Metabolic modulation of sympathetic vasoconstriction in human skeletal muscle: role of tissue hypoxia

Impact of acute hypoxic exposure on neuromuscular and hemodynamic responses during step intensity dynamic constant external resistance leg extension exercise

OBJECTIVES

This study examined the effects of acute normoxic and hypoxic exposure on neuromuscular and hemodynamic physiological responses performed during dynamic step muscle actions.

METHODS

Thirteen recreationally active men (mean ± SD age: 21.2 ± 2.9 yrs) performed dynamic leg extensions unilaterally under Normoxic (FiO2 = 21 %) and Hypoxic (FiO2 = 13 %) conditions in a randomized order at 20 %, 40 %, 60 %, 80 %, and 100 % of their maximal strength. Electromyographic (EMG) amplitude, EMG frequency, (Oxygenated and Deoxygenated hemoglobin; OxyHb, DeoxyHb), Total hemoglobin (TotalHb), and skeletal muscle tissue oxygenation status (StO2) were measured from the vastus lateralis during all contractions.

RESULTS

There were no detectable differences in the neuromuscular responses between normoxia and hypoxia for EMG amplitude (p = 0.37-0.74) and frequency (p = 0.17-0.83). For EMG amplitude there were general increases with intensity (p < 0.01-0.03). EMG frequency remained similar from 20% to 80% and then increased at 100 % effort (p = 0.02). There was no significant difference in patterns of responses for OxyHb (p = 0.870) and TotalHb (p = 0.200) between normoxia and hypoxia. StO2 (p = 0.028) decreased and DeoxyHb (p = 0.006) increased under hypoxia compared to normoxia during dynamic step muscle actions performed in a randomized order.

CONCLUSION

Unlike fatigue, acute hypoxemia in an unfatigued state does not impact the localized neuromuscular responses, but minimally impacts the hemodynamic responses.

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.

Global REACH 2018: increased adrenergic restraint of blood flow preserves coupling of oxygen delivery and demand during exercise at high-altitude

Chronic exposure to hypoxia (high-altitude, HA; >4000 m) attenuates the vasodilatory response to exercise and is associated with a persistent increase in basal sympathetic nerve activity (SNA). The mechanism(s) responsible for the reduced vasodilatation and exercise hyperaemia at HA remains unknown. We hypothesized that heightened adrenergic signalling restrains skeletal muscle blood flow during handgrip exercise in lowlanders acclimatizing to HA. We tested nine adult males (n = 9) at sea-level (SL; 344 m) and following 21-28 days at HA (∼4300 m). Forearm blood flow (FBF; duplex ultrasonography), mean arterial pressure (MAP; brachial artery catheter), forearm vascular conductance (FVC; FBF/MAP), and arterial and venous blood sampling (O₂ delivery ( DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ ) and uptake ( V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ )) were measured at rest and during graded rhythmic handgrip exercise (5%, 15% and 25% of maximum voluntary isometric contraction; MVC) before and after local α- and β-adrenergic blockade (intra-arterial phentolamine and propranolol). HA reduced ΔFBF (25% MVC: SL: 138.3 ± 47.6 vs. HA: 113.4 ± 37.1 ml min^-1 ; P = 0.022) and Δ V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ (25% MVC: SL: 20.3 ± 7.5 vs. HA: 14.3 ± 6.2 ml min^-1 ; P = 0.014) during exercise. Local adrenoreceptor blockade at HA restored FBF during exercise (25% MVC: SL_{α-β blockade} : 164.1 ± 71.7 vs. HA_{α-β blockade} : 185.4 ± 66.6 ml min^-1 ; P = 0.947) but resulted in an exaggerated relationship between DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ and V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ( DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ / V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slope: SL: 1.32; HA: slope: 1.86; P = 0.037). These results indicate that tonic adrenergic signalling restrains exercise hyperaemia in lowlanders acclimatizing to HA. The increase in adrenergic restraint is necessary to match oxygen delivery to demand and prevent over perfusion of contracting muscle at HA. KEY POINTS: In exercising skeletal muscle, local vasodilatory signalling and sympathetic vasoconstriction integrate to match oxygen delivery to demand and maintain arterial blood pressure. Exposure to chronic hypoxia (altitude, >4000 m) causes a persistent increase in sympathetic nervous system activity that is associated with impaired functional capacity and diminished vasodilatation during exercise. In healthy male lowlanders exposed to chronic hypoxia (21-28 days; ∼4300 m), local adrenoreceptor blockade (combined α- and β-adrenergic blockade) restored skeletal muscle blood flow during handgrip exercise. However, removal of tonic adrenergic restraint at high altitude caused an excessive rise in blood flow and subsequently oxygen delivery for any given metabolic demand. This investigation is the first to identify greater adrenergic restraint of blood flow during acclimatization to high altitude and provides evidence of a functional role for this adaptive response in regulating oxygen delivery and demand.

Sex-differences in the sympathetic neurocirculatory responses to chemoreflex activation

Abstract

The purpose of this study was to determine whether there are sex differences in the cardiorespiratory and sympathetic neurocirculatory responses to central, peripheral, and combined central and peripheral chemoreflex activation. Ten women (29 ± 6 years, 22.8 ± 2.4 kg/m²: mean ± SD) and 10 men (30 ± 7 years, 24.8 ± 3.2 kg/m²) undertook randomized 5 min breathing trials of: room air (eucapnia), isocapnic hypoxia (10% oxygen (O₂); peripheral chemoreflex activation), hypercapnic hyperoxia (7% carbon dioxide (CO₂), 50% O₂; central chemoreflex activation) and hypercapnic hypoxia (7% CO₂, 10% O₂; central and peripheral chemoreflex activation). Control trials of isocapnic hyperoxia (peripheral chemoreflex inhibition) and hypocapnic hyperoxia (central and peripheral chemoreflex inhibition) were also included. Muscle sympathetic nerve activity (MSNA; microneurography), mean arterial pressure (MAP; finger photoplethysmography) and minute ventilation (V˙_E; pneumotachometer) were measured. Total MSNA (P = 1.000 and P = 0.616), MAP (P = 0.265) and V˙_E (P = 0.587 and P = 0.472) were not different in men and women during eucapnia and during isocapnic hypoxia. Women exhibited attenuated increases in V˙_E during hypercapnic hyperoxia (27.3 ± 6.3 vs. 39.5 ± 7.5 l/min, P < 0.0001) and hypercapnic hypoxia (40.9 ± 9.1 vs. 53.8 ± 13.3 l/min, P < 0.0001) compared with men. However, total MSNA responses were augmented in women (hypercapnic hyperoxia 378 ± 215 vs. 258 ± 107%, P = 0.017; hypercapnic hypoxia 607 ± 290 vs. 362 ± 268%, P < 0.0001). No sex differences in total MSNA, MAP or V˙_E were observed during isocapnic hyperoxia and hypocapnic hyperoxia. Our results indicate that young women have augmented sympathetic responses to central chemoreflex activation, which explains the augmented MSNA response to combined central and peripheral chemoreflex activation.

Key points

Sex differences in the control of breathing have been well studied, but whether there are differences in the sympathetic neurocirculatory responses to chemoreflex activation between healthy women and men is incompletely understood.

We observed that, compared with young men, young women displayed augmented increases in muscle sympathetic nerve activity during both hypercapnic hyperoxia (central chemoreflex activation) and hypercapnic hypoxia (central and peripheral chemoreflex activation) but had attenuated increases in minute ventilation.

In contrast, no sex differences were found in either muscle sympathetic nerve activity or minute ventilation responses to isocapnic hypoxia (peripheral chemoreceptor stimulation).

Young women have blunted ventilator, but augmented sympathetic responses, to central (hypercapnic hyperoxia) and combined central and peripheral chemoreflex activation (hypercapnic hypoxia), compared with young men. The possible causative association between the reduced ventilation and heightened sympathetic responses in young women awaits validation.

Longitudinal Changes and Recovery in Heart Rate Variability of Young Healthy Subjects When Exposure to a Hypobaric Hypoxic Environment

Background: The autonomic nervous system (ANS) is crucial for acclimatization. Investigating the responses of acute exposure to a hypoxic environment may provide some knowledge of the cardiopulmonary system’s adjustment mechanism.

Objective: The present study investigates the longitudinal changes and recovery in heart rate variability (HRV) in a young healthy population when exposed to a simulated plateau environment.

