Lactate a signal coordinating cell and systemic function
1 Department of Sport and Exercise Sciences, Chelsea School Research Centre,
Welkin Performance Laboratories, Eastbourne, BN20 7SP, UK
2 School of Pharmacy and Biomolecular Sciences, Cockcroft Building,
University of Brighton, Eastbourne, BN20 7SP, UK
* Author for correspondence (e-mail: a.philp{at}brighton.ac.uk)
Accepted 31 October 2005
![]() |
Summary |
---|
The concept of lactate acting as a signalling compound is a relatively new hypothesis stemming from a combination of comparative, cell and whole-organism investigations. It has been clearly demonstrated that lactate is capable of entering cells via the monocarboxylate transporter (MCT) protein shuttle system and that conversion of lactate to and from pyruvate is governed by specific lactate dehydrogenase isoforms, thereby forming a highly adaptable metabolic intermediate system. This review is structured in three sections, the first covering pertinent topics in lactate's history that led to the model of lactate as a waste product. The second section will discuss the potential of lactate as a signalling compound, and the third section will identify ways in which such a hypothesis might be investigated.
In examining the history of lactate research, it appears that periods have occurred when advances in scientific techniques allowed investigation of this metabolite to expand. Similar to developments made first in the 1920s and then in the 1980s, contemporary advances in stable isotope, gene microarray and RNA interference technologies may allow the next stage of understanding of the role of this compound, so that, finally, the fundamental questions of lactate's role in whole-body and localised muscle function may be answered.
Key words: lactate metabolism, signalling mechanism, exercise, mammalian function
![]() |
Introduction |
---|
The formulation of the traditional lactate paradigm intense exercise, lack of oxygen and fatigue
The exploration of intermediatory metabolism is not a new field.
Interpretation of the role of lactic acid can be traced back to its
identification in 1808 by Berzelius
(Berzelius, 1808) and then
later by Araki (1891
), who
showed that lactic acid concentrations in exhausted animal muscle were
proportional to the activation of the exercised muscle, the extent of which
was thought to be associated with O2 availability. In 1907,
Fletcher and Hopkins conducted a series of experiments expanding this
knowledge to the examination of isolated amphibian muscle
(Fletcher and Hopkins, 1907
).
Through progressive investigations, the authors demonstrated that lactic acid
appeared in response to muscle contraction, continuing in the absence of
oxygen. A secondary observation was that, following the stimulated muscle
contraction, the accumulated lactate disappeared when oxygen was present.
During the 1920s, work by three predominant research groups, A. V. Hill's
group in London, the Heidelberg group of Otto Meyerhof, and Dill and
Margaria's group at Harvard (Margaria et
al., 1933), provided much of the basis for our understanding of
lactate metabolism in exercise physiology. In 1923, Hill and Meyerhof combined
their research observations and many of the accepted or hypothesized theories
at the time in a historical review article
(Hill and Meyerhof, 1923
). The
two main theoretical constructs to emerge from this paper were the
identification and naming of the `lactic-acid-cycle' (describing the processes
utilizing the cyclical conversion of glycogen to lactic acid back to glycogen)
and the recognition that `two' distinct pathways supplied the energy required
for muscle contraction, which were deemed aerobic (in the presence) and
anaerobic (in the absence) of oxygen.
Whilst Meyerhof's research concerned the lactic acid cycle on
non-circulated amphibian hemicorpus preparations, Hill et al.
(1924a,b
)
subsequently sought to investigate this phenomenon in humans during exercise.
From a series of experiments and observations, the authors determined the rise
in lactic acid at the onset of exercise to be as a direct result of an
O2 deficit (hypoxia) in exercising skeletal muscle. The `oxygen
debt model' that Hill et al.
(1924a
,b
)
postulated, supported by subsequent work from Hill's laboratory
(Hill, 1932
), became the
primary explanation for the increased appearance of lactic acid during
exercise and ensuing fatigue. Subsequent recognition of these researcher's
contributions (Hill and Meyerhof were jointly awarded the Nobel Prize for
science in 1922) saw the O2 debt hypothesis accepted as a leading
theory in the physiological understanding of prolonged human exercise, whilst
providing the paradigm for the body of further human research that ensued
(Bassett, 2002
).
The interpretation of research conducted during this period is, in many
regards, the reason why lactate has received its label as an end product.
Apart from research such as that generated by Cori's laboratory, which
demonstrated that lactate could be converted back to glucose in the liver
(Cori and Cori, 1929,
1933
), research during the
next 20 years sought to prove lactate as the cause of fatigue, rather than to
question its function.
A prime example of lactate's suggested role during exercise in the years
that followed was the introduction and widespread acceptance of the anaerobic
threshold (AT) concept (Davis,
1985). It was observed that during exercise, an increase in blood
lactate accumulation occurred at a standard relative exercise intensity
(
6075%
O2 max) in
individuals with varying fitness profiles. Combined with the appearance of
lactate in the circulation was an increase in ventilatory drive and energy
expenditure. This transition was seen as the turning point at which the
anaerobic system became the predominant source of energy provision, with a
concomitant increase in lactate concentrations beyond this transition a
consequence of this metabolic switch. Hill et al.'s O2 debt
hypothesis (Hill et al.,
1924a
,b
)
seemed to explain the mechanism behind this increase, as well as progressive
recruitment of glycolytic fibres and changes in substrate utilisation
(Davis, 1985
).
![]() |
So is lactate only an anaerobic product? |
---|
Richardson et al. (1998)
utilised phosphorous magnetic resonance spectroscopy (MRS) and myoglobin
saturation, as measured by 1H nuclear MRS, to address whether
lactate increase during progressive exercise to exhaustion was due to muscle
hypoxia. They observed that net blood lactate efflux was unrelated to
intracellular oxygen partial pressure (PO2) across work
intensities but was linearly related to O2 consumption and
intracellular pH. Therefore, the data provided by Richardson et al.
(1998
) support the notion
that lactate efflux during exercise is unrelated to muscle cytoplasmic
PO2, effectively dissociating lactate production and
hypoxia.
