Muscle blood flow, hypoxia, and hypoperfusion

Electrical Stimulation-Based Twitch Exercise Suppresses Progression of Immobilization-Induced Muscle Fibrosis via Downregulation of PGC-1?/VEGF Pathway

This study aimed to determine whether electrical stimulation-based twitch exercise is effective in inhibiting the progression of immobilization-induced muscle fibrosis. 19 Wistar rats were randomly divided into a control group (n=6), an immobilization group (n=6; with immobilization only), and a Belt group (n=7; with immobilization and twitch exercise through the belt electrode device, beginning 2 weeks after immobilization). The bilateral soleus muscles were harvested after the experimental period. The right soleus muscles were used for histological analysis, and the left soleus muscles were used for biochemical and molecular biological analysis. As a result, in the picrosirius red images, the perimysium and endomysium were thicker in both the immobilization and Belt groups compared to the control group. However, the perimysium and endomysium thickening were suppressed in the Belt group. The hydroxyproline content and alpha-SMA, TGF-beta1, and HIF-1alpha mRNA expressions were significantly higher in the immobilization and belt groups than in the control group. These expressions were significantly lower in the Belt group than in the immobilization group. The capillary-to-myofiber ratio and the mRNA expressions of VEGF and PGC-1alpha were significantly lower in the immobilization and belt groups than in the control group, these were significantly higher in the Belt group than in the immobilization group. From these results, Electrical stimulation-based twitch exercise using the belt electrode device may prevent the progression of immobilization-induced muscle fibrosis caused by downregulating PGC-1alpha/VEGF pathway, we surmised that this intervention strategy might be effective against the progression of muscle contracture. Keywords: Immobilization, Skeletal muscle, Fibrosis, Electrical stimulation-based twitch exercise, PGC-1alpha/VEGF pathway.

Intermittent normobaric hypoxia alters substrate partitioning and muscle oxygenation in individuals with obesity: implications for fat burning

This single-blind, crossover study aimed to measure and evaluate the short-term metabolic responses to continuous and intermittent hypoxic patterns in individuals with obesity. Indirect calorimetry was used to quantify changes in resting metabolic rate (RMR), carbohydrate (CHOox, %CHO), and fat oxidation (FATox, %FAT) in nine individuals with obesity pre and post: 1) breathing normoxic air [normoxic sham control (NS-control)], 2) breathing continuous hypoxia (CH), or 3) breathing intermittent hypoxia (IH). A mean peripheral oxygen saturation ([Formula: see text]) of 80-85% was achieved over a total of 45 min of hypoxia. Throughout each intervention, pulmonary gas exchanges, oxygen consumption (V̇o2) carbon dioxide production (V̇co2), and deoxyhemoglobin concentration (Δ[HHb]) in the vastus lateralis were measured. Both RMR and CHOox measured pre- and postinterventions were unchanged following each treatment: NS-control, CH, or IH (all P > 0.05). Conversely, a significant increase in FATox was evident between pre- and post-IH (+44%, P = 0.048). Although the mean Δ[HHb] values significantly increased during both IH and CH (P < 0.05), the greatest zenith of Δ[HHb] was achieved in IH compared with CH (P = 0.002). Furthermore, there was a positive correlation between Δ[HHb] and the shift in FATox measured pre- and postintervention. It is suggested that during IH, the increased bouts of muscle hypoxia, revealed by elevated Δ[HHb], coupled with cyclic periods of excess posthypoxia oxygen consumption (EPHOC, inherent to the intermittent pattern) played a significant role in driving the increase in FATox post-IH.

Quadriceps fatigue during hypoxic and ischemic knee-extension exercise is similar in males and females

Hypoxia is known to increase muscle fatigue via both central and peripheral mechanisms. Females are typically less fatigable than males during isometric fatiguing contractions due to greater peripheral blood flow. However, sex differences in fatigue are blunted during dynamic fatiguing tasks. Thus, this study determined the interactions of sex and hypoxia on knee extensor muscle contractile function during a dynamic, ischemic fatiguing contraction. Electrical stimulation was used to determine contractile properties of the knee extensor muscles in eight males and eight females before and after an ischemic, dynamic fatiguing task while inspiring room air or a hypoxic gas mixture (10% O2:90% N2). Fatigue (assessed as time-to-task failure) was ∼10% greater during the hypoxic condition (94.3 ± 33.4 s) compared with normoxic condition (107.0 ± 42.8 s, P = 0.041) and ∼40% greater for females than males (77.1 ± 18.8 vs. 124.2 ± 38.7, P < 0.001). Immediately after the dynamic fatiguing task, there were reductions in maximal voluntary contraction force (P = 0.034) and electrically evoked twitch force (P < 0.001), and these reductions did not differ based on sex or inspirate. Cerebral tissue oxygenation showed a significant interaction of time and inspirate (P = 0.003) whereby it increased during normoxia and remained unchanged in hypoxia. No sex-related differences in the changes of cerebral tissue oxygenation were observed (P = 0.528). These data suggest that acute hypoxia increases central fatigue during ischemic single-leg exercise resulting in earlier exercise termination, but the effect does not differ based on sex.NEW & NOTEWORTHY Hypoxia exacerbates fatigue via central mechanisms after ischemic single-leg exercise. The greater fatigue observed during ischemic dynamic fatiguing exercise with hypoxia inspirate did not differ between the sexes. Hypoxia-induced central limitations are present in acute ischemic exercise and do not appear different in males and females.

Neuromuscular and Muscle Tissue Hemodynamic Responses When Exposed to Normobaric Hypoxia during Lower-Body Fatiguing Muscle Actions

OBJECTIVES

This study examined effects of acute hypoxia on the neuromuscular responses (electromyographic (EMG) amplitude and EMG frequency) and localized muscle tissue oxygenated hemoglobin (oxygenated hemoglobin (Oxy_Hb), deoxygenated hemoglobin (Deoxy_Hb), total hemoglobin (Total_Hb), and muscle tissue oxygenation saturation (StO₂) during the process of fatigue.

METHODS

Fifteen male participants (21.4±2.8yr) performed leg extension repetitions to failure at 70% 1-repetition maximum until volitional exhaustion under Normoxic (FiO₂:21%) and Hypoxic (FiO₂:12.9%) conditions. Electromyographic amplitude, EMG frequency, Oxy_Hb, Deoxy_Hb, Total_Hb, and StO₂ were measured from the vastus lateralis at Initial, 20, 40, 60, 80, and 100% of the repetitions to failure.

RESULTS

There was no significant difference in the patterns of responses for EMG amplitude, Oxy_Hb, or Deoxy_Hb between Normoxia and Hypoxia. For EMG frequency, Hypoxia was greater than Normoxia and decreased with fatigue. Total_Hb and StO₂ were greater under Normoxia compared to Hypoxia. The patterns of responses for EMG amplitude, Deoxy_Hb, and Total_Hb increased throughout the repetitions to failure. Oxy_Hb and StO₂ exhibited decreases throughout the repetitions to failure for Normoxic and Hypoxic conditions.

CONCLUSION

The EMG and oxygenation measurements non-invasively suggest a sympathoexcitatory response (indicated by EMG frequency) and provided complimentary information regarding the process of fatigue in normoxic and hypoxic states.

Five Days of Tart Cherry Supplementation Improves Exercise Performance in Normobaric Hypoxia

Previous studies have shown tart cherry (TC) to improve exercise performance in normoxia. The effect of TC on hypoxic exercise performance is unknown. This study investigated the effects of 5 days of tart cherry (TC) or placebo (PL) supplementation on hypoxic exercise performance. Thirteen healthy participants completed an incremental cycle exercise test to exhaustion (TTE) under two conditions: (i) hypoxia (13% O2) with PL and (ii) hypoxia with TC (200 mg anthocyanin per day for 4 days and 100 mg on day 5). Pulmonary gas exchange variables, peripheral arterial oxygen saturation (SpO2), deoxygenated hemoglobin (HHb), and tissue oxygen saturation (StO2) assessed by near-infrared spectroscopy in the vastus lateralis muscle were measured at rest and during exercise. Urinary 8-hydro-2′ deoxyguanosine (8-OHdG) excretion was evaluated pre-exercise and 1 and 5 h post-exercise. The TTE after TC (940 ± 84 s, mean ± standard deviation) was longer than after PL (912 ± 63 s, p < 0.05). During submaximal hypoxic exercise, HHb was lower and StO2 and SpO2 were higher after TC than PL. Moreover, a significant interaction (supplements × time) in urinary 8-OHdG excretion was found (p < 0.05), whereby 1 h post-exercise increases in urinary 8-OHdG excretion tended to be attenuated after TC. These findings indicate that short-term dietary TC supplementation improved hypoxic exercise tolerance, perhaps due to lower HHb and higher StO2 in the working muscles during submaximal exercise.

