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O2 on-kinetics
in isolated in situ canine muscle
Department of Medicine, University of California, San Diego, La Jolla, California 92093-0623; Istituto di Tecnologie Biomediche Avanzate, Consiglio Nazionale delle Ricerche, I-20090 Segrate (MI), Italy; and 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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To test the
hypothesis that muscle O2 uptake
(
O2) on-kinetics is
limited, at least in part, by peripheral
O2 diffusion, we determined the
O2 on-kinetics in
1) normoxia (Control);
2) hyperoxic gas breathing
(Hyperoxia); and 3) hyperoxia and
the administration of a drug (RSR-13, Allos Therapeutics), which
right-shifts the Hb-O2
dissociation curve (Hyperoxia+RSR-13). The study was conducted in
isolated canine gastrocnemius muscles
(n = 5) during transitions from rest
to 3 min of electrically stimulated isometric tetanic contractions
(200-ms trains, 50 Hz; 1 contraction/2 s; 60-70% peak
O2). In all conditions,
before and during contractions, muscle was pump perfused with
constantly elevated blood flow (
), at a level
measured at steady state during contractions in preliminary trials with
spontaneous . Adenosine was infused
intra-arterially to prevent inordinate pressure increases with the
elevated . was measured
continuously, arterial and popliteal venous
O2 concentrations were determined
at rest and at 5- to 7-s intervals during contractions, and
O2 was calculated as
· arteriovenous O2 content difference.
PO2 at 50%
HbO2
saturation (P50) was calculated.
Mean capillary PO2
(
cO2)
was estimated by numerical integration.
P50 was higher in Hyperoxia+RSR-13
[40 ± 1 (SE) Torr] than in Control and in Hyperoxia (31 ± 1 Torr). After 15 s of contractions,
cO2
was higher in Hyperoxia (97 ± 9 Torr) vs. Control (53 ± 3 Torr) and in Hyperoxia+RSR-13 (197 ± 39 Torr) vs. Hyperoxia. The
time to reach 63% of the difference between baseline and steady-state
O2 during contractions was 24.7 ± 2.7 s in Control, 26.3 ± 0.8 s in Hyperoxia, and 24.7 ± 1.1 s in Hyperoxia+RSR-13 (not significant). Enhancement of
peripheral O2 diffusion (obtained
by increased
cO2
at constant O2 delivery) during
the rest-to-contraction (60-70% of peak
O2) transition did not
affect muscle O2
on-kinetics.
gas exchange kinetics; submaximal exercise; muscle oxidative metabolism
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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WE HAVE PREVIOUSLY SHOWN, in the isolated in situ
canine gastrocnemius preparation, that the elimination of any delay in
the rate of adjustment of convective
O2 delivery to muscle
microcirculation in the rest-to-contraction transition, obtained by
pump perfusing the muscle with elevated blood flows
() from the last seconds of rest, did not
significantly accelerate muscle oxygen uptake (O2) on-kinetics (12). Such a
finding represents evidence in favor of the hypothesis that
O2 on-kinetics is not
governed by the rate of adjustment of convective
O2 delivery to the exercising muscle. However, a determinant of
O2 on-kinetics could also be
the finite delivery of O2 to
mitochondria by diffusion, a process that has been shown to contribute
to the limitation of maximal O2
(O2 max) (32). The
hypothesis that peripheral (capillary-to-mitochondria) diffusion of
O2 may be an important determinant
of O2 on-kinetics has not
been directly tested.
We therefore conducted the present study, in which the rate of
peripheral O2 diffusion was
enhanced by increasing the "driving pressure" for
O2 from muscle capillaries to
mitochondria. Such enhancement was obtained by having dogs breathe
hyperoxic gas (100% O2) and by
their hyperoxic gas breathing accompanied by the administration of a
drug,
2-(4-{[(3,5-dimethylanilino)carbonyl]methyl}phenoxy)-2-methylpropionic acid (known as RSR-13; Allos Therapeutics). This agent is an allosteric inhibitor of O2-hemoglobin binding
and has been shown to induce a significant rightward shift of the
Hb-O2 dissociation curve (ODC) (1)
and improve
O2 max (29).
We hypothesized that, if peripheral
O2 diffusion was indeed limiting
O2 on-kinetics, then the
increase in the driving pressure for
O2 from capillaries to
mitochondria would determine, in the presence of constant
convective O2 transport, a
faster O2 on-kinetics.
