Exercise hyperaemia: magnitude and aspects on regulation in humans

NASA SPRINT exercise program efficacy for vastus lateralis and soleus skeletal muscle health during 70 days of simulated microgravity

The efficacy of the NASA SPRINT exercise countermeasures program for quadriceps (vastus lateralis) and triceps surae (soleus) skeletal muscle health was investigated during 70 days of simulated microgravity. Individuals completed 6° head-down-tilt bedrest (BR, n = 9), bedrest with resistance and aerobic exercise (BRE, n = 9), or bedrest with resistance and aerobic exercise and low-dose testosterone (BRE + T, n = 8). All groups were periodically tested for muscle (n = 9 times) and aerobic (n = 4 times) power during bedrest. In BR, surprisingly, the typical bedrest-induced decrements in vastus lateralis myofiber size and power were either blunted (myosin heavy chain, MHC I) or eliminated (MHC IIa), along with no change (P > 0.05) in %MHC distribution and blunted quadriceps atrophy. In BRE, MHC I (vastus lateralis and soleus) and IIa (vastus lateralis) contractile performance was maintained (P > 0.05) or increased (P < 0.05). Vastus lateralis hybrid fiber percentage was reduced (P < 0.05) and energy metabolism enzymes and capillarization were generally maintained (P > 0.05), while not all of these positive responses were observed in the soleus. Exercise offsets 100% of quadriceps and approximately two-thirds of soleus whole muscle mass loss. Testosterone (BRE + T) did not provide any benefit over exercise alone for either muscle and for some myocellular parameters appeared detrimental. In summary, the periodic testing likely provided a partial exercise countermeasure for the quadriceps in the bedrest group, which is a novel finding given the extremely low exercise dose. The SPRINT exercise program appears to be viable for the quadriceps; however, refinement is needed to completely protect triceps surae myocellular and whole muscle health for astronauts on long-duration spaceflights.NEW & NOTEWORTHY This study provides unique exercise countermeasures development information for astronauts on long-duration spaceflights. The NASA SPRINT program was protective for quadriceps myocellular and whole muscle health, whereas the triceps surae (soleus) was only partially protected as has been shown with other programs. The bedrest control group data may provide beneficial information for overall exercise dose and targeting fast-twitch muscle fibers. Other unique approaches for the triceps surae are needed to supplement existing exercise programs.

Acute exercise affects dual-energy X-ray absorptiometry body composition estimates but not standardised ultrasound measurements of subcutaneous adipose tissue

Ultrasound has been demonstrated to be a highly accurate and reliable tool for measuring subcutaneous adipose tissue thickness and is robust against changes in hydration status or acute food or fluid intake. However, the effect of prior acute exercise is unexamined. This study examined the impact of an acute endurance exercise and resistance exercise session on standardised brightness-mode ultrasound measurements of subcutaneous adipose tissue thickness compared to skinfolds and dual-energy X-ray absorptiometry body composition estimates. In a randomised cross-over design, 30 active adults (24.2 ± 4.9 years) undertook physique assessment via standardised brightness-mode ultrasound, skinfolds and dual-energy X-ray absorptiometry before, immediately and 45 min after an acute endurance or resistance exercise session. The mean sum of eight subcutaneous adipose tissue thickness measured via standardised brightness-mode ultrasound increased (0.6 mm, p = 0.04) immediately postendurance exercise but was not meaningful when evaluated against the technical error of measurement of the investigator. A significant (p = 0.01) but not meaningful decrease in the sum of eight skinfolds occurred immediately (-1.1 ± 0.4 mm) and 45 min (-1.3 ± 0.4 mm) postresistance exercise. Comparatively, endurance exercise elicited a meaningful decrease of total mass (460 ± 30 g) and trunk lean mass (680 ± 90 g) dual-energy X-ray absorptiometry estimates. Findings from this study indicate standardised client presentation may be unnecessary when employing either standardised brightness-mode ultrasound or skinfolds for body composition assessment unlike dual-energy X-ray absorptiometry.

Short-Term L-Citrulline Supplementation Does Not Affect Inspiratory Muscle Oxygenation and Respiratory Performance in Older Adults

In sports nutrition, nitric oxide (NO^•) precursors such as L-citrulline are widely used to enhance NO^• bioavailability, which is considered an ergogenic aid. Our study aimed to examine the effect of short-term L-citrulline supplementation on respiratory muscles' performance, fatigue, and oxygenation in older adults. Fourteen healthy older males took 6 g of L-citrulline or a placebo for seven days in a double-blind crossover design. Pulmonary function via spirometry (i.e., forced expired volume in 1 s (FEV₁), forced vital capacity (FVC), and their ratio)), fractional exhaled nitric oxide (NO^•), maximal inspiratory pressure (MIP), rate of perceived exertion, and sternocleidomastoid muscle oxygenation (i.e., oxyhemoglobin (Δ[O₂Hb]) and de-oxyhemoglobin (Δ[HHb]), total hemoglobin concentration (Δ[tHb]), and tissue saturation index (TSI%)) were evaluated at baseline, after seven days of L-citrulline supplementation, and after incremental resistive breathing to task failure of the respiratory muscles. The exhaled NO^• value was only significantly increased after the supplementation (26% p < 0.001) in the L-citrulline condition. Pulmonary function, MIP, rate of perceived exertion, and sternocleidomastoid muscle oxygenation were not affected by the L-citrulline supplementation. In the present study, although short-term L-citrulline supplementation increased exhaled NO^•, no ergogenic aids were found on the examined parameters at rest and after resistive breathing to task failure in older adults.

Do Sports Compression Garments Alter Measures of Peripheral Blood Flow? A Systematic Review with Meta-Analysis

BACKGROUND

One of the proposed mechanisms underlying the benefits of sports compression garments may be alterations in peripheral blood flow.

OBJECTIVE

We aimed to determine if sports compression garments alter measures of peripheral blood flow at rest, as well as during, immediately after and in recovery from a physiological challenge (i.e. exercise or an orthostatic challenge).

METHODS

We conducted a systematic literature search of databases including Scopus, SPORTDiscus and PubMed/MEDLINE. The criteria for inclusion of studies were: (1) original papers in English and a peer-reviewed journal; (2) assessed effect of compression garments on a measure of peripheral blood flow at rest and/or before, during or after a physiological challenge; (3) participants were healthy and without cardiovascular or metabolic disorders; and (4) a study population including athletes and physically active or healthy participants. The PEDro scale was used to assess the methodological quality of the included studies. A random-effects meta-analysis model was used. Changes in blood flow were quantified by standardised mean difference (SMD) [± 95% confidence interval (CI)].

RESULTS

Of the 899 articles identified, 22 studies were included for the meta-analysis. The results indicated sports compression garments improve overall peripheral blood flow (SMD = 0.32, 95% CI 0.13, 0.51, p = 0.001), venous blood flow (SMD = 0.37, 95% CI 0.14, 0.60, p = 0.002) and arterial blood flow (SMD = 0.30, 95% CI 0.01, 0.59, p = 0.04). At rest, sports compression garments did not improve peripheral blood flow (SMD = 0.18, 95% CI - 0.02, 0.39, p = 0.08). However, subgroup analyses revealed sports compression garments enhance venous (SMD = 0.31 95% CI 0.02, 0.60, p = 0.03), but not arterial (SMD = 0.12, 95% CI - 0.16, 0.40, p = 0.16), blood flow. During a physiological challenge, peripheral blood flow was improved (SMD = 0.44, 95% CI 0.19, 0.69, p = 0.0007), with subgroup analyses revealing sports compression garments enhance venous (SMD = 0.48, 95% CI 0.11, 0.85, p = 0.01) and arterial blood flow (SMD = 0.44, 95% CI 0.03, 0.86, p = 0.04). At immediately after a physiological challenge, there were no changes in peripheral blood flow (SMD = - 0.04, 95% CI - 0.43, 0.34, p = 0.82) or subgroup analyses of venous (SMD = - 0.41, 95% CI - 1.32, 0.47, p = 0.35) and arterial (SMD = 0.12, 95% CI - 0.26, 0.51, p = 0.53) blood flow. In recovery, sports compression garments did not improve peripheral blood flow (SMD = 0.25, 95% CI - 0.45, 0.95, p = 0.49). The subgroup analyses showed enhanced venous (SMD = 0.67, 95% CI 0.17, 1.17, p = 0.009), but not arterial blood flow (SMD = 0.02, 95% CI - 1.06, 1.09, p = 0.98).