Muscle oxygenation during normoxic and hypoxic cycling exercise in humans with high affinity haemoglobin

NEW FINDINGS

What is the central question of this study? Do humans with high affinity haemoglobin (HAH) demonstrate attenuated skeletal muscle deoxygenation during normoxic and hypoxic exercise? What is the main finding and its importance? Examination of NIRS-derived muscle oxygenation profiles suggests that fractional oxygen extraction is blunted during hypoxic exercise in humans with HAH compared to controls. However, muscle tissue oxygen saturation levels were higher in humans with HAH during exercise in normoxia compared to controls. These alterations in fractional oxygen extraction in humans with HAH may influence blood flow regulation and exercise capacity during hypoxia.

ABSTRACT

Recently, our lab has shown that humans with genetic mutations resulting in high affinity haemoglobin (HAH) demonstrate better maintained aerobic capacity and peak power output during hypoxic exercise versus normoxic exercise compared to humans with normal affinity haemoglobin. However, the influence of HAH on tissue oxygenation within exercising muscle during normoxia and hypoxia is unknown. Therefore, we examined near-infrared spectroscopy (NIRS)-derived oxygenation profiles of the vastus lateralis during graded cycling exercise in normoxia and hypoxia among humans with HAH (n = 5) and controls with normal affinity haemoglobin (n = 12). The HAH group elicited a blunted increase of deoxygenated haemoglobin+myoglobin during hypoxic exercise compared to the control group (P = 0.03), suggesting reduced fractional oxygen extraction in the HAH group. In addition, the HAH group maintained a higher level of muscle tissue oxygen saturation during normoxic exercise (HAH, 75 ± 4% vs. controls, 65 ± 3%, P = 0.049) and there were no differences between groups in muscle tissue oxygen saturation during hypoxic exercise (HAH, 68 ± 3% vs. controls, 68 ± 2%, P = 0.943). Overall, our results suggest that humans with HAH may demonstrate divergent patterns of fractional oxygen extraction during hypoxic exercise and elevated muscle tissue oxygenation during normoxic exercise compared to controls. This article is protected by copyright. All rights reserved.

Comparisons Between Normobaric Normoxic and Hypoxic Recovery on Post-exercise Hemodynamics After Sprint Interval Cycling in Hypoxia

The aim of this study was to investigate the effects of either normoxic or hypoxic recovery condition on post-exercise hemodynamics after sprint interval leg cycling exercise rather than hemodynamics during exercise. The participants performed five sets of leg cycling with a maximal effort (30 s exercise for each set) with a 4-min recovery of unloaded cycling between the sets in hypoxia [fraction of inspired oxygen (FiO₂) = 0.145]. The load during pedaling corresponded to 7.5% of the individual’s body weight at the first set, and it gradually reduced from 6.5 to 5.5%, 4.5, and 3.5% for the second to fifth sets. After exercise, the participants rested in a sitting position for 30 min under normoxia (room-air) or hypoxia. Mean arterial pressure decreased over time during recovery (p < 0.001) with no condition and interaction effects (p > 0.05). Compared to pre-exercise values, at 30 min after exercise, mean arterial pressure decreased by 5.6 ± 4.8 mmHg (mean ± standard deviation) during hypoxic recovery, and by 5.3 ± 4.6 mmHg during normoxic recovery. Peripheral arterial oxygen saturation (SpO₂) at all time points (5, 10, 20, and 30 min) during hypoxic recovery was lower than during normoxic recovery (all p < 0.05). The area under the hyperemic curve of tissue oxygen saturation (StO₂) at vastus lateralis defined as reperfusion curve above the baseline values during hypoxic recovery was lower than during normoxic recovery (p < 0.05). Collectively, post-exercise hypotension after sprint interval leg cycling exercise was not affected by either normoxic or hypoxic recovery despite marked differences in SpO₂ and StO₂ during recovery between the two conditions.

High altitude exposures and intestinal barrier dysfunction

Gastrointestinal complaints are often reported during ascents to high altitude (> 2500 m), though their etiology is not known. One potential explanation is injury to the intestinal barrier which has been implicated in the pathophysiology of several diseases. High altitude exposures can reduce splanchnic perfusion and blood oxygen levels causing hypoxic and oxidative stress. These stressors might injure the intestinal barrier leading to consequences such as bacterial translocation and local/systemic inflammatory responses. The purpose of this mini review is to 1) discuss the impact of high-altitude exposures on intestinal barrier dysfunction, and 2) present medications and dietary supplements which may have relevant impacts on the intestinal barrier during high-altitude exposures. There is a small but growing body of evidence which shows that acute exposures to high altitudes can damage the intestinal barrier. Initial data also suggests that prolonged hypoxic exposures can compromise the intestinal barrier through alterations in immunological function, microbiota, or mucosal layers. Exertion may worsen high-altitude related intestinal injury via additional reductions in splanchnic circulation and greater hypoxemia. Collectively these responses can result in increased intestinal permeability and bacterial translocation causing local and systemic inflammation. More research is needed to determine the impact of various medications and dietary supplements on the intestinal barrier during high-altitude exposures.

Influence of High Hemoglobin-Oxygen Affinity on Humans During Hypoxia

Humans elicit a robust series of physiological responses to maintain adequate oxygen delivery during hypoxia, including a transient reduction in hemoglobin-oxygen (Hb-O₂) affinity. However, high Hb-O₂ affinity has been identified as a beneficial adaptation in several species that have been exposed to high altitude for generations. The observed differences in Hb-O₂ affinity between humans and species adapted to high altitude pose a central question: is higher or lower Hb-O₂ affinity in humans more advantageous when O₂ availability is limited? Humans with genetic mutations in hemoglobin structure resulting in high Hb-O₂ affinity have shown attenuated cardiorespiratory adjustments during hypoxia both at rest and during exercise, providing unique insight into this central question. Therefore, the purpose of this review is to examine the influence of high Hb-O₂ affinity during hypoxia through comparison of cardiovascular and respiratory adjustments elicited by humans with high Hb-O₂ affinity compared to those with normal Hb-O₂ affinity.

Altitude, Exercise, and Skeletal Muscle Angio-Adaptive Responses to Hypoxia: A Complex Story

Hypoxia, defined as a reduced oxygen availability, can be observed in many tissues in response to various physiological and pathological conditions. As a hallmark of the altitude environment, ambient hypoxia results from a drop in the oxygen pressure in the atmosphere with elevation. A hypoxic stress can also occur at the cellular level when the oxygen supply through the local microcirculation cannot match the cells’ metabolic needs. This has been suggested in contracting skeletal myofibers during physical exercise. Regardless of its origin, ambient or exercise-induced, muscle hypoxia triggers complex angio-adaptive responses in the skeletal muscle tissue. These can result in the expression of a plethora of angio-adaptive molecules, ultimately leading to the growth, stabilization, or regression of muscle capillaries. This remarkable plasticity of the capillary network is referred to as angio-adaptation. It can alter the capillary-to-myofiber interface, which represent an important determinant of skeletal muscle function. These angio-adaptive molecules can also be released in the circulation as myokines to act on distant tissues. This review addresses the respective and combined potency of ambient hypoxia and exercise to generate a cellular hypoxic stress in skeletal muscle. The major skeletal muscle angio-adaptive responses to hypoxia so far described in this context will be discussed, including existing controversies in the field. Finally, this review will highlight the molecular complexity of the skeletal muscle angio-adaptive response to hypoxia and identify current gaps of knowledges in this field of exercise and environmental physiology.

Effects of combined hot and hypoxic conditions on muscle blood flow and muscle oxygenation during repeated cycling sprints

PURPOSE

The purpose of the present study was to determine muscle blood flow and muscle oxygenation during repeated-sprint exercise under combined hot and hypoxic conditions.

METHODS

In a single-blind, cross-over research design, 11 active males performed three sets of 5 × 6-s maximal sprints with 30-s active recovery on a cycling ergometer under control (CON; 23 °C, 50% rH, 20.9% FiO₂), normobaric hypoxic (HYP; 23 °C, 50% rH, 14.5% FiO₂), or hot + normobaric hypoxic (HH; 35 °C, 50% rH, 14.5% FiO₂) conditions. The vastus lateralis muscle blood flow after each set and muscle oxygenation during each sprint were evaluated using near-infrared spectroscopy methods.

RESULTS

Despite similar repeated-sprint performance among the three conditions (peak and mean power outputs, percent decrement score), HH was associated with significantly higher muscle blood flow compared with CON after the first set (CON: 0.61 ± 0.10 mL/min/100 g; HYP: 0.81 ± 0.13 mL/min/100 g; HH: 0.99 ± 0.16 mL/min/100 g; P < 0.05). The tissue saturation index was significantly lower in HYP than in CON during the latter phase of the exercise (P < 0.05), but it did not differ between HH and CON.

CONCLUSION

These findings suggest that a combination of normobaric hypoxia and heat stress partially facilitated the exercise-induced increase in local blood flow, but it did not enhance tissue desaturation.

