Skeletal muscle capillary density and microvascular function are compromised with aging and type 2 diabetes

Skeletal muscle fiber characteristics in patients with chronic heart failure: impact of disease severity and relation with muscle oxygenation during exercise

INTRODUCTION

Skeletal muscle function in patients with heart failure and reduced ejection fraction (HFrEF) greatly determines exercise capacity. However, reports on skeletal muscle fiber dimensions, fiber capillarization, and their physiological importance are inconsistent.

METHODS

Twenty-five moderately-impaired patients with HFrEF and 25 healthy control (HC) subjects underwent muscle biopsy sampling. Type I and type II muscle fiber characteristics were determined by immunohistochemistry. In patients with HFrEF, enzymatic oxidative capacity was assessed, and pulmonary oxygen uptake (VO₂) and skeletal muscle oxygenation during maximal and moderate-intensity exercise were measured using near-infrared spectroscopy.

RESULTS

While muscle fiber cross-sectional area (CSA) was not different between patients with HFrEF and HC, percentage of type I fibers was higher in HC (46±15% versus 37±12%, respectively, P=0.041). Fiber type distribution and CSA were not different between patients in New York Heart Association (NYHA) class II and III. Type I muscle fiber capillarization was higher in HFrEF compared with controls (capillary-to-fiber perimeter exchange (CFPE) index: 5.70±0.92 versus 5.05±0.82, respectively, P=0.027). Patients in NYHA class III had slower VO₂ and muscle deoxygenation kinetics during onset of exercise, and lower muscle oxidative capacity than those in class II (P<0.05). Also, fiber capillarization was lower, but not compared with HC. Higher CFPE index was related to faster deoxygenation (r_spearman=-0.682, P=0.001), however, not to muscle oxidative capacity (r=-0.282, P=0.216).

CONCLUSIONS

Type I muscle fiber capillarization is higher in HFrEF compared with HC, but not in patients with greater exercise impairment. Greater capillarization may positively affect VO₂ kinetics by enhancing muscle oxygen diffusion.

Cardiovascular aging and the microcirculation of skeletal muscle: using contrast-enhanced ultrasound

Skeletal muscle is the largest and most important site of capillary-tissue exchange, especially during high-energy demand tasks such as exercise; however, information regarding the role of the microcirculation in maintaining skeletal muscle health is limited. Changes in microcirculatory function, as observed with aging, chronic and cardiovascular diseases, and exercise, likely precede any alterations that arise in larger vessels, although further investigation into these changes is required. One of the main barriers to addressing this knowledge gap is the lack of methodologies for quantifying microvascular function in vivo; the utilization of valid and noninvasive quantification methods would allow the dynamic evaluation of microvascular flow during periods of clinical relevance such as during increased demand for flow (exercise) or decreased demand for flow (disuse). Contrast-enhanced ultrasound (CEUS) is a promising noninvasive technique that has been used for diagnostic medicine and more recently as a complementary research modality to investigate the response of the microcirculation in insulin resistance, diabetes, and aging. To improve the reproducibility of these measurements, our laboratory has optimized the quantification protocol associated with a bolus injection of the contrast agent for research purposes. This brief report outlines the assessment of microvascular flow using the raw time-intensity curve incorporated into gamma variate response modeling. CEUS could be used to compliment any macrovascular assessments to capture a more complete picture of the aging vasculature, and the modified methods presented here provide a template for the general analysis of CEUS within a research setting.

Sympathoexcitation constrains vasodilation in the human skeletal muscle microvasculature during postocclusive reactive hyperemia

We used diffuse correlation spectroscopy to investigate sympathetic vasoconstriction, local vasodilation, and integration of these two responses in the skeletal muscle microvasculature of 20 healthy volunteers. Diffuse correlation spectroscopy probes were placed on the flexor carpi radialis muscle or vastus lateralis muscle, and a blood flow index was derived continuously. We measured hemodynamic responses during sympathoexcitation induced by forehead cooling, after which the effects of the increased sympathetic tone on vasodilatory responses during postocclusive reactive hyperemia (PORH) were examined. PORH was induced by releasing arterial occlusion (3 min) in an arm or leg. To increase sympathetic tone during PORH, forehead cooling was begun 60 s before the occlusion release and ended 60 s after the release. During forehead cooling, mean arterial pressure rose significantly and was sustained at an elevated level. Significant vasoconstriction and decreases in blood flow index followed by gradual blunting of the vasoconstriction also occurred. The time course of these responses is in good agreement with previous observations in animals. The acute sympathoexcitation diminished the peak vasodilation during PORH only in the vastus lateralis muscle, but it hastened the decline in vasodilation after the peak in both the flexor carpi radialis muscle and vastus lateralis muscle. Consequently, the total vasodilatory response assessed as the area of the vascular conductance during the first minute of PORH was significantly diminished in both regions. We conclude that, in humans, the integrated effects of sympathetic vasoconstriction and local vasodilation have an important role in vascular regulation and control of perfusion in the skeletal muscle microcirculation.

NEW & NOTEWORTHY We used diffuse correlation spectroscopy to demonstrate that acute sympathoexcitation constrains local vasodilation in the human skeletal muscle microvasculature during postocclusive reactive hyperemia. This finding indicates that integration of sympathetic vasoconstriction and local vasodilation is importantly involved in vascular regulation and the control of perfusion of the skeletal muscle microcirculation in humans.

A conceptual framework for predicting and addressing the consequences of disease-related microvascular dysfunction

OBJECTIVE

A growing body of evidence indicates that impaired microvascular perfusion plays a pathological role in a number of diseases. This manuscript aims to better define which aspects of microvascular perfusion are important, what mass transport processes (eg, insulin action, tissue oxygenation) may be impacted, and what therapies might reverse these pathologies.

METHODS

We derive a theory of microvascular perfusion and solute flux drawing from established relationships in mass transport and anatomy. We then apply this theory to predict relationships between microvascular perfusion parameters and microvascular solute flux.

RESULTS

For convection-limited exchange processes (eg, pulmonary oxygen uptake), our model predicts that bulk blood flow is of primary importance. For diffusion-limited exchange processes (eg, insulin action), our model predicts that perfused capillary density is of primary importance. For convection/diffusion co-limited exchange processes (eg, tissue oxygenation), our model predicts that various microvascular perfusion parameters interact in a complex, context-specific manner. We further show that our model can predict established mass transport defects in disease (eg, insulin resistance in diabetes).

CONCLUSIONS

The contributions of microvascular perfusion parameters to tissue-level solute flux can be described using a minimal mathematical model. Our results hold promise for informing therapeutic interventions targeting microvascular perfusion.

Advanced age results in a diminished endothelial glycocalyx

Age-related microvascular dysfunction is well characterized in rodents and humans, but little is known about the properties of the microvascular endothelial glycocalyx in advanced age. We examined the glycocalyx in microvessels of young and old male C57BL6 mice (young: 6.1 ± 0.1 mo vs. old: 24.6 ± 0.2 mo) using intravital microscopy and transmission electron microscopy and in human participants (young: 29 ± 1 yr vs. old: 60 ± 2 yr) using intravital microscopy. Glycocalyx thickness in mesenteric and skeletal muscle microvessels was 51-54% lower in old compared with young mice. We also observed 33% lower glycocalyx thickness in the sublingual microcirculation of humans in advanced age. The perfused boundary region, a marker of glycocalyx barrier function, was also obtained using an automated capture and analysis system. In advanced age, we observed a 10-22% greater perfused boundary region in mice and humans, indicating a more penetrable glycocalyx. Finally, using this automated analysis system, we examined perfused microvascular density and red blood cell (RBC) fraction. Perfused microvascular density is a marker of microvascular function that reflects the length of perfused microvessel segments in a given area; RBC fraction represents the heterogeneity in RBC presence between microvessel segments. Compared with young, the perfused microvascular density was 16-21% lower and RBC fraction was 5-14% lower in older mice and in older humans. These data provide novel evidence that, across mammalian species, a diminished glycocalyx is present in advanced age and is accompanied by markers of impaired microvascular perfusion. Age-related glycocalyx deterioration may be an important contributor to microvascular dysfunction in older adults and subsequent pathophysiology. NEW & NOTEWORTHY Advanced age is characterized by microvascular dysfunction that contributes to age-related cardiovascular diseases, but little is known about endothelial glycocalyx properties in advanced age. This study reveals, for the first time, lower glycocalyx thickness and barrier function that is accompanied by impaired microvascular perfusion in both mice and humans in advanced age.

Perspective: Protein Requirements and Optimal Intakes in Aging: Are We Ready to Recommend More Than the Recommended Daily Allowance?

