Circulating ATP-induced vasodilatation overrides sympathetic vasoconstrictor activity in human skeletal muscle

Hypertension: a problem of organ blood flow supply-demand mismatch

This review introduces a new hypothesis that sympathetically mediated hypertensive diseases are caused, in the most part, by the activation of visceral afferent systems that are connected to neural circuits generating sympathetic activity. We consider how organ hypoperfusion and blood flow supply–demand mismatch might lead to both sensory hyper-reflexia and aberrant afferent tonicity. We discuss how this may drive sympatho-excitatory-positive feedback and extend across multiple organs initiating, or at least amplifying, sympathetic hyperactivity. The latter, in turn, compounds the challenge to sufficient organ blood flow through heightened vasoconstriction that both maintains and exacerbates hypertension.

Cardiovascular Adaptations to Exercise Training

Aerobic exercise training leads to cardiovascular changes that markedly increase aerobic power and lead to improved endurance performance. The functionally most important adaptation is the improvement in maximal cardiac output which is the result of an enlargement in cardiac dimension, improved contractility, and an increase in blood volume, allowing for greater filling of the ventricles and a consequent larger stroke volume. In parallel with the greater maximal cardiac output, the perfusion capacity of the muscle is increased, permitting for greater oxygen delivery. To accommodate the higher aerobic demands and perfusion levels, arteries, arterioles, and capillaries adapt in structure and number. The diameters of the larger conduit and resistance arteries are increased minimizing resistance to flow as the cardiac output is distributed in the body and the wall thickness of the conduit and resistance arteries is reduced, a factor contributing to increased arterial compliance. Endurance training may also induce alterations in the vasodilator capacity, although such adaptations are more pronounced in individuals with reduced vascular function. The microvascular net increases in size within the muscle allowing for an improved capacity for oxygen extraction by the muscle through a greater area for diffusion, a shorter diffusion distance, and a longer mean transit time for the erythrocyte to pass through the smallest blood vessels. The present article addresses the effect of endurance training on systemic and peripheral cardiovascular adaptations with a focus on humans, but also covers animal data.

Mechanical compression during repeated sustained isometric muscle contractions and hyperemic recovery in healthy young males

BACKGROUND

An elevated intramuscular pressure during a single forearm isometric muscle contraction may restrict muscle hyperemia. However, during repeated isometric exercise, it is unclear to what extent mechanical compression and muscle vasodilatation contribute to the magnitude and time course of beat-to-beat limb hemodynamics, due to alterations in leg vascular conductance (LVC).

METHODS

In eight healthy male subjects, the time course of both beat-to-beat leg blood flow (LBF) and LVC in the femoral artery was determined between repeated 10-s isometric thigh muscle contractions and 10-s muscle relaxation (a duty cycle of 20 s) for steady-state 120 s at five target workloads (10, 30, 50, 70, and 90% of maximum voluntary contraction (MVC)). The ratio of restricted LBF due to mechanical compression across workloads was determined by the formula (relaxation LBF--contraction LBF)/relaxation LBF (%).

RESULTS

The exercise protocol was performed completely by all subjects (≤ 50% MVC), seven subjects (≤ 70% MVC), and two subjects (≤ 90% MVC). During a 10-s isometric muscle contraction, the time course in both beat-to-beat LBF and LVC displayed a fitting curve with an exponential increase (P < 0.001, r (2) ≥ 0.956) at each workload but no significant difference in mean LBF across workloads and pre-exercise. During a 10-s muscle relaxation, the time course in both beat-to-beat LBF and LVC increased as a function of workload, followed by a linear decline (P < 0.001, r (2) ≥ 0.889), that was workload-dependent, resulting in mean LBF increasing linearly across workloads (P < 0.01, r (2) = 0.984). The ratio of restricted LBF can be described as a single exponential decay with an increase in workload, which has inflection point distinctions between 30 and 50% MVC.

CONCLUSIONS

In a 20-s duty cycle of steady-state repeated isometric muscle contractions, the post-contraction hyperemia (magnitude of both LBF and LVC) during muscle relaxation was in proportion to the workload, which is in agreement with previous findings. Furthermore, time-dependent beat-to-beat muscle vasodilatation was seen, but not restricted, during isometric muscle contractions through all target workloads. Additionally, the relative contribution of mechanical obstruction and vasodilatation to the hyperemia observed in the repeated isometric exercise protocol was non-linear with regard to workload. In combination with repeated isometric exercise, the findings could potentially prove to be useful indicators of circulatory adjustment by mechanical compression for muscle-related disease.

Effect of PDE5 inhibition on the modulation of sympathetic α-adrenergic vasoconstriction in contracting skeletal muscle of young and older recreationally active humans

Aging is associated with an altered regulation of blood flow to contracting skeletal muscle; however, the precise mechanisms remain unclear. We recently demonstrated that inhibition of cGMP-binding phosphodiesterase 5 (PDE5) increased blood flow to contracting skeletal muscle of older but not young human subjects. Here we examined whether this effect of PDE5 inhibition was related to an improved ability to blunt α-adrenergic vasoconstriction (functional sympatholysis) and/or improved efficacy of local vasodilator pathways. A group of young (23 ± 1 yr) and a group of older (72 ± 1 yr) male subjects performed knee-extensor exercise in a control setting and following intake of the highly selective PDE5 inhibitor sildenafil. During both conditions, exercise was performed without and with arterial tyramine infusion to evoke endogenous norepinephrine release and consequently stimulation of α1- and α2-adrenergic receptors. The level of the sympatholytic compound ATP was measured in venous plasma by use of the microdialysis technique. Sildenafil increased (P < 0.05) vascular conductance during exercise in the older group, but tyramine infusion reduced (P < 0.05) this effect by 38 ± 9%. Similarly, tyramine reduced (P < 0.05) the vasodilation induced by arterial infusion of a nitric oxide (NO) donor by 54 ± 9% in the older group, and this effect was not altered by sildenafil. Venous plasma [ATP] did not change with PDE5 inhibition in the older subjects during exercise. Collectively, PDE5 inhibition in older humans was not associated with an improved ability for functional sympatholysis. An improved efficacy of the NO system may be one mechanism underlying the effect of PDE5 inhibition on exercise hyperemia in aging.

Blood temperature and perfusion to exercising and non-exercising human limbs

NEW FINDINGS

What is the central question of this study? Temperature-sensitive mechanisms are thought to contribute to blood-flow regulation, but the relationship between exercising and non-exercising limb perfusion and blood temperature is not established. What is the main finding and its importance? The close coupling among perfusion, blood temperature and aerobic metabolism in exercising and non-exercising extremities across different exercise modalities and activity levels and the tight association between limb vasodilatation and increases in plasma ATP suggest that both temperature- and metabolism-sensitive mechanisms are important for the control of human limb perfusion, possibly by activating ATP release from the erythrocytes. Temperature-sensitive mechanisms may contribute to blood-flow regulation, but the influence of temperature on perfusion to exercising and non-exercising human limbs is not established. Blood temperature (TB ), blood flow and oxygen uptake (V̇O2) in the legs and arms were measured in 16 healthy humans during 90 min of leg and arm exercise and during exhaustive incremental leg or arm exercise. During prolonged exercise, leg blood flow (LBF) was fourfold higher than arm blood flow (ABF) in association with higher TB and limb V̇O2. Leg and arm vascular conductance during exercise compared with rest was related closely to TB (r(2) = 0.91; P < 0.05), plasma ATP (r(2) = 0.94; P < 0.05) and limb V̇O2 (r(2) = 0.99; P < 0.05). During incremental leg exercise, LBF increased in association with elevations in TB and limb V̇O2, whereas ABF, arm TB and V̇O2 remained largely unchanged. During incremental arm exercise, both ABF and LBF increased in relationship to similar increases in V̇O2. In 12 trained males, increases in femoral TB and LBF during incremental leg exercise were mirrored by similar pulmonary artery TB and cardiac output dynamics, suggesting that processes in active limbs dominate central temperature and perfusion responses. The present data reveal a close coupling among perfusion, TB and aerobic metabolism in exercising and non-exercising extremities and a tight association between limb vasodilatation and increases in plasma ATP. These findings suggest that temperature and V̇O2 contribute to the regulation of limb perfusion through control of intravascular ATP.

