Phenotype of capillaries in skeletal muscle of nNOS-knockout mice

Expression of neuronal NO synthase α- and β-isoforms in skeletal muscle of mice

Knowledge of the primary structure of neuronal NO synthase (nNOS) in skeletal muscle is still conflicting and needs further clarification. To elucidate the expression patterns of nNOS isoforms at both mRNA and protein level, systematic reverse transcription (RT)-PCR and epitope mapping by qualitative immunoblot analysis on skeletal muscle of C57/BL6 mice were performed. The ability of the nNOS isoforms to form aggregates was characterized by native low-temperature polyacrylamide electrophoresis (LT-PAGE). The molecular analysis was focused on the rectus femoris (RF) muscle, a skeletal muscle with a nearly balanced ratio of nNOS α- and β-isoforms. RT-PCR amplificates from RF muscles showed exclusive exon-1d mRNA expression, either with or without exon-μ. Epitope mapping demonstrated the simultaneous expression of the nNOS splice variants α/μ, α/non-μ, β/μ and β/non-μ. Furthermore, immunoblotting suggests that the transition between nNOS α- and β-isoforms lies within exon-3. In LT-PAGE, three protein nNOS associated aggregates were detected in homogenates of RF muscle and tibialis anterior muscle: a 320 kDa band containing nNOS α-isoforms, while 250 and 300 kDa bands consist of nNOS β-isoforms that form homodimers or heterodimers with non-nNOS proteins.

Making the case for resistance training in improving vascular function and skeletal muscle capillarization

Through decades of empirical data, it has become evident that resistance training (RT) can improve strength/power and skeletal muscle hypertrophy. Yet, until recently, vascular outcomes have historically been underemphasized in RT studies, which is underscored by several exercise-related reviews supporting the benefits of endurance training on vascular measures. Several lines of evidence suggest large artery diameter and blood flow velocity increase after a single bout of resistance exercise, and these events are mediated by vasoactive substances released from endothelial cells and myofibers (e.g., nitric oxide). Weeks to months of RT can also improve basal limb blood flow and arterial diameter while lowering blood pressure. Although several older investigations suggested RT reduces skeletal muscle capillary density, this is likely due to most of these studies being cross-sectional in nature. Critically, newer evidence from longitudinal studies contradicts these findings, and a growing body of mechanistic rodent and human data suggest skeletal muscle capillarity is related to mechanical overload-induced skeletal muscle hypertrophy. In this review, we will discuss methods used by our laboratories and others to assess large artery size/function and skeletal muscle capillary characteristics. Next, we will discuss data by our groups and others examining large artery and capillary responses to a single bout of resistance exercise and chronic RT paradigms. Finally, we will discuss RT-induced mechanisms associated with acute and chronic vascular outcomes.

Post-COVID exercise intolerance is associated with capillary alterations and immune dysregulations in skeletal muscles

The SARS-CoV-2 pandemic not only resulted in millions of acute infections worldwide, but also in many cases of post-infectious syndromes, colloquially referred to as "long COVID". Due to the heterogeneous nature of symptoms and scarcity of available tissue samples, little is known about the underlying mechanisms. We present an in-depth analysis of skeletal muscle biopsies obtained from eleven patients suffering from enduring fatigue and post-exertional malaise after an infection with SARS-CoV-2. Compared to two independent historical control cohorts, patients with post-COVID exertion intolerance had fewer capillaries, thicker capillary basement membranes and increased numbers of CD169+ macrophages. SARS-CoV-2 RNA could not be detected in the muscle tissues. In addition, complement system related proteins were more abundant in the serum of patients with PCS, matching observations on the transcriptomic level in the muscle tissue. We hypothesize that the initial viral infection may have caused immune-mediated structural changes of the microvasculature, potentially explaining the exercise-dependent fatigue and muscle pain.

nNOS Increases Fiber Type-Specific Angiogenesis in Skeletal Muscle of Mice in Response to Endurance Exercise

