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

Exercise training alters skeletal muscle microvascular endothelial cell properties in recent postmenopausal females

The present study examined and compared the impact of exercise training on redox and molecular properties of human microvascular endothelial cells derived from skeletal muscle biopsies from sedentary recent (RPF, ≤ 5 years as postmenopausal) and late (LPF, ≥ 10 years as postmenopausal) postmenopausal females. Resting skeletal muscle biopsies were obtained before and after 8 weeks of intense aerobic exercise training for isolation of microvascular endothelial cells and determination of skeletal muscle angiogenic proteins and capillarisation. The microvascular endothelial cells were analysed for mitochondrial respiration and production of reactive oxygen species (ROS), glycolysis and proteins related to vascular function, redox balance and oestrogen receptors. Exercise training led to a reduced endothelial cell ROS formation (∼50%; P = 0.009 and P = 0.020 for intact and permeabilized cells (state 3), respectively) in RPF only, with no effect on endothelial mitochondrial capacity in either group. Basal endothelial cell lactate formation was higher (7%; P = 0.028), indicating increased glycolysis, after compared to before the exercise training period in RPF only. Baseline endothelial G protein-coupled oestrogen receptor (P = 0.028) and muscle capillarisation (P = 0.028) was lower in LPF than in RPF. Muscle vascular endothelial growth factor protein was higher (32%; P = 0.002) following exercise training in LPF only. Exercise training did not influence endothelial cell proliferation or skeletal muscle capillarisation in either group, but the CD31 level in the muscle tissue, indicating endothelial cell content, was higher (>50%; P < 0.05) in both groups. In conclusion, 8 weeks of intense aerobic exercise training reduces ROS formation and enhances glycolysis in microvascular endothelial cells from RPF but does not induce skeletal muscle angiogenesis. KEY POINTS: Late postmenopausal females have been reported to achieve limited vascular adaptations to exercise training. There is a paucity of data on the effect of exercise training on isolated skeletal muscle microvascular endothelial cells (MMECs). In this study the formation of reactive oxygen species in MMECs was reduced and glycolysis increased after 8 weeks of aerobic exercise training in recent but not late postmenopausal females. Late postmenopausal females had lower levels of G protein-coupled oestrogen receptor in MMECs and lower skeletal muscle capillary density at baseline. Eight weeks of intense exercise training altered MMEC properties but did not induce skeletal muscle angiogenesis in postmenopausal females.

Peripheral limitations for performance: Muscle capillarization

Sufficient delivery of oxygen and metabolic substrates, together with removal of waste products, are key elements of muscle performance. Capillaries are the primary site for this exchange in skeletal muscle and the degree of muscle capillarization affects diffusion conditions by influencing mean transit time, capillary surface area and diffusion distance. Muscle capillarization may thus represent a limiting factor for performance. Exercise training increases the number of capillaries per muscle fiber by about 10%-20% within a few weeks in untrained subjects, whereas capillary growth progresses more slowly in well-trained endurance athletes. Studies show that capillaries are tortuous, situated along and across the length of the fibers with an arrangement related to muscle fascicles. Although direct data is lacking, it is possible that years of training not only enhances capillary density but also optimizes the positioning of capillaries, to further improve the diffusion conditions. Muscle capillarization has been shown to increase oxygen extraction during exercise in humans, but direct evidence for a causal link between increased muscle capillarization and performance is scarce. This review covers current knowledge on the implications of muscle capillarization for oxygen and glucose uptake as well as performance. A brief overview of the process of capillary growth and of physical factors, inherent to exercise, which promote angiogenesis, provides the foundation for a discussion on how different training modalities may influence muscle capillary growth. Finally, we identify three areas for future research on the role of capillarization for exercise performance.

Exercise-induced skeletal muscle angiogenesis: impact of age, sex, angiocrines and cellular mediators

Exercise-induced skeletal muscle angiogenesis is a well-known physiological adaptation that occurs in humans in response to exercise training and can lead to endurance performance benefits, as well as improvements in cardiovascular and skeletal tissue health. An increase in capillary density in skeletal muscle improves diffusive oxygen exchange and waste extraction, and thus greater fatigue resistance, which has application to athletes but also to the general population. Exercise-induced angiogenesis can significantly contribute to improvements in cardiovascular and metabolic health, such as the increase in muscle glucose uptake, important for the prevention of diabetes. Recently, our understanding of the mechanisms by which angiogenesis occurs with exercise has grown substantially. This review will detail the biochemical, cellular and biomechanical signals for exercise-induced skeletal muscle angiogenesis, including recent work on extracellular vesicles and circulating angiogenic cells. In addition, the influence of age, sex, exercise intensity/duration, as well as recent observations with the use of blood flow restricted exercise, will also be discussed in detail. This review will provide academics and practitioners with mechanistic and applied evidence for optimising training interventions to promote physical performance through manipulating capillarisation in skeletal muscle.

Exercise promotes angiogenesis by enhancing endothelial cell fatty acid utilization via liver-derived extracellular vesicle miR-122-5p

BACKGROUND

Angiogenesis constitutes a major mechanism responsible for exercise-induced beneficial effects. Our previous study identified a cluster of differentially expressed extracellular vesicle microRNAs (miRNAs) after exercise and found that some of them act as exerkines. However, whether these extracellular vesicle miRNAs mediate the exercise-induced angiogenesis remains unknown.

METHODS

A 9-day treadmill training was used as an exercise model in C57BL/6 mice. Liver-specific adeno-associated virus 8 was used to knock down microRNA-122-5p (miR-122-5p). Human umbilical vein endothelial cells were used in vitro.

RESULTS

Among these differentially expressed extracellular vesicle miRNAs, miR-122-5p was identified as a potent pro-angiogenic factor that activated vascular endothelial growth factor signaling and promoted angiogenesis both in vivo and in vitro. Exercise increased circulating levels of miR-122-5p, which was produced mainly by the liver and shuttled by extracellular vesicles in mice. Inhibition of circulating miR-122-5p or liver-specific knockdown of miR-122-5p significantly abolished the exercise-induced pro-angiogenic effect in skeletal muscles, and exercise-improved muscle performance in mice. Mechanistically, miR-122-5p promoted angiogenesis through shifting substrate preference to fatty acids in endothelial cells, and miR-122-5p upregulated endothelial cell fatty-acid utilization by targeting 1-acyl-sn-glycerol-3-phosphate acyltransferase (AGPAT1). In addition, miR-122-5p increased capillary density in perilesional skin tissues and accelerated wound healing in mice.