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

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

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.

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

KEY POINTS

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.

ABSTRACT

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 2 min 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 Protocol 1 (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 Protocol 2 (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 (Protocol 1) or alone (Protocol 2) did not enhance α1 -mediated vasoconstriction during exercise (Protocol 1: -27 ± 3%; P = 0.2 vs. control, P = 0.4 vs. l-NMMA + ketorolac; Protocol 2: -21 ± 7%; P = 0.9 vs. control). Thus, contracting human skeletal muscle maintains the ability to blunt α1 -adrenergic vasoconstriction during combined KIR channel and Na(+) /K(+) -ATPase inhibition.

Local temperature-sensitive mechanisms are important mediators of limb tissue hyperemia in the heat-stressed human at rest and during small muscle mass exercise

Limb tissue and systemic blood flow increases with heat stress, but the underlying mechanisms remain poorly understood. Here, we tested the hypothesis that heat stress-induced increases in limb tissue perfusion are primarily mediated by local temperature-sensitive mechanisms. Leg and systemic temperatures and hemodynamics were measured at rest and during incremental single-legged knee extensor exercise in 15 males exposed to 1 h of either systemic passive heat-stress with simultaneous cooling of a single leg (n = 8) or isolated leg heating or cooling (n = 7). Systemic heat stress increased core, skin and heated leg blood temperatures (T_b), cardiac output, and heated leg blood flow (LBF; 0.6 ± 0.1 l/min; P < 0.05). In the cooled leg, however, LBF remained unchanged throughout (P > 0.05). Increased heated leg deep tissue blood flow was closely related to T_b (R² = 0.50; P < 0.01), which is partly attributed to increases in tissue V̇O₂ (R² = 0.55; P < 0.01) accompanying elevations in total leg glucose uptake (P < 0.05). During isolated limb heating and cooling, LBFs were equivalent to those found during systemic heat stress (P > 0.05), despite unchanged systemic temperatures and hemodynamics. During incremental exercise, heated LBF was consistently maintained ∼0.6 l/min higher than that in the cooled leg (P < 0.01), with LBF and vascular conductance in both legs showing a strong correlation with their respective local T_b (R² = 0.85 and 0.95, P < 0.05). We conclude that local temperature-sensitive mechanisms are important mediators in limb tissue perfusion regulation both at rest and during small-muscle mass exercise in hyperthermic humans.

Immediate effects of electrical stimulation, diathermy, and physical exercise on lower limb arterial blood flow in diabetic women with peripheral arterial disease: a randomized crossover trial

OBJECTIVE

The purpose of this study was to assess the effects of high-voltage electrical stimulation (HVES), continuous short wave diathermy, and physical exercise on arterial blood flow in the lower limbs of diabetic women with peripheral arterial disease.

METHODS

A crossover study was carried out involving 15 diabetic women (mean age of 77.87 ± 6.20 years) with a diagnosis of peripheral arterial disease. One session of each therapeutic resource was held, with a 7-day washout period between protocols. Blood flow velocity was evaluated before each session and 0, 20, 40 and 60 minutes after the administration of each protocol. Two-way repeated-measures analysis of variance with Bonferroni post hoc test was used for the intragroup and intergroup comparisons.

RESULTS

In the intragroup analysis, a significant reduction (P < .05) was found in blood flow velocity in the femoral and popliteal arteries over time with HVES and physical exercise and in the posterior tibial artery with the physical exercise protocol. However, no significant differences were found in the intergroup analysis (P > .05).

CONCLUSION

Proximal blood circulation in the lower limb of diabetic women with peripheral arterial disease was increased by a single session of HVES and physical exercise, whereas distal circulation was only increased with physical exercise.

Regulation of the skeletal muscle blood flow in humans

In humans, skeletal muscle blood flow is regulated by an interaction between several locally formed vasodilators, including NO and prostaglandins. In plasma, ATP is a potent vasodilator that stimulates the formation of NO and prostaglandins and, very importantly, can offset local sympathetic vasoconstriction. Adenosine triphosphate is released into plasma from erythrocytes and endothelial cells, and the plasma concentration increases in both the feed artery and the vein draining the contracting skeletal muscle. Adenosine also stimulates the formation of NO and prostaglandins, but the plasma adenosine concentration does not increase during exercise. In the skeletal muscle interstitium, there is a marked increase in the concentration of ATP and adenosine, and this increase is tightly coupled to the increase in blood flow. The sources of interstitial ATP and adenosine are thought to be skeletal muscle cells and endothelial cells. In the interstitium, both ATP and adenosine stimulate the formation of NO and prostaglandins, but ATP has also been suggested to induce vasoconstriction and stimulate afferent nerves that signal to increase sympathetic nerve activity. Adenosine has been shown to contribute to exercise hyperaemia, whereas the role of ATP remains uncertain due to lack of specific purinergic receptor blockers for human use. The purpose of this review is to address the interaction between vasodilator systems and to discuss the multiple proposed roles of ATP in human skeletal muscle blood flow regulation.

Chronic hypoxia increases arterial blood pressure and reduces adenosine and ATP induced vasodilatation in skeletal muscle in healthy humans

AIMS

To determine the role played by adenosine, ATP and chemoreflex activation on the regulation of vascular conductance in chronic hypoxia.

METHODS

The vascular conductance response to low and high doses of adenosine and ATP was assessed in ten healthy men. Vasodilators were infused into the femoral artery at sea level and then after 8-12 days of residence at 4559 m above sea level. At sea level, the infusions were carried out while the subjects breathed room air, acute hypoxia (FI O2 = 0.11) and hyperoxia (FI O2 = 1); and at altitude (FI O2 = 0.21 and 1). Skeletal muscle P2Y2 receptor protein expression was determined in muscle biopsies after 4 weeks at 3454 m by Western blot.

RESULTS

At altitude, mean arterial blood pressure was 13% higher (91 ± 2 vs. 102 ± 3 mmHg, P < 0.05) than at sea level and was unaltered by hyperoxic breathing. Baseline leg vascular conductance was 25% lower at altitude than at sea level (P < 0.05). At altitude, the high doses of adenosine and ATP reduced mean arterial blood pressure by 9-12%, independently of FI O2 . The change in vascular conductance in response to ATP was lower at altitude than at sea level by 24 and 38%, during the low and high ATP doses respectively (P < 0.05), and by 22% during the infusion with high adenosine doses. Hyperoxic breathing did not modify the response to vasodilators at sea level or at altitude. P2Y2 receptor expression remained unchanged with altitude residence.

CONCLUSIONS

Short-term residence at altitude increases arterial blood pressure and reduces the vasodilatory responses to adenosine and ATP.

Oral adenosine-5'-triphosphate (ATP) administration increases blood flow following exercise in animals and humans

INTRODUCTION

Extracellular adenosine triphosphate (ATP) stimulates vasodilation by binding to endothelial ATP-selective P2Y2 receptors; a phenomenon, which is posited to be accelerated during exercise. Herein, we used a rat model to examine how different dosages of acute oral ATP administration affected the femoral blood flow response prior to, during, and after an exercise bout. In addition, we performed a single dose chronic administration pilot study in resistance trained athletes.