CONCLUSIONS

Use of sports compression garments enhances venous blood flow at rest, during and in recovery from, but not immediately after, a physiological challenge. Compression-induced changes in arterial blood flow were only evident during a physiological challenge.

Skeletal muscle abnormalities in heart failure with preserved ejection fraction

Almost half of all heart failure (HF) disease burden is due to HF with preserved ejection fraction (HFpEF). The primary symptom in patients with HFpEF, even when well compensated, is severe exercise intolerance and is associated with their reduced quality of life. Recently, studies showed that HFpEF patients have multiple skeletal muscle (SM) abnormalities, and these are associated with decreased exercise intolerance. The SM abnormalities are likely intrinsic to the HFpEF syndrome, not a secondary consequence of an epiphenomenon. These abnormalities are decreased muscle mass, reduced type I (oxidative) muscle fibers, and reduced type I-to-type II fiber ratio as well as a reduced capillary-to-fiber ratio, abnormal fat infiltration into the thigh SM, increased levels of atrophy genes and proteins, reduction in mitochondrial content, and rapid depletion of high-energy phosphate during exercise with markedly delayed repletion of high-energy phosphate during recovery in mitochondria. In addition, patients with HFpEF have impaired nitric oxide bioavailability, particularly in the microvasculature. These SM abnormalities may be responsible for impaired diffusive oxygen transport and/or impaired SM oxygen extraction. To date, exercise training (ET) and caloric restriction are some of the interventions shown to improve outcomes in HFpEF patients. Improvements in exercise tolerance following aerobic ET are largely mediated through peripheral SM adaptations with minimal change in central hemodynamics and highlight the importance of targeting SM to improve exercise intolerance in HFpEF. Focusing on the abnormalities mentioned above may improve the clinical condition of patients with HFpEF.

Comparison Between Treadmill and Bicycle Ergometer Exercises in Terms of Safety of Cardiopulmonary Exercise Testing in Patients With Coronary Heart Disease

Background

Cardiopulmonary exercise testing (CPET) is used widely in the diagnosis, exercise therapy, and prognosis evaluation of patients with coronary heart disease (CHD). The current guideline for CPET does not provide any specific recommendations for cardiovascular (CV) safety on exercise stimulation mode, including bicycle ergometer, treadmill, and total body workout equipment.

Objective

The aim of this study was to explore the effects of different exercise stimulation modes on the occurrence of safety events during CPET in patients with CHD.

Methods

A total of 10,538 CPETs, including 5,674 performed using treadmill exercise and 4,864 performed using bicycle ergometer exercise at Peking University Third Hospital, were analyzed retrospectively. The incidences of CV events and serious adverse events during CPET were compared between the two exercise groups.

Results

Cardiovascular events in enrolled patients occurred during 355 CPETs (3.4%), including 2 cases of adverse events (0.019%), both in the treadmill group. The incidences of overall events [235 (4.1%) vs. 120 (2.5%), P &lt; 0.001], premature ventricular contractions (PVCs) [121 (2.1%) vs. 63 (1.3%), P = 0.001], angina pectoris [45 (0.8%) vs. 5 (0.1%), P &lt; 0.001], and ventricular tachycardia (VT) [32 (0.6%) vs. 14 (0.3%), P = 0.032] were significantly higher in the treadmill group compared with the bicycle ergometer group. No significant difference was observed in the incidence of bradyarrhythmia and atrial arrhythmia between the two groups. Logistic regression analysis showed that the occurrence of overall CV events (P &lt; 0.001), PVCs (P = 0.007), angina pectoris (P &lt; 0.001), and VT (P = 0.008) was independently associated with the stimulation method of treadmill exercise. In male subjects, the occurrence of overall CV events, PVCs, angina pectoris, and VT were independently associated with treadmill exercise, while only the overall CV events and angina pectoris were independently associated with treadmill exercise in female subjects.

Conclusion

In comparison with treadmill exercise, bicycle ergometer exercise appears to be a safer exercise stimulation mode for CPET in patients with CHD.

Matching of O2 Utilization and O2 Delivery in Contracting Skeletal Muscle in Health, Aging, and Heart Failure

Skeletal muscle is one of the most dynamic metabolic organs as evidenced by increases in metabolic rate of >150-fold from rest to maximal contractile activity. Because of limited intracellular stores of ATP, activation of metabolic pathways is required to maintain the necessary rates of ATP re-synthesis during sustained contractions. During the very early phase, phosphocreatine hydrolysis and anaerobic glycolysis prevails but as activity extends beyond ∼1 min, oxidative phosphorylation becomes the major ATP-generating pathway. Oxidative metabolism of macronutrients is highly dependent on the cardiovascular system to deliver O₂ to the contracting muscle fibres, which is ensured through a tight coupling between skeletal muscle O₂ utilization and O₂ delivery. However, to what extent O₂ delivery is ideal in terms of enabling optimal metabolic and contractile function is context-dependent and determined by a complex interaction of several regulatory systems. The first part of the review focuses on local and systemic mechanisms involved in the regulation of O₂ delivery and how integration of these influences the matching of skeletal muscle O₂ demand and O₂ delivery. In the second part, alterations in cardiovascular function and structure associated with aging and heart failure, and how these impact metabolic and contractile function, will be addressed. Where applicable, the potential of exercise training to offset/reverse age- and disease-related cardiovascular declines will be highlighted in the context of skeletal muscle metabolic function. The review focuses on human data but also covers animal observations.

IVIM Imaging of Paraspinal Muscles Following Moderate and High-Intensity Exercise in Healthy Individuals

Background

Quantification of the magnitude and spatial distribution of muscle blood flow changes following exercise may improve our understanding of the effectiveness of various exercise prescriptions. Intravoxel incoherent motion (IVIM) magnetic resonance imaging (MRI) is a technique that quantifies molecular diffusion and microvascular blood flow, and has recently gained momentum as a method to evaluate a muscle's response to exercise. It has also been shown to predict responses to exercise-based physical therapy in individuals with low back pain. However, no study has evaluated the sensitivity of IVIM-MRI to exercise of varying intensity in humans. Here, we aimed to evaluate IVIM signal changes of the paraspinal muscles in response to moderate and high intensity lumbar extension exercise in healthy individuals.

Methods

IVIM data were collected in 11 healthy volunteers before and immediately after a 3-min bout of moderate and high-intensity resisted lumbar extension. IVIM data were analyzed to determine the average perfusion fraction (f), pseudo-diffusion coefficient (D^*), and diffusion coefficient (D) in the bilateral paraspinal muscles. Changes in IVIM parameters were compared between the moderate and high intensity exercise bouts.

Results

Exercise increased all IVIM parameters, regardless of intensity (p < 0.003). Moderate intensity exercise resulted in a 11.2, 19.6, and 3.5% increase in f, D^* and D, respectively. High intensity exercise led to a similar increase in f (12.2%), but much greater changes in D^* (48.6%) and D (7.9%).

Conclusion

IVIM parameter increases suggest that both the moderate and high-intensity exercise conditions elicited measurable changes in blood flow (increased f and D^*) and extravascular molecular diffusion rates (increased D), and that there was a dose-dependence of exercise intensity on D^* and D.

