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O2 on-kinetics in canine muscle
contracting at peak O2
1 Istituto di Tecnologie Biomediche Avanzate, Consiglio Nazionale delle Ricerche, I-20090 Segrate (MI), Italy; 2 Department of Medicine, University of California, San Diego, La Jolla, CA 92093-0623; and 3 Department of Health and Human Performance, Auburn University, Auburn, Alabama 36849-5323
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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A
previous study (Grassi B, Gladden LB, Samaja M, Stary CM, and Hogan MC,
J Appl Physiol 85: 1394-1403, 1998) showed that convective O2 delivery to muscle did not limit
O2 uptake (
O2) on-kinetics
during transitions from rest to contractions at ~60% of peak
O2. The present study aimed to
determine whether this finding is also true for transitions involving
contractions of higher metabolic intensities.
O2 on-kinetics were determined in
isolated canine gastrocnemius muscles in situ (n = 5)
during transitions from rest to 4 min of electrically stimulated
isometric tetanic contractions corresponding to the muscle peak
O2. Two conditions were compared:
1) spontaneous adjustment of muscle blood flow (
)
(Control) and 2) pump-perfused , adjusted
~15-30 s before contractions at a constant level corresponding
to the steady-state value during contractions in Control (Fast
O2 Delivery). In Fast O2 Delivery, adenosine
was infused intra-arterially. was measured continuously in the
popliteal vein; arterial and popliteal venous O2 contents
were measured at rest and at 5- to 7-s intervals during the transition.
Muscle O2 was determined as
times the arteriovenous blood O2 content difference. The time to reach 63% of the O2 difference
between resting baseline and steady-state values during contractions
was 24.9 ± 1.6 (SE) s in Control and 18.5 ± 1.8 s in
Fast O2 Delivery (P < 0.05). Faster
O2 on-kinetics in Fast O2
Delivery was associated with an ~30% reduction in the calculated
O2 deficit and with less muscle fatigue. During transitions
involving contractions at peak O2, convective O2 delivery to muscle, together with an inertia
of oxidative metabolism, contributes in determining the
O2 on-kinetics.
gas-exchange kinetics; skeletal muscle oxidative metabolism; maximal contractions
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IN THE ISOLATED DOG
GASTROCNEMIUS in situ, during transitions from rest to
contractions corresponding to ~60% of the muscle peak oxygen uptake
(O2 peak), convective O2
delivery to muscle and peripheral O2 diffusion do not
represent limiting factors for the kinetics of adjustment of muscle
oxygen uptake (O2)
(O2 on-kinetics) (9, 10).
These findings appear to be in agreement with the hypothesis that the
limiting factors for O2 on-kinetics
mainly reside in an inertia of skeletal muscle oxidative metabolism
(4, 5, 21, 31). Evidence obtained in exercising humans,
however, seems to indicate that, for exercises higher than the
"ventilatory threshold," O2 delivery to muscle may
represent a limiting factor for the O2
on-kinetics, at variance with observations for exercise of lower
metabolic intensities (8, 19). To examine this
discrepancy, we conducted the present study utilizing the isolated dog
gastrocnemius in situ preparation: the aim of the study was to evaluate
the role of convective O2 delivery to muscle as a limiting
factor for O2 on-kinetics during transitions from rest to contractions of higher metabolic intensities (i.e., at the muscle O2 peak) compared
with those in previous work by our laboratory (9). As in
the previous study (9), any delay in the kinetics of
convective O2 delivery during the transition was eliminated
by keeping blood flow () constantly elevated at rest and
throughout the contraction period. We hypothesized that, if muscle
O2 on-kinetics were limited by the rate
of adjustment of convective O2 delivery, the elimination of
any delay in the latter would allow faster
O2 on-kinetics to be observed.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The study was conducted with approval of the Institutional Animal Care and Use Committee of Auburn University, Auburn, Alabama, where the experiments were performed.
