Department of Medicine, McMaster University Medical Centre, Hamilton, Ontario, Canada L8N 3Z5
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ABSTRACT |
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The purpose of the study was to examine the roles of
active pyruvate dehydrogenase
(PDHa), glycogen phosphorylase
(Phos), and their regulators in lactate
(Lac) metabolism during
incremental exercise after ingestion of 0.3 g/kg of either
NaHCO3 [metabolic alkalosis
(ALK)] or CaCO3
[control (CON)]. Subjects
(n = 8) were studied at rest, rest
postingestion, and during constant rate cycling at three stages (15 min
each): 30, 60, 75% of maximal O2
uptake
(
O2 max).
Radial artery and femoral venous blood samples, leg blood flow, and
biopsies of the vastus lateralis were obtained during each power
output. ALK resulted in significantly
(P < 0.05) higher intramuscular
Lac
concentration
([Lac
]; ALK
72.8 vs. CON 65.2 mmol/kg dry wt), arterial whole blood [Lac
] (ALK 8.7 vs. CON 7.0 mmol/l), and leg
Lac
efflux (ALK 10.0 vs.
CON 4.2 mmol/min) at 75%
O2 max. The increased intramuscular
[Lac
] resulted
from increased pyruvate production due to stimulation of glycogenolysis
at the level of Phos a and
phosphofructokinase due to allosteric regulation mediated by increased
free ADP (ADPf), free AMP
(AMPf), and free Pi concentrations.
PDHa increased with ALK at 60%
O2 max but was
similar to CON at 75%
O2 max. The increased
PDHa may have resulted from
alterations in the acetyl-CoA, ADPf, pyruvate, NADH, and
H+ concentrations leading to a
lower relative activity of PDH kinase, whereas the similar values at
75%
O2 max may have
reflected maximal activation. The results demonstrate that imposed
metabolic alkalosis in skeletal muscle results in acceleration of
glycogenolysis at the level of Phos relative to maximal PDH
activation, resulting in a mismatch between the rates of pyruvate
production and oxidation resulting in an increase in
Lac
production.
glycogen phosphorylase; pyruvate dehydrogenase; lactate metabolism
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INTRODUCTION |
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INDUCED METABOLIC ALKALOSIS by sodium bicarbonate
(NaHCO3) ingestion in humans has previously been shown to
increase blood Lac
concentration
([Lac
]) during
exercise (10, 33, 37, 46, 63). Unfortunately, the majority of studies
exploring the effects of metabolic alkalosis on
Lac
metabolism have
focussed on the possible performance-enhancing capabilities of
"bicarbonate loading" (31). Ingestion of NaHCO3 is
thought to enhance performance by buffering the lactic acid produced
with exercise, thereby limiting the effects of the decreased intramuscular pH (pHi; see Ref.
31). Metabolic alkalosis through ingestion or infusion of
NaHCO3 has been shown to enhance
performance for short-duration, high-intensity exercise, but the
mechanisms have not been elucidated. Different mechanisms have been
postulated to explain this, including an increase in muscle
Lac
production (10, 63)
and/or enhanced Lac
efflux
from the muscle (44). Lactate accumulation results from the conversion
of nonoxidized pyruvate to
Lac
by lactate
dehydrogenase (LDH) and as such will be influenced by both
pyruvate production from glycogen via glycogen phosphorylase (Phos) and
pyruvate oxidation by pyruvate dehydrogenase (PDH; see Refs. 18, 38).
In an effort to discern the possible mechanisms responsible for the
increased blood
[Lac] with
metabolic alkalosis, we chose an oral dose of
NaHCO3, previously shown to induce
a significant metabolic alkalosis, to influence plasma
[Lac
], and to
enhance performance (37). Continuous, dynamic constant-rate exercise at
low, moderate, and high intensity was chosen to follow the metabolic
effects, compare fuel utilization with previously described
carbohydrate (CHO) and free fatty acid (FFA) contributions at these
power outputs (50), and to maintain the ATP turnover rate
constant between conditions. This is the first human in vivo study to
examine the key regulatory enzymes, their controllers, and fuel
utilization during continuous dynamic constant rate exercise under
alkalotic conditions.