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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 animal subjects committee of the University of California, San Diego, where the experiments were carried out.
Five adult mongrel dogs of either sex 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). The esophageal temperature was maintained at ~37°C with a heating pad and a heating lamp. After surgical preparation was completed, the animals were treated with heparin (1,500 U/kg). Ventilation was maintained at a level that produced normal arterial PO2 and PCO2 values. PO2 and PCO2 values were also continuously monitored in inspired and expired air by a mass spectrometer (MGA 1100, Perkin-Elmer) and recorded on a strip-chart recorder.
Surgical preparation. The gastrocnemius-flexor digitorum superficialis muscle complex (for convenience referred to as "gastrocnemius") was isolated as described previously (31). 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 the vessels draining into the popliteal vein, except those from the gastrocnemius, were ligated. The popliteal vein was cannulated, and the venous outflow was returned to the animal via a jugular reservoir. The arterial circulation to the gastrocnemius was isolated by ligating all vessels from the femoral and popliteal artery that did not enter the gastrocnemius. The right femoral artery was catheterized for obtaining blood samples. This catheter was extended and placed into the left femoral artery so that the isolated muscle was perfused by blood from this contralateral artery. The arterial blood perfusing the muscle could then come directly from the contralateral artery (systemic pressure, self-perfused) or via a roller pump for controlled-flow perfusion.
The left sciatic nerve, which innervates the gastrocnemius, was doubly ligated and cut between the ties. All exposed tissues were covered with saline-soaked gauze and a plastic sheet to minimize cooling and drying. After the muscle was surgically isolated, the Achilles tendon was attached to an isometric myograph (Statham 1360 transducer) for monitoring tension development. The hindlimb was fixed at the knee and ankle and attached to the myograph with struts to minimize movement. Nails were placed into the femur and tibia bones and attached to a fixed platform next to the leg. A strut was inserted between the force transducer and the bone nail in the femur to further minimize movement. Weights were used at the end of each experiment to calibrate the myograph.Experimental design.
Isometric tetanic muscle contractions were elicited by supramaximal
stimulation of the sciatic nerve with trains of stimuli (4-6 V of
0.2-ms duration at 50 Hz) lasting 200 ms, at a rate of 1 contraction/2
s for a 3-min period. Before each contraction period the resting muscle
was passively stretched to the point at which the highest peak tension
was elicited on stimulation. Preliminary studies showed that this
stimulation pattern elicited 60-70% of the peak metabolic rate
(peak O2) for tetanic
contractions in normoxia in this muscle. Contractions corresponding to
60-70% of peak O2
were chosen to avoid confounding factors deriving from significant
fatiguing of muscles. 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 cycle. For
the purposes of the study, it was indeed 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 contraction by 1.8 s, during which
the muscle was relaxing or relaxed.
, adjusted
~15-30 s before the start of contractions to a level
corresponding to the steady-state value obtained during contractions in
a preliminary trial with spontaneous adjustment of
(Control); 2) same as
1), but the dog inspired from a bag
containing 100% O2 (Hyperoxia);
and 3) same as
2), but with the intra-arterial
administration, over ~15 min before the contraction period, of ~100
ml of half-normal saline solution containing 100 mg/kg body weight of
the sodium salt of the drug 2-(4-{[(3,5-dimethylanilino)carbonyl]methyl}phenoxy)-2-methylpropionic acid (RSR-13, Allos Therapeutics), an allosteric inhibitor of O2 binding to hemoglobin
(Hyperoxia+RSR-13). This drug, administered at the same dosage and in
the same experimental preparation as those in the present study, has
been previously shown to induce a significant rightward shift of the
ODC and an increase in
O2 max (29). The
order of experimental conditions 1 and
2 was randomized, whereas experimental
condition 3 always had to be performed
last because of the duration of RSR-13 effects. In all experimental conditions, to prevent vasoconstriction and inordinate pressure increases with the elevated , 1-2 ml/min of a
10
2 M adenosine solution
(in normal saline) was infused intra-arterially by a pump, beginning
from 20-30 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 obtaining a
significant vasodilation at the muscle level (11) without causing
significant metabolic effects (such as changes in resting
O2,
O2 at the same submaximal
level of contraction, O2 max, and acid-base
status) (12, 16). Elevated and adenosine infusion
prevented the peripheral vasoconstriction and the reduced
described after the administration of other
allosteric inhibitors of O2-Hb
binding (17). Muscle perfusion pressure and muscle vascular resistance
(see Measurements) were
indeed not significantly different in Hyperoxia+RSR-13 compared with the other conditions.