Methods: The study followed a strict experimental paradigm in which physiological signals were collected from 33 healthy college students (26 ± 2 years, 171 cm ± 7 cm, 64 ± 11 kg) using a medical-grade wearable device. The subjects were asked to sit in normoxic (approximately 101 kPa) and hypoxic (4,000 m above sea level, about 62 kPa) environments. The whole experimental process was divided into four stable resting measurement segments in chronological order to analyze the longitudinal changes of physical stress and recovery phases. Seventy-six time-domain, frequency-domain, and non-linear indicators characterizing rhythm variability were analyzed in the four groups.

Results: Compared to normobaric normoxia, participants in hypobaric hypoxia had significantly lower HRV time-domain metrics, such as RMSSD, MeanNN, and MedianNN (p &lt; 0.01), substantially higher frequency domain metrics such as LF/HF ratio (p &lt; 0.05), significantly lower Poincaré plot parameters such as SD1/SD2 ratio and other Poincaré plot parameters are reduced considerably (p &lt; 0.01), and Refined Composite Multi-Scale Entropy (RCMSE) curves are reduced significantly (p &lt; 0.01).

Conclusion: The present study shows that elevated heart rates, sympathetic activation, and reduced overall complexity were observed in healthy subjects exposed to a hypobaric and hypoxic environment. Moreover, the results indicated that Multiscale Entropy (MSE) analysis of RR interval series could characterize the degree of minor physiological changes. This novel index of HRV can better explain changes in the human ANS.

Orthostatic Resiliency During Successive Hypoxic, Hypoxic Orthostatic Challenge: Successful vs. Unsuccessful Cardiovascular and Oxygenation Strategies

Introduction: Rapid environmental changes, such as successive hypoxic-hypoxic orthostatic challenges (SHHOC) occur in the aerospace environment, and the ability to remain orthostatically resilient (OR) relies upon orchestration of physiological counter-responses. Counter-responses adjusting for hypoxia may conflict with orthostatic responses, and a misorchestration can lead to orthostatic intolerance (OI). The goal of this study was to pinpoint specific cardiovascular and oxygenation factors associated with OR during a simulated SHHOC.

Methods: Thirty one men underwent a simulated SHHOC consisting of baseline (P0), normobaric hypoxia (Fi02 = 12%, P1), and max 60 s of hypoxic lower body negative pressure (LBNP, P2). Alongside anthropometric variables, non-invasive cardiovascular, central and peripheral tissue oxygenation parameters, were recorded. OI was defined as hemodynamic collapse during SHHOC. Comparison of anthropometric, cardiovascular, and oxygenation parameters between OR and OI was performed via Student’s t-test. Within groups, a repeated measures ANOVA test with Holm-Sidak post hoc test was performed. Performance diagnostics were performed to assess factors associated with OR/OI (sensitivity, specificity, positive predictive value PPV, and odd’s ratio OR).

Results: Only 9/31 were OR, and 22/31 were OI. OR had significantly greater body mass index (BMI), weight, peripheral Sp02, longer R-R Interval (RRI) and lower heart rate (HR) at P0. During P1 OR exhibited significantly higher cardiac index (CI), stroke volume index (SVI), and lower systemic vascular resistance index (SVRI) than OI. Both groups exhibited a significant decrease in cerebral oxygenation (TOIc) with an increase in cerebral deoxygenated hemoglobin (dHbc), while the OI group showed a significant decrease in cerebral oxygenated hemoglobin (02Hbc) and peripheral oxygenation (TOIp) with an increase in peripheral deoxygenated hemoglobin (dHbp). During P2, OR maintained significantly greater CI, systolic, mean, and diastolic pressure (SAP, MAP, DAP), with a shortened RRI compared to the OI group, while central and peripheral oxygenation were not different. Body weight and BMI both showed high sensitivity (0.95), low specificity (0.33), a PPV of 0.78, with an OR of 0.92, and 0.61. P0 RRI showed a sensitivity of 0.95, specificity of 0.22, PPV 0.75, and OR of 0.99. Delta SVI had the highest performance diagnostics during P1 (sensitivity 0.91, specificity 0.44, PPV 0.79, and OR 0.8). Delta SAP had the highest overall performance diagnostics for P2 (sensitivity 0.95, specificity 0.67, PPV 0.87, and OR 0.9).

Discussion: Maintaining OR during SHHOC is reliant upon greater BMI, body weight, longer RRI, and lower HR at baseline, while increasing CI and SVI, minimizing peripheral 02 utilization and decreasing SVRI during hypoxia. During hypoxic LBNP, the ability to remain OR is dependent upon maintaining SAP, via CI increases rather than SVRI. Cerebral oxygenation parameters, beyond 02Hbc during P1 did not differ between groups, suggesting that the during acute hypoxia, an increase in cerebral 02 consumption, coupled with increased peripheral 02 utilization does seem to play a role in OI risk during SHHOC. However, cardiovascular factors such as SVI are of more value in assessing OR/OI risk. The results can be used to implement effective aerospace crew physiological monitoring strategies.

The influence of short-term high-altitude acclimatization on cerebral and leg tissue oxygenation post-orthostasis

PURPOSE

Orthostasis at sea level decreases brain tissue oxygenation and increases risk of syncope. High altitude reduces brain and peripheral muscle tissue oxygenation. This study determined the effect of short-term altitude acclimatization on cerebral and peripheral leg tissue oxygenation index (TOI) post-orthostasis.

METHOD

Seven lowlanders completed a supine-to-stand maneuver at sea level (450 m) and for 3 consecutive days at high altitude (3776 m). Cardiorespiratory measurements and near-infrared spectroscopy-derived oxygenation of the frontal lobe (cerebral TOI) and vastus lateralis (leg TOI) were measured at supine and 5-min post-orthostasis.

RESULTS

After orthostasis at sea level, cerebral TOI decreased [mean Δ% (95% confidential interval): - 4.5%, (- 7.5, - 1.5), P < 0.001], whilst leg TOI was unchanged [- 4.6%, (- 10.9, 1.7), P = 0.42]. High altitude had no effect on cerebral TOI following orthostasis [days 1-3: - 2.3%, (- 5.3, 0.7); - 2.4%, (- 5.4, 0.6); - 2.1%, (- 5.1, 0.9), respectively, all P > 0.05], whereas leg TOI decreased [days 1-3: - 12.0%, (- 18.3, - 5.7); - 12.1%, (- 18.4, - 5.8); - 10.2%, (- 16.5, - 3.9), respectively, all P < 0.001]. This response did not differ with days spent at high altitude, despite evidence of cardiorespiratory acclimatization [increased peripheral oxygen saturation (supine: P = 0.01; stand: P = 0.02) and decreased end-tidal carbon dioxide (supine: P = 0.003; stand: P = 0.01)].

CONCLUSION

Cerebral oxygenation is preferentially maintained over leg oxygenation post-orthostasis at high altitude, suggesting different vascular regulation between cerebral and peripheral circulations. Short-term acclimatization to high altitude did not alter cerebral and leg oxygenation responses to orthostasis.

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.

Distinct cardiovascular and respiratory responses to short-term sustained hypoxia in juvenile Sprague Dawley and Wistar Hannover rats

Short-term sustained hypoxia (SH) elicits active expiration, augmented late-expiratory (late-E) sympathetic activity, increased arterial pressure and ventilation, and amplified sympathetic and abdominal expiratory responses to chemoreflex activation in rats of the Wistar-Ribeirão Preto (WRP) strain. Herein, we investigated whether SH can differentially affect the cardiovascular and respiratory outcomes of Sprague-Dawley (SD) and Wistar Hannover (WH) rats and compared the results with previous data using WRP rats. For this, we exposed SD and WH rats to SH (FiO₂ = 0.1) for 24 h and evaluated arterial pressure, sympathetic activity, and respiratory pattern. SD rats presented increased arterial pressure, respiratory rate and tidal volume, as well as augmented late-E expiratory motor output and increased sympathetic outflow due to post-inspiratory and late-E sympathetic overactivity. WH rats presented reduced changes, suggesting lower responsiveness of this strain to this SH protocol. The magnitudes of changes in sympathetic and abdominal expiratory motor activities to chemoreflex activation in SD rats were reduced by SH. Pressor responses to chemoreflex activation were shown to be blunted in SD and WH rats after SH. The data are showing that SD, WH, and WRP rat strains exhibit marked differences in their cardiovascular, autonomic and respiratory responses to 24-h SH and draw attention to the importance of rat strain for studies exploring the underlying mechanisms involved in the neuronal changes induced by the experimental model of SH.