CONCLUSION

These findings demonstrated that exercise promotes angiogenesis through upregulation of liver-derived extracellular vesicle miR-122-5p, which enhances fatty acid utilization by targeting AGPAT1 in endothelial cells, highlighting the therapeutic potential of miR-122-5p in tissue repair.

Implication of satellite cell behaviors in capillary growth via VEGF expression-independent mechanism in response to mechanical loading in HeyL-null mice

Angiogenesis and muscle satellite cell (SC)-mediated myonuclear accretion are considered essential for the robust response of contraction-induced muscle hypertrophy. Moreover, both myonucleus and SCs are physically adjacent to capillaries and are the major sites for the expression of proangiogenic factors, such as VEGF, in the skeletal muscle. Thus, events involving the addition of new myonuclei via activation of SCs may play an important role in angiogenesis during muscle hypertrophy. However, the relevance among myonuclei number, capillary supply, and angiogenesis factor is not demonstrated. The Notch effector HeyL is specifically expressed in SCs in the skeletal muscle and is crucial for SC proliferation by inhibiting MyoD in overload-induced muscle hypertrophy. Here, we tested whether the addition of new myonuclei by SC in overloaded muscle is associated with angiogenic adaptation by reanalyzing skeletal muscle from HeyL-knockout (KO) mice, which show blunted responses of SC proliferation, myonucleus addition, and overload-induced muscle hypertrophy. Reanalysis confirmed blunted SC proliferation and myonuclear accretion in the plantaris muscle of HeyL-KO mice 9 wk after synergist ablation. Interestingly, the increase in capillary-to-fiber ratio observed in wild-type (WT) mice was impaired in HeyL-KO mice. In both WT and HeyL-KO mice, the expression of VEGFA and VEGFB was similarly increased in response to overload. In addition, the expression pattern of TSP-1, a negative regulator of angiogenesis, was also not changed between WT and HeyL-KO mice. Collectively, these results suggest that SCs activation-myonuclear accretion plays a crucial role in angiogenesis during overload-induced muscle hypertrophy via independent of angiogenesis regulators.

The Impact of Lower Limb Immobilization and Rehabilitation on Angiogenic Proteins and Capillarization in Skeletal Muscle

PURPOSE

Skeletal muscle vascularization is important for tissue regeneration after injury and immobilization. We examined whether complete immobilization influences capillarization and oxygen delivery to the muscle and assessed the efficacy of rehabilitation by aerobic exercise training.

METHODS

Young healthy males had one leg immobilized for 14 d and subsequently completed 4 wk of intense aerobic exercise training. Biopsies were obtained from musculus vastus lateralis, and arteriovenous blood sampling for assessment of oxygen extraction and leg blood flow during exercise was done before and after immobilization and training. Muscle capillarization, muscle and platelet content of vascular endothelial growth factor (VEGF), and muscle thrombospondin-1 were determined.

RESULTS

Immobilization did not have a significant impact on capillary per fiber ratio or capillary density. The content of VEGF protein in muscle samples was reduced by 36% (P = 0.024), and VEGF to thrombospondin-1 ratio was 94% lower (P = 0.046). The subsequent 4-wk training period increased the muscle VEGF content and normalized the muscle VEGF to thrombospondin-1 ratio but did not influence capillarization. Platelet VEGF content followed the trend of muscle VEGF. At the functional level, oxygen extraction, blood flow, and oxygen delivery at rest and during submaximal exercise were not affected by immobilization or training.

CONCLUSIONS

The results demonstrate that just 2 wk of leg immobilization leads to a strongly reduced angiogenic potential as evidenced by reduced muscle and platelet VEGF content and a reduced muscle VEGF to thrombospondin-1 ratio. Moreover, a subsequent period of intensive aerobic exercise training fails to increase capillarization in the previously immobilized leg, possibly because of the angiostatic condition caused by immobilization.

Effects of exercise training with weight loss on skeletal muscle expression of angiogenic factors in overweight and obese older men

Low skeletal muscle capillarization is associated with impaired glucose tolerance (IGT); however, aerobic exercise training with weight loss (AEX + WL) increases skeletal muscle capillarization and improves glucose tolerance in adults with IGT. Given that the expression of angiogenic growth factors mediates skeletal muscle capillarization, we sought to determine whether angiogenic growth factor levels are associated with low capillarization in those with IGT versus normal glucose tolerance (NGT) or to the benefits of AEX + WL in both groups. Sixteen overweight or obese men 50-75 yr of age completed 6 mo of AEX + WL with oral glucose tolerance tests and vastus lateralis muscle biopsies for measurement of muscle vascular endothelial growth factor (VEGF), placental growth factor (PlGF), soluble fms-like tyrosine kinase receptor-1 (sFlt-1), and basic fibroblast growth factor (bFGF). At baseline, all growth factor levels were numerically lower in IGT than NGT, but these did not reach statistical significance (P = 0.06-0.33). Following AEX + WL, aerobic capacity [maximal oxygen consumption (V̇o_2max)] increased by 16%, whereas body weight and 120-min postprandial glucose levels decreased by 10% and 15%, respectively (P ≤ 0.001 for all). There was a main effect of AEX + WL to increase VEGF (0.095 ± 0.016 vs. 0.114 ± 0.018 ng/µg, P < 0.05), PlGF (0.004 ± 0.001 vs. 0.005 ± 0.001 ng/µg, P < 0.05), and sFlt-1 (0.216 ± 0.029 vs. 0.264 ± 0.036 ng/µg, P < 0.01), with overall increases driven by the IGT group. These results suggest that 6 mo of AEX + WL increases skeletal muscle angiogenic growth factor levels in obese older adults with IGT and NGT, which may contribute to our previous findings that AEX + WL increases capillarization to improve glucose tolerance in those with IGT.NEW & NOTEWORTHY Skeletal muscle capillarization is lower in adults with impaired glucose tolerance than normal controls. This may, in part, be attributable to differential expression of angiogenic growth factors in skeletal muscle. Using a 6-mo aerobic exercise intervention with ∼10% body weight loss (AEX + WL), we show that the expression of angiogenic growth factors tends to be lower in adults with impaired glucose tolerance compared with normal controls and that AEX + WL increased expression of angiogenic growth factors in all participants.