Five adult mongrel dogs of either sex [17.4 ± 1.5 (SE) kg body wt] were anesthetized with pentobarbital sodium (30 mg/kg), with maintenance doses given as required. The dogs were intubated with an endotracheal tube and ventilated with a respirator (model 613, Harvard Apparatus). The rectal temperature was maintained at ~37°C with a heating pad and a heating lamp. After surgical preparation, the animals were treated with heparin (2,000 U/kg). Ventilation was maintained at a level that produced normal arterial PO2 and PCO2 values.
Surgical preparation.
The gastrocnemius-plantaris-flexor digitorum superficialis muscle
complex (for convenience referred to as "gastrocnemius") was
isolated as described previously (26). Briefly, a medial incision was made through the skin of the left hindlimb from midthigh to the ankle. The sartorius, gracilis, semitendinosus, and
semimembranosus muscles, which overlie the gastrocnemius, were doubly
ligated and cut between the ties. To isolate the venous outflow from
the gastrocnemius, all of the vessels draining into the popliteal vein,
except those from the gastrocnemius, were ligated. The popliteal vein
was cannulated, and was measured with a 3-mm cannulating-type electromagnetic flowmeter (model RT-500, Narco Biosystems). Venous outflow was returned to the animal via a reservoir attached to a
cannula in the left jugular vein. The arterial circulation to the
gastrocnemius was isolated by ligation of all vessels from the femoral
and popliteal artery that did not enter the gastrocnemius. The right
femoral artery was also isolated and cannulated. Blood from this artery
was passed through tubing to a roller pump (model 7520-25, head
model 7016-20, Cole-Parmer Masterflex) and then through another
cannula into the contralateral, isolated popliteal artery supplying the
gastrocnemius. Y connectors positioned before and after the pump
allowed either spontaneous perfusion of the gastrocnemius at the
animal's own blood pressure or controlled at any desired level
by adjustment of the pump setting. A T connector in the tubing to the
gastrocnemius was connected to a pressure transducer (model RP-1500,
Narco Biosystems) for measurement of muscle perfusion pressure.
Arterial blood samples were taken from another T connector in the
cannula exiting the right femoral artery before the roller pump.
Experimental design.
To evoke muscle contractions, the nerve was stimulated by supramaximal
square pulses of 4.0- to 6.0-V amplitude and 0.2-ms duration (Grass S48
stimulator) and isolated from ground by a stimulus isolator (Grass
SIU8TB). Before each experiment, the muscle was set at optimal length
by progressively lengthening the muscle as it was stimulated, at a rate
of 0.2 Hz until a peak in developed tension (total tension 
resting tension) was obtained. For the experiments, isometric tetanic
contractions were triggered by stimulation with trains of stimuli
(4-6 V, 200-ms duration, 50-Hz frequency) at a rate of 1 contraction/s for a 4-min period. Prior studies (1, 16)
have shown that this stimulation pattern elicits peak metabolic rate
for this muscle. Tetanic contractions were chosen to allow a rapid
attainment of a steady state of developed force. A steady state of
force was in fact reached from the very first contraction. For the
purposes of the study, it was critical to obtain truly
"rectangular" increases in the forcing function, represented by the
developed force. Each isometric tetanic contraction lasted 200 ms and
was separated from the following by 0.8 s, during which the muscle
was relaxing or relaxed.
[control condition, spontaneous
(Control)], and 2) pump-perfused constant , which
was adjusted ~15-30 s before the start of contractions to a
level corresponding to the steady-state value obtained during
contractions in Control ["treatment" condition, characterized by a
faster adjustment of O2 delivery to the gastrocnemius (Fast
O2 Delivery)]. A schematic representation of the
experimental protocol is shown in Fig. 1.
The order of treatments could not be randomized, since in order to
choose the level during Fast O2 Delivery it was
necessary to know the "spontaneous" level at steady state
during contractions. When the blood supply to the gastrocnemius was
switched from self-perfused to pump perfused, enough time was allowed
for the hemodynamic parameters to stabilize.