Effects of home-based inspiratory muscle training on sickle cell disease (SCD) patients

Introduction

Inspiratory muscle training (IMT) has been shown to be an efficient method of improving exercise tolerance and inspiratory and expiratory muscle strength in several diseases. The effects of IMT on patients with sickle cell anemia (SCD) are relatively unknown. Our study aimed to evaluate the effects of IMT on adult SCD patients, regarding respiratory muscle strength (RMS) variables, lung function, exercise tolerance, blood lactation concentration, limitation imposed by dyspnea during daily activities and impact of fatigue on the quality of life.

Methods

This was a randomized single-blind study, with an IMT design comprising true load (TG) and sham load (SG) groups. Initial assessment included spirometry, volumetric capnography (VCap) and measurement of RMS by maximal inspiratory and expiratory pressure (PImax and PEmax). The Medical Research Council dyspnea scale and modified fatigue impact scale were also applied and blood lactate concentration was measured before and after the 6-minute walk test. After this initial assessment, the patient used the IMT device at home daily, returning every 6 weeks for RMS reassessment. Both groups used the same device and were unaware of which group they were in. After a period totaling 18 weeks, patients underwent the final evaluation, as initially performed.

Results

Twenty-five patients in total participated until the end of the study (median age 42 years). There were no significant differences between TG and SG based on age, sex, body mass index or severity of genotype. At the end of the training, both groups showed a significant increase in PEmax and PImax, improvement in Vcap and in exercise tolerance and dyspnea reduction while performing daily life activities. The same was observed in patients grouped according to disease severity (HbSS and HbSβ⁰vs HbSC and HbSβ⁺), without differences between groups.

Conclusion

Home-based inspiratory muscle training benefits outpatients with SCD, including the sham load group.

Trial registration

http://www.ensaiosclinicos.gov.br; registration number: RBR-6g8n92.

The effects of hypoxia on muscle deoxygenation and recruitment in the flexor digitorum superficialis during submaximal intermittent handgrip exercise

Background

Decreased oxygenation of muscle may be accentuated during exercise at high altitude. Monitoring the oxygen saturation of muscle (SmO₂) during hand grip exercise using near infrared spectroscopy during acute exposure to hypoxia could provide a model for a test of muscle performance without the competing cardiovascular stresses that occur during a cycle ergometer or treadmill test. The purpose of this study was to examine and compare acute exposure to normobaric hypoxia versus normoxia on deoxygenation and recruitment of the flexor digitorum superficialis (FDS) during submaximal intermittent handgrip exercise (HGE) in healthy adults.

Methods

Twenty subjects (11 M/9 F) performed HGE at 50% of maximum voluntary contraction, with a duty cycle of 2 s:1 s until task failure on two occasions one week apart, randomly assigned to normobaric hypoxia (FiO₂ = 12%) or normoxia (FiO₂ = 21%). Near-infrared spectroscopy monitored SmO₂, oxygenated (O₂Hb), deoxygenated (HHb), and total hemoglobin (tHb) over the FDS. Surface electromyography derived root mean square and mean power frequency of the FDS.

Results

Hypoxic compared to normoxic HGE induced a lower FDS SmO₂ (63.8 ± 2.2 vs. 69.0 ± 1.5, p = 0.001) and both protocols decreased FDS SmO₂ from baseline to task failure. FDS mean power frequency was lower during hypoxic compared to normoxic HGE (64.0 ± 1.4 vs. 68.2 ± 2.0 Hz, p = 0.04) and both decreased mean power frequency from the first contractions to task failure (p = 0.000). Under both hypoxia and normoxia, HHb, tHb and root mean square increased from baseline to task failure whereas O₂Hb decreased and then increased during HGE. Arterial oxygen saturation via pulse oximetry (SpO₂) was lower during hypoxia compared to normoxia conditions (p = 0.000) and heart rate and diastolic blood pressure only demonstrated small increases. Task durations and the tension-time index of HGE did not differ between normoxic and hypoxic trials.

Conclusion

Hypoxic compared to normoxic HGE decreased SmO₂ and induced lower mean power frequency in the FDS, during repetitive hand grip exercise however did not result in differences in task durations or tension-time indices. The fiber type composition of FDS, and high duty cycle and intensity may have contributed greater dependence on anaerobiosis.

Preserved ability to blunt sympathetically-mediated vasoconstriction in exercising skeletal muscle of young obese humans

Sympathetic vasoconstriction is attenuated in exercising muscles to assist in matching of blood flow with metabolic demand. This "functional sympatholysis" may be impaired in young obese individuals due to greater sympathetic activation and/or reduced local vasodilatory capacity of both small and large arteries, but this remains poorly understood. We tested the hypothesis that functional sympatholysis is impaired in obese individuals compared with normal-weight counterparts. In 36 obese and normal-weight young healthy adults (n = 18/group), we measured forearm blood flow and calculated forearm vascular conductance (FVC) responses to reflex increases in sympathetic nerve activity induced by lower body negative pressure (LBNP) at rest and during rhythmic handgrip exercise at 15% and 30% of the maximal voluntary contraction (MVC). FVC was normalized to lean forearm mass. In normal-weight individuals, LBNP evoked a decrease in FVC (-16.1 ± 5.7%) in the resting forearm, and the reduction in FVC (15%MVC: -8.1 ± 3.3%; 30%MVC: -1.0 ± 4.0%) was blunted during exercise in an intensity-dependent manner (P < 0.05). Similarly, in obese individuals, LBNP evoked a comparable decrease in FVC (-10.9 ± 5.7%) in the resting forearm, with the reduction in FVC (15%MVC: -9.7 ± 3.3%; 30%MVC: -0.3 ± 4.0%) also blunted during exercise in an intensity-dependent manner (P < 0.05). The magnitude of sympatholysis was similar between groups (P > 0.05) and was intensity-dependent (P < 0.05). Our findings suggest that functional sympatholysis is not impaired in young obese individuals without overt cardiovascular diseases.

Putative Role of Respiratory Muscle Training to Improve Endurance Performance in Hypoxia: A Review

Respiratory/inspiratory muscle training (RMT/IMT) has been proposed to improve the endurance performance of athletes in normoxia. In recent years, due to the increased use of hypoxic training method among athletes, the RMT applicability has also been tested as a method to minimize adverse effects since hyperventilation may cause respiratory muscle fatigue during prolonged exercise in hypoxia. We performed a review in order to determine factors potentially affecting the change in endurance performance in hypoxia after RMT in healthy subjects. A comprehensive search was done in the electronic databases MEDLINE and Google Scholar including keywords: “RMT/IMT,” and/or “endurance performance,” and/or “altitude” and/or “hypoxia.” Seven appropriate studies were found until April 2018. Analysis of the studies showed that two RMT methods were used in the protocols: respiratory muscle endurance (RME) (isocapnic hyperpnea: commonly 10–30′, 3–5 d/week) in three of the seven studies, and respiratory muscle strength (RMS) (Powerbreathe device: commonly 2 × 30 reps at 50% MIP (maximal inspiratory pressure), 5–7 d/week) in the remaining four studies. The duration of the protocols ranged from 4 to 8 weeks, and it was found in synthesis that during exercise in hypoxia, RMT promoted (1) reduced respiratory muscle fatigue, (2) delayed respiratory muscle metaboreflex activation, (3) better maintenance of SaO₂ and blood flow to locomotor muscles. In general, no increases of maximal oxygen uptake (VO_2max) were described. Ventilatory function improvements (maximal inspiratory pressure) achieved by using RMT fostered the capacity to adapt to hypoxia and minimized the impact of respiratory stress during the acclimatization stage in comparison with placebo/sham. In conclusion, RMT was found to elicit general positive effects mainly on respiratory efficiency and breathing patterns, lower dyspneic perceptions and improved physical performance in conditions of hypoxia. Thus, this method is recommended to be used as a pre-exposure tool for strengthening respiratory muscles and minimizing the adverse effects caused by hypoxia related hyperventilation. Future studies will assess these effects in elite athletes.

Acute blood pressure responses after different isometric handgrip protocols in hypertensive patients

OBJECTIVE

The present study analyzed blood pressure responses after a single session of isometric handgrip exercise performed with different volumes and intensities by patients with hypertension.

METHODS

This randomized crossover trial submitted 12 hypertensive patients (58±5 years old) to four isometric handgrip exercise sessions in a random order: 4 x 2 min at 30% of the maximal voluntary contraction (S30%); 4 x 2 min at 50% of the maximal voluntary contraction (S50%2min); 4 x 3 min at 30% of the maximal voluntary contraction (S30%3min); and a control session. The systolic and diastolic blood pressure, heart rate, and rate-pressure product were measured pre- and post-exercise (30th min).

