Exercise Hyperemia Research Articles

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

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. 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. At rest blood flow in the VI (2.6±1.1ml/100g/min) was significantly higher than in VL (1.9±0.6ml/100g/min, p=0.015) and RF (1.7±0.6ml/100g/min, p=0.0015), but comparable to VM (2.4±1.1ml/100g/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±5.5ml/100g/min) and higher compared to VL (35.0±5.5ml/100g/min), RF (38.4±7.4ml/100g/min), and VM (36.2±6.8ml/100g/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). 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.

Potentiation of the NO-cGMP pathway and blood flow responses during dynamic exercise in healthy humans.

Previous work has shown nitric oxide (NO) contributes to ~15% of the hyperemic response to dynamic exercise in healthy humans. This NO-mediated vasodilation occurs, in part, via increases in intracellular cyclic guanosine monophosphate (cGMP), which is catabolized by phosphodiesterase. We sought to examine the effect of phosphodiesterase-5 (PDE-5) inhibition on forearm blood flow (FBF) responses to dynamic handgrip exercise in healthy humans and the role of NO. We hypothesized exercise hyperemia would be augmented by sildenafil citrate (SDF, PDE-5 inhibitor). We further hypothesized any effect of SDF on exercise hyperemia would be abolished with intra-arterial infusion of the NO synthase (NOS) inhibitor L-NG-monomethyl arginine (L-NMMA). FBF (Doppler ultrasound) was assessed at rest and during 5min of dynamic forearm handgrip exercise at 15% of maximal voluntary contraction under control (saline) conditions and during 3 experimental protocols: (1) oral SDF (n=10), (2) intra-arterial L-NMMA (n=20), (3) SDF and L-NMMA (n=10). FBF responses to intra-arterial sodium nitroprusside (NTP, NO donor) were also assessed. FBF increased with exercise (p<0.01). Intra-arterial infusion of L-NMMA resulted in a reduction in exercise hyperemia (17±1 to 15±1mL/dL/min, p<0.01). Although the hyperemic response to NTP was augmented by SDF (area under the curve: 41±7 vs 61±11 AU, p<0.01), there was no effect of SDF on exercise hyperemia (p=0.33). Despite improving NTP-mediated vasodilation, oral SDF failed to augment exercise hyperemia in young, healthy adults. These observations reflect a minor contribution of NO and the cGMP pathway during exercise hyperemia in healthy young humans.

Coronary Exercise Hyperemia Is Impaired in Patients with Peripheral Arterial Disease

Peripheral arterial disease (PAD) is an atherosclerotic vascular disease that affects over 200 million people worldwide. The hallmark of PAD is ischemic leg pain and this condition is also associated with an augmented blood pressure response to exercise, impaired vascular function, and high risk of myocardial infarction and cardiovascular mortality. In this study, we tested the hypothesis that coronary exercise hyperemia is impaired in PAD. Twelve patients with PAD and no overt coronary disease (65±2years, 7 men) and 15 healthy control subjects (64±2years, 9 men) performed supine plantar flexion exercise (30 contractions/min, increasing workload). A subset of subjects (n=7 PAD, n=8 healthy) also performed isometric handgrip exercise (40% of maximum voluntary contraction to fatigue). Coronary blood velocity in the left anterior descending artery was measured by transthoracic Doppler echocardiography; blood pressure and heart rate were monitored continuously. Coronary blood velocity responses to 4min of plantar flexion exercise (PAD: Δ2.4±1.2, healthy: Δ6.0±1.6cm/sec, P=0.039) and isometric handgrip exercise (PAD: Δ8.3±4.2, healthy: Δ16.9±3.6, P=0.033) were attenuated in PAD patients. These data indicate that coronary exercise hyperemia is impaired in PAD, which may predispose these patients to myocardial ischemia.

Blood flow responses to mild-intensity exercise in ectopic vs. orthotopic prostate tumors; dependence upon host tissue hemodynamics and vascular reactivity.

Given the critical role of tumor O2 delivery in patient prognosis and the rise in preclinical exercise oncology studies, we investigated tumor and host tissue blood flow at rest and during exercise as well as vascular reactivity using a rat prostate cancer model grown in two transplantation sites. In male COP/CrCrl rats, blood flow (via radiolabeled microspheres) to prostate tumors [R3327-MatLyLu cells injected in the left flank (ectopic) or ventral prostate (orthotopic)] and host tissue was measured at rest and during a bout of mild-intensity exercise. α-Adrenergic vasoconstriction to norepinephrine (NE: 10(-9) to 10(-4) M) was determined in arterioles perforating the tumors and host tissue. To determine host tissue exercise hyperemia in healthy tissue, a sham-operated group was included. Blood flow was lower at rest and during exercise in ectopic tumors and host tissue (subcutaneous adipose) vs. the orthotopic tumor and host tissue (prostate). During exercise, blood flow to the ectopic tumor significantly decreased by 25 ± 5% (SE), whereas flow to the orthotopic tumor increased by 181 ± 30%. Maximal vasoconstriction to NE was not different between arterioles from either tumor location. However, there was a significantly higher peak vasoconstriction to NE in subcutaneous adipose arterioles (92 ± 7%) vs. prostate arterioles (55 ± 7%). Establishment of the tumor did not alter host tissue blood flow from either location at rest or during exercise. These data demonstrate that blood flow in tumors is dependent on host tissue hemodynamics and that the location of the tumor may critically affect how exercise impacts the tumor microenvironment and treatment outcomes.