Comparative examination of the glycolytic pathway across the animal kingdom
has provided evidence that anaerobic conditions are not essential for lactate
to be produced, demonstrating that energy systems work in unison as opposed to
switching on and off, whilst duly confirming the dissociation between lactate
and hypoxic or anoxic conditions. The tailshaker muscle of the western
diamondback rattlesnake (Crotalus atrox) has provided a model that
clearly demonstrates that aerobic metabolism can meet a high ATP demand.
Species such as the rattlesnake are able to alter the energy requirement of
muscle contraction so that glycolysis may continue. Tailshaker muscles are
capable of sustaining high-frequency contractions in the region of
20100 Hz for several hours with an ATP cost per twitch of 0.015 mmol
l1 ATP per gram of muscle
(Conley and Lindstedt, 1996).
Utilising the same model, this time in ischemia and normoxic situations,
Kemper et al. (2001
)
demonstrated that such elevated rates of glycolysis could happen independently
of O2 levels. Such muscle was capable of exercising without fatigue
due to high blood flow levels allowing the rapid turnover of H+ and
lactate (and presumably other metabolites that might themselves be involved in
a fatigue process) within the cells. Recent research suggests that mechanical
trade-offs between twitch tension and duration and between joint force and
displacement explain how the tailshaker muscle can alter rattling frequency
rates without increasing the metabolic cost of activity
(Moon et al., 2002
).
These data allow for two considerations. Firstly, they allow for the
acceptance that lactate is not only produced as a result of anoxic or hypoxic
conditions but also that it is a metabolite produced during adequate oxygen
provision. Secondly, aerobic ATP provision is a highly adaptable process, with
skeletal muscle possessing an inherent ability to adapt to the energy
requirements of the organism. It appears that many animal species are able to
minimise the cost of muscle contraction so that cellular ATP production can
meet ATP demand and sustain high contractile rates
(Conley and Lindstedt, 2002)
with lactate formed as an integral part of this working system, not as an end
product per se.
![]() |
A metabolite on the move... |
---|
This understanding began to change following initial observations in rodent
studies by Donovan and Brooks
(1983), which demonstrated
that endurance training reduced post-exercise lactate concentrations by
enhancing lactate clearance, strongly suggesting that the major fate of
lactate during or following exercise was probably oxidation. Further research
demonstrated that lactate transport was sensitive to pH, specific transport
inhibitors and temperature (Juel,
1988
; Watt et al.,
1988
; Roth and Brooks,
1990
). To directly measure lactate kinetics in humans, Mazzeo et
al. (1986
) used the stable
isotope tracer [1-13C]lactate to demonstrate that the rate of
lactate disposal (Rd) was directly related to metabolic
clearance rate (MCR). That oxidation, as determined by the appearance of
13C enrichment in CO2, was the major fate of lactate
during exercise, and, subsequent to this, that the interpretation of lactate
kinetics by way of concentrations was inappropriate, as circulatory endpoint
values could not reflect lactate turnover in muscle (rate of production minus
rate of removal). Donovan's findings were supported in humans
(MacRae et al., 1992
), whilst
subsequent animal research in giant sarcolemmal vesicle and perfused hindlimb
preparations added support to a carrier-mediated process for lactate transport
in and out of skeletal muscle, as well as the stimulatory effects of
contraction, pH and blood flow on both processes
(Juel et al., 1991
;
Watt et al., 1994
;
Gladden et al., 1995
).
Previous research in erythrocytes suggested three pathways for lactate
transport. First, carrier-mediated transport by a H+-coupled
transporter; second, exchange with inorganic anions mediated by the band 3
protein Cl/HCO 3 exchange; and
third, passive diffusion of lactic acid across the lipid bilayer. Under
physiological conditions, it was believed that the transport pathway mediated
up to 90% of observed lactate flux (Deuticke et al., 1982). In the early
1990s, Kim et al. (1992)
sequenced a membrane protein (Mev) from met-18b-2 hamster ovarian cells that
exhibited an unusually high uptake of the 6-carbon branched
dihydroxymonocarboxylate mevalonate. When a plasmid expressing a cDNA for Mev
(pMev) was introduced by transfection into wild-type Chinese hamster ovary
cells, an mRNA that hybridizes to the Mev cDNA was identified. Following
cloning and sequencing of the wild-type version of Mev, coupled with the
observation that the cloned protein did not facilitate mevalonate transport,
it was concluded that the wild-type Mev transported other substances,
independently of mevalonate. Further examination identified that this protein
was related to the previously characterised transport system found in
erythrocytes (Garcia et al.,
1994
).
Subsequently, an entire family of monocarboxylate transport (MCT) proteins
(now with 14 isoforms) has been cloned, and their individual roles have been
characterised (for detailed topological characteristics and processes, see
Halestrap and Price, 1999;
Halestrap and Meredith, 2004
).
The predominant MCTs in human skeletal muscle are MCT1 and MCT4, whilst MCT2
has been identified in the liver
(McClelland et al., 2003
).
McCullagh et al. (1996
)
suggested that MCT1 facilitated uptake of lactate into muscle cells for
oxidative metabolism, as such being coordinately expressed with the heart
isoform of lactate dehydrogenase (LDH), with both being found in higher
concentration in type I fibres. At a similar time, Wilson et al.
(1998
) showed that the
low-affinity transporter MCT4 could be responsible for the net export of
lactate from the cell, and as such was predominantly expressed in glycolytic
type IIA fibres, which are known to be the major physiological producers of
lactate when they are contracting.
With the increased knowledge of MCT-facilitated lactate transport, further
evidence in support of the lactate shuttle hypothesis became available. Brooks
(1986) postulated the
framework of the lactate shuttle hypothesis prior to the discoveries of MCT or
their distribution (Fig. 1).