At the end of the experiment, the dogs were killed with an overdose of
pentobarbital sodium. The contralateral gastrocnemius was excised and
weighed, and the weight was utilized to normalize variables to muscle
mass as appropriate.
Measurements.
to the gastrocnemius was continuously measured in
the popliteal vein by an ultrasonic flowmeter (T108, Transonic
Systems). The output of the flowmeter was set in the "pulsatile"
mode, and the filter cutoff frequency was set at 100 Hz. The flowmeter
probe (in-line type) was inserted in the vein as close as possible to the gastrocnemius. The flowmeter probe was calibrated before each experiment, and the calibration was checked against zero-flow and
against timed collection of blood in a graduated cylinder at different
. Values were sampled at 50 Hz by an
analog-to-digital converter and stored on disk via a computerized
data-acquisition system (AcqNowledge 3.0, Biopac Systems). Average
values of were then calculated during discrete 3-s
time intervals. Arterial perfusion pressure of the gastrocnemius
(BPmus) was monitored continuously by a pressure transducer (Gould Statham P23 ID) in a
catheter placed at the head of the muscle and recorded on a strip-chart
recorder. Vascular resistance was calculated as
BPmus/. Systemic arterial blood pressure was monitored continuously by a
pressure transducer (Gould Statham P23 ID) in a catheter placed in the
carotid artery and recorded on a strip-chart recorder.
.
Bubble-free blood samples were immediately stored in ice and analyzed
within 10-60 min of collection. Arterial and venous PO2,
PCO2, and pH were measured at
37°C by using a blood-gas analyzer (IL 1306, Instrumentation
Laboratories). Arterial and venous hemoglobin concentration
([Hb]), percentage Hb saturation with
O2
(%HbO2),
and O2 concentration
(CaO2 and CvO2, respectively) were measured by a
CO-oximeter (IL 282, Instrumentation Laboratories). These instruments
were calibrated before and during each experiment. Dissolved
O2 was accounted for.
Calculations.
Plasma bicarbonate concentration
([HCO3]) was calculated
from the measured pH and PCO2 values
by the Henderson-Hasselbalch equation.
O2 of the gastrocnemius was
calculated from the Fick principle as
O2 = · arteriovenous O2 content difference
(CaO2 
CvO2).
O2 was calculated at the discrete time intervals corresponding to the timing of the blood samples. Arterial and venous values of
PO2 and
%HbO2 were
utilized (after correction for temperature,
PCO2, and pH) to plot the ODC for
Control and Hyperoxia (no RSR-13 administered) and for
Hyperoxia+RSR-13. PO2 at which 50%
Hb is saturated (P50) was then
calculated by using the Hill equation.
cO2)
after ~15 s of contractions (i.e., at a critical timing in the
analysis of O2 on-kinetics) and at steady state during contractions, by the use of Fick's law of
diffusion as a simple model of capillary exchange, as discussed in more
detail elsewhere (14, 30). The important assumptions of this
calculation are as follows: 1)
muscle O2 diffusive conductance (DmusO2) is
constant at each point along the capillary;
2) all the residual
O2 in muscle venous blood is
explained by diffusion limitation of
O2 transport, on the assumption
that arteriovenous O2 shunts are
negligible and
/O2
distribution is relatively homogeneous. The numerical process
iteratively determines that value of
DmusO2 that
produces the measured PvO2. By the use
of the associated PO2 profile,
cO2
can be calculated as an average of all
PO2 values from the arterial to the venous end of the capillary.
cO2
is an estimate for the mean driving pressure for
O2 diffusion in those conditions.