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, 1-2 ml/min
of a 10
2 M adenosine solution (in normal saline) were
infused intra-arterially by a pump, beginning from ~15-30 s
before the onset of contractions. The adenosine infusion was then
continued throughout the contraction period. This dosage of the drug
was previously shown to be effective, in the same preparation as the
present study, in obtaining a significant vasodilation at the muscle
level without causing significant metabolic effects (such as changes in
resting O2,
O2 at the same submaximal level of
contraction, and O2 peak) (9,
17).
At the end of the experiment, the dogs were killed with an overdose of
pentobarbital. The gastrocnemius was excised and weighed, and the
weight was utilized to normalize variables per unit of muscle mass as appropriate.
Measurements.
Output from the pressure transducer was recorded on a strip-chart
recorder, whereas outputs from the load cell and flowmeter were fed
through strain-gauge and transducer couplers, respectively, into a
computerized (PowerComputing PowerBase 240 Macintosh clone) data-acquisition system (GW Instruments, SuperScope II and instruNet model 100B D/A input/output system). The load cell reaches 90% of full
response within 1 ms, whereas the flowmeter has a pulsatile cutoff
frequency of 30 Hz; both signals were sampled at a rate of 100 Hz by
the computerized data-acquisition system. The load cell was calibrated
with known weights before each experiment. The flowmeter was calibrated
with a graduated cylinder and clock during and after each experiment.
Muscle was averaged over every five contractions and then fit
to a smooth curve to allow precise calculation of values
corresponding to the time of venous samples, as described below.
Vascular resistance was calculated as muscle perfusion pressure
(BPm)/.
O2 of the gastrocnemius was calculated
by the Fick principle as O2 = · (CaO2 CvO2), where CaO2 CvO2 is the difference in O2 content
between arterial blood (CaO2) and venous
blood (CaO2). O2 was calculated at discrete time
intervals corresponding to the timing of the blood samples.
Statistical analyses. Values are expressed as means ± SE. To check the statistical significance of differences between two means, we performed paired Student's t-test (2-tailed). The level of significance was set at P < 0.05. Data fitting by exponential or polynomial functions was performed by the squared residuals method.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The mean weight of the gastrocnemius muscles was 75 ± 5 g.
Resting values of the main variables pertinent to O2
transport and utilization, acid-base status, and hemodynamics are shown in Table 1. Arterial Hb concentration,
arterial PO2, arterial Hb saturation,
CaO2, arterial PCO2, and
arterial pH were not significantly different between the two
conditions, thereby excluding any significant ordering effect on these
variables deriving from the sequence of experimental conditions. As
planned, and · CaO2 were higher in Fast O2 Delivery compared with Control. However,
CaO2 CvO2 was lower in Fast
O2 Delivery than in Control so that resting O2 was not significantly different
between the two conditions. BPm was not different between
the two conditions. As a consequence of the adenosine infusion, muscle
vascular resistance (BPm/) was lower in Fast
O2 Delivery compared with Control.
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Steady-state values during contractions for the main variables
pertinent to O2 transport and utilization, acid-base
status, and hemodynamics are shown for the two experimental conditions in Table 2. For variables related to
O2 transport and utilization and acid-base status, no
significant differences were observed between experimental conditions.
It is noteworthy that the O2 values were
not different between the two conditions, excluding any ordering effect
for this variable as well. BPm was slightly higher in Fast
O2 Delivery than in Control. Muscle force values (average
values calculated over 5 contractions) at the beginning of the
contraction period, after 1 min of contractions, and at the end of
contractions were, respectively, 4.85 ± 0.38, 3.67 ± 0.31, and 3.09 ± 0.15 N/100 g in Control and 4.41 ± 0.36, 3.87 ± 0.23, and 3.27 ± 0.17 N/100 g in Fast O2
Delivery. The fatigue index, calculated as the ratio between measured
force and initial force, was after 1 min of contractions 0.76 ± 0.04 (Control) and 0.89 ± 0.03 (Fast O2 Delivery)
(P < 0.05) and at the end of contractions 0.64 ± 0.03 (Control) and 0.75 ± 0.03 (Fast O2 Delivery)
(P < 0.05). A higher fatigue index value indicates
less muscle fatigue.
