Leg and arm lactate and substrate kinetics during
exercise
G.
van Hall1,
M.
Jensen-Urstad2,
H.
Rosdahl3,
H.-C.
Holmberg3,
B.
Saltin1, and
J. A. L.
Calbet1
1 The Copenhagen Muscle Research Centre, University
Hospital, DK-2100 Copenhagen, Denmark; and the Departments of
2 Cardiology and
3 Physiology-Pharmacology, Karolinska Institute, 171 77 Stockholm, Sweden
 |
ABSTRACT |
To
study the role of muscle mass and muscle activity on lactate and energy
kinetics during exercise, whole body and limb lactate, glucose, and
fatty acid fluxes were determined in six elite cross-country skiers
during roller-skiing for 40 min with the diagonal stride (Continuous
Arm + Leg) followed by 10 min of double poling and diagonal stride at
72-76% maximal O2 uptake. A high lactate appearance rate (Ra, 184 ± 17 µmol · kg
1 · min
1)
but a low arterial lactate concentration (~2.5 mmol/l) were observed
during Continuous Arm + Leg despite a substantial net lactate release
by the arm of ~2.1 mmol/min, which was balanced by a similar net
lactate uptake by the leg. Whole body and limb lactate oxidation during
Continuous Arm + Leg was ~45% at rest and ~95% of disappearance
rate and limb lactate uptake, respectively. Limb lactate kinetics
changed multiple times when exercise mode was changed. Whole body
glucose and glycerol turnover was unchanged during the different skiing
modes; however, limb net glucose uptake changed severalfold. In
conclusion, the arterial lactate concentration can be maintained at a
relatively low level despite high lactate Ra during
exercise with a large muscle mass because of the large capacity of
active skeletal muscle to take up lactate, which is tightly correlated
with lactate delivery. The limb lactate uptake during exercise is
oxidized at rates far above resting oxygen consumption, implying that
lactate uptake and subsequent oxidation are also dependent on an
elevated metabolic rate. The relative contribution of whole body and
limb lactate oxidation is between 20 and 30% of total carbohydrate
oxidation at rest and during exercise under the various conditions.
Skeletal muscle can change its limb net glucose uptake severalfold
within minutes, causing a redistribution of the available glucose
because whole body glucose turnover was unchanged.
lactate dehydrogenase; cross-country skiing; tracers
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INTRODUCTION |
AS EARLY AS 1907,
Fletcher and Hopkins (11) not only provided definitive
evidence of the relation between muscle activity and production of
lactic acid in the amphibian skeletal muscle, but they also concluded
that skeletal muscles possess the requisite chemical mechanisms for the
removal of lactic acid once formed. Despite this early finding, lactate
was long considered a metabolic end product, that is, lactate produced
during muscle contraction and released into the circulation for
subsequent uptake by the liver for recycling via gluconeogenesis. The
importance of skeletal muscle in lactate clearance in humans became
clear from experiments starting in the late 1950s. It was shown that,
during exercise, lactate was taken up by nonactive skeletal muscles
(1, 7, 12). Furthermore, when the arterial lactate
concentration was also elevated, active skeletal muscles cleared
lactate (12, 26, 30), and when two-legged cycle ergometer
exercise was performed with one leg having a normal and the other a low
glycogen content, the leg with the normal glycogen content released
lactate, whereas lactate was taken up by the leg with the low glycogen content (10, 13). In addition, the utilization of lactate by skeletal muscle appeared to be higher when light exercise was performed compared with complete rest (14, 24, 26, 27). From these studies it was concluded that skeletal muscles not only
produce lactate, but they are also the major tissue for lactate removal
from the circulation. Further studies with lactate isotopes have shown
a simultaneous limb lactate uptake and release at rest and during
exercise (15, 16, 34). This suggests a dynamic situation,
with exchange of lactate as a carbohydrate source between fibers within
the same muscle but also from one muscle group to another, whether
actively contracting or not. To study the magnitude of such fluxes, six
elite cross-country skiers were studied during roller skiing. This
exercise model provides a unique opportunity to investigate systemic
and skeletal muscle lactate and energy kinetics during exercise with
the majority of the body's skeletal muscle mass engaged in the
exercise. Cross-country skiers have a similar training status in the
upper and the lower body muscles, and to them roller-skiing is a
natural mode of exercise with substantial changes in skeletal muscle
activity without a change in the total work done by the whole body.
Thus the skiers were studied during skiing with the diagonal stride
technique, in which both the legs and arms are active, and during the
double poling technique, in which mainly the arms provide the
speed-generating force. Leg and arm lactate, glucose, and fatty acid
kinetics were determined by measurements across a leg and an arm for
metabolites, tracer dilution, and blood flow at rest and during the two
modes of skiing. In addition, muscle biopsies were obtained from the
vastus lateralis and deltoid for enzyme activities and lactate
dehydrogenase isoform patterns.