The aim of the present study was not to examine the performance effect of an induced metabolic alkalosis during exercise. Rather, the first aim was to determine the effect of metabolic alkalosis on the key regulatory enzymes Phos and PDH and their allosteric regulators. The second aim was to measure the effect of metabolic alkalosis on glycolytic intermediates, muscle pyruvate production, and pyruvate oxidation. The third aim was to measure muscle lactate accumulation, production, and efflux. The last aim was to determine if alkalosis has any effects on glucose uptake and FFA utilization during exercise.
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METHODS |
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Subjects
Eight healthy male volunteers participated in the study [age 23 ± 1.8 (SE) yr; height 173 ± 3.8 cm; weight 75.3 ± 4.4 kg]. Written consent was obtained from each subject after explanation of the purposes and associated risks of the study protocol. The study was approved by the Ethics Committees of both McMaster University and McMaster University Medical Center.Preexperimental Protocol
All subjects completed an initial incremental maximal exercise test on a cycle ergometer to determineExperimental Protocol
Each subject participated in two experimental trials separated by 2-3 wk and was randomized to receive capsules containing either 0.3 g/kg of NaHCO3 [metabolic alkalosis (ALK)] or 0.3 g/kg of CaCO3 [control (CON)]. On the morning of each trial, the subjects reported to the laboratory after consumption of a standard light meal consisting primarily of CHO. The exercise portion of the protocol consisted of three levels of continuous, constant-rate exercise on a cycle ergometer at 30, 60, and 75% of
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A radial artery was catheterized with a Teflon catheter (20 gauge, 3.2 cm; Baxter, Irvine, CA) percutaneously after anesthetizing the area with 0.5 ml of 2% lidocaine without epinephrine (6). A femoral vein was catheterized percutaneously for insertion of the thermodilution catheter (model 93-135-6F; Baxter) using the Seldinger technique (6) after administration of 3-4 ml of lidocaine without epinephrine. Both the arterial and femoral venous catheters were maintained patent with sterile, nonheparinized, isotonic saline solution. Arterial and femoral venous blood samples were simultaneously taken at rest, rest postingestion, and during each of the three exercise bouts at 6 and 11 min. Single leg blood flow measurements were made after blood sampling at the same time points. Single leg blood flow was determined using the thermodilution technique, as described by Andersen and Saltin (1). Nonheparinized isotonic saline (10 ml) was injected, and leg blood flow was calculated by a portable CO monitor (Spacelab, Redmond, VA). At least three measurements were recorded at each time point and then averaged.
A total of five percutaneous needle biopsies of the vastus lateralis
were taken (1 at rest, 1 at rest postingestion, and 3 during exercise
at the end of each power output). The resting biopsies were obtained
with the subject lying on a bed. The resting and exercise biopsies were
obtained on opposite legs and then were reversed for the second trial.
Biopsy sites were prepared by making an incision through the deep
fascia under local anesthetic (2% lidocaine without epinephrine), as
described by Bergström et al. (5). Respiratory measurements of
ventilation (E), O2 uptake
(
O2),
CO2 production
(
CO2), and respiratory
exchange ratio (RER) were measured at 5 and 11 min of each exercise stage.
