Role of adenosine in regulating the heterogeneity of skeletal muscle blood flow during exercise in humans

Alterations in aerobic fitness and muscle deoxygenation during ramp incremental exercise in trained youth cyclists: a ~3-year longitudinal study

Alterations of aerobic fitness and muscle deoxygenation during a ramp incremental exercise test (GXT) were assessed on two occasions within a time-frame of 2.9 ± 0.1y in competitive youth cyclists. Nine cyclists (age, 14.5 ± 1.1y; peak oxygen uptake (V˙O_2peak), 62.6 ± 4.2 mL.min^-1.kg^-1) participated in this investigation. V˙O_2peak, the gas exchange threshold (GET) and the respiratory compensation point (RCP), as well as the muscle deoxygenation response pattern were determined during a GXT using open circuit spirometry and near-infrared spectroscopy, respectively. T-tests and Pearson's correlations were used to assess effects of time on the dependent variables and relationships between changes of parameter estimates of aerobic fitness and the muscle deoxygenation response, respectively. Workrate and metabolic rate at GET (33 ± 20 and 42 ± 23%) and RCP (36 ± 20 and 40 ± 22%), and V˙O_2peak (30 ± 18%) significantly increased throughout the study (P < 0.05). The muscle deoxygenation response showed a significant rightward shift from occasion one to two (P < 0.05). Alterations in the workrate/metabolic rate at RCP, and V˙O_2peak, were correlated with alterations of the muscle deoxygenation response (R = 0.71-0.89, P < 0.05). Together, this is thought to indicate a superior muscle perfusion within the tissue of interrogation at the same metabolic rate on occasion two vs. one, which partially contributed to the improved aerobic fitness in the cyclists herein.

Energetic Contributions Including Gender Differences and Metabolic Flexibility in the General Population and Athletes

Metabolic flexibility includes the ability to perform fat and carbohydrate oxidation, as well as oxidative capacity, which is associated with mitochondrial function, energetic contributions, and physical health and performance. During a session of graded incremental exercise testing (GIET), we investigated metabolic flexibility, the contributions of three energy systems, and performances of individuals with different metabolic characteristics. Fifteen general population (GP; n = 15, male n = 7, female n = 8) and 15 national-level half-marathon and triathlon athletes (A; n = 15, male n = 7, female n = 8) participated in this study. During GIET, heart rate (HR), oxygen uptake (V˙O_2mean and V˙CO_2mean), metabolic equivalents (METs) in V˙O_2mean, and blood glucose and lactate concentrations (La⁻) were measured. Furthermore, jogging/running speeds (S) at specific La⁻, fat and carbohydrate oxidations (FATox and CHOox), and energetic contributions (oxidative; W_Oxi, glycolytic; W_Gly, and phosphagen; W_PCr) were calculated. The percentages of HR_max, relative V˙O_2mean, V˙CO_2mean, and METs in V˙O_2mean were all lower in A than they were in GP. FATox values were lower in GP than in A, while CHOox and La⁻ were higher in GP than in A. Negative correlations between La⁻ and FATox were also observed in both groups. Contributions of W_Oxi, W_Gly, and W_PCr were higher in GP than in A during GIET. Moreover, values of W_Gly, and W_PCr were significantly lower and higher, respectively, in male GP than in female GP. Furthermore, S at specific La⁻ were higher in A than in GP. It is suggested that an individualized low-intensity recovery exercise program be established, to achieve increased metabolic flexibility and oxidative capacity (aerobic base), such as public health improvements and a greater volume of higher exercise intensities; this is the type of exercise that elite athletes worldwide mostly perform during their training period and progression. This may prevent cardiac/metabolic diseases in GP.

A century of exercise physiology: key concepts on coupling respiratory oxygen flow to muscle energy demand during exercise

After a short historical account, and a discussion of Hill and Meyerhof’s theory of the energetics of muscular exercise, we analyse steady-state rest and exercise as the condition wherein coupling of respiration to metabolism is most perfect. The quantitative relationships show that the homeostatic equilibrium, centred around arterial pH of 7.4 and arterial carbon dioxide partial pressure of 40 mmHg, is attained when the ratio of alveolar ventilation to carbon dioxide flow (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{V}}_{A}/{\dot{V}}_{R}{CO}_{2}$$\end{document}V˙A/V˙RCO2) is − 21.6. Several combinations, exploited during exercise, of pertinent respiratory variables are compatible with this equilibrium, allowing adjustment of oxygen flow to oxygen demand without its alteration. During exercise transients, the balance is broken, but the coupling of respiration to metabolism is preserved when, as during moderate exercise, the respiratory system responds faster than the metabolic pathways. At higher exercise intensities, early blood lactate accumulation suggests that the coupling of respiration to metabolism is transiently broken, to be re-established when, at steady state, blood lactate stabilizes at higher levels than resting. In the severe exercise domain, coupling cannot be re-established, so that anaerobic lactic metabolism also contributes to sustain energy demand, lactate concentration goes up and arterial pH falls continuously. The \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\dot{V}}_{A}/{\dot{V}}_{R}{CO}_{2}$$\end{document}V˙A/V˙RCO2 decreases below − 21.6, because of ensuing hyperventilation, while lactate keeps being accumulated, so that exercise is rapidly interrupted. The most extreme rupture of the homeostatic equilibrium occurs during breath-holding, because oxygen flow from ambient air to mitochondria is interrupted. No coupling at all is possible between respiration and metabolism in this case.

Fitness Level- and Sex-related Differences in Macro- and Microvascular Responses during Reactive Hyperemia

PURPOSE

Reactive hyperemia (RH) is widely used for the investigation of macrovascular (Flow-Mediated Dilation, FMD) and microvascular (Near Infrared Spectroscopy-Vascular Occlusion Test, NIRS-VOT) function. Mixed results have been reported on fitness level- and sex-related differences in FMD outcomes, and little is known about microvascular differences in untrained and chronically trained males and females.

METHODS

Fifteen chronically trained (CT: 8 MALES, 7 FEMALES) and sixteen untrained (UT: 8 MALES, 8 FEMALES) individuals participated in this study. Aerobic fitness (V[Combining Dot Above]O2max) was assessed during a cycling incremental exercise test to volitional exhaustion. FMD and NIRS-VOT were performed simultaneously on the lower limb investigating superficial femoral artery and vastus lateralis muscle, respectively.

RESULTS

%FMD was not different between groups (CT MALES, 4.62 ± 1.42; CT FEMALES, 4.15 ± 2.23; UT MALES, 5.10 ± 2.53; CT FEMALES, 3.20 ± 1.67). Peak blood flow showed greater values in CT vs UT (p ≤ 0.0001) and MALES vs FEMALES (p = 0.032). RH blood flow AUC was greater in CT vs UT (p = 0.001). At the microvascular level, desaturation and reperfusion rates were faster in CT vs UT (p = 0.018 and p = 0.013) and MALES vs FEMALES (p = 0.011 and p = 0.005). V[Combining Dot Above]O2max was significantly correlated with reperfusion rate (p = 0.0005) but not with %FMD.

CONCLUSION

Whereas NIRS-VOT outcomes identified fitness- and sex-related differences in vascular responses, %FMD did not. However, when reactive hyperemia-related outcomes from the FMD analysis were considered, fitness- and/or sex-related differences were detected. These data highlight the importance of integrating FMD and NIRS-VOT outcomes for a more comprehensive evaluation of vascular function.