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METHODS |
Subjects
Six Swedish elite cross-country skiers, age 24 (20-31) yr, height 181 (174-190)
cm, and weight 74 (71-83) kg, participated in the
study. Maximal oxygen uptake (
O2 max)
was 5.2 (4.9-5.8) l/min or 71 (65-74)
ml · kg
1 · min
1,
assessed during an incremental intensity test with diagonal stride on
roller skis on a modified treadmill as used in the actual study. One
additional skier was studied with a slightly different exercise
protocol. This subject has been excluded except for the data on the
muscle biopsies. The subjects were informed about the possible risks
and discomfort involved before their voluntary consent to participate
was given. The study was performed according to the Declaration of
Helsinki and was approved by the Ethical Committee of the Karolinska
Institute, Stockholm, Sweden.
Protocol
On the day of the experiment, the athletes reported to the
laboratory at 8 AM, and catheters were placed under local anesthesia. Positioning of the catheters is depicted in Fig.
1. A Swan-Ganz triple-lumen catheter was
inserted into an antecubital vein and under fluoroscopic guidance was
advanced into the subclavian vein ~5 cm before the merger with the
jugular vein. One lumen was used for blood sampling and another for
infusion of ice-cold saline for blood flow measurements with the
thermodilution technique (2). The femoral arterial
catheter (18-gauge, Ohmeda, Wiltshire, UK) was inserted
2-5 cm below the inguinal ligament and advanced 5-10 cm in
the proximal direction. Another catheter was inserted into the right
femoral vein and under fluoroscopic guidance advanced to the right
atrium. In the left femoral vein, a venous catheter with side holes
(radiopack TFE, Cook, Bjaerverskov, Denmark) was inserted and
advanced ~5 cm proximal to the inguinal ligament. A thermistor was
inserted through the venous catheter for blood flow measurements by the
constant infusion thermodilution technique (2). Another
catheter in the femoral vein was inserted ~2 cm below the inguinal
ligament and advanced ~5 cm in the distal direction for blood
sampling. The catheters can slide out during exercise. To prevent this,
catheters were fixed to the skin with stitches. After the
catheterization procedures, the athletes remained in the supine
position until preparations were started for the exercise bout. Thirty
minutes after the final placement of the catheters, blood samples were
obtained for assessment of background enrichment of lactate and
CO2. Immediately after the background samples were taken, a
bolus of H13CO3 (1.5 µmol/kg) was given, and
a primed constant infusion of tracers (Cambridge Isotope Laboratories,
Andover, MA) was started of [1-13C]lactate (0.79 µmol · min
1 · kg
1,
prime 13 µmol/kg), [1,1,2,3,3-2H5]glycerol
(0.14 µmol · min
1 · kg
1,
prime 1.5 µmol/kg) and [6,6-2H2]glucose
(0.38 µmol · min
1 · kg
1,
prime 17.6 µmol/kg). After subjects had spent 1 h in the supine position, a biopsy was taken under local anesthesia of the vastus lateralis in seven skiers. From three of them, an additional biopsy was
taken from the deltoid. Two hours after the start of the constant [1-13C]lactate infusion, three resting blood samples were
obtained 15 min apart. In addition, femoral and subclavian venous blood flows were measured just before blood sampling. After the last resting
blood sample had been taken, the subjects were seated on a chair placed
on the treadmill (Refox, Falun, Sweden) to prepare for exercise. This
preparation took between 30 and 50 min and included connection of
extension lines to the catheters and fixation of the lines. The lines
for blood sampling were fixed on the back of the harness, which was
worn to protect subjects from injuries in case they fell during skiing
(Fig. 2). At the start of the exercise,
the continuous infusion of [1-13C]lactate was increased
fourfold, and the continuous infusion of
[1,1,2,3,3-2H5]glycerol and
[6,6-2H2]glucose was increased twofold.
Classical skiing involves different techniques. The diagonal stride
involves both the arms and the legs and is used uphill (Fig. 2). The
double poling technique mainly involves the upper body (arms) and is
used on flat terrain and slightly uphill. The protocol consisted of 40 min of continuous diagonal style (Continuous Arm + Leg), followed
without breaks by 10 min of double poling (Arm) and 10 min of diagonal
stride (Arm + Leg). Blood samples were taken after 21, 24, and 36 min
of continuous Arm + Leg, and then ~5-7 min after the start of
Arm and Arm + Leg skiing. After the study was finished, the subjects
were moved to the operation theater, and the catheter positions in the
subclavian vein and right atrium were checked with fluoroscopy; no
catheter was found displaced during the study.
Blood was sampled anaerobically in a heparinized syringe and
immediately analyzed for hemoglobin, oxygen saturation (OSM3 hemoxymeter, Radiometer, Copenhagen, Denmark), blood pH,
CO2, and O2 tension (ABL5, Radiometer). Another
blood sample was taken, and the blood was collected in ice-cold tubes
that contained 10 µl of 0.33 M EDTA/ml of blood and was immediately
centrifuged at 4°C for 10 min and stored at
50°C until analysis.