The dog as an exercise science animal model: a review of physiological and hematological effects of exercise conditions

[Purpose]

Exercise is a fundamental way to maintain and improve health and physical fitness. Many human studies have demonstrated the beneficial effects of exercise on various biological parameters. However, studies investigating the effects of exercise in dogs are limited. This review summarized the current data from studies that examined the effects of different exercise conditions (treadmill vs. non-treadmill and acute vs. chronic) on physiological and hematological parameters in dogs.

[Methods]

Papers addressing the effects of exercise in dogs published from January 2000 to October 2020 were retrieved from the online databases of Scopus, Google Scholar, and PubMed and were selected and reviewed.

[Results]

The exercise conditions differentially affected physiological and hematological responses and adaptation in dogs. Therefore, the development and comprehensive evaluation of scientific exercise programs for dogs are necessary.

[Conclusion]

The dog would be a valuable exercise science animal model, and studies aiming at the optimal health, well-being, and quality of life of dogs need to be conducted.

Effect of Age and Acute Exercise on Circulating Angioregulatory Factors

The balance of angiogenic factors, including vascular endothelial growth factor (VEGF), and angiostatic factors, like thrombospondin-1 (TSP-1) and endostatin, controls striated muscle angiogenic responses to exercise training. The effect of age on circulating levels of these factors following a bout of exercise is unclear. The authors hypothesized that older adults would have lower circulating VEGF but higher TSP-1 and endostatin after exercise compared with young adults. Ten young and nine older participants cycled for 45 min at 60% estimated HRmax. Serum [VEGF], [TSP-1], and [endostatin] obtained before (PREX), immediately after (POSTX0), and 3 hr after (POSTX3) exercise were analyzed. [VEGF] increased in older adults only from PREX to POSTX0 (p < .05). [TSP-1] increased in both age groups (p < .05). There was no effect of age or exercise on [endostatin]. In conclusion, immediately after exercise, both groups had a similar increase in [TSP-1], but [VEGF] increased in older adults only.

Early time course of change in angiogenic proteins in human skeletal muscle and vascular cells with endurance training

Angiogenic, mitochondrial, and related transcriptional proteins were assessed in human skeletal muscle and isolated vascular cells during the early phase of endurance training. Thigh muscle biopsies were obtained in healthy young subjects, after one acute bout (n = 9) and after 3, 5, 7, and 14 days (n = 9) of cycle ergometer training. Whole muscle homogenates were analyzed for angiogenic, mitochondrial, and regulatory mRNA and protein levels. Angiogenic proteins were determined in muscle-derived endothelial cells and pericytes sorted by fluorescence-activated cell sorting. Acute exercise induced an increase in whole muscle mRNA of peroxisome proliferator-activated receptor gamma coactivator 1α (4.5-fold; P = .002) and vascular endothelial growth factor (VEGF) (2.4-fold; P = .001) at 2 hours post. After 14 days of training, there was an increase in CD31 protein (63%; P = .010) in whole muscle indicating capillary growth. There was also an increase in muscle VEGF receptor 2 (VEGFR2) (1.5-fold; P = .013), in OXPHOS proteins (complex I, II, IV, V; 1.4- to 1.9-fold; P < .05) after 14 days of training and an increase in estrogen-related receptorα protein (1.5-fold; P = .039) at 14 days compared to 5 days of training. Both endothelial cells and pericytes expressed VEGF and other angiogenic factors at the protein level but with a distinctively lower expression of VEGFR2 and thrombospondin-1 (TSP-1) in pericytes. The findings illustrate that initiation of capillary and mitochondrial adaptations occurs within 14 days of training and suggest that sustained changes in angiogenic proteins including VEGF and TSP-1 are moderate in whole muscle and vascular cells.

Muscle-Specific Sensitivity to Voluntary Physical Activity and Detraining

Aerobic physical activity triggers adaptations in skeletal muscle including a fast-to-slow shift in myosin heavy chain (MHC) isoforms, an enhanced capillary network, and mitochondrial biogenesis to meet increased demands placed upon this tissue. Although the magnitude of these responses appears to be dependent on muscle phenotype as well as training volume and/or intensity, the whole-muscle response to detraining remains mostly unexplored. Here, we hypothesized that the shifts toward slower MHC phentotype and the increased capillarity and mitochondrial oxidative markers induced with training would return toward sedentary (SED) control levels sooner in the fast plantaris than in the slow soleus muscle as a result of detraining. Soleus and plantaris muscles from 8-week (TR 8wk) voluntarily running adult female Sprague–Dawley rats were compared to muscles from SED and detrained rats (DETR) (4 weeks voluntary running followed by 4 weeks of reduced activity), which were subdivided into low- (DETR Lo) and high-running-distance (DETR Hi) groups. We show that maintaining the fast-to-slow MHC isoform shift required consistent aerobic training in the soleus and plantaris muscles: detraining clearly abolished any fast-to-slow gains in the plantaris, whereas the training volume in DETR Hi rats appeared to influence the MHC return to basal levels in the soleus. Total capillary number (per mm²) in the plantaris increased in all groups compared to SED levels, but, in the soleus, this enhancement was observed only in the TR 8wk rats. Generally, increased mitochondrial markers for aerobicitiy were observed in TR 8wk plantaris, but not soleus, muscles. In a second experiment, we show that the muscle-specific adaptations were similar after 4 weeks of voluntary exercise (TR 4wk) as in 4 weeks (TR 8wk). Taken together, our findings suggest that the plantaris muscle is more sensitive to voluntary physical activity and detraining than the soleus muscle; these results also demonstrate that the soleus muscle requires a greater aerobic challenge (i.e., intensity, duration) to trigger phenotypic, angiogenic, or aerobic enzyme adaptations. Our findings generally suggest that muscular aerobic fitness to voluntary running, or its loss during detraining, manifests as changes occurring primarily within fast, rather than slow, muscle phenotypes.