We studied the relationship between neuronal NO synthase (nNOS) expression and capillarity in the tibialis anterior (TA) muscle of mice subjected to treadmill training. The mRNA (+131%) and protein (+63%) levels of nNOS were higher (p ≤ 0.05) in the TA muscle of C57BL/6 mice undergoing treadmill training for 28 days than in those of littermates remaining sedentary, indicating an up-regulation of nNOS by endurance exercise. Both TA muscles of 16 C57BL/6 mice were subjected to gene electroporation with either the pIRES2-ZsGreen1 plasmid (control plasmid) or the pIRES2-ZsGreen1-nNOS gene-inserted plasmid (nNOS plasmid). Subsequently, one group of mice (n = 8) underwent treadmill training for seven days, while the second group of mice (n = 8) remained sedentary. At study end, 12-18% of TA muscle fibers expressed the fluorescent reporter gene ZsGreen1. Immunofluorescence for nNOS was 23% higher (p ≤ 0.05) in ZsGreen1-positive fibers than ZsGreen1-negative fibers from the nNOS-transfected TA muscle of mice subjected to treadmill training. Capillary contacts around myosin heavy-chain (MHC)-IIb immunoreactive fibers (14.2%; p ≤ 0.05) were only higher in ZsGreen1-positive fibers than ZsGreen1-negative fibers in the nNOS-plasmid-transfected TA muscles of trained mice. Our observations are in line with an angiogenic effect of quantitative increases in nNOS expression, specifically in type-IIb muscle fibers after treadmill training.

Distinct structural and functional angiogenic responses are induced by different mechanical stimuli

Objective

Adequacy of the microcirculation is essential for maintaining repetitive skeletal muscle function while avoiding fatigue. It is unclear, however, whether capillary remodelling after different angiogenic stimuli is comparable in terms of vessel distribution and consequent functional adaptations. We determined the physiological consequences of two distinct mechanotransductive stimuli: (1) overload‐mediated abluminal stretch (OV); (2) vasodilator‐induced shear stress (prazosin, PR).

Methods

In situ EDL fatigue resistance was determined after 7 or 14 days of intervention, in addition to measurements of femoral artery flow. Microvascular composition (muscle histology) and oxidative capacity (citrate synthase activity) were quantified, and muscle PO₂ calculated using advanced mathematical modelling.

Results

Compared to controls, capillary‐to‐fiber ratio was higher after OV14 (134%, p < .001) and PR14 (121%, p < .05), although fatigue resistance only improved after overload (7 days: 135%, 14 days: 125%, p < .05). In addition, muscle overload improved local capillary supply indices and reduced CS activity, while prazosin treatment failed to alter either index of aerobic capacity.

Conclusion

Targeted capillary growth in response to abluminal stretch is a potent driver of improved muscle fatigue resistance, while shear stress‐driven angiogenesis has no beneficial effect on muscle function. In terms of capillarity, more is not necessarily better.

Hypoxic training improves blood pressure, nitric oxide and hypoxia-inducible factor-1 alpha in hypertensive patients

PURPOSE

To examine the effects of intermittent hypoxic breathing at rest (IHR) or during exercise (IHT) on blood pressure and nitric oxide metabolites (NOx) and hypoxia-inducible factor-1 alpha levels (HIF-1α) over a 6-week period.

METHODS

47 hypertensive patients were randomly allocated to three groups: hypertensive control (CON: n = 17; IHR: n = 15 and IHT: n = 15. The CON received no intervention; whereas, IH groups received eight events of hypoxia (F_IO₂ 0.14), and normoxia (F_IO₂ 0.21), 24-min hypoxia and 24-min normoxia, for 6 weeks. The baseline data were collected 2 days before the intervention; while, the post-test data were collected at days 2 and 28 after the 6-week intervention.

