Journal of Physiology
Acute systemic hypoxia in resting and exercising humans
markedly increases skeletal muscle sympathetic discharge,
ventilation, heart rate and presumably cardiac sympathetic
tone with no or minor elevations in mean arterial pressure
(Saito et al. 1988; Rowell et al. 1989; Somers et al. 1989;
Seals et al. 1991a; Morgan et al. 1995; Smith et al. 1996;
Hansen et al.2000). Despite this enhanced vasoconstrictor
activity during exercise, cardiac output, contracting
skeletal muscle blood flow and coronary sinus blood flow
increase with systemic hypoxia, thereby contributing to
the maintenance of cardiac and peripheral O2delivery
and O2 uptake (Rowell et al. 1986; Knight et al. 1993;
Grubbström et al. 1993; Richardson et al. 1995; Koskolou
et al. 1997a,b; Roach et al. 1999; González-Alonso et al.
2001, 2002). In resting humans, hypoxaemia also prevents
sympathetic vasoconstriction (Rowell & Blackman 1986;
Weisbrod et al.2001; Dinenno et al.2003). The augmented
sympathetic outflow to skeletal muscle with moderate
acute hypoxaemia is thought to be primarily mediated by
excitation of chemoreceptors within the carotid and aortic
bodies sensing the concomitant fall in arterial blood free
O2(Pa,J) (Marshall et al. 1994; Lahiri & Acker,1999).
To identify the role of Pa,J in cardiovascular control, we
have recently developed a model in vivo in humans that
allows the independent manipulation of the O2dissolved
in the blood (Pa,J) by varying the inspiratory fraction of
O2, while using carbon monoxide (CO) to reduce the
amount of O2 bound to haemoglobin (González-Alonso
et al. 2001). By comparing normoxia, hypoxia, CO +
normoxia, CO + hyperoxia and hyperoxia, we have
demonstrated that alterations in contracting skeletal
muscle blood flow and vascular conductance are unrelated
to pronounced alterations in Pa,J (40–590 mmHg), but
closely linked to the oxygenation state of haemoglobin
(González-Alonso et al. 2001, 2002). Our findings suggest
that the main vascular O2sensor locus is located in the
erythrocyte itself, rather than in the PJ-sensitive regions
of the endothelium or vascular smooth muscle. Whether
Human skeletal muscle sympathetic nerve activity, heart rate
and limb haemodynamics with reduced blood oxygenation
Akiko Hanada, Mikael Sander and José González-Alonso
Copenhagen Muscle Research Centre, Rigshospitalet, University of Copenhagen, Denmark
Acute systemic hypoxia causes significant increases in human skeletal muscle sympathetic nerve
activity (MSNA), heart rate and ventilation. This phenomenon is thought to be primarily mediated
by excitation of peripheral chemoreceptors sensing a fall in arterial free oxygen partial pressure
(Pa,J). We directly tested the role of Pa,J on MSNA (peroneal microneurography), heart rate,
ventilation and leg haemodynamics (n = 7–8) at rest and during rhythmic handgrip exercise by
using carbon monoxide (CO) to mimic the effect of systemic hypoxia on arterial oxyhaemoglobin
(~20% lower O2Hba), while normalising or increasing Pa,J (range 40–620 mmHg). The four
experimental conditions were: (1) normoxia (Pa,J ~110 mmHg; carboxyhaemoglobin (COHb)
~2%); (2) hypoxia (Pa,J~40 mmHg; COHb ~2%); (3) CO + normoxia (Pa,J~110 mmHg; COHb
~23%); and (4) CO + hyperoxia (Pa,J ~620 mmHg; COHb ~24%). Acute hypoxia augmented
sympathetic burst frequency, integrated MSNA, heart rate and ventilation compared to normoxia
over the entire protocol (7–13 bursts min_1, 100–118%, 13–17 beats min_1, 2–4 l min_1, respectively,
P < 0.05). The major new findings were: (1) CO + normoxia and CO + hyperoxia also elevated
MSNA compared to normoxia (63–144% increase in integrated MSNA; P < 0.05) but they did not
increase heart rate (62–67 beats min_1) or ventilation (6.5–6.8 l min_1), and (2) despite the 4-fold
elevation in MSNA with hypoxaemia and exercise, resting leg blood flow, vascular conductance and
O2uptake remained unchanged. In conclusion, the present results suggest that increases in MSNA
with CO are not mediated by activation of the chemoreflex, whereas hypoxia-induced tachycardia
and hyperventilation are mediated by activation of the chemoreflex in response to the decline in
Pa,J. Our findings also suggest that Pa,Jis not an obligatory signal involved in the enhanced MSNA
with reduced blood oxygenation.