Metabolic regulation of exercise-induced angiogenesis

Skeletal muscle relies on an ingenious network of blood vessels, which ensures optimal oxygen and nutrient supply. An increase in muscle vascularization is an early adaptive event to exercise training, but the cellular and molecular mechanisms underlying exercise-induced blood vessel formation are not completely clear. In this review, we provide a concise overview on how exercise-induced alterations in muscle metabolism can evoke metabolic changes in endothelial cells (ECs) that drive muscle angiogenesis. In skeletal muscle, angiogenesis can occur via sprouting and splitting angiogenesis and is dependent on vascular endothelial growth factor (VEGF) signaling. In the resting muscle, VEGF levels are controlled by the estrogen-related receptor γ (ERRγ). Upon exercise, the transcriptional coactivator peroxisome-proliferator-activated receptor-γ coactivator-1α (PGC1α) orchestrates several adaptations to endurance exercise within muscle fibers and simultaneously promotes transcriptional activation of Vegf expression and increased muscle capillary density. While ECs are highly glycolytic and change their metabolism during sprouting angiogenesis in development and disease, a similar role for EC metabolism in exercise-induced angiogenesis in skeletal muscle remains to be elucidated. Nonetheless, recent studies have illustrated the importance of endothelial hydrogen sulfide and sirtuin 1 (SIRT1) activity for exercise-induced angiogenesis, suggesting that EC metabolic reprogramming may be fundamental in this process. We hypothesize that the exercise-induced angiogenic response can also be modulated by metabolic crosstalk between muscle and the endothelium. Defining the underlying molecular mechanisms responsible for skeletal muscle angiogenesis in response to exercise will yield valuable insight into metabolic regulation as well as the determinants of exercise performance.

Aerobic Exercise Preconception and During Pregnancy Enhances Oxidative Capacity in the Hindlimb Muscles of Mice Offspring

Liu, J, Lee, I, Feng, H-Z, Galen, SS, Hüttemann, PP, Perkins, GA, Jin, J-P, Hüttemann, M, and Malek, MH. Aerobic exercise preconception and during pregnancy enhances oxidative capacity in the hindlimb muscles of mice offspring. J Strength Cond Res 32(5): 1391-1403, 2018-Little is known about the effect of maternal exercise on offspring skeletal muscle health. The purpose of this study, therefore, was to determine whether maternal exercise (preconception and during pregnancy) alters offspring skeletal muscle capillarity and mitochondrial biogenesis. We hypothesized that offspring from exercised dams would have higher capillarity and mitochondrial density in the hindlimb muscles compared with offspring from sedentary dams. Female mice in the exercise condition had access to a running wheel in their individual cage 30 days before mating and throughout pregnancy, whereas the sedentary group did not have access to the running wheel before mating and during pregnancy. Male offspring from both groups were killed when they were 2 months old, and their tissues were analyzed. The results indicated no significant (p > 0.05) mean differences for capillarity density, capillarity-to-fiber ratio, or regulators of angiogenesis such as VEGF-A and TSP-1. Compared with offspring from sedentary dams, however, offspring from exercised dams had an increase in protein expression of myosin heavy chain type I (MHC I) (∼134%; p = 0.009), but no change in MHC II. For mitochondrial morphology, we found significant (all p-values ≤ 0.0124) increases in mitochondrial volume density (∼55%) and length (∼18%) as well as mitochondria per unit area (∼19%). For mitochondrial enzymes, there were also significant (all p-values ≤ 0.0058) increases in basal citrate synthase (∼79%) and cytochrome c oxidase activity (∼67%) in the nonoxidative muscle fibers as well as increases in basal (ATP) (∼52%). Last, there were also significant mean differences in protein expression for regulators (FIS1, Lon protease, and TFAM) of mitochondrial biogenesis. These findings suggest that maternal exercise before and during pregnancy enhances offspring skeletal muscle mitochondria functionality, but not capillarity.

Human expression patterns: qualitative and quantitative analysis of thrombospondin-1 under physiological and pathological conditions

Thrombospondin-1 (TSP-1), a matricellular protein and one of the first endogenous anti-angiogenic molecules identified, has long been considered a potent modulator of human diseases. While the therapeutic effect of TSP-1 to suppress cancer was investigated in both research and clinical settings, the mechanisms of how TSP-1 is regulated in cancer remain elusive, and the scientific answers to the question of whether TSP-1 expressions can be utilized as diagnostic or prognostic marker for patients with cancer are largely inconsistent. Moreover, TSP-1 plays crucial functions in angiogenesis, inflammation and tissue remodelling, which are essential biological processes in the progression of many cardiovascular diseases, and therefore, its dysregulated expressions in such conditions may have therapeutic significance. Herein, we critically analysed the literature pertaining to TSP-1 expression in circulating blood and pathological tissues in various types of cancer as well as cardiovascular and inflammation-related diseases in humans. We compare the secretion rates of TSP-1 by different cancer and non-cancer cells and discuss the potential connection between the expression changes of TSP-1 and vascular endothelial growth factor (VEGF) observed in patients with cancer. Moreover, the pattern and emerging significance of TSP-1 profiles in cardiovascular disease, such as peripheral arterial disease, diabetes and other related non-cancer disorders, are highlighted. The analysis of published TSP-1 data presented in this review may have implications for the future exploration of novel TSP-1-based treatment strategies for cancer and cardiovascular-related diseases.

Increased capillary tortuosity and pericapillary basement membrane thinning in skeletal muscle of mice undergoing running wheel training

To work out which microvascular remodeling processes occur in murine skeletal muscle during endurance exercise, we subjected C57BL/6 mice to voluntary running wheel training for 1 week (1 wk-t) or 6 weeks (6 wks-t). By means of morphometry, the capillarity as well as the compartmental and sub-compartmental structure of the capillaries were quantitatively described at the light microscopy level and at the electron microscopy level, respectively, in the plantaris (PLNT) muscle of the exercising mice in comparison to untrained littermates. In the early phase of the training (1 wk-t), angiogenesis [32% higher capillary/fiber (C/F) ratio; P<0.05] in PLNT muscle was accompanied by a tendency for capillary lumen enlargement (30%; P=0.06) and a reduction of the pericapillary basement membrane thickness [(CBMT) 12.7%; P=0.09] as well as a 21% shortening of intraluminal protrusion length (P<0.05), all compared with controls. After long-term training (6 wks-t), when the mice reached a steady state in running activity, additional angiogenesis (C/F ratio: 76%; P<0.05) and a 16.3% increase in capillary tortuosity (P<0.05) were established, accompanied by reversal of the lumen expansion (23%; P>0.05), further reduction of the CBMT (16.5%; P<0.05) and additional shortening of the intraluminal protrusion length (23%; P<0.05), all compared with controls. Other structural indicators, such as capillary profile sizes, profile area densities, perimeters of the capillary compartments and concentrations of endothelium-pericyte peg-socket junctions, were not significantly different between the mouse groups. Besides angiogenesis, increase of capillary tortuosity and reduction of CBMT represent the most striking microvascular remodeling processes in skeletal muscle of mice that undergo running wheel training.