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Average values (± SE) of , CaO2 CvO2, and O2 at rest
and during contractions in the two experimental conditions are shown in
Fig. 2. As planned (see
METHODS), in Fast O2 Delivery was kept
substantially constant throughout the experiment at a level
corresponding to the steady-state value observed during contractions in
Control. CaO2 CvO2
on-kinetics appeared slightly faster in Control than in Fast
O2 Delivery, whereas the opposite was true for
O2 on-kinetics.
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Individual values of O2 in the two
experimental conditions are shown in Fig.
3. Because amplitudes of responses were
slightly different within each dog in the two experimental conditions, to facilitate visual comparison of time courses
O2 values were "normalized" so that
they ranged from 0 (time 0, resting values just before onset
of contractions) to 1 (end of contractions). Visual inspection of data
suggested, in most experiments, the presence of a "slow component"
of O2 kinetics (6) of
delayed onset (1-2 min into the contraction period) and
superimposed on the "earlier" O2
response ("primary component," according to commonly utilized
terminology, see, e.g., Ref. 32). The presence of a
O2 slow component was confirmed in 9 experiments out of 10 (the only exception being dog 3, Fast
O2 Delivery, see Fig. 3) by the observation of a lower sum
of squared residuals after the data were fitted with a function with
two exponential terms [one for the primary (p) and one for
the slow (s) component], compared with that obtained by
utilizing a monoexponential function. Thus to evaluate mathematically
and compare the O2 on-kinetics in the
two experimental conditions, values obtained for each experiment during
the contraction period (as well as the value corresponding to
time 0, i.e., the resting value) were fitted by a function of the type
![y(t)=a+bp·[1−e<SUP>−(<IT>t − cp</IT>)<IT>/dp</IT></SUP>]<IT>+bs·</IT>[<IT>1−e</IT><SUP> − (<IT>t − cs</IT>)<IT>/ds</IT></SUP>]](/content/vol89/issue4/fulltext/1293/img001.gif)
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(1) |
) of the
functions during the two components of the response. Parameter values
(cp, cs, dp, ds) were
determined that yielded the lowest sum of squared residuals.
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In dog 3, Fast O2 Delivery (experiment in which
no slow component was observed) O2 data
were fitted by a monoexponential function of the type
![y(t)=a+bp·[1−e<SUP>−(<IT>t − cp</IT>)<IT>/dp</IT></SUP>]](/content/vol89/issue4/fulltext/1293/img002.gif)
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(2) |
O2 had
reached an asymptote. In fact, differences between the last and the
penultimate O2 data points amounted to
0.12 ml · min1 · 100 g1 in
Control and to 0.04 ml · min1 · 100 g1 in Fast O2 Delivery, corresponding in both
cases to <1% of the total O2 response.
Amplitudes of the O2 slow component were not significantly different in Fast O2 Delivery (1.3 ± 0.5 ml · min1 · 100 g1) and
Control (1.1 ± 0.2 ml · min1 · 100 g1).
To compare the O2 on-kinetics in the two
experimental conditions, Eq. 1 or 2 was solved to
calculate the time necessary to reach 50%
(t50%, corresponding to the half-time of the overall response) and 63% [t63%,
corresponding to the "mean response time" (14) or to
the of a monoexponential response] of the differences between the
resting baselines and the steady-state values obtained during
contractions. The resulting times correspond to the points at which the
O2 response passed through 50 and 63%
of the difference between the resting baseline and steady state toward
the end of contractions (9, 10, 12). In all experiments,
t50% and t63% occurred
during the primary component of the O2
on-kinetics. Both t50% and
t63% were calculated to allow an easier
comparison with previous studies, which utilized either half-time or
(or mean response time) to describe the
O2 on-kinetics. The obtained average
values (±SE) of t50% and
t63% for the two experimental conditions are
shown in Fig.4.