Analytical Procedures
Plasma was analyzed enzymatically for lactate, glucose (Roche
Unikit, Neuss, Germany), and free fatty acids (NEFA-C kit, Wako Chemical) on an automatic analyzer (Cobas Fara, Roche, Basel, Switzerland).
Lactate, glucose, and glycerol enrichment was measured by gas
chromatography-mass spectrometry (GC-MS, Finnigan Automass II and III,
Paris, France; GC column, CP-SIL 8CB, Chrompack,
Middelburg, The Netherlands). In preparation of the GC-MS
analysis, samples were processed to make a trimethylsilyl derivative of
lactate, a butylboronic acid acetate derivative of glucose, and a
trifluorobutyrate derivative of glycerol. For the preparation of the
trimethylsilyl derivative of lactate, 1 ml of ethanol was added to 200 µl of blood extracts and centrifuged for 10 min, and the supernatant was transferred to a new screw-capped tube and evaporated to dryness under a stream of nitrogen. For lactate enrichment, 50 µl of pyridine and 50 µl of
N-(O-bistrimethylsilyl)-trifluoroacetamide, or
BSTFA, with 1% trimethylchlorosilan (or TMCS, Pierce) were added, and the solution was incubated for 30 min at room temperature. The isotopic
enrichment was determined using electron impact ionization, with ions
at mass-to-charge ratios (m/z) 219 and 220 representing the
molecular ions of unlabeled and labeled derivatives, respectively. For
determination of glucose enrichment, 250 µl of water and 3 ml of
chloroform-methanol (2.3:1) were added to 150 µl of blood extract,
vortex mixed for 10 min, and centrifuged at 4°C for 15 min. The upper
layer was washed once by adding 1 ml of water (pH 2, with HCl) and 2 ml
of chloroform and was centrifuged at 4°C for 15 min. The upper layer
was then evaporated to dryness, after which 250 µl of butylboronic
acid (100 mg/10 ml pyridine) were added to the dry residue and
incubated for 30 min at 95°C. After the addition of 250 µl of
acetic anhydride and incubation for 90 min at room temperature, the
solution was evaporated to dryness and redissolved in 100 µl of ethyl
acetate. The isotopic enrichment was determined using electron impact
ionization and ions at m/z 297 and 299, representing the
molecular ions of unlabeled and labeled glucose derivatives,
respectively. For the preparation of the fluorobutyrate derivative of
glycerol, 3 ml of ethanol-chloroform (2.3:1) were added to 200 µl of
plasma, mixed, and centrifuged. The top layer was extracted once more
with 2 ml of chloroform and 1 ml of water (pH 2, with HCl), mixed, and
centrifuged. The top layer was evaporated under a stream of
N2. Two hundred microliters of heptafluorobutyric acid
anhydride in ethyl acetate (1:3, vol/vol) were added to the residue and
heated for 10 min at 70°C. The solution was evaporated under a stream
of N2 and the residue redissolved in 1 ml of ethyl acetate.
The isotopic enrichment of glycerol was determined by electron impact
ionization, and selective monitoring of ions at m/z
252-256, representing the molecular ions of unlabeled (252) and labeled derivatives (256),
respectively, was performed.
Muscle biopsies were analyzed for lactate dehydrogenase (LDH), citrate
synthase (CS), 3-hydroxyacyl-CoA dehydrogenase (HAD) activity, and LDH isoforms.
Calculations
The whole body rates of appearance (Ra) and
disappearance (Rd) of lactate, glucose, and glycerol at
rest and during exercise were calculated using the steady-state
equation
Limb tracer measurements were calculated as
F is the isotopic infusion rate (µmol/min), Ca and
Cv and Ea and Ev are the arterial
and venous concentrations and isotopic enrichments, respectively, in
the tracer/tracee ratio.
The blood flow was used to calculate net leg uptake of lactate
and glucose. The plasma flow, being blood flow × (1
hematocrit), was used to calculate net fatty acid uptake.
Whole body and tissue steady state is a requirement for reliable
quantitative tracer estimates of whole body metabolite turnover rates,
as well as limb balances of tracers and concentrations, usually
referred to as Fick's principle. Complete steady state is difficult to
achieve during exercise; however, the lactate and glucose enrichments
and concentrations (see Figs. 3 and 7) were similar after 12, 24, and 36 min of Continuous Arm + Leg, suggesting a reasonable steady
state. This was clearly not the case for lactate during the 10 min of
Arm and Arm + Leg skiing. However, the lactate turnover rate is high,
and it seems reasonable to assume that a near steady state was achieved
after 7 min, when the blood sample was obtained. The whole body lactate
Rd oxidized to CO2 and the limb lactate uptake
oxidized to CO2 were markedly lower during Arm and Arm + Leg (see Fig. 5); this might be caused by non-steady-state conditions,
most likely in the bicarbonate pool turnover rate and size, since they
are known to be affected by changes in pH and metabolic rate
(32). Thus the changes of the bicarbonate pool turnover
rate and size resulting in 13CO2 output during
the Arm and Arm + Leg are potentially not entirely related to lactate
oxidation.

