Department of Medicine, Division of Respiratory and Critical Care
Physiology and Medicine, Harbor-University of California Los Angeles
Medical Center, Torrance, California 90509
Previous studies have shown that a metabolic
alkalosis develops in the muscle during early exercise. This has been
linked to phosphocreatine hydrolysis. Over a similar time frame, the femoral vein blood pH and plasma
K+ and
HCO
3 concentrations increase without
an increase in PCO2. Thus
CO2 from aerobic metabolism is converted to HCO3 rather than being
eliminated by the lungs. The purpose of this study was to quantify the
increase in early CO2 stores and
the component due to the exercise-induced metabolic alkalosis (E-I
Alk). To avoid masking the increase in CO2 stores by
CO2 released as
HCO3 buffers lactic acid, the
transient increase in CO2 stores
was measured only for work rates (WRs) below the lactic
acidosis threshold (LAT). The increase in
CO2 stores was evident at the
airway starting at ~15 s; the increase reached a peak at ~60 s and
was complete by ~3 min of exercise. The increase in
CO2 stores was greater, but the kinetics were unaffected at the higher WR. Three
components of the change in aerobically generated
CO2 stores were considered relevant: the carbamate component of the Haldane effect, the increase in CO2 stores due to increase in
tissue PCO2, and the E-I Alk. The
Haldane effect was calculated to be ~5%. Physically dissolved
CO2 in the tissues was ~30% of
the store increase. The remaining E-I Alk
CO2 stores averaged 61 and 68%
for 60 and 80% LAT WRs, respectively. The kinetics of
O2 uptake correlated with the time
course of the increase in CO2
stores; the size of the O2 deficit
correlated with the size of the E-I Alk component of the
CO2 stores. We conclude that a
major component of the aerobically generated increase in
CO2 stores is the new
HCO3 generated as phosphocreatine is
converted to creatine.
carbon dioxide stores; gas-exchange kinetics; near-infrared
spectroscopy; lactic acidosis threshold; phosphocreatine hydrolysis; oxygen deficit
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INTRODUCTION |
RECENTLY, AN INCREASE in concentrations of
HCO3 and
K+ and a decline in
H+ concentration (increase in pH)
without an increase in PCO2 was found
in femoral vein blood during early exercise (27) (Fig. 1) and could be attributed to the
hydrolysis of phosphocreatine (PCr), a reaction that takes up
H+ (20). The increase in
HCO3 and
K+ were stoichiometrically linked
(27). This early exercise-induced alkalosis (E-I Alk) must be a
component of the transient increase in
CO2 stores. The fixing of
metabolic CO2 as
HCO3 (22.3 ml/mmol of
HCO3), as a result of this early E-I
Alk, contributes to a reduction in
CO2 production
(
CO2) relative
to O2 uptake
(O2) and to a decrease in
respiratory exchange ratio (RER) during the period of high rate of
HCO3 formation (Fig. 1). The decrease
in RER takes place between 15 and 90 s after the start of
exercise, with the lowest value at ~30 s. Although this phenomenon is
consistent at all work rates (WRs), it is more marked at high WRs.
However, with heavy exercise, buffering of the developing lactic
acidosis partially masks the increase in early
CO2 stores (23, 27).
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Fig. 1.
Respiratory exchange ratio (RER) and femoral vein (FV)
HCO3,
PCO2, and
PO2 in response to 40%
(left) and 85%
(right) of peak
O2 uptake
(O2) during the first 90 s of
leg cycling exercise in an upright position. Data shown are averaged
for 5 subjects. Data reported in this figure come from studies
previously reported from this laboratory (25, 27). The femoral vein
HCO3 and
PCO2 data were reported in Ref 27.
PO2 data come from the same studies
and were reported in Ref 25. RER data are from the same studies but
were not previously reported. Bars show SE at selected times [at
rest and at 30 s (dotted lines) and at 60 s of exercise].
Significant differences: * P < 0.05 from rest;
+ P < 0.05 between values at 30 and at 60 s of exercise.
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To generate high-energy phosphate, muscle metabolizes primarily
glycogen (7, 9, 13, 21). Thus muscle substrate respiratory quotient
(RQm) is relatively high. In
fact, muscle biopsy as well as gas-exchange measurements have shown
that the RQm is equal to ~0.95
(4, 7, 9). Therefore, the decrease in RER at 15-90 s after the
onset of exercise (Fig. 1) must be due to an increase in
CO2 stores. For moderate
[40% of peak O2 or
~80% of lactic acidosis threshold (LAT)] constant-work-rate
(CWR) exercise (Fig. 1, left), RER
reaches a peak value of <1.0 before 4 min and thereafter remains
relatively constant or decreases slowly with time (17, 28). If the WR
is above the subject's LAT (Fig. 1,
right), the initial decrease in RER
is followed by an increase to a value >1.0 because of release of
CO2 from
HCO3 as
HCO3 buffers the newly formed lactic
acid. It then decreases toward a lower value as the rate of lactate increase slows (28).