RESULTS

No significant changes were observed in cardiovascular variables after any session (p>0.05 for all comparisons). Similarly, individual analyses revealed heterogeneity in the responses, including increases in blood pressure observed in some sessions. Patients with reduced blood pressure after an isometric handgrip exercise session exhibited a higher body mass index, diastolic blood pressure and heart rate (p<0.05). They also tended to be younger (p=0.07).

CONCLUSION

Isometric handgrip exercise performed with different intensities and volumes did not reduce the blood pressure of hypertensive patients.

Muscle oxygenation profiles between active and inactive muscles with nitrate supplementation under hypoxic exercise

Whether dietary nitrate supplementation improves exercise performance or not is still controversial. While redistribution of sufficient oxygen from inactive to active muscles is essential for optimal exercise performance, no study investigated the effects of nitrate supplementation on muscle oxygenation profiles between active and inactive muscles. Nine healthy males performed 25 min of submaximal (heart rate ~140 bpm; EX_sub) and incremental cycling (EX_max) until exhaustion under three conditions: (A) normoxia without drink; (B) hypoxia (FiO₂ = 13.95%) with placebo (PL); and (c) hypoxia with beetroot juice (BR). PL and BR were provided for 4 days. Oxygenated and deoxygenated hemoglobin (HbO₂ and HHb) were measured in vastus lateralis (active) and biceps brachii (inactive) muscles, and the oxygen saturation of skeletal muscle (StO₂; HbO₂/total Hb) were calculated. During EX_sub, BR suppressed the HHb increases in active muscles during the last 5 min of exercise. During EX_max, time to exhaustion with BR (513 ± 24 sec) was significantly longer than with PL (490 ± 39 sec, P < 0.05). In active muscles, BR suppressed the HHb increases at moderate work rates during EX_max compared to PL (P < 0.05). In addition, BR supplementation was associated with greater reductions in HbO₂ and StO₂ at higher work rates in inactive muscles during EX_max. Collectively, these findings indicate that short‐term dietary nitrate supplementation improved hypoxic exercise tolerance, perhaps, due to suppressed increases in HHb in active muscles at moderate work rates. Moreover, nitrate supplementation caused greater reductions in oxygenation in inactive muscle at higher work rates during hypoxic exercise.

Hypoxia affects tissue oxygenation differently in the thigh and calf muscles during incremental running

PURPOSE

The present study was performed to determine the impact of hypoxia on working muscle oxygenation during incremental running, and to compare tissue oxygenation between the thigh and calf muscles.

METHODS

Nine distance runners and triathletes performed incremental running tests to exhaustion under normoxic and hypoxic conditions (fraction of inspired oxygen = 0.15). Peak pulmonary oxygen uptake ([Formula: see text]) and tissue oxygen saturation (StO₂) were measured simultaneously in both the vastus lateralis and medial gastrocnemius.

RESULTS

Hypoxia significantly decreased peak running speed and [Formula: see text] (p < 0.01). During incremental running, StO₂ in the vastus lateralis decreased almost linearly, and the rate of decrease from warm-up (180 m min^-1) to [Formula: see text] was significantly greater than in the medial gastrocnemius under both normoxic and hypoxic conditions (p < 0.01). StO₂ in both muscles was significantly decreased under hypoxic compared with normoxic conditions at all running speeds (p < 0.01). The rate at which StO₂ was decreased by hypoxia was greater in the vastus lateralis as the running speed increased, whereas it changed little in the medial gastrocnemius.

CONCLUSIONS

These results suggest that the thigh is more deoxygenated than the calf under hypoxic conditions, and that the effects of hypoxia on tissue oxygenation differ between these two muscles during incremental running.

Genetic determination of the vascular reactions in humans in response to the diving reflex

The purpose of this study was to investigate the genetic mechanisms of the defense vascular reactions in response to the diving reflex in humans with polymorphisms in the genes ADBR2, ACE, AGTR1, BDKRB2, and REN We hypothesized that protective vascular reactions, in response to the diving reflex, are genetically determined and are distinguished in humans with gene polymorphisms of the renin-angiotensin and kinin-bradykinin system. A total of 80 subjects (19 ± 1.4 yr) participated in the study. The intensity of the vascular response was estimated using photoplethysmogram. The I/D polymorphism (rs4340) of ACE was analyzed by PCR. REN (G/A, rs2368564), AGTR1 (A/C, rs5186), BDKRB2 (T/C, rs1799722), and ADBR2 (A/G, rs1042713) polymorphisms were examined using the two-step multiplex PCR followed by carrying allele hybridization on the biochip. Subjects with the BDKRB2 (C/C), ACE (D/D), and ADBR2 (G/G, G/A) genotypes exhibited the strongest peripheral vasoconstriction in response to diving. In subjects with a combination of the BDKRB2 (C/C) plus ACE (D/D) genotypes, we observed the lowest pulse wave amplitude and pulse transit time values and the highest arterial blood pressure during face immersion compared with the heterozygous individuals, suggesting that these subjects are more susceptible to diving hypoxia. This study observed that humans with gene polymorphisms of the renin-angiotensin and kinin-bradykinin systems demonstrate various expressions of protective vascular reactions in response to the diving reflex. The obtained results might be used in estimation of resistance to hypoxia of any origin in human beings or in a medical practice.NEW & NOTEWORTHY Our study demonstrates that the vascular reactions in response to the diving reflex are genetically determined and depend on gene polymorphisms of the kinin-bradykinin and the renin-angiotensin systems.

Toward translating near-infrared spectroscopy oxygen saturation data for the non-invasive prediction of spatial and temporal hemodynamics during exercise

Image-based computational fluid dynamics (CFD) studies conducted at rest have shown that atherosclerotic plaque in the thoracic aorta (TA) correlates with adverse wall shear stress (WSS), but there is a paucity of such data under elevated flow conditions. We developed a pedaling exercise protocol to obtain phase contrast magnetic resonance imaging (PC-MRI) blood flow measurements in the TA and brachiocephalic arteries during three-tiered supine pedaling at 130, 150, and 170 % of resting heart rate (HR), and relate these measurements to non-invasive tissue oxygen saturation [Formula: see text] acquired by near-infrared spectroscopy (NIRS) while conducting the same protocol. Local quantification of WSS indices by CFD revealed low time-averaged WSS on the outer curvature of the ascending aorta and the inner curvature of the descending aorta (dAo) that progressively increased with exercise, but that remained low on the anterior surface of brachiocephalic arteries. High oscillatory WSS observed on the inner curvature of the aorta persisted during exercise as well. Results suggest locally continuous exposure to potentially deleterious indices of WSS despite benefits of exercise. Linear relationships between flow distributions and tissue oxygen extraction calculated from [Formula: see text] were found between the left common carotid versus cerebral tissue [Formula: see text] and the dAo versus leg tissue [Formula: see text]. A resulting six-step procedure is presented to use NIRS data as a surrogate for exercise PC-MRI when setting boundary conditions for future CFD studies of the TA under simulated exercise conditions. Relationships and ensemble-averaged PC-MRI inflow waveforms are provided in an online repository for this purpose.

Cardiovascular Reflexes Activity and Their Interaction during Exercise

Cardiac output and arterial blood pressure increase during dynamic exercise notwithstanding the exercise-induced vasodilation due to functional sympatholysis. These cardiovascular adjustments are regulated in part by neural reflexes which operate to guarantee adequate oxygen supply and by-products washout of the exercising muscles. Moreover, they maintain adequate perfusion of the vital organs and prevent excessive increments in blood pressure. In this review, we briefly summarize neural reflexes operating during dynamic exercise with particular emphasis on their interaction.