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Fig. 3.
Whole body lactate concentration (A) and rates of
appearance and disappearance (Ra/d,
B). Values are means ± SE of 6 subjects.
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Statistical Analysis
All data are expressed as means ± SE for six subjects. The
nonparametric Wilcoxon signed-rank test was applied to determine differences between leg and arm. Statistical significance was set at
P < 0.05.
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RESULTS |
Subject characteristics
The Swedish elite cross-country skiers had a
O2 max of 5.2 (4.9-5.8) l/min or
71 (65-74) ml · kg body
wt
1 · min
1,
assessed during incremental diagonal stride roller skiing.
Oxygen uptake (
O2) during the 40 min of Continuous Arm + Leg and Arm + Leg skiing was 3.95 ± 0.02 l/min or 76 ± 1%
O2 max.
O2 during Arm was
3.75 ± 0.18 l/min or 72 ± 2%
O2 max.
Whole Body and Limb Lactate Kinetics
Rest and Continuous Arm + Leg.
The arterial lactate concentration increased from 0.7 mmol/l at rest to
~2.5 mmol/l with Continuous Arm + Leg skiing at 76%
O2 max and remained nearly constant
over the 40 min (Fig. 3A).
Also, whole body lactate Ra/d increased during change from rest to exercise (Fig. 3B); however, the arterial lactate
concentration increased only ~3.5-fold, whereas lactate
Ra increased ~10-fold. At rest, a small limb net lactate
release was observed (Fig.
4A). During Continuous Arm + Leg, a constant net lactate release of ~2 mmol/min by the arm was
observed, and in contrast, a net lactate uptake was observed of a
similar magnitude and release for the leg. The tracer-estimated limb
lactate uptake and release showed a simultaneous uptake for both limbs
(Fig. 4, B and C). The leg lactate uptake was
markedly higher, and the lactate release was similar, for leg and arm.
However, to compare leg and lactate kinetics, the difference in
skeletal muscle mass has to be taken into account. When lactate uptake
and release are expressed per kilogram of skeletal muscle, the picture
is reversed (Fig. 4, D and E). The leg and arm
lactate uptake values are rather similar, but lactate release is
substantially higher for the arm. Thus the difference in the leg and
arm net lactate exchange, in which the leg is a net lactate consumer
and the arm a net lactate producer, seems to be caused solely by the
difference in magnitude of lactate release. At rest, the 42-55%
of the lactate leaving the circulation and taken up by the limb was
found oxidized, and during Continuous Arm + Leg nearly all was oxidized
(Fig. 5). When limb lactate uptake and
release were plotted as a function of limb lactate delivery (arterial
lactate concentration times limb blood flow), a tight correlation was
observed between limb lactate delivery and limb uptake, and not limb
release (Fig. 6).

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Fig. 4.
Limb lactate kinetics. Limb net lactate exchange (A) is
arteriovenous concentration times blood flow. Limb lactate uptake
(B) is tracer-calculated lactate uptake, and limb lactate
release (C) is tracer-calculated limb lactate uptake
minus net limb lactate exchange. D and E: limb
lactate uptake and release, respectively, expressed per kilogram of
skeletal muscle. Estimations are based on the assumption that muscle
mass of the leg ranges between 8 and 10 kg and that the upper body
muscle mass of the arm ranges between 3 and 4 kg. Solid parts of bars
represent estimates with the low muscle mass, and gray parts of
bars represent estimates with the highest muscle mass. Values are
means ± SE of 6 subjects. * Not significantly different from
zero; #significantly different from leg.
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Fig. 5.
Whole body and limb lactate oxidation. Whole body lactate oxidation
is estimated as the whole body lactate rate of disappearance
(Rd) recovered in breath CO2 (A);
limb lactate oxidation is estimated as the tracer lactate uptake (see
Fig. 4B) recovered in blood CO2 (B).
Values are means ± SE of 6 subjects.
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Fig. 6.
Correlation of limb lactate delivery with limb lactate uptake
(A) and limb lactate release (B). Data points are
averages for each individual of 3 measurements during 40 min of
continuous Arm + Leg skiing of leg (n = 6) and arm
(n = 5) lactate delivery vs. lactate uptake or release.
Limb lactate delivery is arterial concentration times limb blood
flow.
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Arm and Arm + Leg.
Within 7 min of Arm skiing, the arterial lactate concentration
increased more than threefold. A substantial decrease in the lactate
concentration could be observed after the 10 min of Arm + Leg skiing at
the same intensity as the 40 min of Continuous Arm + Leg before the Arm
skiing. The pattern of changes in lactate Ra
imaged the changes observed for the arterial lactate
concentration, even though the relative changes were quantitatively
rather different. When going from the 40 min of Continuous Arm + Leg to
Arm skiing, the arterial lactate concentration increased 2.5-fold
within 5 min, whereas lactate Ra increased only 0.7-fold.