This hypothesis proposed that lactate was able to transfer from its site of
production (cytosol) to neighbouring cells and a variety of organs (e.g.
liver, kidney and heart), where its oxidation or continued metabolism could
occur. Of key importance to this hypothesis was the appreciation that for
lactate shuttling to occur, as suggested, a cellular protein transport system
would be implicated.
|
The original lactate shuttle hypothesis has since seen a number of
revisions, with an intracellular component introduced
(Brooks et al., 1999;
Fig. 1). The extension to an
intracellular shuttle system has not been without its controversy. The
principle depends upon the presence of mitochondrial LDH (mLDH) for the
re-conversion of lactate, once it enters the mitochondrion, to pyruvate and
for mitochondrial located MCTs (Brooks et
al., 1999
). This component has been strongly challenged by two
independent investigations (Rasmussen et
al., 2002
; Sahlin et al.,
2002
). The principal flaw to the Brooks model, detailed by these
authors, was that lactate entering the mitochondria would create a futile
cycle by which pyruvate is reduced to lactate in the mitochondria and vice
versa in the cytosol. It was suggested that this would induce a situation
compromising energy production, as both the redox state of the cell and the
required direction of substrate flow would be reversed.
This suggested scenario, however, seems unlikely. Firstly, in conversion of
pyruvate to lactate, lactate accepts an H+ ion from NADH, thereby
allowing increased availability of NAD and maintenance of the redox state of
the cell. Secondly, within the intracellular model there would not be a futile
cycle formed, as lactate entering the mitochondria would be converted to
pyruvate and oxidised. Lactate acts as an alternative pathway for substrate to
enter the mitochondria, competing with pyruvate for MCT transport. The
intracellular shuttle (Fig. 1)
does not suggest that pyruvate is not present in the intracellular
compartment; instead it suggests that the LDH conversion of lactate to
pyruvate is more than a cytosolic reaction alone. Data provided by Laughlin et
al. (1993) utilising MRS in
working canine hearts have proven that infusion of [13C]pyruvate
labelled cytosolic lactate and alanine pools whereas [13C]lactate
did not label cytosolic pyruvate or alanine. However, the TCA cycle substrate
-ketoglutarate was labelled, suggesting that infused lactate by-passed
the cytosolic LDH reaction and was converted to pyruvate in the mitochondria.
Brooks (2002b
) questioned the
methods used by Rasmussen et al.
(2002
) and Sahlin et al.
(2002
) in obtaining
mitochondria, suggesting that mLDH could easily have been lost during this
subfractionation process and was the main reason for the discrepancies in
results. The controversy over mitochondrial-located MCTs might have been
resolved by two recent studies (Butz et
al., 2004
; Hashimoto et al.,
2005
), with the latter using immunohistochemical analysis in
combination with confocal laser scanning microscopy (CLSM) to clearly
demonstrate the co-localisation of MCT1 and cytochrome oxidase (COX) at both
interfibrillar and subsarcolemmal cell domains. These data would indicate that
MCTs and associated proteins are therefore positioned specifically to
facilitate functions of the lactate shuttle system. For detailed applications
of the lactate shuttle hypothesis, see recent reviews by Brooks
(2002a
,b
)
and Gladden (2004
).
Lactate has been suggested to play an important role in cellular and
organelle redox balance, a function demonstrated in the proposed peroxisomal
lactate shuttle (McClelland et al.,
2003). It has long been known that long-chain ß-oxidation of
fatty acids occurs in mammalian peroxisomes
(Lazarow and de Duve, 1976
);
however, for ß-oxidation to continue, both FADH2 and NADH must
be reoxidized. McGroarty et al.
(1974
) first suggested the
presence of LDH in rat liver peroxisomes, however it was not until the study
of Baumgart et al. (1996
) that
LDH was identified in the peroxisomal matrix. McClelland et al.
(2003
) recently confirmed the
findings of Baumgart et al.
(1996
) identifying the presence
of LDH; further, peroxisomal ß-oxidation was stimulated by pyruvate, with
lactate generated when pyruvate was added to peroxisomes. MCT1 and MCT2 were
identified as facilitating the entry of pyruvate into the peroxisomal matrix
and lactate efflux from the organelle, thus forming the basis for a
peroxisomal lactate shuttle and explaining how lactate and its efflux can
regulate specific cellular and organelle redox balance
(Brooks et al., 1999
).
MCT expression seems to be rapidly modulated to respond to changes in
muscle activity. Many studies have demonstrated increases in MCT content
following a single exercise bout (Green et
al., 2002) or periods of endurance training
(Baker et al., 1998
;
Bergman et al., 1999
;
Pilegaard et al., 1999
;
Dubouchaud et al., 2000
).
Recent research suggests that MCT increases may occur rapidly following
exercise. Zhou et al. (2000
)
provided evidence that MCT4 mRNA was transiently increased during exercise.
Further to this, Green et al.
(2002
) showed an increase in
MCT1 (121%) and MCT4 (120%) protein expression taken from skeletal muscle
biopsies 2 and 4 days after a 56 h 60%
O2 peak exercise
bout in humans. Most recently, Coles et al.
(2004
) have shown that 2 h
exercise (21 m min1, 15% grade) in rats increases MCT1 and
MCT4 mRNA 23-fold, peaking 10 h post exercise. These responses,
however, were observed to be tissue specific [different responses found
between soleus and extensor digitorum longus (EDL) muscles] and, in some
cases, transiently upregulated so that protein levels had returned to
pre-exercise levels 24 h post exercise. Subsequently, these authors suggested
that the MCT family of transporters belong to a group of metabolic genes,
rapidly activated following exercise
(Hildebrandt et al., 2003
).
These gene products (mRNA) are present in small amounts in cells; however,
they have rapid induction times, suggesting that small quantities of each are
required for metabolic function to be supported
(Hildebrandt et al., 2003
). It
does, however, remain to be seen whether such rapid induction of MCTs
following exercise is repeated in human skeletal muscle. By contrast,
denervation (Pilegaard and Juel,
1995
) and inactivity (Wilson
et al., 1998
) lead to a decline in MCT expression.
These discoveries have been important in the recognition of lactate acting as a mobile metabolite, able to move within cellular compartments and adjacent muscle fibres and distributed widely across systemic circulation to inactive tissue and organs. Thus, lactate has the capacity to act as a metabolic signal at the cellular, localised and whole-body level, either directly or through its effects on H+ or other metabolic regulators. Further, the rapid induction of MCT following repeated muscle contraction means that the mechanisms of lactate transport can quickly adapt to an exercise stimulus, resulting in the notion of lactate as a signal to a rapid adaptable process maintaining cell homeostasis.