The Dietary Reference Intakes set the protein RDA for persons >19 y of age at 0.8 g protein ⋅ kg body weight-1 ⋅ d-1. A growing body of evidence suggests, however, that the protein RDA may be inadequate for older individuals. The evidence for recommending a protein intake greater than the RDA comes from a variety of metabolic approaches. Methodologies centered on skeletal muscle are of paramount importance given the age-related decline in skeletal muscle mass and function (sarcopenia) and the degree to which dietary protein could mitigate these declines. In addition to evidence from short-term experimental trials, observational data show that higher protein intakes are associated with greater muscle mass and, more importantly, better muscle function with aging. We are in dire need of more evidence from longer-term intervention trials showing the efficacy of protein intakes that are higher than the RDA in older persons to support skeletal muscle health. We propose that it should be recommended that older individuals consume ≥1.2 g protein · kg-1 · d-1 and that there should be an emphasis on the intake of the amino acid leucine, which plays a central role in stimulating skeletal muscle anabolism. Critically, the often-cited potential negative effects of consuming higher protein intakes on renal and bone health are without a scientific foundation in humans.

Daily muscle stretching enhances blood flow, endothelial function, capillarity, vascular volume and connectivity in aged skeletal muscle

KEY POINTS

In aged rats, daily muscle stretching increases blood flow to skeletal muscle during exercise. Daily muscle stretching enhanced endothelium-dependent vasodilatation of skeletal muscle resistance arterioles of aged rats. Angiogenic markers and capillarity increased in response to daily stretching in muscles of aged rats. Muscle stretching performed with a splint could provide a feasible means of improving muscle blood flow and function in elderly patients who cannot perform regular aerobic exercise.

ABSTRACT

Mechanical stretch stimuli alter the morphology and function of cultured endothelial cells; however, little is known about the effects of daily muscle stretching on adaptations of endothelial function and muscle blood flow. The present study aimed to determine the effects of daily muscle stretching on endothelium-dependent vasodilatation and muscle blood flow in aged rats. The lower hindlimb muscles of aged Fischer rats were passively stretched by placing an ankle dorsiflexion splint for 30 min day^-1 , 5 days week^-1 , for 4 weeks. Blood flow to the stretched limb and the non-stretched contralateral limb was determined at rest and during treadmill exercise. Endothelium-dependent/independent vasodilatation was evaluated in soleus muscle arterioles. Levels of hypoxia-induced factor-1α, vascular endothelial growth factor A and neuronal nitric oxide synthase were determined in soleus muscle fibres. Levels of endothelial nitric oxide synthase and superoxide dismutase were determined in soleus muscle arterioles, and microvascular volume and capillarity were evaluated by microcomputed tomography and lectin staining, respectively. During exercise, blood flow to plantar flexor muscles was significantly higher in the stretched limb. Endothelium-dependent vasodilatation was enhanced in arterioles from the soleus muscle from the stretched limb. Microvascular volume, number of capillaries per muscle fibre, and levels of hypoxia-induced factor-1α, vascular endothelial growth factor and endothelial nitric oxide synthase were significantly higher in the stretched limb. These results indicate that daily passive stretching of muscle enhances endothelium-dependent vasodilatation and induces angiogenesis. These microvascular adaptations may contribute to increased muscle blood flow during exercise in muscles that have undergone daily passive stretch.

Muscle fiber capillarization as determining factor on indices of insulin sensitivity in humans

To investigate the association between muscle fiber capillarization and indices of insulin sensitivity in healthy older adults. A skeletal muscle biopsy was taken from the m. vastus lateralis of 22 healthy (nondiabetic) male older adults. In addition, all participants underwent an Oral Glucose Tolerance Test (OGTT). Muscle fiber capillarization was assessed by immunohistochemistry. Participants were divided into a group with relatively low (LOW) or high (HIGH) muscle fiber capillarization (capillary‐to‐fiber perimeter exchange (CFPE) index), based on the median value for the entire group. All participants were healthy, nonobese, and had a normal glucose tolerance, according to the individual OGTT results. Whereas no differences in blood glucose concentrations were observed between groups during the OGTT, the postprandial increase in plasma insulin concentrations was significantly greater in the LOW compared to the HIGH muscle fiber capillarization group (P < 0.05). Skeletal muscle fiber capillarization may determine insulin sensitivity in humans.

Impairment of an Endothelial NAD+-H2S Signaling Network Is a Reversible Cause of Vascular Aging

A decline in capillary density and blood flow with age is a major cause of mortality and morbidity. Understanding why this occurs is key to future gains in human health. NAD precursors reverse aspects of aging, in part, by activating sirtuin deacylases (SIRT1-SIRT7) that mediate the benefits of exercise and dietary restriction (DR). We show that SIRT1 in endothelial cells is a key mediator of pro-angiogenic signals secreted from myocytes. Treatment of mice with the NAD+ booster nicotinamide mononucleotide (NMN) improves blood flow and increases endurance in elderly mice by promoting SIRT1-dependent increases in capillary density, an effect augmented by exercise or increasing the levels of hydrogen sulfide (H2S), a DR mimetic and regulator of endothelial NAD+ levels. These findings have implications for improving blood flow to organs and tissues, increasing human performance, and reestablishing a virtuous cycle of mobility in the elderly.

Effects of Exercise and Aging on Skeletal Muscle

A substantial loss of muscle mass and strength (sarcopenia), a decreased regenerative capacity, and a compromised physical performance are hallmarks of aging skeletal muscle. These changes are typically accompanied by impaired muscle metabolism, including mitochondrial dysfunction and insulin resistance. A challenge in the field of muscle aging is to dissociate the effects of chronological aging per se on muscle characteristics from the secondary influence of lifestyle and disease processes. Remarkably, physical activity and exercise are well-established countermeasures against muscle aging, and have been shown to attenuate age-related decreases in muscle mass, strength, and regenerative capacity, and slow or prevent impairments in muscle metabolism. We posit that exercise and physical activity can influence many of the changes in muscle during aging, and thus should be emphasized as part of a lifestyle essential to healthy aging.

Age Reduces Microvascular Function in the Leg Independent of Physical Activity

The microvasculature is critical in the control of blood flow. Aging and reduced physical activity (PA) may both decrease microvascular function.

PURPOSE

The primary aim was to evaluate the influence of age on microvascular function in adults with similar PA levels. Secondary aims were to assess the reliability of muscle functional magnetic resonance imaging in older adults (OA) and the relationship between PA and microvascular function in OA.

METHODS

Microvascular blood-oxygen-level dependent (BOLD) responses were measured in young adults (YA, n = 12, mean ± SD age = 21 ± 1 yr old, PA = 239 ± 73 × 10 counts per day) and OA (n = 13, 64 ± 4 yr old, PA = 203 ± 48 × 10 counts per day). Functional magnetic resonance images (3T, echo planar BOLD) of the leg were acquired after brief (1 s) maximal voluntary isometric contractions. The test-retest reliability of BOLD responses and the Pearson correlation between peak BOLD and PA were assessed in a group of OA (OA-r) with a broad range of PA (66 ± 5 yr old, n = 9, PA range = 54 × 10 to 674 × 10 counts per day).

RESULTS

Peak BOLD microvascular responses were reduced for OA compared with YA. OA peak BOLD was 27% lower in the soleus (3.3% ± 0.8% OA vs 4.5% ± 1.4% YA; P = 0.017) and 40% lower in the anterior compartment (1.6% ± 0.6% OA vs 2.7% ± 1.1% YA; P = 0.006). Coefficients of variation were 8.6% and 11.8% for peak BOLD in the soleus and anterior compartment, respectively, with an intraclass correlation of 0.950 for both muscle regions. The correlation between peak BOLD and PA was r ≥ 0.715, P ≤ 0.030.

CONCLUSIONS

Aging was associated with reduced microvascular function in leg muscles, independent of PA. The findings also revealed good reliability for BOLD magnetic resonance imaging in OA for the soleus and anterior compartment muscles.

Dietary nitrate supplementation in cardiovascular health: an ergogenic aid or exercise therapeutic?

Oral consumption of inorganic nitrate, which is abundant in green leafy vegetables and roots, has been shown to increase circulating plasma nitrite concentration, which can be converted to nitric oxide in low oxygen conditions. The associated beneficial physiological effects include a reduction in blood pressure, modification of platelet aggregation, and increases in limb blood flow. There have been numerous studies of nitrate supplementation in healthy recreational and competitive athletes; however, the ergogenic benefits are currently unclear due to a variety of factors including small sample sizes, different dosing regimens, variable nitrate conversion rates, the heterogeneity of participants’ initial fitness levels, and the types of exercise tests used. In clinical populations, the study results seem more promising, particularly in patients with cardiovascular diseases who typically present with disruptions in the ability to transport oxygen from the atmosphere to working tissues and reduced exercise tolerance. Many of these disease-related, physiological maladaptations, including endothelial dysfunction, increased reactive oxygen species, reduced tissue perfusion, and muscle mitochondrial dysfunction, have been previously identified as potential targets for nitric oxide restorative effects. This review is the first of its kind to outline the current evidence for inorganic nitrate supplementation as a therapeutic intervention to restore exercise tolerance and improve quality of life in patients with cardiovascular diseases. We summarize the factors that appear to limit or maximize its effectiveness and present a case for why it may be more effective in patients with cardiovascular disease than as ergogenic aid in healthy populations.