Muscle Analysis
Muscle samples were immediately frozen in liquid N2. A small piece (10-35 mg) was chipped from each biopsy (under liquid N2) for determination of the fraction of PDH in the active form (PDHa), as previously described (20, 51). The remainder of the sample was freeze-dried, dissected free of blood and connective tissue, and powdered. One aliquot was analyzed for Phos activity according to the methods of Young et al. (72). Briefly, a 3- to 4-mg sample of muscle was homogenized at
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Blood Sampling and Analysis
Arterial and femoral venous blood samples (~10 ml) were collected in heparinized plastic syringes and placed on ice. One portion (1-2 ml) of each blood sample was analyzed for blood gas determination (AVL 995 Automatic Blood Gas Analyzer), O2 and CO2 content (Cameron Instrument, Port Arkansas, TX), and hemoglobin (OSM3 Hemoximeter; Radiometer, Copenhagen, Denmark). A second portion of each sample was deproteinized with 6% PCA and stored atLeg Uptake and Release of Metabolites,
O2, and
CO2
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Subjects exercised at a constant rate, and because no significant differences occurred in blood flows or metabolite concentrations between the 6- and the 11-min sampling points at each power output, the two values were averaged to obtain one value for each power output. Reported values are for the single leg only.
Calculations
Flux through Phos and therefore glycogenolysis was calculated from the differences in glycogen utilization divided by time. PDHa flux was estimated from the PDHa as measured in wet tissue and converted to dry tissue using the wet-to-dry ratio. Pyruvate production was calculated from the sum of the rates of glycogen breakdown and glucose uptake minus the sum of the rates of accumulation of muscle glucose, G-6-P, and F-6-P. LacIntramuscular pH.
Intramuscular pH (pHi) was calculated from the
[Lac] and
[pyruvate] according to the methods of Sahlin et al. (55).
Lac gradient and
[H+]
gradient.
The Lac
gradient between
the plasma and muscle for both trials was calculated for arterial and
femoral venous blood at 75%
O2 max only. The
Lac
gradient between the
muscle and arterial plasma was calculated as the difference between the
wet weight intramuscular
[Lac
] and
arterial plasma
[Lac
]. The
Lac
gradient from
muscle-to-femoral venous plasma was calculated as above using the
noncorrected venous values. The arterial-to-muscle and femoral
venous-to-muscle [H+]
gradients were calculated as the difference between the respective blood compartment [H+]
and the calculated intramuscular
[H+].
Statistical Analysis
Data were analyzed using two-way ANOVA with repeated measures (treatment × time), except where otherwise stated. When a significant F ratio was found, the Newman-Keuls post hoc test was used to compare means. The following data were analyzed using a two-tailed paired dependent-sample Student's t-test: Phos a and glycogen utilization at each power output. Data are presented as means ± SE. Differences were considered significant at P < 0.05. ![]() |
RESULTS |
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Muscle Metabolism
Phos.
Phos a activity did not change with
power output during CON. However, with ALK, Phos
a progressively decreased with power output and was significantly lower at 75%
O2 max with ALK
compared with CON (ALK 37.9 ± 5.1 vs. 48.8 ± 5.0 mmol dry
wt · kg
1 · min
1; Fig.
2).
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Lactate and pyruvate.
Intramuscular
[Lac] increased
with each power output but was significantly higher with ALK at both
60% (ALK 40.9 ± 8.3 vs. CON 26.2 ± 5.0 mmol/kg dry wt) and
75% (ALK 72.8 ± 11.8 vs. CON 65.2 ± 10.1 mmol/kg dry wt)
O2 max (Fig.
4). Muscle [pyruvate] increased
with each power output and was similar between conditions at 30 and
75%
O2 max
and significantly higher with ALK during the second power output (Fig.
4).
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PDHa.
Resting (ALK 0.55 ± 0.01 vs. CON 0.55 ± 0.08 mmol wet wt
· kg1 · min
1)
and rest postingestion (ALK 0.55 ± 0.11 vs. CON
0.53 ± 0.05 mmol wet wt · kg
1 · min
1)
PDHa were not different between
conditions. Under both conditions, PDHa increased progressively with
each power output but was significantly higher at 60%
O2 max (4.17 ± 0.23 vs. 3.77 ± 0.27) with ALK compared with CON,
respectively (Fig. 5).
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ATP, ADPf,
AMPf, free Pi,
and PCr.