Alpha adrenergic receptor blockade increases capillarization and fractional O2 extraction and lowers blood flow in contracting human skeletal muscle

AIM

To assess the effect of elevated basal shear stress on angiogenesis in humans and the role of enhanced skeletal muscle capillarization on blood flow and O₂ extraction.

METHODS

Limb haemodynamics and O₂ extraction were measured at rest and during one-leg knee-extensor exercise (12 and 24 W) in 10 healthy untrained young men before and after 4-week treatment with an α₁ receptor-antagonist (Terazosin, 1-2 mg day^-1 ). Corresponding biopsies were taken from the m. vastus lateralis.

RESULTS

Resting leg blood flow was increased by 57% 6 h following Terazosin treatment (P < 0.05), while basal capillary-to-fibre ratio was 1.69 ± 0.08 and increased to 1.90 ± 0.08 after treatment (P < 0.05). Leg O₂ extraction during knee-extensor exercise was higher (4-5%; P < 0.05), leg blood flow and venous lactate levels lower (6-7%; P < 0.05), while leg VO₂ was not different after Terazosin treatment.

CONCLUSIONS

These results demonstrate that daily treatment with an α-adrenergic receptor blocker induces capillary growth in human skeletal muscle, likely due to increased shear stress. The increase in capillarization resulted in an increased fractional O₂ extraction, a lower blood flow and venous lactate levels in the exercising leg. The increase in capillarization, and concomitant functional readouts in the exercising leg, may provide a basis for novel angiotherapy.

Electrical Stimulation of Denervated Rat Skeletal Muscle Retards Capillary and Muscle Loss in Early Stages of Disuse Atrophy

The purpose of the present study is to investigate the effects of low-frequency electrical muscle stimulation (ES) on the decrease in muscle mass, fiber size, capillary supply, and matrix metalloproteinase (MMP) immunoreactivity in the early stages of denervation-induced limb disuse. Direct ES was performed on the tibialis anterior muscle following denervation in seven-week-old male rats. The rats were divided into the following groups: control (CON), denervation (DN), and denervation with direct ES (DN + ES). Direct ES was performed at an intensity of 16 mA and a frequency of 10 Hz for 30 min per day, six days a week, for one week. We performed immunohistochemical staining to determine the expression of dystrophin, CD34, and MMP-2 in transverse sections of TA muscles. The weight, myofiber cross-sectional area (FCSA), and capillary-to-fiber (C/F) ratio of the tibialis anterior (TA) muscle were significantly reduced in the DN group compared to the control and DN + ES groups. The MMP-2 positive area was significantly greater in DN and DN + ES groups compared to the control group. These findings suggest beneficial effects of direct ES in reducing muscle atrophy and capillary regression without increasing MMP-2 immunoreactivity in the early stages of DN-induced muscle disuse in rat hind limbs.

Transcriptional and Post-Transcriptional Regulation of Thrombospondin-1 Expression: A Computational Model

Hypoxia is an important physiological stress signal that drives angiogenesis, the formation of new blood vessels. Besides an increase in the production of pro-angiogenic signals such as vascular endothelial growth factor (VEGF), hypoxia also stimulates the production of anti-angiogenic signals. Thrombospondin-1 (TSP-1) is one of the anti-angiogenic factors whose synthesis is driven by hypoxia. Cellular synthesis of TSP-1 is tightly regulated by different intermediate biomolecules including proteins that interact with hypoxia-inducible factors (HIFs), transcription factors that are activated by receptor and intracellular signaling, and microRNAs which are small non-coding RNA molecules that function in post-transcriptional modification of gene expression. Here we present a computational model that describes the mechanistic interactions between intracellular biomolecules and cooperation between signaling pathways that together make up the complex network of TSP-1 regulation both at the transcriptional and post-transcriptional level. Assisted by the model, we conduct in silico experiments to compare the efficacy of different therapeutic strategies designed to modulate TSP-1 synthesis in conditions that simulate tumor and peripheral arterial disease microenvironment. We conclude that TSP-1 production in endothelial cells depends on not only the availability of certain growth factors but also the fine-tuned signaling cascades that are initiated by hypoxia.

Physiological Capillary Regression is not Dependent on Reducing VEGF Expression

Investigations into physiologically controlled capillary regression report the provocative finding that microvessel regression occurs in the face of persistent elevation of skeletal muscle VEGF expression. TSP-1, a negative angiogenic regulator, is increasingly being observed to temporally correlate with capillary regression, suggesting that increased TSP-1 (and not reduction in VEGF per se) is needed to initiate, and likely regulate, capillary regression. Based on evidence being gleaned from physiologically mediated regression of capillaries, it needs to be recognized that capillary regression (and perhaps capillary rarefaction with disease) is not simply the reversal of factors used to stimulate angiogenesis. Rather, the conceptual understanding that angiogenesis and capillary regression each have specific and unique requirements that are biologically constrained to opposite sides of the balance between positive and negative angioregulatory factors may shed light on why anti-VEGF therapies have not lived up to the promise in reversing angiogenesis and providing the cure that many had hoped toward fighting cancer. Emerging evidence from physiological controlled angiogenesis suggest that cases involving excessive or uncontrolled capillary expansion may be best treated by therapies designed to increase expression of negative angiogenic regulators, whereas those involving capillary rarefaction may benefit from inhibiting negative regulators (like TSP-1).

Role of Growth Factors in Modulation of the Microvasculature in Adult Skeletal Muscle

Post-natal skeletal muscle is a highly plastic tissue that has the capacity to regenerate rapidly following injury, and to undergo significant modification in tissue mass (i.e. atrophy/hypertrophy) in response to global metabolic changes. These processes are reliant largely on soluble factors that directly modulate muscle regeneration and mass. However, skeletal muscle function also depends on an adequate blood supply. Thus muscle regeneration and changes in muscle mass, particularly hypertrophy, also demand rapid changes in the microvasculature. Recent evidence clearly demonstrates a critical role for soluble growth factors in the tight regulation of angiogenic expansion of the muscle microvasculature. Furthermore, exogenous modulation of these factors has the capacity to impact directly on angiogenesis and thus, indirectly, on muscle regeneration, growth and performance. This chapter reviews recent developments in understanding the role of growth factors in modulating the skeletal muscle microvasculature, and the potential therapeutic applications of exogenous angiogenic and anti-angiogenic mediators in promoting effective growth and regeneration, and ameliorating certain diseases, of skeletal muscle.