The Influence of Clinically Diagnosed Neuropathy on Respiratory Muscle Strength in Type 2 Diabetes Mellitus

OBJECTIVES

This cross-sectional study investigated the influence of clinically diagnosed neuropathy (cdNP) on respiratory muscle strength in patients with type 2 diabetes mellitus (T2DM).

METHODS

110 T2DM patients and 35 nondiabetic healthy controls (≥60 years) were allocated to one of three groups depending on the presence of cdNP: T2DM without cdNP (D-; n = 28), T2DM with cdNP (D+; n = 82), and controls without cdNP (C; n = 35). Clinical neurological diagnostic examination consisted of Vibration Perception Threshold and Diabetic Neuropathy Symptom score. Respiratory muscle strength was registered by maximal Inspiratory and Expiratory Pressures (PI_max and PE_max), and respiratory function by Peak Expiratory Flow (PEF). Isometric Handgrip Strength and Short Physical Performance Battery were used to evaluate peripheral skeletal muscle strength and physical performance. Univariate analysis of covariance was used with age, level of physical activity, and body mass index as covariates.

RESULTS

PI_max, PE_max, and PEF were higher in C compared to D- and D+. Exploring more in detail, PI_max, PE_max, and PEF were significantly lower in D+ compared to C. PE_max and PEF were also significantly lower in D- versus C. Measures of peripheral muscle strength and physical performance showed less associations with cdNP and T2DM.

CONCLUSIONS

The presence of cdNP affects respiratory muscle strength in T2DM patients compared to healthy controls. Both cdNP and diabetes in themselves showed a distinctive impact on respiratory muscle strength and function; however, an accumulating effect could not be ascertained in this study. As commonly used measures of peripheral muscle strength and physical performance seemed to be less affected at the given time, the integration of PI_max, PE_max, and PEF measurements in the assessment of respiratory muscle weakness could be of added value in the (early) screening for neuropathy in patients with T2DM.

Berberine Improves Cognitive Deficiency and Muscular Dysfunction via Activation of the AMPK/SIRT1/PGC-1a Pathway in Skeletal Muscle from Naturally Aging Rats

OBJECTIVE

The manifestations of aging include cognitive deficits and muscular dysfunction, which are closely linked to impairment of mitochondrial biogenesis. Berberine, an isoquinoline alkaloid, presents multiple anti-diabetic pharmacological effects. Evidence has indicated that insulin resistance and cognitive impairment share the same pathogenesis, and berberine could reverse glucose metabolism abnormalities and muscle mitochondrial dysfunction induced by a high-fat diet. This study was used to investigate whether berberine could be used as an anti-aging drug to prevent cognitive deficits and muscular dysfunction in natural aging.

METHODS

Biochemical indicators and an intraperitoneal glucose tolerance test were tested in 5-month-old rats (5 mo group), 24-month-old rats (24 mo group) and 24-month-old rats that had undergone 6 months of berberine treatment (BBR group). A Morris water maze test was conducted to assess the cognitive ability of the rats. Insulin resistance in whole-body was evaluated by intraperitoneal glucose tolerance test (IPGTT). The morphology of the skeletal muscle tissue was observed by hematoxylin-eosin (HE) staining. The levels of total cholesterol, triglyceride, ATP and reactive oxygen species (ROS) were assessed with corresponding reagent kits. The protein expressions of GLUT4, AMPK, SIRT1 and PGC-1α in skeletal muscle were examined by Western blot.

RESULTS

The results showed that administration of berberine for 6 months significantly improved cognitive deficits and insulin resistance in naturally aging rats (p<0.01). Furthermore, berberine treatment helped normalize the disordered alignment and the decreased number of muscle fibers (p<0.01) in the skeletal muscle of 24 mo rats. Berberine decreased the levels of ROS in both the serum and the skeletal muscle of 24 mo rats (p<0.01). Berberine increased the protein expression of p-AMPK, SIRT1 and PGC-1α and increased the production of ATP in the skeletal muscle of aging rats (p<0.01).

CONCLUSIONS

Berberine markedly ameliorates aging-related reductions in cognitive ability and muscular function, and the activation of the AMPK/SIRT1/PGC-1α pathway in skeletal muscle may be the underlying protective mechanism of berberine on muscular function.

The Muscle Stem Cell Niche in Health and Disease

The regulation of stem cells that maintain and regenerate postnatal tissues depends on extrinsic signals originating from their microenvironment, commonly referred to as the stem cell niche. Complex higher-order regulatory interrelationships with the tissue and factors in the systemic circulation are integrated and propagated to the stem cells through the niche. The stem cell niche in skeletal muscle tissue is both a paradigm for a structurally and functionally relatively static niche that maintains stem cell quiescence during tissue homeostasis, and a highly dynamic regenerative niche that is subject to extensive structural remodeling and a flux of different support cell populations. Conditions ranging from aging to chronically degenerative skeletal muscle diseases affect the composition of the niche and thereby impair the regenerative potential of muscle stem cells. A holistic and integrative understanding of the extrinsic mechanisms regulating muscle stem cells in health and disease in a broad systemic context will be imperative for the identification of regulatory hubs in the niche interactome that can be targeted to maintain, restore, or enhance the regenerative capacity of muscle tissue. Here, we review the microenvironmental regulation of muscle stem cells, summarize how niche dysfunction can contribute to disease, and discuss emerging therapeutic implications.

Evidence for congruent impairment in micro and macrovascular function in type 1 diabetes

Diabetes affects large and small vessels through mechanisms only partially known. In the present study, we evaluated the function of capillaries and large arteries in subjects with type 1 diabetes mellitus (T1DM) to study the effect of chronic hyperglycemia in the absence of other cardiovascular risk factors. Twenty-five subjects with T1DM and 12 healthy age-matched controls were enrolled. Nine patients had mild or moderate retinopathy. Contrast enhanced ultrasound was used to measure perfusion of the deep forearm flexor muscle of the non-dominant arm at rest (baseline) and after an ischemic stimulus (reactive hyperemia). Perfusion was expressed as Video Intensity (VI) in arbitrary unit (a.u.)/mm². The time to reach peak VI after ischemia was also recorded. The function of large arteries was evaluated using flow-mediated vasodilation (FMD). VI was significantly lower in T1DM compared to control subjects both at baseline (0.22±0.16 vs 0.44±0.35 a.u./mm², p<0.05), and after ischemia (0.33±0.24 vs 0.68±0.46 a.u./mm², p<0.05). The time to reach peak VI after ischemia was markedly longer in T1DM (5.6±2.2 vs 4.0±1.7 seconds, p<0.02). These differences were more marked in T1DM subjects with retinopathy. FMD was lower in TIDM patients compared to controls (5.4±6.4 vs 10.7±4.5%, p<0.01). The present findings demonstrate that T1DM patients have defective peripheral skeletal muscle perfusion both at rest and after ischemia compared with control subjects. Low muscle perfusion associates with low FMD of the brachial artery. Furthermore, T1DM subjects with retinopathy have the least muscle perfusion and blunted response to hyperemia compared to T1DM without retinopathy.

Automated Measurement of Microvascular Function Reveals Dysfunction in Systemic Sclerosis: A Cross-sectional Study

OBJECTIVE

The aim of our study was to determine whether an automated capture and analysis system could detect differences in structure and function of sublingual microvessels in patients with systemic sclerosis (SSc) compared to healthy controls.

METHODS

Intravital microscopy of the sublingual microcirculation was automatically captured and analyzed in 40 patients with SSc and 10 age-matched healthy controls.

RESULTS

Total and perfused microvascular density were lower in patients with SSc compared with controls (total microvascular density: 2471 ± 134 µm/mm² vs 3067 ± 197 µm/mm², p = 0.020; perfused microvascular density: 1708 ± 92 µm/mm² vs 2192 ± 144 µm/mm², p = 0.009). However, the relative percentage of perfused to total microvascular density was similar between SSc and controls (72 ± 2% vs 71 ± 2%, respectively, p = 0.429). Mean red blood cell (RBC) fraction, which indicates the longitudinal tube hematocrit of microvessel segments, was lower in patients with SSc compared with controls (69 ± 1% vs 77 ± 1%, respectively, p < 0.001). Perfused boundary region (PBR), a marker of endothelial glycocalyx barrier properties, was higher in patients with SSc compared with controls (2.1 ± 0.0 µm vs 1.9 ± 0.0 µm, respectively, p = 0.012), suggestive of a dysfunctional glycocalyx. There was an inverse association of PBR with perfused microvascular density (r = -0.40, p = 0.004) and RBC fraction (r = -0.80, p < 0.001).

