Limitations to vasodilatory capacity and .VO2 max in trained human skeletal muscle

Regulation of Skeletal Muscle Resistance Arteriolar Tone: Integration of Multiple Mechanisms

INTRODUCTION

Physiological system complexity represents an imposing challenge to gaining insight into how arteriolar behavior emerges. Further, mechanistic complexity in arteriolar tone regulation requires that a systematic determination of how these processes interact to alter vascular diameter be undertaken.

METHODS

The present study evaluated the reactivity of ex vivo proximal and in situ distal resistance arterioles in skeletal muscle with challenges across the full range of multiple physiologically relevant stimuli and determined the stability of responses over progressive alterations to each other parameter. The five parameters chosen for examination were (1) metabolism (adenosine concentration), (2) adrenergic activation (norepinephrine concentration), (3) myogenic activation (intravascular pressure), (4) oxygen (superfusate PO2), and (5) wall shear rate (altered intraluminal flow). Vasomotor tone of both arteriole groups following challenge with individual parameters was determined; subsequently, responses were determined following all two- and three-parameter combinations to gain deeper insight into how stimuli integrate to change arteriolar tone. A hierarchical ranking of stimulus significance for establishing arteriolar tone was performed using mathematical and statistical analyses in conjunction with machine learning methods.

RESULTS

Results were consistent across methods and indicated that metabolic and adrenergic influences were most robust and stable across all conditions. While the other parameters individually impact arteriolar tone, their impact can be readily overridden by the two dominant contributors.

CONCLUSION

These data suggest that mechanisms regulating arteriolar tone are strongly affected by acute changes to the local environment and that ongoing investigation into how microvessels integrate stimuli regulating tone will provide a more thorough understanding of arteriolar behavior emergence across physiological and pathological states.

Pharmacological modulation of adrenergic tone alters the vasodilatory response to passive leg movement in young but not in old adults

The age-related increase in α-adrenergic tone may contribute to decreased leg vascular conductance (LVC) both at rest and during exercise in the old. However, the effect on passive leg movement (PLM)-induced LVC, a measure of vascular function, which is markedly attenuated in this population, is unknown. Thus, in eight young (25 ± 5 yr) and seven old (65 ± 7 yr) subjects, this investigation examined the impact of systemic β-adrenergic blockade (propanalol, PROP) alone, and PROP combined with either α1-adrenergic stimulation (phenylephrine, PE) or α-adrenergic inhibition (phentolamine, PHEN), on PLM-induced vasodilation. LVC, calculated from femoral artery blood flow and pressure, was determined and PLM-induced Δ peak (LVCΔpeak) and total vasodilation (LVCAUC, area under curve) were documented. PROP decreased LVCΔpeak (PROP: 4.8 ± 1.8, Saline: 7.7 ± 2.7 mL·mmHg-1, P < 0.001) and LVCAUC (PROP: 1.1 ± 0.7, Saline: 2.4 ± 1.6 mL·mmHg-1, P = 0.002) in the young, but not in the old (LVCΔpeak, P = 0.931; LVCAUC, P = 0.999). PE reduced baseline LVC (PE: 1.6 ± 0.4, PROP: 2.3 ± 0.4 mL·min-1·mmHg-1, P < 0.01), LVCΔpeak (PE: 3.2 ± 1.3, PROP: 4.8 ± 1.8 mL·min-1·mmHg-1, P = 0.004), and LVCAUC (PE: 0.5 ± 0.4, PROP: 1.1 ± 0.7 mL·mmHg-1, P = 0.011) in the young, but not in the old (baseline LVC, P = 0.199; LVCΔpeak, P = 0.904; LVCAUC, P = 0.823). PHEN increased LVC at rest and throughout PLM in both groups (drug effect: P < 0.05), however LVCΔpeak was only improved in the young (PHEN: 6.4 ± 3.1, PROP: 4.4 ± 1.5 mL·min-1·mmHg-1, P = 0.004), and not in the old (P = 0.904). Furthermore, the magnitude of α-adrenergic modulation (PHEN - PE) of LVCΔpeak was greater in the young compared with the old (Young: 3.35 ± 2.32, Old: 0.40 ± 1.59 mL·min-1·mmHg-1, P = 0.019). Therefore, elevated α-adrenergic tone does not appear to contribute to the attenuated vascular function with age identified by PLM.NEW & NOTEWORTHY Stimulation of α1-adrenergic receptors eliminated age-related differences in passive leg movement (PLM) by decreasing PLM-induced vasodilation in the young. Systemic β-blockade attenuated the central hemodynamic component of the PLM response in young individuals. Inhibition of α-adrenergic receptors did not improve the PLM response in older individuals, though withdrawal of α-adrenergic modulation augmented baseline and maximal vasodilation in both groups. Accordingly, α-adrenergic signaling plays a role in modulating the PLM vasodilatory response in young but not in old adults, and elevated α-adrenergic tone does not appear to contribute to the attenuated vascular function with age identified by PLM.