Spectral Analysis of Muscle Hemodynamic Responses in Post-Exercise Recovery Based on Near-Infrared Spectroscopy

Spectral analysis of blood flow or blood volume oscillations can help to understand the regulatory mechanisms of microcirculation. This study aimed to explore the relationship between muscle hemodynamic response in the recovery period and exercise quantity. Fifteen healthy subjects were required to perform two sessions of submaximal plantarflexion exercise. The blood volume fluctuations in the gastrocnemius lateralis were recorded in three rest phases (before and after two exercise sessions) using near-infrared spectroscopy. Wavelet transform was used to analyze the total wavelet energy of the concerned frequency range (0.005–2 Hz), which were further divided into six frequency intervals corresponding to six vascular regulators. Wavelet amplitude and energy of each frequency interval were analyzed. Results showed that the total energy raised after each exercise session with a significant difference between rest phases 1 and 3. The wavelet amplitudes showed significant increases in frequency intervals I, III, IV, and V from phase 1 to 3 and in intervals III and IV from phase 2 to 3. The wavelet energy showed similar changes with the wavelet amplitude. The results demonstrate that local microvascular regulators contribute greatly to the blood volume oscillations, the activity levels of which are related to the exercise quantity.

Post-exercise cardiac autonomic and cardiovascular responses to heart rate-matched and work rate-matched hypoxic exercise

Purpose

This study investigated the effect of performing hypoxic exercise at the same heart rate (HR) or work rate (WR) as normoxic exercise on post-exercise autonomic and cardiovascular responses.

Methods

Thirteen men performed three interval-type exercise sessions (5 × 5-min; 1-min recovery): normoxic exercise at 80% of the WR at the first ventilatory threshold (N), hypoxic exercise (FiO₂ = 14.2%) at the same WR as N (H-WR) and hypoxic exercise at the same HR as N (H-HR). Autonomic and cardiovascular assessments were conducted before and after exercise, both at rest and during active squat–stand manoeuvres (SS).

Results

Compared to N, H-WR elicited a higher HR response (≈ 83% vs ≈ 75%HRmax, p < 0.001) and H-HR a reduced exercise WR (− 21.1 ± 9.3%, p < 0.001). Cardiac parasympathetic indices were reduced 15 min after exercise and recovered within 60 min in N and H-HR, but not after H-WR (p < 0.05). H-WR altered cardiac baroreflex sensitivity (cBRS) both at rest and during SS (specifically in the control of blood pressure fall during standing phases) in the first 60 min after the exercise bout (p < 0.05). Post-exercise hypotension (PEH) did not occur in H-HR (p > 0.05) but lasted longer in H-WR than in N (p < 0.05).

Conclusions

Moderate HR-matched hypoxic exercise mimicked post-exercise autonomic responses of normoxic exercise without resulting in significant PEH. This may relate to the reduced WR and the limited associated mechanical/metabolic strain. Conversely, WR-matched hypoxic exercise impacted upon post-exercise autonomic and cardiovascular responses, delaying cardiac autonomic recovery, temporarily decreasing cBRS and evoking prolonged PEH.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00421-021-04678-5.

Muscle Oxygen Delivery in the Forearm and in the Vastus Lateralis Muscles in Response to Resistance Exercise: A Comparison Between Nepalese Porters and Italian Trekkers

Altitude ascending represents an intriguing experimental model reproducing physiological and pathophysiological conditions sharing hypoxemia as the denominator. The aim of the present study was to investigate fractional oxygen extraction and blood dynamics in response to hypobaric hypoxia and to acute resistance exercises, taking into account several factors including different ethnic origin and muscle groups. As part of the “Kanchenjunga Exploration & Physiology” project, six Italian trekkers and six Nepalese porters took part in a high altitude trek in the Himalayas. The measurements were carried out at low (1,450 m) and high altitude (HA; 4,780 m). Near-infrared spectroscopy (NIRS)-derived parameters, i.e., Tot-Hb and tissue saturation index (TSI), were gathered at rest and after bouts of 3-min resistive exercise, both in the quadriceps and in the forearm muscles. TSI decreased with altitude, particularly in forearm muscles (from 66.9 to 57.3%), whereas the decrement was less in the quadriceps (from 62.5 to 57.2%); Nepalese porters were characterized by greater values in thigh TSI than Italian trekkers. Tot-Hb was increased after exercise. At altitude, such increase appeared to be higher in the quadriceps. This effect might be a consequence of the long-term adaptive memory due to the frequent exposures to altitude. Although speculative, we suggest a long-term adaptation of the Nepalese porters due to improved oxygenation of muscles frequently undergoing hypoxic exercise. Muscle structure, individual factors, and altitude exposure time should be taken into account to move on the knowledge of oxygen delivery and utilization at altitude.

Greater α1-adrenergic-mediated vasoconstriction in contracting skeletal muscle of patients with type 2 diabetes

Findings presented in this article are the first to show patients with type 2 diabetes mellitus have blunted hyperemic and vasodilatory responses to dynamic handgrip exercise. Moreover, we illustrate greater α1-adrenergic-mediated vasoconstriction may contribute to our initial observations. Collectively, these data suggest patients with type 2 diabetes may have impaired functional sympatholysis, which can contribute to their reduced exercise capacity.

Highs and lows of sympathetic neurocardiovascular transduction: influence of altitude acclimatization and adaptation

High-altitude (>2,500 m) exposure results in increased muscle sympathetic nervous activity (MSNA) in acclimatizing lowlanders. However, little is known about how altitude affects MSNA in indigenous high-altitude populations. Additionally, the relationship between MSNA and blood pressure regulation (i.e., neurovascular transduction) at high-altitude is unclear. We sought to determine 1) how high-altitude effects neurocardiovascular transduction and 2) whether differences exist in neurocardiovascular transduction between low- and high-altitude populations. Measurements of MSNA (microneurography), mean arterial blood pressure (MAP; finger photoplethysmography), and heart rate (electrocardiogram) were collected in 1) lowlanders (n = 14) at low (344 m) and high altitude (5,050 m), 2) Sherpa highlanders (n = 8; 5,050 m), and 3) Andean (with and without excessive erythrocytosis) highlanders (n = 15; 4,300 m). Cardiovascular responses to MSNA burst sequences (i.e., singlet, couplet, triplet, and quadruplet) were quantified using custom software (coded in MATLAB, v.2015b). Slopes were generated for each individual based on peak responses and normalized total MSNA. High altitude reduced neurocardiovascular transduction in lowlanders (MAP slope: high altitude, 0.0075 ± 0.0060 vs. low altitude, 0.0134 ± 0.080; P = 0.03). Transduction was elevated in Sherpa (MAP slope, 0.012 ± 0.007) compared with Andeans (0.003 ± 0.002, P = 0.001). MAP transduction was not statistically different between acclimatizing lowlanders and Sherpa (MAP slope, P = 0.08) or Andeans (MAP slope, P = 0.07). When resting MSNA is accounted for (ANCOVA), transduction was inversely related to basal MSNA (bursts/minute) independent of population (RRI, r = 0.578 P < 0.001; MAP, r = -0.627, P < 0.0001). Our results demonstrate that transduction is blunted in individuals with higher basal MSNA, suggesting that blunted neurocardiovascular transduction is a physiological adaptation to elevated MSNA rather than an effect or adaptation specific to chronic hypoxic exposure.NEW & NOTEWORTHY This study has identified that sympathetically mediated blood pressure regulation is reduced following ascent to high-altitude. Additionally, we show that high altitude Andean natives have reduced blood pressure responsiveness to sympathetic nervous activity (SNA) compared with Nepalese Sherpa. However, basal sympathetic activity is inversely related to the magnitude of SNA-mediated fluctuations in blood pressure regardless of population or condition. These data set a foundation to explore more precise mechanisms of blood pressure control under conditions of persistent sympathetic activation and hypoxia.

Prolonged treadmill running in normobaric hypoxia causes gastrointestinal barrier permeability and elevates circulating levels of pro- and anti-inflammatory cytokines

This study examined the impact of treadmill running in normobaric hypoxia on gastrointestinal barrier permeability and the systemic inflammatory response. Ten recreationally active participants completed two 1-h bouts of matched-workload treadmill exercise (65% normoxic maximal oxygen consumption) in counterbalanced order. One bout was performed in normoxia (NORM: fraction of inspired oxygen (FIO2) = 20.9%) and the other in normobaric hypoxia (HYP: FIO2 = 13.5%). Minute ventilation, respiratory rate (RR), tidal volume (VT), oxygen consumption, carbon dioxide production, respiratory exchange ratio (RER), and heart rate (HR) were measured with a metabolic cart. Peripheral oxygen saturation (SpO2) was measured with pulse oximetry. Absolute tissue saturation (StO2) was measured with near-infrared spectroscopy. Fatty acid-binding protein (I-FABP) and circulating cytokine concentrations (interleukin (IL)-1Ra, IL-6, IL-10) were assayed from plasma samples that were collected pre-exercise, postexercise, 1 h-postexercise, and 4 h-postexercise. Data were analyzed with 2-way (condition × time) repeated-measures ANOVAs. Newman–Keuls post hoc tests were run where appropriate (p < 0.05). As compared with NORM, 1 h of treadmill exercise in HYP caused greater (p < 0.05) changes in minute ventilation (+30%), RR (+16%), VT (+10%), carbon dioxide production (+18%), RER (+16%), HR (+4%), SpO2 (–16%), and StO2 (–10%). Gut barrier permeability and circulating cytokine concentrations were also greater (p < 0.05) following HYP exercise, where I-FABP was shown increased at postexercise (+68%) and IL-1Ra at 1 h-postexercise (+266%). I-FABP and IL-1Ra did not change (p > 0.05) following NORM exercise. IL-6 and IL-10 increased with exercise in both study conditions but were increased more (p < 0.05) following HYP at postexercise (+705% and +127%, respectively) and 1 h-postexercise (+400% and +128%, respectively).