Effects of isometric handgrip (IHG) training of one forearm on reactive and exercise hyperaemia in the ipsilateral and contralateral arm of White European Young Men

There is evidence that IHG training reduces arterial blood pressure (ABP), particularly in hypertensive subjects and that it may also enhance reactive hyperaemia in the trained arm. We have tested in normotensive, young men, whether IHG improves reactive and exercise hyperaemia in the contralateral, untrained arm, which would be evidence of remote effects on dilator function. The study was performed on 10 recreationally active, White European men (18–25 years). IHG training comprised 4×3 min contractions at 30% Maximum voluntary contraction (MVC) at 5 min intervals, 4 days/week for 4 weeks, undertaken with the dominant arm. Before and after IHG training, forearm blood flow (FBF) was recorded by venous occlusion plethysmography in the trained arm and non‐trained contralateral arm, at rest and at intervals after 3‐mins arterial occlusion (reactive hyperemia), or 3‐min rhythmic handgrip contractions at 60% Maximum voluntary contraction (MVC; exercise hyperemia). As expected, MVC was increased after IHG training in the trained arm (29.0±1.3, vs 33.5±1.5*Kg, *: P&lt; 0.01, paired t‐test), but not in the untrained arm (27.0±0.9 vs 27.1±0.9Kg). IHG training did not affect resting ABP: 124±1.8/70±1.7 vs 125±1.8/72±1.6 mmHg, or resting FBF: 4–9±0.9 vs 5.1±0.8 ml.min−1.100ml−1 in dominant arm before vs after IHG training. However, reactive hyperaemia was increased in both the trained arm and non‐trained arm by IHG training: peak FBF increased from 41.4±1.9 to 50.8±2.0* ml.min−1. 100ml−1 (*: P&lt; 0.01 RMANOVA) in the trained arm, and from 41.3±4.8 to 52.8 ± 1.7* ml.min−1. 100ml−1 in the non‐trained arm. Further, exercise hyperaemia was increased in both arms by IHG training, peak FBF was increased from 75.6±2.5 to 110.2±2.6* ml.min−1. 100ml−1 in the trained arm and from 77.8±9.4 to 101.1±2.9* ml.min−1. 100ml−1. These results indicate that in healthy young men, IHG training of one arm for 4 weeks improves peak muscle power and induces a parallel increase in exercise hyperaemia associated with submaximal isometric contraction in the trained arm. In addition, our results indicate for the first time, that not only is there a concomitant increase in reactive hyperaemia in the trained arm, but also a substantial increase in reactive hyperaemia and in exercise hyperaemia to submaximal contraction in the contralateral, non‐trained arm, in the absence of a change in muscle power. The mechanisms underlying these changes remain to be determined but we propose that they reflect a generalised beneficial effect of IHG training on endothelial dilator function.Support or Funding InformationAlexander S. Onassis Public Benefit Foundation

Measurement of Nitric Oxide with EPR in Whole Blood during Peak Hand Grip Exercise in Women with Metabolic Syndrome

The contribution of bioavailable Nitric Oxide to the change in vascular function during exercise hyperemia is primarily concluded through the lack of functional changes in the presence of eNOS inhibitors. The aim of this study is to utilize EPR techniques to measure the changes in NO production during peak hand grip exercise to identify mechanistic differences of vascular function in women with Metabolic Syndrome compared to healthy controls.MethodsIn this study, 13 participants (5 MetSyn and 8 Controls) performed the procedure of graded handgrip exercise on a dynamic handgrip device while brachial blood flow was continuously measured with Doppler ultrasound and B‐mode (GE Vivid E with 6MHz linear probe). Following a brief rest, individuals were asked to perform handgrip exercise whereby the workload progressively increased in a ramp fashion (0.5 kg·min−1) until the required contraction rate (30 cpm) could no longer be maintained. At rest and immediately after halting exercise at task failure for individual peak resistance, whole blood was drawn into a prepared vaccutainer in a 1:1 venous blood to the metal chelator, defferoxamine (DF), in diethyldithiocarbamate (DETC) Krebs buffer. The samples were immediately flash frozen in liquid nitrogen and stored at −80 degrees Celsius. All pre and peak NO samples were measured and compared with known NO samples using an X‐band electron paramagnetic resonance spectrometer (Bruker Biospin, Karlsruhe, Germany). Whole blood with known concentrations of NO2 (1–10mM) and Na2S2O4 (20 mM) served as a reference.ResultsWhole blood NO increased significantly from rest to peak exercise in both groups (p=0.047) in addition to brachial conductance (p=0.0001). There was no significant difference in the rise in NO during peak exercise between the MetSyn and Control groups (p=0.67) however there is a significant difference in brachial conductance starting, peak and delta between the groups (p=0.021, 0.028, and 0.019).ConclusionThe whole blood Iron Chelated EPR technique is capable of detecting the degree of change in NO potentially contributing exercise hyperemia in different groups and potentially at variable grades of exercise. Although there is a significant difference in the vascular conductance between the groups, with the number of participants included in this study, there was no difference indicated between the groups for NO at rest and peak exercise.Support or Funding InformationResearch reported in this publication was supported by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under grant number P20GM103451.