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A slow component was also observed for in Control (amplitude
7.0 ± 2.4 ml · 100 g1 · min1) and for
CaO2 CvO2 in Fast O2 Delivery (amplitude 0.7 ± 0.2 ml · 100 g1 · min1).
Equation 1 was, therefore, utilized to fit these data. On
the other hand, because no slow component was observed for
CaO2 CvO2 in
Control, Eq. 2 was utilized to fit the latter data. No kinetics analysis could, of course, be performed for in Fast O2 Delivery (the variable was kept constant throughout the
test). TDs and for the primary component
(TDp and
p, respectively) were calculated for
O2, , and
CaO2 CvO2, and the obtained
values are presented in Table 3. In
Control, increased almost immediately at contraction onset
(TDp < 0.5 s) with an exponential time course
characterized by a p of ~25 s, whereas, for
CaO2 CvO2, a very rapid
exponential increase, characterized by a p
of ~6 s, was preceded by an ~6.5 s
TDp. Also for
O2, an exponential increase with a p of ~17 s was preceded by
TDp of ~6 s. The sum of
TDp and p was not
different between and O2,
whereas values for CaO2 CvO2 were significantly lower. TDp and p for
O2 were both significantly lower in Fast
O2 Delivery than in Control, showing that the faster O2 on-kinetics in the former were
attributable to a faster primary component. The
p for
CaO2 CvO2 was
significantly higher in Fast O2 Delivery than in Control.
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The "O2 deficit" for the investigated on-transition,
i.e., the time integral of the difference between the O2
requirement (considered, as a first approximation, corresponding to the
asymptotic O2 value
during the slow component) and the measured
O2, was calculated for the two
experimental conditions as the product between the overall amplitude of
the response (i.e., the O2 difference
between the resting baseline and the asymptotic value during the slow
component) and t63% (taken as a mean response time for the non-steady-state O2
changes) (32). The O2 deficit was 6.4 ± 0.6 ml/100 g in Control and 4.8 ± 0.6 ml/100 g in Fast O2
Delivery. Thus the faster O2 on-kinetics
in Fast O2 Delivery resulted in an ~30% reduction in the
O2 deficit compared with that observed in Control.
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DISCUSSION |
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|
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The main finding of the present study was that, in the isolated
dog gastrocnemius in situ, during transitions from rest to electrically
stimulated contractions corresponding to the muscle O2 peak, abolishment of delays in
convective O2 delivery to muscle resulted in significantly
faster muscle O2 on-kinetics. This
suggests that, for transitions involving contractions of high-metabolic
intensity, convective O2 delivery plays a role as a
limiting factor for O2 on-kinetics.
In previous studies conducted utilizing the same preparation but
involving transitions from rest to contractions of lower metabolic
intensity (~60% of O2 peak), our
laboratory showed that convective O2 delivery (as well as
peripheral O2 diffusion) did not play a role as limiting
factors for the O2 on-kinetics (9,
10). On the other hand, studies conducted in exercising humans
suggested that, for exercise higher than the ventilatory threshold,
enhanced O2 delivery to muscle may indeed determine faster
O2 on-kinetics, which is at variance
with observations for transitions involving exercise of lower metabolic
intensities (8, 11, 19). The results of the present study,
in conjunction with those mentioned above (8, 9, 11, 19),
suggest that the issue of O2 delivery or O2
utilization as the main limiting factor for the
O2 on-kinetics (see, e.g., Refs.