Four mechanisms can cause tissue
CO2 stores to increase:
1) blood
CO2 that results from increase in
PCO2,
2)
CO2 bound to hemoglobin (Hb) as
carbamate due to oxyhemoglobin
(HbO2) desaturation (Haldane
effect) (8), 3) increase in
CO2 stored in tissues as
PCO2 increases, and
4) fixation of metabolic CO2 as
HCO3 due to the alkalinization
resulting from the conversion of PCr to creatine and inorganic
phosphate (2, 20, 27, 31). The purpose of this study was to determine the dynamics and volume of the increase in
CO2 stored during moderate exercise and to apportion the quantities attributable to each mechanism. Because exercise above the LAT will result in additional CO2 as
HCO3 buffers lactic acid, and
hyperventilation resulting from ventilatory compensation for
metabolic acidosis (32), we restricted our studies to WRs that were
below the LAT.
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METHODS |
Subjects
Twelve healthy nonsmokers, between 24 and 71 yr of age (mean age
38 ± 13 yr) and at different levels of fitness, were enrolled (Table 1). The subjects were familiar with
the equipment and did a preliminary exercise study. The research
protocol was approved by the Human Subjects Committee at Harbor-UCLA
Medical Center. All subjects gave informed consent.
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Table 1.
Subject characteristics, aerobic parameters, and work rates at 60 and 80% of lactate acidosis threshold (LAT)
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Preliminary Ramp Exercise Testing
Each subject performed a ramp exercise test before the days of study.
Exercise was performed on an electromagnetically braked cycle ergometer
(CPE 2000, Medical Graphics, St. Paul, MN) for determination of
exercise capacity, LAT by the V-slope method (5), and peak
O2 (defined as the
O2 averaged over the last 15 s of exercise). The rate of WR increase (range 15-25 W/min) was
chosen so that the subject would be exhausted by ~10 min of progressively increasing WR. During each test, a pedaling frequency of
60 rpm was maintained with the aid of a visual indicator of pedal rate.
Measurements of gas exchange and heart rate were made breath by breath
during 2 min of rest, during 3 min of unloaded pedaling, and during the
progressively increasing WR test.
The slope (S) of the increase of
O2 as a function of
increase in WR was derived for each individual by least squares
regression line of O2 vs.
WR. The mean value and SD were 10.1 ± 0.7 ml · min1 · W1
and were not significantly different from the value of 10.2 ± 1.0 ml · min1 · W1
previously reported for normal subjects (15). The WRs at
O2 of 60 and 80% LAT were
calculated as follows
where
b is the
O2 for unloaded cycling.
CWR Test
Each subject performed two sessions of periodic CWR exercise. Each
session consisted of 3 min of unloaded cycling followed by four
repetitions of 4-min periods of CWR separated by 5 min of unloaded
cycling, as shown in Fig. 2. The 4-min WR
periods for one session were at 60% of the subject's LAT above the
unloaded cycling; for the other session, the periods were at 80% of
the LAT above unloaded cycling. The two sessions were
separated by 1-7 days, and the order of work level was randomized.
The WRs performed by each subject are shown in Table 1. The unloaded baseline cycling
O2 was defined as
the mean of the last 30 s before the four CWR periods, as shown in Fig.
2. There was no progressive increase in
O2 and
CO2 during unloaded cycling before the load, as would be anticipated if recovery from loaded exercise was not complete by 5 min. Earlier studies (3, 17, 29, 30)
found that the O2 and
CO2 increased in an
exponential fashion after the transition from unloaded to loaded CWR
exercise below the LAT, and a steady state was achieved by 4 min (26).
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Fig. 2.
Schematic representation of testing protocol. Vertical bars indicate
time of loaded exercise (load). Each subject performed 2 protocols on
different days, in random order, one at 60% of the lactic acidosis
threshold (LAT) and the other at 80% LAT. Subjects maintained a
cycling rate of 60 rpm on the unloaded ergometer (marked as
1, 2, 3, 4) before the workload was imposed.
Workload was repeated 4 times for 4-min workload periods, with 5-min
recovery periods of unloaded cycling (unl) between each workload.
Unloaded O2 averaged 372 ± 9 ml/min for the 60% LAT protocol and 379 ± 6 ml/min for the
80% LAT protocol.
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Measurements and Calculations
Pulmonary gas exchange.
The O2,
CO2, and other respiratory
variables were measured breath by breath (Medical Graphics). These data
were converted by interpolation to points at intervals of 1 s. The
points at the beginning of each WR increase were time aligned and then
averaged, second by second, for the four WR repetitions. Because all
work repetitions were not exactly equal to the second in time (they may
differ by several seconds, despite computer control, because of
variable breath lengths), the averaging was restricted to the length of
the shortest work repetition. Similarly, all transitions to unloaded
cycling were aligned at the end of the exercise load and were averaged
over the length of the shortest unloaded repetition. The transition
points were correctly aligned so that the transition behavior was
averaged accurately. The resulting averaged exercise and recovery
repetitions were joined to give a true picture of O2 and
CO2 dynamics over
the entire cycle of loaded cycling (exercise) followed by unloaded
cycling (recovery).
The CO2 and
O2 volumes for the exercise and
recovery periods were calculated by integration of the averaged
second-by-second O2 and
CO2 measurements. To obtain
the volume of CO2 and
O2 that resulted from exercise,
the baseline (end of unloaded cycling O2 and
CO2 × 9 min)
CO2 and
O2 volumes were subtracted from the total exercise and recovery
CO2 and
O2 volumes to obtain the volumes
of O2 and
CO2 caused by the exercise load.
RQm.