RESULTS

We observed a significant decrease of the SBP in both IH groups: IHR (- 12.0 ± 8.0 mmHg, p = 0.004 and - 9.9 ± 8.8 mmHg, p = 0.028, mean ± 95% CI) and IHT (- 13.0 ± 7.8 mmHg, p = 0.002 and - 10.0 ± 8.4 mmHg, p = 0.016) at days 2 and 28 post-intervention, respectively. Compared to CON, IHR and IHT had increased of NOx (IHR; 8.5 ± 7.6 μmol/L, p = 0.031 and IHT; 20.0 ± 9.1 μmol/L, p < 0.001) and HIF-1α (IHR; 170.0 ± 100.0 pg/mL, p = 0.002 and IHT; 340.5 ± 160.0 pg/mL, p < 0.001). At 2 days post-intervention, NOx and HIF-1α were negatively correlated with SBP in IHT.

CONCLUSION

IH programs may act as an alternative therapeutic strategy for hypertension patients probably through elevation of NOx and HIF-1α production.

Oral Administration of Sodium Nitrate to Metabolic Syndrome Patients Attenuates Mild Inflammatory and Oxidative Responses to Acute Exercise

The beneficial effects of exercise for the treatment and prevention of metabolic syndrome pathologies have been related to its anti-inflammatory and antioxidant effects. Dietary nitrate supplementation is an emerging treatment strategy to alleviate the symptoms of metabolic syndrome affections and to improve vascular function. In this double-blind crossover trial, metabolic syndrome patients performed two exercise tests for 30 min at 60–70% maximal heart rate after the intake of a placebo or a nitrate-enriched beverage. Acute exercise increased the plasma concentration of TNFα, intercellular adhesion molecule ICAM1, PGE1, PGE2 and the newly detected 16-hydroxypalmitic acid (16-HPAL) in metabolic syndrome patients. The cytokine and oxylipin production by peripheral blood mononuclear cells (PBMCs) and neutrophils could be responsible for the plasma concentrations of TNFα and IL6, but not for the plasma concentration of oxylipins nor its post-exercise increase. The intake of sodium nitrate 30 min before exercise increased the concentration of nitrate and nitrite in the oral cavity and plasma and reduced the oxygen cost of exercise. Additionally, nitrate intake prevented the enhancing effects of acute exercise on the plasma concentration of TNFα, ICAM1, PGE1, PGE2 and 16-HPAL, while reducing the capabilities of PBMCs and neutrophils to produce oxylipins.

The effect of two exercise modalities on skeletal muscle capillary ultrastructure in individuals with type 2 diabetes

Type 2 diabetes is associated with microvascular dysfunction, but little is known about how capillary ultrastructure is affected by exercise training. To investigate the effect of two types of exercise training on skeletal muscle capillary ultrastructure and capillarization in individuals with type 2 diabetes, 21 individuals with type 2 diabetes were allocated (randomized controlled trial) to 11 weeks of aerobic exercise training consisting of either moderate-intensity endurance training (END; n = 10) or low-volume high-intensity interval training (HIIT; n = 11). Skeletal muscle biopsies (m vastus lateralis) were obtained before and after the training intervention. At baseline, there was no difference in capillarization, capillary structure, and exercise hyperemia between the two groups. After the training intervention, capillary-to-fiber ratio increased by 8% ± 3% in the END group (P < 0.05) and was unchanged in the HIIT group with no difference between groups. Endothelium thickness increased (P < 0.05), basement membrane thickness decreased (P < 0.05), and the capillary lumen tended (P = 0.07) to increase in the END group, whereas these structural indicators were unchanged after HIIT. In contrast, skeletal muscle endothelial nitric oxide synthase (eNOS) increased after HIIT (P < 0.05), but not END, whereas there was no change in vascular endothelial growth factor (VEGF), superoxide dismutase (SOD)-2, or NADPH oxidase after both training protocols. In contrast to END training, HIIT did not alter capillarization or capillary structure in individuals with type 2 diabetes. In conclusion, HIIT appears to be a less effective strategy to treat capillary rarefaction and reduce basement thickening in type 2 diabetes.