METHODS

ANIMAL STUDY

Male Wistar rats were gavage-fed the body surface area, species adjusted human equivalent dose (HED) of either 100 mg (n=4), 400 mg (n=4), 1,000 mg (n=5) or 1,600 mg (n=5) of oral ATP as a disodium salt (Peak ATP®, TSI, Missoula, MT). Rats that were not gavage-fed were used as controls (CTL, n=5). Blood flow was monitored continuously: a) 60 min prior to, b) during and c) 90 min following an electrically-evoked leg-kicking exercise. Human Study: In a pilot study, 12 college-aged resistance-trained subjects were given 400 mg of ATP (Peak ATP®, TSI, Missoula, MT) daily for 12 weeks, and prior to an acute arm exercise bout at weeks 1, 4, 8, and 12. Ultrasonography-determined volumetric blood flow and vessel dilation in the brachial artery was measured at rest, at rest 30 minutes after supplementation, and then at 0, 3, and 6 minutes after the exercise.

RESULTS

ANIMAL STUDY

Rats fed 1,000 mg HED demonstrated significantly greater recovery blood flow (p < 0.01) and total blood flow AUC values (p < 0.05) compared to CTL rats. Specifically, blood flow was elevated in rats fed 1,000 mg HED versus CTL rats at 20 to 90 min post exercise when examining 10-min blood flow intervals (p < 0.05). When examining within-group differences relative to baseline values, rats fed the 1,000 mg and 1,600 mg HED exhibited the most robust increases in blood flow during exercise and into the recovery period. Human study: At weeks 1, 8, and 12, ATP supplementation significantly increased blood flow, along with significant elevations in brachial dilation.

CONCLUSIONS

Oral ATP administration can increase post-exercise blood flow, and may be particularly effective during exercise recovery.

Exercise training modulates functional sympatholysis and α-adrenergic vasoconstrictor responsiveness in hypertensive and normotensive individuals

Essential hypertension is linked to an increased sympathetic vasoconstrictor activity and reduced tissue perfusion. We investigated the role of exercise training on functional sympatholysis and postjunctional α-adrenergic responsiveness in individuals with essential hypertension. Leg haemodynamics were measured before and after 8 weeks of aerobic training (3-4 times per week) in eight hypertensive (47 ± 2 years) and eight normotensive untrained individuals (46 ± 1 years) during arterial tyramine infusion, arterial ATP infusion and/or one-legged knee extensions. Before training, exercise hyperaemia and leg vascular conductance (LVC) were lower in the hypertensive individuals (P < 0.05) and tyramine lowered exercise hyperaemia and LVC in both groups (P < 0.05). Training lowered blood pressure in the hypertensive individuals (P < 0.05) and exercise hyperaemia was similar to the normotensive individuals in the trained state. After training, tyramine did not reduce exercise hyperaemia or LVC in either group. When tyramine was infused at rest, the reduction in blood flow and LVC was similar between groups, but exercise training lowered the magnitude of the reduction in blood flow and LVC (P < 0.05). There was no difference in the vasodilatory response to infused ATP or in muscle P2Y2 receptor content between the groups before and after training. However, training lowered the vasodilatory response to ATP and increased skeletal muscle P2Y2 receptor content in both groups (P < 0.05). These results demonstrate that exercise training improves functional sympatholysis and reduces postjunctional α-adrenergic responsiveness in both normo- and hypertensive individuals. The ability for functional sympatholysis and the vasodilator and sympatholytic effect of intravascular ATP appear not to be altered in essential hypertension.

Strategies for quantitation of endogenous adenine nucleotides in human plasma using novel ion-pair hydrophilic interaction chromatography coupled with tandem mass spectrometry

We present here a novel and highly sensitive ion-pair hydrophilic interaction chromatography-tandem mass spectrometry (IP-HILIC-MS/MS) method for quantitation of highly polar acid metabolites like adenine nucleotides. A mobile phase based on diethylamine (DEA) and hexafluoro-2-isopropanol (HFIP) and an aminopropyl (NH2) column were applied for a novel chromatographic separation for the determination of AMP, ADP and ATP in biological matrices. This novel IP-HILIC mechanism could be hypothesized by the ion-pairing reagent (DEA) in the mobile phase forming neutral and hydrophilic complexes with the analytes of polar organic acids. The IP-HILIC-MS/MS assay for adenine nucleotides was successfully validated with satisfactory linearity, sensitivity, accuracy, reproducibility and matrix effects. The lower limit of quantitation (LLOQ) at 2.00ng/mL obtained for ATP showed a least 10-fold higher sensitivity than previous LC-MS/MS assays except nano-LC-MS/MS assay. In summary, this novel IP-HILIC-MS/MS assay provides a sensitive method for nucleotides bioanalysis and shows great potential to determine a number of organic acids in biological matrices.

Maximal heart rate does not limit cardiovascular capacity in healthy humans: insight from right atrial pacing during maximal exercise

In humans, maximal aerobic power (VO2 max ) is associated with a plateau in cardiac output (Q), but the mechanisms regulating the interplay between maximal heart rate (HRmax) and stroke volume (SV) are unclear. To evaluate the effect of tachycardia and elevations in HRmax on cardiovascular function and capacity during maximal exercise in healthy humans, 12 young male cyclists performed incremental cycling and one-legged knee-extensor exercise (KEE) to exhaustion with and without right atrial pacing to increase HR. During control cycling, Q and leg blood flow increased up to 85% of maximal workload (WLmax) and remained unchanged until exhaustion. SV initially increased, plateaued and then decreased before exhaustion (P < 0.05) despite an increase in right atrial pressure (RAP) and a tendency (P = 0.056) for a reduction in left ventricular transmural filling pressure (LVFP). Atrial pacing increased HRmax from 184 ± 2 to 206 ± 3 beats min(-1) (P < 0.05), but Q remained similar to the control condition at all intensities because of a lower SV and LVFP (P < 0.05). No differences in arterial pressure, peripheral haemodynamics, catecholamines or VO2 were observed, but pacing increased the rate pressure product and RAP (P < 0.05). Atrial pacing had a similar effect on haemodynamics during KEE, except that pacing decreased RAP. In conclusion, the human heart can be paced to a higher HR than observed during maximal exercise, suggesting that HRmax and myocardial work capacity do not limit VO2 max in healthy individuals. A limited left ventricular filling and possibly altered contractility reduce SV during atrial pacing, whereas a plateau in LVFP appears to restrict Q close to VO2 max .

Blood pressure regulation II: what happens when one system must serve two masters--oxygen delivery and pressure regulation?

During high-intensity dynamic exercise, O2 delivery to active skeletal muscles is enhanced through marked increases in both cardiac output and skeletal muscle blood flow. When the musculature is vigorously engaged in exercise, the human heart lacks the pumping capacity to meet the blood flow demands of both the skeletal muscles and other organs such as the brain. Vasoconstriction must therefore be induced through activation of sympathetic nervous activity to maintain blood flow to the brain and to produce the added driving pressure needed to increase flow to the skeletal muscles. In this review, we first briefly summarize the local vascular and neural control mechanisms operating during high-intensity exercise. This is followed by a review of the major neural mechanisms regulating blood pressure during high-intensity exercise, focusing mainly on the integrated activities of the arterial baroreflex and muscle metaboreflex. In high cardiac output situations, such as during high-intensity dynamic exercise, small changes in total peripheral resistance can induce large changes in blood pressure, which means that rapid and fine regulation is necessary to avoid unacceptable drops in blood pressure. To accomplish this rapid regulation, arterial baroreflex function may be modulated in various ways through activation of the muscle metaboreflex and/or other neural mechanisms. Moreover, this modulation of the arterial baroreflex may change over the time course of an exercise bout, or to accommodate changes in exercise intensity. Within this model, integration of arterial baroreflex modulation with other neural mechanisms plays an important role in cardiovascular control during high-intensity exercise.