Elevated Extracellular Potassium Prior to Muscle Contraction Reduces Exercise Hyperemia in Humans

During muscle contractions, an increase in interstitial potassium (K+) and subsequent activation of inwardly‐rectifying K+ (KIR) channels is thought to be a functional vasodilatory signal contributing to exercise hyperemia. Indeed, our laboratory has recently demonstrated that inhibition of KIR channels blunts exercise hyperemia ~30% in humans. In theory, the vasodilatory response to changes in interstitial K+ during muscle contraction could be minimized by artificially elevating the concentration of K+ perfusing the vasculature prior to the onset of muscle contractions, thus limiting further recruitment of this pathway and blunting the dilatory response to exercise. We tested the hypothesis that infusion of potassium chloride (KCl) to elevate extracellular potassium concentrations and induce hyperpolarization prior to the onset of muscle contractions would reduce the contribution of this mechanism to the dilatory response to exercise. In young adults (n = 10), we measured forearm blood flow (FBF; Doppler ultrasound) and calculated changes in vascular conductance (FVC; FBF/intra‐arterial blood pressure) during 4 minutes of rhythmic handgrip exercise at 10% of maximal voluntary contraction during 1) control conditions (CTRL), 2) local, intra‐arterial infusion of KCl to elevate the concentration of K+ prior to the initiation of exercise, and 3) intra‐arterial infusion of sodium nitroprusside (SNP) as a high flow control to match the K+‐induced vasodilation observed at rest. As intended, infusion of KCl or SNP elevated resting FVC similarly prior to the onset of exercise (CTRL: 40 ± 7 vs. KCl: 84 ± 13 and SNP: 85 ± 14 ml·min−1·100 mmHg−1; both P &lt; 0.01 vs. CTRL). Infusion of KCl diminished the subsequent vasodilatory response to handgrip exercise by ~31% when compared to control conditions (ΔFVC: KCl: 92 ± 14 vs. CTRL: 133 ± 16 ml·min−1·100 mmHg−1; P &lt; 0.01), whereas elevation of resting FVC with SNP prior to contractions did not change the vasodilatory response to exercise (ΔFVC, SNP: 117 ± 15 vs. CTRL: 133 ± 16 ml·min−1·100 mmHg−1; P = NS). These findings indicate that K+‐mediated hyperpolarization prior to the onset of muscle contractions blunts the normal vasodilatory response to exercise, which supports a role for elevated K+ signaling in the hyperemic response to exercise in humans.Support or Funding InformationHL119337, HL095573

Flow‐Induced Dilation of Skeletal Muscle Feed Arteries: Relevance to Exercise Hyperemia

Flow‐induced dilation is the dilatory response seen in arteries as a result of an increase in shear stress along the endothelium. It is often thought that flow‐induced dilation contributes to arterial dilation during exercise as blood flow is increased to working muscle. Previous research compared in vivo shear stress values during rest and exercise to in vitro shear stress values and concluded that flow‐induced dilation of rat soleus feed arteries (Jasperse &amp; Laughlin) is unlikely to contribute to exercise hyperemia. However, the predominantly slow‐twitch soleus is unique in that it contributes to postural maintenance and receives higher amounts of blood flow during stance compared to other hindlimb muscles. Therefore, the aim of the present study was to determine if feed arteries from the predominantly fast‐twitch rat gastrocnemius muscle (GFA) demonstrated flow‐induced dilation and to further assess the potential role of flow‐induced dilation in GFA during exercise. GFA (n=12) from male Sprague‐Dawley rats were isolated and cannulated on two glass micropipettes and intraluminal diameter was measured using videomicroscopy. Pressure was maintained at 90 cmH2O using two independent fluid reservoirs. Changing the height of the fluid reservoirs created a pressure gradient that induced flow through the arteries. Pressure gradients of 2, 4, 6, 8, 10, 15, 20, 30, and 40 cmH2O were used. GFA dilated to increasing flow and shear stress values ranged up to 8.0 ± 0.8 dyn/cm2. The in vitro shear stress response range was compared to calculated in vivo shear stress values from previously published studies. Estimated GFA blood flow values in standing rats are 431 μl/min, in walking rats 816 μl/min, and in running rats 1510 μl/min. Corresponding shear stress values were 95 dyn/cm2 (standing), 180 dyn/cm2 (walking), and 332 dyn/cm2 (running). These in vivo shear stress values far exceed the shear stress values to which GFA dilated in vitro. This comparison suggests that maximal flow‐induced dilation occurs at shear stress values far below those present in GFA in non‐exercising rats and that flow‐induced dilation of GFA is an unlikely contributor to the increase in blow flow seen during exercise.Support or Funding InformationSeaver Research Council of Pepperdine University