Physiological Function during Exercise and Environmental Stress in Humans-An Integrative View of Body Systems and Homeostasis

Claude Bernard's milieu intérieur (internal environment) and the associated concept of homeostasis are fundamental to the understanding of the physiological responses to exercise and environmental stress. Maintenance of cellular homeostasis is thought to happen during exercise through the precise matching of cellular energetic demand and supply, and the production and clearance of metabolic by-products. The mind-boggling number of molecular and cellular pathways and the host of tissues and organ systems involved in the processes sustaining locomotion, however, necessitate an integrative examination of the body's physiological systems. This integrative approach can be used to identify whether function and cellular homeostasis are maintained or compromised during exercise. In this review, we discuss the responses of the human brain, the lungs, the heart, and the skeletal muscles to the varying physiological demands of exercise and environmental stress. Multiple alterations in physiological function and differential homeostatic adjustments occur when people undertake strenuous exercise with and without thermal stress. These adjustments can include: hyperthermia; hyperventilation; cardiovascular strain with restrictions in brain, muscle, skin and visceral organs blood flow; greater reliance on muscle glycogen and cellular metabolism; alterations in neural activity; and, in some conditions, compromised muscle metabolism and aerobic capacity. Oxygen supply to the human brain is also blunted during intense exercise, but global cerebral metabolism and central neural drive are preserved or enhanced. In contrast to the strain seen during severe exercise and environmental stress, a steady state is maintained when humans exercise at intensities and in environmental conditions that require a small fraction of the functional capacity. The impact of exercise and environmental stress upon whole-body functions and homeostasis therefore depends on the functional needs and differs across organ systems.

Acute L-Citrulline Supplementation Increases Nitric Oxide Bioavailability but Not Inspiratory Muscle Oxygenation and Respiratory Performance

The present study aimed to investigate whether acute L-citrulline supplementation would affect inspiratory muscle oxygenation and respiratory performance. Twelve healthy males received 6 g of L-citrulline or placebo in a double-blind crossover design. Pulmonary function (i.e., forced expired volume in 1 s, forced vital capacity and their ratio), maximal inspiratory pressure (MIP), fractional exhaled nitric oxide (NO^•), and sternocleidomastoid muscle oxygenation were measured at baseline, one hour post supplementation, and after an incremental resistive breathing protocol to task failure of the respiratory muscles. The resistive breathing task consisted of 30 inspirations at 70% and 80% of MIP followed by continuous inspirations at 90% of MIP until task failure. Sternocleidomastoid muscle oxygenation was assessed using near-infrared spectroscopy. One-hour post-L-citrulline supplementation, exhaled NO^• was significantly increased (19.2%; p < 0.05), and this increase was preserved until the end of the resistive breathing (16.4%; p < 0.05). In contrast, no difference was observed in the placebo condition. Pulmonary function and MIP were not affected by the L-citrulline supplementation. During resistive breathing, sternocleidomastoid muscle oxygenation was significantly reduced, with no difference noted between the two supplementation conditions. In conclusion, a single ingestion of 6 g L-citrulline increased NO^• bioavailability but not the respiratory performance and inspiratory muscle oxygenation.

Microvascular blood flow changes of the abductor pollicis brevis muscle during sustained static exercise

A practical assessment of the general health and microvascular function of the palm muscle, abductor pollicis brevis (APB), is important for the diagnosis of different conditions. In this study, we have developed a protocol and a probe to study microvascular blood flow using near-infrared diffuse correlation spectroscopy (DCS) in APB during and after thumb abduction at 55% of maximum voluntary contraction (MVC). Near-infrared time resolved spectroscopy (TRS) was also used to characterize the baseline optical and hemodynamic properties. Thirteen (n=13) subjects were enrolled and subdivided in low MVC (N=6, MVC<2.3 kg) and high MVC (N=7, MVC≥2.3 kg) groups. After ruling out significant changes in the systemic physiology that influence the muscle hemodynamics, we have observed that the high MVC group showed a 56% and 36% decrease in the blood flow during exercise, with respect to baseline, in the long and short source-detector (SD) separations (p=0.031 for both). No statistical differences were shown for the low MVC group (p=1 for short and p=0.15 for long SD). These results suggest that the mechanical occlusion, due to increased intramuscular pressure, exceeded the vasodilation elicited by the higher metabolic demand. Also, blood flow changes during thumb contraction negatively correlated (R=-0.7, p<0.01) with the absolute force applied by each subject. Furthermore, after the exercise, muscular blood flow increased significantly immediately after thumb contractions in both high and low MVC groups, with respect to the recorded values during the exercise (p=0.031). An increase of 251% (200%) was found for the long (short) SD in the low MVC group. The high MVC groups showed a significant 90% increase in blood flow only after 80 s from the start of the protocol. For both low and high MVC groups, blood flow recovered to baseline values within 160 s from starting the exercise. In conclusion, DCS allows the study of the response of a small muscle to static exercise and can be potentially used in multiple clinical conditions scenarios for assessing microvascular health.

Regenerated Microvascular Networks in Ischemic Skeletal Muscle

Skeletal muscle is the largest organ in humans. The viability and performance of this metabolically demanding organ are exquisitely dependent on the integrity of its microcirculation. The architectural and functional attributes of the skeletal muscle microvasculature are acquired during embryonic and early postnatal development. However, peripheral vascular disease in the adult can damage the distal microvasculature, together with damaging the skeletal myofibers. Importantly, adult skeletal muscle has the capacity to regenerate. Understanding the extent to which the microvascular network also reforms, and acquires structural and functional competence, will thus be critical to regenerative medicine efforts for those with peripheral artery disease (PAD). Herein, we discuss recent advances in studying the regenerating microvasculature in the mouse hindlimb following severe ischemic injury. We highlight new insights arising from real-time imaging of the microcirculation. This includes identifying otherwise hidden flaws in both network microarchitecture and function, deficiencies that could underlie the progressive nature of PAD and its refractoriness to therapy. Recognizing and overcoming these vulnerabilities in regenerative angiogenesis will be important for advancing treatment options for PAD.

Exercise Testing, Physical Training and Fatigue in Patients with Mitochondrial Myopathy Related to mtDNA Mutations

Mutations in mitochondrial DNA (mtDNA) cause disruption of the oxidative phosphorylation chain and impair energy production in cells throughout the human body. Primary mitochondrial disorders due to mtDNA mutations can present with symptoms from adult-onset mono-organ affection to death in infancy due to multi-organ involvement. The heterogeneous phenotypes that patients with a mutation of mtDNA can present with are thought, at least to some extent, to be a result of differences in mtDNA mutation load among patients and even among tissues in the individual. The most common symptom in patients with mitochondrial myopathy (MM) is exercise intolerance. Since mitochondrial function can be assessed directly in skeletal muscle, exercise studies can be used to elucidate the physiological consequences of defective mitochondria due to mtDNA mutations. Moreover, exercise tests have been developed for diagnostic purposes for mitochondrial myopathy. In this review, we present the rationale for exercise testing of patients with MM due to mutations in mtDNA, evaluate the diagnostic yield of exercise tests for MM and touch upon how exercise tests can be used as tools for follow-up to assess disease course or effects of treatment interventions.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Redox basis of exercise physiology

Redox reactions control fundamental processes of human biology. Therefore, it is safe to assume that the responses and adaptations to exercise are, at least in part, mediated by redox reactions. In this review, we are trying to show that redox reactions are the basis of exercise physiology by outlining the redox signaling pathways that regulate four characteristic acute exercise-induced responses (muscle contractile function, glucose uptake, blood flow and bioenergetics) and four chronic exercise-induced adaptations (mitochondrial biogenesis, muscle hypertrophy, angiogenesis and redox homeostasis). Based on our analysis, we argue that redox regulation should be acknowledged as central to exercise physiology.

Amplification of endothelium-dependent vasodilatation in contracting human skeletal muscle: role of KIR channels

KEY POINTS

In humans, the vasodilatory response to skeletal muscle contraction is mediated in part by activation of inwardly rectifying potassium (K_IR ) channels. Evidence from animal models suggest that K_IR channels serve as electrical amplifiers of endothelium-dependent hyperpolarization (EDH). We found that skeletal muscle contraction amplifies vasodilatation to the endothelium-dependent agonist ACh, whereas there was no change in the vasodilatory response to sodium nitroprusside, an endothelium-independent nitric oxide donor. Blockade of K_IR channels reduced the exercise-induced amplification of ACh-mediated vasodilatation. Conversely, pharmacological activation of K_IR channels in quiescent muscle via intra-arterial infusion of KCl independently amplified the vasodilatory response to ACh. This study is the first in humans to demonstrate that specific endothelium-dependent vasodilatory signalling is amplified in the vasculature of contracting skeletal muscle and that K_IR channels may serve as amplifiers of EDH-like vasodilatory signalling in humans.