Vasoconstrictor responsiveness during hyperbaric hyperoxia in contracting human muscle

Large increases in systemic oxygen content cause substantial reductions in exercising forearm blood flow (FBF) due to increased vascular resistance. We hypothesized that 1) functional sympatholysis (blunting of sympathetic α-adrenergic vasoconstriction) would be attenuated during hyperoxic exercise and 2) α-adrenergic blockade would limit vasoconstriction during hyperoxia and increase FBF to levels observed under normoxic conditions. Nine male subjects (age 28 ± 1 yr) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Studies were performed in a hyperbaric chamber at 1 atmosphere absolute (ATA; sea level) while breathing 21% O(2) and at 2.82 ATA while breathing 100% O(2) (estimated change in arterial O(2) content ∼6 ml O(2)/100 ml). FBF (ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC) was calculated from FBF and blood pressure (arterial catheter). Vasoconstrictor responsiveness was determined using intra-arterial tyramine. FBF and FVC were substantially lower during hyperoxic exercise than normoxic exercise (∼20-25%; P < 0.01). At rest, vasoconstriction to tyramine (% decrease from pretyramine values) did not differ between normoxia and hyperoxia (P > 0.05). During exercise, vasoconstrictor responsiveness was slightly greater during hyperoxia than normoxia (-22 ± 3 vs. -17 ± 2%; P < 0.05). However, during α-adrenergic blockade, hyperoxic exercise FBF and FVC remained lower than during normoxia (P < 0.01). Therefore, our data suggest that although the vasoconstrictor responsiveness during hyperoxic exercise was slightly greater, it likely does not explain the majority of the large reductions in FBF and FVC (∼20-25%) during hyperbaric hyperoxic exercise.

Carotid chemoreceptor modulation of blood flow during exercise in healthy humans

Carotid chemoreceptor (CC) inhibition reduces sympathetic nervous outflow in exercising dogs and humans. We sought to determine if CC suppression increases muscle blood flow in humans during exercise and hypoxia. Healthy subjects (N = 13) were evaluated at rest and during constant-work leg extension exercise while exposed to either normoxia or hypoxia (inspired O(2) tension, F(IO(2)), ≈ 0.12, target arterial O(2) saturation = 85%). Subjects breathed hyperoxic gas (F(IO(2)) ≈ 1.0) and/or received intravenous dopamine to inhibit the CC while femoral arterial blood flow data were obtained continuously with pulsed Doppler ultrasound. Exercise increased heart rate, mean arterial pressure, femoral blood flow and conductance compared to rest. Transient hyperoxia had no significant effect on blood flow at rest, but increased femoral blood flow and conductance transiently during exercise without changing blood pressure. Similarly, dopamine had no effect on steady-state blood flow at rest, but increased femoral blood flow and conductance during exercise. The transient vasodilatory response observed by CC inhibition with hyperoxia during exercise could be blocked with simultaneous CC inhibition with dopamine. Despite evidence of dopamine reducing ventilation during hypoxia, no effect on femoral blood flow, conductance or mean arterial pressure was observed either at rest or during exercise with CC inhibition with dopamine while breathing hypoxia. These findings indicate that the carotid chemoreceptor contributes to skeletal muscle blood flow regulation during normoxic exercise in healthy humans, but that the influence of the CC on blood flow regulation in hypoxia is limited.

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.

Hyperbaric hyperoxia reduces exercising forearm blood flow in humans

Hypoxia during exercise augments blood flow in active muscles to maintain the delivery of O(2) at normoxic levels. However, the impact of hyperoxia on skeletal muscle blood flow during exercise is not completely understood. Therefore, we tested the hypothesis that the hyperemic response to forearm exercise during hyperbaric hyperoxia would be blunted compared with exercise during normoxia. Seven subjects (6 men/1 woman; 25 ± 1 yr) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Forearm blood flow (FBF; in ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC; in ml·min(-1)·100 mmHg(-1)) was calculated from FBF and blood pressure (in mmHg; brachial arterial catheter). Studies were performed in a hyperbaric chamber with the subjects supine at 1 atmospheres absolute (ATA) (sea level) while breathing normoxic gas [21% O(2), 1 ATA; inspired Po(2) (Pi(O(2))) ≈ 150 mmHg] and at 2.82 ATA while breathing hyperbaric normoxic (7.4% O(2), 2.82 ATA, Pi(O(2)) ≈ 150 mmHg) and hyperoxic (100% O(2), 2.82 ATA, Pi(O(2)) ≈ 2,100 mmHg) gas. Resting FBF and FVC were less during hyperbaric hyperoxia compared with hyperbaric normoxia (P < 0.05). The change in FBF and FVC (Δ from rest) during exercise under normoxia (204 ± 29 ml/min and 229 ± 37 ml·min(-1)·100 mmHg(-1), respectively) and hyperbaric normoxia (203 ± 28 ml/min and 217 ± 35 ml·min(-1)·100 mmHg(-1), respectively) did not differ (P = 0.66-0.99). However, the ΔFBF (166 ± 21 ml/min) and ΔFVC (163 ± 23 ml·min(-1)·100 mmHg(-1)) during hyperbaric hyperoxia were substantially attenuated compared with other conditions (P < 0.01). Our data suggest that exercise hyperemia in skeletal muscle is highly dependent on oxygen availability during hyperoxia.

Modulation of postjunctional α-adrenergic vasoconstriction during exercise and exogenous ATP infusions in ageing humans

The ability to modulate sympathetic α-adrenergic vasoconstriction in contracting muscle is impaired with age. In young adults, adenosine triphosphate (ATP) has been shown to blunt sympathetic vasoconstrictor responsiveness similar to exercise. Therefore, we tested the hypothesis that modulation of postjunctional α-adrenergic vasoconstriction to exogenous ATP is impaired in ageing humans.We measured forearm blood flow (FBF; Doppler ultrasound) and calculated vascular conductance (FVC) to intra-arterial infusions of phenylephrine (α₁-agonist) and dexmedetomidine (α₂-agonist) during rhythmic handgrip exercise (15% MVC), a control non-exercise vasodilator condition (adenosine), and ATP infusion in seven older (64 ± 3 years) and seven young (22 ± 1 years) healthy adults. Forearm hyperaemia was matched across all vasodilatating conditions. During adenosine, forearm vasoconstrictor responses to direct α₁-stimulation were lower in older compared with young adults (ΔFVC=-25 ± 3% vs. -41 ± 5%; P <0.05), whereas the responses to α₂-stimulation were not different (-35±6% vs. -44 ± 8%; NS). During exercise, α₁-mediated vasoconstriction was significantly blunted compared with adenosine in both young (-9 ± 2% vs. -41 ± 5%) and older adults (-15 ± 2% vs. -25 ± 3%); however, the magnitude of sympatholysis was reduced in older adults (32 ± 13 vs. 74 ± 8%; P <0.05). Similarly, α₂-mediated vasoconstriction during exercise was significantly blunted in both young (-15 ± 4% vs. -44 ± 8%) and older adults (-26 ± 3% vs. -35 ± 6%), however the magnitude of sympatholysis was reduced in older adults (19 ± 8% vs. 60 ± 10%; P <0.05). During ATP, both α₁- and α₂-mediated vasoconstriction was nearly abolished in young and older adults (ΔFVC ∼ -5%), and the magnitude of sympatholysis was similar in both age groups (∼85-90%). Our findings indicate that the ability to modulate postjunctional α-adrenergic vasoconstriction during exercise is impaired with age, whereas the sympatholytic effect of exogenous ATP is preserved. Thus, if impairments in vascular control during exercise in older adults involve vasoactive ATP, we speculate that circulating ATP is reduced with advancing age.

Multi-site and multi-depth near-infrared spectroscopy in a model of simulated (central) hypovolemia: lower body negative pressure

Purpose

To test the hypothesis that the sensitivity of near-infrared spectroscopy (NIRS) in reflecting the degree of (compensated) hypovolemia would be affected by the application site and probing depth. We simultaneously applied multi-site (thenar and forearm) and multi-depth (15–2.5 and 25–2.5 mm probe distance) NIRS in a model of simulated hypovolemia: lower body negative pressure (LBNP).

Methods

The study group comprised 24 healthy male volunteers who were subjected to an LBNP protocol in which a baseline period of 30 min was followed by a step-wise manipulation of negative pressure in the following steps: 0, −20, −40, −60, −80 and −100 mmHg. Stroke volume and heart rate were measured using volume-clamp finger plethysmography. Two multi-depth NIRS devices were used to measure tissue oxygen saturation (StO2) and tissue hemoglobin index (THI) continuously in the thenar and the forearm. To monitor the shift of blood volume towards the lower extremities, calf THI was measured by single-depth NIRS.

Results

The main findings were that the application of LBNP resulted in a significant reduction in stroke volume which was accompanied by a reduction in forearm StO2 and THI.

Conclusions

NIRS can be used to detect changes in StO2 and THI consequent upon central hypovolemia. Forearm NIRS measurements reflect hypovolemia more sensitively than thenar NIRS measurements. The sensitivity of these NIRS measurements does not depend on NIRS probing depth. The LBNP-induced shift in blood volume is reflected by a decreased THI in the forearm and an increased THI in the calf.