3-5, 14, 21, 28, 31) might lead to partially different
conclusions, depending on the intensity of the exercise (or
contractions) involved in the transition. More specifically, for
transitions to exercise (or contractions) of low-metabolic intensity,
the limiting factor would be represented by an inertia of muscle
oxidative metabolism, whereas for transitions to exercise (or
contractions) of high-metabolic intensity, O2 delivery
would contribute, together with the inertia of oxidative metabolism, in
determining the O2 on-kinetics. All of
this appears quite reasonable, in physiological terms, considering
that, during contractions close to or at
O2 peak, the mechanisms responsible for
convective O2 delivery to muscle [which is known to be the
main limiting factor for maximum O2 (see, e.g., Ref. 30)] must be stressed to a greater extent than during
contractions of lower metabolic intensity.
The O2 on-kinetics in Fast
O2 Delivery [i.e., after eliminating possible restraints
by convective O2 delivery and presumably also by
intramuscular /O2 maldistribution
(see below)] was faster than in Control, but the adjustment to the new
steady state still followed a time course characterized by a half-time
of ~12.5 s. As shown in Fig. 2, a complete dissociation of the
independent variable, i.e., of O2 delivery to muscle (as
represented by ), in the two experimental conditions, determined
only a relatively minor change in the dependent variable, i.e., of
O2.
CaO2 CvO2
was slightly slower to adjust to the new steady state in Fast
O2 Delivery than in Control, suggesting an intrinsic
limitation of the rate at which muscle can utilize the delivered
O2. An inertia in the adjustment of skeletal muscle
oxidative phosphorylation to the new metabolic level could be
determined by levels of cellular metabolic controllers (see, e.g.,
Refs. 4, 21, 31) and/or enzyme activation (see, e.g., Ref. 27). Other
factors potentially responsible could be the following. 1)
There could be a limitation in O2 delivery to muscle fibers
related to peripheral O2 diffusion. The latter has been
shown not to be a limiting factor for O2 kinetics during transitions from rest to 60% of
O2 peak (10), but the
situation could be different during transitions from rest to
contractions of higher metabolic intensity, because it is known that
peripheral O2 diffusion represents one of the limiting
factors for maximum O2 (see, e.g., Ref.
30). 2) There could be methodological factors related to the
fact that O2 was necessarily determined
across the muscle and not inside of it, where gas exchange occurs. Van
Beek and Westerhof (29) applied mathematical methods to
their isolated rabbit heart experimental model to account for
O2 diffusive and vascular transport delays in the
interpretation of the observed venous O2 concentration transients during step changes in heart rate. The resulting
"mitochondrial" O2 response time was
significantly faster than that of O2 determined across the whole heart (29). 3)
There could be utilization of myoglobin (Mb) O2 stores.
O2 stored with Mb could indeed contribute to oxidative
metabolism early during the transition (7), and the
resulting O2 would not of course be
detected by our measurements, which were carried out across the muscle.
However, the contributions of Mb O2 stores, with respect to
the measured O2 across muscle, should be
small. Assuming a Mb concentration of ~7 g/kg of dog muscle
(22), Mb O2 stores in 80 g of muscle
would be ~0.7 ml of O2. By assuming a 50% Mb
desaturation, occurring "early" (first minute) during the
contraction period (25), this would correspond to a
O2 of only ~0.3-0.4
ml · min1 · 100 g1.
It is well known that there is spatial heterogeneity of within
active muscle (23), but at present it is not known whether this corresponds to O2 heterogeneity.
/O2 maldistribution within muscle
could, in theory, influence the O2
on-kinetics by determining areas of tissue anaerobiosis. In our
experimental model, all muscle fibers were synchronously activated by
electrical stimulation so that O2
heterogeneity was presumably absent or significantly reduced compared
with the situation of asynchronous and heterogeneous fiber activation
in physiologically contracting muscle. This aspect represents an
intrinsic limitation of the present experimental model. In Fast
O2 Delivery, this absent or reduced
O2 heterogeneity was associated with
high and with vasodilation induced by adenosine. Thus, if any
/O2 maldistribution was present
in Control, it must have been significantly reduced or eliminated in
Fast O2 Delivery. In theory, this could be one factor
causing the faster O2 on-kinetics
observed in Fast O2 Delivery.