ABSTRACT

The local vasodilatory response to muscle contraction is due in part to the activation of inwardly rectifying potassium (K_IR ) channels. Evidence from animal models suggest that K_IR channels function as 'amplifiers' of endothelium-dependent vasodilators. We tested the hypothesis that contracting muscle selectively amplifies endothelium-dependent vasodilatation via activation of K_IR channels. We measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to local intra-arterial infusion of ACh (endothelium-dependent dilator) during resting conditions, handgrip exercise (5% maximum voluntary contraction) or sodium nitroprusside (SNP; endothelium-independent dilator) which served as a high-flow control condition (n = 7, young healthy men and women). Trials were performed before and after blockade of K_IR channels via infusion of barium chloride. Exercise augmented peak ACh-mediated vasodilatation (ΔFVC saline: 117 ± 14; exercise: 236 ± 21 ml min^-1 (100 mmHg)^-1 ; P < 0.05), whereas SNP did not impact ACh-mediated vasodilatation. Blockade of K_IR channels attenuated the exercise-induced augmentation of ACh. In eight additional subjects, SNP was administered as the experimental dilator. In contrast to ACh, exercise did not alter SNP-mediated vasodilatation (ΔFVC saline: 158 ± 35; exercise: 121 ± 22 ml min^-1 (100 mmHg)^-1 ; n.s.). Finally, in a subset of six subjects, direct pharmacological activation of K_IR channels in quiescent muscle via infusion of KCl amplified peak ACh-mediated vasodilatation (ΔFVC saline: 97 ± 15, KCl: 142 ± 16 ml min^-1  (100 mmHg)^-1 ; respectively; P < 0.05). These findings indicate that skeletal muscle contractions selectively amplify endothelium-dependent vasodilatory signalling via activation of K_IR channels, and this may be an important mechanism contributing to the normal vasodilatory response to exercise in humans.

Neuropeptide Y1 and alpha-1 adrenergic receptor-mediated decreases in functional vasodilation in gluteus maximus microvascular networks of prediabetic mice

Prediabetes is associated with impaired contraction‐evoked dilation of skeletal muscle arterioles, which may be due to increased sympathetic activity accompanying this early stage of diabetes disease. Herein, we sought to determine whether blunted contraction‐evoked vasodilation resulted from enhanced sympathetic neuropeptide Y1 receptor (Y1R) and alpha‐1 adrenergic receptor (α1R) activation. Using intravital video microscopy, second‐, third‐, and fourth‐order (2A, 3A, and 4A) arteriolar diameters were measured before and following electrical field stimulation of the gluteus maximus muscle (GM) in prediabetic (PD, Pound Mouse) and control (CTRL, c57bl6, CTRL) mice. Baseline diameter was similar between groups; however, single tetanic contraction (100 Hz; 400 and 800 msec) and sustained rhythmic contraction (2 and 8 Hz, 30 sec) evoked rapid onset vasodilation and steady‐state vasodilatory responses that were blunted by 50% or greater in PD versus CTRL. Following Y1R and α1R blockade with sympathetic antagonists BIBP3226 and prazosin, contraction‐evoked arteriolar dilation in PD was restored to levels observed in CTRL. Furthermore, arteriolar vasoconstrictor responses to NPY (10⁻¹³–10⁻⁸ mol/L) and PE (10⁻⁹–10⁻⁵ mol/L) were greater in PD versus CTRL at higher concentrations, especially at 3A and 4A. These findings suggest that contraction‐evoked vasodilation in PD is blunted by Y1R and α1R receptor activation throughout skeletal muscle arteriolar networks.

Size Exponents for Scaling Maximal Oxygen Uptake in Over 6500 Humans: A Systematic Review and Meta-Analysis

BACKGROUND

Maximal oxygen uptake ([Formula: see text] _2max) is conventionally normalized to body size as a simple ratio or using an allometric exponent < 1. Nevertheless, the most appropriate body size variable to use for scaling and the value of the exponent are still enigmatic. Studies tend to be based on small samples and can, therefore, lack precision.

OBJECTIVE

The objective of this systematic review was to provide a quantitative synthesis of reported static allometric exponents used for scaling [Formula: see text] _2max to whole body mass and fat-free mass.

METHODS

Eight electronic databases (CINAHL, Cochrane Central Register of Controlled Trials, EMBASE, MEDLINE, PubMed, Scopus, SPORTDiscus and Web of Science) were searched for relevant studies published up to January 2016. Search terms included 'oxygen uptake', 'cardiorespiratory fitness', '[Formula: see text] _2max', '[Formula: see text] _2peak', 'scaling' and all interchangeable terms. Inclusion criteria included human cardiorespiratory fitness data; cross-sectional study designs; an empirical derivation of the exponent; reported precision statistics; and reported information regarding participant sex, age and sports background, [Formula: see text] _2max protocol, whole body composition protocol and line-fitting methods. A random-effects model was used to quantify weighted pooled exponents and 95% confidence limits (Cls). Heterogeneity was quantified with the tau-statistic (τ). Meta-regression was used to quantify the impact of selected moderator variables on the exponent effect size. A 95% prediction interval was calculated to quantify the likely range of true fat-free mass exponents in similar future studies, with this distribution used to estimate the probability that an exponent would be above theorised universal values of [Formula: see text].

RESULTS

Thirty-six studies, involving 6514 participants, met the eligibility criteria. Whole body mass and fat-free mass were used as the scaling denominator in 27 and 15 studies, respectively. The pooled allometric exponent (95% Cls) was found to be 0.70 (0.64 to 0.76) for whole body mass and 0.90 (0.83 to 0.96) for fat-free mass. The between-study heterogeneity was greater for whole body mass (τ = ±0.15) than for fat-free mass (τ = ±0.11). Participant sex explained 30% of the between-study variability in the whole body mass exponent, but the influence on the fat-free mass exponent was trivial. The whole body mass exponent of 0.52 (0.40 to 0.64) for females was substantially lower than the 0.76 (0.70 to 0.83) for males, whereas the fat-free mass exponent was similar for both sexes. The effects of all other moderators were trivial. The 95% PI for fat-free mass ranged from 0.68 to 1.12. The estimated probability of a true fat-free mass exponent in a future study being greater than [Formula: see text] power scaling is 0.98 (very likely) and 0.92 (likely), respectively.

CONCLUSIONS

In this quantitative synthesis of published studies involving over 6500 humans, the whole body mass exponent was found to be spuriously low and prone to substantial heterogeneity. We conclude that the scaling of [Formula: see text] _2max in humans is consistent with the allometric cascade model with an estimated prediction interval for the fat-free mass exponent not likely to be consistent with the [Formula: see text] power laws.

Endothelial dysfunction correlates with exaggerated exercise pressor response during whole body maximal exercise in chronic kidney disease

Chronic kidney disease (CKD) patients have exercise intolerance associated with increased cardiovascular mortality. Previous studies demonstrate that blood pressure (BP) and sympathetic nerve responses to handgrip exercise are exaggerated in CKD. These patients also have decreased nitric oxide (NO) bioavailability and endothelial dysfunction, which could potentially lead to an impaired ability to vasodilate during exercise. We hypothesized that CKD patients have exaggerated BP responses during maximal whole body exercise and that endothelial dysfunction correlates with greater exercise pressor responses in these patients. Brachial artery flow-mediated dilation (FMD) was assessed before maximal treadmill exercise in 56 participants: 38 CKD (56.7 ± 1.2 yr old, 38 men) and 21 controls (52.8 ± 1.8 yr old, 20 men). During maximal treadmill exercise, the slope-of-rise in systolic BP (+10.32 vs. +7.75 mmHg/stage, P < 0.001), mean arterial pressure (+3.50 vs. +2.63 mmHg/stage, P = 0.004), and heart rate (+11.87 vs. +10.69 beats·min^-1·stage^-1, P = 0.031) was significantly greater in CKD compared with controls. Baseline FMD was significantly lower in CKD (2.76 ± 0.42% vs. 5.84 ± 0.97%, P = 0.008). Lower FMD values were significantly associated with a higher slope-of-rise in systolic BP (+11.05 vs. 8.71 mmHg/stage, P = 0.003) during exercise in CKD, as well as poorer exercise capacity measured as peak oxygen uptake (V̇o_2peak; 19.47 ± 1.47 vs. 24.57 ± 1.51 ml·min^-1·kg^-1, P < 0.001). These findings demonstrate that low FMD in CKD correlates with augmented BP responses during exercise and lower V̇o_2peak, suggesting that endothelial dysfunction may contribute to exaggerated exercise pressor responses and poor exercise capacity in CKD patients.

