Haemodynamic responses to exercise, ATP infusion and thigh compression in humans: insight into the role of muscle mechanisms on cardiovascular function
- José González-Alonso1,2,
- Stefan P. Mortensen2,
- Tina D. Jeppesen2,
- Leena Ali4,
- Horace Barker4,
- Rasmus Damsgaard2,
- Niels H. Secher2,3,
- Ellen A. Dawson2,3 and
- Stéphane P. Dufour1
- 1Centre for Sports Medicine and Human Performance, Brunel University, Uxbridge, Middlesex, UK2The Copenhagen Muscle Research Centre, Rigshospitalet, University of Copenhagen, Denmark3Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Denmark4Department of Anaesthetics, Ealing Hospital NHS Trust, Southall, Middlesex, UK
- Corresponding author J. González-Alonso: Centre for Sports Medicine and Human Performance, Brunel University, Uxbridge, Middlesex UB8 PH3, UK. Email: j.gonzalez-alonso{at}brunel.ac.uk
Abstract
The muscle pump and muscle vasodilatory mechanims are thought to play important roles in increasing and maintaining muscle
perfusion and cardiac output 
during exercise, but their actual contributions remain uncertain. To evaluate the role of the skeletal muscle pump and vasodilatation
on cardiovascular function during exercise, we determined leg and systemic haemodynamic responses in healthy men during (1)
incremental one-legged knee-extensor exercise, (2) step-wise femoral artery ATP infusion at rest, (3) passive exercise (n = 10), (4) femoral vein or artery ATP infusion (n = 6), and (5) cyclic thigh compressions at rest and during passive and voluntary exercise (n = 7). Incremental exercise resulted in progressive increases in leg blood flow (ΔLBF 7.4 ± 0.7 l min−1), cardiac output (
8.7 ± 0.7 l min−1), mean arterial pressure (ΔMAP 51 ± 5 mmHg), and leg and systemic oxygen delivery and 
. Arterial ATP infusion resulted in similar increases in 
, LBF, and systemic and leg oxygen delivery, but central venous pressure and muscle metabolism remained unchanged and MAP
was reduced. In contrast, femoral vein ATP infusion did not alter LBF, 
or MAP. Passive exercise also increased blood flow (ΔLBF 0.7 ± 0.1 l min−1), yet the increase in muscle and systemic perfusion, unrelated to elevations in aerobic metabolism, accounted only for ∼5%
of peak exercise hyperaemia. Likewise, thigh compressions alone or in combination with passive exercise increased blood flow
(ΔLBF 0.5–0.7 l min−1) without altering 
, MAP or 
. These findings suggest that the skeletal muscle pump is not obligatory for sustaining venous return, central venous pressure,
stroke volume and 
or maintaining muscle blood flow during one-legged exercise in humans. Further, its contribution to muscle and systemic peak
exercise hyperaemia appears to be minimal in comparison to the effects of muscle vasodilatation.
During exercise, blood flow to contracting skeletal muscles increases to ensure supply of oxygen and substrates and enhance
removal of metabolic by-products. This exercise-induced elevation in muscle perfusion takes place in conjunction with an increase
in cardiac output 
, brought about by an increase in both stroke volume (SV) and heart rate (HR; Higginbotham et al. 1986). The mechanisms controlling local and systemic perfusion are thought to implicate mechanical, neurological and metabolic
factors (Shepherd, 1983; Laughlin et al. 1996; Rowell, 2004; Saltin, 2007). Conventionally, the skeletal muscle pump (i.e. the lumped functions including local and central circulatory effects (Sheriff, 2005)) is deemed to be vital in coordinating the local and systemic blood flow responses by enhancing venous return, central venous
pressure (CVP), end-diastolic volume of the heart and thus SV and 
during exercise (Rowell et al. 1986; Notarius & Magder, 1996). Further, the muscle pump is thought to play an important role in muscle hyperaemia from the onset of exercise, although
its significance remains contentious with some reports supporting (Pollack & Wood, 1949; Folkow et al. 1971; Shiotani et al. 2002) and other refuting (Laughlin & Schrage, 1999; Dodson & Gladden, 2003; Hamann et al. 2003) a significant contribution (for review, Laughlin, 1987; Laughlin & Joyner, 2003).