Electronic supplementary material

The online version of this article (doi:10.1007/s00134-010-2128-6) contains supplementary material, which is available to authorized users.

Muscle metaboreflex control of the circulation during exercise

This review covers the control of blood pressure, cardiac output and muscle blood flow by the muscle metaboreflex which involves chemically sensitive nerves located in muscle parenchyma activated by metabolites accumulating in the muscle during contraction. The efferent response to metaboreflex activation is an increase in sympathetic nerve activity that constricts the systemic vasculature and also evokes parallel inotropic and chronotropic effects on the heart to increase cardiac output. The metaboreflex elicits a significant blood pressure elevating response during exercise and functions to redistribute blood flow and blood volume. Regional specificity in the efferent response to the metaboreflex activated from either the leg or the arm is seen in the balance between signals for vasoconstriction to curtail blood flow and signals to increase cardiac output. The metaboreflex has dual functions. It can both elevate and decrease muscle blood flow depending on (1) the intensity and mode of contraction, (2) the limb in which the reflex is evoked, (3) the strength of the signal defined by the muscle mass, (4) the extent to which blood flow is redistributed from inactive vascular beds to increase central blood volume and (5) the extent to which cardiac output can be increased.

Delayed vasoconstrictor response to venous pooling in the calf is associated with high orthostatic tolerance to LBNP

Central blood volume loss to venous pooling in the lower extremities and vasoconstrictor response are commonly viewed as key factors to distinguish between individuals with high and low tolerance to orthostatic stress. In this study, we analyzed calf vasoconstriction as a function of venous pooling during simulated orthostatic stress. We hypothesized that high orthostatic tolerance (OT) would be associated with greater vasoconstrictor responses to venous pooling compared with low OT. Nineteen participants underwent continuous stepped lower body negative pressure at -10, -20, -30, -40, -50, and -60 mmHg each for 5 min or until exhibiting signs of presyncope. Ten participants completed the lower body negative pressure procedure without presyncope and were categorized with high OT; the remaining nine were categorized as having low OT. Near-infrared spectroscopy measurements of vasoconstriction (Hachiya T, Blaber A, Saito M. Acta Physiologica 193: 117-127, 2008) in calf muscles, along with heart rate (HR) responses for each participant, were evaluated in relation to calf blood volume, estimated by plethysmography. The slopes of this relationship between vasoconstriction and blood volume were not different between the high- and low-tolerance groups. However, the onset of vasoconstriction in the high-tolerance group was delayed. Greater HR increments in the low-tolerance group were also observed as a function of lower limb blood pooling. The delayed vasoconstriction and slower HR increments in the high-tolerance group to similar venous pooling in the low group may suggest a greater vascular reserve and possible delayed reduction in venous return.

Impairment of maximal aerobic power with moderate hypoxia in endurance athletes: do skeletal muscle mitochondria play a role?

This study investigates the role of central vs. peripheral factors in the limitation of maximal oxygen uptake (V̇o2max) with moderate hypoxia [inspired fraction (FiO2) =14.5%]. Fifteen endurance-trained athletes performed maximal cycle incremental tests to assess V̇o2max, maximal cardiac output (Q̇max), and maximal arteriovenous oxygen (a-vO2) difference in normoxia and hypoxia. Muscle biopsies of vastus lateralis were taken 1 wk before the cycling tests to evaluate maximal muscle oxidative capacity (V̇max) and sensitivity of mitochondrial respiration to ADP ( Km) on permeabilized muscle fibers in situ. Those athletes exhibiting the largest reduction of V̇o2max in moderate hypoxia (Severe Loss group: −18 ± 2%) suffered from significant reductions in Q̇max (−4 ± 1%) and maximal a-vO2 difference (−14 ± 2%). Athletes who well tolerated hypoxia, as attested by a significantly smaller drop of V̇o2max with hypoxia (Moderate Loss group: −7 ± 1%), also display a blunted Q̇max (−9 ± 2%) but, conversely, were able to maintain maximal a-vO2 difference (+1 ± 2%). Though V̇max was similar in the two experimental groups, the smallest reduction of V̇o2max with moderate hypoxia was observed in those athletes presenting the lowest apparent Km for ADP in the presence of creatine ( Km+Cr). In already-trained athletes with high muscular oxidative capacities, the qualitative, rather than quantitative, aspects of the mitochondrial function may constitute a limiting factor to aerobic ATP turnover when exercising at low FiO2, presumably through the functional coupling between the mitochondrial creatine kinase and ATP production. This study suggests a potential role for peripheral factors, including the alteration of cellular homeostasis in active muscles, in determining the tolerance to hypoxia in maximally exercising endurance-trained athletes.

Sympathetic restraint of muscle blood flow during hypoxic exercise

Control of exercising muscle blood flow is a balance between local vasodilatory factors and the increase in global sympathetic vasoconstrictor outflow. Hypoxia has been shown to potentiate the muscle sympathetic nerve response to exercise, potentially limiting the increase in muscle blood flow. Accordingly, we investigated sympathetic restraint to exercising muscle during whole body exercise in hypoxia. Six dogs chronically instrumented with ascending aortic and hindlimb flow probes and a terminal aortic catheter were studied at rest and mild [2.5 miles/h (mph), 5% grade] and moderate (4.0 mph, 10% grade) exercise while breathing room air or hypoxia (Pa(O(2)) approximately 45 mmHg) in the intact control condition and following systemic alpha-adrenergic blockade (phentolamine). Hypoxia caused an increase in cardiac output (CO), hindlimb flow (Flow(L)), and blood pressure (BP), while total (Cond(T)) and hindlimb conductance (Cond(L)) were unchanged at rest and mild exercise but increased with moderate exercise. During both mild and moderate exercise, alpha-blockade in normoxia resulted in significant vasodilation as evidenced by increases in CO (10%), Flow(L) (17%), Cond(T) (33%), Cond(L) (43%), and a decrease in BP (-18%), with the increase in Cond(L) greater than the increase in Cond(T) during mild exercise. Compared with the normoxic response, alpha-blockade in hypoxia during exercise resulted in a significantly greater increase in Cond(T) (59%) and Cond(L) (74%) and a correspondingly greater decrease in BP (-34%) from baseline. These findings indicate that there is considerable hypoxia-induced sympathetic restraint of muscle blood flow during both mild and moderate exercise, which helps to maintain arterial blood pressure in hypoxia.

The ergogenic effect of recombinant human erythropoietin on VO2max depends on the severity of arterial hypoxemia

Treatment with recombinant human erythropoietin (rhEpo) induces a rise in blood oxygen-carrying capacity (CaO₂) that unequivocally enhances maximal oxygen uptake (V̇O₂max) during exercise in normoxia, but not when exercise is carried out in severe acute hypoxia. This implies that there should be a threshold altitude at which V̇O₂max is less dependent on CaO₂. To ascertain which are the mechanisms explaining the interactions between hypoxia, CaO₂ and V̇O₂max we measured systemic and leg O₂ transport and utilization during incremental exercise to exhaustion in normoxia and with different degrees of acute hypoxia in eight rhEpo-treated subjects. Following prolonged rhEpo treatment, the gain in systemic V̇O₂max observed in normoxia (6–7%) persisted during mild hypoxia (8% at inspired O₂ fraction (F_IO₂) of 0.173) and was even larger during moderate hypoxia (14–17% at F_IO₂ = 0.153–0.134). When hypoxia was further augmented to F_IO₂ = 0.115, there was no rhEpo-induced enhancement of systemic V̇O₂max or peak leg V̇O₂. The mechanism highlighted by our data is that besides its strong influence on CaO₂, rhEpo was found to enhance leg V̇O₂max in normoxia through a preferential redistribution of cardiac output toward the exercising legs, whereas this advantageous effect disappeared during severe hypoxia, leaving augmented CaO₂ alone insufficient for improving peak leg O₂ delivery and V̇O₂. Finally, that V̇O₂max was largely dependent on CaO₂ during moderate hypoxia but became abruptly CaO₂-independent by slightly increasing the severity of hypoxia could be an indirect evidence of the appearance of central fatigue.