Contributions of Central Command and Muscle Feedback to Sympathetic Nerve Activity in Contracting Human Skeletal Muscle

During voluntary contractions, muscle sympathetic nerve activity (MSNA) to contracting muscles increases in proportion to force but the underlying mechanisms are not clear. To shed light on these mechanisms, particularly the influences of central command and muscle afferent feedback, the present study tested the hypothesis that MSNA is greater during voluntary compared with electrically-evoked contractions. Seven male subjects performed a series of 1-min isometric dorsiflexion contractions (left leg) separated by 2-min rest periods, alternating between voluntary and electrically-evoked contractions at similar forces (5-10% of maximum). MSNA was recorded continuously (microneurography) from the left peroneal nerve and quantified from cardiac-synchronized, negative-going spikes in the neurogram. Compared with pre-contraction values, MSNA increased by 51 ± 34% (P < 0.01) during voluntary contractions but did not change significantly during electrically-evoked contractions (-8 ± 12%, P > 0.05). MSNA analyzed at 15-s intervals revealed that this effect of voluntary contraction appeared 15-30 s after contraction onset (P < 0.01), remained elevated until the end of contraction, and disappeared within 15 s after contraction. These findings suggest that central command, and not feedback from contracting muscle, is the primary mechanism responsible for the increase in MSNA to contracting muscle. The time-course of MSNA suggests that there is a longer delay in the onset of this effect compared with its cessation after contraction.

Tetrahydrobiopterin ameliorates the exaggerated exercise pressor response in patients with chronic kidney disease: a randomized controlled trial

Chronic kidney disease (CKD) patients have an exaggerated increase in blood pressure (BP) during rhythmic handgrip exercise (RHG 20%) and static handgrip exercise (SHG 30%). Nitric oxide levels increase during exercise and help prevent excessive hypertension by both increasing vasodilation and reducing sympathetic nerve activity (SNA). Therefore, we hypothesized that tetrahydrobiopterin (BH4), an essential cofactor for nitric oxide synthase, would ameliorate the exaggerated exercise pressor response in CKD patients. In a randomized, double-blinded, placebo-controlled trial, we tested the effects of 12 wk of sapropterin dihydrochloride (6R-BH4; n = 18) versus placebo (n = 14) treatement on BP and muscle SNA (MSNA) responses during RHG 20% and SHG 30% in CKD patients. The 6R-BH4-treated group had a significantly lower systolic BP (+6 ± 1 vs. +13 ± 2 mmHg, P = 0.002) and mean arterial pressure response (+5 ± 1 vs. +10 ± 2 mmHg, P = 0.020) during RHG 20% and a significantly lower systolic BP response (+19 ± 3 vs. +28 ± 3 mmHg, P = 0.043) during SHG 30%. Under baseline conditions, there was no significant difference in MSNA responses between the groups; however, when the BP response during exercise was equalized between the groups using nitroprusside, the 6R-BH4-treated group had a significantly lower MSNA response during RHG 20% (6R-BH4 vs. placebo, +12 ± 1 vs. +21 ± 2 bursts/min, P = 0.004) but not during SHG 30%. These findings suggest that 6R-BH4 ameliorates the augmented BP response during RHG 20% and SHG 30% in CKD patients. A reduction in reflex activation of SNA may contribute to the decreased exercise pressor response during RHG 20% but not during SHG 30% in CKD patients.

Absence of Dystrophin Disrupts Skeletal Muscle Signaling: Roles of Ca2+, Reactive Oxygen Species, and Nitric Oxide in the Development of Muscular Dystrophy

Dystrophin is a long rod-shaped protein that connects the subsarcolemmal cytoskeleton to a complex of proteins in the surface membrane (dystrophin protein complex, DPC), with further connections via laminin to other extracellular matrix proteins. Initially considered a structural complex that protected the sarcolemma from mechanical damage, the DPC is now known to serve as a scaffold for numerous signaling proteins. Absence or reduced expression of dystrophin or many of the DPC components cause the muscular dystrophies, a group of inherited diseases in which repeated bouts of muscle damage lead to atrophy and fibrosis, and eventually muscle degeneration. The normal function of dystrophin is poorly defined. In its absence a complex series of changes occur with multiple muscle proteins showing reduced or increased expression or being modified in various ways. In this review, we will consider the various proteins whose expression and function is changed in muscular dystrophies, focusing on Ca(2+)-permeable channels, nitric oxide synthase, NADPH oxidase, and caveolins. Excessive Ca(2+) entry, increased membrane permeability, disordered caveolar function, and increased levels of reactive oxygen species are early changes in the disease, and the hypotheses for these phenomena will be critically considered. The aim of the review is to define the early damage pathways in muscular dystrophy which might be appropriate targets for therapy designed to minimize the muscle degeneration and slow the progression of the disease.

Reduced blood flow to contracting skeletal muscle in ageing humans: is it all an effect of sand through the hourglass?

The ability to sustain a given absolute submaximal workload declines with advancing age, likely to be due to a lower level of blood flow and O2 delivery to the exercising muscles. Given that physical inactivity mimics many of the physiological changes associated with ageing, separating the physiological consequences of ageing and physical inactivity can be challenging; yet, observations from cross-sectional and longitudinal studies on the effects of physical activity have provided some insight. Physical activity has the potential to offset the age-related decline in blood flow to contracting skeletal muscle during exercise where systemic blood flow is not limited by cardiac output, thereby improving O2 delivery and allowing for an enhanced energy production from oxidative metabolism. The mechanisms underlying the increase in blood flow with regular physical activity include improved endothelial function and the ability for functional sympatholysis - an attenuation of the vasoconstrictor effect of sympathetic nervous activity. These vascular adaptations with physical activity are likely to be an effect of improved nitric oxide and ATP signalling. Collectively, precise matching of blood flow and O2 delivery to meet the O2 demand of the active skeletal muscle of aged individuals during conditions where systemic blood flow is not limited by cardiac output seems to a large extent to be related to the level of physical activity.

Thermoregulatory modeling for cold stress

Modeling for cold stress has generated a rich history of innovation, has exerted a catalytic influence on cold physiology research, and continues to impact human activity in cold environments. This overview begins with a brief summation of cold thermoregulatory model development followed by key principles that will continue to guide current and future model development. Different representations of the human body are discussed relative to the level of detail and prediction accuracy required. In addition to predictions of shivering and vasomotor responses to cold exposure, algorithms are presented for thermoregulatory mechanisms. Various avenues of heat exchange between the human body and a cold environment are reviewed. Applications of cold thermoregulatory modeling range from investigative interpretation of physiological observations to forecasting skin freezing times and hypothermia survival times. While these advances have been remarkable, the future of cold stress modeling is still faced with significant challenges that are summarized at the end of this overview.

Interactive effect of acute sympathetic activation and exercise intensity on the dynamic response characteristics of vascular conductance in the human calf muscle

PURPOSE

The effect of acute activation of the sympathetic nervous system on the dynamic response of muscle hyperaemia during exercise at different intensities is not clear.

METHODS

To explore this, six men performed 16, 5-min bouts of intermittent calf contractions at two intensities (25 and 50 % MVC) and two levels of sympathetic activation (CPT cold pressor test, CON control). Mean arterial pressure (MAP) and leg vascular conductance (LVC leg blood flow/MAP) were measured during rest and contractions (3 s intervals), and dynamic response characteristics of LVC were estimated using curve-fitting and empirical modeling.

RESULTS

MAP was ~20 % greater (P ≤ 0.05) during CPT than CON before and during initial contractions at both intensities. At 25 % MVC, CPT reduced the exercise-induced change in LVC (0.109 vs 0.125 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), an effect attributed to the reduction in the amplitude of the fast growth phase (0.091 vs 0.128 1 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05). At 50 % MVC, CPT also blunted the fast growth phase (0.147 vs 0.189 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), but the total change in LVC during exercise was unaffected because of a significant reduction in the amplitude of the rapid decay phase and tendency (P = 0.1) for a lower amplitude of the slow decay phase.