Effects of impaired microvascular flow regulation on metabolism-perfusion matching and organ function

Impaired tissue oxygen delivery is a major cause of organ damage and failure in critically ill patients, which can occur even when systemic parameters, including cardiac output and arterial hemoglobin saturation, are close to normal. This review addresses oxygen transport mechanisms at the microcirculatory scale, and how hypoxia may occur in spite of adequate convective oxygen supply. The structure of the microcirculation is intrinsically heterogeneous, with wide variations in vessel diameters and flow pathway lengths, and consequently also in blood flow rates and oxygen levels. The dynamic processes of structural adaptation and flow regulation continually adjust microvessel diameters to compensate for heterogeneity, redistributing flow according to metabolic needs to ensure adequate tissue oxygenation. A key role in flow regulation is played by conducted responses, which are generated and propagated by endothelial cells and signal upstream arterioles to dilate in response to local hypoxia. Several pathophysiological conditions can impair local flow regulation, causing hypoxia and tissue damage leading to organ failure. Therapeutic measures targeted to systemic parameters may not address or may even worsen tissue oxygenation at the microvascular level. Restoration of tissue oxygenation in critically ill patients may depend on restoration of endothelial cell function, including conducted responses.

α-Adrenergic receptor regulation of skeletal muscle blood flow during exercise in heart failure patients with reduced ejection fraction

Patients suffering from heart failure with reduced ejection fraction (HFrEF) experience impaired limb blood flow during exercise, which may be due to a disease-related increase in α-adrenergic receptor vasoconstriction. Thus, in eight patients with HFrEF (63 ± 4 yr) and eight well-matched controls (63 ± 2 yr), we examined changes in leg blood flow (Doppler ultrasound) during intra-arterial infusion of phenylephrine (PE; an α₁-adrenergic receptor agonist) and phentolamine (Phen; a nonspecific α-adrenergic receptor antagonist) at rest and during dynamic single-leg knee-extensor exercise (0, 5, and 10 W). At rest, the PE-induced reduction in blood flow was significantly attenuated in patients with HFrEF (-15 ± 7%) compared with controls (-36 ± 5%). During exercise, the controls exhibited a blunted reduction in blood flow induced by PE (-12 ± 4, -10 ± 4, and -9 ± 2% at 0, 5, and 10 W, respectively) compared with rest, while the PE-induced change in blood flow was unchanged compared with rest in the HFrEF group (-8 ± 5, -10 ± 3, and -14 ± 3%, respectively). Phen administration increased leg blood flow to a greater extent in the HFrEF group at rest (+178 ± 34% vs. +114 ± 28%, HFrEF vs. control) and during exercise (36 ± 6, 37 ± 7, and 39 ± 6% vs. 13 ± 3, 14 ± 1, and 8 ± 3% at 0, 5, and 10 W, respectively, in HFrEF vs. control). Together, these findings imply that a HFrEF-related increase in α-adrenergic vasoconstriction restrains exercising skeletal muscle blood flow, potentially contributing to diminished exercise capacity in this population.