The role of vascular function on exercise capacity in health and disease

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V ̇ O 2 max ), critical power (CP) and speed of the V ̇ O 2 kinetics following exercise onset. Of these, the latter is, perhaps, the cardinal test of integrated function along the O2 transport pathway from lungs to skeletal muscle mitochondria. Fast V ̇ O 2 kinetics demands that the cardiovascular system distributes exercise-induced blood flow elevations among and within those vascular beds subserving the contracting muscle(s). Ideally, this process must occur at least as rapidly as mitochondrial metabolism elevates V ̇ O 2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q ̇ O 2 ) dependency that slows V ̇ O 2 kinetics, decreasing CP and V ̇ O 2 max , increasing the O2 deficit and sowing the seeds of exercise intolerance. Exercise training, in contrast, does the opposite. Within the context of these three parameters (see Graphical Abstract), this brief review examines the training-induced plasticity of key elements in the O2 transport pathway. It asks how structural and functional vascular adaptations accelerate and redistribute muscle Q ̇ O 2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V ̇ O 2 values. Recent discoveries, especially in the muscle microcirculation and Q ̇ O 2 -to- V ̇ O 2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V ̇ O 2 kinetics and determine CP and V ̇ O 2 max in health and how vascular dysfunction in disease predicates exercise intolerance. Finally, the latest evidence that nitrate supplementation improves vascular and therefore aerobic function in health and disease is presented.

Krogh’s capillary recruitment hypothesis, 100 years on: Is the opening of previously closed capillaries necessary to ensure muscle oxygenation during exercise?

In 1919, August Krogh published his seminal work on skeletal muscle oxygenation. Krogh’s observations indicated that muscle capillary diameter is actively regulated, rather than a passive result of arterial blood flow regulation. Indeed, combining a mathematical model with the number of ink-filled capillaries he observed in muscle cross sections taken at different workloads, Krogh was able to account for muscle tissue’s remarkably efficient oxygen extraction during exercise in terms of passive diffusion from nearby capillaries. Krogh was awarded the 1920 Nobel Prize for his account of muscle oxygenation. Today, his observations are engrained in the notion of capillary recruitment: the opening of previously closed capillaries. While the binary distinction between “closed” and “open” was key to Krogh’s model argument, he did in fact report a continuum of capillary diameters, degrees of erythrocyte deformation, and perfusion states. Indeed, modern observations question the presence of closed muscle capillaries. We therefore examined whether changes in capillary flow patterns and hematocrit among open capillaries can account for oxygen extraction in muscle across orders-of-magnitude changes in blood flow. Our four-compartment model of oxygen extraction in muscle confirms this notion and provides a framework for quantifying the impact of changes in microvascular function on muscle oxygenation in health and disease. Our results underscore the importance of capillary function for oxygen extraction in muscle tissue as first proposed by Krogh. While Krogh’s model calculations still hold, our model predictions support that capillary recruitment can be viewed in the context of continuous, rather than binary, erythrocyte distributions among capillaries.

NEW & NOTEWORTHY Oxygen extraction in working muscle is extremely efficient in view of single capillaries properties. The underlying mechanisms have been widely debated. Here, we develop a four-compartment model to quantify the influence of each of the hypothesized mechanisms on muscle oxygenation. Our results show that changes in capillary flow pattern and hematocrit can account for the high oxygen extraction observed in working muscle, while capillary recruitment is not required to account for these extraction properties.

Increase of Glucose Uptake in Human Bone Marrow With Increasing Exercise Intensity

Human bone marrow is a metabolically active tissue that responds to acute low-intensity exercise by having increased glucose uptake (GU). Here, the authors studied whether bone marrow GU increases more with increased exercise intensities. Femoral bone marrow GU was measured using positron emission tomography and [18F]-fluorodeoxyglucose in six healthy young men during cycling at intensities of 30% (low), 55% (moderate), and 75% (high) of maximal oxygen consumption on three separate days. Bone marrow GU at low was 17.2 µmol·kg−1·min−1 (range 9.0–25.4) and increased significantly (p = .003) at moderate (31.2 µmol·kg−1·min−1, 22.9–39.4) but was not significant from moderate to high (37.4 µmol·kg−1·min−1, 29.0–45.7, p = .26). Furthermore, the ratio between bone and muscle GU decreased from low to moderate exercise intensity (p < .01) but not (p = .99) from moderate to high exercise intensity. In conclusion, these results show that although the increase is not as large as observed in exercising skeletal muscle, GU in femoral bone marrow increases with increasing exercise intensity at least from low- to moderate-intensity effort, which may be important for bone and whole-body metabolic health.

Plasma Nucleotide Dynamics during Exercise and Recovery in Highly Trained Athletes and Recreationally Active Individuals

Circulating plasma ATP is able to regulate local skeletal muscle blood flow and 0₂ delivery causing considerable vasodilatation during exercise. We hypothesized that sport specialization and specific long-term training stimuli have an impact on venous plasma [ATP] and other nucleotides concentration. Four athletic groups consisting of sprinters (n=11; age range 21–30 yr), endurance-trained athletes (n=16; age range 18–31 yr), futsal players (n=14; age range 18–30 yr), and recreationally active individuals (n=12; age range 22–33 yr) were studied. Venous blood samples were collected at rest, during an incremental treadmill test, and during recovery. Baseline [ATP] was 759±80 nmol·l⁻¹ in competitive athletes and 680±73 nmol·l⁻¹ in controls and increased during exercise by ~61% in competitive athletes and by ~31% in recreationally active participants. We demonstrated a rapid increase in plasma [ATP] at exercise intensities of 83–87% of VO₂max in competitive athletes and 94% in controls. Concentrations reported after 30 minutes of recovery were distinct from those obtained preexercise in competitive athletes (P < 0.001) but not in controls (P = 0.61). We found a correlation between total-body skeletal muscle mass and resting and maximal plasma [ATP] in competitive athletes (r=0.81 and r=0.75, respectively). In conclusion, sport specialization is significantly related to plasma [ATP] at rest, during exercise, and during maximal effort. Intensified exercise-induced plasma [ATP] increases may contribute to more effective vessel dilatation during exercise in highly trained athletes than in recreational runners. The most rapid increase in ATP concentration was associated with the respiratory compensation point. No differences between groups of competitive athletes were observed during the recovery period suggesting a similar pattern of response after exercise. Total-body skeletal muscle mass is indirectly related to plasma [ATP] in highly trained athletes.

Methods for the determination of skeletal muscle blood flow: development, strengths and limitations

Since the first measurements of limb blood flow at rest and during nerve stimulation were conducted in the late 1800s, a number of methods have been developed for the determination of limb and skeletal muscle blood flow in humans. The methods, which have been applied in the study of aspects such as blood flow regulation, oxygen uptake and metabolism, differ in terms of strengths and degree of limitations but most have advantages for specific settings. The purpose of this review is to describe the origin and the basic principles of the methods, important aspects and requirements of the procedures. One of the earliest methods, venous occlusion plethysmography, is a noninvasive method which still is extensively used and which provides similar values as other more direct blood flow methods such as ultrasound Doppler. The constant infusion thermodilution method remains the most appropriate for the determination of blood flow during maximal exercise. For resting blood flow and light-to-moderate exercise, the non-invasive ultrasound Doppler methodology, if handled by a skilled operator, is recommendable. Positron emission tomography with radiolabeled water is an advanced method which requires highly sophisticated equipment and allows for the determination of muscle-specific blood flow, regional blood flows and estimate of blood flow heterogeneity within a muscle. Finally, the contrast-enhanced ultrasound method holds promise for assessment of muscle-specific blood flow, but the interpretation of the data obtained remains uncertain. Currently lacking is high-resolution methods for continuous visualization and monitoring of the skeletal muscle microcirculation in humans.