Nitrate as a source of nitrite and nitric oxide during exercise hyperemia in rat skeletal muscle

The presence of nitric oxide (NO) synthase enzymes, mainly the NOS1 isoform, in skeletal muscle had been well established; however in the last decade it has been realized that NO may also be produced by reduction of nitrate and tissue nitrite. We have recently shown that rodent skeletal muscle contains unusually high concentrations of nitrate, compared to blood and other tissues, likely produced by oxidation of NOS1-produced NO. In the present study we measured nitrate and nitrite levels in Wistar rat leg tissue before and after acute and chronic exercise of the animals on a treadmill. We found a very large decrease of muscle nitrate levels immediately after exercise accompanied by a transient increase of nitrite levels. A significant decrease in blood nitrate levels accompanied the changes in muscle levels. Using skeletal muscle tissue homogenates we established that xanthine oxidoreductase (XOR) is at least partially responsible for the generation of nitrite and/or NO from nitrate and that this effect is increased by slight lowering of pH and by other processes related to the exercise itself. We hypothesize that the skeletal muscle nitrate reservoir contributes significantly to the generation of nitrite and then, probably via formation of NO, exercise-induced functional hyperemia. A model for these metabolic interconversions in mammals is presented. These reactions could explain the muscle-generated vasodilator causing increased blood flow, with induced contraction, exercise, or hypoxia, postulated more than 100 years ago.

Blood Flow during Handgrip Exercise in COPD: Effect of Vitamin C.

Chronic obstructive pulmonary disease (COPD) is associated with vascular dysfunction, possibly related to increased oxidative stress. Exercise hyperemia may similarly be impaired, which could have implications for exercise limitations in COPD. We tested if brachial blood flow (BBF) was reduced during handgrip exercise in COPD and if this response would be improved after vitamin C infusion. Doppler ultrasound was used to measure brachial blood flow and vascular conductance (BBF and BVC, respectively) during mild, rhythmic handgrip exercise (EX) under conditions of sham-saline and vitamin C. Measures of flow-mediated dilation (FMD) and nitroglycerine-mediated dilation were used to assess endothelial-dependent and independent dilation, respectively. Biomarkers of antioxidants (vitamin C, superoxide dismutase [SOD], catalase), oxidative stress (malondialdehyde [MDA], advanced oxidation protein products [AOPP]), and nitric oxide metabolism (NOx) were measured in blood plasma. Ten COPD patients with moderate COPD and 10 healthy age-matched controls participated. COPD patients had similar increases in BBF and BVC during EX, compared with controls. Vitamin C was not found to have an effect on blood flow parameters during exercise (P > 0.05). Markers of endothelial-dependent dilation (FMD) and nitroglycerin-mediated dilation were similar between groups at baseline; FMD improved similarly in both groups after vitamin C. Moderate COPD patients have a preserved BBF response during handgrip exercise and do not exhibit endothelial dysfunction. Despite an increase in endothelial-dependent dilation after vitamin C, BBF remained unchanged, suggesting a limited impact of endothelial-derived NO in determining the blood flow response to handgrip exercise in older individuals. COPD patients of moderate severity, screened for cardiovascular disease, do not exhibit endothelial dysfunction and have similar exercise blood flow responses to healthy controls.

Positional differences in reactive hyperemia provide insight into initial phase of exercise hyperemia.

Studies have reported a greater blood flow response to muscle contractions when the limb is below the heart compared with above the heart, and these results have been interpreted as evidence for a skeletal muscle pump contribution to exercise hyperemia. If limb position affects the blood flow response to other vascular challenges such as reactive hyperemia, this interpretation may not be correct. We hypothesized that the magnitude of reactive hyperemia would be greater with the limb below the heart. Brachial artery blood flow (Doppler ultrasound) and blood pressure (finger-cuff plethysmography) were measured in 10 healthy volunteers. Subjects lay supine with one arm supported in two different positions: above or below the heart. Reactive hyperemia was produced by occlusion of arterial inflow for varying durations: 0.5 min, 1 min, 2 min, or 5 min in randomized order. Peak increases in blood flow were 77 ± 11, 178 ± 24, 291 ± 25, and 398 ± 33 ml/min above the heart and 96 ± 19, 279 ± 62, 550 ± 60, and 711 ± 69 ml/min below the heart (P < 0.05). Thus a standard stimulus (vascular occlusion) elicited different responses depending on limb position. To determine whether these differences were due to mechanisms intrinsic to the arterial wall, a second set of experiments was performed in which acute intraluminal pressure reduction for 0.5 min, 1 min, 2 min, or 5 min was performed in isolated rat soleus feed arteries (n = 12). The magnitude of dilation upon pressure restoration was greater when acute pressure reduction occurred from 85 mmHg (mimicking pressure in the arm below the heart; 28.3 ± 7.9, 37.5 ± 5.9, 55.1 ± 9.9, and 68.9 ± 8.6% dilation) than from 48 mmHg (mimicking pressure in the arm above the heart; 20.8 ± 4.8, 22.6 ± 4.4, 31.2 ± 5.8, and 49.2 ± 7.1% dilation). These data support the hypothesis that arm position differences in reactive hyperemia are at least partially mediated by mechanisms intrinsic to the arterial wall. Overall, these results suggest the need to reevaluate studies employing positional changes to examine muscle pump influences on exercise hyperemia.