Analysis of TDp and p
for O2, , and
CaO2 CvO2 obtained in the
present study shows interesting similarities with homologous data
obtained by Grassi et al. (12) in exercising legs in
humans. Both in the present study (Control condition) and in that
study, indeed, increased immediately at the contraction's onset, following a substantially exponential time course,
whereas CaO2 CvO2 showed a "biphasic" time course in which a TD
of several seconds was followed by a very rapid exponential increase to
the new steady state. As mentioned before, in the present study this exponential increase was slower in the presence of an excess of delivered O2 (i.e., in Fast O2 Delivery).
In our laboratory's previous paper (9), we observed, in
the Control condition, a faster on-kinetics compared with
O2 on-kinetics. In the present study,
TD + of on-kinetics and O2 on-kinetics was substantially the
same. This observation should provide further (although indirect)
support for the concept that, for the type of transition investigated
in the present study, convective O2 delivery to muscle is
one of the limiting factors for the O2
on-kinetics.
TD + for the primary component of the
O2 on-kinetics in Control (~23 s), in
the present study, appears remarkably similar to the
t63% O2
on-kinetics value observed in our laboratory's previous work
(9) dealing with transitions to ~60% of
O2 peak (in which no slow component was
observed), thus suggesting constancy of the primary component
of the O2 on-kinetics in the isolated dog gastrocnemis in situ at different metabolic outputs.
In 9 out of 10 experiments (see Fig. 3), a slow component of the
O2 response was observed. To our
knowledge, this is the first time that such component was observed in
this preparation. Possible mechanistic bases for the
O2 slow component, its physiological significance, and its consequences for exercise capacity were discussed
in several recent studies and reviews (see, e.g., Ref. 6). As discussed
above, in the present study the faster
O2 on-kinetics observed in Fast
O2 Delivery was essentially due to a faster "primary"
(32) response.
Faster O2 on-kinetics in Fast
O2 Delivery resulted in an ~30% reduction in the
calculated O2 deficit compared with Control. A faster
adjustment of skeletal muscle oxidative metabolism during increases in
metabolic demand reduces the need for substrate level phosphorylation,
with lesser disturbance of cellular and organ homeostasis (lower
degradation of phosphocreatine and glycogen stores, lower accumulation
of lactate and H+) and obvious implications for work
performance and muscle fatigue. In the present study, muscle fatigue
was indeed less in Fast O2 Delivery than in Control. In
theory, higher force production (less muscle fatigue) in Fast
O2 Delivery could be associated with a higher ATP demand
and, therefore, with higher O2, thereby
possibly influencing the comparison of
O2 kinetics in the two experimental conditions. Against this possibility is the observation of very similar
O2 values at steady state during
contractions in the two conditions. According to Gaesser and Poole
(6), complexities associated with the
O2 slow component may confound accurate calculation and interpretation of the O2 deficit. Critical
for such calculation is the assumption that the O2
requirement for exercise is known. In the present study, as a first
approximation, we assumed that the O2 requirement for
contractions corresponded to the asymptotic
O2 value at the end of the contraction
period. This assumption cannot be directly tested. However, because the amplitudes of both the primary and the slow components of the O2 response were not different in the
two conditions, and because we were mainly interested in comparing the
O2 deficit between the two conditions, we believe that such
comparison was substantially correct.