The discrepancy between blood flows achieved by maximal pharmacological vasodilatation and exercise is one piece of evidence
supporting the muscle pump hypothesis (Rowell, 2004; Sheriff, 2005). Studies in upright seated healthy humans, however, show that femoral artery infusion of ATP or adenosine increase leg blood
flow (LBF) to levels similar to peak exercise hyperaemia (i.e. 7–8 l min−1) (Rådegran & Calbet, 2001; González-Alonso et al. 2002; Rosenmeier et al. 2004), but their effects on central cardiovascular variables are unknown. According to the muscle pump hypothesis, pharmacologically
induced muscle hyperaemia would be limited at least partially by insufficient venous return, CVP, SV and thus 
as the skeletal muscle pump is not assisting venous return from the lower extremities to the heart (Rowell et al. 1996). To date, no study has compared leg vasodilatation to exercise hyperaemia in humans to determine whether or not the muscle
pump is necessary for the maintenance of central haemodynamic function.
Partition of the muscle pump from other mechanical and metabolic effects of muscle contraction on blood flow is crucial to
determine its independent contribution to exercise hyperaemia. Studies of humans using passive and voluntary knee-extensor
exercise in the upright and supine positions (Wray et al. 2005) and external whole forearm compressions (Kirby et al. 2007) indicate a contribution of mechanically induced vasodilatation to onset exercise hyperaemia. These studies, however, did
not simultaneously measure muscle metabolism to ensure that the proposed influences of the muscle pump and mechanical deformation
on perfusion are independent of metabolic mechanisms. Supporting an independent mechanically driven vasodilatory pathway,
mechanical deformation of rat skeletal muscle feed arteries in vitro via increases in extravascular pressure induces rapid vasodilatation (Clifford et al. 2006). Whether increases in extravascular pressure in the human thigh lead to muscle vasodilatation and elevations in 
beyond what is found during passive and dynamic knee-extensor exercise has not been investigated.
The principal aim of this study was to determine the influence of leg vasodilatation on central cardiovascular responses and,
specifically, on CVP, SV and 
and compare these responses with those observed during exercise. We hypothesized that arterial ATP infusion increases SV
and 
to levels seen during exercise without the activation of the muscle pump. A second aim of the study was to examine the contribution
of the skeletal muscle pump and mechanical deformation on local and systemic exercise hyperaemia by comparing haemodynamic
responses during passive and active knee-extensor exercise, cyclic thigh compressions alone or in combination with passive
and voluntary exercise and separate femoral vein and artery ATP infusion. We hypothesized that the contribution of the skeletal
muscle pump and mechanical vasodilatatory mechanisms to local and systemic peak exercise hyperaemia is small in comparison
to the effects of muscle vasodilatation.
Methods
Twenty-three healthy subjects participated in three studies. Ten recreationally active males participated in the first study
(Study 1). They had a mean (± s.d.) age of 26 ± 3 years, body weight of 83 ± 11 kg, height of 189 ± 8 cm and a maximal oxygen uptake 
of 4.8 ± 0.4 l min−1. In Study 2, the subjects were three healthy males and three females aged 39 ± 3 years, weight 67 ± 4 kg, height 171 ± 2
cm and a 
of 2.8 ± 0.1 l min−1. In Study 3, the subjects were seven recreationally active males aged 27 ± 5 years, weight 81 ± 9 kg and height 181 ± 7 cm.
The subjects were fully informed of any risks and discomforts associated with the experiments before providing their informed
written consent to participate. The studies conformed to the code of Ethics of the World Medical Association (Declaration of Helsinki) and were approved by the Research Ethics Committees of Copenhagen and Frederiksberg communities (Studies 1 and 2) and Brunel
University (Study 3).