CONCLUSION

Increased sympathetic constraint of vasodilation persists during initial contractions but is overcome at the high intensity by a mechanism apparently related to hyperaemic decay.

Sympathetic neural activity to the cardiovascular system: integrator of systemic physiology and interindividual characteristics

The sympathetic nervous system is a ubiquitous, integrating controller of myriad physiological functions. In the present article, we review the physiology of sympathetic neural control of cardiovascular function with a focus on integrative mechanisms in humans. Direct measurement of sympathetic neural activity (SNA) in humans can be accomplished using microneurography, most commonly performed in the peroneal (fibular) nerve. In humans, muscle SNA (MSNA) is composed of vasoconstrictor fibers; its best-recognized characteristic is its participation in transient, moment-to-moment control of arterial blood pressure via the arterial baroreflex. This property of MSNA contributes to its typical "bursting" pattern which is strongly linked to the cardiac cycle. Recent evidence suggests that sympathetic neural mechanisms and the baroreflex have important roles in the long term control of blood pressure as well. One of the striking characteristics of MSNA is its large interindividual variability. However, in young, normotensive humans, higher MSNA is not linked to higher blood pressure due to balancing influences of other cardiovascular variables. In men, an inverse relationship between MSNA and cardiac output is a major factor in this balance, whereas in women, beta-adrenergic vasodilation offsets the vasoconstrictor/pressor effects of higher MSNA. As people get older (and in people with hypertension) higher MSNA is more likely to be linked to higher blood pressure. Skin SNA (SSNA) can also be measured in humans, although interpretation of SSNA signals is complicated by multiple types of neurons involved (vasoconstrictor, vasodilator, sudomotor and pilomotor). In addition to blood pressure regulation, the sympathetic nervous system contributes to cardiovascular regulation during numerous other reflexes, including those involved in exercise, thermoregulation, chemoreflex regulation, and responses to mental stress.

Adrenergic and non-adrenergic control of active skeletal muscle blood flow: implications for blood pressure regulation during exercise

Blood flow to active skeletal muscle increases markedly during dynamic exercise. However, despite the massive capacity of skeletal muscle vasculature to dilate, arterial blood pressure is well maintained. Sympathetic nerve activity is elevated with increased intensity of dynamic exercise, and is essential for redistribution of cardiac output to active skeletal muscle and maintenance of arterial blood pressure. In addition, aside from the sympathetic nervous system, evidence from human studies is now emerging that supports roles for non-adrenergic vasoconstrictor pathways that become active during exercise and contribute to vasoconstriction in active skeletal muscle. Neuropeptide Y and adenosine triphosphate are neurotransmitters that are co-released with norepinephrine from sympathetic nerve terminals capable of producing vasoconstriction. Likewise, plasma concentrations of arginine vasopressin, angiotensin II (Ang II) and endothelin-1 (ET-1) increase during dynamic exercise, particularly at higher intensities. Ang II and ET-1 have both been shown to be important vasoconstrictor pathways for restraint of blood flow in active skeletal muscle and the maintenance of arterial blood pressure during exercise. Indeed, although both adrenergic and non-adrenergic vasoconstriction can be attenuated in exercising muscle with greater intensity of exercise, with the higher volume of blood flow, the active skeletal muscle vasculature remains capable of contributing importantly to the maintenance of blood pressure. In this brief review we provide an update on skeletal muscle blood flow regulation during exercise with an emphasis on adrenergic and non-adrenergic vasoconstrictor pathways and their potential capacity to offset vasodilation and aid in the regulation of blood pressure.

Abnormal neurocirculatory control during exercise in humans with chronic renal failure

Abnormal neurocirculatory control during exercise is one important mechanism leading to exercise intolerance in patients with both end-stage renal disease (ESRD) and earlier stages of chronic kidney disease (CKD). This review will provide an overview of mechanisms underlying abnormal neurocirculatory and hemodynamic responses to exercise in patients with kidney disease. Recent studies have shown that ESRD and CKD patients have an exaggerated increase in blood pressure (BP) during both isometric and rhythmic exercise. Subsequent studies examining the role of the exercise pressor reflex in the augmented pressor response revealed that muscle sympathetic nerve activity (MSNA) was not augmented during exercise in these patients, and metaboreflex-mediated increases in MSNA were blunted, while mechanoreflex-mediated increases were preserved under basal conditions. However, normalizing the augmented BP response during exercise via infusion of nitroprusside (NTP), and thereby equalizing baroreflex-mediated suppression of MSNA, an important modulator of the final hemodynamic response to exercise, revealed that CKD patients had an exaggerated increase in MSNA during isometric and rhythmic exercise. In addition, mechanoreflex-mediated control was augmented, and metaboreceptor blunting was no longer apparent in CKD patients with baroreflex normalization. Factors leading to mechanoreceptor sensitization, and other mechanisms underlying the exaggerated exercise pressor response, such as impaired functional sympatholysis, should be investigated in future studies.

KIR channel activation contributes to onset and steady-state exercise hyperemia in humans

We tested the hypothesis that activation of inwardly rectifying potassium (KIR) channels and Na(+)-K(+)-ATPase, two pathways that lead to hyperpolarization of vascular cells, contributes to both the onset and steady-state hyperemic response to exercise. We also determined whether after inhibiting these pathways nitric oxide (NO) and prostaglandins (PGs) are involved in the hyperemic response. Forearm blood flow (FBF; Doppler ultrasound) was determined during rhythmic handgrip exercise at 10% maximal voluntary contraction for 5 min in the following conditions: control [saline; trial 1 (T1)]; with combined inhibition of KIR channels and Na(+)-K(+)-ATPase alone [via barium chloride (BaCl2) and ouabain, respectively; trial 2 (T2)]; and with additional combined nitric oxide synthase (N(G)-monomethyl-l-arginine) and cyclooxygenase inhibition [ketorolac; trial 3 (T3)]. In T2, the total hyperemic responses were attenuated ~50% from control (P < 0.05) at exercise onset, and there was minimal further effect in T3 (protocol 1; n = 11). In protocol 2 (n = 8), steady-state FBF was significantly reduced during T2 vs. T1 (133 ± 15 vs. 167 ± 17 ml/min; Δ from control: -20 ± 3%; P < 0.05) and further reduced during T3 (120 ± 15 ml/min; -29 ± 3%; P < 0.05 vs. T2). In protocol 3 (n = 8), BaCl2 alone reduced FBF during onset (~50%) and steady-state exercise (~30%) as observed in protocols 1 and 2, respectively, and addition of ouabain had no further impact. Our data implicate activation of KIR channels as a novel contributing pathway to exercise hyperemia in humans.

Inefficient functional sympatholysis is an overlooked cause of malperfusion in contracting skeletal muscle

Contracting skeletal muscle can overcome sympathetic vasoconstrictor activity (functional sympatholysis), which allows for a blood supply that matches the metabolic demand. This ability is thought to be mediated by locally released substances that modulate the effect of noradrenaline (NA) on the α-receptor. Tyramine induces local NA release and can be used in humans to investigate the underlying mechanisms and physiological importance of functional sympatholysis in the muscles of healthy and diseased individuals as well as the impact of the active muscles' training status. In sedentary elderly men, functional sympatholysis and muscle blood flow are impaired compared to young men, but regular physical activity can prevent these age related impairments. In young subjects, two weeks of leg immobilization causes a reduced ability for functional sympatholysis, whereas the trained leg maintained this function. Patients with essential hypertension have impaired functional sympatholysis in the forearm, and reduced exercise hyperaemia in the leg, but this can be normalized by aerobic exercise training. The effect of physical activity on the local mechanisms that modulate sympathetic vasoconstriction is clear, but it remains uncertain which locally released substance(s) block the effect of NA and how this is accomplished. NO and ATP have been proposed as important inhibitors of NA mediated vasoconstriction and presently an inhibitory effect of ATP on NA signalling via P2 receptors appears most likely.