Effect of extraluminal ATP application on vascular tone and blood flow in skeletal muscle: implications for exercise hyperemia

During skeletal muscle contractions, the concentration of ATP increases in muscle interstitial fluid as measured by microdialysis probes. This increase is associated with the magnitude of blood flow, suggesting that interstitial ATP may be important for contraction-induced vasodilation. However, interstitial ATP has solely been described to induce vasoconstriction in skeletal muscle. To examine whether interstitial ATP induces vasodilation in skeletal muscle and to what extent this vasoactive effect is mediated by formation of nitric oxide (NO) and prostanoids, three different experimental models were studied. The rat gluteus maximus skeletal muscle model was used to study changes in local skeletal muscle hemodynamics. Superfused ATP at concentrations found during muscle contractions (1-10 μM) increased blood flow by up to 400%. In this model, the underlying mechanism was also examined by inhibition of NO and prostanoid formation. Inhibition of these systems abolished the vasodilator effect of ATP. Cell-culture experiments verified ATP-induced formation of NO and prostacyclin in rat skeletal muscle microvascular endothelial cells, and ATP-induced formation of NO in rat skeletal muscle cells. To confirm these findings in humans, ATP was infused into skeletal muscle interstitium of healthy subjects via microdialysis probes and found to increase muscle interstitial concentrations of NO and prostacyclin by ~60% and ~40%, respectively. Collectively, these data suggest that a physiologically relevant elevation in interstitial ATP concentrations increases muscle blood flow, indicating that the contraction-induced increase in skeletal muscle interstitial [ATP] is important for exercise hyperemia. The vasodilator effect of ATP application is mediated by NO and prostanoid formation.

Persistence of functional sympatholysis post-exercise in human skeletal muscle

Blunting of sympathetic vasoconstriction in exercising muscle is well-established. Whether it persists during the early post-exercise period is unknown. This study tested the hypothesis that it persists in human skeletal muscle during the first 10 min of recovery from exercise. Eight healthy young males (21.4 ± 0.8 yrs, SE) performed 7 min of forearm rhythmic isometric handgrip exercise at 15% below forearm critical force (fCF). In separate trials, a cold pressor test (CPT) of 2 min duration was used to evoke forearm sympathetic vasoconstriction in each of Rest (R), Steady State Exercise (Ex), 2-4 min Post-Exercise (PEearly), and 8-10 min Post-Exercise (PElate). A 7 min control exercise trial with no CPT was also performed. Exercising forearm brachial artery blood flow, arterial blood pressure, cardiac output (CO), heart rate (HR), forearm deep venous catecholamine concentration, and arterialized venous catecholamine concentration were obtained immediately prior to and following the CPT in each trial. CPT resulted in a significant increase in forearm venous plasma norepinephrine concentration in all trials (P = 0.007), but no change in arterialized plasma norepinephrine (P = 0.32). CPT did not change forearm venous plasma epinephrine (P = 0.596) or arterialized plasma epinephrine concentration (P = 0.15). As assessed by the %reduction in forearm vascular conductance (FVC) the CPT evoked a robust vasoconstriction at rest that was severely blunted in exercise (R = -39.9 ± 4.6% vs. Ex = 5.5 ± 7.4%, P < 0.001). This blunting of vasoconstriction persisted at PEearly (-12.3 ± 10.1%, P = 0.02) and PElate (-18.1 ± 8.2%, P = 0.03) post-exercise. In conclusion, functional sympatholysis remains evident in human skeletal muscle as much as 10 min after the end of a bout of forearm exercise. Persistence of functional sympatholysis may have important implications for blood pressure regulation in the face of a challenge to blood pressure following exercise.

Mechanical effects of muscle contraction increase intravascular ATP draining quiescent and active skeletal muscle in humans

Intravascular adenosine triphosphate (ATP) evokes vasodilation and is implicated in the regulation of skeletal muscle blood flow during exercise. Mechanical stresses to erythrocytes and endothelial cells stimulate ATP release in vitro. How mechanical effects of muscle contractions contribute to increased plasma ATP during exercise is largely unexplored. We tested the hypothesis that simulated mechanical effects of muscle contractions increase [ATP](venous) and ATP effluent in vivo, independent of changes in tissue metabolic demand, and further increase plasma ATP when superimposed with mild-intensity exercise. In young healthy adults, we measured forearm blood flow (FBF) (Doppler ultrasound) and plasma [ATP](v) (luciferin-luciferase assay), then calculated forearm ATP effluent (FBF×[ATP](v)) during rhythmic forearm compressions (RFC) via a blood pressure cuff at three graded pressures (50, 100, and 200 mmHg; Protocol 1; n = 10) and during RFC at 100 mmHg, 5% maximal voluntary contraction rhythmic handgrip exercise (RHG), and combined RFC + RHG (Protocol 2; n = 10). [ATP](v) increased from rest with each cuff pressure (range 144-161 vs. 64 ± 13 nmol/l), and ATP effluent was graded with pressure. In Protocol 2, [ATP](v) increased in each condition compared with rest (RFC: 123 ± 33; RHG: 51 ± 9; RFC + RHG: 96 ± 23 vs. Mean Rest: 42 ± 4 nmol/l; P < 0.05), and ATP effluent was greatest with RFC + RHG (RFC: 5.3 ± 1.4; RHG: 5.3 ± 1.1; RFC + RHG: 11.6 ± 2.7 vs. Mean Rest: 1.2 ± 0.1 nmol/min; P < 0.05). We conclude that the mechanical effects of muscle contraction can 1) independently elevate intravascular ATP draining quiescent skeletal muscle without changes in local metabolism and 2) further augment intravascular ATP during mild exercise associated with increases in metabolism and local deoxygenation; therefore, it is likely one stimulus for increasing intravascular ATP during exercise in humans.

Differential effects of nebivolol versus metoprolol on functional sympatholysis in hypertensive humans

In young healthy humans, sympathetic vasoconstriction is markedly blunted during exercise to optimize blood flow to the metabolically active muscle. This phenomenon known as functional sympatholysis is impaired in hypertensive humans and rats by angiotensin II-dependent mechanisms, involving oxidative stress and inactivation of nitric oxide (NO). Nebivolol is a β1-adrenergic receptor blocker that has NO-dependent vasodilatory and antioxidant properties. We therefore asked whether nebivolol would restore functional sympatholysis in hypertensive humans. In 21 subjects with stage 1 hypertension, we measured muscle oxygenation and forearm blood flow responses to reflex increases in sympathetic nerve activity evoked by lower body negative pressure at rest, and during rhythmic handgrip exercise at baseline, after 12 weeks of nebivolol (5-20 mg/d) or metoprolol (100-300 mg/d), using a double-blind crossover design. We found that nebivolol had no effect on lower body negative pressure-induced decreases in oxygenation and forearm blood flow in resting forearm (from -29±5% to -30±5% and from -29±3% to -29±3%, respectively; P=NS). However, nebivolol attenuated the lower body negative pressure-induced reduction in oxygenation and forearm blood flow in exercising forearm (from -14±4% to -1±5% and from -15±2% to -6±2%, respectively; both P<0.05). This effect of nebivolol on oxygenation and forearm blood flow in exercising forearm was not observed with metoprolol in the same subjects, despite a similar reduction in blood pressure. Nebivolol had no effect on sympathetic nerve activity at rest or during handgrip, suggesting a direct effect on vascular function. Thus, our data demonstrate that nebivolol restored functional sympatholysis in hypertensive humans by a mechanism that does not involve β1-adrenergic receptors. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT01502787.