Similar acute physiological responses from effort and duration matched leg press and recumbent cycling tasks

The present study examined the effects of exercise utilising traditional resistance training (leg press) or ‘cardio’ exercise (recumbent cycle ergometry) modalities upon acute physiological responses. Nine healthy males underwent a within session randomised crossover design where they completed both the leg press and recumbent cycle ergometer conditions. Conditions were approximately matched for effort and duration (leg press: 4 × 12RM using a 2 s concentric and 3 s eccentric repetition duration controlled with a metronome, thus each set lasted  60 s; recumbent cycle ergometer: 4 × 60 s bouts using a resistance level permitting 80–100 rpm but culminating with being unable to sustain the minimum cadence for the final 5–10 s). Measurements included VO₂, respiratory exchange ratio (RER), blood lactate, energy expenditure, muscle swelling, and electromyography. Perceived effort was similar between conditions and thus both were well matched with respect to effort. There were no significant effects by ‘condition’ in any of the physiological responses examined (all p > 0.05). The present study shows that, when both effort and duration are matched, resistance training (leg press) and ‘cardio’ exercise (recumbent cycle ergometry) may produce largely similar responses in VO₂, RER, blood lactate, energy expenditure, muscle swelling, and electromyography. It therefore seems reasonable to suggest that both may offer a similar stimulus to produce chronic physiological adaptations in outcomes such as cardiorespiratory fitness, strength, and hypertrophy. Future work should look to both replicate the study conducted here with respect to the same, and additional physiological measures, and rigorously test the comparative efficacy of effort and duration matched exercise of differing modalities with respect to chronic improvements in physiological fitness.

Vascular function and endothelin-1: tipping the balance between vasodilation and vasoconstriction

Endothelin-1 (ET-1), a potent vasoconstrictor secreted by vascular endothelial cells, has been implicated in the pathophysiology of numerous cardiovascular diseases, yet the direct impact of ET-1 on vascular function remains unclear. Therefore, in seven young (23 ± 1 yr) healthy subjects, we investigated the effect of an intra-arterial infusion of ET-1 on reactive hyperemia (RH) and flow-mediated dilation (FMD) in the popliteal artery following 5 min of suprasystolic cuff occlusion. ET-1 infusion significantly attenuated basal leg blood flow (control: 62 ± 4 ml/min, ET-1: 47 ± 9 ml/min), RH [area-under-curve (AUC); control: 162 ± 15 ml, ET-1: 104 ± 16 ml], and peak RH (control: 572 ± 51 ml/min, ET-1: 412 ± 32 ml/min) (P < 0.05). Administration of ET-1 also reduced FMD (control: 2.4 ± 0.3%, ET-1: 0.5 ± 0.5%) and FMD normalized for shear rate (control: 10.5 × 10^-4 ± 2.0 × 10^-4%/s^-1, ET-1: 0.9 × 10^-4 ± 2.8 ×10^-4%/s^-1). These findings reveal that elevated levels of ET-1 have a significant impact on vascular function, indicating that studies employing RH and FMD as markers of microvascular function and nitric oxide bioavailability, respectively, should exercise caution, as ET-1 can impact these assessments by tipping the balance between vasodilation and vasoconstriction, in favor of the latter.NEW & NOTEWORTHY Endothelin-1 (ET-1) is recognized as the body's most potent endogenous vasoconstrictor, but the impact of this peptide on vascular function is not well understood. The present study revealed that the intra-arterial administration of ET-1 impaired both microvascular and conduit vessel function of the leg in young, healthy, humans. Studies employing vascular testing in patient cohorts that experience a disease-related increase in ET-1 should thus exercise caution, as ET-1 clearly impairs vascular function.