Muscle ATP concentration ([ATP]) was unaltered by exercise
or as a result of ALK. Muscle
[ADPf] and
[AMPf] increased with each power output, but both were significantly higher with ALK at 60 and 75%
O2 max (Table
3). Free
Pi increased with each power
output, but to a significantly greater degree with ALK (Fig. 6). The [PCr] decreased with increasing
power output but was significantly more depleted at each power output
during ALK compared with CON (Fig. 6).
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Pyruvate production and oxidation and lactate production and
oxidation.
Pyruvate production increased with exercise but was significantly
higher during both the second and third power outputs with ALK.
Pyruvate oxidation increased with exercise similarly between conditions
at both 30 and 75%
O2 max. At 60%
O2 max,
pyruvate oxidation was significantly higher during ALK (Table
4). Relative pyruvate oxidation expressed
as the percentage of pyruvate produced that was oxidized was similar
between conditions for both 30 and 75%
O2 max. However,
relative pyruvate oxidation was significantly higher during 60%
O2 max with ALK (Table
4). Lactate production was similar between conditions during the first
two power outputs but was significantly higher at 75%
O2 max during ALK
(Table 4).
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Blood Metabolites, Blood Flow, and Exchange Across the Leg
Blood pH, PCO2, and HCO
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Leg blood flow and leg respiratory
quotient. Leg blood flow increased progressively with
exercise similarly between conditions (Table
6). Leg
O2,
CO2, and leg respiratory
quotient were not different between conditions, increasing with each
power output (Table 6).
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Blood lactate and flux.
Arterial [Lac]
increased progressively with each power output but was significantly
higher at both 60 and 75%
O2 max with ALK (Table
5). Net Lac
release across
the leg increased with each power output but was significantly higher
during 75%
O2 max with
ALK (Fig. 8).
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Lactate Gradient, H+ Gradient, and pHi
The lactate gradient between both the arterial and femoral venous plasma and muscle was not different between conditions at 75%
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Respiratory Gas Exchange Variables
Whole body
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DISCUSSION |
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The present study examined the effects of induced alkalosis on the
metabolic responses of skeletal muscle during continuous, dynamic, constant-rate exercise at three power outputs (30, 60, and
75% O2 max). The main
effects of alkalosis during this type of exercise occurred during the
two higher power outputs and included an enhanced glycogen utilization
with a concomitant increase in pyruvate production, increased
intramuscular Lac
accumulation, enhanced Lac
efflux from the exercising leg, and a greater relative activation of
Phos than PDHa.
Lactate production within the muscle is dependent on the balance
between the rates of pyruvate production and oxidation. Intramuscular Lac is formed from pyruvate
by the action of the near-equilibrium enzyme LDH (47). Although greater
pyruvate production was observed at the two highest power outputs,
Lac
production was elevated
over CON values only during 75%
O2 max (Table
4). Lactate production at the highest power output resulted from a
significant degree of mismatch between the rates of
glycogenolysis/glycolysis and maximal
PDHa activation. At 60%
O2 max, the absence of
an increase in lactate production despite an increased pyruvate
production resulted from an enhanced pyruvate oxidation relative to
production due to greater PDHa
with ALK. These changes resulted from the effect of alkalosis on the
rate-limiting enzymes Phos, phosphofructokinase (PFK), and PDH. The
main control points for glycogenolysis/glycolysis involve Phos and PFK,
respectively, whereas entry into the oxidative pathway is controlled by
PDHa (47).