Experimental protocols
In Study 1, leg and systemic haemodynamic responses were examined during both incremental femoral artery infusions of ATP (1, 4, 16 and 64 μmol min−1) and incremental one-legged knee-extensor exercise to 93 ± 3% of peak power (26 ± 3, 52 ± 6, 79 ± 9 and 95 ± 11 W). ATP (Sigma A7699) was dissolved in isotonic saline and infused into the femoral artery during stepwise 1.5 min periods. Each exercise stage lasted 1.5 min. In addition, leg and systemic haemodynamics were examined during passive leg-kicking exercise. All protocols were separated by 30 min of recovery. In Study 2, leg and systemic haemodynamics were examined (1) at rest, (2) during and after ATP infusion in the femoral artery or vein (1 and 15 μmol min−1), and (3) during and after recovery from peak knee-extensor exercise (53 ± 7 W; ∼60 r.p.m.). In Study 3, leg and systemic haemodynamics were examined with and without thigh compressions (at a rate of 60 Hz): (1) at rest, (2) during passive leg-kicking exercise, and (3) during light one-legged knee-extensor exercise (20 W; ∼20% of peak power). In Studies 1 and 2, the subjects were examined in the upright-seated position and in Study 3 in the semirecumbent position. The haemodynamic effects of cyclic thigh compressions were also examined when subjects were in the upright-seated and the supine positions.
Instrumentation of subjects
On the first visit to the laboratory of Study 1, the subjects performed incremental knee-extensor and cycle ergometer exercise
(Excalibur, Lode, the Netherlands) to determine peak power and 
. On the day of the experiment, the subjects reported to the laboratory at 8 a.m. following ingestion of their usual breakfast.
Upon arrival, subjects rested in the supine position while catheters were placed under local anaesthesia into the femoral
artery and vein of the exercising thigh and the brachial artery and an antecubital vein, with the latter catheter being advanced
to the right atrium. The femoral artery and vein catheters were positioned 1–2 cm proximal or distal from the inguinal ligament.
A thermistor to measure venous blood temperature was inserted through the femoral catheter orientated in the anterograde direction
for the determination of femoral blood flow. The subjects then walked to the experimental room and sat on the knee-extensor
ergometer while instrumentation was attached. Following ∼10 min of seated rest, baseline leg and systemic haemodynamics and
blood samples were obtained. During each stage of either ATP infusion or incremental knee-extensor exercise, arterial and
femoral venous blood samples were withdrawn after 40 s followed by the measurement of LBF at 60 s. Measurements were also
obtained after 10 min of recovery. During the passive-exercise protocol, the subject's leg strapped to the ergometer was moved
passively with the same frequency and range of motion as during dynamic knee-extensor exercise. In Study 2, catheters for
blood sampling and ATP infusion were inserted in the femoral artery and vein whereas in Study 3 they were inserted in the
radial artery and femoral vein following the procedures described for Study 1. Active exercise in the subsequent studies was
performed as described above. Passive exercise, however, was performed with the leg free from the ergometer to minimize muscle
metabolic activity. In all studies, the lower legs were below heart level.
Thigh compressions
In Study 3, increases in extravascular pressure were induced using two custom-made blood pressure cuffs that were wrapped around the thigh and rapidly inflated and deflated with a cuff inflation unit (Hokanson E20, Bellevue, WA, USA). To simulate knee-extensor exercise closely, the cuff was inflated to 200 mmHg at a rate of 60 Hz, which is the same frequency as during exercise. This was the highest pressure-to-frequency ratio that could be produced. During exercise, cuff inflation and force development were synchronized using a triggering system (PowerLab 16/30, ADInstruments, Bella Vista, NSW 2153, Australia), such that the cuff inflation was superimposed upon quadriceps muscle contraction and deflation occurred during relaxation. During dynamic knee-extensor exercise, the relaxation phase is passive as the leg is moved backwards by the ergometer's motor.