CONCLUSION

Our results indicate that automated capture and analysis of sublingual microvessel segments produces detailed, objective microvascular structural and functional data that have allowed us to distinguish patients with SSc from controls. These data suggest that microvascular structural and functional abnormalities present in patients with SSc could be at least partly due to a dysfunctional glycocalyx.

A non-invasive magnetic resonance imaging approach for assessment of real-time microcirculation dynamics

We present a novel, non-invasive magnetic resonance imaging (MRI) technique to assess real-time dynamic vasomodulation of the microvascular bed. Unlike existing perfusion imaging techniques, our method is sensitive only to blood volume and not flow velocity. Using graded gas challenges and a long-life, blood-pool T₁-reducing agent gadofosveset, we can sensitively assess microvascular volume response in the liver, kidney cortex, and paraspinal muscle to vasoactive stimuli (i.e. hypercapnia, hypoxia, and hypercapnic hypoxia). Healthy adult rats were imaged on a 3 Tesla scanner and cycled through 10-minute gas intervals to elicit vasoconstriction followed by vasodilatation. Quantitative T₁ relaxation time mapping was performed dynamically; heart rate and blood oxygen saturation were continuously monitored. Laser Doppler perfusion measurements confirmed MRI findings: dynamic changes in T₁ corresponded with perfusion changes to graded gas challenges. Our new technique uncovered differential microvascular response to gas stimuli in different organs: for example, mild hypercapnia vasodilates the kidney cortex but constricts muscle vasculature. Finally, we present a gas challenge protocol that produces a consistent vasoactive response and can be used to assess vasomodulatory capacity. Our imaging approach to monitor real-time vasomodulation may be extended to other imaging modalities and is valuable for investigating diseases where microvascular health is compromised.

Dissociation of local and global skeletal muscle oxygen transport metrics in type 2 diabetes

AIMS

Exercise capacity is impaired in type 2 diabetes, and this impairment predicts excess morbidity and mortality. This defect appears to involve excess skeletal muscle deoxygenation, but the underlying mechanisms remain unclear. We hypothesized that reduced blood flow, reduced local recruitment of blood volume/hematocrit, or both contribute to excess skeletal muscle deoxygenation in type 2 diabetes.

METHODS

In patients with (n=23) and without (n=18) type 2 diabetes, we recorded maximal reactive hyperemic leg blood flow, peak oxygen utilization during cycling ergometer exercise (VO_2peak), and near-infrared spectroscopy-derived measures of exercise-induced changes in skeletal muscle oxygenation and blood volume/hematocrit.

RESULTS

We observed a significant increase (p<0.05) in skeletal muscle deoxygenation in type 2 diabetes despite similar blood flow and recruitment of local blood volume/hematocrit. Within the control group skeletal muscle deoxygenation, local recruitment of microvascular blood volume/hematocrit, blood flow, and VO_2peak are all mutually correlated. None of these correlations were preserved in type 2 diabetes.

CONCLUSIONS

These results suggest that in type 2 diabetes 1) skeletal muscle oxygenation is impaired, 2) this impairment may occur independently of bulk blood flow or local recruitment of blood volume/hematocrit, and 3) local and global metrics of oxygen transport are dissociated.

Understanding Age-Related Changes in Skeletal Muscle Metabolism: Differences Between Females and Males

Skeletal muscle is the largest metabolic organ system in the human body. As such, metabolic dysfunction occurring in skeletal muscle impacts whole-body nutrient homeostasis. Macronutrient metabolism changes within the skeletal muscle with aging, and these changes are associated in part with age-related skeletal muscle remodeling. Moreover, age-related changes in skeletal muscle metabolism are affected differentially between males and females and are likely driven by changes in sex hormones. Intrinsic and extrinsic factors impact observed age-related changes and sex-related differences in skeletal muscle metabolism. Despite some support for sex-specific differences in skeletal muscle metabolism with aging, more research is necessary to identify underlying differences in mechanisms. Understanding sex-specific aging skeletal muscle will assist with the development of therapies to attenuate adverse metabolic and functional outcomes.

Changes in microvascular density differentiate metabolic health outcomes in monkeys with prior radiation exposure and subsequent skeletal muscle ECM remodeling

Radiation exposure accelerates the onset of age-related diseases such as diabetes, cardiovascular disease, and neoplasia and, thus, lends insight into in vivo mechanisms common to these disorders. Fibrosis and extracellular matrix (ECM) remodeling, which occur with aging and overnutrition and following irradiation, are risk factors for development of type 2 diabetes mellitus. We previously demonstrated an increased incidence of skeletal muscle insulin resistance and type 2 diabetes mellitus in monkeys that had been exposed to whole body irradiation 5-9 yr prior. We hypothesized that irradiation-induced fibrosis alters muscle architecture, predisposing irradiated animals to insulin resistance and overt diabetes. Rhesus macaques (Macaca mulatta, n = 7-8/group) grouped as nonirradiated age-matched controls (Non-Rad-CTL), irradiated nondiabetic monkeys (Rad-CTL), and irradiated monkeys that subsequently developed diabetes (Rad-DM) were compared. Prior radiation exposure resulted in persistent skeletal muscle ECM changes, including a relative overabundance of collagen IV and a trend toward increased transforming growth factor-β1. Preservation of microvascular markers differentiated the irradiated diabetic and nondiabetic groups. Microvascular density and plasma nitrate and heat shock protein 90 levels were lower in Rad-DM than Rad-CTL. These results are consistent with a protective effect of abundant microvasculature in maintaining glycemic control within radiation-induced fibrotic muscle.

Muscle fibre capillarization is a critical factor in muscle fibre hypertrophy during resistance exercise training in older men

BACKGROUND

Adequate muscle fibre perfusion is critical for the maintenance of muscle mass; it is essential in the rapid delivery of oxygen, nutrients and growth factors to the muscle, stimulating muscle fibre growth. Muscle fibre capillarization is known to decrease substantially with advancing age. However, whether (relative) low muscle fibre capillarization negatively impacts the muscle hypertrophic response following resistance exercise training in older adults is unknown.

METHODS

Twenty-two healthy older men (71 ± 1 years) performed 24 weeks of progressive resistance type exercise training. To assess the change in muscle fibre characteristics, percutaneous biopsies from the vastus lateralis muscle were taken before and following 12 and 24 weeks of the intervention programme. A comparison was made between participants who had a relatively low type II muscle fibre capillary-to-fibre perimeter exchange index (CFPE; LOW group) and high type II muscle fibre CFPE (HIGH group) at baseline. Type I and type II muscle fibre size, satellite cell, capillary content and distance between satellite cells to the nearest capillary were determined by immunohistochemistry.

RESULTS

Overall, type II muscle fibre size (from 5150 ± 234 to 6719 ± 446 µm² , P < 0.05) and satellite cell content (from 0.058 ± 0.006 to 0.090 ± 0.010 satellite cells per muscle fibre, P < 0.05) had increased significantly in response to 24 weeks of resistance exercise training. However, these improvements where mainly driven by differences in baseline type II muscle fibre capillarization, whereas muscle fibre size (from 5170 ± 390 to 7133 ± 314 µm² , P < 0.05) and satellite cell content (from 0.059 ± 0.009 to 0.102 ± 0.017 satellite cells per muscle fibre, P < 0.05) increased significantly in the HIGH group, no significant changes were observed in LOW group following exercise training. No significant changes in type I and type II muscle fibre capillarization were observed in response to 12 and 24 weeks of resistance exercise training in both the LOW and HIGH group.

CONCLUSIONS

Type II muscle fibre capillarization at baseline may be a critical factor for allowing muscle fibre hypertrophy to occur during prolonged resistance exercise training in older men.

Effects of exercise-induced apelin levels on skeletal muscle and their capillarization in type 2 diabetic rats

INTRODUCTION

Exercise-induced apelin as a myokine is believed to play a role in the improvement of type 2 diabetes mellitus (T2DM) and capillarization. In this study, we evaluated the association between exercise-induced apelin and muscle capillarization.

METHODS

Zucker rats underwent a treadmill exercise program. Body composition, muscle strength, muscle size, muscle capillarization, and insulin resistance (homeostatic model assessment [HOMA-IR]) were measured. Apelin levels of skeletal muscle and plasma were then analyzed.

RESULTS

Exercise improved body composition (P < 0.05), HOMA-IR (P < 0.05), and grip strength (P < 0.001). In the soleus, the fiber size of T2DM was decreased (P < 0.001), but it increased in fiber size and capillarization after exercise (P < 0.001) occurred. We identified an increase in plasma apelin (P < 0.05) and a decrease in soleus apelin (P < 0.01), as well as an association between soleus apelin and angiogenesis (P < 0.01).

DISCUSSION

A role for exercise-induced apelin in improving metabolism indicates the possibility of a new drug target for the treatment of metabolic diseases and repairing skeletal muscle damage. Muscle Nerve 56: 1155-1163, 2017.