The leg net lactate uptake and arm net lactate release values were
exaggerated during Arm exercise. The increase in leg net lactate uptake
was caused by an increased lactate uptake and a decreased leg lactate
release, whereas the increase in arm net lactate release was due to an increase in both lactate uptake and release, even though the latter increased more. During the 10 min of Arm + Leg skiing following Arm
skiing, a limb net lactate exchange could not be observed. In the case
of the leg, the net lactate uptake was minimized because of a
substantial increased lactate release that was far higher then the
increase in lactate uptake. In the case of the arm, the net lactate
uptake was minimized because of a decrease in lactate release and an
increase in lactate uptake.
Whole Body and Limb Carbohydrate and Fat Utilization
Rest and Continuous Arm + Leg.
At rest, whole body respiratory exchange ratio (RER) and limb
respiratory quotient (RQ) were rather similar. However, it is clear
that the contribution of the limbs to energy expenditure, and thus RER,
is relatively small at rest (~40%) compared with exercise (~80%).
The relative contribution of lactate oxidation to total carbohydrate
utilization was ~30% for the whole body and 20% for the limbs. The
rate of leg net fatty acid uptake was similar to total leg fat
utilization. This calculation could not be made for the arm, since the
positioning of the subclavian venous catheter did not exclude
contamination by subcutaneous adipose tissue and skin. During
Continuous Arm + Leg, whole body and limb total carbohydrates and fat
utilization were rather similar over time (Table
1). However, during Continuous Arm + Leg,
the whole body glucose Ra/d, and especially leg net glucose
uptake, increased with exercise duration (Fig.
7, B and
C), whereas the leg net fatty
acid uptake did not change (Fig. 8D). This
suggests that the leg muscle glycogen utilization decreased with
exercise duration. The relative contribution of lactate oxidation to
carbohydrate utilization was rather similar for the whole body
(28%), legs (35%), and arms (27%) (Fig. 9).

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Fig. 7.
Whole body and limb glucose kinetics. A: arterial
glucose concentration; B: whole body glucose rates of
appearance and disappearance; C: limb net glucose uptake
presented as the arteriovenous concentration times blood flow;
D: limb net glucose uptake expressed per kg of skeletal
muscle. Estimations are based on the assumption that the muscle mass of
the leg ranges between 8 and 10 kg and the upper body muscle mass of
the arm ranges between 3 and 4 kg. In D, solid part of bars
represents estimates with low muscle mass, and gray part of bars
represents estimates with the highest muscle mass. Values are
means ± SE of 6 subjects. * Not significantly different from
zero; #significantly different from leg.
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Fig. 8.
Whole body glycerol kinetics, fatty acid (FA) concentration, and
limb net exchange. A: arterial concentration of glycerol;
B: rate of appearance of glycerol as an indicator of whole
body lipolysis; C: arterial concentration of fatty acids;
D: limb net fatty acid uptake. Values are means ± SE
of 6 subjects. * Not significantly different from zero;
#significantly different from leg.
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Fig. 9.
Whole body and limb carbohydrate and fat utilization. Values show
absolute and relative whole body and limb carbohydrate (A)
and fat (B) utilization at rest and the average utilization
during continuous Arm + Leg (C and D).
Contributions of fatty acid (FA) to total fat utilization and of net
glucose uptake to total carbohydrate utilization are estimated with the
assumption that limb net FA uptake is completely oxidized. The
contribution of glycogen is the limb carbohydrate oxidation net
glucose uptake lactate oxidation.
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Arm and Arm + Leg.
The RER increased during Arm skiing but returned during Arm + Leg
to levels similar to those observed during Continuous Arm + Leg (Table
1). This indicates that Arm skiing elicited
an increase in carbohydrate utilization. Despite considerable
variability in limb RQ, the enhanced RER during Arm skiing is caused by
enhanced arm carbohydrate utilization. Blood glucose was the main
carbohydrate source for the arm during Arm, whereas the contribution of
lactate oxidation was unchanged. In contrast, leg RQ decreased and net glucose uptake diminished during Arm compared with Continuous Arm + Leg. Thus the lowering of "work" by the legs elicited an enhanced
reliance on fat. Because the leg net fatty acid uptake was similar
during Arm and Continuous Arm + Leg, the contribution of muscle
triacylglycerol must have increased. The main differences observed
during Arm + Leg compared with Continuous Arm + Leg were the higher arm
lactate uptake and net glucose uptake. These were most likely caused by
the previous exercise, resulting in a substantial reduction in muscle
glycogen, hence reduced utilization.