Graded sympatholytic effect of exogenous ATP on postjunctional alpha-adrenergic vasoconstriction in the human forearm: implications for vascular control in contracting muscle

Recent evidence suggests that adenosine triphosphate (ATP) can inhibit vasoconstrictor responses to endogenous noradrenaline release via tyramine in the skeletal muscle circulation, similar to what is observed in contracting muscle. Whether this involves direct modulation of postjunctional alpha-adrenoceptor responsiveness, or is selective for alpha(1)- or alpha(2)-receptors remains unclear. Therefore, in Protocol 1, we tested the hypothesis that exogenous ATP can blunt direct postjunctional alpha-adrenergic vasoconstriction in humans. We measured forearm blood flow (FBF; Doppler ultrasound) and calculated the vascular conductance (FVC) responses to local intra-arterial infusions of phenylephrine (alpha(1)-agonist) and dexmedetomidine (alpha(2)-agonist) during moderate rhythmic handgrip exercise (15% maximum voluntary contraction), during a control non-exercise vasodilator condition (adenosine), and during ATP infusion in eight young adults. Forearm hyperaemia was matched across all conditions. Forearm vasoconstrictor responses to direct alpha(1)-receptor stimulation were blunted during exercise versus adenosine (DeltaFVC = -11 +/- 3% versus -39 +/- 5%; P< 0.05), and were abolished during ATP infusion (-3 +/- 2%). Similarly, vasoconstrictor responses to alpha(2)-receptor stimulation were blunted during exercise versus adenosine (-13 +/- 4% versus -40 +/- 8%; P< 0.05), and were abolished during ATP infusion (-4 +/- 4%). In Prototol 2 (n = 10), we tested the hypothesis that graded increases in ATP would reduce alpha(1)-mediated vasoconstriction in a dose-dependent manner compared with vasodilatation evoked via adenosine. Forearm vasoconstrictor responses during low dose adenosine (-38 +/- 3%) and ATP (-33 +/- 2%) were not significantly different from rest (-40 +/- 3%; P> 0.05). In contrast, vasoconstrictor responses during moderate (-22 +/- 6%) and high dose ATP (-8 +/- 5%) were significantly blunted compared with rest, whereas the responses during adenosine became progressively greater (moderate = -48 +/- 4%, P = 0.10; high = -53 +/- 6%, P< 0.05). We conclude that exogenous ATP is capable of blunting direct postjunctional alpha-adrenergic vasoconstriction, that this involves both alpha(1)- and alpha(2)-receptor subtypes, and that this is graded with ATP concentrations. Collectively, these data are consistent with the conceptual framework regarding how muscle blood flow and vascular tone are regulated in contracting muscles of humans.

Functional changes in cerebral and paraspinal muscle physiology of healthy women during exposure to whole-body vibration

The objective of this study was to investigate the effects of whole-body vibration on multiple tissues simultaneously in fourteen healthy women. On three separate days, participants were exposed to frequencies, 3, 4.5, or 6 Hz (at 0.9 g(r.m.s) acceleration in vertical direction) per day on a simulator for 16 min. While sitting 'with' and 'without' backrest support, participants also performed handgrip contractions for 1 min. Cerebral and lumbar muscle oxygenation and blood volume responses were measured using near-infrared spectroscopy. Cardiorespiratory responses were collected using a metabolic cart. In general, cerebral and cardiorespiratory responses increased with vibration compared to without vibration, whereas in the lumbar region oxygenation and blood volume responses decreased. Greatest cerebral responses were observed at 6 Hz (P<0.05). When compared to exposure to vibration without performing work, significant decrease in lumbar responses was observed during handgrip contractions in both conditions of sitting 'with' and 'without' a backrest (P<0.05). Such decreases in the lumbar responses suggest postural load due to prolonged sitting combined with physical activity during vibration, might reduce vascular supply to the paraspinal muscles. This study reiterates the importance of understanding the physiological basis for various health disorders in women due to exposure to whole-body vibration.

Metabolic forearm vasodilation is enhanced following Bier block with phentolamine

The extent to which sympathetic nerve activity restrains metabolic vasodilation in skeletal muscle remains unclear. We determined forearm blood flow (FBF; ultrasound/Doppler) and vascular conductance (FVC) responses to 10 min of ischemia [reactive hyperemic blood flow (RHBF)] and 10 min of systemic hypoxia (inspired O(2) fraction = 0.1) before and after regional sympathetic blockade with the alpha-receptor antagonist phentolamine via Bier block in healthy humans. In a control group, we performed sham Bier block with saline. Consistent with alpha- receptor inhibition, post-phentolamine, basal FVC (FBF/mean arterial pressure) increased (pre vs. post: 0.42 +/- 0.05 vs. 1.03 +/- 0.21 units; P < 0.01; n = 12) but did not change in the saline controls (pre vs. post: 0.56 +/- 0.14 vs. 0.53 +/- 0.08 units; P = not significant; n = 5). Post-phentolamine, total RHBF (over 3 min) increased substantially (pre vs. post: 628 +/- 75 vs. 826 +/- 92 ml/min; P < 0.01) but did not change in the controls (pre vs. post: 618 +/- 66 vs. 661 +/- 35 ml/min; P = not significant). In all conditions, compared with peak RHBF, peak skin reactive hyperemia was markedly delayed. Furthermore, post-phentolamine (pre vs. post: 0.43 +/- 0.06 vs. 1.16 +/- 0.17 units; P < 0.01; n = 8) but not post-saline (pre vs. post: 0.93 +/- 0.16 vs. 0.87 +/- 0.19 ml/min; P = not significant; n = 5), the FVC response to hypoxia (arterial O(2) saturation = 77 +/- 1%) was markedly enhanced. These data suggest that sympathetic vasoconstrictor nerve activity markedly restrains skeletal muscle vasodilation induced by local (forearm ischemia) and systemic (hypoxia) vasodilator stimuli.

Skeletal Muscle Morphology in Patients with Restless Legs Syndrome

AIM

The aim of the study was to assess the cellular and structural properties of skeletal muscle in restless legs syndrome (RLS).

METHOD

Twenty patients and 16 controls were included. Aerobic performance was assessed using a submaximal test. On muscle biopsies taken from the tibialis anterior, fiber distribution and fiber area were analyzed together with parameters surveying the microvascularization, especially the tortuosity, which is expressed as a percent of muscle fiber perimeter in contact with the wall of the microvessel, length of capillary/perimeter of fiber (LC/PF) index.

RESULTS

The RLS group had significantly lower predicted maximal oxygen uptake (p = 0.01) and significantly higher LC/PF index (p = 0.01) compared to the controls.

CONCLUSION

The higher capillary tortuosity in RLS patients indicates the occurrence of significant remodeling in capillary geometry in RLS.

The role of hypoxia in erectile dysfunction mechanisms

Chronic hypoxia is related to many pathological conditions: aging, heart and respiratory failure, sleep apneas, smoke, chronic obstructive pulmonary disease (COPD), diabetes, hypertension and arteriosclerosis, all characterized by reductions of sleep-related erections (SREs) and by erectile dysfunction (ED). Sleep-related erections occur naturally during rapid eye movement (REM) sleep in sexually potent men. Hypoxia is also a physiological condition at altitude. The level of inspired oxygen decreases progressively with the increase of altitude; for this reason, this study was performed to evaluate the relationship of SREs with hypoxic environment. SREs have been recorded by an erectometer (RigiScan) on three mountain climbers (mean age: 32.5) during a 26-day stay at an altitude ranging from 2000 to 5600 m above sea level. Twenty-four records have been made at progressively increasing altitudes. A data analysis was carried out on a statistical mean of the three values of each variable and an analysis of variance (ANOVA) and Newman-Keuls test were carried out for multiple comparison among groups. At altitudes over 4450 m, we found lack of rigidity at 80-100% and 60-79%. Mean % of rigidity and rigidity time of 80-100% (tip and base) decreased progressively with altitude. No significant reductions were shown in rigidity time at 0-19% and at 20-39% (tip and base), of total number, of total and mean duration of SREs. Pathological rigidometric records at high altitude in sexually potent men at sea level clarify the primary role of hypoxia in physiopathological ED pathway.