Influence of hypoxic stimulation on angiogenesis and satellite cells in mouse skeletal muscle

We clarified in our previous study that hypoxic training promotes angiogenesis in skeletal muscle, but the mechanism of angiogenesis in skeletal muscle remains unknown. In this study, we investigated the influence of differences in hypoxia exposure on angiogenesis in skeletal muscles at differing ages and metabolic characteristics at which the production of reactive oxygen species and nitric oxide may differ. Ten-week-old (young) and 20-month-old (old) mice were separated into control (N), continuous hypoxia (H), and intermittent hypoxia (IH) groups. The H group was exposed to 16% O₂ hypoxia for 5 days and the IH group was exposed to 16% O₂ hypoxia at one-hour intervals during the light period for 5 days. After completion of hypoxia exposure, the soleus and gastrocnemius muscles were immediately excised, and mRNA expression of angiogenesis- and satellite cell-related genes was investigated using real-time RT-PCR. In addition, muscle fiber type composition, muscle fiber area, number of satellite cells, and capillary density were measured immunohistochemically. In the young soleus muscle, the muscle fiber area was decreased in the H group, and mRNA expression of satellite cell activation-related MyoD, MHCe, and BDNF was significantly increased. On the other hand, in the old soleus muscle, nNOS and VEGF-A mRNA expression, and the capillary density were significantly increased in the H group. In the superficial portion of the gastrocnemius, mRNA expression of FGF2, an angiogenic factor secreted by satellite cells, was significantly increased in the young IH group. In addition, a positive correlation between VEGF-A mRNA expression and nNOS mRNA expression in the soleus muscle and eNOS mRNA expression in the superficial portion of the gastrocnemius was noted. These data demonstrated that age, hypoxia exposure method and muscle metabolic characteristics are related, which results in significant differences in angiogenesis.

Relation of nNOS isoforms to mitochondrial density and PGC-1alpha expression in striated muscles of mice

The expression of neuronal NO synthase (nNOS) alpha- and beta-isoforms in skeletal muscle is well documented but only little information is available about their regulation/functions. Using different mouse models, we now assessed whether the expression of nNOS-isoforms in muscle fibers is related to mitochondria content/activity and regulated by peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha). Catalytic histochemistry revealed highest nNOS-concentrations to be present in type-2 oxidative muscle fibers. Differences in mitochondrial density between nNOS-KO-mice and WT-littermates established by morphometry after transmission electron microscopy were significant in the oxidative portion of the tibialis anterior muscle (TA) but not in rectus femoris muscle (RF) indicating an nNOS-dependent mitochondrial pool in TA. Quantitative immunoblotting displayed the nNOS alpha-isoform to preponderate in those striated muscles of C57BL/6-mice that comprise of many type-2 oxidative fibers, e.g. TA, while roughly even levels of the two nNOS-isoforms were expressed in those muscles that mainly consist of type-2 glycolytic fibers, e.g. RF. Differences in citrate synthase-activity in muscle homogenates between nNOS-KO-mice and WT-littermates were positively related to nNOS alpha-isoform levels. In transgenic-mice over-expressing muscular PGC-1alpha compared to WT-littermates, immunoblotting revealed a significant shift in nNOS-expression in favor of the alpha-isoform in six out of eight striated muscles (exceptions: soleus muscle and tongue) without consistent relationship to changes in the expression of mitochondrial markers. In summary, our study demonstrated the nNOS alpha-isoform expression to be related to mitochondrial content/activity and to be up-regulated by up-stream PGC-1alpha in striated muscles, particularly in those enriched with type-2 oxidative fibers implying a functional convergence of the two signaling systems in these fibers.