(Resubmitted 28 March 2003; accepted after revision 12 June 2003; first published online 8 August 2003)
Corresponding authorJ. González-Alonso: Copenhagen Muscle Research Centre, Rigshospitalet, Section 7652,
Blegdamsvej9, DK-2100 Copenhagen Ø, Denmark. Email: firstname.lastname@example.org
J Physiol(2003), 551.2, pp.635–647
© The Physiological Society 2003
Journal of Physiology
muscle sympathetic nerve activity (MSNA) at rest and
during exercise reflects changes in O2bound to haemo-
globin, alterations in arterial free O2or changes in whole
blood O2content has never been explored. Moreover, it
remains unknown whether a mild CO load, which mimics
the effect of systemic hypoxia on arterial oxyhaemoglobin
(~20% lower O2Hba), results in an increase in MSNA
without compromising blood flow, O2delivery and muscle
O2uptake in the inactive limbs.
In contrast to the stimulatory effects of acute systemic
hypoxia (Rowell et al. 1989; Somers et al. 1989; Seals et al.
1991b), MSNA, heart rate and ventilation have been
shown to be unaltered or reduced with hyperoxia
(breathing 100% O2) (Seals et al.1991b; Hansen & Sander,
2003). We therefore hypothesised that if Pa,Jwas an essential
signal mediating the increased sympathoexcitation outflow
to skeletal muscle, the cardiac acceleration and the hyper-
ventilation normally observed with systemic hypoxia
(Rowell et al. 1989; Seals et al. 1991a) would, with a very
large increase in Pa,Jfrom ~40 mmHg in systemic hypoxia
to ~620 mmHg in CO + hyperoxia, completely restore
these responses to normoxic levels despite the persistent
equal reduction in arterial oxyhaemoglobin. Systemic
hypoxia is thought to potentiate exercise-induced
sympathetic neural activation (Seals et al. 1991a). However,
this concept is based on studies using brief periods of
hypoxia exposure (3–4 min), which are not sufficient to
stabilise blood oxygenation and increase MSNA above
control (Seals et al. 1991a). This finding clearly contrasts
with the 260% increase in MSNA in resting humans after
20 min of 10% O2under hypocapnic conditions (Rowell et
al. 1989). Therefore, we hypothesised that the stimulatory
effects of systemic hypoxia and CO inhalation would be
similar at rest and during handgrip exercise when
sufficient time for equilibration of bodily O2 stores is
given. Lastly, we hypothesised that resting limb blood flow
and aerobic metabolism will be preserved despite an
augmented sympathetic vasoconstrictor activity with
hypoxia and CO inhalation. To test these hypotheses, we
measured skeletal MSNA (peroneal microneurography),
heart rate, mean arterial pressure, ventilation and leg
haemodynamics during supine rest, handgrip exercise and
recovery in healthy males exposed to conditions producing
the same reduction in arterial oxyhaemoglobin (i.e. hypoxia,
CO + normoxia, CO + hyperoxia) compared to normoxia,
but vastly different Pa,J(40–620 mmHg).