Functional sympatholysis and sympathetic escape in a theoretical model for blood flow regulation

A mathematical simulation of flow regulation in vascular networks is used to investigate the interaction between arteriolar vasoconstriction due to sympathetic nerve activity (SNA) and vasodilation due to increased oxygen demand. A network with 13 vessel segments in series is used, each segment representing a different size range of arterioles or venules. The network includes five actively regulating arteriolar segments with time-dependent diameters influenced by shear stress, wall tension, metabolic regulation, and SNA. Metabolic signals are assumed to be propagated upstream along vessel walls via a conducted response. The model exhibits functional sympatholysis, in which sympathetic vasoconstriction is partially abrogated by increases in metabolic demand, and sympathetic escape, in which SNA elicits an initial vasoconstriction followed by vasodilation. In accordance with experimental observations, these phenomena are more prominent in small arterioles than in larger arterioles when SNA is assumed to act equally on arterioles of all sizes. The results imply that a mechanism based on the competing effects on arteriolar tone of SNA and conducted metabolic signals can account for several observed characteristics of functional sympatholysis, including the different responses of large and small arterioles.

Endothelin-A-mediated vasoconstriction during exercise with advancing age

The endothelin-1 vasoconstrictor pathway contributes to age-related elevations in resting peripheral vascular tone primarily through activation of the endothelin subtype A (ET(A)) receptor. However, the regulatory influence of ET(A)-mediated vasoconstriction during exercise in the elderly is unknown. Thus, in 17 healthy volunteers (n = 8 young, 24±2 years; n = 9 old, 70±2 years), we examined leg blood flow, mean arterial pressure, leg arterial-venous oxygen (O2) difference, and leg O2 consumption (VO2) at rest and during knee-extensor exercise before and after intra-arterial administration of the ET(A) antagonist BQ-123. During exercise, BQ-123 administration increased leg blood flow to a greater degree in the old (+29±5 mL/min/W) compared with the young (+16±3 mL/min/W). The increase in leg blood flow with BQ-123 was accompanied by an increase in leg VO2 in both groups, suggesting a reduced efficiency following ET(A) receptor blockade. Together, these findings have identified an age-related increase in ET(A)-mediated vasoconstrictor activity that persists during exercise, suggesting an important role of this pathway in the regulation of exercising skeletal muscle blood flow and maintenance of arterial blood pressure in the elderly.

Capacity and hypoxic response of subcutaneous adipose tissue blood flow in humans

BACKGROUND

The blood flow capacity in subcutaneous adipose tissue in humans remains largely unknown, and therefore the aim of this study was to determine the physiological range of blood flow in this tissue.

METHODS AND RESULTS

The subcutaneous adipose tissue blood flow (ATBF) was measured in 9 healthy young men by positron emission tomography using radiowater tracer. Subcutaneous ATBF was determined in regions adjacent to knee extensors at rest and during dynamic knee extensor exercise, and with 2 physiological perturbations: while breathing moderate systemic hypoxic air (14% O2) at rest and during exercise, and during intra-femoral artery infusion of high-dose adenosine infusion. ATBF was 1.3±0.6ml·100g(-1)·min(-1) at rest and increased with exercise (8.0±3.0ml·100g(-1)·min(-1), P<0.001) and adenosine infusion (10.5±4.9ml·100g(-1)·min(-1), P=0.001), but not when breathing moderate systemic hypoxic air (1.5±0.4ml·100g(-1)·min(-1)). ATBF was similar during exercise and adenosine infusion, but vascular conductance was lower during adenosine infusion. Finally, ATBF during exercise in moderate systemic hypoxia was reduced (6.3±2.2ml·100g(-1)·min(-1)) compared to normoxic exercise (P=0.004).

CONCLUSIONS

The vasodilatation capacity of human subcutaneous adipose blood flow appears to be comparable to, or even higher, than that induced by moderate intensity exercise. Furthermore, the reduced blood flow response in subcutaneous adipose tissue during systemic hypoxia is likely to contribute, in part, to the redistribution of blood flow to exercising muscle in a condition of reduced oxygen availability.