Skeletal muscle signaling and the heart rate and blood pressure response to exercise: insight from heart rate pacing during exercise with a trained and a deconditioned muscle group

Endurance training lowers heart rate and blood pressure responses to exercise, but the mechanisms and consequences remain unclear. To determine the role of skeletal muscle for the cardioventilatory response to exercise, 8 healthy young men were studied before and after 5 weeks of 1-legged knee-extensor training and 2 weeks of deconditioning of the other leg (leg cast). Hemodynamics and muscle interstitial nucleotides were determined during exercise with the (1) deconditioned leg, (2) trained leg, and (3) trained leg with atrial pacing to the heart rate obtained with the deconditioned leg. Heart rate was ≈ 15 bpm lower during exercise with the trained leg (P<0.05), but stroke volume was higher (P<0.05) and cardiac output was similar. Arterial and central venous pressures, rate-pressure product, and ventilation were lower during exercise with the trained leg (P<0.05), whereas pulmonary capillary wedge pressure was similar. When heart rate was controlled by atrial pacing, stroke volume decreased (P<0.05), but cardiac output, peripheral blood flow, arterial pressures, and pulmonary capillary wedge pressure remained unchanged. Circulating [norepinephrine], [lactate] and [K(+)] were lower and interstitial [ATP] and pH were higher in the trained leg (P<0.05). The lower cardioventilatory response to exercise with the trained leg is partly coupled to a reduced signaling from skeletal muscle likely mediated by K(+), lactate, or pH, whereas the lower cardiac afterload increases stroke volume. These results demonstrate that skeletal muscle training reduces the cardioventilatory response to exercise without compromising O2 delivery, and it can therefore be used to reduce the load on the heart during physical activity.

Haemodynamic responses to dehydration in the resting and exercising human leg

Dehydration and hyperthermia reduces leg blood flow (LBF), cardiac output ([Formula: see text]) and arterial pressure during whole-body exercise. It is unknown whether the reductions in blood flow are associated with dehydration-induced alterations in arterial blood oxygen content (C aO2) and O2-dependent signalling. This study investigated the impact of dehydration and concomitant alterations in C aO2 upon LBF and [Formula: see text]. Haemodynamics, arterial and femoral venous blood parameters and plasma [ATP] were measured at rest and during one-legged knee-extensor exercise in 7 males in four conditions: (1) control, (2) mild dehydration, (3) moderate dehydration, and (4) rehydration. Relative to control, C aO2 and LBF increased with dehydration at rest and during exercise (C aO2: from 199 ± 1 to 208 ± 2, and 202 ± 2 to 210 ± 2 ml L(-1) and LBF: from 0.38 ± 0.04 to 0.77 ± 0.09, and 1.64 ± 0.09 to 1.88 ± 0.1 L min(-1), respectively). Similarly, [Formula: see text] was unchanged or increased with dehydration at rest and during exercise, whereas arterial and leg perfusion pressures declined. Following rehydration, C aO2 declined (to 193 ± 2 mL L(-1)) but LBF remained elevated. Alterations in LBF were unrelated to C aO2 (r (2) = 0.13-0.27, P = 0.48-0.64) and plasma [ATP]. These findings suggest dehydration and concomitant alterations in C aO2 do not compromise LBF despite reductions in plasma [ATP]. While an additive or synergistic effect cannot be excluded, reductions in LBF during exercise with dehydration may not necessarily be associated with alterations in C aO2 and/or intravascular [ATP].

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

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

cAMP/protein kinase A activates cystic fibrosis transmembrane conductance regulator for ATP release from rat skeletal muscle during low pH or contractions

We have shown that cystic fibrosis transmembrane conductance regulator (CFTR) is involved in ATP release from skeletal muscle at low pH. These experiments investigate the signal transduction mechanism linking pH depression to CFTR activation and ATP release, and evaluate whether CFTR is involved in ATP release from contracting muscle. Lactic acid treatment elevated interstitial ATP of buffer-perfused muscle and extracellular ATP of L6 myocytes: this ATP release was abolished by the non-specific CFTR inhibitor, glibenclamide, or the specific CFTR inhibitor, CFTR_inh-172, suggesting that CFTR was involved, and by inhibition of lactic acid entry to cells, indicating that intracellular pH depression was required. Muscle contractions significantly elevated interstitial ATP, but CFTR_inh-172 abolished the increase. The cAMP/PKA pathway was involved in the signal transduction pathway for CFTR-regulated ATP release from muscle: forskolin increased CFTR phosphorylation and stimulated ATP release from muscle or myocytes; lactic acid increased intracellular cAMP, pCREB and PKA activity, whereas IBMX enhanced ATP release from myocytes. Inhibition of PKA with KT5720 abolished lactic-acid- or contraction-induced ATP release from muscle. Inhibition of either the Na⁺/H⁺-exchanger (NHE) with amiloride or the Na⁺/Ca²⁺-exchanger (NCX) with SN6 or KB-R7943 abolished lactic-acid- or contraction-induced release of ATP from muscle, suggesting that these exchange proteins may be involved in the activation of CFTR. Our data suggest that CFTR-regulated release contributes to ATP release from contracting muscle in vivo, and that cAMP and PKA are involved in the activation of CFTR during muscle contractions or acidosis; NHE and NCX may be involved in the signal transduction pathway.

Skeletal muscle vasodilatation during maximal exercise in health and disease

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

Effects of Dăoyĭn Qìgōng in postpolio syndrome patients with cold intolerance

Postpolio syndrome (PPS) is characterized by progressive muscle weakness due to former infection with poliomyelitis and can be associated with other symptoms such as cold intolerance (CI). Dăoyĭn Qìgōng (DQ) is a technique in Traditional Chinese Medicine that impacts the circulation of energy and blood.

OBJECTIVE

It was to verify the effects of DQ in PPS patients complaining of cold intolerance.

METHODS

Ten PPS patients were assessed using the visual analogue scale (VAS) adapted for CI before and after intervention with DQ; patients practiced it in a sitting position for 40 minutes, 3 times per week over 3 consecutive months. Patients were reassessed three months after ceasing DQ.

RESULTS

There was a statistically significant difference in local and systemic VAS-Cold both at the end of DQ training and three months past the end of this.

CONCLUSION

The DQ technique ameliorated CI complaints in patients with PPS.

Lifelong physical activity preserves functional sympatholysis and purinergic signalling in the ageing human leg

Ageing is associated with an impaired ability to modulate sympathetic vasoconstrictor activity (functional sympatholysis) and a reduced exercise hyperaemia. The purpose of this study was to investigate whether a physically active lifestyle can offset the impaired functional sympatholysis and exercise hyperaemia in the leg and whether ATP signalling is altered by ageing and physical activity. Leg haemodynamics, interstitial [ATP] and P2Y(2) receptor content was determined in eight young (23 ± 1 years), eight lifelong sedentary elderly (66 ± 2 years) and eight lifelong active elderly (62 ± 2 years) men at rest and during one-legged knee extensions (12 W and 45% maximal workload (WL(max))) and arterial infusion of ACh and ATP with and without tyramine. The vasodilatory response to ACh was lowest in the sedentary elderly, higher in active elderly (P < 0.05) and highest in the young men (P < 0.05), whereas ATP-induced vasodilatation was lower in the sedentary elderly (P < 0.05). During exercise (12 W), leg blood flow, vascular conductance and VO2 was lower and leg lactate release higher in the sedentary elderly compared to the young (P < 0.05), whereas there was no difference between the active elderly and young. Interstitial [ATP] during exercise and P2Y(2) receptor content were higher in the active elderly compared to the sedentary elderly (P < 0.05). Tyramine infusion lowered resting vascular conductance in all groups, but only in the sedentary elderly during exercise (P < 0.05). Tyramine did not alter the vasodilator response to ATP infusion in any of the three groups. Plasma [noradrenaline] increased more during tyramine infusion in both elderly groups compared to young (P < 0.05). A lifelong physically active lifestyle can maintain an intact functional sympatholysis during exercise and vasodilator response to ATP despite a reduction in endothelial nitric oxide function. A physically active lifestyle increases interstitial ATP levels and skeletal muscle P2Y(2) receptor content.