Age-related differences in skeletal muscle microvascular response to exercise as detected by contrast-enhanced ultrasound (CEUS)

Background

Aging involves reductions in exercise total limb blood flow and exercise capacity. We hypothesized that this may involve early age-related impairments of skeletal muscle microvascular responsiveness as previously reported for insulin but not for exercise stimuli in humans.

Methods

Using an isometric exercise model, we studied the effect of age on contrast-enhanced ultrasound (CEUS) parameters, i.e. microvascular blood volume (MBV), flow velocity (MFV) and blood flow (MBF) calculated from replenishment of Sonovue contrast-agent microbubbles after their destruction. CEUS was applied to the vastus lateralis (VLat) and intermedius (VInt) muscle in 15 middle-aged (MA, 43.6±1.5 years) and 11 young (YG, 24.1±0.6 years) healthy males before, during, and after 2 min of isometric knee extension at 15% of peak torque (PT). In addition, total leg blood flow as recorded by femoral artery Doppler-flow. Moreover, fiber-type-specific and overall capillarisation as well as fiber composition were additionally assessed in Vlat biopsies obtained from CEUS site. MA and YG had similar quadriceps muscle MRT-volume or PT and maximal oxygen uptake as well as a normal cardiovascular risk factors and intima-media-thickness.

Results

During isometric exercise MA compared to YG reached significantly lower levels in MFV (0.123±0.016 vs. 0.208±0.036 a.u.) and MBF (0.007±0.001 vs. 0.012±0.002 a.u.). In the VInt the (post-occlusive hyperemia) post-exercise peaks in MBV and MBF were significantly lower in MA vs. YG. Capillary density, capillary fiber contacts and femoral artery Doppler were similar between MA and YG.

Conclusions

In the absence of significant age-related reductions in capillarisation, total leg blood flow or muscle mass, healthy middle-aged males reveal impaired skeletal muscle microcirculatory responses to isometric exercise. Whether this limits isometric muscle performance remains to be assessed.

Muscle Characteristics and Substrate Energetics in Lifelong Endurance Athletes

PURPOSE

The goal of this study was to explore the effect of lifelong aerobic exercise (i.e., chronic training) on skeletal muscle substrate stores (intramyocellular triglyceride [IMTG] and glycogen), skeletal muscle phenotypes, and oxidative capacity (ox), in older endurance-trained master athletes (OA) compared with noncompetitive recreational younger (YA) athletes matched by frequency and mode of training.

METHODS

Thirteen OA (64.8 ± 4.9 yr) exercising 5 times per week or more were compared with 14 YA (27.8 ± 4.9 yr) males and females. IMTG, glycogen, fiber types, succinate dehydrogenase, and capillarization were measured by immunohistochemistry in vastus lateralis biopsies. Fat-ox and carbohydrate (CHO)-ox were measured by indirect calorimetry before and after an insulin clamp and during a cycle ergometer graded maximal test.

RESULTS

V˙O2peak was lower in OA than YA. The OA had greater IMTG in all fiber types and lower glycogen stores than YA. This was reflected in greater proportion of type I and less type II fibers in OA. Type I fibers were similar in size, whereas type II fibers were smaller in OA compared with YA. Both groups had similar succinate dehydrogenase content. Numbers of capillaries per fiber were reduced in OA but with a higher number of capillaries per area. Metabolic flexibility and insulin sensitivity were similar in both groups. Exercise metabolic efficiency was higher in OA. At moderate exercise intensities, carbohydrate-ox was lower in OA but with similar Fat-ox.

CONCLUSIONS

Lifelong exercise is associated with higher IMTG content in all muscle fibers and higher metabolic efficiency during exercise that are not explained by differences in muscle fibers types and other muscle characteristics when comparing older with younger athletes matched by exercise mode and frequency.

Skeletal muscle satellite cells are located at a closer proximity to capillaries in healthy young compared with older men

BACKGROUND

Skeletal muscle satellite cells (SC) are instrumental in maintenance of muscle fibres, the adaptive responses to exercise, and there is an age-related decline in SC. A spatial relationship exists between SC and muscle fibre capillaries. In the present study, we aimed to investigate whether chronologic age has an impact on the spatial relationship between SC and muscle fibre capillaries. Secondly, we determined whether this spatial relationship changes in response to a single session of resistance exercise.

METHODS

Muscle biopsies were obtained from the vastus lateralis of previously untrained young men (YM, 24 ± 3 years; n = 23) and older men (OM, 67 ± 4 years; n = 22) at rest. A subset of YM (n = 9) performed a single bout of resistance exercise, where additional muscle biopsies taken at 24 and 72 h post-exercise recovery. Skeletal muscle fibre capillarization, SC content, and activation status were assessed using immunofluorescent microscopy of muscle cross sections.

RESULTS

Type II muscle fibre SC and capillary content was significantly lower in the YM compared with OM (P < 0.05). Furthermore, type II muscle fibre SC were located at a greater distance from the nearest capillary in OM compared with YM (21.6 ± 1.3 vs. 17.0 ± 0.8 µm, respectively; P < 0.05). In response to a single bout of exercise, we observed a significant increase in SC number and activation status (P < 0.05). In addition, activated vs. quiescent SC were situated closer (P < 0.05) to capillaries.

CONCLUSIONS

We demonstrate that there is a greater distance between capillaries and type II fibre-associated SC in OM as compared with YM. Furthermore, quiescent SC are located significantly further away from capillaries than active SC after single bout of exercise. Our data have implications for how muscle adapts to exercise and how aging may affect such adaptations.

Altered muscle satellite cell activation following 16 wk of resistance training in young men

Skeletal muscle satellite cells (SC) play an important role in muscle adaptation. In untrained individuals, SC content and activation status have been observed to increase in response to a single bout of exercise. Muscle fiber characteristics change considerably when resistance exercise is performed chronically, but whether training status affects the activity of SC in response to a single bout of exercise remains unknown. We examined the changes in SC content and activation status following a single bout of resistance exercise, before and following a 16-wk progressive resistance training (RT) program in 14 young (25 ± 3 yr) men. Before and after RT, percutaneous biopsies from the vastus lateralis muscle were taken before a single bout of resistance exercise and after 24 and 72 h of postexercise recovery. Muscle fiber size, capillarization, and SC response were determined by immunohistochemistry. Following RT, there was a greater activation of SC after 24 h in response to a single bout of resistance exercise (Pre, 1.4 ± 0.3; 24 h, 3.1 ± 0.3 Pax7⁺/MyoD⁺ cells per 100 fibers) compared with before RT (Pre, 1.4 ± 0.3; 24 h, 2.2 ± 0.3 Pax7⁺/MyoD⁺ cells per 100 fibers, P < 0.05); no difference was observed 72 h postexercise. Following 16 wk of RT, MyoD mRNA expression increased from basal to 24 h after the single bout of exercise (P < 0.05); this change was not observed before training. Individual capillary-to-fiber ratio (C/Fi) increased in both type I (1.8 ± 0.3 to 2.0 ± 0.3 C/Fi, P < 0.05) and type II (1.7 ± 0.3 to 2.2 ± 0.3 C/Fi, P < 0.05) fibers in response to RT. After RT, enhanced activation of SC in response to resistance exercise is accompanied by increases in muscle fiber capillarization.

Skeletal Muscle Regeneration, Repair and Remodelling in Aging: The Importance of Muscle Stem Cells and Vascularization

Sarcopenia is the age-related loss of skeletal muscle mass and strength. Ultimately, sarcopenia results in the loss of independence, which imposes a large financial burden on healthcare systems worldwide. A critical facet of sarcopenia is the diminished ability for aged muscle to regenerate, repair and remodel. Over the years, research has focused on elucidating underlying mechanisms of sarcopenia and the impaired ability of muscle to respond to stimuli with aging. Muscle-specific stem cells, termed satellite cells (SC), play an important role in maintaining muscle health throughout the lifespan. It is well established that SC are essential in skeletal muscle regeneration, and it has been hypothesized that a reduction and/or dysregulation of the SC pool, may contribute to accelerated loss of skeletal muscle mass that is observed with advancing age. The preservation of skeletal muscle tissue and its ability to respond to stimuli may be impacted by reduced SC content and impaired function observed with aging. Aging is also associated with a reduction in capillarization of skeletal muscle. We have recently demonstrated that the distance between type II fibre-associated SC and capillaries is greater in older compared to younger adults. The greater distance between SC and capillaries in older adults may contribute to the dysregulation in SC activation ultimately impairing muscle's ability to remodel and, in extreme circumstances, regenerate. This viewpoint will highlight the importance of optimal SC activation in addition to skeletal muscle capillarization to maximize the regenerative potential of skeletal muscle in older adults.