Contracting human skeletal muscle maintains the ability to blunt α1‐adrenergic vasoconstriction during KIR channel and Na+/K+‐ATPase inhibition

During exercise there is a balance between vasoactive factors that facilitate increases in blood flow and oxygen delivery to the active tissue and the sympathetic nervous system, which acts to limit muscle blood flow for the purpose of blood pressure regulation. Functional sympatholysis describes the ability of contracting skeletal muscle to blunt the stimulus for vasoconstriction, yet the underlying signalling of this response in humans is not well understood. We tested the hypothesis that activation of inwardly rectifying potassium channels and the sodium-potassium ATPase pump, two potential vasodilator pathways within blood vessels, contributes to the ability to blunt α1 -adrenergic vasoconstriction. Our results show preserved blunting of α1 -adrenergic vasconstriction despite blockade of these vasoactive factors. Understanding this complex phenomenon is important as it is impaired in a variety of clinical populations. Sympathetic vasoconstriction in contracting skeletal muscle is blunted relative to that which occurs in resting tissue; however, the mechanisms underlying this 'functional sympatholysis' remain unclear in humans. We tested the hypothesis that α1 -adrenergic vasoconstriction is augmented during exercise following inhibition of inwardly rectifying potassium (KIR ) channels and Na(+) /K(+) -ATPase (BaCl2 +ouabain). In young healthy humans, we measured forearm blood flow (Doppler ultrasound) and calculated forearm vascular conductance (FVC) at rest, during steady-state stimulus conditions (pre-phenylephrine), and after 2min of phenylephrine (PE; an α1 -adrenoceptor agonist) infusion via brachial artery catheter in response to two different stimuli: moderate (15% maximal voluntary contraction) rhythmic handgrip exercise or adenosine infusion. In Protocol1 (n=11 subjects) a total of six trials were performed in three conditions: control (saline), combined enzymatic inhibition of nitric oxide (NO) and prostaglandin (PG) synthesis (l-NMMA+ketorolac) and combined inhibition of NO, PGs, KIR channels and Na(+) /K(+) -ATPase (l-NMMA+ketorolac+BaCl2 +ouabain). In Protocol2 (n=6) a total of four trials were performed in two conditions: control (saline), and combined KIR channel and Na(+) /K(+) -ATPase inhibition. All trials occurred after local β-adrenoceptor blockade (propranolol). PE-mediated vasoconstriction was calculated (%ΔFVC) in each condition. Contrary to our hypothesis, despite attenuated exercise hyperaemia of ∼30%, inhibition of KIR channels and Na(+) /K(+) -ATPase, combined with inhibition of NO and PGs (Protocol1) or alone (Protocol2) did not enhance α1 -mediated vasoconstriction during exercise (Protocol1: -27±3%; P=0.2vs. control, P=0.4vs. l-NMMA+ketorolac; Protocol2: -21±7%; P=0.9vs. control). Thus, contracting human skeletal muscle maintains the ability to blunt α1 -adrenergic vasoconstriction during combined KIR channel and Na(+) /K(+) -ATPase inhibition.

Effects of modest hyperoxia and oral vitamin C on exercise hyperaemia and reactive hyperaemia in healthy young men.

We have argued that breathing 40 % O2 attenuates exercise hyperaemia by decreasing production of O2-dependent vasodilators. However, breathing 100 % O2 attenuated endothelium-dependent vasodilatation evoked by acetylcholine and this effect was prevented by vitamin C, implicating reactive oxygen species (ROS). We have therefore used vitamin C to test the hypothesis that 40 % O2 modulates exercise hyperaemia and reactive hyperaemia independently of ROS. In a cross-over study on 10 male subjects (21.1 ± 0.84 years), we measured forearm blood flow (venous occlusion plethysmography) and calculated forearm vascular conductance (FVC) at rest and following static handgrip at 60 % maximum voluntary contraction for 2 min and following arterial occlusion for 2 min, after placebo or oral vitamin C (2000 mg), and when breathing air or 40 % O2. During air breathing, vitamin C augmented the peak increase in FVC following static contraction, or release of arterial occlusion, by ~50 or 60 %, respectively (P < 0.05). Breathing 40 % O2 in the presence of placebo attenuated post-contraction hyperaemia by ~25 % (P < 0.05), but had no effect on reactive hyperaemia. By contrast, in the presence of vitamin C, 40 % O2 attenuated the peak increase in FVC following static contraction, or release of arterial occlusion by ~25 and 50 %, respectively (P < 0.05). These results indicate that in young men, exercise hyperaemia following strenuous muscle contraction and reactive hyperaemia are blunted by ROS. However, they are also consistent with the view that modest hyperoxia induced by breathing 40 % O2 acts independently of ROS to attenuate not only post-contraction hyperaemia, but also reactive hyperaemia, by decreasing release of O2-dependent vasodilators.