Phos
Phos is the flux-generating enzyme responsible for glycogenolysis within skeletal muscle and is subject to both covalent and allosteric regulation. Phos a, considered the active form, is active in the absence of AMPf, whereas Phos b, the less active form, requires AMPf (12). Covalent b-to-a transformation is mediated by Phos kinase a, which is activated by either an increase in epinephrine or cytosolic Ca2+ concentration ([Ca2+]) via cAMP-dependent and -independent mechanisms, respectively (54). Posttransformational allosteric activation of Phos b is mediated by AMP and IMP, whereas inhibition is mediated by ATP and G-6-P. Substrate regulation of both forms of Phos by the free Pi and glycogen concentrations is equally important (12, 18).Phos a was significantly reduced at
the highest power output with ALK. This reduction presumably reflects
the inhibition of Phos kinase a by the
increased [H+] (Table
3). Previous studies have demonstrated this relationship between
[H+] and Phos
a in intensely exercising muscle (13).
However, the mole fraction of Phos in the
a form is not the sole determinant of
glycogenolytic flux, as previous studies examining the relationship between Phos a transformation and
glycogenolysis have demonstrated (14, 18, 34, 54). The present results
of a higher glycogen utilization at 75%
O2 max
(133 ± 6 vs. 113 ± 12; Fig. 3) despite a lower Phos
a (Fig. 2) and the observed greater
glycogen utilization (132 ± 15 vs. 75 ± 6; Fig. 3) despite
similar Phos a (Fig. 2) at 60%
O2 max with ALK are in
agreement with these previous studies. Additionally, there may have
been a hormonal effect throughout exercise due to a decrease in the
circulating epinephrine concentration ([epinephrine]) with
ALK, which may have contributed to the lower observed Phos
a. Bouissou and colleagues (10)
observed a 34% reduction in the plasma [epinephrine]
and higher intramuscular [Lac
] with
alkalosis in humans who cycled to exhaustion. The enhanced glycogenolysis despite similar or lower Phos
a transformation with ALK likely
resulted from posttransformational modulation by increases in the
AMPf, and IMP concentrations and
an increase in its substrate free Pi.
AMP and IMP have been shown to stimulate Phos a (2, 54) and Phos b (2, 15). AMP acts on Phos a by reducing the Km for Pi from 26.8 to 11.8 mM in the presence of as little as 0.01 mM AMP (2, 54). AMP acts in a similar manner on Phos b but with a higher Km. Both the [AMPf] and free [Pi] were significantly greater with ALK during the second and third power outputs and were well above the required concentrations for activation (Table 3 and Fig. 6). In support of the close relationship between Phos activity, glycogenolytic flux, and the [AMPf], other studies using caffeine ingestion (17), increased FFA availability (24, 48), and short-term training (16) have demonstrated glycogen sparing during exercise associated with blunted AMPf accumulation. The increased [AMPf] may have augmented activation of both Phos a and Phos b. IMP activates Phos b with a Km of 1.2 mM (2). IMP concentration ([IMP]) was not measured in the present study but has been shown previously to increase with exercise and when the [ADPf] increases (64). The [ADPf] was significantly elevated with ALK, which may have led to an increase in the [IMP], which in turn may have activated Phos b.
ATP and G-6-P are both inhibitors of
Phos b. ATP has an inhibitory constant
(Ki) of ~2
mM, whereas G-6-P has a
Ki of ~0.3 mM
(25). In the present study, [ATP] remained constant
throughout exercise and between conditions at levels above the
Ki.
[G-6-P] was significantly
lower with ALK at 75%
O2 max but remained above the Ki.
These combined results should favor a reduction in Phos activity and
glycogenolysis. However, it has been previously demonstrated that the
inhibition of Phos b by both ATP and
G-6-P can be overcome when the
[AMPf] increases
sufficiently (18). In addition, previous studies have demonstrated that
glycogenolytic flux is closely tied to the availability of its
substrate, free Pi, and the
allosteric regulator, AMPf, both
of which were elevated with ALK (16, 17).
In summary, Phos activity and therefore glycogenolytic flux results from the combination of covalent, allosteric, and substrate regulation. During ALK, despite a lower transformation of Phos a, glycogenolytic flux was enhanced, and glycogen utilization increased during the second and third power outputs due to the maintenance of flux through posttransformational allosteric activation of Phos a + b by an increased [AMPf], and possibly [IMP], and an increase in the concentration of its substrate, free Pi.