RQm was calculated as the ratio of the total
CO2 produced during the 9 min of the exercise-recovery
period
(CO2 work + CO2rec) minus the baseline volume of CO2
(CO2 baseline)
to the total O2 consumed during
the 9 min of the exercise-recovery period
(O2 work + O2 rec)
minus the baseline volume of O2
consumed
(O2 baseline)
for each subject, i.e.
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CO2 stores from pulmonary gas-exchange
measurements.
The increase in aerobically generated
CO2 per unit of time
(
CO2 work)
in response to exercise is described by the increase in
O2 per unit of time above
baseline
(O2 work)
times RQm. The
O2 work
is equal to the difference between pulmonary
O2 during unloaded and
loaded exercise
(O2 work)
plus the decrease in venous O2 stores
(O2 venous)
and muscle myoglobin (Mb) O2 stores
(O2 Mb).
O2 Mb
is relatively small, being ~24 ml for 80% LAT WR (see
DISCUSSION). Adding this to the
O2 measured by external
respiration would result in an additional 24 ml (less for the 60% LAT
WR) to the E-I Alk CO2 stores.
Because it is a small and fixed value, we have not added this volume to
the volume of the increase in CO2
stored, as determined from external respiration measurements. In turn
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(1)
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CO2 work
is also equal to the sum of the increase in pulmonary
CO2 output
(CO2 work)
in response to loaded exercise plus
CO2 from aerobic metabolism stored
in the tissues
(CO2 store) plus venous blood
(CO2 venous),
i.e.
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(2)
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Substituting
CO2 work
of Eq. 1 for
CO2 work
of Eq. 2
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(3)
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Assuming RQm × O2 venous = CO2 venous,
these terms would cancel each other; hence
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(4)
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Thus,
by not including
O2 venous
in the total
O2 work,
we do not include
CO2 venous
in the increase in CO2 stores calculated from external respiration. This leaves three components of
the increase in CO2 stores that
are determined from measurements of external respiration. These include
1) the increase in blood CO2 content due to Hb
deoxygenation (Haldane effect); 2)
the increase in CO2 stored in
tissues because of increase in tissue
PCO2; and
3) the chemically bound
CO2 due to tissue alkalinization
from hydrolysis of PCr.
The accumulated CO2 stores
(CumCO2) value, as related to
exercise time, is the integral of the
CO2 store,
as follows
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(5)
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where
dt is 1 s.
Calculation of components of the increase in
CO2 stores.
haldane effect.
We utilized near-infrared spectroscopy (NIRS) (RunMan; NIM,
Philadelphia, PA) to monitor kinetics of muscle
HbO2 and
MbO2 saturation.
Changes in differences between the two wavelengths monitored
[(760-850 nm)] are used to estimate changes in the relative desaturation of the
(HbO2 + MbO2)
in the tissue under the probe (18). The linearity of the desaturation
output from the NIRS unit had been validated previously in our
laboratory (6). The probe was positioned over the vastus lateralis
muscle, 10-12 cm above the knee and parallel to the major axis of
the thigh. All studies were performed after calibration with similar balance of the signal and difference-gain settings
on the spectrometer. The muscle oxygenation signal was continuously
monitored and recorded second by second throughout the protocol on a
computerized system (DI200 PGH/PGC; Dataq Instruments, Akron, OH).
The magnitude of the Haldane effect was calculated from the estimated
change in mixed venous content between unloaded cycling and 60 and 80%
LAT WRs. On the basis of the studies of Stringer et al. (24) for normal
subjects, mixed venous O2 contents
(means ± SE) during unloaded cycling, 60% LAT, and 80% LAT were
estimated to be 12.78 ± 0.10, 12 ± 0.10, and 11.11 ± 0.05 ml/dl, respectively. Therefore, mixed venous
HbO2 saturation
(
O2)
would be (in %) 62.6 (range, 62-63), 58.8 (range,
58.3-59.3), and 54.5 (range, 54.2-54.7) for unloaded
and for 60 and 80% LAT exercise, respectively, if Hb = 15 g/dl (Hb was
not measured in this study).
Thus
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(6)
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where
0.022 is the change in CO2 content
(in mmol) for a 1% decrease in
O2
per liter of blood (19), Vvenous
is the volume (in liters) of venous blood estimated to be 5%
of body weight (3), and 22.3 ml is the
CO2
STPD of 1 mmol of
CO2. Thus total CO2 derived from the Haldane
effect for the change in
O2
from unloaded cycling to 60 and 80% LAT exercise would be 6 (range, 4-7) and 13 (range, 12-14) ml, respectively.
PHYSICALLY DISSOLVED CO2
STORES.
Assuming that the increase in leg tissue
PCO2 is equal to the increase in
femoral vein PCO2
(PfvCO2)
(11)
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(7)
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where 0.0294 is solubility of
CO2 at 39°C in
mmol · l1 · mmHg1
(19), skeletal muscle temperature is estimated to be 38-39°C at 60 and 80% LAT (22), 22.3 ml is the volume of
CO2
STPD of 1 mmol
CO2, and estimated leg muscle
fluid volume (in liters) is 60% of leg muscle mass. Skeletal muscle
mass was estimated by the method of Gallagher et al. (12).