Cardiovascular Adaptations to Exercise Training

Aerobic exercise training leads to cardiovascular changes that markedly increase aerobic power and lead to improved endurance performance. The functionally most important adaptation is the improvement in maximal cardiac output which is the result of an enlargement in cardiac dimension, improved contractility, and an increase in blood volume, allowing for greater filling of the ventricles and a consequent larger stroke volume. In parallel with the greater maximal cardiac output, the perfusion capacity of the muscle is increased, permitting for greater oxygen delivery. To accommodate the higher aerobic demands and perfusion levels, arteries, arterioles, and capillaries adapt in structure and number. The diameters of the larger conduit and resistance arteries are increased minimizing resistance to flow as the cardiac output is distributed in the body and the wall thickness of the conduit and resistance arteries is reduced, a factor contributing to increased arterial compliance. Endurance training may also induce alterations in the vasodilator capacity, although such adaptations are more pronounced in individuals with reduced vascular function. The microvascular net increases in size within the muscle allowing for an improved capacity for oxygen extraction by the muscle through a greater area for diffusion, a shorter diffusion distance, and a longer mean transit time for the erythrocyte to pass through the smallest blood vessels. The present article addresses the effect of endurance training on systemic and peripheral cardiovascular adaptations with a focus on humans, but also covers animal data.

Advances and challenges in skeletal muscle angiogenesis

The role of capillaries is to serve as the interface for delivery of oxygen and removal of metabolites to/from tissues. During the past decade there has been a proliferation of studies that have advanced our understanding of angiogenesis, demonstrating that tissue capillary supply is under strict control during health but poorly controlled in disease, resulting in either excessive capillary growth (pathological angiogenesis) or losses in capillarity (rarefaction). Given that skeletal muscle comprises nearly 40% of body mass in humans, skeletal muscle capillary density has a significant impact on metabolism, endocrine function, and locomotion and is tightly regulated at many different levels. Skeletal muscle is also high adaptable and thus one of the few organ systems that can be experimentally manipulated (e.g., by exercise) to study physiological regulation of angiogenesis. This review will focus on the methodological concerns that have arisen in determining skeletal muscle capillarity and highlight the concepts that are reshaping our understanding of the angio-adaptation process. We also summarize selected new findings (physical influences, molecular changes, and ultrastructural rearrangement of capillaries) that identify areas of future research with the greatest potential to expand our understanding of how angiogenesis is normally regulated, and that may also help to better understand conditions of uncontrolled (pathological) angiogenesis.

Regulation of skeletal muscle capillary growth in exercise and disease

Capillaries, which are the smallest and most abundant type of blood vessel, form the primary site of gas, nutrient, and waste transfer between the vascular and tissue compartments. Skeletal muscle exhibits the capacity to generate new capillaries (angiogenesis) as an adaptation to exercise training, thus ensuring that the heightened metabolic demand of the active muscle is matched by an improved capacity for distribution of gases, nutrients, and waste products. This review summarizes the current understanding of the regulation of skeletal muscle capillary growth. The multi-step process of angiogenesis is coordinated through the integration of a diverse array of signals associated with hypoxic, metabolic, hemodynamic, and mechanical stresses within the active muscle. The contributions of metabolic and mechanical factors to the modulation of key pro- and anti-angiogenic molecules are discussed within the context of responses to a single aerobic exercise bout and short-term and long-term training. Finally, the paradoxical lack of angiogenesis in peripheral artery disease and diabetes and the implications for disease progression and muscle health are discussed. Future studies that emphasize an integrated analysis of the mechanisms that control skeletal muscle capillary growth will enable development of targeted exercise programs that effectively promote angiogenesis in healthy individuals and in patient populations.

Abnormalities in three-dimensional capillary architecture and imbalance between vascular endothelial growth factor-A and thrombospondin-1 in soleus muscle of ovariectomized rat

Reduced ovarian hormone levels associated with menopause or ovariectomy (OVX) not only result in vascular dysfunction but also lead to structural abnormalities in capillaries. Therefore, the effect of OVX on the three-dimensional (3-D) architecture of capillary networks and the underlying molecular mechanisms were investigated in rat soleus muscle. Seven-week-old female Wistar rats were divided into the OVX and sham-treated (Sham) groups. The OVX group exhibited lower endurance exercise capacity compared to the sham group and resulted in decreased capillary diameter, number of anastomoses and capillary/anastomosis volume in soleus muscle, indicating 3-D structural abnormalities of capillary networks. Furthermore, OVX led to increased concentrations of thrombospondin-1 (TSP-1) protein and a decreased VEGF-A/TSP-1 ratio, an indicator of angio-adaptations, in soleus muscle compared with the Sham group. These results indicate OVX may induce 3-D capillary regression in soleus muscle through an imbalance between VEGF-A and TSP-1 expression, possibly associated with decreased exercise tolerance in ovariectomized rats.

Capillary growth in human skeletal muscle: physiological factors and the balance between pro-angiogenic and angiostatic factors

In human skeletal muscle, the capillary net readily adapts according to the level of muscular activity to allow for optimal diffusion conditions for oxygen from the blood to the muscle. Animal studies have demonstrated that stimulation of capillary growth in skeletal muscle can occur either by mechanical or by chemical signalling. Mechanical signals originate from shear stress forces on the endothelial cell layer induced by the blood flowing through the vessel, but include also mechanical stretch and compression of the vascular structures and the surrounding tissue, as the muscle contracts. Depending on the mechanical signal provided, capillary growth may occur either by longitudinal splitting (shear stress) or by sprouting (passive stretch). The mechanical signals initiate angiogenic processes by up-regulation or release of angioregulatory proteins that either promote, modulate or inhibit angiogenesis. A number of such regulatory proteins have been described in skeletal muscle in animal and cell models but also in human skeletal muscle. Important pro-angiogenic factors in skeletal muscle are vascular endothelial growth factor, endothelial nitric oxide synthase and angiopoietin 2, whereas angiostatic factors include thrombospondin-1 and tissue inhibitor of matrix metalloproteinase. Which of these angiogenic factors are up-regulated in the muscle tissue depends on the mechanical and chemical stimulus provided and, consequently, the process by which capillary growth occurs. The present review addresses physiological signals and angiogenic factors in skeletal muscle with a focus on human data.