Novelty Normobaric hypoxia caused significant desaturation and increased most cardiopulmonary responses by 10%–30%. Significant gut barrier permeability and increased pro- and anti-inflammatory cytokine concentrations could promote an “open window” in the hours following HYP exercise.

The hypoxic microenvironment: a driving force for heterotopic ossification progression

Heterotopic ossification (HO) refers to the formation of bone tissue outside the normal skeletal system. According to its pathogenesis, HO is divided into hereditary HO and acquired HO. There currently lack effective approaches for HO prevention or treatment. A deep understanding of its pathogenesis will provide promising strategies to prevent and treat HO. Studies have shown that the hypoxia-adaptive microenvironment generated after trauma is a potent stimulus of HO. The hypoxic microenvironment enhances the stability of hypoxia-inducible factor-1α (HIF-1α), which regulates a complex network including bone morphogenetic proteins (BMPs), vascular endothelial growth factor (VEGF), and neuropilin-1 (NRP-1), which are implicated in the formation of ectopic bone. In this review, we summarize the current understanding of the triggering role and underlying molecular mechanisms of the hypoxic microenvironment in the initiation and progression of HO, focusing mainly on HIF-1 and it's influenced genes  BMP, VEGF, and NRP-1. A better understanding of the role of hypoxia in HO unveils novel therapeutic targets for HO that reduce the local hypoxic microenvironment and inhibit HIF-1α activity.

Muscle Oxygenation During Hypoxic Exercise in Children and Adults

INTRODUCTION

While hypoxia is known to decrease peak oxygen uptake ( V . ⁢ o _{2 max}) and maximal power output in both adults and children its influence on submaximal exercise cardiorespiratory and, especially, muscle oxygenation responses remains unclear.

METHODS

Eight pre-pubertal boys (age = 8 ± 2 years.; body mass (BM) = 29 ± 7 kg) and seven adult males (age = 39 ± 4 years.; BM = 80 ± 8 kg) underwent graded exercise tests in both normoxic (P_iO₂ = 134 ± 0.4 mmHg) and hypoxic (P_iO₂ = 105 ± 0.6 mmHg) condition. Continuous breath-by-breath gas exchange and near infrared spectroscopy measurements, to assess the vastus lateralis oxygenation, were performed during both tests. The gas exchange threshold (GET) and muscle oxygenation thresholds were subsequently determined for both groups in both conditions.

RESULTS

In both groups, hypoxia did not significantly alter either GET or the corresponding V . ⁢ o ₂ at GET. In adults, higher V . _E levels were observed in hypoxia (45 ± 6 l/min) compared to normoxia (36 ± 6 l/min, p < 0.05) at intensities above GET. In contrast, in children both the hypoxic V . _E and V . ⁢ o ₂ responses were significantly greater than those observed in normoxia only at intensities below GET (p < 0.01 for V . _E and p < 0.05 for V . ⁢ o ₂). Higher exercise-related heart rate (HR) levels in hypoxia, compared to normoxia, were only noted in adults (p < 0.01). Interestingly, hypoxia per se did not influence the muscle oxygenation thresholds during exercise in neither group. However, and in contrast to adults, the children exhibited significantly higher total hemoglobin concentration during hypoxic as compared to normoxic exercise (tHb) at lower exercise intensities (30 and 60 W, p = 0.01).

CONCLUSION

These results suggest that in adults, hypoxia augments exercise ventilation at intensities above GET and might also maintain muscle blood oxygenation via increased HR. On the other hand, children exhibit a greater change of muscle blood perfusion, oxygen uptake as well as ventilation at exercise intensities below GET.

Dissociating the effects of oxygen pressure and content on the control of breathing and acute hypoxic response

Arterial oxygen tension and oxyhemoglobin saturation (S a O 2 ) decrease in parallel during hypoxia. Distinguishing between changes in oxygen tension and oxygen content as the relevant physiological stimulus for cardiorespiratory alterations remains challenging. To overcome this, we recruited nine individuals with hemoglobinopathy manifesting as high-affinity hemoglobin [HAH; partial pressure at 50% S a O 2 (P50) = 16 ± 0.4 mmHg] causing greater S a O 2 at a given oxygen partial pressure compared with control subjects (n = 12, P50 = 26 ± 0.4 mmHg). We assessed ventilatory and cardiovascular responses to acute isocapnic hypoxia, iso-oxic hypercapnia, and 20 min of isocapnic hypoxia (arterial Po2 = 50 mmHg). Blood gas alterations were achieved with dynamic end-tidal forcing. When expressed as a function of the logarithm of oxygen partial pressure, ventilatory sensitivity to hypoxia was not different between groups. However, there was a significant difference when expressed as a function of S a O 2 . Conversely, the rise in heart rate was blunted in HAH subjects when expressed as a function of partial pressure but similar when expressed as a function of S a O 2 . Ventilatory sensitivity to hypercapnia was not different between groups. During sustained isocapnic hypoxia, the rise in minute ventilation was similar between groups; however, heart rate was significantly greater in the controls during 3 to 9 min of exposure. Our results support the notion that oxygen tension, not content, alters cellular Po2 in the chemosensors and drives the hypoxic ventilatory response. Our study suggests that in addition to oxygen partial pressure, oxygen content may also influence the heart rate response to hypoxia.NEW & NOTEWORTHY We dissociated the effects of oxygen content and pressure of cardiorespiratory regulation studying individuals with high-affinity hemoglobin (HAH). During hypoxia, the ventilatory response, expressed as a function of oxygen tension, was similar between HAH variants and controls; however, the rise in heart rate was blunted in the variants. Our work supports the notion that the hypoxic ventilatory response is regulated by oxygen tension, whereas cardiovascular regulation may be influenced by arterial oxygen content and tension.

Ischemic Preconditioning, O2 Kinetics, and Performance in Normoxia and Hypoxia

INTRODUCTION

Ischemic preconditioning (IPC) before exercise has been shown to be a novel approach to improve performance in different exercise modes in normoxia (NORM). Few studies have been conducted examining potential mechanisms behind these improvements, and less has been done examining its influence during exercise in hypoxia (HYP). Oxygen uptake and extraction kinetics are factors that have been implicated as possible determinants of cycling performance. We hypothesized that IPC would lead to improvements in oxygen extraction and peripheral blood flow kinetics, and this would translate to improvements in cycling time trial (TT) performance in both NORM and HYP.

METHODS

Thirteen men (age, 24 ± 7 yr; V˙O2max, 63.1 ± 5.1 mL·kg·min) participated in the study. Subjects completed trials of each combination of normobaric HYP (FiO2 = 0.16, simulating ~8000 ft/2500 m) or NORM (FiO2 = 0.21) with preexercise IPC protocol (4 × 5 min at 220 mm Hg) or SHAM procedure. Trials included submaximal constant load cycle exercise bouts (power outputs of 15% below gas exchange threshold, and 85% of V˙O2max), and a 5-km cycling performance TT.

RESULTS

Ischemic preconditioning significantly improved 5-km TT time in NORM by 0.9% ± 1.8% compared with SHAM (IPC, 491.2 ± 35.2 s vs SHAM, 495.9 ± 36.0 s; P < 0.05). Ischemic preconditioning did not alter 5-km TT performance times in HYP (P = 0.231). Ischemic preconditioning did, however, improve tissue oxygen extraction in HYP (deoxygenated hemoglobin/myoglobin: IPC, 21.23 ± 10.95 μM; SHAM, 19.93 ± 9.91 μM; P < 0.05) during moderate-intensity exercise.

CONCLUSIONS

Our data confirm that IPC is an effective ergogenic aid for athletes performing 5-km cycling TT bouts in NORM. Ischemic preconditioning did mitigate the declines in tissue oxygen during moderate-intensity exercise in HYP, but this did not translate to a significant effect on mean group performance. These data suggest that IPC may be of benefit for athletes training and competing in NORM.