Twenty subjects participated in three studies. All subjects were
healthy caucasian males, 26 ± 1 years of age, body weight
74 ± 4 kg, and height 182 ± 3 cm. The subjects were fully
informed of any risks and discomforts associated with the
experiments before giving 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 Ethics Committee for Copenhagen and
In all three investigations, the subjects were studied in the supine
position. In the first study, eight of the subjects performed
rhythmic handgrip (RHG) exercise under the following four
conditions: (1) normoxia (inspiratory O2 fraction, FI,J
21.4 ± 0.2%; inspiratory O2tension, PI,J 163 ± 1 mmHg; blood
carboxyhaemoglobin fraction, COHb 2.3 ± 0.1%); (2) hypoxia
(FI,J 10.3 ± 0.1%; PI,J 78 ± 1 mmHg; COHb 2.2 ± 0.1%);
(3) CO breathing combined with normoxia (CO + normoxia;
FI,J 21.1 ± 0.1%; PI,J 160 ± 1 mmHg; COHb 20.3 ± 0.9%); and
(4) CO breathing combined with hyperoxia (CO + hyperoxia;
FI,J 95.9 ± 0.3%; PI,J 729 ± 3 mmHg; COHb 21.2 ± 0.9%).
Each of the four conditions was separated from the next by
~20 min of supine rest. The normoxic and hypoxic trials were
performed first and second, respectively, whereas the two CO
trials were counterbalanced across subjects. The CO trials were
performed last, due to the relatively long period required to return
to baseline COHb levels. During each condition, MSNA, blood
pressure, heart rate and respiratory rate were recorded during
(1) 10 min of rest; and (2) 5 min of RHG (40% of maximal
voluntary contraction) followed by 2 min of post-exercise
ischaemia and 3 min of recovery. In a second control study, five
subjects were studied during four consecutive periods of
normoxia to assess the effect of time and repeated exercise on
MSNA and haemodynamics, while also breathing in the closed-
circuit system. In a third study, seven of the subjects were studied
under the same experimental conditions as in study1 with the aim
of assessing the ventilatory response as well as the resting leg
On arrival at the laboratory in studies 1 and 2, the subjects rested
in the supine position while a catheter was inserted in an
antecubital vein for blood sampling. In each condition, blood
pressure was measured every 30 s by automated sphygmo-
manometry (Dinamap Pro 100, Critikon, Tampa, FL, USA).
Mean arterial pressure (MAP) was calculated as [(2 w diastolic
blood pressure) + systolic blood pressure]/3. Heart rate was
determined from an electrocardiogram and respiratory rate from
a strain-gauge pneumobelt tracing. These tracings and MSNA
measured by peroneal microneurography were continuously
recorded using a Powerlab system (ADInstruments, Sydney,
Australia). In study 3, two catheters were placed under local
anaesthesia into the femoral artery and vein of the resting right leg
using the Seldinger technique. 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 venous catheter
orientated in the anterograde direction for the determination of
femoral venous blood flow. Femoral venous blood flow (an index
of leg blood flow) was determined by the constant infusion
thermodilution technique (Andersen & Saltin, 1985; González-
Alonso et al. 2000). Arterial blood pressure was continuously
monitored by a pressure transducer (Pressure Monitoring Kit,
Baxter) at the level of the inguinal region. The ventilatory response
was measured continuously in each condition using a pneumotach
placed in the expiratory side of the closed-circuit system (OCM-2
metabolic cart, Applied Electrochemistry, USA).
Multiunit efferent postganglionic sympathetic nerve activity to
the resting skeletal muscle bed was obtained with unipolar
A. Hanada, M. Sander and J. Gonzaléz-Alonso
Journal of Physiology
human studies showing an elevation in ventilation and
cardiac output with systemic hypoxia but a normal
cardiorespiratory response with CO-hypoxia (Asmussen
& Chiodi, 1941; Chiodi et al. 1941). Because the hyper-
ventilation with systemic hypoxia was accompanied by a
large lowering in Pa,J and Pa,CO2and a lesser increase in
arterial pH, we are tempted to suggest that the unchanged
ventilation with CO interventions was largely associated
with the normalisation of Pa,J, Pa,CO2and pH.