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

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

Systemic and vastus lateralis muscle blood flow and O2 extraction during ramp incremental cycle exercise

During ramp incremental cycling exercise increases in pulmonary O2 uptake (Vo2p) are matched by a linear increase in systemic cardiac output (Q). However, it has been suggested that blood flow in the active muscle microvasculature does not display similar linearity in blood flow relative to metabolic demand. This study simultaneously examined both systemic and regional (microvascular) blood flow and O2 extraction during incremental cycling exercise. Ten young men (Vo2 peak = 4.2 ± 0.5 l/min) and 10 young women (Vo2 peak = 3.2 ± 0.5 l/min) were recruited to perform two maximal incremental cycling tests on separate days. The acetylene open-circuit technique and mass spectrometry and volume turbine were used to measure Q (every minute) and breath-by-breath Vo2p, respectively; systemic arterio-venous O2 difference (a-vO2diff) was calculated as Vo2p/Q on a minute-by-minute basis. Changes in near-infrared spectroscopy-derived muscle deoxygenation (Δ[HHb]) were used (in combination with Vo2p data) to estimate the profiles of peripheral O2 extraction and blood flow of the active muscle microvasculature. The systemic Q-to-Vo2p relationship was linear (~5.8 l/min increase in Q for a 1 l/min increase in Vo2p) with a-vO2diff displaying a hyperbolic response as exercise intensity increased toward Vo2 peak. The peripheral blood flow response profile was described by an inverted sigmoid curve, indicating nonlinear responses relative to metabolic demand. The Δ[HHb] profile increased linearly with absolute Vo2p until high-intensity exercise, thereafter displaying a "near-plateau". Results indicate that systemic blood flow and thus O2 delivery does not reflect the profile of blood flow changes at the level of the microvasculature.

Taming the "sleeping giant": the role of endothelin-1 in the regulation of skeletal muscle blood flow and arterial blood pressure during exercise

The cardiovascular response to exercise is governed by a combination of vasodilating and vasoconstricting influences that optimize exercising muscle perfusion while protecting mean arterial pressure (MAP). The degree to which endogenous endothelin (ET)-1, the body's most potent vasoconstrictor, participates in this response is unknown. Thus, in eight young (24 ± 2 yr), healthy volunteers, we examined leg blood flow, MAP, tissue oxygenation, heart rate, leg arterial-venous O(2) difference, leg O(2) consumption, pH, and net ET-1 and lactate release at rest and during knee extensor exercise (0, 5, 10, 15, 20, and 30 W) before and after an intra-arterial infusion of BQ-123 [ET subtype A (ET(A)) receptor antagonist]. At rest, BQ-123 did not evoke a change in leg blood flow or MAP. During exercise, net ET-1 release across the exercising leg increased approximately threefold. BQ-123 increased leg blood flow by ~20% across all work rates (changes of 113 ± 76, 176 ± 83, 304 ± 108, 364 ± 130, 502 ± 117, and 570 ± 178 ml/min at 0, 5, 10, 15, 20, and 30 W, respectively) and attenuated the exercise-induced increase in MAP by ~6%. The increase in leg blood flow was accompanied by a ~9% increase in leg O(2) consumption with an unchanged arterial-venous O(2) difference and deoxyhemoglobin, suggesting a decline in intramuscular efficiency after ET(A) receptor blockade. Together, these findings identify a significant role of the ET-1 pathway in the cardiovascular response to exercise, implicating vasoconstriction via the ET(A) receptor as an important mechanism for both the restraint of blood flow in the exercising limb and maintenance of MAP in healthy, young adults.

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 adenosine, exercise, and moderate acute hypoxia on energy substrate utilization of human skeletal muscle