Effects of heat and cold on health, with special reference to Finnish sauna bathing

Environmental stress such as extremely warm or cold temperature is often considered a challenge to human health and body homeostasis. However, the human body can adapt relatively well to heat and cold environments, and recent studies have also elucidated that particularly heat stress might be even highly beneficial for human health. Consequently, the aim of the present brief review is first to discuss general cardiovascular and other responses to acute heat stress, followed by a review of beneficial effects of Finnish sauna bathing on general and cardiovascular health and mortality as well as dementia and Alzheimer's disease risk. Plausible mechanisms included are improved endothelial and microvascular function, reduced blood pressure and arterial stiffness, and possibly increased angiogenesis in humans, which are likely to mediate the health benefits of sauna bathing. In addition to heat exposure with physiological adaptations, cold stress-induced physiological responses and brown fat activation on health are also discussed. This is important to take into consideration, as sauna bathing is frequently associated with cooling periods in cold(er) environments, but their combination remains poorly investigated. We finally propose, therefore, that possible additive effects of heat- and cold-stress-induced adaptations and effects on health would be worthy of further investigation.

Regulation of bone blood flow in humans: The role of nitric oxide, prostaglandins, and adenosine

The mechanisms that regulate bone blood flow (BBF) in humans are largely unknown. Animal studies suggest that nitric oxide (NO) could be involved, and in this study, we investigated the effects of inhibition of nitric oxide synthase (NOS) alone and in combination with inhibition of cyclooxygenase (COX) enzyme, thus prostaglandin (PG) synthesis on femoral bone marrow blood flow by positron emission tomography in healthy young men at rest and during one-leg dynamic exercise. In an additional group of healthy men, the role of adenosine (ADO) in the regulation of BBF during exercise was investigated by use of an adenosine receptor blocker (aminophylline). Inhibitors were directly infused into the femoral artery. Resting BBF was 1.1 ± 0.4 mL 100 g^-1 min^-1 and increased to almost sixfold in response to exercise (6.3 ± 1.5 mL 100 g^-1  min^-1 ). Inhibition of NOS reduced BBF at rest to 0.7 ± 0.3 mL 100 g^-1  min^-1 (P = .036), but did not affect BBF significantly during exercise (5.5 ± 1.4 mL 100 g^-1  min^-1 , P = .25). On the other hand, while combined NOS and COX inhibition did not cause any further reduction of blood flow at rest (0.6 ± 0.2 mL 100 g^-1 min^-1 ), the combined blockade reduced BBF during exercise by ~21%, to 5.0 ± 1.8 mL 100 g^-1  min^-1 (P = .014). Finally, the ADO inhibition during exercise reduced BBF from 5.5 ± 1.9 mL 100 g^-1  min^-1 to 4.6 ± 1.2 mL 100 g^-1  min^-1 (P = .045). In conclusion, our results support the view that NO is involved in controlling bone marrow blood flow at rest, and NO, PG, and ADO play important roles in controlling human BBF during exercise.

The noninvasive simultaneous measurement of tissue oxygenation and microvascular hemodynamics during incremental handgrip exercise

Limb blood flow increases linearly with exercise intensity; however, invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We tested the hypotheses that 1) brachial artery blood flow (Q̇BA) would increase with increasing exercise intensity until task failure, 2) blood flow index of the flexor digitorum superficialis (BFIFDS) measured noninvasively via diffuse correlation spectroscopy would plateau at a submaximal work rate, and 3) muscle oxygenation characteristics (total-[heme], deoxy-[heme], and percentage saturation) measured noninvasively with near-infrared spectroscopy would demonstrate a plateau at a similar work rate as BFIFDS. Sixteen subjects (23.3 ± 3.9 yr, 170.8 ± 1.9 cm, 72.8 ± 3.4 kg) participated in this study. Peak power (Ppeak) was determined for each subject (1.8 ± 0.4 W) via an incremental handgrip exercise test. Q̇BA, BFIFDS, total-[heme], deoxy-[heme], and percentage saturation were measured during each stage of the exercise test. On a subsequent testing day, muscle activation measurements of the FDS (RMSFDS) were collected during each stage of an identical incremental handgrip exercise test via electromyography from a subset of subjects ( n = 7). Q̇BA increased with exercise intensity until the final work rate transition ( P < 0.05). No increases in BFIFDS or muscle oxygenation characteristics were observed at exercise intensities greater than 51.5 ± 22.9% of Ppeak. No submaximal plateau in RMSFDS was observed. Whereas muscle activation of the FDS increased until task failure, noninvasively measured indices of perfusive and diffusive muscle microvascular oxygen delivery demonstrated submaximal plateaus.

NEW & NOTEWORTHY Invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We demonstrate that indices of perfusive and diffusive microvascular oxygen transport to skeletal muscle, measured completely noninvasively, plateau at submaximal work rates during incremental exercise, even though limb blood flow and muscle recruitment continued to increase.

Evidence of a metabolic reserve in the skeletal muscle of elderly people

The purpose of the present study was to determine whether mitochondrial function is limited by O₂ availability or the intrinsic capacity of mitochondria to synthesize ATP in elderly individuals. To this aim, we examined, in comparison to free-flow conditions (FF), the effect of superimposing reactive hyperemia (RH), induced by a period of brief ischemia during the last min of exercise, on O₂ availability and mitochondrial function in the calf muscle. 12 healthy, untrained, elderly subjects performed dynamic plantar flexion exercise and phosphorus magnetic resonance spectroscopy (³¹P-MRS), near-infrared spectroscopy (NIRS), and Doppler ultrasound were used to assess muscle metabolism and peripheral hemodynamics. Limb blood flow [area under the curve (AUC), FF: 1.5±0.5L; RH: 3.2±1.1L, P<0.01] and convective O₂ delivery (AUC, FF: 0.30±0.13L; RH: 0.64±0.29L, P<0.01) were significantly increased in RH in comparison to FF. RH was also associated with significantly higher capillary blood flow (P<0.05) and this resulted in a 33% increase in estimated peak mitochondrial ATP synthesis rate (FF: 24±11 mM.min⁻¹; RH: 31±7 mM.min⁻¹, P<0.05). These results document a hemodynamic reserve in the contracting calf muscle of the elderly accessible by superimposing reactive hyperemia. Furthermore, this increase in O₂ availability enhanced mitochondrial function thus indicating a skeletal muscle metabolic reserve despite advancing age and low level of physical activity.

The physiology of submaximal exercise: The steady state concept

The steady state concept implies that the oxygen flow is invariant and equal at each level along the respiratory system. The same is the case with the carbon dioxide flow. This condition has several physiological consequences, which are analysed. First, we briefly discuss the mechanical efficiency of exercise and the energy cost of human locomotion, as well as the roles played by aerodynamic work and frictional work. Then we analyse the equations describing the oxygen flow in lungs and in blood, the effects of ventilation and of the ventilation - perfusion inequality, and the interaction between diffusion and perfusion in the lungs. The cardiovascular responses sustaining gas flow increase in blood are finally presented. An equation linking ventilation, circulation and metabolism is developed, on the hypothesis of constant oxygen flow in mixed venous blood. This equation tells that, if the pulmonary respiratory quotient stays invariant, any increase in metabolic rate is matched by a proportional increase in ventilation, but by a less than proportional increase in cardiac output.

The effect of nitric oxide synthase inhibition with and without inhibition of prostaglandins on blood flow in different human skeletal muscles

PURPOSE

Animal studies suggest that the inhibition of nitric oxide synthase (NOS) affects blood flow differently in different skeletal muscles according to their muscle fibre type composition (oxidative vs glycolytic). Quadriceps femoris (QF) muscle consists of four different muscle parts: vastus intermedius (VI), rectus femoris (RF), vastus medialis (VM), and vastus lateralis (VL) of which VI is located deep within the muscle group and is generally regarded to consist mostly of oxidative muscle fibres.