![]() |
Lactate as the cause or consequence of fatigue? |
---|
Some of the methods employed by Robergs et al.
(2004) to illustrate their
argument have been questioned by subsequent papers
(Boning et al., 2005
;
Kemp, 2005
); however, the
general consensus from a variety of experimental approaches appears to be that
lactate has minimal involvement in the onset of fatigue. Instead, recent
research suggests an increase of inorganic phosphate (Pi) produced
during contraction as the leading contender responsible for initiating muscle
fatigue at the level of muscle function (see review by
Westerblad et al., 2002
).
Contemporary explanation of fatigue certainly points to a combination of
effects, as opposed to one mechanism, causing fatigue, certainly in
whole-organism function. Accordingly, it is probably premature to also accept
the Pi hypothesis as the sole cause of fatigue until further
research is carried out, particularly in vivo
(Gladden, 2004
), just as care
should be taken when dismissing H+ accumulation from the aetiology
of fatigue until our overall understanding of fatigue is improved
(Fitts, 2003
;
Boning et al., 2005
).
It would now seem that lactate ions may in fact have a protective effect on
contraction force, as first demonstrated by Nielsen et al.
(2001). In their experiments,
it was observed that a reduction in tetanic force of intact isolated muscle
fibres caused by elevated potassium (K+) could be almost completely
reversed when incubated in lactate (20 mmol l1). The
substrate concentration used within this experiment led the authors to
hypothesise that at high exercise intensities, where intra-muscle lactate is
known to range between
15 and 25 mmol l1, lactate acts
to increase force, counteracting the force-depressing effects of high
extracellular K+ whilst having no effect on the membrane potential
or Ca2+ handling of the muscle. Further research has shown that at
a K+ incubation of 11 mmol l1 and a temperature
of 30°C, a 16% decline in force production of intact rat soleus or EDL can
be seen compared with controls. At the same K+ concentration, the
previously observed force decrement was restored to control values when
temperature (3035°C), lactate (10 mmol l1) and
catecholamine concentrations were all elevated, suggesting involvement of each
of these factors in force restoration
(Pedersen et al., 2003
).
Further, Karelis et al. (2004
)
have shown that maximum dynamic and isometric in situ force
production of electrically stimulated rat plantaris muscle is elevated during
intravenous lactate infusion (12 mmol l1) compared with
controls. The authors attributed this observation to increased maintenance of
M-wave characteristics during electrical stimulation and lactate infusion
trials compared with controls.
Nielsen et al.'s original lactate protection hypothesis
(Nielsen et al., 2001) has
recently been supported by further work from this group. Pedersen et al.
(2004
) reported that, in the
presence of chloride (Cl), intracellular acidosis increased
the excitability of the T system in depolarized muscle fibres, counteracting
fatigue at a critical phase in the excitationcontraction coupling
process. Acidification reduced Cl permeability, thereby
reducing the stimulus needed to generate a propagating action potential. This
view is not recognised by all. By contrast, Kristensen et al.
(2005
) questioned whether this
phenomenon can be extended to a whole-system model during exercise. These
authors reported that muscle preparations in vitro were unable to
produce a similar amount of force compared with controls when incubated in a
20 mmol l1 Na-lactate, 12 mmol l1
Na-lactate + 8 mmol l1 lactic acid or a 20 mmol
l1 lactic acid solution and stimulated to fatigue. It was
concluded that, although lactate regenerates force in passive muscle, this
process is not apparent when muscle is exercised. The authors suggest that the
depolarizing effect of lactate incubation observed by Nielsen et al.
(2001
) was not replicated, as
K+ depolarisation was less pronounced in vivo when muscle
was stimulated. These data seem to suggest that the extension of the Nielsen
et al. (2001
) hypothesis to a
full-system model is difficult due to the number of confounding systems that
operate during exercise in vivo. It appears that lactate may delay
the onset of fatigue by maintaining the excitability of muscle and that this
situation may occur during extremely intensive exercise. The basis and
understanding of this role, however, still remains poorly understood, whilst
the methods to transfer isolated muscle research into full-system physiology
are currently lacking. Clearly, further approaches to investigate this topic
are warranted to establish whether Nielsen's hypothesis can be extended to
whole-muscle function in vivo.
Peripheral or localised fatigue is characterised by metabolic change in
specific skeletal muscle or muscle groups, whether it be a reduction in pH or
an increased accumulation of a compound such as Pi. The classical
theory of exercise-induced fatigue proposes that exercise is limited only
after oxygen delivery to the exercising skeletal muscle becomes inadequate,
inducing anaerobiosis (Mitchell and
Blomqvist, 1971; Bassett and
Howley, 2000
). Noakes and colleagues have suggested an alternative
hypothesis, implicating a `central governor' (CNS), which regulates the mass
of skeletal muscle recruited during exercise through motoneurone pool
recruitment, a consequence of which would be to protect the heart from
ischaemia during maximal exercise (Noakes,
1998
; Noakes et al.,
2001
,
2004
;
St Clair Gibson et al.,
2003
). This model predicts that the ultimate control of exercise
performance resides in the brain's ability to vary the work rate and metabolic
demand by altering the number of skeletal muscle motor units recruited during
exercise (Noakes et al.,
2004
). Some attempts have been made by this group to address
physiological parameters in peripheral tissue that may act as the signal to
the CNS to regulate exercise intensity
(Rauch et al., 2005
);
however, this mechanism still remains unclear.
So could lactate have a role as a peripheral signal to the CNS during
exercise? We now know that lactate is a mobile metabolite capable of cell and
intracellular shuttling, with the circulation able to shift this metabolite to
a number of facultative sites for oxidation or recycling. There is also
mounting evidence in support of lactate utilisation in the brain
(Ide and Secher, 2000)
via the astrocyteneurone lactate shuttle, a system
clearly capable of affecting substrate delivery and neurone function
(Pellerin et al., 1998
;
Pellerin and Magistretti,
2003
). So could lactate be one of the peripheral exercise signals
that might be incorporated into Noakes' model
(Noakes et al., 2004
)?