Glucose metabolism increases in hypoxia and can be influenced by endogenous adenosine, but the role of adenosine for regulating glucose metabolism at rest or during exercise in hypoxia has not been elucidated in humans. We studied the effects of exogenous adenosine on human skeletal muscle glucose uptake and other blood energy substrates [free fatty acid (FFA) and lactate] by infusing adenosine into the femoral artery in nine healthy young men. The role of endogenous adenosine was studied by intra-arterial adenosine receptor inhibition (aminophylline) during dynamic one-leg knee extension exercise in normoxia and acute hypoxia corresponding to ∼3,400 m of altitude. Extraction and release of energy substrates were studied by arterial-to-venous (A-V) blood samples, and total uptake or release was determined by the product of A-V differences and muscle nutritive perfusion measured by positron emission tomography. The results showed that glucose uptake increased from a baseline value of 0.2 ± 0.2 to 2.0 ± 2.2 μmol·100 g(-1)·min(-1) during adenosine infusion (P < 0.05) at rest. Although acute hypoxia enhanced arterial FFA levels, it did not affect muscle substrate utilization at rest. During exercise, glucose uptake was higher (195%) during acute hypoxia compared with normoxia (P = 0.058), and aminophylline had no effect on energy substrate utilization during exercise, despite that arterial FFA levels were increased. In conclusion, exogenous adenosine at rest and acute moderate hypoxia during low-intensity knee-extension exercise increases skeletal muscle glucose uptake, but the increase in hypoxia appears not to be mediated by adenosine.

Disparity in regional and systemic circulatory capacities: do they affect the regulation of the circulation?

In this review we integrate ideas about regional and systemic circulatory capacities and the balance between skeletal muscle blood flow and cardiac output during heavy exercise in humans. In the first part of the review we discuss issues related to the pumping capacity of the heart and the vasodilator capacity of skeletal muscle. The issue is that skeletal muscle has a vast capacity to vasodilate during exercise [approximately 300 mL (100 g)(-1) min(-1)], but the pumping capacity of the human heart is limited to 20-25 L min(-1) in untrained subjects and approximately 35 L min(-1) in elite endurance athletes. This means that when more than 7-10 kg of muscle is active during heavy exercise, perfusion of the contracting muscles must be limited or mean arterial pressure will fall. In the second part of the review we emphasize that there is an interplay between sympathetic vasoconstriction and metabolic vasodilation that limits blood flow to contracting muscles to maintain mean arterial pressure. Vasoconstriction in larger vessels continues while constriction in smaller vessels is blunted permitting total muscle blood flow to be limited but distributed more optimally. This interplay between sympathetic constriction and metabolic dilation during heavy whole-body exercise is likely responsible for the very high levels of oxygen extraction seen in contracting skeletal muscle. It also explains why infusing vasodilators in the contracting muscles does not increase oxygen uptake in the muscle. Finally, when approximately 80% of cardiac output is directed towards contracting skeletal muscle modest vasoconstriction in the active muscles can evoke marked changes in arterial pressure.

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

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

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

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

Role of {alpha}1-adrenergic vasoconstriction in the regulation of skeletal muscle blood flow with advancing age

alpha(1)-Adrenergic vasoconstriction during dynamic leg exercise is diminished in younger individuals, although the extent of this exercise-induced "sympatholysis" in the elderly remains uncertain. Thus, in nine young (25 +/- 1 yr) and six older (72 +/- 2 yr) healthy volunteers, we evaluated changes in leg blood flow (ultrasound Doppler) during blood flow-adjusted intra-arterial infusion of phenylephrine (PE; a selective alpha(1)-adrenergic agonist) at rest and during knee-extensor leg exercise at 20, 40, and 60% of maximal work rate (WR(max)). To probe the potential contributors to exercise-induced changes in alpha(1)-adrenergic receptor sensitivity, exercising leg O(2) consumption (Vo(2)) and lactate efflux were also evaluated (n = 10). At rest, the PE-induced vasoconstriction (i.e., decrease in leg blood flow) was diminished in older (-37 +/- 3%) compared with young (-54 +/- 4%) subjects. During exercise, the magnitude of alpha(1)-adrenergic vasoconstriction in the active leg decreased in both groups. However, compared with young, older subjects maintained a greater vasoconstrictor response to PE at 40% WR(max) (-14 +/- 3%, older; -7 +/- 2%, young) and 60% WR(max) (-11 +/- 3%, older; -4 +/- 3%, young). It is possible that this observation may be attributed to lower absolute work rates in the older group, because, for a similar absolute work rate ( approximately 10 W) and leg Vo(2) ( approximately 0.36 l/min), vasoconstriction to PE was not different between groups (-14 +/- 3%; older; -17 +/- 5%, young). Together, these data challenge the concept of reduced sympatholysis in the elderly, suggesting instead that the inhibition of alpha(1)-adrenergic vasoconstriction in the exercising leg is associated with work performed and, therefore, more closely related to the rate of oxidative metabolism than to age per se.