METHODS

We studied the effect of NOS inhibition on blood flow in these four different muscles by positron emission tomography in eight young healthy men at rest and during one-leg dynamic exercise, with and without combined blockade with prostaglandins.

RESULTS

At rest blood flow in the VI (2.6 ± 1.1 ml/100 g/min) was significantly higher than in VL (1.9 ± 0.6 ml/100 g/min, p = 0.015) and RF (1.7 ± 0.6 ml/100 g/min, p = 0.0015), but comparable to VM (2.4 ± 1.1 ml/100 g/min). NOS inhibition alone or with prostaglandins reduced blood flow by almost 50% (p < 0.001), but decrements were similar in all four muscles (drug × muscle interaction, p = 0.43). During exercise blood flow was also the highest in VI (45.4 ± 5.5 ml/100 g/min) and higher compared to VL (35.0 ± 5.5 ml/100 g/min), RF (38.4 ± 7.4 ml/100 g/min), and VM (36.2 ± 6.8 ml/100 g/min). NOS inhibition alone did not reduce exercise hyperemia (p = 0.51), but combined NOS and prostaglandin inhibition reduced blood flow during exercise (p = 0.002), similarly in all muscles (drug × muscle interaction, p = 0.99).

CONCLUSION

NOS inhibition, with or without prostaglandins inhibition, affects blood flow similarly in different human QF muscles both at rest and during low-to-moderate intensity exercise.

The Spatial Distribution of Absolute Skeletal Muscle Deoxygenation During Ramp-Incremental Exercise Is Not Influenced by Hypoxia

Time-resolved near-infrared spectroscopy (TRS-NIRS) allows absolute quantitation of deoxygenated haemoglobin and myoglobin concentration ([HHb]) in skeletal muscle. We recently showed that the spatial distribution of peak [HHb] within the quadriceps during moderate-intensity cycling is reduced with progressive hypoxia and this is associated with impaired aerobic energy provision. We therefore aimed to determine whether reduced spatial distribution of skeletal muscle [HHb] was associated with impaired aerobic energy transfer during exhaustive ramp-incremental exercise in hypoxia. Seven healthy men performed ramp-incremental cycle exercise (20 W/min) to exhaustion at 3 fractional inspired O2 concentrations (FIO2): 0.21, 0.16, 0.12. Pulmonary O2 uptake ([Formula: see text]) was measured using a flow meter and gas analyser system. Lactate threshold (LT) was estimated non-invasively. Absolute muscle deoxygenation was quantified by multichannel TRS-NIRS from the rectus femoris and vastus lateralis (proximal and distal regions). [Formula: see text] and LT were progressively reduced (p<0.05) with hypoxia. There was a significant effect (p<0.05) of FIO2 on [HHb] at baseline, LT, and peak. However the spatial variance of [HHb] was not different between FIO2 conditions. Peak total Hb ([Hbtot]) was significantly reduced between FIO2 conditions (p<0.001). There was no association between reductions in the spatial distribution of skeletal muscle [HHb] and indices of aerobic energy transfer during ramp-incremental exercise in hypoxia. While regional [HHb] quantified by TRS-NIRS at exhaustion was greater in hypoxia, the spatial distribution of [HHb] was unaffected. Interestingly, peak [Hbtot] was reduced at the tolerable limit in hypoxia implying a vasodilatory reserve may exist in conditions with reduced FIO2.

Heterogeneity of Muscle Blood Flow and Metabolism: Influence of Exercise, Aging, and Disease States

The systematic increase in V˙O2 uptake and O2 extraction with increasing work rates conceals a substantial heterogeneity of O2 delivery (Q˙O2)-to- V˙O2 matching across and within muscles and other organs. We hypothesize that whether increased/decreased Q˙O2/V˙O2 heterogeneity can be judged as "good" or "bad," for example, after exercise training or in aged individuals or with disease (heart failure, diabetes) depends on the resultant effects on O2 transport and contractile performance.

Muscle deoxygenation in the quadriceps during ramp incremental cycling: Deep vs. superficial heterogeneity

Muscle deoxygenation (i.e., deoxy[Hb + Mb]) during exercise assesses the matching of oxygen delivery (Q̇O2) to oxygen utilization (V̇O2). Until now limitations in near-infrared spectroscopy (NIRS) technology did not permit discrimination of deoxy[Hb + Mb] between superficial and deep muscles. In humans, the deep quadriceps is more highly vascularized and oxidative than the superficial quadriceps. Using high-power time-resolved NIRS, we tested the hypothesis that deoxygenation of the deep quadriceps would be less than in superficial muscle during incremental cycling exercise in eight males. Pulmonary V̇O2 was measured and muscle deoxy[Hb + Mb] was determined in the superficial vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF-s) and the deep rectus femoris (RF-d). deoxy[Hb + Mb] in RF-d was significantly less than VL at 70% (67.2 ± 7.0 vs. 75.5 ± 10.7 μM) and 80% (71.4 ± 11.0 vs. 79.0 ± 15.4 μM) of peak work rate (WR(peak)), but greater than VL and VM at WR(peak) (87.7 ± 32.5 vs. 76.6 ± 17.5 and 75.1 ± 19.9 μM). RF-s was intermediate at WR(peak) (82.6 ± 18.7 μM). Total hemoglobin and myoglobin concentration and tissue oxygen saturation were significantly greater in RF-d than RF-s throughout exercise. The slope of deoxy[Hb + Mb] increase (proportional to Q̇O2/V̇O2) in VL and VM slowed markedly above 70% WR(peak), whereas it became greater in RF-d. This divergent deoxygenation pattern may be due to a greater population of slow-twitch muscle fibers in the RF-d muscle and the differential recruitment profiles and vascular and metabolic control properties of specific fiber populations within superficial and deeper muscle regions.

Organ-specific physiological responses to acute physical exercise and long-term training in humans

Virtually all tissues in the human body rely on aerobic metabolism for energy production and are therefore critically dependent on continuous supply of oxygen. Oxygen is provided by blood flow, and, in essence, changes in organ perfusion are also closely associated with alterations in tissue metabolism. In response to acute exercise, blood flow is markedly increased in contracting skeletal muscles and myocardium, but perfusion in other organs (brain and bone) is only slightly enhanced or is even reduced (visceral organs). Despite largely unchanged metabolism and perfusion, repeated exposures to altered hemodynamics and hormonal milieu produced by acute exercise, long-term exercise training appears to be capable of inducing effects also in tissues other than muscles that may yield health benefits. However, the physiological adaptations and driving-force mechanisms in organs such as brain, liver, pancreas, gut, bone, and adipose tissue, remain largely obscure in humans. Along these lines, this review integrates current information on physiological responses to acute exercise and to long-term physical training in major metabolically active human organs. Knowledge is mostly provided based on the state-of-the-art, noninvasive human imaging studies, and directions for future novel research are proposed throughout the review.

Reduction of V̇O2 slow component by priming exercise: novel mechanistic insights from time-resolved near-infrared spectroscopy

Novel time-resolved near-infrared spectroscopy (TR-NIRS), with adipose tissue thickness correction, was used to test the hypotheses that heavy priming exercise reduces the V̇_O2 slow component (V̇_O2SC) (1) by elevating microvascular [Hb] volume at multiple sites within the quadriceps femoris (2) rather than reducing the heterogeneity of muscle deoxygenation kinetics. Twelve subjects completed two 6-min bouts of heavy work rate exercise, separated by 6 min of unloaded cycling. Priming exercise induced faster overall V̇_O2 kinetics consequent to a substantial reduction in the V̇_O2SC (0.27 ± 0.12 vs. 0.11 ± 0.09 L·min⁻¹, P < 0.05) with an unchanged primary V̇_O2 time constant. An increased baseline for the primed bout [total (Hb + Mb)] (197.5 ± 21.6 vs. 210.7 ± 22.5 μmol L⁻¹, P < 0.01), reflecting increased microvascular [Hb] volume, correlated significantly with the V̇_O2SC reduction. At multiple sites within the quadriceps femoris, priming exercise reduced the baseline and slowed the increase in [deoxy (Hb + Mb)]. Changes in the intersite coefficient of variation in the time delay and time constant of [deoxy (Hb + Mb)] during the second bout were not correlated with the V̇_O2SC reduction. These results support a mechanistic link between priming exercise-induced increase in muscle [Hb] volume and the reduced V̇_O2SC that serves to speed overall V̇_O2 kinetics. However, reduction in the heterogeneity of muscle deoxygenation kinetics does not appear to be an obligatory feature of the priming response.