Resistance exercise order does not determine postexercise delivery of testosterone, growth hormone, and IGF-1 to skeletal muscle

Does resistance exercise order affect hormone availability? Participants performed arm exercise before and after leg exercise. Hormone delivery was estimated by multiplying brachial artery blood flow and hormone concentrations. Blood flow increased after arm (276%) and leg (193%; both p < 0.001) exercise. Testosterone, growth hormone, and insulin-like growth factor 1 showed with distinct delivery patterns between conditions; however (interactions all p < 0.001), net exposure was similar. The anabolic potential of postexercise hormones was not affected by exercise order.

A Mechanism-Based Approach to Prevention of and Therapy for Fibromyalgia

Fibromyalgia syndrome (FMS) is characterized by pain referred to deep tissues. Diagnosis and treatment of FMS are complicated by a variable coexistence with regional pain, fatigue, sleep disruption, difficulty with mentation, and depression. The widespread, deep pain of FMS can be a consequence of chronic psychological stress with autonomic dysregulation. Stress acts centrally to facilitate pain and acts peripherally, via sympathetic vasoconstriction, to establish painful muscular ischemia. FMS pain, with or without a coexistent regional pain condition, is stressful, setting up a vicious circle of reciprocal interaction. Also, stress interacts reciprocally with systems of control over depression, mentation, and sleep, establishing FMS as a multiple-system disorder. Thus, stress and the ischemic pain it generates are fundamental to the multiple disorders of FMS, and a therapeutic procedure that attenuates stress and peripheral vasoconstriction should be highly beneficial for FMS. Physical exercise has been shown to counteract peripheral vasoconstriction and to attenuate stress, depression, and fatigue and improve mentation and sleep quality. Thus, exercise can interrupt the reciprocal interactions between psychological stress and each of the multiple-system disorders of FMS. The large literature supporting these conclusions indicates that exercise should be considered strongly as a first-line approach to FMS therapy.

Skeletal muscle vasodilatation during maximal exercise in health and disease

Maximal exercise vasodilatation results from the balance between vasoconstricting and vasodilating signals combined with the vascular reactivity to these signals. During maximal exercise with a small muscle mass the skeletal muscle vascular bed is fully vasodilated. During maximal whole body exercise, however, vasodilatation is restrained by the sympathetic system. This is necessary to avoid hypotension since the maximal vascular conductance of the musculature exceeds the maximal pumping capacity of the heart. Endurance training and high-intensity intermittent knee extension training increase the capacity for maximal exercise vasodilatation by 20-30%, mainly due to an enhanced vasodilatory capacity, as maximal exercise perfusion pressure changes little with training. The increase in maximal exercise vascular conductance is to a large extent explained by skeletal muscle hypertrophy and vascular remodelling. The vasodilatory capacity during maximal exercise is reduced or blunted with ageing, as well as in chronic heart failure patients and chronically hypoxic humans; reduced vasodilatory responsiveness and increased sympathetic activity (and probably, altered sympatholysis) are potential mechanisms accounting for this effect. Pharmacological counteraction of the sympathetic restraint may result in lower perfusion pressure and reduced oxygen extraction by the exercising muscles. However, at the same time fast inhibition of the chemoreflex in maximally exercising humans may result in increased vasodilatation, further confirming a restraining role of the sympathetic nervous system on exercise-induced vasodilatation. This is likely to be critical for the maintenance of blood pressure in exercising patients with a limited heart pump capacity.

Sildenafil improves microvascular O2 delivery-to-utilization matching and accelerates exercise O2 uptake kinetics in chronic heart failure

Nitric oxide (NO) can temporally and spatially match microvascular oxygen (O(2)) delivery (Qo(2mv)) to O(2) uptake (Vo(2)) in the skeletal muscle, a crucial adjustment-to-exercise tolerance that is impaired in chronic heart failure (CHF). To investigate the effects of NO bioavailability induced by sildenafil intake on muscle Qo(2mv)-to-O(2) utilization matching and Vo(2) kinetics, 10 males with CHF (ejection fraction = 27 ± 6%) undertook constant work-rate exercise (70-80% peak). Breath-by-breath Vo(2), fractional O(2)extraction in the vastus lateralis {∼deoxygenated hemoglobin + myoglobin ([deoxy-Hb + Mb]) by near-infrared spectroscopy}, and cardiac output (CO) were evaluated after sildenafil (50 mg) or placebo. Sildenafil increased exercise tolerance compared with placebo by ∼20%, an effect that was related to faster on- and off-exercise Vo(2) kinetics (P < 0.05). Active treatment, however, failed to accelerate CO dynamics (P > 0.05). On-exercise [deoxy-Hb + Mb] kinetics were slowed by sildenafil (∼25%), and a subsequent response "overshoot" (n = 8) was significantly lessened or even abolished. In contrast, [deoxy-Hb + Mb] recovery was faster with sildenafil (∼15%). Improvements in muscle oxygenation with sildenafil were related to faster on-exercise Vo(2) kinetics, blunted oscillations in ventilation (n = 9), and greater exercise capacity (P < 0.05). Sildenafil intake enhanced intramuscular Qo(2mv)-to-Vo(2) matching with beneficial effects on Vo(2) kinetics and exercise tolerance in CHF. The lack of effect on CO suggests that improvement in blood flow to and within skeletal muscles underlies these effects.

Nitrite and nitrite reductases: from molecular mechanisms to significance in human health and disease

Nitrite, previously considered physiologically irrelevant and a simple end product of endogenous nitric oxide (NO) metabolism, is now envisaged as a reservoir of NO to be activated in response to oxygen (O(2)) depletion. In the first part of this review, we summarize and compare the mechanisms of nitrite-dependent production of NO in selected bacteria and in eukaryotes. Bacterial nitrite reductases, which are copper or heme-containing enzymes, play an important role in the adaptation of pathogens to O(2) limitation and enable microrganisms to survive in the human body. In mammals, reduction of nitrite to NO under hypoxic conditions is carried out in tissues and blood by an array of metalloproteins, including heme-containing proteins and molybdenum enzymes. In humans, tissues play a more important role in nitrite reduction, not only because most tissues produce more NO than blood, but also because deoxyhemoglobin efficiently scavenges NO in blood. In the second part of the review, we outline the significance of nitrite in human health and disease and describe the recent advances and pitfalls of nitrite-based therapy, with special attention to its application in cardiovascular disorders, inflammation, and anti-bacterial defence. It can be concluded that nitrite (as well as nitrate-rich diet for long-term applications) may hold promise as therapeutic agent in vascular dysfunction and ischemic injury, as well as an effective compound able to promote angiogenesis.

ATP as a mediator of erythrocyte-dependent regulation of skeletal muscle blood flow and oxygen delivery in humans

In healthy human beings, blood flow to dynamically contracting skeletal muscle is regulated primarily to match oxygen (O(2)) delivery closely with utilisation. This occurs across a wide range of exercise intensities, as well as when exercise is combined with conditions that modify blood O(2) content. The red blood cells (RBCs), the primary O(2) carriers in the blood, contribute to the regulation of the local processes matching O(2) supply and demand. This is made possible by the ability of RBCs to release the vasoactive substance adenosine triphosphate (ATP) in response to reductions in erythrocyte and plasma O(2), as well as to other adjuvant metabolic and mechanical stimuli. The regulatory role of RBCs in human beings is supported by the observations that, i) exercising skeletal muscle blood flow responds primarily to changes in the amount of O(2) bound to the erythrocyte haemoglobin molecules, rather than the amount of O(2) in plasma, and ii) exercising muscle blood flow can almost double (from 260 to 460 ml min(-1) 100 g(-1)) with alterations in blood O(2) content, such that O(2) delivery and are kept constant. Besides falling blood O(2) content, RBCs release ATP when exposed to increased temperature, reduced pH, hypercapnia, elevated shear stress and augmented mechanical deformation, i.e. conditions that exist in the microcirculation of active skeletal muscle. ATP is an attractive mediator signal for skeletal muscle blood flow regulation, not only because it can act as a potent vasodilator, but also because of its sympatholytic properties in the human limb circulations. These properties are essential to counteract the vasoconstrictor effects of concurrent increases in muscle sympathetic nerve activity and circulating vasoconstrictor substances during exercise. Comparison of the relative vasoactive potencies and sympatholytic properties of ATP, other nucleotides, and adenosine in human limbs, suggests that intravascular ATP exerts its vasodilator and sympatholytic effects directly, and not via its degradation compounds. In conclusion, current evidence clearly indicates that RBCs are involved directly in the regulation of O(2) supply to human skeletal muscle during dynamic exercise. Further, intravascular ATP might be an important mediator in local metabolic sensing and signal transduction between the RBCs and the endothelial and smooth muscle cells in the vascular beds of skeletal muscle.