Restrictions in systemic and locomotor skeletal muscle perfusion, oxygen supply and VO2 during high-intensity whole-body exercise in humans

Perfusion to exercising skeletal muscle is regulated to match O(2) delivery to the O(2) demand, but this regulation might be compromised during or approaching maximal whole-body exercise as muscle blood flow for a given work rate is blunted. Whether muscle perfusion is restricted when there is an extreme metabolic stimulus to vasodilate during supramaximal exercise remains unknown. To examine the regulatory limits of systemic and muscle perfusion in exercising humans, we measured systemic and leg haemodynamics, O(2) transport, and , and estimated non-locomotor tissue perfusion during constant load supramaximal cycling (498 +/- 16 W; 110% of peak power; mean +/- S.E.M.) in addition to both incremental cycling and knee-extensor exercise to exhaustion in 13 trained males. During supramaximal cycling, cardiac output (Q), leg blood flow (LBF), and systemic and leg O(2) delivery and reached peak values after 60-90 s and thereafter levelled off at values similar to or approximately 6% (P < 0.05) below maximal cycling, while upper body blood flow remained unchanged (approximately 5.5 l min(-1)). In contrast, Q and LBF increased linearly until exhaustion during one-legged knee-extensor exercise accompanying increases in non-locomotor tissue blood flow to approximately 12 l min(-1). At exhaustion during cycling compared to knee-extensor exercise, Q, LBF, leg vascular conductance, leg O(2) delivery and leg for a given power were reduced by 32-47% (P < 0.05). In conclusion, locomotor skeletal muscle perfusion is restricted during maximal and supramaximal whole-body exercise in association with a plateau in Q and limb vascular conductance. These observations suggest that limits of cardiac function and muscle vasoconstriction underlie the inability of the circulatory system to meet the increasing metabolic demand of skeletal muscles and other tissues during whole-body exercise.

Endothelin-1-mediated vasoconstriction at rest and during dynamic exercise in healthy humans

It is now generally accepted that alpha-adrenoreceptor-mediated vasoconstriction is attenuated during exercise, but the efficacy of nonadrenergic vasoconstrictor pathways during exercise remains unclear. Thus, in eight young (23 +/- 1 yr), healthy volunteers, we contrasted changes in leg blood flow (ultrasound Doppler) before and during intra-arterial infusion of the alpha(1)-adrenoreceptor agonist phenylephrine (PE) with that of the nonadrenergic endothelin A (ET(A))/ET(B) receptor agonist ET-1. Heart rate, arterial blood pressure, common femoral artery diameter, and mean blood velocity were measured at rest and during knee-extensor exercise at 20%, 40%, and 60% of maximal work rate (WR(max)). Drug infusion rates were adjusted for blood flow to maintain comparable doses across all subjects and conditions. At rest, PE infusion (8 ng x ml(-1) x min(-1)) provoked a rapid and significant decrease in leg blood flow (-51 +/- 3%) within 2.5 min. Resting ET-1 infusion (40 pg x ml(-1) x min(-1)) significantly decreased leg blood flow within 5 min, reaching a maximal vasoconstriction (-34 +/- 3%) after 25-30 min of continuous infusion. Compared with rest, an exercise intensity-dependent attenuation to PE-mediated vasoconstriction was observed (-18 +/- 5%, -7 +/- 2%, and -1 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). Vasoconstriction in response to ET-1 was also blunted in an exercise intensity-dependent manner (-13 +/- 3%, -7 +/- 4%, and 2 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). These findings support a significant contribution of ET-1 and alpha-adrenergic receptors in the regulation of skeletal muscle blood flow in the human leg at rest and suggest a similar, intensity-dependent "lysis" of peripheral ET and alpha-adrenergic vasoconstriction during dynamic exercise.