Leg and systemic haemodynamics and oxygenation
Throughout all studies, pulmonary 
was measured online (Cosmed Quark b 2, Italy), HR was obtained from an electrocardiogram while vascular pressures were measured
at heart level in artery and right atrium or femoral vein with transducers (Pressure Monitoring Kit, Baxter) connected to
a monitor (Dialogue 2000, Danica Electronic, Copenhagen, Denmark) (Studies 1 and 2) or BP amplifiers and a data acquisition
system (Power Laboratory 16/30, ADInstruments; Study 3). The LBF was measured by the constant-infusion thermodilution method
in Study 1 (Andersen & Saltin, 1985; González-Alonso et al. 2000) and by ultrasound Doppler in Studies 2 and 3 (GE Logiq 5 and GE Vivid 7, General Electrics, USA) (Rådegran, 1997). In all studies, 
was calculated by multiplying SV by HR, using the modelflow method to determine SV (Beat Scope 1.1a; Finapres Medical Systems
BV, Amsterdam, the Netherlands) (Bogert & van Lieshout, 2005; González-Alonso et al. 2006). Systemic and leg vascular conductance was calculated as the ratio between 
or LBF and MAP. For systemic O2 delivery, 
was multiplied by the arterial O2 content whereas systemic O2 extraction was the ratio between the systemic arterial–venous (a–v) O2 difference and the arterial O2 content. Blood gas variables and haemoglobin concentrations were measured using ABL700 or ABL 825 analysers (Radiometer,
Copenhagen, Denmark).
Statistical analysis
One- or two-way repeated measure of variance (ANOVA) was performed to test significant differences between treatments. Following a significant F test, pair-wise differences were identified using Tukey's post hoc test. The significant level was set at P < 0.05 and data are presented as mean ± s.e.m.
Results
Incremental knee-extensor exercise and arterial ATP infusion
During incremental one-legged knee-extensor exercise, 
, LBF, MAP and systemic and leg O2 delivery and 
increased progressively and returned to baseline values after 10 min of recovery (Fig. 1). The rate of increase in LBF and leg 
was 79 ± 8 and 13 ± 1 ml min−1 W−1, respectively (r2 = 0.99; P < 0.001) and the increase in LBF and 
was 6.1 ± 0.3 and 5.5 ± 0.4 l min−1
−1, respectively (P = 0.73; r2 = 0.99; P < 0.001). The increase in 
above ∼30% peak power was associated with a plateau in SV (P = 0.15 from 30% to 93% peak power) and an exponential rise in HR. The increase in LBF reflected an increase in leg vascular
conductance and perfusion pressure.
Graded femoral artery infusion of ATP evoked significant increases in 
, LBF, and leg and systemic O2 delivery, even though MAP, CVP and perfusion pressure remained stable. When plotted against the increase in LBF, the rise
in 
and O2 delivery were not different during ATP infusion and exercise (Fig. 2). However, in contrast to the plateau observed during exercise, arterial ATP infusion resulted in a linear increase in SV
accompanying a lower rate of rise in HR. Therefore, SV at LBF > 5 l min−1 and 
> 11 l min−1 was higher during ATP infusion compared with exercise even though CVP was lower at peak leg perfusion. Also, in contrast
to exercise, arterial ATP infusion did not change metabolism across the leg, as indicated by the unchanged leg 
, glucose uptake, lactate release and the declining femoral venous temperature (Table 1). Blood flow to non-exercising or non-infused leg tissues (i.e. 
– LBF) remained at ∼6 l min−1 during mild and moderate exercise and mild and moderate arterial ATP infusion, but tended to increase at peak exercise and
ATP-induced hyperaemia (6.6–7.6 l min−1). Lastly, both leg and systemic vascular conductance were elevated at LBF > 7 l min−1 during ATP infusion compared to exercise. The elevated systemic vascular conductance at the two highest levels of hyperaemia
during arterial ATP infusion compared to exercise was associated with higher vascular conductance in both the leg and the
rest of the body tissues, with the former accounting for ∼60% of the systemic difference.
Passive knee-extensor exercise
Passive exercise resulted in increases in LBF, HR, SV, leg 
and MAP, a trend (P = 0.09) for an increase in 
and a parallel reduction in the leg a–v O2 difference (P < 0.05; Table 2). The increase in blood flow and 
was the same at the leg and systemic levels (i.e. 0.6–0.7 l min−1 and 30–37 ml min−1, respectively). Thus, the increase in 
and LBF unrelated to enhanced metabolism represented only 0.3–0.4 l min−1, or ∼5% of the peak exercise leg and systemic hyperaemia (i.e. 7.2–7.9 l min−1). In support of the estimate, passive exercise only increased 
and LBF by 0.2–0.3 l min−1 when 
remained unchanged in Study 3.