Contribution of intravascular versus interstitial purines and nitric oxide in the regulation of exercise hyperaemia in humans

The regulation of blood flow to skeletal muscle involves a complex interaction between several locally formed vasodilators that are produced both in the skeletal muscle interstitium and intravascularly. The gas nitric oxide (NO) and the purines ATP and adenosine, are potent vasodilators that are formed by multiple cell types and released into the skeletal muscle interstitium and in plasma in response to muscle contraction. Cellular sources of ATP and NO in plasma are erythrocytes and endothelial cells, whereas interstitial sources are skeletal muscle cells and endothelial cells. Adenosine originates primarily from extracellular degradation of ATP. During exercise the concentrations of ATP and adenosine increase markedly in the interstitium with smaller increases occurring in plasma, and thus the interstitial concentration during exercise is severalfold higher than in plasma. The concentration of NO metabolites (NOx) in interstitium and plasma does not change during exercise and is similar in the two compartments. Adenosine and NO have been shown to contribute to exercise hyperaemia whereas the role of ATP remains unclear due to lack of specific purinergic receptor blockers. The relative role of intravascular versus interstitial vasodilators is not known but evidence suggests that both compartments are important. In cardiovascular disease, a reduced capacity to form adenosine in the muscle interstitium may be a contributing factor in increased peripheral vascular resistance.

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

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

Impaired skeletal muscle blood flow control with advancing age in humans: attenuated ATP release and local vasodilation during erythrocyte deoxygenation

RATIONALE

Skeletal muscle blood flow is coupled with the oxygenation state of hemoglobin in young adults, whereby the erythrocyte functions as an oxygen sensor and releases ATP during deoxygenation to evoke vasodilation. Whether this function is impaired in humans of advanced age is unknown.

OBJECTIVE

To test the hypothesis that older adults demonstrate impaired muscle blood flow and lower intravascular ATP during conditions of erythrocyte deoxygenation.

METHODS AND RESULTS

We showed impaired forearm blood flow responses during 2 conditions of erythrocyte deoxygenation (systemic hypoxia and graded handgrip exercise) with age, which was caused by reduced local vasodilation. In young adults, both hypoxia and exercise significantly increased venous [ATP] and ATP effluent (forearm blood flow×[ATP]) draining the skeletal muscle. In contrast, hypoxia and exercise did not increase venous [ATP] in older adults, and both venous [ATP] and ATP effluent were substantially reduced compared with young people despite similar levels of deoxygenation. Next, we demonstrated that this could not be explained by augmented extracellular ATP hydrolysis in whole blood with age. Finally, we found that deoxygenation-mediated ATP release from isolated erythrocytes was essentially nonexistent in older adults.

CONCLUSIONS

Skeletal muscle blood flow during conditions of erythrocyte deoxygenation was markedly reduced in aging humans, and reductions in plasma ATP and erythrocyte-mediated ATP release may be a novel mechanism underlying impaired vasodilation and oxygen delivery during hypoxemia with advancing age. Because aging is associated with elevated risk for ischemic cardiovascular disease and exercise intolerance, interventions that target erythrocyte-mediated ATP release may offer therapeutic potential.

Two weeks of muscle immobilization impairs functional sympatholysis but increases exercise hyperemia and the vasodilatory responsiveness to infused ATP

During exercise, contracting muscles can override sympathetic vasoconstrictor activity (functional sympatholysis). ATP and adenosine have been proposed to play a role in skeletal muscle blood flow regulation. However, little is known about the role of muscle training status on functional sympatholysis and ATP- and adenosine-induced vasodilation. Eight male subjects (22 ± 2 yr, Vo(2max): 49 ± 2 ml O(2)·min(-1)·kg(-1)) were studied before and after 5 wk of one-legged knee-extensor training (3-4 times/wk) and 2 wk of immobilization of the other leg. Leg hemodynamics were measured at rest, during exercise (24 ± 4 watts), and during arterial ATP (0.94 ± 0.03 μmol/min) and adenosine (5.61 ± 0.03 μmol/min) infusion with and without coinfusion of tyramine (11.11 μmol/min). During exercise, leg blood flow (LBF) was lower in the trained leg (2.5 ± 0.1 l/min) compared with the control leg (2.6 ± 0.2 l/min; P < 0.05), and it was higher in the immobilized leg (2.9 ± 0.2 l/min; P < 0.05). Tyramine infusion lowers LBF similarly at rest, but, when tyramine was infused during exercise, LBF was blunted in the immobilized leg (2.5 ± 0.2 l/min; P < 0.05), whereas it was unchanged in the control and trained leg. Mean arterial pressure was lower during exercise with the trained leg compared with the immobilized leg (P < 0.05), and leg vascular conductance was similar. During ATP infusion, the LBF response was higher after immobilization (3.9 ± 0.3 and 4.5 ± 0.6 l/min in the control and immobilized leg, respectively; P < 0.05), whereas it did not change after training. When tyramine was coinfused with ATP, LBF was reduced in the immobilized leg (P < 0.05) but remained similar in the control and trained leg. Training increased skeletal muscle P2Y2 receptor content (P < 0.05), whereas it did not change with immobilization. These results suggest that muscle inactivity impairs functional sympatholysis and that the magnitude of hyperemia and blood pressure response to exercise is dependent on the training status of the muscle. Immobilization also increases the vasodilatory response to infused ATP.

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

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

Role of nitric oxide and prostanoids in the regulation of leg blood flow and blood pressure in humans with essential hypertension: effect of high-intensity aerobic training

We examined the role of nitric oxide (NO) and prostanoids in the regulation of leg blood flow and systemic blood pressure before and after 8 weeks of aerobic high-intensity training in individuals with essential hypertension (n = 10) and matched healthy control subjects (n = 11). Hypertensive subjects were found to have a lower (P < 0.05) blood flow to the exercising leg than normotensive subjects (30 W: 2.92 ± 0.16 vs. 3.39 ± 0.37 l min(−1)). Despite the lower exercise hyperaemia, pharmacological inhibition of the NO and prostanoid systems reduced leg blood flow to a similar extent during exercise in the two groups and vascular relaxation to the NO-dependent vasodilator acetylcholine was also similar between groups. High-intensity aerobic training lowered (P < 0.05) resting systolic (∼9 mmHg) and diastolic (∼12 mmHg) blood pressure in subjects with essential hypertension, but this effect of training was abolished when the NO and prostanoid systems were inhibited. Skeletal muscle vascular endothelial NO synthase uncoupling, expression and phosphorylation status were similar in the two groups before and after training. These data demonstrate that a reduction in exercise hyperaemia in hypertensive subjects is not associated with a reduced capacity of the NO and prostanoid systems to induce vasodilatation or with altered acetylcholine-induced response. However, our data suggest that the observed reduction in blood pressure is related to a training-induced change in the tonic effect of NO and/or prostanoids on vascular tone.