Influence of Vigorous Physical Activity on Structure and Function of the Cardiovascular System in Young Athletes-The MuCAYA-Study

Objective: Moderate physical activity (PA) is associated with a reduced risk to develop cardiovascular disease. However, junior athletes exercise between 10 and 20 h a week with intensities exceeding moderate levels by far. In this regard, the cardiovascular system has to increase its work five to six times compared to moderate intensities. This may result in potentially pathological adaptations of the cardiovascular system. The underlying process of vascular adaptations to exercise is yet not fully understood and hardly investigated in junior athletes. An increased blood pressure and pulse wave velocity, ventricular hypertrophy, arrhythmia, and even sudden cardiac death (SCD) has been reported in adult athletes. Studies, examining the cardiovascular system in children, its association to intensity and type of exercise, are rare. Therefore, we present the study protocol of a prospective cross-sectional study that investigates the influence of PA on the cardiovascular system in young athletes.

Methods and Design: Children and adolescents, 7–18 years, presenting for their annual pre-participation screening at the Institute of Preventive Pediatrics, Faculty of Sports and Health Sciences, Technical University of Munich (TUM), are examined in this prospective cross-sectional study. Vascular parameters measured by ultrasound are carotid intima-media thickness (cIMT), vascular stiffness (AC, Ep, β, PWV β), and the vascular diameter (D) to calculate the IMT:Diameter-Ratio (IDR). Cardiac function is evaluated by a 12-lead ECG, and echocardiographic parameters (end-diastolic left ventricular diameter, left ventricular diastolic posterior wall thickness, diastolic septal thickness, left ventricular mass and relative wall thickness, ejection fraction, and shortening fraction). A cardiopulmonary exercise test is performed on a bicycle ergometer, muscular strength is assessed with the handgrip test, and physical activity with the MoMo questionnaire.

Discussion: It is essential to follow young athletes over the course of their career in order to detect pathophysiological changes in the myocardium as soon as possible. If these changes are preceded or followed by changes in vascular structure and function is not known yet. Therefore, we present the study protocol of the Munich Cardiovascular adaptations in young athletes study (MuCAYA-Study) which investigates the association between vascular and cardiac adaptation to intensive exercise in junior athletes.

Muscle Oxygenation During Repeated Double-Poling Sprint Exercise in Normobaric Hypoxia and Normoxia

We compared upper limb muscle oxygenation responses during repeated double-poling sprint exercise in normobaric hypoxia and normoxia. Eight male kayakers completed a repeated double-poling sprint exercise (3 × 3 × 20-s maximal sprints, 40-s passive recovery, 5-min rest) in either hypoxia (HYP, FiO₂ = 14.5%) or normoxia (NOR, FiO₂ = 20.9%). Power output, muscle oxygenation of triceps brachii muscle (using near infrared spectroscopy), arterial oxygen saturation, and cardiorespiratory variables were monitored. Mean power output tended to be lower (-5.2%; P = 0.06) in HYP compared with NOR, while arterial oxygen saturation (82.9 ± 0.9% vs. 90.5 ± 0.8%) and systemic oxygen uptake (1936 ± 140 vs. 2408 ± 83 mL⋅min^-1) values were lower (P < 0.05). Exercise-induced increases in deoxygenated hemoglobin (241.7 ± 46.9% vs. 175.8 ± 27.2%) and total hemoglobin (138.0 ± 18.1% vs. 112.1 ± 6.7%) were greater in HYP in reference to NOR (P < 0.05). Despite moderate hypoxia exacerbating exercise-induced elevation in blood perfusion of active upper limb musculature, power output during repeated double-poling exercise only tended to be lower.

The effects of 4 weeks normobaric hypoxia training on microvascular responses in the forearm flexor

Intermittent exposure to hypoxia can lead to improved endurance performance. Currently, it is unclear whether peripheral adaptions play a role in improving oxygen delivery and utilization following both training and detraining. This study aimed to characterize skeletal muscle blood flow (mBF), oxygen consumption (mV̇O₂), and perfusion adaptations to i) 4-weeks handgrip training in hypoxic and normoxic conditions, and ii) following 4-weeks detraining. Using a randomised crossover design, 9 males completed 30-min handgrip training four times a week in hypoxic (14% FiO₂ ~ 3250m altitude) and normoxic conditions. mBF, mV̇O₂ and perfusion were assessed pre, post 4-weeks training, and following 4-weeks detraining. Hierarchical linear modelling found that mV̇O₂ increased at a significantly faster rate (58%) with hypoxic training (0.09 mlO₂·min^-1 · 100g^-1 per week); perfusion increased at a significantly (69%) faster rate with hypoxic training (3.72 μM per week). mBF did not significantly change for the normoxic condition, but there was a significant increase of 0.38 ml· min^-1 · 100ml^-1 per week (95% CI: 0.35, 0.40) for the hypoxic condition. During 4-weeks detraining, mV̇O₂ and perfusion significantly declined at similar rates for both conditions, whereas mBF decreased significantly faster following hypoxic training. Four weeks hypoxic training increases the delivery and utilisation of oxygen in the periphery.

The Effects of Hyperoxia on Sea-Level Exercise Performance, Training, and Recovery: A Meta-Analysis

BACKGROUND

Acute exercise performance can be limited by arterial hypoxemia, such that hyperoxia may be an ergogenic aid by increasing tissue oxygen availability. Hyperoxia during a single bout of exercise performance has been examined using many test modalities, including time trials (TTs), time to exhaustion (TTE), graded exercise tests (GXTs), and dynamic muscle function tests. Hyperoxia has also been used as a long-term training stimulus or a recovery intervention between bouts of exercise. However, due to the methodological differences in fraction of inspired oxygen (F_iO₂), exercise type, training regime, or recovery protocols, a firm consensus on the effectiveness of hyperoxia as an ergogenic aid for exercise training or recovery remains unclear.

OBJECTIVES

The aims of this study were to (1) determine the efficacy of hyperoxia as an ergogenic aid for exercise performance, training stimulus, and recovery before subsequent exercise; and (2) determine if a dose-response exists between F_iO₂ and exercise performance improvements.

DATA SOURCE

The PubMed, Web of Science, and SPORTDiscus databases were searched for original published articles up to and including 8 September 2017, using appropriate first- and second-order search terms.

STUDY SELECTION

English-language, peer-reviewed, full-text manuscripts using human participants were reviewed using the process identified in the preferred reporting items for systematic reviews and meta-analyses (PRISMA) statement.

DATA EXTRACTION

Data for the following variables were obtained by at least two of the authors: F_iO₂, wash-in time for gas, exercise performance modality, heart rate, cardiac output, stroke volume, oxygen saturation, arterial and/or capillary lactate, hemoglobin concentration, hematocrit, arterial pH, arterial oxygen content, arterial partial pressure of oxygen and carbon dioxide, consumption of oxygen and carbon dioxide, minute ventilation, tidal volume, respiratory frequency, ratings of perceived exertion of breathing and exercise, and end-tidal oxygen and carbon dioxide partial pressures.

DATA GROUPING

Data were grouped into type of intervention (acute exercise, recovery, and training), and performance data were grouped into type of exercise (TTs, TTE, GXTs, dynamic muscle function), recovery, and training in hyperoxia.

DATA ANALYSIS

Hedges' g effect sizes and 95% confidence intervals were calculated. Separate Pearson's correlations were performed comparing the effect size of performance versus F_iO₂, along with the effect size of arterial content of oxygen, arterial partial pressure of oxygen, and oxygen saturation.

RESULTS

Fifty-one manuscripts were reviewed. The most common F_iO₂ for acute exercise was 1.00, with GXTs the most investigated exercise modality. Hyperoxia had a large effect improving TTE (g = 0.89), and small-to-moderate effects increasing TTs (g = 0.56), GXTs (g = 0.40), and dynamic muscle function performance (g = 0.28). An F_iO₂ ≥ 0.30 was sufficient to increase general exercise performance to a small effect or higher; a moderate positive correlation (r = 0.47-0.63) existed between performance improvement of TTs, TTE, and dynamic muscle function tests and F_iO₂, but not GXTs (r = 0.06). Exercise training and recovery supplemented with hyperoxia trended towards a large and small ergogenic effect, respectively, but the large variability and limited amount of research on these topics prevented a definitive conclusion.

CONCLUSION

Acute exercise performance is increased with hyperoxia. An F_iO₂ ≥ 0.30 appears to be beneficial for performance, with a higher F_iO₂ being correlated to greater performance improvement in TTs, TTE, and dynamic muscle function tests. Exercise training and recovery supplemented with hyperoxic gas appears to have a beneficial effect on subsequent exercise performance, but small sample size and wide disparity in experimental protocols preclude definitive conclusions.