Catecholamines
Arterial epinephrine and norepinephrine concentrations
increased from rest to exercise but remained unchanged during all
skiing modes. The arterial epinephrine concentration was 1-1.2
nmol/l, and norepinephrine was 10-12 nmol/l.
Skeletal Muscle LDH Activity and Isoforms, and CS and HAD
Activities
Muscle biopsies were obtained from the vastus
lateralis of seven skiers. The LDH activity was low (Fig.
10), with a very high contribution of
the heart-type LDH isoform (H-LDH, 70% of the total LDH). The LDH-2
type is the most predominant isoform, and only 6% consisted of the
pure muscle type LDH (M-LDH), the LDH-5. In the three skiers from whom
biopsies of both muscles were obtained, the vastus lateralis LDH
activity was nearly one-half that of the deltoid; however, the
contribution of the LDH isoforms was almost identical in both muscles.
The activity of the oxidative enzymes CS and HAD was lower in the
vastus lateralis than in the deltoid (Table
2).

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Fig. 10.
Skeletal muscle lactate dehydrogenase (LDH) activity and
isoform distribution. A: limb LDH activity is measured in
vastus lateralis only (n = 7) and in both vastus
lateralis and deltoid (n = 3). B: LDH
isoform distribution (LDH-1 to -5) in vastus lateralis and deltoid
(n = 3), and total heart-type (H) and muscle-type (M)
LDH isoforms; H-LDH was calculated as (LDH-1 + 0.75 LDH-2 + 0.5 LDH-3 + 0.25 LDH-4)/total LDH; M-LDH was calculated as
(LDH-5 + 0.75 LDH-4 + 0.5 LDH-3 + 0.25 LDH-2)/total LDH.
C: LDH isoform and total H- and M-LDH distribution in vastus
lateralis (n = 7).
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DISCUSSION |
There are four main findings of the present study.
1) The arterial lactate concentration can be maintained
at a relatively low level despite a high lactate Ra during
exercise with a large muscle mass because of the large capacity of
active skeletal muscle to take up lactate. 2) Leg
lactate uptake is tightly correlated with lactate delivery. However,
the limb lactate uptake during exercise is oxidized at rates far above
resting
O2, implying that lactate uptake
and subsequent oxidation are also dependent on an elevated metabolic
rate. 3) The contribution of whole body and limb lactate
oxidation was between 20 and 30% of total carbohydrate oxidation at
rest and during exercise. 4) Skeletal muscle can change its
glucose uptake severalfold within minutes, causing a redistribution of
the available glucose when whole body glucose turnover is unchanged.
At rest, the systemic lactate Ra of the athletes (18 ± 3 µmol · kg body
mass
1 · min
1) was
rather similar to the Ra observed in active young subjects (17 ± 2 µmol · kg body
mass
1 · min
1), and so
was the relative contribution of leg lactate release to systemic
lactate Ra (34). This suggests that no
difference exists between highly trained and active young individuals
in whole body and skeletal muscle lactate appearance and clearance at
rest. The 40 min of Continuous Arm + Leg exercise was performed at a
high relative intensity (76%
O2 max).
Despite this high work intensity, the arterial lactate concentration
was only moderately increased. It could have been anticipated that
these athletes would produce a small amount of lactate compared with less-trained individuals. However, the arterial lactate concentration is not a good indicator of lactate production during exercise. During
the 40 min of Continuous Arm + Leg, the lactate Ra was 184 ± 17 µmol · kg body
mass
1 · min
1. During
bicycle exercise in active subjects, we have observed an arterial
lactate concentration of 1.8 mmol/l at 46%
O2 max, with a lactate Ra
of ~60 µmol · kg body
mass
1 · min
1, and an
arterial lactate concentration of ~6 mmol/l at 82%
O2 max with a lactate Ra of
160 µmol · kg body
mass
1 · min
1
(34). The latter lactate Ra is thus even lower
than during the 40 min of Continuous Arm + Leg. This is the case
despite a 2.5-fold difference in the arterial concentration. It is
clear that the higher lactate uptake compared with lactate release of the legs is the reason for a low lactate concentration despite a high
lactate turnover rate. It has been shown that, in recovery from
exercise, the lactate concentration decreases faster when exercise is
performed (14, 24). In addition, when lactate was infused
at the same rate at rest and during exercise at different intensities,
the increase in lactate concentration became lower the higher the
exercise intensity (27). These studies indicated that
active skeletal muscle is an important consumer of lactate, not only a
producer of lactate, despite a net lactate release, as shown previously
(15, 16, 31). The current study shows not only that
skeletal muscle is the most important tissue for lactate clearance
during exercise but also that active skeletal muscle can be a
substantial net lactate consumer. In fact, if inactive leg skeletal
muscle had to take up and oxidize the amounts of lactate observed
during exercise in the present study, it would require an oxygen
utilization about fourfold higher than that actually observed for the
resting leg. This implies that only active skeletal muscle, due to its
multiple times of increased energy expenditure, and not resting
skeletal muscle, is able to oxidize the large amounts of lactate
produced during exercise.