GSNOR Deficiency Enhances In Situ Skeletal Muscle Strength, Fatigue Resistance, and RyR1 S-Nitrosylation Without Impacting Mitochondrial Content and Activity

AIM

Nitric oxide (NO) plays important, but incompletely defined roles in skeletal muscle. NO exerts its regulatory effects partly though S-nitrosylation, which is balanced by denitrosylation by enzymes such as S-nitrosoglutathione reductase (GSNOR), whose functions in skeletal muscle remain to be fully deciphered.

RESULTS

GSNOR null (GSNOR^-/-) tibialis anterior (TA) muscles showed normal growth and were stronger and more fatigue resistant than controls in situ. However, GSNOR^-/- lumbrical muscles showed normal contractility and Ca²⁺ handling in vitro, suggesting important differences in GSNOR function between muscles or between in vitro and in situ environments. GSNOR^-/- TA muscles exhibited normal mitochondrial content, and capillary densities, but reduced type IIA fiber content. GSNOR inhibition did not impact mitochondrial respiratory complex I, III, or IV activities. These findings argue that enhanced GSNOR^-/- TA contractility is not driven by changes in mitochondrial content or activity, fiber type, or blood vessel density. However, loss of GSNOR led to RyR1 hypernitrosylation, which is believed to increase muscle force output under physiological conditions. cGMP synthesis by soluble guanylate cyclase (sGC) was decreased in resting GSNOR^-/- muscle and was more responsive to agonist (DETANO, BAY 41, and BAY 58) stimulation, suggesting that GSNOR modulates cGMP production in skeletal muscle.

INNOVATION

GSNOR may act as a "brake" on skeletal muscle contractile performance under physiological conditions by modulating nitrosylation/denitrosylation balance.

CONCLUSIONS

GSNOR may play important roles in skeletal muscle contractility, RyR1 S-nitrosylation, fiber type specification, and sGC activity. Antioxid. Redox Signal. 26, 165-181.

Rac1 governs exercise-stimulated glucose uptake in skeletal muscle through regulation of GLUT4 translocation in mice

KEY POINT

Exercise increases skeletal muscle energy turnover and one of the important substrates for the working muscle is glucose taken up from the blood. The GTPase Rac1 can be activated by muscle contraction and has been found to be necessary for insulin-stimulated glucose uptake, although its role in exercise-stimulated glucose uptake is unknown. We show that Rac1 regulates the translocation of the glucose transporter GLUT4 to the plasma membrane in skeletal muscle during exercise. We find that Rac1 knockout mice display significantly reduced glucose uptake in skeletal muscle during exercise.

ABSTRACT

Exercise increases skeletal muscle energy turnover and one of the important substrates for the working muscle is glucose taken up from the blood. Despite extensive efforts, the signalling mechanisms vital for glucose uptake during exercise are not yet fully understood, although the GTPase Rac1 is a candidate molecule. The present study investigated the role of Rac1 in muscle glucose uptake and substrate utilization during treadmill exercise in mice in vivo. Exercise-induced uptake of radiolabelled 2-deoxyglucose at 65% of maximum running capacity was blocked in soleus muscle and decreased by 80% and 60% in gastrocnemius and tibialis anterior muscles, respectively, in muscle-specific inducible Rac1 knockout (mKO) mice compared to wild-type littermates. By developing an assay to quantify endogenous GLUT4 translocation, we observed that GLUT4 content at the sarcolemma in response to exercise was reduced in Rac1 mKO muscle. Our findings implicate Rac1 as a regulatory element critical for controlling glucose uptake during exercise via regulation of GLUT4 translocation.

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.

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).