The present numerical integration technique was originally utilized for
maximal exercises, during which mitochondrial
PO2
(PmitO2) was considered to be sufficiently close to zero to be neglected. During submaximal contractions, however,
PmitO2 could be
higher. Gayeski et al. (9) obtained in dog gracilis muscle
intracellular PO2 values ranging
(median values) from ~13 Torr after 15 s of contractions to ~10
Torr after 180 s of contractions, corresponding to ~70% of
O2 max (i.e., the
same metabolic intensity of the present study). Recent work by
Richardson et al. (28), however, suggests that in the human quadriceps
muscle intracellular PO2 is very low
(<5 Torr) even during submaximal exercise. The same authors (28) also
showed that the low PO2 levels are reached within 20 s after the beginning of exercise. In the present study the numerical integration process was carried out by assuming PmitO2 values
of 0, 5, and 15 Torr to examine the validity of the assumptions. With
the different
PmitO2 values,
however,
cO2 values varied by only 1-4 Torr, because they are primarily
determined by the measured values of
PaO2 and
PvO2. Such a systematic error would
correspond to only ~6 and 2% of the
cO2
changes induced, respectively, by Hyperoxia and Hyperoxia+RSR-13
compared with Control (after 15 s of contractions) (see
RESULTS). All
cO2
values reported herein were obtained by assuming a
PmitO2 of
5 Torr.
Statistical analyses. Values were expressed as means ± SE. To determine the statistical significance of differences between two means, a paired Student's t-test (2-tailed) was performed. To determine the statistical significance of differences among more than two means, a repeated-measures analysis of variance was performed. A Tukey's post hoc test was utilized to discriminate where significant differences occurred. The level of significance was set at P < 0.05. Statistical analyses were performed by utilizing a commercially available software package (InStat, GraphPad Software).
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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The weight of the gastrocnemius muscles was 63 ± 5 g.
Resting values of the main variables pertinent to O2 transport and utilization, acid-base status, and hemodynamics are shown in Table 1 for the three experimental conditions. PaO2 and PvO2 were higher in Hyperoxia and in Hyperoxia+RSR-13 than in Control, as a consequence of hyperoxic breathing. A greater volume of dissolved O2 was responsible for the slightly higher CaO2 observed in Hyperoxia compared with Control. All other variables were not significantly different among the three experimental conditions, with the exception of PaCO2, which was higher in Hyperoxia+RSR-13 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, biomechanics, and hemodynamics are shown in Table 2 for the three experimental conditions. PaO2 and PvO2 were higher in Hyperoxia and in Hyperoxia+RSR-13 than in Control, as a consequence of hyperoxic breathing. Higher volumes of dissolved O2 were responsible for the slightly higher CaO2 observed in Hyperoxia and in Hyperoxia+RSR-13 compared with Control. All other variables were not significantly different among the three experimental conditions.
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RSR-13 administration caused a significant increase in P50 (31.3 ± 0.7 Torr in Control and in Hyperoxia and 40.1 ± 1.4 Torr in Hyperoxia+RSR-13), indicating a rightward shift of the ODC.
Average (±SE)
cO2
values after 15 s of contractions and at steady state during
contractions, calculated by the numerical integration technique, are
shown in Fig. 1 for the three experimental conditions. At both time points,
cO2
values were significantly higher in Hyperoxia and in Hyperoxia+RSR-13
than in Normoxia, and they were also significantly higher in
Hyperoxia+RSR-13 than in Hyperoxia.
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Average values (±SE) of ,
· CaO2,
CaO2 CvO2, and
O2 at rest and during
contractions are shown in Fig. 2 for the
three experimental conditions. and
· CaO2
were kept constant throughout the experiment at a level corresponding
to the steady-state value observed during contractions in preliminary
trials with spontaneous adjustment of (see
METHODS), thereby eliminating any
delay in convective O2 delivery to
muscle in the rest-to-contractions transition. Despite the marked
increases in the driving pressures for
O2 peripheral diffusion, in
Hyperoxia vs. Control, and, even more markedly so, in Hyperoxia+RSR-13
vs. Control, the kinetics of CaO2 CvO2 and
O2 (i.e., the variables
related to O2 utilization by
muscle) appeared remarkably similar in all experimental conditions.