The elite cross-country skiers in the present study have well-trained
leg and arm muscles, which makes them suitable in the comparison of
metabolic properties of leg and arm skeletal muscle without having to
take the confounding effect of more trained leg than arm muscles into
consideration. However, to compare leg and arm muscles, the energy
turnover of both the legs and the arms has to be known. The legs
contain more muscle than arms. Thus the data have to be expressed per
kilogram of muscle or, more specifically, the muscle mass over which
the arteriovenous difference for lactate is measured needs to be known.
Leg muscle mass is 8-10 kg, and arm muscle mass is 3-4 kg in
young trained subjects. We have used this range of muscle mass to
express leg and arm data per kilogram of muscle (Table 1, Figs. 4 and
7). If blood flow to and
O2 by
the limbs were expressed per kilogram of limb muscle, then leg and arm
blood flow and oxygen utilization would be similar during Continuous
Arm + Leg. This indicates that, during Continuous Arm + Leg, the leg
and arm muscles performed a similar absolute amount of work per
kilogram of muscle. In addition, during Continuous Arm + Leg, the leg
and arm net glucose uptake values were similar. Despite similar work
and net glucose uptake, the leg lactate uptake was higher and the leg
lactate release lower than for the arm (Fig. 4). This would suggest
that the arm muscles have a lower ability to utilize lactate and a
higher ability to produce lactate when moderately to highly active.
However, although the data may suggest a similar amount of work by legs and arms, they do not provide information as to whether the relative workloads were similar. Arterial lactate and net lactate release across
an exercising limb strongly correlate with relative workload. The
athletes indicated that the work performed by the arms during Arm
skiing was close to what they could maximally do with their arms. This
would mean that the relative arm intensity during continuous Arm + Leg
was ~60% of maximal arm
O2 and thus
higher than the relative intensity by the legs, which was estimated to
be ~45%. On the other hand, this would imply that arm muscles could
do less work than leg muscles per kilogram of skeletal muscle. It is of
note that, when maximal leg exercise with a large muscle mass (bicycle
exercise) is compared with leg exercise with a small muscle mass
(one-leg knee-extensor exercise), work performed and
O2 per kilogram of muscle are about
twofold higher during exercise with the small muscle mass
(35). Alternatively, the lower lactate utilization and
higher production of the arm vs. the leg muscle might be explained by
differences in arm and leg muscle fiber type and LDH isoform
composition. Mygind (22) reported a fairly small
difference in slow-twitch fiber content in vastus lateralis (69%) and
triceps brachii (51%) of elite cross-country skiers. The lower LDH
activity in the vastus lateralis compared with deltoid in the present
study also seems to suggest a markedly higher slow-twitch fiber content
in leg compared with arm muscles. However, the high and similar H-LDH
isoform contribution to total LDH actually suggests a similar metabolic
profile in legs and arms of the present subjects. Skeletal muscle H-LDH
has been considered to catalyze the reaction of lactate to pyruvate,
whereas the muscle-type LDH (M-LDH) catalyzes pyruvate to lactate
(9). However, the role of H-LDH and M-LDH in muscle
lactate utilization or production has been questioned (33). It has been suggested that all LDH isoforms are
equally able to utilize and produce lactate; this is supported by the observation in LDH M-subunit-deficient patients that muscle lactate production is proportional to total LDH activity compared with healthy controls after ischemic work (17).
Lactate is simultaneously taken up and released by skeletal muscle. A
variety of explanations could account for this observation. It has been
suggested that, because of the rapid equilibrium between lactate and
pyruvate, the release of unlabeled lactate is a result of simple
isotopic exchange by LDH (18, 19, 36). If isotopic exchange equilibrium were the sole mechanism leading to unlabeled lactate formation, the muscle enrichment of lactate and pyruvate would
be similar. However, it has been shown that pyruvate enrichment in
skeletal muscle is substantially lower than lactate (Refs. 8 and 20, and van Hall, unpublished
observations). More importantly, if the lactate release were to be
explained by a rapid isotopic exchange, then the lactate tracer would
provide a measure for pyruvate flux. Thus the lactate oxidation would
have provided an estimate of pyruvate oxidation, which is entirely
carbohydrate oxidation. However, in the present study, it is clearly
demonstrated that whole body and limb lactate oxidation is between 20 and 30% of carbohydrate oxidation under the various conditions (for a detailed discussion see Ref. 21). In addition, the tight
correlation between lactate uptake and lactate delivery but not between
lactate delivery and lactate release suggests that the
pathways of lactate release in muscle are separate and regulated
independently. Another explanation is muscle fiber heterogeneity, with
lactate production (release) occurring in fast-glycolytic (type IIB)
fibers and uptake and oxidation in slow-oxidative (type I) fibers
(3). This model, however, does not seem to be adequate for
predicting the simultaneous lactate uptake and release at rest and the
tight correlation between lactate uptake and lactate delivery.