Certainly, lactate's production characteristics allow it to perform such a
role. It is elevated during exercise and reaches maximal levels at or just
following the termination of exercise. Further, shuttling mechanisms would
allow for an influence of lactate, centrally and peripherally, again
fulfilling roles as part of the central governor hypothesis. It will be of
interest to see whether the peripheral signal for the central governor is
identified in future research and whether lactate has a role to play in this
scenario.
![]() |
Lactate as a signal? |
---|
Suggestion of a role for lactate as a metabolic signal at the
whole-organism level has been postulated by Brooks
(2002a), who proposed that
lactate may operate as a pseudo-hormone. Within this model, blood glucose and
glycogen reserves in diverse tissues are regulated to provide lactate, which
may then be used within the cells where it is made or transported through the
interstitium and vasculature to adjacent or anatomically distributed cells for
utilization. In this role, lactate becomes a quantitatively important
oxidizable substrate and gluconeogenic precursor, as well as a means by which
metabolism in diverse tissues may be coordinated. Lactate has the ability to
regulate cellular redox state, via exchange and conversion into its
more readily oxidized analogue, pyruvate, and effects on NAD+/NADH
ratios. Lactate is released into the systemic circulation and taken up by
distal tissues and organs, where it also affects the redox state in those
cells.
Further evidence for lactate acting as something more than a metabolite or
metabolic by-product comes from wound repair research, where lactate appears
to induce a biochemical `perception' effect
(Trabold et al., 2003). It
had been suggested that the elevated acidosis associated with wound
regeneration was a result of localised hypoxia. However, Trabold et al.
(2003
) provided evidence that
lactate may act as a stimulus similar to hypoxia without any compromise to
O2 levels. Green and Goldberg
(1964
) demonstrated that
collagen synthesis rose
2-fold in lactate-incubated (15 mmol
l1) fibroblasts, whilst Constant et al.
(2000
) showed that increased
lactate was capable of upregulating vascular endothelial growth factor (VEGF)
in similar proportions. To examine this apparent relationship, Trabold et al.
(2003
) elevated extracellular
lactate in the wounds of male Sprague-Dawley rats by implanting purified
solid-state, hydrolysable polyglycolide. This substance raised localised
lactate to a maintained 23 mmol l1. Elevating lactate
resulted in elevations in VEGF and a 50% increase in collagen deposition over
a 3-week period. These data suggest that lactate is capable of inducing
responses characteristic of O2 lack, operating to instigate a
pseudo-hypoxic (as far as concentration of lactate is concerned) environment.
In combination with this action, the continued presence of molecular oxygen
(as the tissue was not hypoxic) allows endothelial cells and fibroblasts to
promote increased collagen deposition and neovascularization.
The possibility that lactate acts as a metabolic signal is important to
take research further. Based on the hypotheses of Trabold et al.
(2003) and Brooks
(2002a
), can a working model
of lactate signalling be extended to systemic and localised exercise
function?
|
![]() |
A role for lactate in fuel selection? |
---|
Brooks and Mercier (1994)
recognised that a clear crossing point where fuel utilisation came from fat
and CHO equally was observable in fuel selection. The `crossover concept'
suggests that the proportion of substrate utilization in an individual at any
point in time depends on a trade-off between exercise-intensity-induced
responses (which increase CHO utilization) and endurance-training-induced
responses (which promote lipid mobilisation and oxidation). The crossover
point may be taken as the power output at which energy from CHO-derived fuels
predominates over that from lipids, with increases in power eliciting further
increments in CHO utilization and decrements in lipid oxidation.
The exercise intensity at which a transitional shift in substrate supply
might occur was originally examined in dogs and goats by Roberts et al.
(1996) through calculated
rates of fat and CHO oxidation from respiratory exchange ratio (RER) data.
Maximal fat oxidation rates were observed at 40% of maximal exercise intensity
in both species, with fat oxidation shown to provide around 77% of total
energy requirements. Bergman and Brooks
(1999
) studied this in humans
and found the highest lipid oxidation rate in the fed state at 40%
O2 peak. Taken
together, the data provided by Roberts et al.
(1996
) and Bergman and Brooks
(1999
) would suggest that
humans and other mammals, regardless of differences in aerobic capacities,
genotype and training adaptation, demonstrate similar substrate utilization
patterns when relative exercise intensity is considered
(Bergman and Brooks, 1999
).
Van Loon et al. (2001
)
utilised a continuous infusion of [U-13C]palmitate and
[6,6-2H2]glucose to provide direct measures of
whole-body fat oxidation, which were increased from rest at approximately 8 kJ
min1 up to a maximum rate of 32±2 kJ
min1 at 55% maximal workload (Wmax) or
approximately 6075% maximal oxygen consumption. As exercise intensity
increased to 75% Wmax fat oxidation declined by 34% to
19±2 kJ min1. Free fatty acid (FFA) concentrations
and blood flow were maintained at the highest exercise intensity, suggesting
ample FFA arterial availability.
Three possibilities have been suggested to explain the decline in FFA acid
oxidation in the face of sufficient supply. Firstly, gradual depletion or
limited turnover of the cytosolic free carnitine pool could alter long-chain
fatty acid (LCFA) transport across the mitochondrial membrane
(Harris and Foster, 1990).
Secondly, reduced transport of FFAs by escalating cellular or systemic
acidosis may limit FFA uptake due to downregulation of the fatty acid
transporter, carnitine palmitoyl-transferase 1 (CPT1)
(Sidossis et al., 1998
;
Bonen et al., 1999
). Finally,
changes in glucose flux and energy expenditure may regulate the amount of
available malonyl-CoA, an allosteric inhibitor of CPT1, which has been shown
to regulate fat oxidation (Ruderman and
Dean, 1998
; Roepstorff et
al., 2005
). To date, the exact mechanism regulating the relative
contribution of CHO and fat to energy provision during exercise still remains
unknown. The most recent examination of fuel balance during exercise was
conducted by Roepstorff et al.