For Caucasians and Asians, estimated leg muscle mass
is
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(8)
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For African-Americans, estimated leg muscle mass is
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(9)
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where 1 and 0 in both equations are codes for men and women,
respectively; and SEE is standard error of the estimate.
In reference to Eq. 7, the dilemma is
how to estimate
PfvCO2 in
different subjects at 60 and 80% LAT without actually measuring it in
each subject. Data collected from a previous study (24) showed that an
increase in end-tidal PCO2
(PETCO2), or
PETCO2, correlated with
the increase in
PfvCO2 (PfvCO2)
for WRs below the LAT in normal subjects with an average slope (means ± SE) of 1.45 ± 0.06 (see
APPENDIX). Using this
relationship, we calculated
PfvCO2 as
the product
of PETCO2 × 1.45 (range, 1.39-1.51) for each subject, on
the basis of the subject's increase in
PETCO2 from unloaded
cycling to the same subject's CWR exercise.
E-I ALK CO2 STORES.
The E-I Alk CO2 stores were
calculated as the difference between the increase in total
CO2 stores and the increase in
CO2 stores due to the Haldane
effect plus the increase in the physically dissolved
CO2 stores.
Femoral vein blood-gas changes.
In constructing Fig. 1 to illustrate the relationship between the
changes in RER and the gas-exchange events at the level of the muscle,
we used previously reported data from studies by this laboratory (25,
27). The femoral venous HCO3 and
PfvCO2 were
reported in Ref. 27. To compare the magnitude and timing of the
metabolic alkalosis (see Fig. 7) with the E-I Alk
CO2 stores, we calculated standard
HCO3 from the data reported in Ref.
27. The PO2 was reported in Ref. 25.
The gas-exchange data for calculating RER are previously unreported but
come from the same experiments.
O2 time constant.
The increase in O2 from
unloaded cycling to steady-state exercise was fit with a
monoexponential model from the following equation by using least
squares criteria (2, 30)
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(10)
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where
Y0 is the
baseline (unloaded cycling)
O2,
A1 is the
O2 above baseline at
steady-state
(O2 ss),
and 
is the time constant for
O2.
O2 deficit.
The O2 deficit is simply the
product of
O2 ss
and (30), i.e.
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(11)
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Data Analysis and Statistics
The dynamics of the increase in
CO2 stores were compared with the
rate of leg deoxygenation from measurements of time of peak rate of
increase in CO2 stores and the
half-time of muscle deoxygenation, respectively.
Comparison of multiple unloaded
O2 and
CO2 values was carried out
by using a repeated-measures ANOVA. Comparison of variables between the
two levels of exercise was done by using the paired t-test. Significance of differences
among the three components in the
CumCO2 stores calculation was
determined by using ANOVA. Relationships among variables were studied
by using linear regression techniques and by calculating Pearson
product-moment correlation coefficients. Significant change was
determined to be P < 0.05. All
values reported are means ± SD.
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RESULTS |
Gas Exchange During Exercise and Recovery
All subjects (n = 12) carried out four
repetitions of square-wave WR exercise at a
O2 of 80% LAT (mean values:
WR, 56 ± 13 W; O2, 1,028 ± 185 ml/min) for 4 min, followed by unloaded cycling for 5 min
(Table 1). Nine of the 12 subjects completed the protocol at a
O2 of 60% LAT (mean values:
WR, 39 ± 11 W; O2, 835 ± 188 ml/min). Three subjects were not included in the 60% LAT
exercise study because of technical difficulties. Complete recovery in
O2 and
CO2 in the 5 min of unloaded
cycling after each of the WR repetitions was evidenced by no
progressive increase in these measurements at the end of the unloaded
cycling recovery period. The group mean
O2 and
CO2 values during the last 30 s of 5 min of unloaded cycling at
O2 of 60% LAT CWR exercise were 372 ± 73 and 351 ± 85 ml/min, respectively; for 80% LAT, the group mean O2 and
CO2 values were 379 ± 50 and 351 ± 56 ml/min, respectively.
Dynamics of RER and Exercise-Induced Changes in
CO2 Stores
Figure 3 illustrates the second-by-second
time-averaged changes in O2,
CO2, RER, and calculated
dCO2
store/dt and the increase in CumCO2 store for a
representative subject (subject 5)
at 80% LAT. O2 reached a
plateau by 2.5 min, and CO2
reached a plateau by 3 min. RER was the same as during unloaded cycling for the first 10 s of exercise and then decreased to a nadir at 55 s
before returning to a value just under 1.0 by 3 min. The increase in
CO2 stores became evident at the
lung at ~10 s after the start of loaded exercise, and the rate of
increase peaked at 55 s (at the nadir of RER); it reached a value of
230 ml/min at that time. The rate of increase declined gradually until
the end of the third minute, when
dCO2
store/dt returned to zero, indicating
no further change in CO2 stores.
The CumCO2 store during exercise
has a sigmoid contour, with the period of most rapid increase being
between 30 and 80 s; it reaches a maximal value of 340 ml at 3 min. At
the start of recovery, RER changed in the opposite direction from the
change seen at the start of exercise; consequently, the direction of
dCO2
store/dt reversed. The
CO2 stores that accumulated during
exercise were eliminated by the last minute of unloaded cycling
recovery.
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Fig. 3.