"Beet-ing" the Mountain: A Review of the Physiological and Performance Effects of Dietary Nitrate Supplementation at Simulated and Terrestrial Altitude

Exposure to altitude results in multiple physiological consequences. These include, but are not limited to, a reduced maximal oxygen consumption, drop in arterial oxygen saturation, and increase in muscle metabolic perturbations at a fixed sub-maximal work rate. Exercise capacity during fixed work rate or incremental exercise and time-trial performance are also impaired at altitude relative to sea level. Recently, dietary nitrate (NO₃⁻) supplementation has attracted considerable interest as a nutritional aid during altitude exposure. In this review, we summarise and critically evaluate the physiological and performance effects of dietary NO₃⁻ supplementation during exposure to simulated and terrestrial altitude. Previous investigations at simulated altitude indicate that NO₃⁻ supplementation may reduce the oxygen cost of exercise, elevate arterial and tissue oxygen saturation, improve muscle metabolic function, and enhance exercise capacity/performance. Conversely, current evidence suggests that NO₃⁻ supplementation does not augment the training response at simulated altitude. Few studies have evaluated the effects of NO₃⁻ at terrestrial altitude. Current evidence indicates potential improvements in endothelial function at terrestrial altitude following NO₃⁻ supplementation. No effects of NO₃⁻ supplementation have been observed on oxygen consumption or arterial oxygen saturation at terrestrial altitude, although further research is warranted. Limitations of the present body of literature are discussed, and directions for future research are provided.

Integration of intracellular signaling: Biological analogues of wires, processors and memories organized by a centrosome 3D reference system

BACKGROUND

Myriads of signaling pathways in a single cell function to achieve the highest spatio-temporal integration. Data are accumulating on the role of electromechanical soliton-like waves in signal transduction processes. Theoretical studies strongly suggest feasibility of both classical and quantum computing involving microtubules.

AIM

A theoretical study of the role of the complex composed of the plasma membrane and the microtubule-based cytoskeleton as a system that transmits, stores and processes information.

METHODS

Theoretical analysis presented here refers to (i) the Penrose-Hameroff theory of consciousness (Orchestrated Objective Reduction; Orch OR), (ii) the description of the centrosome as a reference system for construction of the 3D map of the cell proposed by Regolini, (iii) the Heimburg-Jackson model of the nerve pulse propagation along axons' lipid bilayer as soliton-like electro-mechanical waves.

RESULTS AND CONCLUSION

The ideas presented in this paper provide a qualitative model for the decision-making processes in a living cell undergoing a differentiation process.

OUTLOOK

This paper paves the way for the real-time live-cell observation of information processing by microtubule-based cytoskeleton and cell fate decision making.

Horizon meeting on cardiovascular physiology: Dedicated to Dr. Mike Sharratt

This perspective document summarizes discussions held at the Canadian Society for Exercise Physiology Annual Meeting in Winnipeg on October 27, 2017, when an expert panel was assembled to discuss the key questions and challenges for future research in cardiovascular exercise physiology. We were inspired by the example of the late Dr. Mike Sharratt, an accomplished and impactful Professor in the Faculty of Kinesiology at the University of Waterloo. Dr. Sharratt had a unique ability to bring experts together and translate theory into action, with a central goal of optimizing the health benefits of exercise, particularly in the fields of cardiac rehabilitation and aging (University of Waterloo Applied Health Science Department 2016; University of Waterloo Health Science Newsletter, 10-1-2017 ( http://uwaterloo.ca/applied-health-sciences/news/remembering-mike-sharratt )).

Conduit Artery Diameter During Exercise Is Enhanced After Local, but Not Remote, Ischemic Preconditioning

Introduction: The ability of ischemic preconditioning (IPC) to enhance exercise capacity may be mediated through altering exercise-induced blood flow and/or vascular function. This study investigated the hypothesis that (local) IPC enhances exercise-induced blood flow responses and prevents decreases in vascular function following exercise.

Methods: Eighteen healthy, recreationally trained, male participants (mean ±SD: age 32 ± 8 years; BMI 24.2 ± 2.3; blood pressure 122 ± 10/72 ± 8 mmHg; resting HR 58 ± 9 beats min^-1) received IPC (220 mmHg; 4 × 5-min bilateral arms), REMOTE IPC (220 mmHg; 4 × 5-min bilateral legs), or SHAM (20 mmHg; 4 × 5-min bilateral arms) in a counterbalanced order prior to 30-min of submaximal (25% maximal voluntary contraction) unilateral rhythmic handgrip exercise. Brachial artery diameter and blood flow were assessed every 5-min throughout the 30-min submaximal exercise using high resolution ultrasonography. Pre- and post-exercise vascular function was measured using flow-mediated dilation (FMD).

Results: IPC resulted in enlarged brachial artery diameter during exercise [0.016 cm (0.003–0.03 cm), P = 0.015] compared to REMOTE IPC, but blood flow during exercise was similar between conditions (P > 0.05). Blood flow (l/min) increased throughout exercise (time: P < 0.005), but there was no main effect of condition (P = 0.29) or condition ^∗ time interaction (P = 0.83). Post-exercise FMD was similar between conditions (P > 0.05).

Conclusion: Our data show that local (but not remote) IPC, performed as a strategy prior to exercise, enhanced exercise-induced conduit artery diameter dilation, but these changes do not translate into increased blood flow during exercise nor impact post-exercise vascular function.

Inhibition of Na+ /K+ -ATPase and KIR channels abolishes hypoxic hyperaemia in resting but not contracting skeletal muscle of humans

KEY POINTS

Increasing blood flow (hyperaemia) to exercising muscle helps match oxygen delivery and metabolic demand. During exercise in hypoxia, there is a compensatory increase in muscle hyperaemia that maintains oxygen delivery and tissue oxygen consumption. Nitric oxide (NO) and prostaglandins (PGs) contribute to around half of the augmented hyperaemia during hypoxic exercise, although the contributors to the remaining response are unknown. In the present study, inhibiting NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels did not blunt augmented hyperaemia during hypoxic exercise beyond previous observations with NO/PG block alone. Furthermore, although inhibition of only Na⁺ /K⁺ -ATPase and K_IR channels abolished hyperaemia during hypoxia at rest, it had no effect on augmented hyperaemia during hypoxic exercise. This is the first study in humans to demonstrate that Na⁺ /K⁺ -ATPase and K_IR channel activation is required for augmented muscle hyperaemia during hypoxia at rest but not during hypoxic exercise, thus providing new insight into vascular control.

ABSTRACT

Exercise hyperaemia in hypoxia is augmented relative to the same exercise intensity in normoxia. During moderate-intensity handgrip exercise, endothelium-derived nitric oxide (NO) and vasodilating prostaglandins (PGs) contribute to ∼50% of the augmented forearm blood flow (FBF) response to hypoxic exercise (HypEx), although the mechanism(s) underlying the remaining response are unclear. We hypothesized that combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels would abolish the augmented hyperaemic response in HypEx. In healthy young adults, FBF responses were measured (Doppler ultrasound) and forearm vascular conductance was calculated during 5 min of rhythmic handgrip exercise at 20% maximum voluntary contraction under regional sympathoadrenal inhibition in normoxia and isocapnic HypEx (O₂ saturation ∼80%). Compared to control, combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and K_IR channels (l-NMMA + ketorolac + ouabain + BaCl_2; Protocol 1; n = 10) blunted the compensatory increase in FBF during HypEx by ∼50% (29 ± 6 mL min^-1 vs. 62 ± 8 mL min^-1 , respectively, P < 0.05). By contrast, ouabain + BaCl₂ alone (Protocol 2; n = 10) did not affect this augmented hyperaemic response (50 ± 11 mL min^-1 vs. 60 ± 13 mL min^-1 , respectively, P > 0.05). However, the blocked condition in both protocols abolished the hyperaemic response to hypoxia at rest (P < 0.05). We conclude that activation of Na⁺ /K⁺ -ATPase and K_IR channels is involved in the hyperaemic response to hypoxia at rest, although it does not contribute to the augmented exercise hyperaemia during hypoxia in humans.

Baseline values of cardiovascular and respiratory parameters predict response to acute hypoxia in young healthy men

The majority of the available works have studied distinct hypoxic responses of respiratory and cardiovascular systems. This study examines how these systems interact while responding to hypoxia and whether baseline metrics moderate reactions to a hypoxic challenge. Central hemodynamic, aortic wave reflection, and gas exchange parameters were measured in 27 trained young men before and after 10-min normobaric isocapnic hypoxia (10 % O2). Associations were assessed by correlation and multiple regression analyses. Hypoxic changes in the parameters of pulse wave analysis such as augmentation index (-114 %, p=0.007), pulse pressure amplification (+6 %, p=0.020), time to aortic reflection wave (+21 %, p<0.001) report on the increase in arterial distensibility. Specifically, initially compliant arteries blunt the positive cardiac chronotropic response to hypoxia and facilitate the myocardial workload. The degree of blood oxygen desaturation is directly correlated with both baseline values and hypoxic responses of aortic and peripheral blood pressures. The hypoxia-induced gain in ventilation (VE), while controlling for basal VE and heart rate (HR), is inversely associated with deltaHR and deltasystolic blood pressure. The study suggests that cardiovascular and respiratory systems mutually supplement each other when responding to hypoxic challenge.