Angiogenesis-related ultrastructural changes to capillaries in human skeletal muscle in response to endurance exercise

The ultrastructure of capillaries in skeletal muscle was morphometrically assessed in vastus lateralis muscle (VL) biopsies taken before and after exercise from 22 participants of two training studies. In study 1 (8 wk of ergometer training), light microscopy revealed capillary-fiber (C/F) ratio (+27%) and capillary density (+16%) to be higher (P ≤ 0.05) in postexercise biopsies than in preexercise biopsies from all 10 participants. In study 2 (6 mo of moderate running), C/F ratio and capillary density were increased (+23% and +20%; respectively, P ≤ 0.05) in VL biopsies from 6 angiogenesis responders (AR) after training, whereas 6 nonangiogenesis responders (NR) showed nonsignificant changes in these structural indicators (-4%/-4%, respectively). Forty capillary profiles per participant were evaluated by point and intersection counting on cross sections after transmission electron microscopy. In study 1, volume density (Vv) and mean arithmetic thickness (T) of endothelial cells (ECs; +19%/+17%, respectively) and pericytes (PCs; +20%/+21%, respectively) were higher (P ≤ 0.05), whereas Vv and T of the pericapillary basement membrane (BM) were -23%/-22% lower (P ≤ 0.05), respectively, in posttraining biopsies. In study 2, exercise-related differences between AR and NR-groups were found for Vv and T of PCs (AR, +26%/+22%, respectively, both P ≤ 0.05; NR, +1%/-3%, respectively, both P > 0.05) and BM (AR, -14%/-13%, respectively, both P ≤ 0.05; NR, -9%/-11%, respectively, P = 0.07/0.10). Vv and T of ECs were higher (AR, +16%/+18%, respectively; NR, +6%/+6%, respectively; all P ≤ 0.05) in both groups. The PC coverage was higher (+13%, P ≤ 0.05) in VL biopsies of individuals in the AR group but nonsignificantly altered (+3%, P > 0.05) in those of the NR group after training. Our study suggests that intensified PC mobilization and BM thinning are related to exercise-induced angiogenesis in human skeletal muscle, whereas training per se induces EC-thickening.

Fibroblast Growth Factor 9 Imparts Hierarchy and Vasoreactivity to the Microcirculation of Renal Tumors and Suppresses Metastases

Tumor vessel normalization has been proposed as a therapeutic paradigm. However, normal microvessels are hierarchical and vasoreactive with single file transit of red blood cells through capillaries. Such a network has not been identified in malignant tumors. We tested whether the chaotic tumor microcirculation could be reconfigured by the mesenchyme-selective growth factor, FGF9. Delivery of FGF9 to renal tumors in mice yielded microvessels that were covered by pericytes, smooth muscle cells, and a collagen-fortified basement membrane. This was associated with reduced pulmonary metastases. Intravital microvascular imaging revealed a haphazard web of channels in control tumors but a network of arterioles, bona fide capillaries, and venules in FGF9-expressing tumors. Moreover, whereas vasoreactivity was absent in control tumors, arterioles in FGF9-expressing tumors could constrict and dilate in response to adrenergic and nitric oxide releasing agents, respectively. These changes were accompanied by reduced hypoxia in the tumor core and reduced expression of the angiogenic factor VEGF-A. FGF9 was found to selectively amplify a population of PDGFRβ-positive stromal cells in the tumor and blocking PDGFRβ prevented microvascular differentiation by FGF9 and also worsened metastases. We conclude that harnessing local mesenchymal stromal cells with FGF9 can differentiate the tumor microvasculature to an extent not observed previously.

Capillary growth, ultrastructure remodelling and exercise training in skeletal muscle of essential hypertensive patients

AIM

The aim was to elucidate whether essential hypertension is associated with altered capillary morphology and density and to what extent exercise training can normalize these parameters.

METHODS

To investigate angiogenesis and capillary morphology in essential hypertension, muscle biopsies were obtained from m. vastus lateralis in subjects with essential hypertension (n = 10) and normotensive controls (n = 11) before and after 8 weeks of aerobic exercise training. Morphometry was performed after transmission electron microscopy, and protein levels of several angioregulatory factors were determined.