Exercise preconditioning improves traumatic brain injury outcomes

PURPOSE

To determine whether 6 weeks of exercise performed prior to traumatic brain injury (TBI) could improve post-TBI behavioral outcomes in mice, and if exercise increases neuroprotective molecules (vascular endothelial growth factor-A [VEGF-A], erythropoietin [EPO], and heme oxygenase-1 [HO-1]) in brain regions responsible for movement (sensorimotor cortex) and memory (hippocampus).

METHODS

120 mice were randomly assigned to one of four groups: (1) no exercise+no TBI (NOEX-NOTBI [n=30]), (2) no exercise+TBI (NOEX-TBI [n=30]), (3) exercise+no TBI (EX-NOTBI [n=30]), and (4) exercise+TBI (EX-TBI [n=30]). The gridwalk task and radial arm water maze were used to evaluate sensorimotor and cognitive function, respectively. Quantitative real time polymerase chain reaction and immunostaining were performed to investigate VEGF-A, EPO, and HO-1 mRNA and protein expression in the right cerebral cortex and ipsilateral hippocampus.

RESULTS

EX-TBI mice displayed reduced post-TBI sensorimotor and cognitive deficits when compared to NOEX-TBI mice. EX-NOTBI and EX-TBI mice showed elevated VEGF-A and EPO mRNA in the cortex and hippocampus, and increased VEGF-A and EPO staining of sensorimotor cortex neurons 1 day post-TBI and/or post-exercise. EX-TBI mice also exhibited increased VEGF-A staining of hippocampal neurons 1 day post-TBI/post-exercise. NOEX-TBI mice demonstrated increased HO-1 mRNA in the cortex (3 days post-TBI) and hippocampus (3 and 7 days post-TBI), but HO-1 was not increased in mice that exercised.

CONCLUSIONS

Improved TBI outcomes following exercise preconditioning are associated with increased expression of specific neuroprotective genes and proteins (VEGF-A and EPO, but not HO-1) in the brain.

High intensity interval and endurance training have opposing effects on markers of heart failure and cardiac remodeling in hypertensive rats

There has been re-emerging interest and significant work dedicated to investigating the metabolic effects of high intensity interval training (HIIT) in recent years. HIIT is considered to be a time efficient alternative to classic endurance training (ET) that elicits similar metabolic responses in skeletal muscle. However, there is a lack of information on the impact of HIIT on cardiac muscle in disease. Therefore, we determined the efficacy of ET and HIIT to alter cardiac muscle characteristics involved in the development of diastolic dysfunction, such as ventricular hypertrophy, fibrosis and angiogenesis, in a well-established rodent model of hypertension-induced heart failure before the development of overt heart failure. ET decreased left ventricle fibrosis by ~40% (P < 0.05), and promoted a 20% (P<0.05) increase in the left ventricular capillary/fibre ratio, an increase in endothelial nitric oxide synthase protein (P<0.05), and a decrease in hypoxia inducible factor 1 alpha protein content (P<0.05). In contrast, HIIT did not decrease existing fibrosis, and HIIT animals displayed a 20% increase in left ventricular mass (P<0.05) and a 20% decrease in cross sectional area (P<0.05). HIIT also increased brain natriuretic peptide by 50% (P<0.05), in the absence of concomitant angiogenesis, strongly suggesting pathological cardiac remodeling. The current data support the longstanding belief in the effectiveness of ET in hypertension. However, HIIT promoted a pathological adaptation in the left ventricle in the presence of hypertension, highlighting the need for further research on the widespread effects of HIIT in the presence of disease.

(-)-Epicatechin combined with 8 weeks of treadmill exercise is associated with increased angiogenic and mitochondrial signaling in mice

The purpose of this study was to conduct an 8 week endurance training program with and without (–)–epicatechin treatment and to determine whether there is a possible cumulative effect on protein markers of angiogenesis and mitochondrial biogenesis. Thirty-four 14-month old male mice (C57BL/6N) were randomized into four groups: control (C); (–)–epicatechin only ((–)–Epi); control with endurance training (CE); and (–)–epicatechin with endurance training ((–)–Epi-Ex). Mice in the training groups performed treadmill exercise for 8 weeks (5 × /week for 60 min/session), whereas mice in the (–)–epicatechin group received 1.0 mg/kg of body mass twice daily during the training period. At 8 weeks, distance ran on the treadmill increased by 46, 69, and 84% in the (–)–Epi, CE, and (–)–Epi-Ex groups, respectively compared to the control group (p < 0.001 for all comparisons). Furthermore, the (–)–Epi-Ex group had significantly higher exercise capacity than the (–)–Epi and CE group. For angiogenic regulators, the (–)–Epi-Ex group had significantly higher VEGF-R2 protein expression with a concomitant reduction in TSP-1 protein expression than the exercise group. Interestingly, FoxO1 protein expression was significantly reduced for all three experimental groups compared to the control group. Protein markers such as PGC-1β and TFAM were significantly higher in the (–)–Epi-Ex group compared to the three other groups. These findings suggest that (–)–epicatechin treatment combined with 8 weeks of endurance training provide a cumulative effect on a number of angiogenic and mitochondrial signaling which functionally translates to enhanced exercise tolerance.

Nox4 supports proper capillary growth in exercise and retina neo-vascularization

KEY POINTS

We provide evidence for two distinct functions of the NADPH oxidase Nox4 in angiogenesis using Nox4 knockout mice. First, Nox4 maintains vascular endothelial growth factor expression and prevents an increase in angiopoietin 1 expression, thereby contributing to angiogenesis in exercise. Second, deletion of Nox4, via an enhanced angiopoietin 1 expression, contributes to stabilization of new formed vessels and prevents an exacerbated neo-angiogenesis in oxygen-induced retinopathy. By contrast, Nox4 does not influence developmental angiogenesis.

ABSTRACT

By producing H2 O2 , the NADPH oxidase Nox4 is involved in hypoxia-induced angiogenesis, as present in vascular remodelling of the hypertrophic heart or blood flow recovery after hind limb ischaemia. In the present study, we hypothesized that Nox4 contributes to proper capillary growth in the retina and in exercised muscles and investigated this in wild-type and Nox4(-/-) mice. Exercise, as induced by voluntary running in a running wheel or forced running on a treadmill, stimulated capillary growth in wild-type but not Nox4(-/-) mice. As an underlying mechanism, we identified both vascular endothelial growth factor (VEGF) expression to be reduced and angiopoietin 1 (Ang1) expression to be increased in response to Nox4 knockout. To differentiate the two factors, oxygen-induced retinopathy was investigated. In this model, deletion of Nox4 protected from neo-angiogenesis and stabilized the network of regrown vessels, which is a typical feature of Ang1. However the angiogenesis in the developing retina was similar between Nox4(-/-) and wild-type mice. Thus, Nox4 contributes to exercise- and hypoxia-induced angiogenesis through a dual mechanism of maintaining VEGF and preventing Ang-1 expression, whereas the developmental angiogenesis is Nox4 independent.