Nitrite and S -Nitrosohemoglobin Exchange Across the Human Cerebral and Femoral Circulation

Background:

The mechanisms underlying red blood cell (RBC)–mediated hypoxic vasodilation remain controversial, with separate roles for nitrite ( ) and S -nitrosohemoglobin (SNO-Hb) widely contested given their ability to transduce nitric oxide bioactivity within the microcirculation. To establish their relative contribution in vivo, we quantified arterial-venous concentration gradients across the human cerebral and femoral circulation at rest and during exercise, an ideal model system characterized by physiological extremes of O 2 tension and blood flow.

Methods:

Ten healthy participants (5 men, 5 women) aged 24±4 (mean±SD) years old were randomly assigned to a normoxic (21% O 2 ) and hypoxic (10% O 2 ) trial with measurements performed at rest and after 30 minutes of cycling at 70% of maximal power output in hypoxia and equivalent relative and absolute intensities in normoxia. Blood was sampled simultaneously from the brachial artery and internal jugular and femoral veins with plasma and RBC nitric oxide metabolites measured by tri-iodide reductive chemiluminescence. Blood flow was determined by transcranial Doppler ultrasound (cerebral blood flow) and constant infusion thermodilution (femoral blood flow) with net exchange calculated via the Fick principle.

Results:

Hypoxia was associated with a mild increase in both cerebral blood flow and femoral blood flow ( P <0.05 versus normoxia) with further, more pronounced increases observed in femoral blood flow during exercise ( P <0.05 versus rest) in proportion to the reduction in RBC oxygenation ( r =0.680–0.769, P <0.001). Plasma gradients reflecting consumption (arterial>venous; P <0.05) were accompanied by RBC iron nitrosylhemoglobin formation (venous>arterial; P <0.05) at rest in normoxia, during hypoxia ( P <0.05 versus normoxia), and especially during exercise ( P <0.05 versus rest), with the most pronounced gradients observed across the bioenergetically more active, hypoxemic, and acidotic femoral circulation ( P <0.05 versus cerebral). In contrast, we failed to observe any gradients consistent with RBC SNO-Hb consumption and corresponding delivery of plasma S -nitrosothiols ( P >0.05).

Conclusions:

These findings suggest that hypoxia and, to a far greater extent, exercise independently promote arterial-venous delivery gradients of intravascular nitric oxide, with deoxyhemoglobin-mediated reduction identified as the dominant mechanism underlying hypoxic vasodilation.

Predicting blood flow responses to rhythmic handgrip exercise from one second isometric contractions

The aim of this work was to predict blood flow responses to rhythmic handgrip exercise from one second isometric contractions. Seven healthy men were studied. Each subject performed a single 1 s handgrip contraction at 10 %, 20 % and 40 % of the maximum handgrip strength. We then repeatedly summed hyperaemic responses from single contractions to predict hyperaemic response to a prolonged bout of rhythmic exercise. There was similarity between steady state brachial blood flow velocity (BBV) extrapolated from single handgrip contractions and during 2 min of rhythmic exercise for 20 % (10.0+/-3.8 cm/s vs. 10.2+/-2.6 cm/s, r=0.93, p=0.003) and 40 % of maximum contractions (14.2+/-5.5 cm/s vs. 15.6+/-3.4 cm/s, r=0.88, p=0.009), but not for 10 % (7.5+/-4.1 cm/s vs. 5.7+/-3.3 cm/s, r=0.94, p=0.018). BBV progressively rose substantially higher during rhythmic contractions than peak BBV observed during single contractions at matched intensity. Respective peak BBV during single contractions and steady state BBV rhythmic contractions were 4.4+/-2.1 and 5.7+/-3.3 cm.s(-1) at 10 % forearm strength (p=0.14), 5.6+/-2.4 and 10.2+/-2.8 cm.s(-1) at 20 % (p=0.002), and 7.0+/-2.5 and 15.6+/-3.6 cm.s(-1) at 40 % (p=0.003). In conclusion, there is similarity between the summated blood flow velocity calculated from a single 1 s muscle contraction and the steady state blood flow velocity response of rhythmic exercise.

Joint blood flow is more sensitive to inflammatory arthritis than oxyhemoglobin, deoxyhemoglobin, and oxygen saturation

Joint hypoxia plays a central role in the progression and perpetuation of rheumatoid arthritis (RA). Thus, optical techniques that can measure surrogate markers of hypoxia such as blood flow, oxyhemoglobin, deoxyhemoglobin, and oxygen saturation are being developed to monitor RA. The purpose of the current study was to compare the sensitivity of these physiological parameters to arthritis. Experiments were conducted in a rabbit model of RA and the results revealed that joint blood flow was the most sensitive to arthritis and could detect a statistically significant difference (p<0.05, power = 0.8) between inflamed and healthy joints with a sample size of only four subjects. Considering that this a quantitative technique, the high sensitivity to arthritis suggests that joint perfusion has the potential to become a potent tool for monitoring disease progression and treatment response in RA.

Prolonged adenosine triphosphate infusion and exercise hyperemia in humans

In humans, intra-arterial ATP infusion in limbs mimics many features of exercise hyperemia. However, it remains unknown whether ATP can evoke the prolonged vasodilation seen during exercise. Therefore, we addressed two questions during a continuous 3-h brachial artery infusion of ATP [20 μg·100 ml forearm volume (FAV)(-1)·min(-1)]: 1) would skeletal muscle blood flow remain robust or wane over time (tachyphylaxis); and 2) would the hyperemic response to moderate-intensity exercise performed during the ATP administration be blunted compared with that during control (saline) infusion. Nine participants (25 ± 1 yr) performed one trial consisting of seven bouts of rhythmic handgrip exercise (20 contractions/min at 20% of maximum), two bouts during saline (control), and five bouts during 180 min of continuous ATP infusion. Five minutes of ATP infusion resulted in a 710% increase in forearm vascular conductance (FVC) from control (4.8 ± 0.77 vs. 35.0 ± 5.7 ml·min(-1)·100 mmHg(-1)·dl FAV(-1), P < 0.05). Contrary to our expectations, FVC did not wane over time with values of 35.0 ± 5.7 and 36.0 ± 7.7 ml·min(-1)·100 mmHg(-1)·dl FAV(-1) (P > 0.05), seen prior to the exercise bouts at 5 vs. 150 min, respectively. During superimposed exercise, FVC increased from 35.0 ± 5.7 to 49.6 ± 5.4 ml·min(-1)·100 mmHg(-1)·dl FAV(-1) at 5 min and 36.0 ± 7.7 to 54.5 ± 5.0 at 150 min (P < 0.05). Our findings demonstrate ATP vasodilation is prolonged over time without tachyphylaxis; however, exercise hyperemia responses remain intact. Our results challenge the metabolic theory of exercise hyperemia, suggesting a disconnect between matching of blood flow and metabolic demand.

Exercise capacity and cardiac hemodynamic response in female ApoE/LDLR(-/-) mice: a paradox of preserved V'O2max and exercise capacity despite coronary atherosclerosis

We assessed exercise performance, coronary blood flow and cardiac reserve of female ApoE/LDLR^−/− mice with advanced atherosclerosis compared with age-matched, wild-type C57BL6/J mice. Exercise capacity was assessed as whole body maximal oxygen consumption (V’O_2max), maximum running velocity (v_max) and maximum distance (DIST_max) during treadmill exercise. Cardiac systolic and diastolic function in basal conditions and in response to dobutamine (mimicking exercise-induced cardiac stress) were assessed by Magnetic Resonance Imaging (MRI) in vivo. Function of coronary circulation was assessed in isolated perfused hearts. In female ApoE/LDLR^−/− mice V’O_2max, v_max and DIST_max were not impaired as compared with C57BL6/J mice. Cardiac function at rest and systolic and diastolic cardiac reserve were also preserved in female ApoE/LDLR^−/− mice as evidenced by preserved fractional area change and similar fall in systolic and end diastolic area after dobutamine. Moreover, endothelium-dependent responses of coronary circulation induced by bradykinin (Bk) and acetylcholine (ACh) were preserved, while endothelium-independent responses induced by NO-donors were augmented in female ApoE/LDLR^−/− mice. Basal COX-2-dependent production of 6-keto-PGF_1α was increased. Concluding, we suggest that robust compensatory mechanisms in coronary circulation involving PGI₂- and NO-pathways may efficiently counterbalance coronary atherosclerosis-induced impairment in V’O_2max and exercise capacity.