Performance effects and metabolic consequences of caffeine and caffeinated energy drink consumption on glucose disposal

This review documents two opposing effects of caffeine and caffeine-containing energy drinks, i.e., their positive effects on athletic performance and their negative impacts on glucose tolerance in the sedentary state. Analysis of studies examining caffeine administration prior to performance-based exercise showed caffeine improved completion time by 3.6%. Similar analyses following consumption of caffeine-containing energy drinks yielded positive, but more varied, benefits, which were likely due to the diverse nature of the studies performed, the highly variable composition of the beverages consumed, and the range of caffeine doses administered. Conversely, analyses of studies administering caffeine prior to either an oral glucose tolerance test or insulin clamp showed a decline in whole-body glucose disposal of ~30%. The consequences of this resistance are unknown, but there may be implications for the development of a number of chronic diseases. Both caffeine-induced performance enhancement and insulin resistance converge with the primary actions of caffeine on skeletal muscle.

Dynamic heterogeneity of exercising muscle blood flow and O2 utilization

Resolving the bases for different physiological functioning or exercise performance within a population is dependent on our understanding of control mechanisms. For example, when most young healthy individuals run or cycle at moderate intensities, oxygen uptake (VO2) kinetics are rapid and the amplitude of the VO2 response is not constrained by O2 delivery. For this to occur, muscle O2 delivery (i.e., blood flow × arterial O2 concentration) must be coordinated superbly with muscle O2 requirements (VO2), the efficacy of which may differ among muscles and distinct fiber types. When the O2 transport system succumbs to the predations of aging or disease (emphysema, heart failure, and type 2 diabetes), muscle O2 delivery and O2 delivery-VO2 matching and, therefore, muscle contractile function become impaired. This forces greater influence of the upstream O2 transport pathway on muscle aerobic energy production, and the O2 delivery-VO2 relationship(s) assumes increased importance. This review is the first of its kind to bring a broad range of available techniques, mostly state of the art, including computer modeling, radiolabeled microspheres, positron emission tomography, magnetic resonance imaging, near-infrared spectroscopy, and phosphorescence quenching to resolve the O2 delivery-VO2 relationships and inherent heterogeneities at the whole body, interorgan, muscular, intramuscular, and microvascular/myocyte levels. Emphasis is placed on the following: 1) intact humans and animals as these provide the platform essential for framing and interpreting subsequent investigations, 2) contemporary findings using novel technological approaches to elucidate O2 delivery-VO2 heterogeneities in humans, and 3) future directions for investigating how normal physiological responses can be explained by O2 delivery-VO2 heterogeneities and the impact of aging/disease on these processes.

Maximal oxygen consumption in healthy humans: theories and facts

This article reviews the concept of maximal oxygen consumption ([Formula: see text]) from the perspective of multifactorial models of [Formula: see text] limitation. First, I discuss procedural aspects of [Formula: see text] measurement: the implications of ramp protocols are analysed within the theoretical work of Morton. Then I analyse the descriptive physiology of [Formula: see text], evidencing the path that led to the view of monofactorial cardiovascular or muscular [Formula: see text] limitation. Multifactorial models, generated by the theoretical work of di Prampero and Wagner around the oxygen conductance equation, represented a radical change of perspective. These models are presented in detail and criticized with respect to the ensuing experimental work. A synthesis between them is proposed, demonstrating how much these models coincide and converge on the same conclusions. Finally, I discuss the cases of hypoxia and bed rest, the former as an example of the pervasive effects of the shape of the oxygen equilibrium curve, the latter as a neat example of adaptive changes concerning the entire respiratory system. The conclusion is that the concept of cardiovascular [Formula: see text] limitation is reinforced by multifactorial models, since cardiovascular oxygen transport provides most of the [Formula: see text] limitation, at least in normoxia. However, the same models show that the role of peripheral resistances is significant and cannot be neglected. The role of peripheral factors is greater the smaller is the active muscle mass. In hypoxia, the intervention of lung resistances as limiting factors restricts the role played by cardiovascular and peripheral factors.

Positron emission tomography detects greater blood flow and less blood flow heterogeneity in the exercising skeletal muscles of old compared with young men during fatiguing contractions

The purpose of this study was to investigate blood flow and its heterogeneity within and among the knee muscles in five young (26 ± 6 years) and five old (77 ± 6 years) healthy men with similar levels of physical activity while they performed two types of submaximal fatiguing isometric contraction that required either force or position control. Positron emission tomography (PET) and [(15)O]-H2O were used to determine blood flow at 2 min (beginning) and 12 min (end) after the start of the tasks. Young and old men had similar maximal forces and endurance times for the fatiguing tasks. Although muscle volumes were lower in the older subjects, total muscle blood flow was similar in both groups (young men: 25.8 ± 12.6 ml min(-1); old men: 25.1 ± 15.4 ml min(-1); age main effect, P = 0.77) as blood flow per unit mass of muscle in the exercising knee extensors was greater in the older (12.5 ± 6.2 ml min(-1) (100 g)(-1)) than the younger (8.6 ± 3.6 ml min(-1) (100 g)(-1)) men (age main effect, P = 0.001). Further, blood flow heterogeneity in the exercising knee extensors was significantly lower in the older (56 ± 27%) than the younger (67 ± 34%) men. Together, these data show that although skeletal muscles are smaller in older subjects, based on the intact neural drive to the muscle and the greater, less heterogeneous blood flow per gram of muscle, old fit muscle achieves adequate exercise hyperaemia.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

AltitudeOmics: on the consequences of high-altitude acclimatization for the development of fatigue during locomotor exercise in humans

The development of muscle fatigue is oxygen (O2)-delivery sensitive [arterial O2 content (CaO2) × limb blood flow (QL)]. Locomotor exercise in acute hypoxia (AH) is, compared with sea level (SL), associated with reduced CaO2 and exaggerated inspiratory muscle work (Winsp), which impairs QL, both of which exacerbate fatigue individually by compromising O2 delivery. Since chronic hypoxia (CH) normalizes CaO2 but exacerbates Winsp, we investigated the consequences of a 14-day exposure to high altitude on exercise-induced locomotor muscle fatigue. Eight subjects performed the identical constant-load cycling exercise (138 ± 14 W; 11 ± 1 min) at SL (partial pressure of inspired O2, 147.1 ± 0.5 Torr), in AH (73.8 ± 0.2 Torr), and in CH (75.7 ± 0.1 Torr). Peripheral fatigue was expressed as pre- to postexercise percent reduction in electrically evoked potentiated quadriceps twitch force (ΔQtw,pot). Central fatigue was expressed as the exercise-induced percent decrease in voluntary muscle activation (ΔVA). Resting CaO2 at SL and CH was similar, but CaO2 in AH was lower compared with SL and CH (17.3 ± 0.5, 19.3 ± 0.7, 20.3 ± 1.3 ml O2/dl, respectively). Winsp during exercise increased with acclimatization (SL: 387 ± 36, AH: 503 ± 53, CH: 608 ± 67 cmH2O·s(-1)·min(-1); P < 0.01). Exercise at SL did not induce central or peripheral fatigue. ΔQtw,pot was significant but similar in AH and CH (21 ± 2% and 19 ± 3%; P = 0.24). ΔVA was significant in both hypoxic conditions but smaller in CH vs. AH (4 ± 1% vs. 8 ± 2%; P < 0.05). In conclusion, acclimatization to severe altitude does not attenuate the substantial impact of hypoxia on the development of peripheral fatigue. In contrast, acclimatization attenuates, but does not eliminate, the exacerbation of central fatigue associated with exercise in severe AH.