Exercise, ageing and the lung

This review provides a pulmonary-focused description of the age-associated changes in the integrative physiology of exercise, including how declining lung function plays a role in promoting multimorbidity in the elderly through limitation of physical function. We outline the ageing of physiological systems supporting endurance activity: 1) coupling of muscle metabolism to mechanical power output; 2) gas transport between muscle capillary and mitochondria; 3) matching of muscle blood flow to its requirement; 4) oxygen and carbon dioxide carrying capacity of the blood; 5) cardiac output; 6) pulmonary vascular function; 7) pulmonary oxygen transport; 8) control of ventilation; and 9) pulmonary mechanics and respiratory muscle function. Deterioration in function occurs in many of these systems in healthy ageing. Between the ages of 25 and 80 years pulmonary function and aerobic capacity each decline by ∼40%. While the predominant factor limiting exercise in the elderly likely resides within the function of the muscles of ambulation, muscle function is (at least partially) rescued by exercise training. The age-associated decline in pulmonary function, however, is not recovered by training. Thus, loss in pulmonary function may lead to ventilatory limitation in exercise in the active elderly, limiting the ability to accrue the health benefits of physical activity into senescence.

Morphometry of skeletal muscle capillaries: the relationship between capillary ultrastructure and ageing in humans

AIM

To determine whether the ultrastructure of the capillary system in human skeletal muscle changes during advancing senescence, we evaluated the compartmental and subcompartmental organization of capillaries from vastus lateralis muscle (VL) biopsies of 41 non-diseased persons aged 23-75 years.

METHODS

From each VL biopsy, 38-40 randomly selected capillaries were assessed by transmission electron microscopy and subsequent morphometry with a newly established tablet-based image analysis technique.

RESULTS

Quantification of the compartmental organization revealed most indicators of the capillary ultrastructure to be only non-significantly altered (P > 0.05) over age. However, the peri-capillary basement membrane (BM) was thicker in the older participants than in the younger ones (P ≤ 0.05). Regression analysis revealed a bipartite relationship between the two parameters: a homogenous slight increase in BM thickness up to the age of approximately 50 years was followed by a second phase with more scattered BM thickness values. In 44.5% of the capillary profiles, projections/filopodia of the pericytes (PCs) traversed the BM and invaded endothelial cells (ECs) visible as PC pegs in pale cytoplasm holes (EC sockets). Strikingly, PC pegs were often in proximity to the EC nucleus. In PC profiles, sockets were likewise detected in 14.2% of the capillaries. Within these PC sockets, cellular profiles were frequently seen, which could be assigned to EC filopodia, internal PC curling or PC-PC interactions. Quantification of the occurrence of peg-socket junctions revealed the proportions of empty EC sockets and empty PC sockets to increase (P ≤ 0.05) during ageing.

CONCLUSION

Our investigation demonstrates advancing senescence to be associated with increase in BM thickness and loss of EC and PC filopodia length in skeletal muscle capillaries.

One Week of Bed Rest Leads to Substantial Muscle Atrophy and Induces Whole-Body Insulin Resistance in the Absence of Skeletal Muscle Lipid Accumulation

Short (<10 days) periods of muscle disuse, often necessary for recovery from illness or injury, lead to various negative health consequences. The current study investigated mechanisms underlying disuse-induced insulin resistance, taking into account muscle atrophy. Ten healthy, young males (age: 23 ± 1 years; BMI: 23.0 ± 0.9 kg · m(-2)) were subjected to 1 week of strict bed rest. Prior to and after bed rest, lean body mass (dual-energy X-ray absorptiometry) and quadriceps cross-sectional area (CSA; computed tomography) were assessed, and peak oxygen uptake (VO2peak) and leg strength were determined. Whole-body insulin sensitivity was measured using a hyperinsulinemic-euglycemic clamp. Additionally, muscle biopsies were collected to assess muscle lipid (fraction) content and various markers of mitochondrial and vascular content. Bed rest resulted in 1.4 ± 0.2 kg lean tissue loss and a 3.2 ± 0.9% decline in quadriceps CSA (both P < 0.01). VO2peak and one-repetition maximum declined by 6.4 ± 2.3 (P < 0.05) and 6.9 ± 1.4% (P < 0.01), respectively. Bed rest induced a 29 ± 5% decrease in whole-body insulin sensitivity (P < 0.01). This was accompanied by a decline in muscle oxidative capacity, without alterations in skeletal muscle lipid content or saturation level, markers of oxidative stress, or capillary density. In conclusion, 1 week of bed rest substantially reduces skeletal muscle mass and lowers whole-body insulin sensitivity, without affecting mechanisms implicated in high-fat diet-induced insulin resistance.

Sodium nitrate co-ingestion with protein does not augment postprandial muscle protein synthesis rates in older, type 2 diabetes patients

The age-related anabolic resistance to protein ingestion is suggested to be associated with impairments in insulin-mediated capillary recruitment and postprandial muscle tissue perfusion. The present study investigated whether dietary nitrate co-ingestion with protein improves muscle protein synthesis in older, type 2 diabetes patients. Twenty-four men with type 2 diabetes (72 ± 1 yr, 26.7 ± 1.4 m/kg(2) body mass index, 7.3 ± 0.4% HbA1C) received a primed continuous infusion of l-[ring-(2)H5]phenylalanine and l-[1-(13)C]leucine and ingested 20 g of intrinsically l-[1-(13)C]phenylalanine- and l-[1-(13)C]leucine-labeled protein with (PRONO3) or without (PRO) sodium nitrate (0.15 mmol/kg). Blood and muscle samples were collected to assess protein digestion and absorption kinetics and postprandial muscle protein synthesis rates. Upon protein ingestion, exogenous phenylalanine appearance rates increased in both groups (P < 0.001), resulting in 55 ± 2% and 53 ± 2% of dietary protein-derived amino acids becoming available in the circulation over the 5h postprandial period in the PRO and PRONO3 groups, respectively. Postprandial myofibrillar protein synthesis rates based on l-[ring-(2)H5]phenylalanine did not differ between groups (0.025 ± 0.004 and 0.021 ± 0.007%/h over 0-2 h and 0.032 ± 0.004 and 0.030 ± 0.003%/h over 2-5 h in PRO and PRONO3, respectively, P = 0.7). No differences in incorporation of dietary protein-derived l-[1-(13)C]phenylalanine into de novo myofibrillar protein were observed at 5 h (0.016 ± 0.002 and 0.014 ± 0.002 mole percent excess in PRO and PRONO3, respectively, P = 0.8). Dietary nitrate co-ingestion with protein does not modulate protein digestion and absorption kinetics, nor does it further increase postprandial muscle protein synthesis rates or the incorporation of dietary protein-derived amino acids into de novo myofibrillar protein in older, type 2 diabetes patients.

Impaired metabolic profile is a predictor of capillary rarefaction in a population of hypertensive and normotensive individuals

Capillary rarefaction is typically encountered in essential hypertension, yet identification of factors interfering with this phenomenon remains substantially underinvestigated. We examined whether components of metabolic profile (dyslipidemia, insulin resistance), inflammatory (high-sensitivity C-reactive protein, high-sensitivity C-reactive protein), and angiogenic (vascular endothelial growth factor) factors are implicated in this phenomenon in a population of newly diagnosed, never-treated hypertensive patients and normotensive controls. Nailfold capillary density was estimated with nailfold capillaroscopy using specifically designed software. A total of 159 individuals, 93 hypertensives, and 66 normotensives were included. Nailfold capillary density was lower among hypertensives compared to normotensives (146.4 ± 31.0 vs. 155.4 ± 26.9, respectively; P = .047). In the total population, capillary density significantly correlated with high-density lipoprotein (HDL) (r = 0.232; P = .003), HDL/low-density lipoprotein ratio (r = 0.175; P = .025), age (r = 0.236; P = .003), but neither with vascular endothelial growth factor or high-sensitivity C-reactive protein. An inverse association was found with body mass index (r = -0.174; P = .029), insulin levels (r = -0.200; P = .018), and homeostasis model assessment-insulin resistance (r = -0.223; P = .009). In the separate analysis for the hypertensive population, sex (P = .014) and homeostasis model assessment-insulin resistance (P = .011) were identified as significant predictors of capillary rarefaction after adjustment for other factors. On the contrary, only HDL levels (P = .036) predicted capillary density in the multiple regression model for the normotensive population. Different aspects of impaired metabolic profile, that is, insulin resistance and low HDL levels, but not angiogenic or inflammatory markers, appear to be independently associated with capillary rarefaction in hypertensive and normotensive individuals.

Skeletal Muscle Vascular Function: A Counterbalance of Insulin Action

Insulin is a vasoactive hormone that regulates vascular homeostasis by maintaining balance of endothelial-derived NO and ET-1. Although there is general agreement that insulin resistance and the associated hyperinsulinemia disturb this balance, the vascular consequences for hyperinsulinemia in isolation from insulin resistance are still unclear. Presently, there is no simple answer for this question, especially in a background of mixed reports examining the effects of experimental hyperinsulinemia on endothelial-mediated vasodilation. Understanding the mechanisms by which hyperinsulinemia induces vascular dysfunction is essential in advancing treatment and prevention of insulin resistance-related vascular complications. Thus, we review literature addressing the effects of hyperinsulinemia on vascular function. Furthermore, we give special attention to the vasoregulatory effects of hyperinsulinemia on skeletal muscle, the largest insulin-dependent organ in the body. This review also characterizes the differential vascular effects of hyperinsulinemia on large conduit vessels versus small resistance microvessels and the effects of metabolic variables in an effort to unravel potential sources of discrepancies in the literature. At the cellular level, we provide an overview of insulin signaling events governing vascular tone. Finally, we hypothesize a role for hyperinsulinemia and insulin resistance in the development of CVD.