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To evaluate mathematically and to compare the
O2 on-kinetics in the three
experimental conditions, the values obtained for each experiment during
the contraction period were fitted by a monoexponential function of the
type
![<IT>y</IT> = <IT>a</IT>+<IT>b</IT> [1 − <IT>e</IT><SUP>−(<IT>t</IT> − <IT>c</IT>)/<IT>d</IT></SUP>]](/content/vol85/issue4/fulltext/1404/img002.gif)
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(1) |
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To compare the O2 on-kinetics
in the three experimental conditions, Eq. 1 obtained with analysis
B was solved to calculate the time necessary to reach
50% (t50%,
corresponding to the half-time of the response) and 63%
[t63%,
corresponding to the time constant (
) of a monoexponential
response] of the differences between the resting baselines and
the steady-state values obtained during contractions. The times elapsed
during the first one to three
O2 points, neglected in
analysis B, were added to the times
obtained by solving the functions, so that the resulting times
corresponded to the points at which the
O2 response passed through 50 and 63% of the difference between the resting baseline and the steady
state during contractions (13). Both
t50% and
t63% were
calculated to allow an easier comparison with previous studies, which
utilized either half-time or to describe
O2 on-kinetics. The obtained
average values (±SE) of
t50% and
t63% for the
three experimental conditions are shown in Fig.
4. For both parameters, no significant differences were observed among the three conditions.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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In the present study we hypothesized that if the rate of peripheral
O2 diffusion was indeed limiting
O2 on-kinetics, any increase
in the driving pressure for O2
from capillaries to mitochondria would elicit a faster
O2 on-kinetics. We
significantly enhanced the potential for peripheral
O2 diffusion by increasing the
driving pressure for O2 from
capillaries to mitochondria. Such an increase was obtained by having
the dog breathe a hyperoxic mixture (which significantly increased
PaO2 and
PvO2 and therefore the estimated cO2)
and by administering (associated with hyperoxia) a drug (RSR-13), which
resulted in a significant rightward shift of the ODC. Indeed, for the
same O2 delivery,
O2, and therefore
CaO2 CvO2, the rightward shift of the ODC
determined a further increase in
cO2,
compared with that obtained by hyperoxia alone. The main result of the
present study was that, during the transition from rest to contractions
corresponding to 60-70% of peak
O2, enhanced peripheral O2 diffusion did not
have any significant effect on muscle
O2 on-kinetics, indicating
that the latter was not limited by
O2 diffusion from muscle
capillaries to mitochondria.
All experimental conditions utilized in the present study were
characterized by the elimination of any delay in convective O2 delivery to muscle during the
rest-to-exercise transition. This was obtained by pump perfusing the
muscle, from the last 15 s of rest and throughout the contraction
period, at levels reached at steady state during
contractions in the presence of a spontaneus adjustment of
. In a previous study we showed in our laboratory
that, with this intervention, muscle
O2 on-kinetics was not
significantly different from that obtained with a spontaneous adjustment of (12), allowing us to conclude that
convective O2 delivery to muscle
was not limiting O2
on-kinetics. Taken together, the results of the previous
study (12) and of the present one indicate that, in the isolated in
situ dog gastrocnemius preparation, during the transition from rest to
contractions corresponding to 60-70% of peak
O2, neither
convective nor diffusive O2
delivery limits muscle O2
on-kinetics.
Peripheral O2 diffusion from the interior of the red blood cell (RBC) to the inner membrane of myocyte mitochondria depends on several factors, such as the partial pressure difference between the two sites, the time available for diffusion (RBC transit time), the length of the diffusion path, and the conductance through the path. O2 conductance, in turn, depends on factors such as the solubility of the gas in plasma and tissue membranes, RBC spacing in the capillary, the off-loading kinetics of O2 from Hb, and the presence of myoglobin in the cytoplasm. In the present study, as mentioned above, we enhanced the potential for peripheral O2 diffusion by increasing the driving pressure for O2 from muscle capillaries to mitochondria. The methods utilized to obtain this increase should not negatively influence the other factors determining peripheral O2 diffusion.
Peripheral O2 diffusion has been
shown to be one of the limiting factors for
O2 max (32). In
particular,
DmusO2 has been shown to be closely related to the capillary-to-fiber number ratio, a
parameter indicating the capillary surface area per fiber (3). The
observation in the present study that peripheral
O2 diffusion does not affect
muscle O2 on-kinetics appears
then in agreement with recent work by Chilibeck et al. (7), who did not
find in humans a significant correlation between capillary-to-fiber number ratio and pulmonary
O2 on-kinetics,
although many other factors in intact humans could confound such a
conclusion.