Effect of nitric oxide synthase inhibition on the exchange of glucose and fatty acids in human skeletal muscle

Background

The role of nitric oxide in controlling substrate metabolism in humans is incompletely understood.

Methods

The present study examined the effect of nitric oxide blockade on glucose uptake, and free fatty acid and lactate exchange in skeletal muscle of eight healthy young males. Exchange was determined by measurements of muscle perfusion by positron emission tomography and analysis of arterial and femoral venous plasma concentrations of glucose, fatty acids and lactate. The measurements were performed at rest and during exercise without (control) and with blockade of nitric oxide synthase (NOS) with N^G-monomethyl-l-arginine (L-NMMA).

Results

Glucose uptake at rest was 0.40 ± 0.21 μmol/100 g/min and increased to 3.71 ± 2.53 μmol/100 g/min by acute one leg low intensity exercise (p < 0.01). Prior inhibition of NOS by L-NMMA did not affect glucose uptake, at rest or during exercise (0.40 ± 0.26 and 4.74 ± 2.69 μmol/100 g/min, respectively). In the control trial, there was a small release of free fatty acids from the limb at rest (−0.05 ± 0.09 μmol/100 g/min), whereas during inhibition of NOS, there was a small uptake of fatty acids (0.04 ± 0.05 μmol/100 g/min, p < 0.05). During exercise fatty acid uptake was increased to (0.89 ± 1.07 μmol/100 g/min), and there was a non-significant trend (p = 0.10) for an increased FFA uptake with NOS inhibition 1.23 ± 1.48 μmol/100 g/min) compared to the control condition. Arterial concentrations of all substrates and exchange of lactate over the limb at rest and during exercise remained unaltered during the two conditions.

Conclusion

In conclusion, inhibition of nitric oxide synthesis does not alter muscle glucose uptake during low intensity exercise, but affects free fatty acid exchange especially at rest, and may thus be involved in the modulation of energy metabolism in the human skeletal muscle.

Inhibition of α-adrenergic tone disturbs the distribution of blood flow in the exercising human limb

The role of neuronal regulation of human cardiovascular function remains incompletely elucidated, especially during exercise. Here we, by positron emission tomography, monitored tissue-specific blood flow (BF) changes in nine healthy young men during femoral arterial infusions of norepinephrine (NE) and phentolamine. At rest, the α-adrenoceptor agonist NE reduced BF by ~40%, similarly in muscles (from 3.2 ± 1.9 to 1.4 ± 0.3 ml·min(-1)·100 g(-1) in quadriceps femoris muscle), bone (from 1.1 ± 0.4 to 0.5 ± 0.2 ml·min(-1)·100 g(-1)) and adipose tissue (AT) (from 1.2 ± 0.7 to 0.7 ± 0.3 ml·min(-1)·100 g(-1)). During exercise, NE reduced exercising muscle BF by ~16%. BF in AT was reduced similarly as rest. The α-adrenoceptor antagonist phentolamine increased BF similarly in the different muscles and other tissues of the limb at rest. During exercise, BF in inactive muscle was increased 3.4-fold by phentolamine compared with exercise without drug, but BF in exercising muscles was not influenced. Bone and AT (P = 0.055) BF were also increased by phentolamine in the exercise condition. NE increased and phentolamine decreased oxygen extraction in the limb during exercise. We conclude that inhibition of α-adrenergic tone markedly disturbs the distribution of BF and oxygen extraction in the exercising human limb by increasing BF especially around inactive muscle fibers. Moreover, although marked functional sympatholysis also occurs during exercise, the arterial NE infusion that mimics the exaggerated sympathetic nerve activity commonly seen in patients with cardiovascular disease was still capable of directly limiting BF in the exercising leg muscles.

Bone blood flow and metabolism in humans: effect of muscular exercise and other physiological perturbations

Human bone blood flow and metabolism during physical exercise remains poorly characterized. In the present study we measured femoral bone blood flow and glucose uptake in young healthy subjects by positron emission tomography in three separate protocols. In 6 women, blood flow was measured in femoral bone at rest and during one-leg intermittent isometric exercise with increasing exercise intensities. In 9 men, blood flow in the femur was determined at rest and during dynamic one-leg exercise and two other physiological perturbations: moderate systemic hypoxia (14 O2 ) at rest and during exercise, and during intrafemoral infusion of high-dose adenosine. Bone glucose uptake was measured at rest and during dynamic one-leg exercise in 5 men. The results indicate that isometric exercise increased femoral bone blood flow from rest (1.8 ± 0.6 mL/100 g/min) to low intensity exercise (4.1 ± 1.5 mL/100 g/min, p = 0.01), but blood flow did not increase further with increasing intensity. Resting femoral bone blood flow in men was similar to that of women and dynamic one-leg exercise increased it to 4.2 ± 1.2 mL/100 g/min, p < 0.001. Breathing of hypoxic air did not change femoral bone blood flow at rest or during exercise, but intra-arterial infusion of adenosine during resting conditions increased bone blood flow to 5.7 ± 2.4 mL/100 g/min, to the level of moderate-intensity dynamic exercise. Dynamic one-leg exercise increased femoral bone glucose uptake 4.7-fold compared to resting contralateral leg. In conclusion, resting femoral bone blood flow increases by physical exercise, but appears to level off with increasing exercise intensities. Moreover, although moderate systemic hypoxia does not change bone blood flow at rest or during exercise, intra-arterially administered adenosine during resting conditions is capable of markedly enhancing bone blood flow in humans. Finally, bone glucose uptake also increases substantially in response to exercise.

Increasing exercise intensity reduces heterogeneity of glucose uptake in human skeletal muscles

Proper muscle activation is a key feature of survival in different tasks in daily life as well as sports performance, but can be impaired in elderly and in diseases. Therefore it is also clinically important to better understand the phenomenon that can be elucidated in humans non-invasively by positron emission tomography (PET) with measurements of spatial heterogeneity of glucose uptake within and among muscles during exercise. We studied six healthy young men during 35 minutes of cycling at relative intensities of 30% (low), 55% (moderate), and 75% (high) of maximal oxygen consumption on three separate days. Glucose uptake in the quadriceps femoris muscle group (QF), the main force producing muscle group in recreational cycling, and its four individual muscles, was directly measured using PET and 18F-fluoro-deoxy-glucose. Within-muscle heterogeneity was determined by calculating the coefficient of variance (CV) of glucose uptake in PET image voxels within the muscle of interest, and among-muscles heterogeneity of glucose uptake in QF was expressed as CV of the mean glucose uptake values of its separate muscles. With increasing intensity, within-muscle heterogeneity decreased in the entire QF as well as within its all four individual parts. Among-muscles glucose uptake heterogeneity also decreased with increasing intensity. However, mean glucose uptake was consistently lower and heterogeneity higher in rectus femoris muscle that is known to consist of the highest percentage of fast twitch type II fibers, compared to the other three QF muscles. In conclusion, these results show that in addition to increased contribution of distinct muscle parts, with increases in exercise intensity there is also an enhanced recruitment of muscle fibers within all of the four heads of QF, despite established differences in muscle-part specific fiber type distributions. Glucose uptake heterogeneity may serve as a useful non-invasive tool to elucidate muscle activation in aging and diseased populations.