Contribution of alpha2-adrenoceptors and Y1 neuropeptide Y receptors to the blunting of sympathetic vasoconstriction induced by systemic hypoxia in the rat

There is evidence that sympathetically evoked vasoconstriction in skeletal muscle is blunted in systemic hypoxia, but the mechanisms underlying this phenomenon are not clear. In Saffan-anaesthetized Wistar rats, we have studied the role of α₂-adrenoceptors and neuropeptide Y (NPY) Y₁ receptors in mediating vasoconstriction evoked by direct stimulation of the lumbar sympathetic chain by different patterns of impulses in normoxia (N) and systemic hypoxia (H: breathing 8% O₂). Patterns comprised 120 impulses delivered in bursts over a 1 min period at 40 or 20 Hz, or continuously at 2 Hz. Hypoxia attenuated the evoked increases in femoral vascular resistance (FVR) by all patterns, the response to 2 Hz being most affected (40 Hz bursts: N = 3.25 ± 0.75 arbitrary resistance units (RU); H = 1.14 ± 0.29 RU). Yohimbine (Yoh, α₂-adrenoceptor antagonist) or BIBP 3226 (Y₁-receptor antagonist) did not affect baseline FVR. In normoxia, Yoh attenuated the responses evoked by high frequency bursts and 2 Hz, whereas BIBP 3226 only attenuated the response to 40 Hz (40 Hz bursts: N + Yoh = 2.1 ± 0.59 RU; N + BIBP 3226 = 1.9 ± 0.4 RU). In hypoxia, Yoh did not further attenuate the evoked responses, but BIBP 3226 further attenuated the response to 40 Hz bursts. These results indicate that neither α₂-adrenoceptors nor Y₁ receptors contribute to basal vascular tone in skeletal muscle, but both contribute to constrictor responses evoked by high frequency bursts of sympathetic activity. We propose that in systemic hypoxia, the α₂-mediated component represents about 50% of the sympathetically evoked constriction that is blunted, whereas the contribution made by Y₁ receptors is resistant. Thus we suggest the importance of NPY in the regulation of FVR and blood pressure increases during challenges such as systemic hypoxia.

Forelimb postischaemic reactive hyperaemia is impaired by hypotensive low body negative pressure in healthy subjects

Local metabolic conditions adapt blood supply to metabolic requirement by a direct effect on vascular smooth muscles and indirectly by modulating sympathetic vasoconstrictor effectiveness. During exercise, sympathetic nervous activity could in turn interfere on local metabolic control of vascular tone and restrain blood flow to active muscles. In order to investigate that interaction non-invasively, we measured postischaemic reactive hyperaemia (RH) in the forelimb of eight healthy young men (22.7 +/- 2.1 years) at rest and during two levels of sympathetic stimulation using low body negative pressure (LBNP -15 and -30 mmHg). During every stages, RH was measured after 40, 60, 90 and 180 s of arterial occlusion, respectively. In control conditions, RH rose with duration of ischaemia (18.9, 24.2, 30.4, 33.1 ml min(-1) per 100 ml(-1) for 40, 60, 90 and 180 s of ischaemia, respectively). During non-hypotensive LBNP (-15 mmHg) sympathetic activation was associated with decreased forelimb blood flow (6.4 +/- 0.9 versus 3.9 +/- 0.6 ml min(-1) per 100 ml(-1), P<0.01), but RH were not significantly different from control conditions. During hypotensive tachycardia LBNP (-30 mmHg), RH were significantly lower than under the previous LBNP stage. This fall in RH was greater after the shortest gap of ischaemia and tapered off as arterial occlusion gap increased (-22.3, -13.1, -10.5 and -8.7% for 40, 60, 90 and 180 s of ischaemia, respectively). These results suggested that vascular tone adaptation to local metabolic conditions was modified by sympathetic nervous activation. This was particularly marked when an hypotensive-mediated sympathetic stimulation was opposed to short gaps of ischaemia.

Systemic hypoxia and vasoconstrictor responsiveness in exercising human muscle

Exercise blunts sympathetic alpha-adrenergic vasoconstriction (functional sympatholysis). We hypothesized that sympatholysis would be augmented during hypoxic exercise compared with exercise alone. Fourteen subjects were monitored with ECG and pulse oximetry. Brachial artery and antecubital vein catheters were placed in the nondominant (exercising) arm. Subjects breathed hypoxic gas to titrate arterial O2 saturation to 80% while remaining normocapnic via a rebreath system. Baseline and two 8-min bouts of rhythmic forearm exercise (10 and 20% of maximum) were performed during normoxia and hypoxia. Forearm blood flow, blood pressure, heart rate, minute ventilation, and end-tidal CO2 were measured at rest and during exercise. Vasoconstrictor responsiveness was determined by responses to intra-arterial tyramine during the final 3 min of rest and each exercise bout. Heart rate was higher during hypoxia (P < 0.01), whereas blood pressure was similar (P = 0.84). Hypoxic exercise potentiated minute ventilation compared with normoxic exercise (P < 0.01). Forearm blood flow was higher during hypoxia compared with normoxia at rest (85 +/- 9 vs. 66 +/- 7 ml/min), at 10% exercise (276 +/- 33 vs. 217 +/- 27 ml/min), and at 20% exercise (464 +/- 32 vs. 386 +/- 28 ml/min; P < 0.01). Arterial epinephrine was higher during hypoxia (P < 0.01); however, venoarterial norepinephrine difference was similar between hypoxia and normoxia before (P = 0.47) and during tyramine administration (P = 0.14). Vasoconstriction to tyramine (%decrease from pretyramine values) was blunted in a dose-dependent manner with increasing exercise intensity (P < 0.01). Interestingly, vasoconstrictor responsiveness tended to be greater (P = 0.06) at rest (-37 +/- 6% vs. -33 +/- 6%), at 10% exercise (-27 +/- 5 vs. -22 +/- 4%), and at 20% exercise (-22 +/- 5 vs. -14 +/- 4%) between hypoxia and normoxia, respectively. Thus sympatholysis is not augmented by moderate hypoxia nor does it contribute to the increased blood flow during hypoxic exercise.

Maximal exercise and muscle oxygen extraction in acclimatizing lowlanders and high altitude natives

The tight relation between arterial oxygen content and maximum oxygen uptake (Vv(o2max)within a given person at sea level is diminished with altitude acclimatization. An explanation often suggested for this mismatch is impairment of the muscle O(2) extraction capacity with chronic hypoxia, and is the focus of the present study. We have studied six lowlanders during maximal exercise at sea level (SL) and with acute (AH) exposure to 4,100 m altitude, and again after 2 (W2) and 8 weeks (W8) of altitude sojourn, where also eight high altitude native (Nat) Aymaras were studied. Fractional arterial muscle O(2) extraction at maximal exercise was 90.0+/-1.0% in the Danish lowlanders at sea level, and remained close to this value in all situations. In contrast to this, fractional arterial O(2) extraction was 83.2+/-2.8% in the high altitude natives, and did not change with the induction of normoxia. The capillary oxygen conductance of the lower extremity, a measure of oxygen diffusing capacity, was decreased in the Danish lowlanders after 8 weeks of acclimatization, but was still higher than the value obtained from the high altitude natives. The values were (in ml min(-1) mmHg(-1)) 55.2+/-3.7 (SL), 48.0+/-1.7 (W2), 37.8+/-0.4 (W8) and 27.7+/-1.5 (Nat). However, when correcting oxygen conductance for the observed reduction in maximal leg blood flow with acclimatization the effect diminished. When calculating a hypothetical leg V(o2max)at altitude using either the leg blood flow or the O(2) conductance values obtained at sea level, the former values were almost completely restored to sea level values. This would suggest that the major determinant V(o2max)for not to increase with acclimatization is the observed reduction in maximal leg blood flow and O(2) conductance.

Skin blood flow influences near-infrared spectroscopy-derived measurements of tissue oxygenation during heat stress

Near-infrared (NIR) spectroscopy is a noninvasive optical technique that is increasingly used to assess muscle oxygenation during exercise with the assumption that the contribution of skin blood flow to the NIR signal is minor or nonexistent. We tested this assumption in humans by monitoring forearm tissue oxygenation during selective cutaneous vasodilation induced by locally applied heat ( n = 6) or indirect whole body heating (i.e., heating subject but not area surrounding NIR probes; n = 8). Neither perturbation has been shown to cause a measurable change in muscle blood flow or metabolism. Local heating (∼41°C) caused large increases in the NIR-derived tissue oxygenation signal [before heating = 0.82 ± 0.89 optical density (OD), after heating = 18.21 ± 2.44 OD; P < 0.001]. Similarly, whole body heating (increase internal temperature 0.9°C) also caused large increases in the tissue oxygenation signal (before heating = −0.31 ± 1.47 OD, after heating = 12.48 ± 1.82 OD; P < 0.001). These increases in the tissue oxygenation signal were closely correlated with increases in skin blood flow during both local heating (mean r = 0.95 ± 0.02) and whole body heating (mean r = 0.89 ± 0.04). These data suggest that the contribution of skin blood flow to NIR measurements of tissue oxygenation can be significant, potentially confounding interpretation of the NIR-derived signal during conditions where both skin and muscle blood flows are elevated concomitantly (e.g., high-intensity and/or prolonged exercise).