Advantages and limitations of the present preparation (isolated dog
gastrocnemius in situ) for the study of
O2 on-kinetics were discussed at length
in our laboratory's previous studies (9, 10). In short,
the main disadvantages are represented by the unphysiological
contraction pattern (synchronous tetanic contractions), by the
intrinsic invasiveness of the preparation, and by the problem of
extrapolating the results to exercising humans. Whereas caution is
warranted in this respect, it must be noted that 1) basic
mechanisms of regulation of oxidative phosphorylation appear the same
across mammalian species (2), and 2) the
patterns of and O2 increase at
contraction onset appear remarkably similar in canine [as shown by the
present and by previous studies (see, e.g., Ref. 9)] and in human
muscles (see, e.g., Refs. 12, 15), with the only difference being
faster kinetics in dogs, presumably as a consequence of the higher
percentage of oxidative fibers (20). On the other hand,
the experimental model offers the obvious advantages of allowing the
manipulation (and a direct measurement) of O2 delivery to
the contracting muscle, as well as the measurement of
O2 directly across the muscle. Resting
to muscle was higher, and O2 extraction was lower, than the values usually observed in skeletal muscle (see, e.g., Ref.
18). The elevated resting is likely attributable to some loss
of vascular tone related to the surgical denervation of the muscle
(18). It must be noted, however, that the proposed mechanisms determining and regulating the increase in muscle at
the onset of contractions (muscle pump, "myogenic" control, "propagated vasodilation," endothelium-derived vasodilation,
increased concentration of metabolites in the interstitium)
(18) would be unaffected by the surgical denervation of
the muscle. As pointed out above, indeed, the time course of
increase during the transition, as described in the present study,
appears remarkably similar to that observed in human studies (see,
e.g., Refs. 12, 15, 24). In strict terms, however, it cannot be
excluded that the magnitude of increase in the
O2 on-kinetics observed in Fast O2 Delivery (vs. Control) in the present study might have
been even greater if muscles had not been relatively hyperperfused in
Control. Caution in the extrapolation of the present data to exercising
humans should derive also from the fact that, in our Control condition,
the relative hyperperfusion of muscle and the likely reduction
of O2/ heterogeneity might cause
higher intracellular PO2 values at rest and at
the onset of contractions, compared with that observed in humans.
Although we employed stimulation parameters that have been associated
with the highest O2 values ever reported
for this muscle preparation (1, 16), our
O2 peak values were considerably lower
than those reported by those investigators (1, 16). One
possible reason for this result is the requirement of contralateral
perfusion through tubing and a pump in the present study, as well as in
those performed over the years by the San Diego group (see, e.g., Ref.
13). Therefore, it remains possible that the conclusions of the present
study are peculiar to these experimental conditions.
In conclusion, in previous studies our laboratory showed that, in the
isolated dog gastrocnemius in situ, during transitions from rest to
~60% of O2 peak, convective
O2 delivery and peripheral O2 diffusion do not
represent limiting factors for the O2
on-kinetics, which, therefore, appears mainly set by an inertia of
muscle oxidative metabolism (9, 10). In the present study,
we showed that the situation was different during transitions to higher
metabolic intensity, i.e., from rest to the muscle
O2 peak. In this case, indeed,
elimination of all delays in convective O2 delivery to
muscle during the transition resulted in faster
O2 on-kinetics, in a lower
O2 deficit, and in less muscle fatigue. This demonstrates
that, during transitions to contractions at
O2 peak, convective O2
delivery contributes, together with an inertia of oxidative metabolism,
in determining the O2 on-kinetics. The
limiting factors for muscle O2
on-kinetics are at least in part different, depending on the intensity
of the metabolic transition.
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ACKNOWLEDGEMENTS |
|---|
This study was supported by National Institute of Arthritis and Musculoskeletal and Skin Diseases Grants 1RO1-AR-40342 and AR-40155, by NATO Collaborative Research Grant 972111, and by Telethon-Italy Grant 1161C.
| |
FOOTNOTES |
|---|
Address for reprint requests and other correspondence: B. Grassi, ITBA-CNR, Palazzo LITA, Via Fratelli Cervi 93, I-20090 Segrate (MI), Italy (E-mail: grassi{at}itba.mi.cnr.it).
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.
Received 27 August 1999; accepted in final form 1 May 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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