Example of kinetics of increase in
CO2 stores and its accumulation
(CumCO2 store) derived from the
simultaneous changes in O2
and CO2 during a 4-min 80%
LAT (60 W) constant work rate exercise and 5 min of recovery. Data are
for subject 5. All data are calculated
second by second (see text for details).
dCO2
store/dt, rate of
CO2 store increase in ml/min;
CumCO2 store, accumulated
CO2 stores as related to time; TD,
time delay; T90%, time to 90% of
response.
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The dynamics of CO2 store change
in response to exercise are shown in Table
2 for all of the subjects. The average
RQm values were 0.91 ± 0.04 and 0.97 ± 0.06 for the 60 and 80% LAT WRs, respectively. The
increase in CO2 stores became
evident at the lung, with a time delay of 16 ± 6 and 12 ± 6 s
from the start of exercise for the 60 and 80% LAT WRs, respectively.
It then increased to a peak rate of 103 ± 27 and 184 ± 74 ml/min at 61 ± 8 and 55 ± 6 s, respectively, for the 60 and
80% WRs. The peak rate of increase in
CO2 stores was significantly
larger (P < 0.01), and the time
delay was significantly shorter (P < 0.05) for the 80% compared with the 60% LAT exercise (Table 2).
However, the peak time for dCO2
store/dt was not significantly different for the two WRs after subtracting the time delays for each
exercise. The CumCO2 store of 244 ± 93 ml for 80% LAT was significantly higher than the 141 ± 48 ml for the 60% LAT WR (P < 0.001;
Table 2). The 90% time (T90)
for CumCO2 stores for 80% LAT was
the same as that for 60% LAT. During unloaded recovery, RER,
CO2 stores, and Cum
CO2 stores behaved with similar
but reverse dynamics to the accumulating
CO2 stores.
Components of the Increase in CO2 Stores
Table 3 apportions the increase in total
CumCO2 stores into its components.
The Haldane effect.
The Haldane effect should have similar kinetics to that of muscle
deoxygenation, because this effect takes place in venous blood. Figure
4 shows the average dynamics of the leg
deoxygenation immediately after the 80% LAT exercise load was imposed
for the 12 subjects. It reached a constant value by 50 s (Fig. 4). The group half-time (T50%) values for
the leg deoxygenation were 21 ± 5 and 19 ± 4 s for the 60 and
80% LAT exercise, respectively (P > 0.05). The group T90% for the leg
deoxygenation was 37 ± 11 and 39 ± 12 s for the 60 and 80% LAT exercise, respectively (P > 0.05). Compared with the time course of increase in
CO2 stores (which had a
T90% of 133 ± 14 and 133 ± 18 s for the 60 and 80%-LAT exercise, respectively), the
kinetics of the leg deoxygenation was much faster. The increase in
CO2 stores from the Haldane effect (Eq. 6) was 6 ± 1 and 13 ± 2 ml for 60 and 80% LAT WRs, respectively.
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Fig. 4.
Leg deoxygenation measured with near-infrared spectroscopy (NIRS) and
the simultaneous RER for 4 min of 80% LAT constant-work-intensity
exercise. Data are average for 12 subjects in the study. Dotted line
(left), start of loaded exercise
from unloaded cycling; dotted line
(right), time transition from loaded
to unloaded cycling. Half-time
(T50%) and 90% of response
time (T90%) of the NIRS signal
are indicated. Nadir of RER response was at 50 s and then gradually
increased to a steady state by 3 min. A.U., arbitrary unit.
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Physically dissolved CO2.
The increase in physically dissolved
CO2 (Eq. 7) was 50 ± 25 and 60 ± 21 ml for 60 and 80%
LAT WRs, respectively.
E-I Alk CO2.
The difference between the total and the sum of physically dissolved
and Haldane effect is 84 ± 27 ml (61% of total) and 171 ± 88 ml (68% of total) for 60 and 80% LAT WRs, respectively (Table 3).
This is attributable to the E-I Alk
CO2 store (fixed as
HCO3). It is the largest fraction of
the various components of the increase in total
CO2 stores measured by external
respiration. The absolute, but not the percentage, values were
significantly higher for the 80% LAT compared with the 60% LAT exercise.
The Accumulated CO2 Stores and Fitness
Because the early CO2 stores
should be closely linked to the contribution of the splitting of PCr as
well as the rate of O2 transport
to muscle at the start of exercise, it might be anticipated that the
amount and, probably, the time course of the increase in
CO2 stores are related to fitness.
Because the size of the O2 deficit
and the time constant are larger for the less-fit individual, we
correlated the size of the increase in total
CO2 stores and E-I Alk
CO2 stores to the
O2 deficit (Fig.
5). For both the 60 and 80% LAT levels of
work, the increase in total CO2
stores and E-I Alk CO2 stores
correlated positively with the O2
deficit.
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Fig. 5.
Changes () in total CO2 stores
and exercise-induced metabolic alkali (E-I Alk)
CO2 stores (in ml) as a function
of O2 deficit (in ml) for 60%
(n = 9) and 80% LAT
(n = 12) constant work rate exercise.
In each panel, each symbol represents a different subject. Solid lines
are the least squares best fit correlation between total
CO2 stores or E-I Alk
CO2 stores and
O2 deficit.