Ex vivo measures of muscle mitochondrial capacity reveal quantitative limits of oxygen delivery by the circulation during exercise

Muscle mitochondrial respiratory capacity measured ex vivo provides a physiological reference to assess cellular oxidative capacity as a component in the oxygen cascade in vivo. In this article, the magnitude of muscle blood flow and oxygen uptake during exercise involving a small-to-large fraction of the body mass will be discussed in relation to mitochondrial capacity measured ex vivo. These analyses reveal that as the mass of muscle engaged in exercise increases from one-leg knee extension, to 2-arm cranking, to 2-leg cycling and x-country skiing, the magnitude of blood flow and oxygen delivery decrease. Accordingly, a 2-fold higher oxygen delivery and oxygen uptake per unit muscle mass are seen in vivo during 1-leg exercise compared to 2-leg cycling indicating a significant limitation of the circulation during exercise with a large muscle mass. This analysis also reveals that mitochondrial capacity measured ex vivo underestimates the maximal in vivo oxygen uptake of muscle by up to ∼2-fold. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Alveolar gas exchange and tissue oxygenation during incremental treadmill exercise, and their associations with blood O(2) carrying capacity

The magnitude and timing of oxygenation responses in highly active leg muscle, less active arm muscle, and cerebral tissue, have not been studied with simultaneous alveolar gas exchange measurement during incremental treadmill exercise. Nor is it known, if blood O(2) carrying capacity affects the tissue-specific oxygenation responses. Thus, we investigated alveolar gas exchange and tissue (m. vastus lateralis, m. biceps brachii, cerebral cortex) oxygenation during incremental treadmill exercise until volitional fatigue, and their associations with blood O(2) carrying capacity in 22 healthy men. Alveolar gas exchange was measured, and near-infrared spectroscopy (NIRS) was used to monitor relative concentration changes in oxy- (Δ[O(2)Hb]), deoxy- (Δ[HHb]) and total hemoglobin (Δ[tHb]), and tissue saturation index (TSI). NIRS inflection points (NIP), reflecting changes in tissue-specific oxygenation, were determined and their coincidence with ventilatory thresholds [anaerobic threshold (AT), respiratory compensation point (RC); V-slope method] was examined. Blood O(2) carrying capacity [total hemoglobin mass (tHb-mass)] was determined with the CO-rebreathing method. In all tissues, NIPs coincided with AT, whereas RC was followed by NIPs. High tHb-mass associated with leg muscle deoxygenation at peak exercise (e.g., Δ[HHb] from baseline walking to peak exercise vs. tHb-mass: r = 0.64, p < 0.01), but not with arm muscle- or cerebral deoxygenation. In conclusion, regional tissue oxygenation was characterized by inflection points, and tissue oxygenation in relation to alveolar gas exchange during incremental treadmill exercise resembled previous findings made during incremental cycling. It was also found out, that O(2) delivery to less active m. biceps brachii may be limited by an accelerated increase in ventilation at high running intensities. In addition, high capacity for blood O(2) carrying was associated with a high level of m. vastus lateralis deoxygenation at peak exercise.

Regulation of subcutaneous adipose tissue blood flow during exercise in humans

Regulation of subcutaneous adipose tissue blood flow (ATBF) remains poorly elucidated in humans, especially during exercise. In the present study we tested the role of adenosine in the regulation of ATBF adjacent to active and inactive thigh muscles during intermittent isometric knee-extension exercise (1 s contraction followed by 2 s rest with workloads of 50, 100, and 150 N) in six healthy young women. ATBF was measured using positron emission tomography (PET) without and with unspecific adenosine receptor inhibitor theophylline infused intravenously. Adipose regions were localized from fused PET and magnetic resonance images. Blood flow in subcutaneous adipose tissue adjacent to active muscle increased from rest (1.0 ± 0.3 ml·100 g(-1)·min(-1)) to exercise (P < 0.001) and along with increasing exercise intensity (50 N = 4.1 ± 1.4, 100 N = 5.4 ± 1.8, and 150 N = 6.9 ± 3.0 ml·100 g(-1)·min(-1), P = 0.03 for the increase). In contrast, ATBF adjacent to inactive muscle remained at resting levels with all intensities (∼1.0 ± 0.5 ml·100 g(-1)·min(-1)). During exercise theophylline prevented the increase in ATBF adjacent to active muscle especially during the highest exercise intensity (50 N = 4.3 ± 1.8 ml·100 g(-1)·min(-1), 100 N = 4.0 ± 1.5 ml·100 g(-1)·min(-1), and 150 N = 4.9 ± 1.8 ml·100 g(-1)·min(-1), P = 0.06 for an overall effect) but had no effect on blood flow adjacent to inactive muscle or adipose blood flow in resting contralateral leg. In conclusion, we report in the present study that 1) blood flow in subcutaneous adipose tissue of the leg is increased from rest to exercise in an exercise intensity-dependent manner, but only in the vicinity of working muscle, and 2) adenosine receptor antagonism attenuates this blood flow enhancement at the highest exercise intensities.

Local heating, but not indirect whole body heating, increases human skeletal muscle blood flow

For decades it was believed that direct and indirect heating (the latter of which elevates blood and core temperatures without directly heating the area being evaluated) increases skin but not skeletal muscle blood flow. Recent results, however, suggest that passive heating of the leg may increase muscle blood flow. Using the technique of positron-emission tomography, the present study tested the hypothesis that both direct and indirect heating increases muscle blood flow. Calf muscle and skin blood flows were evaluated from eight subjects during normothermic baseline, during local heating of the right calf [only the right calf was exposed to the heating source (water-perfused suit)], and during indirect whole body heat stress in which the left calf was not exposed to the heating source. Local heating increased intramuscular temperature of the right calf from 33.4 ± 1.0°C to 37.4 ± 0.8°C, without changing intestinal temperature. This stimulus increased muscle blood flow from 1.4 ± 0.5 to 2.3 ± 1.2 ml·100 g⁻¹·min⁻¹ (P < 0.05), whereas skin blood flow under the heating source increased from 0.7 ± 0.3 to 5.5 ± 1.5 ml·100 g⁻¹·min⁻¹ (P < 0.01). While whole body heat stress increased intestinal temperature by ∼1°C, muscle blood flow in the calf that was not directly exposed to the water-perfused suit (i.e., indirect heating) did not increase during the whole body heat stress (normothermia: 1.6 ± 0.5 ml·100 g⁻¹·min⁻¹; heat stress: 1.7 ± 0.3 ml·100 g⁻¹·min⁻¹; P = 0.87). Whole body heating, however, reflexively increased calf skin blood flow (to 4.0 ± 1.5 ml·100 g⁻¹·min⁻¹) in the area not exposed to the water-perfused suit. These data show that local, but not indirect, heating increases calf skeletal muscle blood flow in humans. These results have important implications toward the reconsideration of previously accepted blood flow distribution during whole body heat stress.