Temperature-dependent release of ATP from human erythrocytes: mechanism for the control of local tissue perfusion

Human limb muscle and skin blood flow increases significantly with elevations in temperature, possibly through physiological processes that involve temperature-sensitive regulatory mechanisms. Here we tested the hypothesis that the release of the vasodilator ATP from human erythrocytes is sensitive to physiological increases in temperature both in vitro and in vivo, and examined potential channel/transporters involved. To investigate the source of ATP release, whole blood, red blood cells (RBCs), plasma and serum were heated in vitro to 33, 36, 39 and 42°C. In vitro heating augmented plasma or ‘bathing solution’ ATP in whole blood and RBC samples, but not in either isolated plasma or serum samples. Heat-induced ATP release was blocked by niflumic acid and glibenclamide, but was not affected by inhibitors of nucleoside transport or anion exchange. Heating blood to 42°C enhanced (P < 0.05) membrane protein abundance of cystic fibrosis transmembrane conductance regulator (CFTR) in RBCs. In a parallel in vivo study in humans exposed to whole-body heating at rest and during exercise, increases in muscle temperature from 35 to 40°C correlated strongly with elevations in arterial plasma ATP (r² = 0.91; P = 0.0001), but not with femoral venous plasma ATP (r² = 0.61; P = 0.14). In vitro, however, the increase in ATP release from RBCs was similar in arterial and venous samples heated to 39°C. Our findings demonstrate that erythrocyte ATP release is sensitive to physiological increases in temperature, possibly via activation of CFTR-like channels, and suggest that temperature-dependent release of ATP from erythrocytes might be an important mechanism regulating human limb muscle and skin perfusion in conditions that alter blood and tissue temperature.

Influence of erythrocyte oxygenation and intravascular ATP on resting and exercising skeletal muscle blood flow in humans with mitochondrial myopathy

Oxygen (O₂) extraction is impaired in exercising skeletal muscle of humans with mutations of mitochondrial DNA (mtDNA), but the muscle hemodynamic response to exercise has never been directly investigated. This study sought to examine the extent to which human skeletal muscle perfusion can increase without reductions in blood oxygenation and to determine whether erythrocyte O₂ off-loading and related ATP vascular mechanisms are impaired in humans with mutations of mtDNA. Leg vascular hemodynamic, oxygenation and ATP were investigated in ten patients with mtDNA mutations and ten matched healthy control subjects: 1) at rest during normoxia, hypoxia, hyperoxia and intra-femoral artery ATP infusion, and 2) during passive and dynamic one-legged knee-extensor exercises. At rest, blood flow (LBF), femoral arterial and venous blood oxygenation and plasma ATP were similar in the two groups. During dynamic exercise, LBF and vascular conductance increased 9-10 fold in the patients despite erythrocyte oxygenation and leg O₂ extraction remained unchanged (p<0.01). In the patients, workload-adjusted LBF was 28% to 62% higher during submaximal- and maximal exercises and was associated with augmented plasma ATP. The appropriate hemodynamic adjustments during severe hypoxia and ATP infusion suggest that erythrocyte O₂ off-loading and related ATP vascular mechanisms are intact in patients with mtDNA mutations. Furthermore, greater increase in plasma ATP and LBF at a given metabolic demand in the patients, in concert with unchanged oxyhemoglobin, suggest that erythrocyte O₂ off-loading is not obligatory for the exercise-induced increase in blood flow and intravascular ATP concentration.

Mechanisms of ATP-mediated vasodilation in humans: modest role for nitric oxide and vasodilating prostaglandins

ATP is an endothelium-dependent vasodilator, and findings regarding the underlying signaling mechanisms are equivocal. We sought to determine the independent and interactive roles of nitric oxide (NO) and vasodilating prostaglandins (PGs) in ATP-mediated vasodilation in young, healthy humans and determine whether any potential role was dependent on ATP dose or the timing of inhibition. In protocol 1 (n = 18), a dose-response curve to intrabrachial infusion of ATP was performed before and after both single and combined inhibition of NO synthase [N(G)-monomethyl-L-arginine (L-NMMA)] and cyclooxygenase (ketorolac). Forearm blood flow (FBF) was measured via venous occlusion plethysmography and forearm vascular conductance (FVC) was calculated. In this protocol, neither individual nor combined NO/PG inhibition had any effect on the vasodilatory response (P = 0.22-0.99). In protocol 2 (n = 16), we determined whether any possible contribution of both NO and PGs to ATP vasodilation was greater at low vs. high doses of ATP and whether inhibition during steady-state infusion of the respective dose of ATP impacted the dilation. FBF in this protocol was measured via Doppler ultrasound. In protocol 2, infusion of low (n = 8)- and high-dose (n = 8) ATP for 5 min evoked a significant increase in FVC above baseline (low = 198 ± 24%; high = 706 ± 79%). Infusion of L-NMMA and ketorolac together reduced steady-state FVC during both low- and high-dose ATP (P < 0.05), and in a subsequent trial with continuous NO/PG blockade, the vasodilator response from baseline to 5 min of steady-state infusion was similarly reduced for both low (ΔFVC = -31 ± 11%)- and high-dose ATP (ΔFVC -25 ± 11%; P = 0.70 low vs. high dose). Collectively, our findings indicate a potential modest role for NO and PGs in the vasodilatory response to exogenous ATP in the human forearm that does not appear to be dose or timing dependent; however, this is dependent on the method for assessing forearm vascular responses. Importantly, the majority of ATP-mediated vasodilation is independent of these putative endothelium-dependent pathways in humans.

Functional sympatholysis during exercise in patients with type 2 diabetes with intact response to acetylcholine

OBJECTIVE

Sympathetic vasoconstriction is blunted in contracting human skeletal muscles (functional sympatholysis). In young subjects, infusion of adenosine and ATP increases blood flow, and the latter compound also attenuates α-adrenergic vasoconstriction. In patients with type 2 diabetes and age-matched healthy subjects, we tested 1) the sympatholytic capacity during one-legged exercise, 2) the vasodilatory capacity of adenosine and ATP, and 3) the ability to blunt α-adrenergic vasoconstriction during ATP infusion.

RESEARCH DESIGN AND METHODS

In 10 control subjects and 10 patients with diabetes and normal endothelial function, determined by leg blood flow (LBF) response to acetylcholine infusion, we measured LBF and venous NA, with and without tyramine-induced sympathetic vasoconstriction, during adenosine-, ATP-, and exercise-induced hyperemia.

RESULTS

LBF during acetylcholine did not differ significantly. LBF increased ninefold during exercise and during adenosine- and ATP-induced hyperemia. Infusion of tyramine during exercise did not reduce LBF in either the control or the patient group. During combined ATP and tyramine infusions, LBF decreased by 30% in both groups. Adenosine had no sympatholytic effect.

CONCLUSIONS

In patients with type 2 diabetes and normal endothelial function, functional sympatholysis was intact during moderate exercise. The vasodilatory response for adenosine and ATP did not differ between the patients with diabetes and the control subjects; however, the vasodilatory effect of adenosine and ATP and the sympatholytic effect of ATP seem to decline with age.

Arterial dysfunction and functional performance in patients with peripheral artery disease: A review

Functional performance influences quality of life in individuals with peripheral artery disease (PAD) and is also a powerful prognostic marker in these patients. The pathophysiology of impaired functional performance in patients with PAD is incompletely understood. The severity of atherosclerotic burden, non-invasively assessed by the ankle—brachial index (ABI), does not reliably predict the degree of functional impairment observed in PAD patients. We review associations of measures of arterial function (arterial stiffness and endothelial dysfunction) with functional performance in PAD patients, and also review potential therapies for arterial stiffness and endothelial dysfunction that could improve functional performance in PAD. Recent studies suggest that measures of arterial function, such as arterial stiffness and endothelial function, are associated with exercise performance in the setting of PAD. These studies have provided new insights into (1) the pathophysiology of functional impairment in PAD, (2) mechanisms of strategies known to be effective such as walking programs, and (3) potential new therapeutic interventions for improving functional performance. Thus, therapies aimed at arterial ‘de-stiffening’ and improving endothelial function (such as aerobic exercise, statins and angiotensin-converting enzyme inhibitors) may improve functional performance in patients with PAD; however, further investigations are needed.

