low-fit rats, regardless of ovarian hormone status.

Vertical sleeve gastrectomy corrects metabolic perturbations in a low-exercise capacity rat model

OBJECTIVE

Bariatric surgery is currently our most effective strategy at weight loss, yet the mechanisms for its success remain unknown. Low exercise capacity, in humans and rodents, predicts poor metabolic outcome. The objective of this manuscript was to determine if bariatric surgery could restore metabolic perturbations in rats with low intrinsic exercise capacity.

METHODS

We performed vertical sleeve gastrectomy (VSG) or sham surgery in high fat-fed rats selectively bred for low running capacity.

RESULTS

We found that VSG reduced body mass through a reduction in fat mass, caused early reductions in food intake, and shifted macronutrient preference away from fat and toward carbohydrates. VSG had no impact on basal glucose but did improve the return to baseline after an oral glucose load. As has been shown previously, VSG increased postprandial insulin, GLP-1, and bile acids. There was no significant impact of VSG on plasma triglycerides, hepatic triglycerides, or cholesterol. Interestingly, the brown adipose tissue to white adipose tissue ratio tended to be greater in VSG compared to sham surgery animals. While VSG positively impacted several aspects of metabolism, it did not enhance maximal oxygen capacity and seemed to lower metabolic efficiency as indicated by lower resting oxygen consumption and fat and carbohydrate oxidation.

CONCLUSION

VSG can improve the metabolic status of animals with a low exercise capacity independently of exercise capacity.

Inherently Lean Rats Have Enhanced Activity and Skeletal Muscle Response to Central Melanocortin Receptors

OBJECTIVE

Activity thermogenesis and energy expenditure (EE) are elevated in intrinsically lean rats (high-capacity runners [HCR]) and are also stimulated by melanocortin receptor activation in the ventromedial hypothalamus (VMH). This study determined whether HCR are more responsive to central modulation of activity EE compared with low-capacity runners (LCR).

METHODS

HCR and LCR rats received intra-VMH microinjections of melanotan II (MTII), a mixed melanocortin receptor agonist. Changes in EE, respiratory exchange ratio, activity EE, muscle heat, norepinephrine turnover, and muscle energetic modulators were compared.

RESULTS

HCR were significantly more responsive to intra-VMH MTII-induced changes in EE, activity EE, norepinephrine turnover to some muscle subgroups, and muscle mRNA expression of some energetic modulators. Though HCR had high muscle activity thermogenesis, limited MTII-induced modulation of muscle thermogenesis during activity was seen in LCR only.

CONCLUSIONS

An inherently lean, high-capacity rat phenotype showed elevated response to central melanocortin stimulation of activity EE and use of fat as fuel. This may be driven by sympathetic outflow to skeletal muscle, which was elevated after MTII. Central melanocortin receptor activation also altered skeletal muscle energetic modulators in a manner consistent with elevated EE and lowered respiratory exchange ratio.

Region-specific differences in bioenergetic proteins and protein response to acute high fat diet in brains of low and high capacity runner rats

Aerobic capacity is a strong predictor of mortality. Low capacity runner (LCR) rats exhibit reduced mitochondrial function in peripheral organs. A high fat diet (HFD) can worsen metabolic phenotype in LCR rats. Little is known about metabolic changes in the brains of these rats, however. This study examined protein markers of mitochondrial function and metabolism as a function of aerobic running capacity and an acute HFD in four brain regions: the striatum, hippocampus, hypothalamus, and substantia nigra. After 3 days HFD or chow diets, we measured peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1-α), nuclear respiratory factors 1 (Nrf-1), mitochondrial transcription factor A (TFAM), and phosphorylated (activated) AMP-activated protein kinase (p-AMPK) protein levels in the four brain regions. LCR rats exhibited lower levels of mitochondrial proteins (PGC1-α, Nrf-1, TFAM), and greater p-AMPK, in striatum, but not in the other brain regions. Mitochondrial protein levels were greater in HFD LCR striatum, while p-AMPK was lower in this group. Markers of lower mitochondrial biogenesis and increased metabolic demand were limited to the LCR striatum, which nevertheless maintained the capacity to respond to an acute HFD challenge.