RESULTS

At baseline, capillary density and capillary-to-fibre ratio were not different between the two groups. However, the hypertensive subjects had 9% lower capillary area (12.7 ± 0.4 vs. 13.9 ± 0.2 μm(2)) and tended to have thicker capillary basement membranes (399 ± 16 vs. 358 ± 13 nm; P = 0.094) than controls. Protein expression of vascular endothelial growth factor (VEGF), VEGF receptor-2 and thrombospondin-1 were similar in normotensive and hypertensive subjects, but tissue inhibitor of matrix metalloproteinase was 69% lower in the hypertensive group. After training, angiogenesis was evident by 15% increased capillary-to-fibre ratio in the hypertensive subjects only. Capillary area and capillary lumen area were increased by 7 and 15% in the hypertensive patients, whereas capillary basement membrane thickness was decreased by 17% (P < 0.05). VEGF expression after training was increased in both groups, whereas VEGF receptor-2 was decreased by 25% in the hypertensive patients(P < 0.05).

CONCLUSION

Essential hypertension is associated with decreased lumen area and a tendency for increased basement membrane thickening in capillaries of skeletal muscle. Exercise training may improve the diffusion conditions in essential hypertension by altering capillary structure and capillary number.

Physical exercise associated with NO production: signaling pathways and significance in health and disease

Here we review available data on nitric oxide (NO)-mediated signaling in skeletal muscle during physical exercise. Nitric oxide modulates skeletal myocyte function, hormone regulation, and local microcirculation. Nitric oxide underlies the therapeutic effects of physical activity whereas the pharmacological modulators of NO-mediated signaling are the promising therapeutic agents in different diseases. Nitric oxide production increases in skeletal muscle in response to physical activity. This molecule can alter energy supply in skeletal muscle through hormonal modulation. Mitochondria in skeletal muscle tissue are highly abundant and play a pivotal role in metabolism. Considering NO a plausible regulator of mitochondrial biogenesis that directly affects cellular respiration, we discuss the mechanisms of NO-induced mitochondrial biogenesis in the skeletal muscle cells. We also review available data on myokines, the molecules that are expressed and released by the muscle fibers and exert autocrine, paracrine and/or endocrine effects. The article suggests the presence of putative interplay between NO-mediated signaling and myokines in skeletal muscle. Data demonstrate an important role of NO in various diseases and suggest that physical training may improve health of patients with diabetes, chronic heart failure, and even degenerative muscle diseases. We conclude that NO-associated signaling represents a promising target for the treatment of various diseases and for the achievement of better athletic performance.

Deficient nitric oxide signalling impairs skeletal muscle growth and performance: involvement of mitochondrial dysregulation

BACKGROUND

Nitric oxide (NO), generated in skeletal muscle mostly by the neuronal NO synthases (nNOSμ), has profound effects on both mitochondrial bioenergetics and muscle development and function. The importance of NO for muscle repair emerges from the observation that nNOS signalling is defective in many genetically diverse skeletal muscle diseases in which muscle repair is dysregulated. How the effects of NO/nNOSμ on mitochondria impact on muscle function, however, has not been investigated yet.

METHODS

In this study we have examined the relationship between the NO system, mitochondrial structure/activity and skeletal muscle phenotype/growth/functions using a mouse model in which nNOSμ is absent. Also, NO-induced effects and the NO pathway were dissected in myogenic precursor cells.

RESULTS

We show that nNOSμ deficiency in mouse skeletal muscle leads to altered mitochondrial bioenergetics and network remodelling, and increased mitochondrial unfolded protein response (UPR(mt)) and autophagy. The absence of nNOSμ is also accompanied by an altered mitochondrial homeostasis in myogenic precursor cells with a decrease in the number of myonuclei per fibre and impaired muscle development at early stages of perinatal growth. No alterations were observed, however, in the overall resting muscle structure, apart from a reduced specific muscle mass and cross sectional areas of the myofibres. Investigating the molecular mechanisms we found that nNOSμ deficiency was associated with an inhibition of the Akt-mammalian target of rapamycin pathway. Concomitantly, the Akt-FoxO3-mitochondrial E3 ubiquitin protein ligase 1 (Mul-1) axis was also dysregulated. In particular, inhibition of nNOS/NO/cyclic guanosine monophosphate (cGMP)/cGMP-dependent-protein kinases induced the transcriptional activity of FoxO3 and increased Mul-1 expression. nNOSμ deficiency was also accompanied by functional changes in muscle with reduced muscle force, decreased resistance to fatigue and increased degeneration/damage post-exercise.