Arterial and venous ATP infusion
In contrast to infusion in the femoral artery, ATP infusion in the femoral vein (1 and 15 μmol min−1) did not alter LBF, 
or MAP (Table 3). Likewise, LBF did not change in the non-infused leg during either femoral artery or venous ATP infusion. During recovery
from exercise and arterial ATP infusion, LBF kinetics was similar.
Cyclic external compressions
Whole thigh compressions doubled resting LBF (i.e. ΔLBF = 0.7 ± 0.1 l min−1) and vascular conductance without inducing significant alterations in HR, SV, 
, MAP, mean femoral venous pressure or 
(Table 4). No differences in LBF were observed when cyclic thigh compressions were performed in the upright and supine positions (ΔLBF
0.96 ± 0.17 versus 0.75 ± 0.04 l min−1, respectively; P = 0.35). Superimposing thigh compressions upon exercise increased LBF when exercise was passive but not during voluntary
exercise (Fig. 3).
Discussion
We compared the haemodynamic responses to one-legged knee-extensor exercise, intravascular ATP infusion and external thigh
compressions to gain insight into the role of the muscle pump and vasodilatory mechanisms on cardiovascular function during
exercise. A major finding was that arterial ATP infusion mimicked the increases in 
and LBF observed during incremental knee-extensor exercise without altering CVP or muscle metabolism. In contrast, infusion
of ATP in the femoral vein did not change 
, LBF or MAP, suggesting that skeletal muscle vasodilatation drives the increase in 
during exercise. Another salient finding was that passive knee-extensor exercise evoked a small increase in leg muscle and
systemic perfusion, which was partly related to an enhanced muscle aerobic metabolism. Likewise, thigh compressions alone
or in combination with passive exercise, but not voluntary exercise, doubled LBF without altering 
, MAP or 
. Collectively, these findings suggest that the skeletal muscle pump is not obligatory for sustaining venous return, CVP,
SV and 
or maintaining muscle blood flow during one-legged exercise in humans. Further, the contribution of the muscle pump and mechanically
induced vasodilatation to local and systemic peak exercise hyperaemia appears to be small in comparison to the effects of
limb muscle vasodilatation.
Cardiovascular function during exercise and pharmacologically induced vasodilatation
During exercise, there was the expected increase in 
, LBF, MAP and leg and systemic O2 delivery and 
, which returned to baseline values after 10 min of recovery. The rate of increase in LBF, O2 delivery and leg 
was similar to that reported during one-legged knee-extensor exercise (Andersen & Saltin, 1985; Richardson et al. 1995; Rådegran, 1997; González-Alonso et al. 2002). The increase in LBF during incremental exercise matched the increase in 
, indicating that the augmented systemic variables reflected the enhanced metabolic demand of the exercising knee-extensor
muscles (Savard et al. 1988; Magnusson et al. 1994; González-Alonso et al. 2006). In other words, global blood flow to the upper body and the contralateral leg remained unchanged with only a tendency to
increase at peak exercise when regional 
increased. The close matching between O2 supply and metabolic demand was therefore mediated largely by local quadriceps muscle vascular mechanisms, which are thought
to involve mechanical, neurological and metabolic signals (Guyton et al. 1962; Laughlin et al. 1996; Rowell, 2004; Saltin, 2007).
An important observation was that step-wise femoral artery infusion of ATP evoked the same increase in 
and LBF as incremental exercise, albeit the contribution of HR and SV to 
differed markedly. During incremental exercise, HR increased in a curvilinear fashion from 64 to 134 beats min−1, driving all the increase in 
above 30% of peak power as SV plateaued or tended to decline (Åstrand et al. 1964; Poliner et al. 1980; Keul et al. 1981; Higginbotham et al. 1986; Mortensen et al. 2005). In contrast, the same increase in 
with graded arterial ATP infusion was associated with both progressive elevations in HR (70–95 beats min−1) and gradual increases in SV (103–155 ml beat−1). Consequently, SV was higher during arterial ATP infusion at the two highest levels of 
, suggesting that tachycardia blunted the increase in SV during exercise. This is in agreement with findings on heart transplant
patients demonstrating that the SV response to exercise is augmented because their HR is retarded, but the rise in 
with respect to metabolic demand is normal (Stinson et al. 1972; Clark et al. 1973; Verani et al. 1994). Another striking observation was that systemic and leg vascular conductance was much greater at the two highest levels
of hyperaemia during ATP infusion than during exercise. The lack of vasoconstriction in the leg and other bodily tissues during
ATP infusion appears to account for this response. These observations, on one hand, support the idea that peripheral, rather
than central factors, control the circulatory responses to exercise (Guyton et al. 1962; Laughlin et al. 1996; Rowell, 2004; Rowland, 2005) and, on the other hand, cast doubts about the necessity of the muscle pump to increase and maintain SV and 
during exercise.