Effect of hypoxia on the dynamic response of hyperaemia in the contracting human calf muscle

Although systemic hypoxia increases the muscle hyperaemic response during 'steady-state' exercise, its effect on the dynamic characteristics of this response is not clear. In the present study, we first established that hypoxia increases the steady-state hyperaemic response at low workloads during calf exercise. To study dynamic aspects of this response, eight subjects performed eight exercise trials while breathing a normoxic (fractional inspired O(2) = 0.2094) or hypoxic gas mixture (fractional inspired O(2) = 0.105). Subjects performed intermittent contractions (1 s) of the calf muscle at 20% maximal voluntary contraction, and the leg blood flow (LBF), leg vascular conductance (LVC) and EMG activities of the triceps surae muscles were measured during each contraction-relaxation period (3 s). The LBF and LVC responses were averaged for each subject and fitted using a four-phase, exponential growth and decay function. Hypoxia evoked significant increases in the change in LBF (15%) and LVC (23%) from the start to the end of exercise, as well as the amplitude of the rapid growth phase of LBF and LVC (21%). Similar, but non-significant, effects on the amplitude of the slow growth phase of LBF (P = 0.08) and LVC (P = 0.10) were observed. By contrast, hypoxia had no effect on temporal parameters of these growth phases, parameters defining the decay phases or EMG activities. These results suggest that the effect of hypoxia on exercise hyperaemia is targeted at the rapid and perhaps the slow growth phase of the response, and is not mediated by a change in the level of muscle activation.

Peripheral vasodilatation determines cardiac output in exercising humans: insight from atrial pacing

In dogs, manipulation of heart rate has no effect on the exercise-induced increase in cardiac output. Whether these findings apply to humans remain uncertain, because of the large differences in cardiovascular anatomy and regulation. To investigate the role of heart rate and peripheral vasodilatation in the regulation of cardiac output during steady-state exercise, we measured central and peripheral haemodynamics in 10 healthy male subjects, with and without atrial pacing (100–150 beats min(−1)) during: (i) resting conditions, (ii) one-legged knee extensor exercise (24 W) and (iii) femoral arterial ATP infusion at rest. Exercise and ATP infusion increased cardiac output, leg blood flow and vascular conductance (P < 0.05), whereas cerebral perfusion remained unchanged. During atrial pacing increasing heart rate by up to 54 beats min(−1), cardiac output did not change in any of the three conditions, because of a parallel decrease in stroke volume (P < 0.01). Atrial pacing increased mean arterial pressure (MAP) at rest and during ATP infusion (P < 0.05), whereas MAP remained unchanged during exercise. Atrial pacing lowered central venous pressure (P < 0.05) and pulmonary capillary wedge pressure (P < 0.05) in all conditions, whereas it did not affect pulmonary mean arterial pressure. Atrial pacing lowered the left ventricular contractility index (dP/dt) (P < 0.05) in all conditions and plasma noradrenaline levels at rest (P < 0.05), but not during exercise and ATP infusion. These results demonstrate that the elevated cardiac output during steady-state exercise is regulated by the increase in skeletal muscle blood flow and venous return to the heart, whereas the increase in heart rate appears to be secondary to the regulation of cardiac output.

Neuronal inhibition and excitation, and the dichotomic control of brain hemodynamic and oxygen responses

Brain's electrical activity correlates strongly to changes in cerebral blood flow (CBF) and the cerebral metabolic rate of oxygen (CMRO(2)). Subthreshold synaptic processes correlate better than the spike rates of principal neurons to CBF, CMRO(2) and positive BOLD signals. Stimulation-induced rises in CMRO(2) are controlled by the ATP turnover, which depends on the energy used to fuel the Na,K-ATPase to reestablish ionic gradients, while stimulation-induced CBF responses to a large extent are controlled by mechanisms that depend on Ca(2+) rises in neurons and astrocytes. This dichotomy of metabolic and vascular control explains the gap between the stimulation-induced rises in CMRO(2) and CBF, and in turn the BOLD signal. Activity-dependent rises in CBF and CMRO(2) vary within and between brain regions due to differences in ATP turnover and Ca(2+)-dependent mechanisms. Nerve cells produce and release vasodilators that evoke positive BOLD signals, while the mechanisms that control negative BOLD signals by activity-dependent vasoconstriction are less well understood. Activation of both excitatory and inhibitory neurons produces rises in CBF and positive BOLD signals, while negative BOLD signals under most conditions correlate to excitation of inhibitory interneurons, but there are important exceptions to that rule as described in this paper. Thus, variations in the balance between synaptic excitation and inhibition contribute dynamically to the control of metabolic and hemodynamic responses, and in turn the amplitude and polarity of the BOLD signal. Therefore, it is not possible based on a negative or positive BOLD signal alone to decide whether the underlying activity goes on in principal or inhibitory neurons.

Local control of skeletal muscle blood flow during exercise: influence of available oxygen

Reductions in oxygen availability (O(2)) by either reduced arterial O(2) content or reduced perfusion pressure can have profound influences on the circulation, including vasodilation in skeletal muscle vascular beds. The purpose of this review is to put into context the present evidence regarding mechanisms responsible for the local control of blood flow during acute systemic hypoxia and/or local hypoperfusion in contracting muscle. The combination of submaximal exercise and hypoxia produces a "compensatory" vasodilation and augmented blood flow in contracting muscles relative to the same level of exercise under normoxic conditions. A similar compensatory vasodilation is observed in response to local reductions in oxygen availability (i.e., hypoperfusion) during normoxic exercise. Available evidence suggests that nitric oxide (NO) contributes to the compensatory dilator response under each of these conditions, whereas adenosine appears to only play a role during hypoperfusion. During systemic hypoxia the NO-mediated component of the compensatory vasodilation is regulated through a β-adrenergic receptor mechanism at low-intensity exercise, while an additional (not yet identified) source of NO is likely to be engaged as exercise intensity increases during hypoxia. Potential candidates for stimulating and/or interacting with NO at higher exercise intensities include prostaglandins and/or ATP. Conversely, prostaglandins do not appear to play a role in the compensatory vasodilation during exercise with hypoperfusion. Taken together, the data for both hypoxia and hypoperfusion suggest NO is important in the compensatory vasodilation seen when oxygen availability is limited. This is important from a basic biological perspective and also has pathophysiological implications for diseases associated with either hypoxia or hypoperfusion.

Interactions between intracellular calcium and phosphate in intact mouse muscle during fatigue

Fatigue was studied in intact tibialis anterior muscle of anesthetized mice. The distal tendon was detached and connected to a force transducer while blood flow continued normally. The muscle was stimulated with electrodes applied directly to the muscle surface and fatigued by repeated (1 per 4 s), brief (0.4 s), maximal (100-Hz stimulation frequency) tetani. Force declined monotonically to 49 ± 5% of the initial value with a half time of 36 ± 5 s and recovered to 86 ± 4% after 4 min. Intracellular phosphate concentration ([Pi]) was measured by 31P-NMR on perchloric acid extracts of muscles. [Pi] increased during fatigue from 7.6 ± 1.7 to 16.0 ± 1.6 mmol/kg muscle wet wt and returned to control during recovery. Intracellular Ca2+ was measured with cameleons whose plasmids had been transfected in the muscle 2 wk before the experiment. Yellow cameleon 2 was used to measure myoplasmic Ca2+, and D1ER was used to measure sarcoplasmic reticulum (SR) Ca2+. The myoplasmic Ca2+ during tetani declined steadily during the period of fatigue and showed complete recovery over 4 min. The SR Ca2+ also declined monotonically during fatigue and showed a partial recovery with rest. These results show that the initial phase of force decline is accompanied by a rise in [Pi] and a reduction in the tetanic myoplasmic Ca2+. We suggest that both changes contribute to the fatigue. A likely cause of the decline in tetanic myoplasmic Ca2+ is precipitation of CaPi in the SR.