CONCLUSIONS

Our results indicate that nNOSμ/NO is required to regulate key homeostatic mechanisms in skeletal muscle, namely mitochondrial bioenergetics and network remodelling, UPR(mt) and autophagy. These events are likely associated with nNOSμ-dependent impairments of muscle fibre growth resulting in a deficit of muscle performance.

Effects of systemic hypoxia on human muscular adaptations to resistance exercise training

Hypoxia is an important modulator of endurance exercise‐induced oxidative adaptations in skeletal muscle. However, whether hypoxia affects resistance exercise‐induced muscle adaptations remains unknown. Here, we determined the effect of resistance exercise training under systemic hypoxia on muscular adaptations known to occur following both resistance and endurance exercise training, including muscle cross‐sectional area (CSA), one‐repetition maximum (1RM), muscular endurance, and makers of mitochondrial biogenesis and angiogenesis, such as peroxisome proliferator‐activated receptor‐γ coactivator‐1α (PGC‐1α), citrate synthase (CS) activity, nitric oxide synthase (NOS), vascular endothelial growth factor (VEGF), hypoxia‐inducible factor‐1 (HIF‐1), and capillary‐to‐fiber ratio. Sixteen healthy male subjects were randomly assigned to either a normoxic resistance training group (NRT, n =7) or a hypoxic (14.4% oxygen) resistance training group (HRT, n =9) and performed 8 weeks of resistance training. Blood and muscle biopsy samples were obtained before and after training. After training muscle CSA of the femoral region, 1RM for bench‐press and leg‐press, muscular endurance, and skeletal muscle VEGF protein levels significantly increased in both groups. The increase in muscular endurance was significantly higher in the HRT group. Plasma VEGF concentration and skeletal muscle capillary‐to‐fiber ratio were significantly higher in the HRT group than the NRT group following training. Our results suggest that, in addition to increases in muscle size and strength, HRT may also lead to increased muscular endurance and the promotion of angiogenesis in skeletal muscle.

This study investigated the effect of resistance exercise training performed under systemic hypoxia or normoxia on biochemical and molecular muscular adaptations in healthy male subjects. Our findings demonstrate that resistance training under systemic hypoxia led not only muscle hypertrophy, but most interestingly, to a greater increase in muscular endurance. This increase in muscular endurance was potentially caused by the increased angiogenesis as determined by capillary‐to‐fiber ratio.

Defective homocysteine metabolism: potential implications for skeletal muscle malfunction

Hyperhomocysteinemia (HHcy) is a systemic medical condition and has been attributed to multi-organ pathologies. Genetic, nutritional, hormonal, age and gender differences are involved in abnormal homocysteine (Hcy) metabolism that produces HHcy. Homocysteine is an intermediate for many key processes such as cellular methylation and cellular antioxidant potential and imbalances in Hcy production and/or catabolism impacts gene expression and cell signaling including GPCR signaling. Furthermore, HHcy might damage the vagus nerve and superior cervical ganglion and affects various GPCR functions; therefore it can impair both the parasympathetic and sympathetic regulation in the blood vessels of skeletal muscle and affect long-term muscle function. Understanding cellular targets of Hcy during HHcy in different contexts and its role either as a primary risk factor or as an aggravator of certain disease conditions would provide better interventions. In this review we have provided recent Hcy mediated mechanistic insights into different diseases and presented potential implications in the context of reduced muscle function and integrity. Overall, the impact of HHcy in various skeletal muscle malfunctions is underappreciated; future studies in this area will provide deeper insights and improve our understanding of the association between HHcy and diminished physical function.