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Also, the time to the peak rate of increase in
CO2 stores was significantly
correlated with the time constants for
O2 for the 60 and 80% LAT
exercise (r = 0.74 and 0.79, P = 0.02 and < 0.01, respectively;
Fig. 6). This correlation remained
significant after subtracting the tissue-to-lung time delay (Table 2)
from the time of peak rate increase for both levels of exercise (Fig. 6, bottom).
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Fig. 6.
Time to peak change in CO2 stores
(dCO2
store/dt)
(top) and time to peak minus time
delay (TD) (bottom) as a function of
time constants () for O2
for 60% LAT (n = 9) and 80% LAT
(n = 12) constant work rate exercise
(in s). In each panel, each symbol represents a different subject.
Regression lines for data are also shown.
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DISCUSSION |
Dynamics of RER and CO2 Stores During CWR
Exercise
The development of a metabolic alkalosis during early exercise, with
reversal in early recovery, helps us to understand the mechanism of the
slower increase in CO2 than
O2 and decrease in RER
shortly after the onset of exercise and the increase in RER during
early recovery (17, 26, 29). Because it is difficult to study the
magnitude of the increase in CO2
stores when CO2 stores are
simultaneously being depleted by HCO3 buffering of lactic acid, the WRs selected for this study were below
the LAT (60 and 80% of the LAT). The typical RER changes during the
transition from unloaded cycling to the relatively low work intensities
studied here are shown in Figs. 3 and 4.
O2 and
CO2 abruptly increase at the
start of exercise with the same RER as for unloaded cycling (resting
RER if starting exercise from rest; e.g., Fig 1). RER then consistently
decreases for all work intensities, starting at ~15 s, reaches a
nadir between 30 and 60 s, and approaches a constant value by 180 s
(Fig. 1, left; Figs. 3 and 4) (17, 26,
28, 30). The 15-s delay approximates the time required for blood to be
transported from the muscle capillaries to the pulmonary capillaries.
During the time of decreasing RER, some of the
CO2 generated from aerobic metabolism is stored rather than being delivered to the lungs in
proportion to O2.
It should be pointed out that the decrease and then increase in RER is
not caused by a switch from carbohydrate to fatty acid metabolism in
muscle followed by a switch back to carbohydrate metabolism. If that
were the case, CO2 would not be
available in the stores for RER to increase in the early recovery
period (no hyperventilation at these WRs below the LAT) and
CO2 would not increase above
O2 in recovery as shown in Figs. 3
and 4. In fact, the CO2-balance
studies during exercise and recovery shown here support a constant
RQm during exercise. That the
recharging of PCr stores at early recovery is the major cause for the
postexercise rise in RER is supported by the systematic release of the
stored CO2 (Fig. 3) and reuptake
of K+ by the muscle (see Fig. 5 in
Ref. 27) during recovery, at the time that PCr is
resynthesized (see Figs. 2 and 4 in Ref 31).
Components of the Early CO2 Stores and
Calculation Assumptions
The increase in total tissue CO2
stores (244 ± 93 ml) for the CWR exercise at 80% LAT in this study
is consistent with a previous report (272 ± 92 ml),
despite a very different technique employed for
RQm measurement (3). The increase
in total tissue CO2 stores was
partitioned into three potential components, i.e., the increase in
CO2 in the venous blood due to the
Haldane effect, the physically dissolved
CO2, and the E-I Alk formation due
to PCr hydrolysis. See Potential miscellaneous
factors for further discussion.
The Haldane effect.
O2 desaturation allows Hb to bind
additional CO2 without increasing
blood PCO2. Thus, despite no increase
in PCO2 in the femoral vein blood
during the first 30 s of exercise (Fig. 1), the decrease in
PO2 and, therefore,
O2 saturation can bind additional
CO2 as carbamate and account for
some of stores. However, the kinetics of
HbO2 desaturation
(T50 ~19-21 s;
T90 ~37-39 s) measured by
NIRS (Fig. 4) were clearly faster than the kinetics of the increase in
CO2 stores (Fig. 3, Table 2).
Femoral vein O2 desaturation for
moderate-intensity exercise was shown to be complete within 30-50
s after its start (25). This finding is consistent with the kinetics of
muscle deoxygenation reported here (Fig. 4). By using data reported
previously (24), it can be calculated that the increase in volume of
CO2 stored as carbamate due to Hb
deoxygenation (Haldane effect) at the same
PCO2 is relatively small, being only
~5% of the total increase in tissue CO2 stores. The Haldane effect
also causes changes in HCO3 (16) when
accompanied by a change in PCO2.
Change in the HCO3 component in venous
blood due to uptake or release of
CO2 when Hb is
deoxygenated during exercise and oxygenated during recovery is offset
by changes in venous O2 content, as reflected in the reasonable assumption that
CO2 venous = RQm × O2 venous.
Physically dissolved CO2.
The change in physically dissolved
CO2 depends on
1) the volume of the tissue exposed
to the change in PCO2,
2) the solubility of
CO2 in the tissue, and
3) the increase in tissue PCO2 (10). By using equations
previously reported (12), we estimated the leg muscles weighed 18 ± 3 kg. We assumed that the volume of both legs does not change during 4 min of moderate exercise. The muscle tissue fluid volume of both legs
would be ~11 liters, assuming that the fluid component is 60% of the
leg muscle weight. The solubility of
CO2 is changed with the change in
body temperature. During light and moderate exercise, the body temperature does not rise >2°C, for which the solubility of
CO2 would fall from 0.0301 to
0.0294 mmol · l1 · mmHg1 (19, 22). The
increase in
PfvCO2
approximated from the increase in
PETCO2 above unloaded
cycling for each subject at each WR (see
APPENDIX) averaged 7 ± 3 and 8 ± 2 Torr for the 60 and 80% LAT WRs, respectively. Thus, given
these approximations, the increase in the physically dissolved
CO2 stores averaged 50 and 60 ml
for the 60 and 80% LAT, respectively (Table 3).