Contribution Of The Skeletal Muscle Pump To Blood Flow At The Onset Of Contractions

There is a substantial reserve for increased skeletal muscle blood flow in response to exercise; however, the interaction of neural regulation, vasoactive metabolites, and mechanical characteristics are incompletely understood. The potential contribution of skeletal muscle contraction, the muscle pump (MP), to exercise hyperemia remains in question. PURPOSE: To determine MP contribution to muscle blood flow during transitions from rest to various metabolic rates. METHODS: Canine gastrocnemius muscle (n=7) was isolated and stimulated to produce rhythmic isometric tetanic contractions. The total blood flow (Qwc) kinetics were fitted by a mono-exponential function (tau), and the blood flow response to contraction with the mechanical contribution of MP removed (Qnc) was calculated. The Qnc calculation estimated blood flow in each contraction cycle as a linear variation of blood flow from a point immediately prior to, until the end of each contraction. The following transitions were investigated with spontaneous blood flow response intact: rest to tetanic contractions of 1/3 s; rest to 2/3 s; rest to 1/1 s; and during the transition from 1/3 s to 2/3s. RESULTS: During 1/3 s stimulation, Qnc exceeded Qwc by the second contraction, indicating a negative contribution of MP to total flow from that point on. During 2/3 s stimulation, Qwc exceeded Qnc for at least six contractions but declined rapidly to a negative contribution beyond 20 s. During 1/1 s stimulation, MP contribution declined from onset but Qwc remained greater than Qnc throughout indicating a positive contribution to total flow. In all transitions tau was not different (p=ns) between Qwc and Qnc: 1/3 s stimulation rate, tau = 12.8 ± 5.5 s vs 11.8 ± 3.2 s, respectively; rest to 2/3 s, tau = 25.6 ± 12.0 s vs 22.1 ± 1.3 s; rest to 1/1 s, tau = 16.7 ± 3.0 s vs 22.1 ± 1.3 s; transition from 1/3 s to 2/3s, tau = 21.2 ± 3.4 s vs 21.7 ± 4.8 s. CONCLUSION: MP likely contributes to local perfusion at exercise onset with diminishing returns as rhythmic contractions continue. The benefit to total skeletal muscle blood flow contributed by MP is dependent on contraction frequency. Supported by BBSRC UK, BB/F019521/1.

Response to: control of muscle exercise hyperaemia: are the mechanisms found in transition?

I would like to thank Drs Poole and Behnke for their interest in our review and for initiating a scientific discussion (Poole & Behnke, 2015). If the mechanisms regulating exercise hyperaemia are to be identified in the transition from rest to exercise, it requires that the regulatory mechanisms at the onset of exercise and during ‘steady-state’ exercise conditions are the same. Our knowledge of the regulatory mechanisms during both conditions is incomplete, but it is clear that metabolic processes are too slow to account for the initial blood flow response. It therefore seems likely that the mechanism(s) initiating exercise hyperaemia are, at least in part, different from the ones that sustain it (Clifford & Hellsten, 2004). Thus, although the mechanisms governing the initial rise in exercise hyperaemia are intriguing and important, they do not necessarily provide insight into the regulatory mechanism(s) during ‘steady-state’ conditions or vice versa. It is postulated by Drs Poole and Behnke that ‘steady-state’ exercise hyperaemia for a given work rate is similar in sedentary people, elite athletes and patients and, based on this assumption, the authors make the bald statement that ‘steady-state’ blood flows may not be particularly insightful for resolving the control mechanisms. However, there is evidence demonstrating that exercise hyperaemia and heterogeneity are affected by training status (Saltin et al. 1976; Kalliokoski et al. 2001) and ageing (Proctor & Parker, 2006) as well as in disease states (Sullivan et al. 1989; Nyberg et al. 2012b). Moreover, when combined with other measurements (such as blood samples, microdialysis and/or biopsies), differences in metabolism and regulatory mechanisms can be revealed despite a similar exercise hyperaemia (Mortensen et al. 2012; Nyberg et al. 2012a). Based on the assumption that exercise hyperaemia is similar among populations, Drs Poole and Behnke suggest it is the oxygen uptake () kinetics that are insightful for explaining differences in exercise tolerance between an athlete and a heart failure patient, but neglect the importance of O2 delivery and its impact on muscle metabolism when the fatigue and thus exercise intolerance become apparent. In this light, it is critical to remark that the fatigue processes during walking/running are temporally more closely related to than the kinetics. Understanding why exercise hyperaemia and consequently O2 delivery are impaired in heart failure is key to understanding O2 utilization and exercise intolerance (Esposito et al. 2010). Our human studies have often been inspired by results obtained in advanced animal studies, and we have turned to animal models when limited in our human models. That said, there are clear anatomical and physiological differences between animals and humans that are important to cardiovascular regulation, and one should be very careful before extrapolating findings in animals to humans (see Rowell, 2007). It is my opinion that observations from animal studies should never be seen as definitive before they have been confirmed in humans. Drs Poole and Behnke indicate that the mechanisms governing exercise hyperaemia are different between populations, but the same holds true for species. Skeletal muscle perfusion rates can reach similar values in animals and humans, but the size of the heart in relation to body weight is much smaller in humans. How does this affect the kinetics and the contribution of O2-dependent and independent limitations to O2 utilization? It is clearly relevant to relate not only findings in humans to findings in animals, but also the other way around. I agree with Drs Poole and Behnke that a blend of animal and human investigations is the key to new insight into the regulatory mechanisms in health and how they are altered in disease states. I would like to reassure Drs Poole and Behnke that if we had been asked to write a more extensive review, we would certainly have included more animal work, multiple exercise conditions (for example transition, low and high intensities) and populations (for example, training status and patients). However, we were asked to write a short review on the Scandinavian perspective of blood flow regulation, and this is why our review is limited to the mechanisms regulating ‘steady-state’ exercise hyperaemia in healthy humans. None declared.