Hemodynamic responses to heat stress in the resting and exercising human leg: insight into the effect of temperature on skeletal muscle blood flow

Heat stress increases limb blood flow and cardiac output (Q) in humans, presumably in sole response to an augmented thermoregulatory demand of the skin circulation. Here we tested the hypothesis that local hyperthermia also increases skeletal muscle blood flow at rest and during exercise. Hemodynamics, blood and tissue oxygenation, and muscle, skin, and core temperatures were measured at rest and during exercise in 11 males across four conditions of progressive whole body heat stress and at rest during isolated leg heat stress. During whole body heat stress, leg blood flow (LBF), Q, and leg (LVC) and systemic vascular conductance increased gradually with elevations in muscle temperature both at rest and during exercise (r(2) = 0.86-0.99; P < 0.05). Enhanced LBF and LVC were accompanied by reductions in leg arteriovenous oxygen (a-vO(2)) difference and increases in deep femoral venous O(2) content and quadriceps tissue oxygenation, reflecting elevations in muscle and skin perfusion. The increase in LVC occurred despite an augmented plasma norepinephrine (P < 0.05) and was associated with elevations in muscle temperature (r(2) = 0.85; P = 0.001) and arterial plasma ATP (r(2) = 0.87; P < 0.001). Isolated leg heat stress accounted for one-half of the increase in LBF with severe whole body heat stress. Our findings suggest that local hyperthermia also induces vasodilatation of the skeletal muscle microvasculature, thereby contributing to heat stress and exercise hyperemia. The increased limb muscle vasodilatation in these conditions of elevated muscle sympathetic vasoconstrictor activity is closely related to the rise in arterial plasma ATP and local tissue temperature.

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

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

Local release of ATP into the arterial inflow and venous drainage of human skeletal muscle: insight from ATP determination with the intravascular microdialysis technique

Intraluminal ATP could play an important role in the local regulation of skeletal muscle blood flow, but the stimuli that cause ATP release and the levels of plasma ATP in vessels supplying and draining human skeletal muscle remain unclear. To gain insight into the mechanisms by which ATP is released into plasma, we measured plasma [ATP] with the intravascular microdialysis technique at rest and during dynamic exercise (normoxia and hypoxia), passive exercise, thigh compressions and arterial ATP, tyramine and ACh infusion in a total of 16 healthy young men. Femoral arterial and venous [ATP] values were 109 ± 34 and 147 ± 45 nmol l(−1) at rest and increased to 363 ± 83 and 560 ± 111 nmol l(−1), respectively, during exercise (P < 0.05), whereas these values did not increase when exercise was performed with the other leg. Hypoxia increased venous plasma [ATP] at rest compared to normoxia (P < 0.05), but not during exercise. Arterial ATP infusion (≤1.8 μmol min(−1) increased arterial plasma [ATP] from 74 ± 17 to 486 ± 82 nmol l(−1) (P < 0.05), whereas it remained unchanged in the femoral vein at ∼150 nmol l(−1). Both arterial and venous plasma [ATP] decreased during acetylcholine infusion (P < 0.05). Rhythmic thigh compressions increased arterial and venous plasma [ATP] compared to baseline conditions, whereas these values did not change during passive exercise or tyramine infusion. These results demonstrate that ATP is released locally into arterial and venous plasma during exercise and during hypoxia at rest. Compression of the vascular system could contribute to the increase during exercise whereas there appears to be little ATP release in response to increased blood flow, vascular stretch or sympathetic ATP release. Furthermore, the half-life of arterially infused ATP is <1 s.

Vasodilatory responsiveness to adenosine triphosphate in ageing humans

Endothelium-dependent vasodilatation is reduced with advancing age in humans, as evidenced by blunted vasodilator responsiveness to acetylcholine (ACh). Circulating adenosine triphosphate (ATP) has been implicated in the control of skeletal muscle vascular tone during mismatches in oxygen delivery and demand (e.g. exercise) via binding to purinergic receptors (P2Y) on the endothelium evoking subsequent vasodilatation, and ageing is typically associated with reductions in muscle blood flow under such conditions. Therefore, we tested the hypothesis that ATP-mediated vasodilatation is impaired with age in healthy humans. We measured forearm blood flow (venous occlusion plethysmography) and calculated vascular conductance (FVC) responses to local intra-arterial infusions of ACh, ATP, and sodium nitroprusside (SNP) before and during ascorbic acid (AA) infusion in 13 young and 13 older adults. The peak increase in FVC to ACh was significantly impaired in older compared with young adults (262 ± 71% vs. 618 ± 97%; P < 0.05), and this difference was abolished during AA infusion (510 ± 82% vs. 556 ± 71%; not significant, NS). In contrast, peak FVC responses were not different between older and young adults to either ATP (675 ± 105% vs. 734 ± 126%) or SNP (1116 ± 111% vs. 1138 ± 148%) and AA infusion did not alter these responses in either age group (both NS). In another group of six young and six older adults, we determined whether vasodilator responses to adenosine and ATP were influenced by P1-receptor blockade via aminophylline. The peak FVC responses to adenosine were not different in young (350 ± 65%) versus older adults (360 ± 80%), and aminophylline blunted these responses by ∼50% in both groups. The peak FVC responses to ATP were again not different in young and older adults, and aminophylline did not impact the vasodilatation in either group. Thus, in contrast to the observed impairments in ACh responses, the vasodilatory response to exogenous ATP is not reduced with age in healthy humans. Further, our data also indicate that adenosine mediated vasodilatation is not reduced with age, and that ATP-mediated vasodilatation is independent of P1-receptor stimulation in both young and older adults.

Nitric oxide-mediated vasodilation becomes independent of beta-adrenergic receptor activation with increased intensity of hypoxic exercise

Hypoxic vasodilation in skeletal muscle at rest is known to include β-adrenergic receptor-stimulated nitric oxide (NO) release. We previously reported that the augmented skeletal muscle vasodilation during mild hypoxic forearm exercise includes β-adrenergic mechanisms. However, it is unclear whether a β-adrenergic receptor-stimulated NO component exists during hypoxic exercise. We hypothesized that NO-mediated vasodilation becomes independent of β-adrenergic receptor activation with increased exercise intensity during hypoxic exercise. Ten subjects (7 men, 3 women; 23 ± 1 yr) breathed hypoxic gas to titrate arterial O(2) saturation to 80% while remaining normocapnic. Subjects performed two consecutive bouts of incremental rhythmic forearm exercise (10% and 20% of maximum) with local administration (via a brachial artery catheter) of propranolol (β-adrenergic receptor inhibition) alone and with the combination of propranolol and nitric oxide synthase inhibition [N(G)-monomethyl-l-arginine (l-NMMA)] under normoxic and hypoxic conditions. Forearm blood flow (FBF, ml/min; Doppler ultrasound) and blood pressure [mean arterial pressure (MAP), mmHg; brachial artery catheter] were assessed, and forearm vascular conductance (FVC, ml·min(-1)·100 mmHg(-1)) was calculated (FBF/MAP). During propranolol alone, the rise in FVC (Δ from normoxic baseline) due to hypoxic exercise was 217 ± 29 and 415 ± 41 ml·min(-1)·100 mmHg(-1) (10% and 20% of maximum, respectively). Combined propranolol-l-NMMA infusion during hypoxic exercise attenuated ΔFVC at 20% (352 ± 44 ml·min(-1)·100 mmHg(-1); P < 0.001) but not at 10% (202 ± 28 ml·min(-1)·100 mmHg(-1); P = 0.08) of maximum compared with propranolol alone. These data, when integrated with earlier findings, demonstrate that NO contributes to the compensatory vasodilation during mild and moderate hypoxic exercise; a β-adrenergic receptor-stimulated NO component exists during low-intensity hypoxic exercise. However, the source of the NO becomes less dependent on β-adrenergic mechanisms as exercise intensity increases.