The observation that both convective and diffusive
O2 delivery do not affect muscle
O2 on-kinetics supports the
hypothesis that the latter is mainly set by inertia of muscle oxidative
metabolism (6, 33). The essentially monoexponential increase in muscle O2 during the
rest-to-exercise transition, observed in the present study after
convective and diffusive O2
contraints were eliminated or significantly reduced, appears in
agreement with metabolic models of muscle respiratory control during
exercise (20, 24), according to which a single reaction with
first-order kinetics controls muscle
O2. Such a reaction can be
identified with ATP resynthesis, the rate of which is directly
proportional to creatine concentration, i.e., to one of the products of
phosphocreatine splitting. A monoexponential decrease in
phosphocreatine concentration during the on-transition has indeed been
described by several authors (4, 22, 25).
O2 on-kinetics could also be
influenced by intramuscular maldistribution of
/O2. It is
indeed well known that there is both spatial and temporal heterogeneity
of within active muscle (21, 27), and at present it
is not known whether this corresponds to
O2 heterogeneity. In the
present experimental model, however, all fibers in the muscle were
synchronously activated, so that the constantly elevated
, associated with adenosine administration, is likely
to have reduced
/O2
maldistribution, if any was present.
As in the previous study from our laboratory (12), a monoexponential
curve did not satisfactorily fit the first one to three O2 values (corresponding to
the first 5-10 s) of the contraction period. Indeed,
during the initial phase of contractions, the O2 increase was, in most
cases, less pronounced compared with the ensuing phase of
monoexponential increase, confirming previous observations both in
canine (2, 9) and in human muscle (13). On the other hand, an initial
delay was not described by Mahler (20) in the isolated frog sartorius
or by Piiper et al. (26) in the dog gastrocnemius. Some methodological
factors that could in part explain this initial delay were discussed in
a previous study from our laboratory (12). In short, these
factors relate to temporal distortions introduced by the fact that
O2 was necessarily determined
across the muscle, and not inside of it, where gas exchange occurs. It
must also be noted that the Fick equation to calculate
O2 may not be strictly
rigorous during non-steady-state conditions, although, with the adopted
timing of blood samples, any error should be reduced. In any case, the
observed sluggish O2 increase
during the initial phase of contractions may indicate that,
intramuscularly as well, the
O2 increase does
not follow a monoexponential pattern from the very beginning of work.
Such a finding, if confirmed by future studies specifically aimed at evaluating the initial phase of the transition, could shed some new
light on the metabolic control models mentioned above.
Methodological considerations and limitations.
Some intrinsic limitations of the experimental model and protocol were
discussed at length in a previous paper from our laboratory (12). Dog gastrocnemius is predominantly made up of slow
oxidative (type I) or fast oxidative-glycolytic (type IIa) fibers (23). In strict terms, therefore, the application of the results of the
present study should be limited to muscles characterized by a high
aerobic potential. Mainly as a consequence of the fiber pattern, in dog
muscle the contribution of the so-called "early lactate" to the
energy balance during the on-transition could be lower than that
described in humans. The metabolic transition considered in the present
study was from rest to submaximal (60-70% of peak
O2) contractions. Some
caution is therefore warranted in the extrapolation of the present
results to metabolic transitions involving exercises heavier than the
so-called "lactate threshold," in which the determinants of
muscle O2 on-kinetics might
be different, at least in part, from those involving exercises of lower
metabolic intensity (10). Useful information could perhaps have been
gained from a comparison of
O2 on- and off-kinetics. The
latter, however, was not determined because we had to reduce to a
minimum the number of blood samples to avoid inducing conditions of
acute anemia in the dogs.
cO2,
went in the right direction in allowing us to test the hypothesis of
the study, i.e., whether enhanced peripheral
O2 diffusion would influence
O2 on-kinetics. In the
present study, we were able to further test this hypothesis by
utilizing two "treatment" conditions of increasing intensity, e.g., Hyperoxia alone and Hyperoxia+RSR-13, which established further
significant increases in
cO2.
Previous authors determined
O2 on-kinetics in hyperoxia
in humans, obtaining conflicting results: indeed, according to
Linnarsson (18), O2
on-kinetics was essentially unchanged in hyperoxia, whereas, according
to others (19), hyperoxic breathing elicited a faster kinetics. A major
limitation to these studies, as far as testing peripheral
O2 diffusion as a limiting factor
for O2 on-kinetics
is concerned, was that they could not measure or make any inference on
convective O2 delivery to muscle.