Muscle pump and cardiovascular control
Conventionally, the muscle pump is thought to play an important role in maintaining CVP, end-diastolic volume and SV when
is increased during exercise (Notarius & Magder, 1996). Guyton et al. (1962) and Rowell (1992, 1993, 2004) argue that translocation of blood volume from the vasculature to the heart increases 
because the heart itself cannot raise its output to the level produced during exercise. In line with this notion, Sheriff et al. (1993) showed that the increase in HR with ventricular pacing was accompanied by a fall in CVP and SV, such that 
increased only slightly. They also showed that similar attempts to raise 
above its normal level in voluntarily exercising dogs were counteracted by a fall in CVP and SV. According to Rowell (1992, 2004), these findings of Sheriff et al. support the need for the muscle pump to counteract the flow-dependent redistribution of blood volume from the central to
peripheral vessels and thereby prevent the negative effects of increased 
on end-diastolic volume of the heart and SV. The applicability of these findings in instrumented animals to exercising humans
remains debated (Laughlin & Joyner, 2003; Magder, 2005; Brengelmann, 2005; Clifford et al. 2005; Rothe, 2005; Sheriff, 2005), particularly in the light of the observations that CVP does not need to decline during incremental upright whole-body exercise
in humans (Mortensen et al. 2005), nor during incremental knee-extensor exercise, passive knee-extensor exercise or ATP infusion (Fig. 2). Arguing against a crucial role of the muscle pump in the cardiovascular responses to exercise, patients with a congenital
absence of valves in the deep veins of the legs also show an increase in SV and 
during exercise whilst CVP is maintained or slightly reduced (Bevegård & Lodin, 1962). Therefore, the current data with arterial ATP infusion challenge the muscle pump hypothesis by demonstrating that SV can
increase even above exercise levels and 
can increase to 15 l min−1 to account for the 
of humans with a 
of ∼2 l min−1 without activation of the skeletal muscle pump.
Muscle vasodilatory mechanisms and cardiovascular control
Another significant observation was that passive knee-extensor exercise and cyclic thigh compressions increased LBF but the
magnitude of the response was small compared to the values at peak exercise. For instance, the increase in LBF and 
during passive exercise was 0.6–0.7 l min−1 (representing a 120% increase in LBF) and was associated with a 73% increase in muscle 
. This value agrees closely with the ∼0.5 l min−1 LBF increase found during passive supine and upright knee-extensor exercise (Wray et al. 2005), but is higher than the 0.3 l min−1 increase we found when leg 
did not increase during passive exercise in Study 3. Thus, these findings during passive exercise point towards a small independent
contribution of mechanical factors to exercise hyperaemia. Passive leg motions alone do not adequately simulate activation
of the muscle pump, which involves squeezing the blood out of the veins and venules of the dependent leg. In a subsequent
study, we therefore applied external cyclic compressions to the whole thigh via inflation of a cuff to 200 mmHg at a rate
of 60 Hz to increase intramuscular pressure to peak leg-kicking exercise levels. This was done independently and in combination
with passive and voluntary knee-extensor exercise.