The E-I Alk production.
As soon as exercise starts, PCr hydrolyzes and exerts an alkalinizing
effect as shown by studies that employed
31P-nuclear magnetic resonance
spectroscopy (1, 2, 31). This alkalinizing effect causes an increase in
femoral vein pH and HCO3 without an
increase in PCO2 during the first 30 s of leg cycling exercise (27). As shown in Fig.
7, standard
HCO3 increases in the femoral vein but
not in the arterial blood during this early period of exercise.
Wasserman et al. (27) pointed out that the major mechanism for the
exercise hyperkalemia is the reduction in nondiffusible intracellular
anions in the myocytes when PCr hydrolyzes, because creatine is
essentially neutral, whereas PCr is an acid (pK = 3.87) at the pH of
cell. Thus new anions are needed to balance the free
K+ made available when highly
dissociated PCr is converted to neutral creatine. The new anion
available to the myocyte is HCO3, because the H+ of carbonic acid
would be consumed in the hydrolysis of PCr (20). In this way, the
splitting of PCr slows the rate of muscle
CO2 output from aerobic metabolism
into the exhaled gas (increases the
CO2 stores). Thus venous-arterial
standard HCO3 difference increases
transiently during the time of a high rate of PCr hydrolysis (Fig. 7).
For a WR similar to 80% LAT exercise, the average increase in femoral
vein standard HCO3 during the first 2 min of exercise is ~0.8 mmol/l (Fig. 7) (25, 27). Because the leg
blood flow would be on the order of 8 l/min in steady-state (from an
assumed leg arteriovenous O2
content difference of 100 ml/l and leg
O2 of 800 ml/min, calculated from total body O2),
increasing from 3 l/min at unloaded cycling (14), a WR of 80% LAT
would result in an E-I Alk component of the increase in
CO2 stores of ~178 ml (2 min × 0.8 mmol/l × 22.3 ml/mmol × 5 l/min). Thus the
average value that we obtained for the E-I Alk (171 ml for the 80% LAT
WR shown in Table 3) is of the expected order of magnitude.
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Fig. 7.
Arterial (Art; circles) and femoral vein (Fem V; squares) standard
HCO3
(top; solid symbols) and actual
HCO3
(bottom; open symbols) in response to
80% LAT upright leg cycling exercise. Data are average for 5 subjects
from materials of Ref. 27. Bars, SE. Significant differences are
provided at rest and at 30, 60, 90, and 180 s of exercise:
* P < 0.05 compared with rest;
+ P < 0.05 compared with 30 s;
# P < 0.05 compared with 60 s.
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Potential miscellaneous factors.
For the reasons described in METHODS,
the increase in venous blood CO2,
which is a true increase in CO2
stores, does not enter into our estimate of the increase in total
CO2 stores calculated from
external respiration because it is offset by the consumption of
O2 from the venous
O2 stores that we did not measure.
Similarly, changes in lung CO2
stores from changing functional residual capacity have not been
included, because this should be very small (<10 ml) and offset by
reciprocal changes in lung O2 stores.
We also did not take into account
CO2 produced due to
O2 consumed from the decrease in
physically dissolved O2. However,
this will result in a relatively small additional
CO2 and lead to our
underestimation of the change in
CO2 stores. For example, the leg
venous PO2 decreases only 5 Torr, on
average, in response to exercise of 80% LAT (Fig. 1)
(25). Thus the amount of O2
consumed and therefore CO2
produced from that source would be <3 ml.
The increase in O2 consumed from
unloaded cycling to 80% LAT (~40% of maximal
O2) from the
MbO2 source, although difficult to
estimate, would also be relatively small. If we assume a linear decrease in MbO2 with increasing
O2, a Mb concentration of 0.5 mmol/l, an O2 capacity of 11.2 ml/l for Mb, and MbO2 decreasing by 20% from unloaded cycling to 80% LAT exercise, the
O2 and CO2 per liter of muscle water
would be 2.2 ml. Assuming the leg muscle mass to be 18 kg, of which
60% contains Mb (uniformly distributed), the maximum
CO2 that can come from the
MbO2 source of the lower extremity
is 10.8 × 2.2 = 24 ml (from unloaded to 80% LAT exercise).
If the physically dissolved O2
consumption and O2 consumption
from use of MbO2 were added to the
measured O2, the calculated increase in CO2 stores would be
larger than that reported in Table 3 by a maximum of 27 ml. Thus the
increase in early CO2 stores and
E-I Alk, as reported in Table 3, are underestimated. However, the
assumptions suggest that the underestimates are relatively small.
Possible estimation errors.