(2005
) who utilised high or
low CHO diets to influence glycogen stores and substrate utilisation during 60
min bicycle exercise at 65%
O2 peak in eight
healthy male subjects. The authors observed a decline in muscle malonyl-CoA
concentrations from rest to moderate intensity exercise; however, there was no
change observed when fat oxidation rates were altered by the pre-exercise
meal. Thus, the authors concluded that malonyl-CoA may have a role in
increasing absolute levels of fat oxidation; however, it would not appear to
play a major part in fine-tuning the shifts in CHO and fat oxidation during
the rest-to-exercise transition or during sustained exercise. By contrast, the
availability of free carnitine to CPT1 appears to participate in regulating
fat oxidation during exercise, as muscle carnintine and fat oxidation rates
were both lower during exercise with high compared with low glycogen
conditions (Roepstorff et al.,
2005
).
So, is there potential for lactate to play a role in effecting this
transition? Previous research has shown that, in isolated mitochondria, a
reduction in pH decreases the activity of CPT1 by increasing the
Km of CPT1 for carnitine
(Mills et al., 1984). Starritt
et al. (2000
) have shown that
a decrease in pH from 7.0 to 6.8 reduces CPT1 activity by 40% in
vitro, thereby offering a potential mechanism for extracellular acidosis
to inhibit fat oxidation by reducing supply to the mitochondria or reducing
the rate of fat oxidation at lower exercise intensities where a fall in pH of
approximately 0.10.3 units is common
(Starritt et al., 2000
).
There is a host of research suggesting a direct effect of lactate on
inhibition of lipolysis and increased reesterification of FFA
(Issekutz et al., 1975
;
Ahlborg et al., 1976
;
Jeukendrup, 2002
). Whilst it
seems that this evidence supports a role for acidification in reducing fatty
acid metabolism, it is not clear whether this can be attributed to an increase
in H+, lactate alone or a combination of each. Most recently,
Corbett et al. (2004
) have
shown that as plasma lactate increases at progressive exercise intensities, so
NEFA levels decline. If we put these data into a physiological context, it is
known that the lactate threshold (a sustained increased in systemic lactate
from resting levels) during exercise occurs in most subjects at 6075%
O2 max, with the
accumulation of circulatory lactate known to increase non-uniformly beyond
this exercise transition. This relationship could, of course, be chance, with
lactate increase solely due to increased CHO oxidation or glycolytic flux.
However, if we examine the increase in lactate in the context of a signalling
hypothesis, lactate's role could be perceived as something very different. We
know that ample tissue oxygenation is available in skeletal muscle at
intensities of approximately 6075%
O2 max, allowing
oxidative phosphorylation to proceed
(Richardson et al., 1998
), so
lactate is not released as a result of tissue hypoxia. Similarly, lactate will
be maintained at a steady state beyond the lactate threshold, up to a maximal
lactate steady state, indicating that lactate clearance capacity is not
exceeded at these conditions (Billat et
al., 2003
). Could it be that lactate is released to signal a
progressive switch in fuel utilisation from fat to CHO, reducing FFA substrate
availability for the CPT complex whilst also acting, perhaps in combination
with H+ accumulation, to reduce pH, subsequently downregulating
CPT1-facilitated FFA transport? This model may provide an efficient way of
regulating fuel supply as lactate is produced, signals to its targets and is
then re-used as a fuel, allowing continuation of glycolysis and oxidative
phosphorylation.
As previously discussed, lactate is preferentially utilised, compared with
glucose and pyruvate, in cardiac muscle
(Laughlin et al., 1993).
Further, Chatham et al. (2001
)
have reported a similar selectivity for [13C]lactate to be
preferentially oxidised ahead of [13C]glucose, again in cardiac
muscle preparations. Miller et al.
(2002
) extended this
observation when they reported that infused lactate was preferentially
oxidised in preference to glucose at rest and during whole-body exercise in
humans. The authors concluded that lactate, provided by intravenous infusion,
acted in a glucose sparing role, allowing glucose and glycogen stores to be
maintained, to be utilized later in periods of increased exercise stress.
Artificially elevating lactate concentrations, such as the lactate clamp
method utilised by Miller et al.
(2002
), allows for the
investigation of lactate's role in a variety of processes; however, it does
provide a non-physiological situation, as lactate is added independently of
glucose usage. The elevated lactate concentrations could therefore serve to
stunt glycolysis, as opposed to sparing glucose concentrations. Infused
lactate, if the intracellular lactate shuttle is indeed correct, will bypass
glycolysis, becoming readily accepted into the mitochondria, where it is
converted to pyruvate via mLDH. Therefore, lactate synthesis in the
cytosol would be reduced, and an increase in H+ would follow, since
lactate production from pyruvate normally accepts an H+ from NADH.
This increased acidification could suppress glycolysis by inhibiting
phosphofructokinase (PFK) activity whilst affecting the redox state of the
cell. Glucose and glycogen would then be spared by lactate oxidation; however,
this process cannot occur during regular exercise as, without the infusion,
the only source of lactate production would be as a consequence of glycolysis
(Fig. 3).
|
![]() |
Lactate and pain |
---|
Recent research could, however, implicate lactate as influential in the
sensation of pain during exercise. Following the discovery of a receptor for
protons in the nerve cell membrane
(Krishtal and Pidoplichko,
1980), a family of receptor channel molecules has been identified
and cloned (Waldmann and Lazdunski,
1998
). These are the acid-sensitive ion channel family, or ASICs.
Four ASIC isoforms have been identified in the human genome, each displaying a
characteristic biophysical behaviour with respect to gating properties and pH
dependence (see Krishtal, 2003
for a review). There has been a suggestion that lactate, in combination with
extracellular H+, may influence sensory mechanotransduction
via an ASIC pathway, which in turn may modulate targeting of
nociceptive sensation (Immke and McClesky, 2001).