Exercise Training Reverses Age‐induced Coronary Microvascular Dysfunction

The risk for diastolic dysfunction increases with advancing age. We tested the hypotheses that 1) microvascular dysfunction underlies age‐related diastolic dysfunction, and 2) exercise training (ET) reverses age‐related microvascular and diastolic dysfunction. Old and young male Fischer 344 rats underwent 10 weeks of daily ET or remained sedentary cage controls. Cardiac function was assessed before and after ET. At the end of training, coronary blood flow was measured at rest and during treadmill exercise. The reactivity of the coronary resistance vasculature was assessed in isolated coronary arterioles. E/a ratio decreased with age and was partially restored by ET. Isovolumic relaxation time increased with age and this increase was reversed by ET. Resting blood flow to the left ventricular (LV) endocardium and the right ventricular free wall was decreased in old rats; however, ET increased resting blood flow to the septum, papillary muscles, LV endocardium and epicardium. Exercise hyperemia was reduced by age in the septum, and the LV endocardium and epicardium, but these age‐related reductions were reversed by ET. Age‐induced impairments of both contractile and relaxation responses of coronary arterioles were reversed by ET. Thus, microvascular dysfunction contributes to development of diastolic dysfunction with advancing age; however, ET at an advanced age reverses microvascular dysfunction and improves diastolic function.

O2 Dependent Contributions of Prostaglandins (PGI2 PGE2) to Exercise Hyperaemia in Young (Y) and Older (O) Men

We have shown that breathing 40% O2 and cyclooxygenase (COX) inhibition attenuate exercise hyperaemia to similar extents in Y and O men. PGI2 synthesis is generally attributed to endothelium and PGE2, to skeletal muscle fibres. Thus, in recreationally active Y (18‐25 yrs; n=12) and O (60‐76 yrs; n=12) males, we assayed venous efflux of PGs by ELISA following rhythmic and static handgrip contractions (60% MVC for 3 min, or exhaustion), during air breathing, 40%O2, and after COX blockade with aspirin (A) in a cross over design. In Y males, forearm vascular conductance (FVC) increased from 0.06±0.01 to 0.56±0.06 conductance units (CU) after rhythmic contractions in P/air, but to only 0.44±0.04*, 0.44±0.04* and 0.45±0.03* CU in P/O2, A/air and A/O2 (*P &lt; 0.05 vs P/air). Meanwhile, PGI2 increased from 18.0±6.1 to 349.8±61.7 pg/dl/min in P/air, but to 94.3±26.5*, 44.7±18.4* and 50.8±11.2* pg/dl/min in P/O2, A/air and A/O2, while PGE increased from 20.5±6.6 to 559.5±133.4 pg/dl/min in P/air, but to 189.8±33.8*, 165.4±52.9* and 170.3±43.0* pg/dl/min in P/O2, A/air and A/O2. Similarly in O males, FVC increased from 0.08±0.02 to 0.64±0.06 CU in P/air after rhythmic contraction, but to 0.44±0.06*, 0.46±0.05* and 0.52±0.06* CU in P/O2, A/air and A/O2. Meanwhile, PGI2 increased from 14.4±6.8 to 284.4±43.4 in P/air but to 84.6±25.9*, 52.8±20.3* and 74.6±38.1* pg/dl/min in P/O2, A/air and A/O2, while PGE increased from 16.6±6.6 to 781.6±192.5 in P/air, but to 368.8±188.8*, 357.9±106.4* and 382.3±131.0* pg/dl/min in P/O2, A/air and A/O2. Static contractions yielded similar data. Thus, we propose that PGI2 and PGE2 are released in O2‐dependent ways from endothelium and muscle fibres respectively and contribute to exercise hyperaemia in both Y and O men.