The role of perfusion in the oxygen extraction capability of skin and skeletal muscle

Oxygen extraction (OE) by all cells is dependent on an adequate supply of oxygen in proximal blood vessels and the cell's need and ability to uptake that oxygen. Here the role of blood flow in regulating OE in skin and skeletal muscle was investigated in lean and obese men. OE was derived by two optical reflectance spectroscopy techniques: 1) from the rate of fall in mean blood saturation during a 4 min below knee arterial occlusion, and thus no blood flow, in calf skin and skeletal muscle and 2) in perfused, unperturbed skin, using the spontaneous falls in mean blood saturation induced by vasomotion in calf and forearm skin of 24 subjects, 12 lean and 12 obese. OE in perfused skin was significantly higher in lean compared with obese subjects in forearm (Mann-Whitney, P < 0.004) and calf (P < 0.001) and did not correlate with OE in unperfused skin (ρ = -0.01, P = 0.48). With arterial occlusion and thus no blood flow, skin OE in lean and obese subjects no longer differed (P = 0.23, not significant). In contrast in skeletal muscle with arterial occlusion and no blood flow, the difference in OE between lean and obese subjects occurred, with obese subjects exhibiting significantly higher OE (P < 0.012). The classic model of metabolic blood flow regulation to support oxygen extraction is evident in perfused skin; OE is perturbed without blood flow and reduced in obesity. In resting skeletal muscle other mechanism(s), independent of blood flow, are implicated in oxygen extraction.

The increasingly complex regulation of adipocyte differentiation

Adipose (AD) tissue development and function relies on the ability of adipocytes to proliferate and differentiate into lipid-containing cells that also have endocrine function. Research suggests that certain conditions can induce AD tissue stem cells to differentiate into various cell types and that the microenvironment of the cell, including the extracellular matrix (ECM), is essential in maintaining cell and tissue function. This review provides an overview of factors involved in the proliferation and differentiation of adipocytes. A brief review of the numerous factors that influence PPARγ, the transcription factor thought to be the master regulator of adipocyte differentiation, provides context of established pathways that regulate adipogenesis. Thought provoking findings from research with hypoxia that is supported by earlier research that vascular development is related to adipogenesis are reviewed. Finally, our understanding of the critical role of the ECM and environment in adipogenesis is discussed and compared with studies that suggest that adipocytes may dedifferentiate and can convert into other cell types.

Regulation of Vascular and Renal Function by Metabolite Receptors

To maintain metabolic homeostasis, the body must be able to monitor the concentration of a large number of substances, including metabolites, in real time and to use that information to regulate the activities of different metabolic pathways. Such regulation is achieved by the presence of sensors, termed metabolite receptors, in various tissues and cells of the body, which in turn convey the information to appropriate regulatory or positive or negative feedback systems. In this review, we cover the unique roles of metabolite receptors in renal and vascular function. These receptors play a wide variety of important roles in maintaining various aspects of homeostasis-from salt and water balance to metabolism-by sensing metabolites from a wide variety of sources. We discuss the role of metabolite sensors in sensing metabolites generated locally, metabolites generated at distant tissues or organs, or even metabolites generated by resident microbes. Metabolite receptors are also involved in various pathophysiological conditions and are being recognized as potential targets for new drugs. By highlighting three receptor families-(a) citric acid cycle intermediate receptors, (b) purinergic receptors, and

Beneficial and detrimental role of adenosine signaling in diseases and therapy

Adenosine is a major signaling nucleoside that orchestrates cellular and tissue adaptation under energy depletion and ischemic/hypoxic conditions by activation of four G protein-coupled receptors (GPCR). The regulation and generation of extracellular adenosine in response to stress are critical in tissue protection. Both mouse and human studies reported that extracellular adenosine signaling plays a beneficial role during acute states. However, prolonged excess extracellular adenosine is detrimental and contributes to the development and progression of various chronic diseases. In recent years, substantial progress has been made to understand the role of adenosine signaling in different conditions and to clarify its significance during the course of disease progression in various organs. These efforts have and will identify potential therapeutic possibilities for protection of tissue injury at acute stage by upregulation of adenosine signaling or attenuation of chronic disease progression by downregulation of adenosine signaling. This review is to summarize current progress and the importance of adenosine signaling in different disease stages and its potential therapeutic effects.

Coronary and muscle blood flow during physical exercise in humans; heterogenic alliance

In this review, we present the relation between power generation capabilities and pulmonary oxygen uptake during incremental cycling exercise in humans and the effect of exercise intensity on the oxygen cost of work. We also discuss the importance of oxygen delivery to the working muscles as a factor determining maximal oxygen uptake in humans. Subsequently, we outline the importance of coronary blood flow, myocardial oxygen uptake and myocardial metabolic stability for exercise tolerance. Finally, we describe mechanisms of endothelium-dependent regulation of coronary and skeletal muscle blood flow, dysregulation of which may impair exercise capacity and increase the cardiovascular risk of exercise.

Skeletal muscle vasodilation during systemic hypoxia in humans

In humans, the net effect of acute systemic hypoxia in quiescent skeletal muscle is vasodilation despite significant reflex increases in muscle sympathetic vasoconstrictor nerve activity. This vasodilation increases tissue perfusion and oxygen delivery to maintain tissue oxygen consumption. Although several mechanisms may be involved, we recently tested the roles of two endothelial-derived substances during conditions of sympathoadrenal blockade to isolate local vascular control mechanisms: nitric oxide (NO) and prostaglandins (PGs). Our findings indicate that 1) NO normally plays a role in regulating vascular tone during hypoxia independent of the PG pathway; 2) PGs do not normally contribute to vascular tone during hypoxia, however, they do affect vascular tone when NO is inhibited; 3) NO and PGs are not independently obligatory to observe hypoxic vasodilation when assessed as a response from rest to steady-state hypoxia; and 4) combined NO and PG inhibition abolishes hypoxic vasodilation in human skeletal muscle. When the stimulus is exacerbated via combined submaximal rhythmic exercise and systemic hypoxia to cause further red blood cell (RBC) deoxygenation, skeletal muscle blood flow is augmented compared with normoxic exercise via local dilator mechanisms to maintain oxygen delivery to active tissue. Data obtained in a follow-up study indicate that combined NO and PG inhibition during hypoxic exercise blunts augmented vasodilation and hyperemia compared with control (normoxic) conditions by ∼50%; however, in contrast to hypoxia alone, the response is not abolished, suggesting that other local substances are involved. Factors associated with greater RBC deoxygenation such as ATP release, or nitrite reduction to NO, or both likely play a role in regulating this response.

Modulating the vascular response to limb ischemia: angiogenic and cell therapies

The age-adjusted prevalence of peripheral arterial disease in the US population has been estimated to approach 12%. The clinical consequences of occlusive peripheral arterial disease include pain on walking (claudication), pain at rest, and loss of tissue integrity in the distal limbs; the latter may ultimately lead to amputation of a portion of the lower extremity. Surgical bypass techniques and percutaneous catheter-based interventions may successfully reperfuse the limbs of certain patients with peripheral arterial disease. In many patients, however, the anatomic extent and distribution of arterial occlusion is too severe to permit relief of pain and healing of ischemic ulcers. No effective medical therapy is available for the treatment of such patients, for many of whom amputation represents the only hope for alleviation of symptoms. The ultimate failure of medical treatment and procedural revascularization in significant numbers of patients has led to attempts to develop alternative therapies for ischemic disease. These strategies include administration of angiogenic cytokines, either as recombinant protein or as gene therapy, and more recently, to investigations of stem/progenitor cell therapy. The purpose of this review is to provide an outline of the preclinical basis for angiogenic and stem cell therapies, review the clinical research that has been done, summarize the lessons learned, identify gaps in knowledge, and suggest a course toward successfully addressing an unmet medical need in a large and growing patient population.

Neuronal nitric oxide synthase is dislocated in type I fibers of myalgic muscle but can recover with physical exercise training

Trapezius myalgia is the most common type of chronic neck pain. While physical exercise reduces pain and improves muscle function, the underlying mechanisms remain unclear. Nitric oxide (NO) signaling is important in modulating cellular function, and a dysfunctional neuronal NO synthase (nNOS) may contribute to an ineffective muscle function. This study investigated nNOS expression and localization in chronically painful muscle. Forty-one women clinically diagnosed with trapezius myalgia (MYA) and 18 healthy controls (CON) were included in the case-control study. Subsequently, MYA were randomly assigned to either 10 weeks of specific strength training (SST, n = 18), general fitness training (GFT, n = 15), or health information (REF, n = 8). Distribution of fiber type, cross-sectional area, and sarcolemmal nNOS expression did not differ between MYA and CON. However, MYA showed increased sarcoplasmic nNOS localization (18.8 ± 12 versus 12.8 ± 8%, P = 0.049) compared with CON. SST resulted in a decrease of sarcoplasm-localized nNOS following training (before 18.1 ± 12 versus after 12.0 ± 12%; P = 0,027). We demonstrate that myalgic muscle displays altered nNOS localization and that 10 weeks of strength training normalize these disruptions, which supports previous findings of impaired muscle oxygenation during work tasks and reduced pain following exercise.