Furthermore, it implies that the LDH equilibrium in type IIB fibers is
toward lactate formation as opposed to pyruvate formation in type I
fibers, which is highly unlikely. Thus isotopic exchange and muscle
heterogeneity do not seem to explain the simultaneous lactate uptake
and release by skeletal muscle, although some contribution cannot be
completely excluded. We believe that results of this study are most
consistent with the concept of muscle intracellular compartmentation,
with at least two functional lactate pools: a myofibrillar pool, to be
considered as "glycolytic" and associated with pyruvate derived from blood glucose and glycogen, and an intermyofibrillar pool, to be
considered as "oxidative" and associated with extracellular lactate
and the pyruvate pool destined for mitochondrial oxidation. This
concept is analogous to suggestions for pyruvate compartmentation in
skeletal muscle (29) and heart (6, 23).
Lactate of the glycolytic pool originates mainly from newly
synthesized lactate from pyruvate produced from glucose, and during
exercise from glucose and glycogen. Lactate formation via LDH is
favored, since both pyruvate and NADH are relatively high because of
glycolysis. Lactate from this pool is the lactate predominantly
released from muscle. On the other hand, lactate of the oxidative pool
consists mainly of extracellular lactate taken up from the circulation. Lactate from this pool is converted to pyruvate via LDH, because pyruvate may be relatively low and NAD+ relatively high in
close proximity to the mitochondria. The subsarcolemmal mitochondria
may be most involved because of their spatial advantage for this
process. Lactate from this pool is the lactate taken up by muscle. No
role for mitochondrial LDH is foreseen in this concept, as proposed
previously (4, 5), because evidence has recently been
presented that LDH is absent in human, mouse (25), and rat
(28) skeletal muscle mitochondria. In addition, the
NAD+-NADH redox couple is highly reduced. Thus if LDH were
present in the mitochondrial matrix, it would reduce pyruvate to
lactate and NADH would be oxidized, depriving the tricarboxylic acid
cycle and the respiratory chain of their main substrates.
The leg net glucose uptake was higher than the arm net glucose uptake
during Continuous Arm + Leg, but when expressed per kilogram of muscle,
the arm net glucose uptake was similar. However, when work performed by
the arms was increased, going from Continuous Arm + Leg to Arm skiing,
the skeletal muscle glucose uptake increased severalfold within
minutes, and the opposite was true for the legs. These quantitatively
large and fast changes in limb net glucose uptake occurred without a
change in whole body glucose Rd, glucose concentration, and
catecholamines. The data suggest that the severalfold increase in arm
net glucose uptake is mediated by an enhanced
interstitial-intracellular glucose gradient caused by an enhanced
glucose utilization, since it seems unlikely that the number of
sarcolemmal GLUT4 transporters would increase severalfold within
minutes. Furthermore, an enhanced glucose delivery does not seem to
have an important role, as the increase in blood flow to the arm
(~50%) was far less than the changes in net glucose uptake. Richter
et al. (26) observed that the active leg changed from a
net lactate release to an uptake when more limbs became active. In
addition, a substantial decrease in the leg net glucose uptake was
observed when more limbs became active. However, the combined net
lactate and glucose uptake was rather similar. In the present study,
leg net glucose uptake was reduced to zero during Arm skiing; however,
the combined net lactate and glucose uptake was higher than with
Continuous Arm + Leg. Thus lactate should be looked upon as an
important carbohydrate source in the blood, next to glucose, since its
oxidation contributed 20-30% of total carbohydrate oxidation at
rest and during exercise. This study clearly shows that skeletal muscle
has the ability to change rapidly its substrate source when the work
performed by the arms and legs changes. There is the possibility that
the increase in arm blood lactate and glucose utilization might not be
a result of changes in work intensity or availability (lactate) but
rather a result of diminished glycogen stores and, hence, reduced
contribution of glycogen to carbohydrate oxidation.
 |
ACKNOWLEDGEMENTS |
The help from the staff and use of the facilities at the Department
of Cardiology and Clinical Physiology of the Karolinska Hospital,
Sweden, were highly appreciated. We also thank Carsten Juel and Laurant
Messonnier for the LDH isoform measurements.
 |
FOOTNOTES |
This study was supported by grants from the Swedish Olympic Committee,
Team Danmark, and the Sport Research Council of the Ministry of
Culture. The Copenhagen Muscle Research Centre is funded by a grant
from the Danish National Research Foundation (Grant no. 504-14).
Address for reprint requests and other correspondence: G. van Hall, The Copenhagen Muscle Research Centre, Rigshospitalet, Section 7652, 9 Blegdamsvej, DK-2100 Copenhagen Ø, Denmark
(E-mail: gvhall{at}cmrc.dk).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
September 11, 2002;10.1152/ajpendo.00273.2002
Received 20 June 2002; accepted in final form 5 September 2002.
 |
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