ASICs are Na+ channels. Immke and McClesky (2003) proposed that
the ASIC channel is blocked at a site, near the external entry to the pore, by
Ca2+. Binding of hydrogen ions diminishes the affinity for
Ca2+, which promotes Ca2+ release, thus allowing
Na+ flow through the channel, where it will act to depolarize the
excitable tissue. At a pH of 7.4, Ca2+ affinity remains high
(Kd=12 µmol l1) so that few channels
can open; however, at pH 7.0 the affinity is low enough
(Kd=100 µmol l1) that ASIC channels
open. Lactic acid (it was not clarified whether it was lactate or
H+) seems to enhance the sensitivity of ASIC3, allowing the ASIC
channel to open at lower H+ levels and making the pore more
sensitive to lactic acidosis (Immke and McClesky, 2001). This process has been
implicated in the aetiology of stroke and seizure (ASICs have been detected
throughout the CNS). The drop in pH and increased Ca2+ in both
conditions are likely to affect CNS and peripheral nerve (e.g. nociceptor)
function (Akaike and Ueno,
1994). Drew et al.
(2004
) recently utilised
wild-type and ASIC2/3 double-knockout mice to conclude that the ASIC mechanism
does not contribute to mechanically activated currents in mammalian sensory
neurones. It was suggested that an alternative ion channel type was the
most likely source of mechanotransduction, with receptor classes of the
transient receptor potential (TRP) channel family suggested as a potential
candidate (Clapham, 2003
).
The recent detection of ASIC isoforms in a cell line of skeletal muscle
characteristics points to other roles for ASIC isoforms apart from pain
sensation. Gitterman et al.
(2005) demonstrated that the
rhabdomyosarcoma cell line (SJ-RH30) possesses endogenous acid-gated currents,
similar to the properties of currents arising from ASIC1
subunits
(Gunthorpe et al., 2001
).
Further blocking of the acid-gated current was demonstrated firstly by 30
µmol l1 of the known ASIC1
inhibitor amiloride and
then secondly by a 1:1000 dilution of the ASIC1
antagonist psalmotoxin
1, found in Psalmopoeus venom
(Escoubas et al., 2000
). It
was further demonstrated by these authors that the removal of extracellular
Ca2+ enhanced channel conductance at pH 6.5 by
250%.
Preliminary investigation using TaqManTM (Applied Systems, Warrington,
UK) mRNA quantification provided evidence for expression of both ASIC1 and
ASIC3 mRNA in adult human muscle
(Gitterman et al., 2005
). The
question of whether human muscle is subject to quick fluctuations of pH of a
magnitude capable of activating ASICs has been raised previously by Krishtal
(2003
) and is clearly
paramount if lactate is involved in skeletal muscle ASIC activation in
vivo. It has been suggested that blood lactate concentrations following
strenuous exercise can rise to the region of
20 mmol l1
(Fitts, 1994
); however, common
levels range between 10 and 15 mmol l1 in healthy active
subjects. Concentrations of this magnitude alongside a pH change could be
hypothesised to produce some degree of channel activation and increase in
membrane Na+ conductance or membrane depolarization. Such changes
in membrane ion conductance and polarization could be a signal in themselves
for changes in metabolite use and intracellular signalling pathways, either
directly or through their modulation.
Microdialysis might allow further investigation of the Immke and McClesky
(2001) hypothesis, having been used by a number of research groups
(Rosdahl et al., 1993;
Maclean et al., 1999
;
Green et al., 2000
;
Street et al., 2001
;
Rooyackers, 2005
). Maclean et
al. (1999
) confirmed that a
substantial increase in interstitial lactate occurs during the transition from
rest to exercise, exceeding values seen in plasma. Street et al.
(2001
) added to these data by
observing that interstitial pH declined in a near linear manner as intensity
increased. The lowest pH observed 1 min after a 5 min bout of one-legged knee
extensor exercise (70 W) was 6.93, with a mean of 7.04. A pH change of this
nature could alter ASIC activation (Immke and McClesky, 2003) and act to
increase muscle contractility, delay the onset of fatigue or act as a signal
to cease exercise.
There clearly are discrepancies in research findings between whole-body and localised fatigue. Whilst our understanding of lactate action on ASIC function in vivo and the presence of ASIC protein in nerve and skeletal muscle is in its infancy, the revisiting of lactate's involvement in pain sensation is an interesting renewal of a long debate. It could be that, instead of pain, as such, lactate assists in the detection of severe exercise stress, signalling the termination or scaling down of exercise before muscle or other organ damage occurs. Lactate could potentially signal to nerve cells indicating the exercise stress, to which the sensation of pain would be produced and exercise would be reduced or cease.
![]() |
Signalling with regard to in vivo processes a working hypothesis |
---|
|
As exercise progresses towards exhaustion, whole-body lactate levels
continue to rise (detectable as 820 mmol l1 in blood
and higher in muscle). ATP provision in active muscle is approaching its
maximal capacity and there is a gradual decline in cellular and systemic pH.
Elevated lactate helps reduce glucose usage and glycogenolysis, minimising
depletion of these stores as escalating acidosis reduces PFK function.
Further, H+ ions combined with lactate cause an opening in ASIC
pores, signalling exercise termination. In this role, lactate is filling a
dual purpose. Firstly, its release is indicating stress placed upon active
muscle, whilst, secondly, high concentrations of intracellular lactate could
potentially be acting in a protective manner. Acting as a peripheral signal,
lactate could therefore provide a mechanism by which the CNS detects
localised, at the level of muscle or muscle group, exercise stress and causes
exercise to terminate (Noakes et al.,
2004).
![]() |
Future research |
---|
The lactate clamp (LC) method (Gao et
al., 1998) has been used to demonstrate that artificially elevated
lactate levels during moderate exercise may increase lactate oxidation, spare
blood glucose, reduce glucose production
(Miller et al., 2002
) whilst
also increasing gluconeogenesis (Roef et
al., 2003
). Miller et al.
(2005
) have reported that the
LC method allows an increase in lactate without causing acidosis; in fact, LC
caused a mild alkalosis. LC did not increase ventilation or rating of
perceived exertion, suggesting that the LC can be used to solely study the
effect of lactate, rather than acidosis, on metabolic functions.