Does Adenosine make an O2 Dependent Contribution to Exercise Hyperaemia?

Our previous studies showed that breathing 40% O2 attenuated hyperaemia evoked in human forearm by static or rhythmic contractions and that cyclooxygenase (COX) blockade alone, or together with 40% O2 produced similar attenuation. This suggested the contribution of prostaglandins (PGs) to exercise hyperaemia is O2 dependent. Adenosine is also implicated in exercise hyperaemia; in the rat, its contribution is ~20% and is mediated by A2A receptors. We have now performed similar experiments to test its O2 dependency in male Wistar rats anesthetised with Alfaxan (20‐23 mg/kg/hr i.v.). 5 periods of rhythmic contraction were evoked by stimulating a sciatic nerve for 5 min each at 4 Hz (S1‐5) whilst recording arterial blood pressure (ABP) and femoral blood flow (FBF). In a Time control group (n‐7), responses evoked by S2‐5 were fully comparable. In the Experimental group (n‐8), integrated femoral vascular conductance (FVC; FBF/ABP) increased from 0.016±0.002 to 0.037±0.003 ml/min/mmHg in response to S2 when breathing air, but to only 0.029±0.002* (*P &lt; 0.01 vs. S2; 2‐way repeated measures ANOVA) during S3 when breathing 40% O2. After A2A receptor blockade with ZM241385 (0.05 mg/kg i.v.) when breathing 40% O2, S4 similarly increased FVC to only 0.03±0.004* ml/min/mmHg. Further, after subsequent blockade of COX with diclofenac (1 mg/kg i.v.), S5 again increased FVC to 0.03±0.003* ml/min/mmHg during 40% O2. Baseline FVC and the evoked muscle tension were unchanged. Thus, the increase in FVC was attenuated ~20% by breathing 40% O2 and in contrast to known effects during air breathing, A2A receptor blockade and COX blockade caused no further reductions during 40% O2. These results indicate that the contribution of adenosine to exercise hyperaemia, like that of PGs, is O2 dependent.

Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs.

This review focuses on how blood flow to contracting skeletal muscles is regulated during exercise in humans. The idea is that blood flow to the contracting muscles links oxygen in the atmosphere with the contracting muscles where it is consumed. In this context, we take a top down approach and review the basics of oxygen consumption at rest and during exercise in humans, how these values change with training, and the systemic hemodynamic adaptations that support them. We highlight the very high muscle blood flow responses to exercise discovered in the 1980s. We also discuss the vasodilating factors in the contracting muscles responsible for these very high flows. Finally, the competition between demand for blood flow by contracting muscles and maximum systemic cardiac output is discussed as a potential challenge to blood pressure regulation during heavy large muscle mass or whole body exercise in humans. At this time, no one dominant dilator mechanism accounts for exercise hyperemia. Additionally, complex interactions between the sympathetic nervous system and the microcirculation facilitate high levels of systemic oxygen extraction and permit just enough sympathetic control of blood flow to contracting muscles to regulate blood pressure during large muscle mass exercise in humans.

Coronary Blood Flow Regulation is Impaired During Exercise in Patients with Peripheral Arterial Disease

Peripheral arterial disease (PAD) is associated with an augmented blood pressure response while walking, impaired vascular function, and increased risk of myocardial infarction. We hypothesized that coronary exercise hyperemia is impaired in PAD. Seven PAD patients (67± 2 yrs) and 11 healthy controls (64 ± 2 yrs) performed dynamic plantar flexion exercise at progressive workloads (0.5 ‐ 7.0 kg) with their most symptomatic leg. Coronary blood flow velocity (CBV) in the left anterior descending artery was measured by transthoracic Doppler echocardiography and blood pressure and heart rate were monitored continuously. Rate Pressure Product (RPP) and the myocardial O2 supply/demand ratio (CBV/RPP *1000) were calculated. Since PAD patients exercised for a shorter duration, data were compared at the 2.0 kg workload, which all subjects performed with minimal pain. The change in RPP from rest to exercise at 2.0 kg was augmented in PAD patients vs. controls (2417 ± 489 vs. 1026 ± 143 mmHg*bpm, P = 0.01). The change in the myocardial O2 supply/demand ratio from rest to exercise was impaired in PAD patients vs. controls (2.94 ± 0.99 vs. 5.17 ± 1.35 au, P = 0.05). These data suggest that coronary hyperemia is inadequate for the large rise in myocardial metabolism during low intensity exercise in PAD, which may increase risk for adverse cardiovascular events.