The O2 consumption and
CO2 production of skeletal muscles
that support respiration and posture are included in the measured O2 and
CO2 during exercise. These
muscles contribute <6% of the increase in whole body
O2 and
CO2 during moderate CWR leg
muscle exercise (14). The magnitude of error was estimated for the
Haldane effect and the physically dissolved
CO2 stores by applying previously
reported measurements. For sensitivity analysis involving one SE of the
mean, the Haldane effect is estimated to be 3 ± 1 to 6 ± 2%
for the 60% LAT WR exercise and 5 ± 2 to 7 ± 3% for 80% LAT
WR exercise. The physically dissolved
CO2 store is 30 ± 6 to 39 ± 7% for 60% LAT and 23 ± 8 to 30 ± 10% for the 80% LAT
WR exercise. The EI-Alk CO2 is 55 ± 7 to 66 ± 6% for the 60% LAT WR exercise and 64 ± 13 to
72 ± 10% for the 80% LAT WR exercise.
CO2 Stores and Muscle Bioenergetics
A linkage between O2 increase
and PCr splitting during exercise has been proposed previously (29).
The similarity of the time constant for
O2 increase during phase
2 (the exponential increase in
O2 after the first 15 s of
exercise) with those for PCr and change in the cytosolic free energy of
ATP hydrolysis suggests that phase 2 O2 kinetics reflects muscle
respiration (O2)
kinetics (2, 14). The correlation of the time for peak generation of
CO2 stores with for
O2
(P = 0.02 to <0.0001; Fig. 6) might
also be interpreted as an indication that the dynamics of increase in
CO2 stores correlates with that of
PCr hydrolysis.
O2 Deficit and
CO2 Stores
The O2 deficit arises from
high-energy phosphate-bound energy generated from venous and muscle
O2 stores as well as PCr splitting during early exercise (28). It increases with decreasing fitness. The
total increase in both CO2 stores
and E-I Alk CO2 stores correlated with the O2 deficit; this is
consistent with a common mechanism. Thus, if exercise were performed
below the LAT, a slow rate of increase in
O2 would presumably be
accompanied by a still slower rate of increase in
CO2 and a larger difference
between CO2 and
O2.
Conclusions
By analyzing O2 and
CO2 kinetics simultaneously
during early exercise and recovery, and by a special technique to
determine the respiratory quotient of the skeletal muscle
on the basis of total CO2 produced
and total O2 consumed above
baseline for the exercise and recovery period, we estimated the rate of
increase in total CO2 stores
during two levels of exercise below the LAT. The components of the
increase in CO2 stores were
further analyzed to determine the contribution of each to the increase
in total CO2 stores. It was
estimated that the alkalinizing reaction of PCr hydrolysis might
account for about two-thirds of the increase in total
CO2 stores. This analysis provides
insight into the mechanisms that determine the increase in
CO2 stores during moderate
exercise and a gas-exchange approach to quantify the contribution of
PCr hydrolysis to early-exercise bioenergetics.
The calculation assumptions have already been discussed except for the
estimation of change in femoral vein
PCO2 (PfvCO2)
for determination of increase in tissue
PCO2. From a retrospective review of
previously collected data (24), we found that
PfvCO2
correlated with change in end-tidal
PCO2 (PETCO2) for exercise
work rates (WRs) below lactic acidosis threshold (LAT). To obtain
PfvCO2 to
estimate the increase in physically dissolved
CO2, we analyzed the relationship
between PfvCO2 and
PETCO2 for exercise below
the LAT from the data of five normal healthy male subjects (aged 25 ± 6 yr) performing an incremental exercise test (25-40 W/min)
by using a computer-controlled, electromagnetically braked, cycle
ergometer (type 18070, Gould-Godart, Bilthoven, The Netherlands) (23). Pulmonary gas exchange and
PETCO2 were measured breath by breath, and
PfvCO2 was
measured at 30-s intervals. The details of the catheter placement were
reported previously (23). Briefly, the 8-cm 10-Fr sheath (Cordis,
Miami, FL) was inserted percutaneously into the right femoral vein, 2 cm below the inguinal ligment, by the Seldinger technique and was
located 4-6 cm above the inguinal ligment. The sampled blood was
immediately iced and was analyzed for
PCO2,
PO2, and pH by using an
Instrumentation Laboratories 1306 blood-gas machine.
Figure 8 shows the
correlation of simultaneous
PfvCO2 and
PETCO2 for WRs for
exercise at below-LAT WR. The average slope (mean ± SD) of the regression relating
PfvCO2 to
PETCO2 was 1.45 ± 0.14 (r = 0.74, P < 0.0001). Therefore, to determine
PfvCO2, we
multiplied the average slope (1.45) by the
PETCO2 during the fourth
minute of loaded exercise for each study. This gave an increase in
PfvCO2 that was
consistent with values reported in the literature for the level of work
performed (27), and values for physically dissolved
CO2 that were relatively uniform
among subjects. Initially, we assumed one value for all subjects, on the basis of average
PfvCO2
values in the literature. Use of the individual
PETCO2 values to estimate
PfvCO2 gave us data that were much more uniform than selection of an average
PfvCO2 for
all subjects.
During this research, M. L. Chuang was a visiting scientist from
Chang Gung Memorial Hospital, Taipei, Taiwan; H. Ting was a visiting
scientist from Chung Shan Medical and Dental College, Taichung, Taiwan;
T. Otsuka was a visiting
TOP
ABSTRACT
INTRODUCTION