PFK
PFK plays a key role in the regulation of glycolysis and therefore pyruvate production. PFK catalyzes the conversion of F-6-P to fructose 1,6-bisphosphate with the use of ATP (47), with the relative enzyme activity reflected by changes in the [F-6-P] and [G-6-P], with which it is in equilibrium. PFK is subject to regulation by a large number of metabolites that function to either inhibit or activate the enzyme complex. ATP and H+ inhibit, whereas ADP, AMP, Pi, and F-6-P activate, the enzyme complex, with the net enzyme activity resulting from the combination of these inputs (66). These are the most potent regulators, which reflect energy state and fuel utilization within the cell and thereby provide feedback regulation to adjust glycolytic flux.[ATP] remained constant between trials and across power
outputs, which provided a small degree of inhibition. Also, during the
highest power output, intramuscular
[H+] was significantly
elevated for both trials (Table 3), which would provide inhibition, as
previous in vitro studies using constant [ATP] with
declining pH have demonstrated (22, 65). Human exhaustive exercise
protocols have found similar changes reflecting reductions in PFK
activity with reduced intramuscular
[H+] (36, 59). The
magnitude of the pH inhibition can be modulated by increases in
[F-6-P] and the activators
ADP, AMP, and free Pi. Alkalosis led to increases in
[F-6-P] and
[G-6-P] during the third
power output but to a significantly lower magnitude than CON. At this
power output, pHi was
significantly lower compared with CON but may have failed to inhibit
PFK activity due to positive modulation by the significantly elevated
[AMPf],
[ADPf], and
[Pi]. AMP acts by
augmenting PFK's affinity for its substrate,
F-6-P (9). The increased
[F-6-P], although lower
than CON, was elevated above the
Km of
0.1-0.2 mM, which could have opposed the pH inhibition by
decreasing the affinity of the ATP binding site (39). The accompanying
rise in the [G-6-P],
although above the
Ki for Phos, may
have been overridden by the positive modulation of Phos by the
increased [AMPf] and
[Pi]. Previous in
vitro studies have also demonstrated substantial acceleration of
glycolysis with the lowering of the ATP-to-ADP ratio, a situation
present with ALK at both 60% (CON 132 vs. ALK 87) and 75%
O2 max (CON
80 vs. ALK 55) due to the increase in
[ADPf] (71). The
stimulatory effect of an increase in
[ADPf] on PFK activity
has been shown to substantially increase with as little as a 2 mM
increase in Pi, which also
occurred in the present study with ALK (71).
The combined results demonstrate enhanced glycogenolytic/glycolytic flux with ALK due to allosteric upregulation of both Phos and PFK activity, leading to the increased glycogen utilization and pyruvate production at the two highest power outputs.
PDH
PDHc is a mitochondrial enzyme complex that catalyzes the decarboxylation of glycolytically derived pyruvate and therefore reflects the rate of CHO entry into the tricarboxylic acid (TCA) cycle. PDHc transformation between the active (PDHa) and inactive (PDHb) forms is regulated by the balance between PDH kinase (PDHK; deactivating) and PDH phosphatase (PDHP; activating; see Refs. 52, 68). The relative phosphatase/kinase activity is controlled by the mitochondrial acetyl-CoA-to-CoASH, ATP-to-ATP, and NADH-to-NAD+ ratios and the allosteric regulators Ca2+, pyruvate, and H+. Increases in the ratios decrease PDHa transformation, whereas decreases in the ratios have the opposite effect. Increases in [pyruvate] inhibit the kinase only, increases in [Ca2+] inhibit the kinase and activate the phosphatase, and increases in [H+] activate the phosphatase only (51, 52, 68). Due to the complex interaction of regulators and the observed differences in PDHa between conditions, the changes occurring at 60 and 75%Changes in PDHa at 60%
O2 max.