In keeping with the passive exercise results, we found that whole thigh cyclic compressions alone and in combination with
passive exercise, but not with voluntary exercise, doubled LBF (ΔLBF 0.5–0.7 l min−1) without significantly altering other cardiovascular variables. A possible explanation for the lack of effect during voluntary
exercise might be that the additional compression did not have any influence on venous return since compressions were applied
when the veins were already empty (Tschakovsky et al. 2004). These findings, however, are in close agreement with a study in the human forearm demonstrating a 2- to 3-fold increase
in forearm blood flow and vascular conductance immediately after single, sustained and repeated whole forearm compressions
(Kirby et al. 2007). The present observations in the thigh support a contribution of mechanical influences to exercise hyperaemia in the human
leg (Clifford et al. 2006; Dufour et al. 2006; Kirby et al. 2007). Although the relative responses in the upper and lower limbs appear to be similar, the absolute increase in blood flow
is much greater in the leg than in the forearm. This makes it possible to assess the central haemodynamic effects of external
compressions in this study. Interestingly, the increases in LBF and 
in all experimental conditions were highly correlated to increases in leg 
(slopes, 6.1 and 7.1 l min−1
−1, respectively; Fig. 4), suggesting that the major contributor to local and systemic blood flow during one-legged kicking exercise is oxygen-related
vasodilatation. We estimated that the increase in LBF and 
during passive exercise and thigh compressions, unrelated to the rate of muscle metabolism, accounts for ∼5% of peak knee-extensor
exercise or peak ATP-induced hyperaemia (Figs 3 and 4). This is in congruence with recent findings showing that the increase in forearm blood flow and vascular conductance during
repeated forearm compressions (200 mmHg) only accounted for ∼20% of contraction-induced hyperaemia (Kirby et al. 2007). The failure of cyclic thigh compressions to elicit a larger increase in LBF in the upright position where the veins are
loaded compared to the supine position where they are unloaded is another important piece of evidence refuting a significant
contribution of the skeletal muscle pump. Collectively, these findings during cyclic thigh compressions and passive knee-extensor
exercise suggest that the muscle pump and mechanically induced vasodilatation account for a small fraction of the leg and
systemic exercise hyperaemia.
Leg muscle hyperaemia during both exercise and arterial ATP infusion is brought about by local vasodilatation overriding concomitant augmented sympathetic vasoconstrictor outflow (Von Euler, 1974; Joyner & Halliwill, 2000; Rosenmeier et al. 2004; Ichinose et al. 2006). During exercise, however, sympathetic activation is associated with robust increases in HR and MAP (the ‘exercise pressor reflex’; Fig. 1) whereas during arterial ATP and adenosine infusions, MAP remains unchanged, or declines slightly, and the HR response is blunted in association with a higher SV as discussed above (Rådegran & Hellsten, 2000; Rådegran & Calbet, 2001; Rosenmeier et al. 2004) (Fig. 4). Intravascular ATP infusion does not enhance muscle metabolism or plasma ATP, nor does it activate muscle contraction. It is therefore doubtful that metabolic by-products and ATP accumulate in the interstitium and chemo- and mechanoreceptors are activated with ATP infusion (González-Alonso et al. 2002; Rosenmeier et al. 2004; SP Mortensen, J González-Alonso, L Brune, B Saltin & H Hellsten, unpublished observation). The distinct blood pressure response during exercise might be associated with activation of mechanosensitive afferent type III fibres and chemosensitive afferent type IV fibres (Mitchell et al. 1989; Kindig et al. 2007). The similar blood flows but very different blood pressure responses during arterial ATP infusion and exercise suggest distinct regulatory loci and signalling pathways in human skeletal muscle for blood flow and blood pressure regulation.
The present results raise the question of how is preload to the heart maintained in the ATP condition? As discussed above,
neither arterial nor venous ATP infusion is likely to stimulate the metaboreflex and, given the lack of change in MAP and
CVP with increasing infusion rate of ATP, it is also unlikely that arterial or cardiopulmonary baroreceptors are stimulated
to a significant extent. The possibility that vasoconstriction in other vascular beds maintains preload to the heart by ATP
infusion is also at odds with the observations that: (i) the control leg blood flow and vascular conductance are unchanged
with arterial ATP infusion (González-Alonso et al. 2002; Fig. 3), (ii) non-exercising and non-infused bodily tissues blood flow (difference between 
and LBF) is the same with graded ATP infusion and exercise (Fig. 2), and (iii) non-exercising and non-infused tissue vascular conductance remained unchanged with increasing rates of ATP infusion.