Insulin resistance and sarcopenia: mechanistic links between common co-morbidities

Insulin resistance (IR) in skeletal muscle is a key defect mediating the link between obesity and type 2 diabetes, a disease that typically affects people in later life. Sarcopenia (age-related loss of muscle mass and quality) is a risk factor for a number of frailty-related conditions that occur in the elderly. In addition, a syndrome of 'sarcopenic obesity' (SO) is now increasingly recognised, which is common in older people and is applied to individuals that simultaneously show obesity, IR and sarcopenia. Such individuals are at an increased risk of adverse health events compared with those who are obese or sarcopenic alone. However, there are no licenced treatments for sarcopenia or SO, the syndrome is poorly defined clinically and the mechanisms that might explain a common aetiology are not yet well characterised. In this review, we detail the nature and extent of the clinical syndrome, highlight some of the key physiological processes that are dysregulated and discuss some candidate molecular pathways that could be implicated in both metabolic and anabolic defects in skeletal muscle, with an eye towards future therapeutic options. In particular, the potential roles of Akt/mammalian target of rapamycin signalling, AMP-activated protein kinase, myostatin, urocortins and vitamin D are discussed.

C5b-9 deposits on endomysial capillaries in non-dermatomyositis cases

Deposits of the terminal-membrane-attack-complex (MAC) C5b-9 on perfascicular endomysial capillaries are generally regarded as diagnostic hallmark of dermatomyositis (DM). Although the pathophysiology is not clear, C5b-9 deposits on capillaries seem to be associated with microinfarctions and vascular damage. Here, we report on a series of 19 patients presenting with C5b-9 accumulation on endomysial capillaries in the absence of features for DM. To decipher differences in the capillary C5b-9 accumulation pattern between DM and non-DM cases, we assessed the extent of endomysial capillary C5b-9 deposits related to capillary density and extent of myofiber necrosis by immunohistochemistry in 12 DM and 8 control patients. We found similar numbers of C5b-9-positive myofibers in both DM and non-DM C5b-9(+) cases. The distribution pattern differed as DM cases showed significantly more perifascicular capillary C5b-9 deposits as compared to non-DM cases, which presented stronger endomysial capillary C5b-9 deposits in a diffuse pattern. While total capillary density was not differing, DM patients displayed significantly more C5b-9(+) necrotic fibers as compared to non-DM C5b-9(+). In summary, endomysial capillary C5b-9 deposits are present in a variety of non-DM cases, however with differing distribution pattern. In conclusion, capillary C5b-9(+) deposits should be assessed critically, taking into consideration the distribution pattern.

Sarcopenia Is Associated With Lower Skeletal Muscle Capillarization and Exercise Capacity in Older Adults

BACKGROUND

Skeletal muscle capillary rarefaction limits the transcapillary transport of nutrients and oxygen to muscle and may contribute to sarcopenia and functional impairment in older adults. We tested the hypothesis that skeletal muscle capillarization and exercise capacity (VO2max) are lower in sarcopenic than in nonsarcopenic older adults and that the degree of sarcopenia is related to lower skeletal muscle capillarization.

METHODS

Body composition, VO2max, and vastus lateralis capillarization were determined in 76 middle-aged and older men and women (age = 61±1 years, body mass index [BMI] = 30.7±0.5kg/m(2) [mean ± SEM]). Participants were classified as sarcopenic if appendicular lean mass divided by BMI (ALMBMI) was less than 0.789 for men or less than 0.512 for women.

RESULTS

Sarcopenic subjects (ALMBMI = 0.65±0.04, n = 16) had 20% lower capillary-to-fiber ratio, as well as 13% and 15% lower VO2max expressed as mL/kg/min or L/min, respectively, compared with sex-, race-, and age-matched participants without sarcopenia (ALMBMI = 0.81±0.05, n = 16; p < .05). In all 76 subjects, ALMBMI, thigh muscle cross-sectional area, and VO2max correlated directly with capillarization (r = .30-.37, p ≤ .05), after accounting for age, sex, and race.

CONCLUSIONS

These findings suggest that low skeletal muscle capillarization is one factor that may contribute to sarcopenia and reduced exercise capacity in older adults by limiting diffusion of substrates, oxygen, hormones, and nutrients. Strategies to prevent the aging-related decline in skeletal muscle capillarization may help to prevent or slow the progression of sarcopenia and its associated functional declines in generally healthy older adults.

Lower skeletal muscle capillarization in hypertensive elderly men

Aging strongly affects the skeletal muscle and is associated with microvascular dysfunctions. Age is also a primary risk factor for the metabolic syndrome, which is a cluster of metabolic and cardiovascular symptoms. Among the metabolic syndrome components, hypertension is the most prevalent in elderly subjects and has a central role in vascular alterations. Despite critical clinical outcomes, the effects of hypertension and metabolic syndrome on skeletal muscle capillarization have poorly been investigated during aging. In the present study, muscle biopsies from normotensive young (YO) and elderly (ELc) men, and elderly men with hypertension (EL-HT) or metabolic syndrome (EL-MS) were assessed for the number of capillaries around a fiber (CAF), capillary-to-fiber perimeter exchange (CFPE), length of contact to perimeter of fiber ratio (LC/PF), capillary tortuosity, and for extracellular matrix (ECM) embedding capillaries. As capillarization and muscle mitochondrial oxidative capacity may be associated, we also investigated cytochrome c oxidase (COX) content. Our findings indicate that capillarization and COX did not change between normotensive adult and old individuals. They further reveal that hypertension in elderly men is associated with reduced CAF (ELc: 5.2 ± 0.4, EL-HT: 4.1 ± 0.2, P<0.02 for type I fibers; ELc: 4.1 ± 0.2, EL-HT: 3.1 ± 0.3, P<0.03 for type IIA fibers), CFPE (ELc: 7.9 ± 0.7, EL-HT: 6.4 ± 0.4 capillaries/1000 μm, P<0.03 for type I fibers; ELc: 6.5 ± 0.4, EL-HT: 5.2 ± 0.4 capillaries/1000 μm, P<0.03 for type IIA fibers), LC/PF (ELc: 23.3 ± 1.2, EL-HT: 17.8 ± 0.6%, P<0.01 for type I fibers; ELc: 19.8 ± 1.1, EL-HT: 15.6 ± 0.8%, P<0.01 for type IIA fibers) and capillary tortuosity, and with ECM endomysium fibrosis. Capillary rarefaction also correlated with lower COX content in the old hypertensive muscle. No further modification occurred with metabolic syndrome in elderly men. Collectively, our results suggest that hypertension plays a central role in muscle capillarization during aging, and that the other components of metabolic syndrome do not make major additional changes in the aged skeletal muscle capillary network.

Post-Prandial Protein Handling: You Are What You Just Ate

Background

Protein turnover in skeletal muscle tissue is highly responsive to nutrient intake in healthy adults.

Objective

To provide a comprehensive overview of post-prandial protein handling, ranging from dietary protein digestion and amino acid absorption, the uptake of dietary protein derived amino acids over the leg, the post-prandial stimulation of muscle protein synthesis rates, to the incorporation of dietary protein derived amino acids in de novo muscle protein.

Design

12 healthy young males ingested 20 g intrinsically [1-¹³C]-phenylalanine labeled protein. In addition, primed continuous L-[ring-²H₅]-phenylalanine, L-[ring-²H₂]-tyrosine, and L-[1-¹³C]-leucine infusions were applied, with frequent collection of arterial and venous blood samples, and muscle biopsies throughout a 5 h post-prandial period. Dietary protein digestion, amino acid absorption, splanchnic amino acid extraction, amino acid uptake over the leg, and subsequent muscle protein synthesis were measured within a single in vivo human experiment.

Results

55.3±2.7% of the protein-derived phenylalanine was released in the circulation during the 5 h post-prandial period. The post-prandial rise in plasma essential amino acid availability improved leg muscle protein balance (from -291±72 to 103±66 μM·min^-1·100 mL leg volume^-1; P<0.001). Muscle protein synthesis rates increased significantly following protein ingestion (0.029±0.002 vs 0.044±0.004%·h^-1 based upon the muscle protein bound L-[ring-²H₅]-phenylalanine enrichments (P<0.01)), with substantial incorporation of dietary protein derived L-[1-¹³C]-phenylalanine into de novo muscle protein (from 0 to 0.0201±0.0025 MPE).