In the present study, PDHa
increased with each power output as a result of contraction-induced
increases in the
[Ca2+] (19, 21, 34).
However, increases in the
[Ca2+] cannot be the
sole mechanism responsible for the increased
PDHa with ALK, since the power
outputs were identical between trials. The elevated
PDHa with ALK resulted from
changes in the allosteric regulators acetyl-CoA, ADPf,
H+, pyruvate, and the NADH-to-NAD+ ratio.
Changes in PDHa at 75%
O2 max.
At this power output, PDHa
transformation was similar between conditions and reflects the
attainment of maximal PDHa (Fig. 5). Previous studies have shown maximal activation at this power output
(34, 50). The increased pyruvate production with ALK resulted from a
slightly higher glycogen utilization. The absence of a difference in
the relative pyruvate oxidation rates between conditions was due to the
similar rates of PDHa. The higher
lactate production rate observed with ALK resulted from the higher
glycogenolytic/glycolytic rate. However, at this power output with ALK,
the major fate of the lactate produced was efflux from the muscle and
not intramuscular accumulation, which will be discussed later.
Cellular energetics.
The rate of mitochondrial ATP production is regulated by
O2 availability and the
[NADH]-to-[NAD+] and
the [ATP]-to-[ADP] × [Pi] ratios (70).
O2 availability was not limiting in the present study in
either trial, as neither the mouth
O2 nor O2
uptake across the leg was different (Tables 6 and
8). However, differences in glycogen and FFA
utilization were apparent. During the two higher power outputs with
ALK, there was a decrease in FFA utilization, as evidenced by
decreased [acetyl-CoA] and the markedly higher glycogen
utilization. During CON, the significantly lower glycogen utilization
necessitated an increase in FFA utilization to match energy production
to ATP demand. The reduced FFA utilization with ALK may have decreased
the mitochondrial [NADH], which would necessitate a higher
[ADPf] and
[Pi] to drive oxidative phosphorylation according to the equation
(70)
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(2) |
Lactate Metabolism and Transport
Intramuscular lactate accumulation reflects the balance between the rates of lactate production and efflux from the muscle (7, 38). Previous studies in humans employing metabolic alkalosis have focused on the effects of alkalosis on blood [LacLactate production during ALK at 75%
O2 max was increased
compared with CON and resulted from the mismatch between the rates of
glycogenolysis and PDHa flux, as
evidenced by enhanced pyruvate production and significantly higher
glycogen utilization in the absence of a difference in
PDHa between trials (Fig. 3 and
Table 4). The similar PDHa between
trials reflects maximal activation and is supported by the similar
rates of pyruvate oxidation at this power output (Table 4). Therefore,
the only difference between trials at this power output was the
augmented CHO utilization and thus glycogenolytic flux, which greatly
exceeded the maximal PDHa flux.
Previous studies have demonstrated that a mismatch does exist between
the maximal rates of Phos and PDHa
at higher power outputs, resulting in a significant increase in lactate production (34, 51).
Intramuscular lactate accumulation is also a function of the rate of
efflux from the muscle (38). ALK significantly increased arterial whole
blood [Lac] and
plasma [Lac
]
(Table 5) during both the second and third power outputs and enhanced
efflux from the exercising leg during the highest power output only
(Fig. 8). Blood
[Lac
]
represents the balance of lactate entry from muscle and uptake by
inactive tissue (7). The present results can be explained by both
enhanced lactate transport out of the exercising leg and possibly
reduced uptake by nonexercising tissue.
Lactate transport across the sarcolemma occurs via a monocarboxylate
lactate-proton cotransport protein and as such is the rate-limiting
step in lactate efflux (38, 45). Kinetic studies of the transporter
using isolated sarcolemmal vesicle preparations have shown it to have a
high affinity for L-lactate and to be sensitive to changes
in both the Lac and
H+ concentration gradients (38).