To summarise, the present results showed that CO in
conjunction with either normoxia or hyperoxia increased
MSNA at rest and during exercise in a similar manner
to acute systemic hypoxia, yet this did not evoke the
well-characterised hypoxia-induced tachycardia, hyper-
ventilation and hypocapnia. Secondly, the markedly
augmented MSNA with combined hypoxaemia and
exercise did not cause local skeletal muscle vaso-
constriction or suppressed oxygen uptake in the resting
or exercising limbs. This suggests the existence of
compensatory mechanisms in resting and contracting
skeletal muscle capable of over-riding the elevated
vasoconstrictor stimuli. Finally, MSNA at rest and during
exercise was more closely related to O2 bound to
haemoglobin than to O2dissolved in plasma, which fuels
the hypothesis that the erythrocyte might act as an O2
sensor for MSNA activation. In stark contrast to MSNA,
heart rate and ventilation appear closely related to the
alterations in Pa,J. Collectively, the present findings
suggest that increases in MSNA with CO are not mediated
by activation of the chemoreflex, whereas hypoxia-
induced tachycardia and hyperventilation are mediated by
activation of the chemoreflex in response to the decline in
Pa,J. Future experiments should address the question of
whether the present results in male caucasians can be
generalised to other populations.
Andersen P & Saltin B (1985). Maximal perfusion of skeletal muscle
in man. J Physiol366, 233–246.
Asmussen E & Chiodi H (1941). The effect of hypoxemia on
ventilation and circulation in man. Am J Physiol132, 426–436.
Chiodi H, Dill DB, Consolazio F & Harvath SM (1941). Respiratory
and circulatory responses to acute carbon monoxide poising. Am J
Christensen P, Rasmussen JW & Henneberg SW (1993). Accuracy of
a new bedside method for estimation of circulating blood volume.
Clin Care Med21, 1535–1540.
Dally M, Deb M & Scott M J (1962). An analysis of the pulmonary
cardiovascular reflex effects of stimulation of the carotid body
chemoreceptors in the dog. J Physiol162, 555–573.
Dampney RAL, Blessing WW & Tan E (1988). Origin of tonic
GABAergic inputs to vasopressor neurons in the subretrofacial
nucleous of the rabbit. J Auton Nerv Syst24, 227–239.
Davidson NS, Goldner S & McCloskey DI (1976). Respiratory
modulation of baroreceptor and chemoreceptor reflexes affecting
heart rate and cardiac vagal efferent nerve activity. J Physiol259,
Dinenno FA, Joyner MJ & Halliwill JR (2003). Failure of systemic
hypoxia to blunt a-adrenergic vasocontriction in the human
forearm. J Physiol 549, 985–994.
Escourrou P, Johnson DG & Rowell LB (1984). Hypoxemia increases
plasma catecholamine concentrations in exercising humans. J Appl
Gonzalez C, Almaraz L, Obeso A & Rigual R (1994). Carotid body
chemoreceptors: from natural stimuli to sensory discharges.
Physiol Rev74, 829–899.
González-Alonso J, Olson DB & Saltin B (2002). Erythrocyte and the
regulation of human skeletal muscle blood flow and oxygen
delivery: role of circulating ATP. Circ Res91, 1046–1055.
González-Alonso J, Quistorff B, Krustrup P, Bangsbo J & Saltin B
(2000). Heat production in human skeletal muscle at onset of
intense dynamic exercise. J Physiol524, 603–615.
González-Alonso J, Richardson RS & Saltin B (2001). Exercising
skeletal muscle blood flow in humans responds to reduction in
arterial oxyhaemoglobin, but not to altered free oxygen. J Physiol
Grubbström J, Berglund B & Kaiser L (1993). Myocardial oxygen
supply and lactate metabolism during marked arterial
hypoxaemia. Acta Physiol Scand149, 303–310.
Hansen J & Sander M (2003). Sympathetic neural overactivity in
healthy humans after prolonged exposure to hypobaric hypoxia.
Hansen J, Sander M & Thomas G (2000). Metabolic modulation of
sympathetic vasoconstriction in exercising skeletal muscle. Acta
Physiol Scand168, 489–503.
Hansen J, Thomas GD, Harris SA, Parsons WJ & Victor R (1996).