Conclusion

Ingestion of a single meal-like amount of protein allows ~55% of the protein derived amino acids to become available in the circulation, thereby improving whole-body and leg protein balance. About 20% of the dietary protein derived amino acids released in the circulation are taken up in skeletal muscle tissue following protein ingestion, thereby stimulating muscle protein synthesis rates and providing precursors for de novo muscle protein synthesis.

Trial Registration

trialregister.nl 3638

Postprandial Protein Handling Is Not Impaired in Type 2 Diabetes Patients When Compared With Normoglycemic Controls

CONTEXT

The progressive loss of muscle mass with aging is accelerated in type 2 diabetes patients. It has been suggested that this is attributed to a blunted muscle protein synthetic response to food intake.

OBJECTIVE

The objective of the study was to test the hypothesis that the muscle protein synthetic response to protein ingestion is impaired in older type 2 diabetes patients when compared with healthy, normoglycemic controls.

DESIGN

A clinical intervention study with two parallel groups was conducted between August 2011 and July 2012.

SETTING

The study was conducted at the research unit of Maastricht University, The Netherlands. Intervention, Participants, and Main Outcome Measures: Eleven older type 2 diabetes males [diabetes; age 71 ± 1 y, body mass index (BMI) 26.2 ± 0.5 kg/m(2)] and 12 age- and BMI-matched normoglycemic controls (control; age 74 ± 1 y, BMI 24.8 ± 1.1 kg/m(2)) participated in an experiment in which they ingested 20 g intrinsically L-[1-(13)C]phenylalanine-labeled protein. Continuous iv L-[ring-(2)H5]phenylalanine infusion was applied, and blood and muscle samples were obtained to assess amino acid kinetics and muscle protein synthesis rates in the postabsorptive and postprandial state.

RESULTS

Plasma insulin concentrations increased after protein ingestion in both groups, with a greater rise in the diabetes group. Postabsorptive and postprandial muscle protein synthesis rates did not differ between groups and averaged 0.029 ± 0.003 vs 0.029 ± 0.003%/h(1) and 0.031 ± 0.002 vs 0.033 ± 0.002%/h(1) in the diabetes versus control group, respectively. Postprandial L-[1-(13)C]phenylalanine incorporation into muscle protein did not differ between groups (0.018 ± 0.001 vs 0.019 ± 0.002 mole percent excess, respectively).

CONCLUSIONS

Postabsorptive muscle protein synthesis and postprandial protein handling is not impaired in older individuals with type 2 diabetes when compared with age-matched, normoglycemic controls.

Pericyte NF-κB activation enhances endothelial cell proliferation and proangiogenic cytokine secretion in vitro

Pericytes are skeletal muscle resident, multipotent stem cells that are localized to the microvasculature. In vivo, studies have shown that they respond to damage through activation of nuclear-factor kappa-B (NF-κB), but the downstream effects of NF-κB activation on endothelial cell proliferation and cell-cell signaling during repair remain unknown. The purpose of this study was to examine pericyte NF-κB activation in a model of skeletal muscle damage; and use genetic manipulation to study the effects of changes in pericyte NF-κB activation on endothelial cell proliferation and cytokine secretion. We utilized scratch injury to C2C12 cells in coculture with human primary pericytes to assess NF-κB activation and monocyte chemoattractant protein-1 (MCP-1) secretion from pericytes and C2C12 cells. We also cocultured endothelial cells with pericytes that expressed genetically altered NF-κB activation levels, and then quantified endothelial cell proliferation and screened the conditioned media for secreted cytokines. Pericytes trended toward greater NF-κB activation in injured compared to control cocultures (P = 0.085) and in comparison to C2C12 cells (P = 0.079). Second, increased NF-κB activation in pericytes enhanced the proliferation of cocultured endothelial cells (1.3-fold, P = 0.002). Finally, we identified inflammatory signaling molecules, including MCP-1 and interleukin 8 (IL-8) that may mediate the crosstalk between pericytes and endothelial cells. The results of this study show that pericyte NF-κB activation may be an important mechanism in skeletal muscle repair with implications for the development of therapies for musculoskeletal and vascular diseases, including peripheral artery disease.

Characteristics and determinants of the sublingual microcirculation in populations of different ethnicity

No previous population study assessed sublingual capillary density (CD) or perfused boundary region (PBR). Lower PBR indicates greater glycocalyx width. In 252 Han and 220 She Chinese and 254 Flemish people (mean age, 51.1 years; 54.7% women), representing random population samples, we measured total and perfused CD and PBR in the sublingual capillary bed, using oblique profiled epi-illumination, and cardiovascular risk factors. In multivariable analyses, we modeled ethnicity as random effect. Significance level was α ≤ 0.05. Compared with Chinese, Flemish had lower total (577 versus 546 n°/mm(2)) and perfused (338 versus 320 n°/mm(2)) CD, but similar perfused-to-total CD ratio (mean, 0.59). Perfused-to-total CD ratio increased with age (effect size per 1-SD increase, +0.015 per year), body mass index (+0.008 per kg/m(2)), total cholesterol (+0.012 per mmol/L), and Framingham risk score (+0.018 per point) with no ethnic differences in these associations. For age and Framingham risk score, associations with perfused-to-total CD ratio were driven by positive relationships with perfused CD, whereas associations with total CD were nonsignificant. Chinese when compared with Flemish had higher hematocrit (43.0 versus 41.1%), PBR (2010 versus 1876 nm), and pulse rate (72.6 versus 63.3 bpm). PBR standardized for hematocrit, perfused CD, and pulse rate decreased with body mass index (-26.7 nm/kg/m2), mean arterial pressure (-30.6 nm/mm Hg), and diastolic pressure (-28.5 nm/mm Hg) with no ethnic differences in these associations. In conclusion, a higher cardiovascular risk profile is associated with functional recruitment of capillaries with preserved glycocalyx that protects the endothelial lining.

Gene therapy in peripheral artery disease

INTRODUCTION

Despite the remarkable progress of medicine and endovascular procedures for revascularization, patients with critical limb ischemia (CLI) remain at high risk for amputation and often have a low quality of life due to pain and ulcers in the ischemic leg. Thus, a novel strategy for generating new blood vessels in CLI patients without treatment options is vital. Pre-clinical studies and Phase I clinical trials using VEGF and fibroblast growth factor (FGF) demonstrated promising results; however, more rigorous Phase II and III clinical trials failed to demonstrate benefits for CLI patients. Recently, two multicenter, double-blind, placebo-controlled clinical trials in Japan (Phase III) and the USA (Phase II) showed the benefits of hepatocyte growth factor (HGF) gene therapy for CLI patients. Although the number of patients included in these trials was relatively small, these results imply a distinct beneficial function for HGF over other angiogenic growth factors in a clinical setting.

AREAS COVERED

In this review, data from Phase I-III clinical trials of gene therapy for patients with peripheral artery disease (PAD) are examined. In addition, the potential mechanisms behind the success or failure of clinical trials are discussed.

EXPERT OPINION

Compared with VEGF and FGF, HGF has a unique molecular effect on inflammation, fibrosis and cell senescence under pathological conditions. These features may explain the clinical benefits of HGF in PAD patients.

Capillary rarefaction: an early marker of microvascular disease in young hemodialysis patients

BACKGROUND

Pediatric patients with chronic kidney disease (CKD) are at increased risk of early cardiovascular disease and premature death. Abnormalities in microvascular structure and function may presage end-organ damage including vascular calcification and myocardial ischemia associated with disordered mineral metabolism. Early detection of microvascular rarefaction (reduced density of capillaries) may identify at-risk patients and prompt timely therapeutic interventions. Our objective was to study capillary rarefaction in pediatric hemodialysis (HD) patients and to determine possible associations with mineral metabolism and cardiac risk biomarkers.

METHODS

Capillary density (CD) was measured by nailfold capillaroscopy in 19 pediatric HD patients and 20 healthy controls. Demographic and biochemical markers were collected at entry and 6-month follow-up.

RESULTS

CD was significantly decreased in HD patients compared with controls with a deficit of 24 and 31% at baseline and subsequent follow-up. Maximal CD correlated significantly with intact parathyroid hormone (iPTH) (r = -0.45; P = 0.005), serum calcium (r = -0.38; P = 0.02) and 25(OH) vitamin D levels (r = +0.36; P = 0.03) in HD patients. Capillary functional measures were similar to controls. By multivariate analysis, the primary negative determinants of CD were African American race and hyperparathyroidism; whereas, glomerular disease had a positive influence on capillary rarefaction (R (2) = 64.2% variance; P = 0.001).

CONCLUSION

Pediatric HD patients demonstrate a 'structural deficit' in CD but show preserved 'functional integrity'. Capillary rarefaction, an early risk factor of incipient vascular calcification, was strongly associated with biomarkers of altered mineral metabolism. Further studies are warranted to determine the impact of optimizing blood pressure and metabolic control on changes in capillary rarefaction in young CKD patients.