In the present study with ALK, there was an increase in the
intramuscular
[Lac], but the
[Lac
] gradient
between the muscle and both the arterial plasma and femoral venous
plasma compartments was similar between conditions (Table 7).
Therefore, the enhanced lactate efflux observed with ALK was not a
function of the increase in the intramuscular
[Lac
]. This
means that some other factor likely had an effect on the transporter
(8). The most plausible effector is
H+, as the
[H+] gradient between
muscle and both the arterial and femoral venous blood was significantly
elevated with ALK (Table 7; see Refs. 38, 40). ALK also induced a
significant elevation in extracellular HCO
3 concentration
([HCO
3]) that may have
contributed to the enhanced lactate efflux from the muscle. The
importance of external
[HCO
3] on lactate efflux
has been demonstrated in isolated muscle preparations with low external
[HCO
3] yielding reduced lactate efflux (32, 43, 58). In addition, studies employing metabolic
alkalosis in exercising humans (10, 27, 33, 37, 46, 63, 69) have
consistently demonstrated increases in the appearance of lactate in the
plasma when accompanied by increases in
[HCO
3]. During ALK in the
present study, extracellular
[HCO
3] was significantly
elevated in both arterial and femoral venous blood at rest
postingestion and remained elevated throughout each of the three power
outputs, which is an agreement with the above evidence (Table 5).
Additionally, decreased lactate uptake by inactive tissue may have
contributed to the higher arterial
[Lac
] with ALK.
Uptake by inactive tissue has been shown to occur during exercise (28).
Normally, with exercise, the increased blood
[H+] and
[Lac
] create an
inwardly directed [H+]
and [Lac
]
gradient, which facilitates uptake into inactive tissue. However, alkalosis decreases the
[H+] gradient and
therefore may reduce uptake compared with control conditions.
Summary and Conclusions
Induction of a metabolic alkalosis results in a complex series of metabolic effects during exercise reflecting changes in the activity of key regulatory enzymes and fuel utilization. The main findings of the study demonstrate that alkalosis during 60%High-intensity exercise (75%
O2 max) with metabolic
alkalosis leads to significantly increased lactate production,
intramuscular accumulation, and efflux. The increased lactate
production and increased intramuscular accumulation results from
the absence of downregulation of glycogenolysis and glycolysis that
typically occurs as pHi declines.
Instead, the increased ADPf,
AMPf, and free
Pi concentrations competed with
and/or negated the pH effect, resulting in the maintenance of
glycogenolysis and therefore pyruvate production. However, the
glycogenolytic rate exceeded the maximal PDHa rate, resulting in increased
lactate production.
The elevated blood
[Lac]
accompanying alkalosis likely resulted from the effects of an altered
[H+] gradient on the
transporters and is not due to changes in the [Lac
] gradient.
Reduced uptake of lactate by inactive tissue may also have contributed
to the increased arterial
[Lac
] with ALK,
but this contribution was not assessed in the present study.
The present data demonstrate that the increased blood
[Lac] commonly
observed with metabolic alkalosis results from a complex series of
events that modulate the activities of the key regulatory enzymes Phos,
PFK, and PDHa.
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ACKNOWLEDGEMENTS |
---|
We acknowledge G. Obminski, Dr. M. Ganagaragah, T. M. Bragg, and J. Otis for technical assistance.
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FOOTNOTES |
---|
This work was supported by grants from the Medical Research Council of Canada (MRC). M. G. Hollidge-Horvat was supported by an MRC Studentship, M. L. Parolin was supported by a Natural Science and Engineering Council studentship, and G. J. F. Heigenhauser is a Career Investigator of the Heart and Stroke Foundation of Ontario (no. I-2576).
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. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: G. J. F. Heigenhauser, Dept. of Medicine, McMaster University Medical Centre, 1200 Main St. West, Hamilton, Ontario, Canada L8N 3Z5 (E-mail: heigeng{at}fhs.csu.McMaster.ca).
Received 5 May 1999; accepted in final form 15 September 1999.
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