Apart from the robust vasodilator response (27-fold increase in leg blood flow and vascular conductance) and significant increase
in HR, graded arterial ATP infusion results in a 2.4-fold increase in muscle sympathetic nerve activity and circulating noradrenaline
(Rosenmeier et al. 2004). Here, we found that venous ATP infusion did not alter leg or systemic haemodynamics and since HR did not change, it is
likely that sympathetic activity was also unaltered (Table 3). Hence, it is unlikely that intravascular ATP infusion is increasing 
through a direct effect of circulating ATP on the heart. This implies that increases in venous return, SV and 
with ATP infusion might involve mechanisms linking the functioning of skeletal and cardiac muscle, independently of the established
contraction-activated mechanical, neurological and metabolic signalling pathways.
In conclusion, this study provides evidence in humans supporting the idea that the muscle pump is not necessary for sustaining
venous return, CVP and 
and maintaining muscle blood flow during one-legged exercise. Further, the contribution of the muscle pump and mechanically
induced vasodilatation to local and systemic peak exercise hyperaemia appears to be small in comparison to the effects of
limb muscle vasodilatation.
Authors’ present addresses
E. A. Dawson: Research Institute for Sport and Exercise Science. Liverpool John Moores University, Liverpool L3 2ET, UK.
S. P. Dufour: UFR STAPS, Université Marc Bloch, 14 Rue René Descartes, 67084 Strasbourg Cedex, France.
Acknowledgements
We give special thanks to the volunteer subjects. We also thank Troels Munch, Jacob Mørkeberg, Peter Nissen, James Pearson, Eric Stöhr, David Low and Dr Lotlikar for their assistance. Special thanks are given to Roger Paton from the Brunel Institute of Bioengineering for his help with the cuff inflation–deflation unit. The studies were supported by the Novo Nordisk Foundation, The Copenhagen Hospital System and Brunel University.
Footnotes
-
(Resubmitted 1 February 2008; accepted after revision 12 March 2008; first published online 13 March 2008)
References
Figure 1. Leg and systemic haemodynamics during incremental knee-extensor exercise Cardiovascular variables plotted against increases in power output during one-legged knee-extensor exercise. Data are mean ± s.e.m. for 9–10 subjects.
Figure 2. Leg and systemic haemodynamics with incremental exercise and graded intrafemoral artery ATP infusion Cardiovascular variables plotted against leg blood flow during incremental one-legged knee-extensor exercise and graded intrafemoral artery ATP infusion. Open circles depict systemic haemodynamics whereas filled circles depict leg haemodynamics. Data are mean ± s.e.m. for 9 subjects. *Different from exercise, P < 0.05.
Figure 3. Haemodynamic effects of exercise, ATP infusion and external compressions Change in blood flow and arterial pressure from baseline values during peak and light exercise, intrafemoral artery or vein ATP infusion, and thigh compressions alone and in combination with passive exercise or light exercise. Note that leg vasodilatation with arterial ATP infusion matches exercise hyperaemia, but not the increase in arterial blood pressure. ATP acts locally as intrafemoral vein ATP infusion did not alter these responses. Further, effect of mechanical-induced vasodilatation via cyclic external compressions at the same rate as exercise induces significant increases in leg and systemic blood flow, but this increase only accounts for ∼10% of the peak exercise hyperaemia. Data are mean ± s.e.m. *Significant change from corresponding baseline values, P < 0.05.
Figure 4. Leg blood flow and blood pressure as a function of leg aerobic metabolism Changes in blood flow and arterial pressure from baseline values during incremental knee-extensor exercise, step-wise intrafemoral
artery ATP infusion, passive exercise and thigh compressions alone and in combination with passive exercise or light exercise.
Noteworthy is the significant correlations between the increases in leg blood flow, cardiac output and mean arterial pressure
and the increases in leg 
during the exercise and thigh compressions protocols, but the dissociation in these responses during arterial ATP infusion.
Table 1. Femoral blood variables during one-legged knee-extensor exercise and femoral artery ATP infusion
Table 2. Leg and systemic haemodynamic effects of passive exercise
Table 3. Leg and systemic haemodynamic effects of femoral artery and vein ATP infusions in resting human subjects
Table 4. Leg and systemic haemodynamic effects of thigh compressions