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.

Exercise-induced capillary growth in human skeletal muscle and the dynamics of VEGF

In skeletal muscle, growth of capillaries is an important adaptation to exercise training that secures adequate diffusion capacity for oxygen and nutrients even at high-intensity exercise when increases in muscle blood flow are profound. Mechanical forces present during muscle activity, such as shear stress and passive stretch, lead to cellular signaling, enhanced expression of angiogenic factors, and initiation of capillary growth. The most central angiogenic factor in skeletal muscle capillary growth is VEGF. During muscle contraction, VEGF increases in the muscle interstitium, acts on VEGF receptors on the capillary endothelium, and thereby stimulates angiogenic processes. A primary source of muscle interstitial VEGF during exercise is the skeletal muscle fibers which contain large stores of VEGF within vesicles. We propose that, during muscle activity, these VEGF-containing vesicles are redistributed toward the sarcolemma where the contents are secreted into the extracellular fluid. VEGF mRNA expression is increased primarily after exercise, which allows for a more rapid replenishment of VEGF stores lost through secretion during exercise. Future studies should focus on elucidating mechanisms and regulation of VEGF secretion.

Muscle microvascular adaptation and angiogenic gene induction in response to exercise training are attenuated in middle-aged rats

This study was designed to investigate exercise-induced changes in muscle capillarisation, the mRNA expression of angiogenic genes, and microRNA levels in young and middle-aged rats. Rats in the training groups were subjected to treadmill running 5 days a week for 3 weeks. The exercise protocol for the young (12-week old) group was 20-25 m/min, 40-60 min/day with a gradient of 15%, and for the middle-aged (12-month old) group was 18-20 m/min, 40-60 min/day with a gradient of 5%. The enzyme histochemical identification of capillary profiles was performed on cross-sections of gastrocnemius muscle. Total RNA was isolated, reverse transcription was performed, and mRNA and microRNA levels were determined by real-time PCR. The capillary-to-fibre ratio was significantly increased by exercise training in the young group (by 10%), but only slightly in the middle-aged (by 5%) group. Vascular endothecial growth factor (VEGF) mRNA levels were at significantly higher values after acute exercise (1.6-fold) and the 3-week training protocol (1.9-fold) in the young group, but not in the middle-aged group. VEGF protein expression levels were significantly increased after training in the young group only. Endothelial nitric oxide synthase, VEGF-R2 and thrombospondin-1 mRNA levels were significantly lower in the middle-aged group than in the young group. Anti-angiogenic miR-195 levels were significantly enhanced by exercise training in the middle-aged group only. These results indicated that the exercise-induced adaptation of muscle capillarity was attenuated in middle-aged rats, possibly by the lower induction of VEGF and up-regulation of anti-angiogenic miRNA expression.

Aging and the Skeletal Muscle Angiogenic Response to Exercise in Women

Whether aging lowers skeletal muscle basal capillarization and angiogenesis remains controversial. To investigate the effects of aging on skeletal muscle capillarization, eight young (YW) and eight aged (AW) women completed 8 weeks of exercise training. The response and relationships of muscle capillarization, interstitial vascular endothelial growth factor (VEGF), and microvascular blood flow to aerobic exercise training were investigated. Vastus lateralis biopsies were obtained before and after exercise training for the measurement of capillarization. Muscle interstitial VEGF protein and microvascular blood flow were measured at rest and during submaximal exercise at PRE, 1-WK, and 8-WKS by microdialysis. Exercise training increased (20%-25%) capillary contacts of type I, IIA, and IIB fibers in YW and AW. Interstitial VEGF protein was higher in AW than YW at rest and was higher in YW than AW during exercise independent of training status. Differences in muscle capillarization were not explained by secreted VEGF nor were differences in VEGF explained by microvascular blood flow. These results confirm that aging (57-76 years age range) does not impair the muscle angiogenic response to exercise training, although sex differences may exist in similarly trained women and men.

Effects of detraining on the temporal expression of positive and negative angioregulatory proteins in skeletal muscle of mice

Temporal expression of positive and negative angiogenic factors in response to detraining is poorly understood. We report the protein expression of anti-angiogenic peptides (thrombospondin-1, TSP-1; and endostatin) as well as pro-angiogenic factors (vascular endothelial growth factor, VEGF; matrix metalloproteinases-2 and -9), and nucleolin (a nuclear protein involved with synthesis and maturation of ribosomes) in response to detraining in triceps surae muscles of C57BL/6 mice. Male mice were allowed to exercise voluntarily for 21 days, and then basal and acute response to exercise were evaluated at 1, 7, 14 and 28 days detraining (D1, D7, D14, D28, respectively, n = 12/group). As seen in the D1 mice, training resulted in the increased muscle capillary-to-fibre ratio (C/F), increased maximal running time and elevated basal expression of VEGF and matrix metalloproteinase-9 (P < 0.05). After 7 days of detraining (D7), C/F levels were similar to control levels, but both basal VEGF and TSP-1 were elevated (P < 0.05). At D14 and D28, TSP-1 protein was not different compared to baseline levels; however, VEGF was elevated in gastrocnemius (GA), but not the soleus (SOL) or plantaris (PLT) muscles, of D14 mice. Endostatin tended to decrease in D14 and D28 compared to controls. Timing of nucleolin protein expression differed between muscle groups, with increases at D1, D7 and D14 in the PLT, SOL and GA muscles, respectively. The response of VEGF and nucleolin to acute exercise was blunted with training, and remained blunted in the PLT and SOL even after 28 days of detraining, at a time point long after muscle capillarization was observed to be similar to pre-training levels. These data suggest that TSP-1 may be a mediator of capillary regression with detraining, even in the face of elevated VEGF, suggesting that pro-angiogenic regulators may not be able to prevent the regression of skeletal muscle capillaries under physiological conditions. The responses of matrix metalloproteinases, endostatin and nucleolin poorly correlated with detraining-induced capillary regression.