Differential sympathetic neural control of muscle oxygenation in
resting and exercising human skeletal muscle. J Clin Invest98,
Hansen J, Thomas GD, Jacobson TN & Victor RG (1994). Muscle
metaboreflex triggers parallel sympathetic activation in exercising
and resting human skeletal muscle. Am J Physiol266, 2508–2514.
Hartley LH, Vogel JA & Cohen MI (1973). Central, femoral, and
brachial circulation during exercise in hypoxia. J Appl Physiol34,
Hatcher JD, Chiu LK & Jennings DB (1978). Anemia as a stimulus to
aortic and carotid chemoreceptors in the cat. J Appl Physiol44,
Hausberg M & Somers VK (1997). Neural circulatory responses to
carbon monoxide in healthy humans. Hypertension29,
Heistad DD & Wheeler RC (1970). Effect of acute hypoxia on
vascular responsiveness in man. J Clin Invest49, 1252–1265.
Jewett DL (1964). Activity of single efferent fibres in the cervical
vagues of the dog with special reference to possible cardio-
inhibitory fibres. J Physiol175, 321–357.
Knight DR, Schffartizik W, Poole DC, Hogan MC, Bebout DE &
Wagner P (1993). Effects of hyperoxia on maximal leg O2supply
and utilization in man. J Appl Physiol75, 2586–2594.
Koskolou MD, Calbet JAL, Rådegran G & Roach RC (1997a).
Hypoxia and the cardiovascular response to dynamic knee-
extensor exercise. Am J Physiol272, H2655–2663.
Koskolou MD, Roach RC, Calbet JAL, Rådegran G & Saltin B
(1997b). Cardiovascular responses to dynamic exercise with acute
anemia in humans. Am J Physiol273, H1787–1793.
Lahiri S & Acker H (1999). Oxygen sensing in the body. Respir
A. Hanada, M. Sander and J. Gonzaléz-Alonso
Journal of Physiology
Lahiri S, Mulligan E, Nishino T, Mokashi A & Davies RO (1981).
Relative responses of aortic body and carotid body
chemoreceptors to carboxyhemoglobinemia. J Appl Physiol50,
Leuenbenger UA, Gray K & Herr MD (1999). Adenosine contributes
to hypoxia-induced forearm vasodilation in humans. J Appl
Leuenbenger UA, Hardy JC, Herr MD, Gray KS & Sinoway LI (2001).
Hypoxia augments apnea-induced peripheral vasoconstriction in
humans. J Appl Physiol90, 1516–1522.
McLeod RDM& Scott MJ (1964). The heart rate responses to carotid
body chemoreceptor stimulation in the cat. J Physiol175, 193–202.
Marshall JM (1994). Peripheral chemoreceptors and cardiovascular
regulation. Physiol Rev74, 543–594.
Mitchell JH, Kaufmann MP, Iwamoto GA (1983). The exercise
pressor reflex: its cardiovascular effects, afferent mechanisms, and
central pathways. Annu Rev Physiol45, 229–242.
Morgan BJ, Crabtree DC, Palta M & Skatrud JB (1995). Combined
hypoxia and hypercapnia evokes long-lasting sympathetic
activation in humans. J Appl Physiol79, 205–213.
O’Donell CP & Bower EA (1992). Heart rate changes evoked by
hypoxia in the anesthetized, artificially ventilated cat. Exp Physiol
Remensnyder JP, Mitchell JH & Sarnoff SJ (1962). Functional
sympatholysis during muscular activity. Circ Res11, 370–380.
Richardson RS, Noyszewski EA, Kendrick KF, Leigh JS & Wagner PD
(1995). Myoglobin O2desaturation during exercise: evidence of
limited O2transport. J Clin Invest96, 1916–1926.
Richardson RS, Noyszewski EA, Saltin B & González-Alonso J
(2002). Effect of mild carboxy-hemoglobin on exercising skeletal
muscle: intravascular and intracellular evidence. Am J Physiol283,
Roach RC, Koskolou MD, Calbet JAL & Saltin B (1999). Arterial O2
content and tension in regulation of cardiac output and leg blood
flow during exercise in humans. Am J Physiol276, H438–445.