Skeletal muscle blood flow and oxygen uptake at rest and during exercise in humans: a pet study with nitric oxide and cyclooxygenase inhibition

The aim of the present study was to determine the effect of nitric oxide and prostanoids on microcirculation and oxygen uptake, specifically in the active skeletal muscle by use of positron emission tomography (PET). Healthy males performed three 5-min bouts of light knee-extensor exercise. Skeletal muscle blood flow and oxygen uptake were measured at rest and during the exercise using PET with H2O15 and 15O2 during: 1) control conditions; 2) nitric oxide synthase (NOS) inhibition by arterial infusion of NG-monomethyl-l-arginine (l-NMMA), and 3) combined NOS and cyclooxygenase (COX) inhibition by arterial infusion of l-NMMA and indomethacin. At rest, inhibition of NOS alone and in combination with indomethacin reduced ( P < 0.05) muscle blood flow. NOS inhibition increased ( P < 0.05) limb oxygen extraction fraction (OEF) more than the reduction in muscle blood flow, resulting in an ∼20% increase ( P < 0.05) in resting muscle oxygen consumption. During exercise, muscle blood flow and oxygen uptake were not altered with NOS inhibition, whereas muscle OEF was increased ( P < 0.05). NOS and COX inhibition reduced ( P < 0.05) blood flow in working quadriceps femoris muscle by 13%, whereas muscle OEF and oxygen uptake were enhanced by 51 and 30%, respectively. In conclusion, by specifically measuring blood flow and oxygen uptake by the use of PET instead of whole limb measurements, the present study shows for the first time in humans that inhibition of NO formation enhances resting muscle oxygen uptake and that combined inhibition of NOS and COX during exercise increases muscle oxygen uptake.

Regulation of human skeletal muscle perfusion and its heterogeneity during exercise in moderate hypoxia

Although many effects of both acute and chronic hypoxia on the circulation are well characterized, the distribution and regulation of blood flow (BF) heterogeneity in skeletal muscle during systemic hypoxia is not well understood in humans. We measured muscle BF within the thigh muscles of nine healthy young men using positron emission tomography during one-leg dynamic knee extension exercise in normoxia and moderate physiological systemic hypoxia (14% O(2) corresponding to approximately 3,400 m of altitude) without and with local adenosine receptor inhibition with femoral artery infusion of aminophylline. Systemic hypoxia reduced oxygen extraction of the limb but increased muscle BF, and this flow increment was confined solely to the exercising quadriceps femoris muscle. Exercising muscle BF heterogeneity was reduced from rest (P = 0.055) but was not affected by hypoxia. Adenosine receptor inhibition had no effect on capillary BF during exercise in either normoxia or hypoxia. Finally, one-leg exercise increased muscle BF heterogeneity both in the resting posterior hamstring part of the exercising leg and in the resting contralateral leg, whereas mean BF was unchanged. In conclusion, the results show that increased BF during one-leg exercise in moderate hypoxia is confined only to the contracting muscles, and the working muscle hyperemia appears not to be directly mediated by adenosine. Increased flow heterogeneity in noncontracting muscles likely reflects sympathetic nervous constraints to curtail BF increments in areas other than working skeletal muscles, but this effect is not potentiated in moderate systemic hypoxia during small muscle mass exercise.

Comparison of exogenous adenosine and voluntary exercise on human skeletal muscle perfusion and perfusion heterogeneity

Adenosine is a widely used pharmacological agent to induce a “high-flow” control condition to study the mechanisms of exercise hyperemia, but it is not known how well an adenosine infusion depicts exercise-induced hyperemia, especially in terms of blood flow distribution at the capillary level in human muscle. Additionally, it remains to be determined what proportion of the adenosine-induced flow elevation is specifically directed to muscle only. In the present study, we measured thigh muscle capillary nutritive blood flow in nine healthy young men using PET at rest and during the femoral artery infusion of adenosine (1 mgmin−1l thigh volume−1), which has previously been shown to induce a maximal whole thigh blood flow of ∼8 l/min. This response was compared with the blood flow induced by moderate- to high-intensity one-leg dynamic knee extension exercise. Adenosine increased muscle blood flow on average to 40 ± 7 ml·min−1·100 g muscle−1 with an aggregate value of 2.3 ± 0.6 l/min for the whole thigh musculature. Adenosine also induced a substantial change in blood flow distribution within individuals. Muscle blood flow during the adenosine infusion was comparable with blood flow in moderate- to high-intensity exercise (36 ± 9 ml·min−1·100 g muscle−1), but flow heterogeneity was significantly higher during the adenosine infusion than during voluntary exercise. In conclusion, a substantial part of the flow increase in the whole limb blood flow induced by a high-dose adenosine infusion is conducted through the physiological non-nutritive shunt in muscle and/or also through tissues of the limb other than muscle. Additionally, an intra-arterial adenosine infusion does not mimic exercise hyperemia, especially in terms of muscle capillary flow heterogeneity, while the often-observed exercise-induced changes in capillary blood flow heterogeneity likely reflect true changes in nutritive flow linked to muscle fiber and vascular unit recruitment.

Adenosine receptor antagonist and augmented vasodilation during hypoxic exercise

We tested the hypothesis that adenosine contributes to augmented skeletal muscle vasodilation during hypoxic exercise. In separate protocols, subjects performed incremental rhythmic forearm exercise (10% and 20% of maximum) during normoxia and normocapnic hypoxia (80% arterial O2 saturation). In protocol 1 (n = 8), subjects received an intra-arterial administration of saline (control) and aminophylline (adenosine receptor antagonist). In protocol 2 (n = 10), subjects received intra-arterial phentolamine (alpha-adrenoceptor antagonist) and combined phentolamine and aminophylline administration. Forearm vascular conductance (FVC; in ml x min(-1).100 mmHg(-1)) was calculated from forearm blood flow (in ml/min) and blood pressure (in mmHg). In protocol 1, the change in FVC (DeltaFVC; change from normoxic baseline) during hypoxic exercise with saline was 172 +/- 29 and 314 +/- 34 ml x min(-1) x 100 mmHg(-1) (10% and 20%, respectively). Aminophylline administration did not affect DeltaFVC during hypoxic exercise at 10% (190 +/- 29 ml x min(-1)x100 mmHg(-1), P = 0.4) or 20% (287 +/- 48 ml x min(-1) x 100 mmHg(-1), P = 0.3). In protocol 2, DeltaFVC due to hypoxic exercise with phentolamine infusion was 313 +/- 30 and 453 +/- 41 ml x min(-1) x 100 mmHg(-1) (10% and 20% respectively). DeltaFVC was similar at 10% (352 +/- 39 ml min(-1) x 100 mmHg(-1), P = 0.8) and 20% (528 +/- 45 ml x min(-1) x 100 mmHg(-1), P = 0.2) hypoxic exercise with combined phentolamine and aminophylline. In contrast, DeltaFVC to exogenous adenosine was reduced by aminophylline administration in both protocols (P < 0.05 for both). These observations suggest that adenosine receptor activation is not obligatory for the augmented hyperemia during hypoxic exercise in humans.

Measuring muscle blood flow: a key link between systemic and regional metabolism

PURPOSE OF REVIEW

To provide a brief overview of the main techniques to measure muscle blood flow in humans and highlight some of the strengths and weaknesses associated with each technique.

RECENT FINDINGS

Peak muscle blood flow values of 300 ml/min per 100 g are possible in humans during heavy exercise performed with small muscle mass. This value is far higher than that which appears in most textbooks. Accurate and reliable techniques are therefore essential in measuring muscle blood flow. Current invasive techniques commonly used include indicator dilution (thermodilution and dye dilution) and radiolabel tracer washout (e.g. 133Xe washout) methods. Although invasive techniques have provided valuable insight into tissue blood flow, noninvasive techniques such as venous occlusion plethysmography and Doppler ultrasound are frequently used and provide accurate measurements of blood flow. Newer imaging techniques (MRI, positron emission tomography, and contrast-enhanced ultrasonography) promise increased resolution of measurements of local blood flow, including in discrete tissues in which more classical techniques are not able to be used.

SUMMARY

Muscle blood flow is a key link in the interplay and regulation of systemic and local muscle metabolism. Recognizing the advantages and limitations of each technique is essential to translational researchers studying the effects of nutrition and metabolism on muscle blood flow.