It has indeed been shown that O2
extraction, as well as to skeletal muscles, is
somewhat impeded in hyperoxic dogs (8). According to other authors, the
negative effects of hyperoxia on muscle
O2 delivery and utilization are
attributable to intramuscular restriction and
maldistribution (5). Several lines of reasoning, however, allow us to
exclude the notion that the factors mentioned above significantly affected the main findings of the present study. 1) Throughout the hyperoxic
experiments, muscle was kept constantly elevated by
pump perfusing the muscle. Any vasoconstriction at the arteriolar level
was prevented by the adenosine administration. Muscle perfusion
pressure and vascular resistance were indeed not significally different
during the hyperoxic experiments compared with the normoxic Control.
This allows us to exclude any impairment in bulk
O2 delivery to muscle deriving
from hyperoxic breathing. 2)
Adenosine infusion, associated with the elevated ,
should have counteracted any effects of hyperoxia on
distribution within the muscle. In any case, even if some
maldistribution occurred, it did not affect muscle
O2 utilization, as shown by the
fact that muscle O2, for the
same intensity of contractions, was not significantly different in
hyperoxic conditions compared with the normoxic Control.
3) In any case, even if hyperoxia
had some negative effects on O2
delivery and utilization, the comparison between Hyperoxia and
Hyperoxia+RSR-13 would be "safe."
Koike et al. (15) attempted to reduce the driving pressure for
O2 from muscle capillaries to
mitochondria through a leftward shift of the ODC, obtained by
increasing the percentage of carboxyhemoglobin (%HbCO). The authors
observed slower O2
on-kinetics, in exercising humans, in the presence of increased %HbCO.
The observation of slower O2
kinetics in conditions of presumably impeded peripheral O2 diffusion, however, does not
demonstrate per se that the kinetics, in normal conditions, is limited
by peripheral O2 diffusion.
Moreover, in their protoco, Koike et al. (15) could not control for
other variables that would have gone in the direction of enhancing
peripheral O2 diffusion. For
example, muscle O2 delivery
presumably increased as a compensation for the increased HbCO. This
would go in the direction of increasing muscle capillary
PO2. Moreover, exercise with
increased HbCO was associated with increases in blood lactate levels,
and the associated lactic acidosis would shift the ODC rightward.
Conclusions.
In the present study we showed that, in the isolated in situ dog
gastrocnemius preparation, after peripheral
O2 diffusion was enhanced by
increasing the driving pressure for
O2 from muscle capillaries to
mitochondria, muscle O2
on-kinetics during the transition from rest to contractions,
corresponding to 60-70% of peak
O2, was unchanged compared
with control conditions. This indicates that peripheral
O2 diffusion was not limiting the kinetics. In a previous study (12) from our laboratory, it was shown
that, for the same work range, convective
O2 delivery to muscle was not
limiting muscle O2
on-kinetics as well. Taken together, the results of these
two studies indicate that, in this preparation, during the transition
from rest to contractions, corresponding to 60-70% of peak
O2, neither convective nor
diffusive O2 delivery affects
muscle O2 on-kinetics,
supporting the hypothesis that the latter is mainly set by an intrinsic
inertia of muscle oxidative metabolism.
| |
ACKNOWLEDGEMENTS |
|---|
The authors are grateful to Nick Busan and Jeffrey Struthers for skillful technical assistance.
| |
FOOTNOTES |
|---|
This study was supported by National Institutes of Health Grants HL-17731, AR-40155, and 1R01-AR-40342; an American Heart Association, Kentucky Affiliate Grant; and by North Atlantic Treaty Organization Collaborative Research Grant 972111. Partial support by Allos Therapeutics, Inc., is acknowledged.
Address for reprint requests: B. Grassi, ITBA, CNR, Palazzo LITA, Via Fratelli Cervi 93; I-20090 Segrate (MI), Italy (E-mail: grassi{at}itba.mi.cnr.it).
Received 24 October 1997; accepted in final form 1 June 1998.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
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) at
rest and during contractions in 3 experimental conditions.
A: dogs
1-3 (left to
right, respectively).
B: dogs
4 and 5 (left and
right, respectively). Monoexponential
functions (solid line) that yielded lowest average values of squared
residuals (analysis B) are also
shown. Vertical hatched lines, onset of contractions. See text for
further details.