Rowell LB & Blackmon JR (1986). Lack of sympathetic
vasoconstriction in hypoxemic humans at rest. Am J Physiol251,
Rowell LB, Johnson DG, Chase PB, Comess KA & Seals DR (1989).
Hypoxemia raises muscle sympathetic activity but not
norepinephrine in resting humans. J Appl Physiol66, 1736–1743.
Rowell LB, Saltin B, Kiens B & Christiansen NJ (1986). Is peak
quadriceps blood flow in humans even higher during exercise in
hypoxemia? Am J Physiol251, H1038–1044.
St Croix CM, Morgan BJ, Wetter TJ & Dempsey JA (2000). Reflex
effects from fatiguing diaphragm increase sympathetic efferent
activity (MSNA) to limb muscle in humans. J Physiol529,
Saito M, Abe H, Iwase S, Koga K & Mano T (1991). Muscle
sympathetic nerve responsiveness to static contraction is not
altered under hypoxia. Jpn J Physiol41, 775–783.
Saito M, Mano S, Iwase K, Koga K, Abe H & Yamazaki Y (1988).
Responses in muscle sympathetic activity to acute hypoxia in
humans. J Appl Physiol65, 1548–1552.
Schmidt HD, Raychhaus M, Francis DP, Davies CL, Nuding S,
Peschel T, Schmidt DS, Coats AJ, Opitz H & Werdan K (2001).
Assessment of chemoreflex sensitivity in free breathing young
subjects by correction for respiratory influence. Int J Cardiol78,
Seals DR (1989). Sympathetic neural discharge and vascular
resistance during exercise in humans. J Appl Physiol66,
Seals DR, Johnson DG & Fregosi RF (1991a). Hypoxia potentiates
exercise-induced sympathetic neural activation in humans. J Appl
Seals DR, Johnson DG & Fregosi RF (1991b). Hyperoxia lowers
sympathetic activity at rest but not during exercise in humans. Am
J Physiol260, R873–878.
Sinoway L, Prophet S, Gorman I, Mosher T, Shenberger J, Dolecki
M, Briggs R & Zelis R (1989). Muscle acidosis during static
exercise is associated with calf vasoconstriction. J Appl Physiol66,
Smith ML, Niedermaier ON, Hardy SM, Decker MJ & Strohl KP
(1996). Role of hypoxemia in sleep apnea-induced
sympathoexcitation. J Auton Nerv Syst56, 184–190.
Somers VK, Mark AL, Zavala DC, Abboud FM (1989). Influence of
ventilation and hypocapnia on sympathetic nerve responses to
hypoxia in normal humans. J Appl Physiol67, 2095–2100.
Trzebski A, Smith ML, Beightol LA, Fritsch-Yelle JM, Rea RF &
Eckberg DL (1995). Modulation of human sympathetic periodicity
by mild, brief hypoxia and hypercapnia. J Physiol Pharmacol46,
Vallbo Å, Hagbarth K-E, Torebjork H & Wallin B (1979).
Somatosensory, proprioceptive, and sympathetic activity in
human peripheral nerves. Physiol Rev59, 919–957.
Weisbrod CJ, Minson CT, Joyner MJ & Halliwill JR (2001). Effects of
regional phentolamine on hypoxic vasodilatation in healthy
humans. J Physiol537, 613–621.
This study was supported by a grant from The Danish National
Research Foundation (504–14). We thank Dr Jaya Rosenmeier,
Dr José A. L. Calbet and Professor Bengt Saltin for their help with
the catheterisations in the last study. We also thank Professor
Bengt Saltin for his comments and support in this difficult
endeavour. In addition, we thank Dr Niels H. Secher for helping
us with experiments assessing central venous pressure. A.H. was
on leave from the Department of Cardiovascular Medicine.
Hokkaido University, Sapporo, Japan. M.S. was supported by a
fellowship from the Kaj Hansen Foundation and by grants from
the Danish Heart Foundation, the Novo Foundation and the
Danish Medical Research Council.
Sympathetic nerve activity and circulating oxygen