A number of exercise scenarios, as well as pathological conditions, exist
that may also allow many of the ideas suggested in this paper to be
scrutinised. Does lactate function in a signalling role during hypoxic stress?
We know that following prolonged exposure to hypoxia, lactate levels have been
shown to decline, in what is termed `the lactate paradox'
(Hochachka et al., 2002). This
condition could test further the role of lactate during exercise. Also, what
happens during exercise in myophosphorylase-deficient patients (McArdle's
disease), who are unable to increase their production of lactate during
exercise, or during chronic hyperlactatemia such as that experienced by type
II diabetes and HIV patients? Why might lactate be elevated in these
scenarios?
As is evident from much of the research discussed in this review,
improvements in gene analysis and manipulation technologies have occurred over
the past decade. The sequencing of the human genome, in combination with
RT-PCR and oligonucleotide array technology, now allows the rapid screening of
a host of signalling pathways and novel ion channels from a relatively small
tissue sample. There are also strategies for examining signalling pathways
that may be involved if phosphorylation is a key event
(Knebel et al., 2001;
Haydon et al., 2002
).
Researchers also have the added benefit of gene knock-out technology and the
use of transgenic approaches to allow proof of concept previously
unattainable. Further to this, the introduction of siRNA approaches in whole
body systems will allow researchers to examine and manipulate certain pathways
and analyse a large variety of targets in vivo. The ability to
artificially elevate lactate concentrations safely in vivo, and then
limiting or removing pathways using siRNA manipulation, will allow direct
assessment of lactate's role in a variety of local and whole-system processes
whilst limiting the confounding influence of parallel energy systems and
pathways that, to date, greatly restrict the scope of in vivo
investigation.
![]() |
Conclusions |
---|
Lactate should be appreciated not as a sink for glycolytic waste to accumulate or as an acidifier but as an effective mechanism for coordinated fuel sensing and tissue function. This fuel is shuttled to a variety of sites where it is directly oxidised, re-converted back to pyruvate or glucose and oxidised, allowing the process of glycolysis to restart and ATP provision maintained. The shuttling facilitators MCT1 and MCT4, and possibly others in the MCT family, are proteins with rapid induction capabilities and the ability to respond to a host of contraction and environmental stimuli. Lactate production and MCT transport characteristics could allow them to operate as a signal mechanism activating a variety of functions during exercise and recovery.
Research should now be directed towards understanding the function of lactate during exercise in humans. The idea of lactate signalling to a variety of targets has stemmed from data across a variety of research areas, from cell and organelle to whole-body and system based experiments. Lactate's potential role in a variety of processes has been clearly demonstrated; however, the mechanisms underlying these observations in many cases remain undetermined. Lactate's role in fuel selection should be clarified, with the LC method appearing to be a suitable method of investigating this process. Studies utilising siRNA application in combination with microarray analysis could be used to address signalling targets that lactate may influence during exercise, whilst examination of ASICs in skeletal muscle may provide a channel by which lactate acts to increase muscle contractility during in vivo function or to signal to nerve cells protecting against exercise damage. With these areas warranting investigation, it certainly seems feasible that lactate has a few tricks left to show us of its role in exercise function.
![]() |
Acknowledgments |
---|
![]() |
References |
---|
Ahlborg, G., Hagenfeldt, L. and Wahren, J. (1976). Influence of lactate infusion on glucose and FFA metabolism in man. Scand. J. Clin. Lab. Invest. 36,193 -201.[Medline]
Akaike, N. and Ueno, S. (1994). Proton-induced current in neuronal cells. Prog. Neurobiol. 43, 73-83.[CrossRef][Medline]
Allen, D. G., Lannergren, J. and Westerblad, H.
(1995). Muscle cell function during prolonged activity: cellular
mechanisms of fatigue. Exp. Physiol.
80,497
-527.
Araki, T. (1891). Ueber die bildung von milchsaure und glucose im organismus bei sauerstoffmangel. Zeitschr. Phys. Chem. 15,335 -370.
Baker, S. K., McCullagh, K. J. A. and Bonen, A.
(1998). Training intensity dependent and tissue specific
increases in lactate uptake and MCT1 in heart and muscle. J. Appl.
Physiol. 84,987
-994.
Bangsbo, J., Juel, C., Hellsten, Y. and Saltin, B. (1997). Dissociation between lactate and proton exchange in muscle during intense exercise in man. J. Physiol. 504,489 -499.[Abstract]
Bassett, D. R., Jr (2002). Scientific
contributions of A. V. Hill: exercise physiology pioneer. J. Appl.
Physiol. 93,1567
-1582.
Bassett, D. R., Jr and Howley, E. T. (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med. Sci. Sports. Exerc. 32, 70-84.[CrossRef][Medline]
Baumgart, E., Fahimi, H. D., Stich, A. and Volkl, A.
(1996). L-Lactate dehydrogenase A4 and
A3B isoforms are bona fide peroxisomal enzymes in rat liver.
J. Biol. Chem. 271,3846
-3855.
Bergman, B. C. and Brooks, G. A. (1999).
Respiratory gas-exchange ratios during graded exercise in fed and fasted
trained and untrained men. J. Appl. Physiol.
86,479
-487.
Bergman, B. C., Wolfel, E. E., Butterfield, G. E., Lopaschuk, G.
D., Casazza, G. A., Horning, M. A. and Brooks, G. A.
(1999). Active muscle and whole body lactate kinetics after
endurance training in men. J. Appl. Physiol.
87,1684
-1696.
Berzelius, J. J. (1808). Djurkemien. Stockholm: Marquard.
Billat, V. L. S. P., Py, G., Koralsztein, J.-P. and Mercier, J. (2003). The concept of the maximal lactate steady state: a bridge between biochemistry, physiology and sport science. Sports Med. 33,407 -426.[Medline]
Bonen, A., Dyck, D. J., Ibrahimi, A. and Abumrad, N. A. (1999). Muscle contractile activity increases fatty acid metabolism and transport and FAT/CD36. Am. J. Physiol. 276,E642 -E649.
Boning, D., Strobel, G., Beneke, R. and Maassen, N. (2005). Lactic acid still remains the real cause of exercise-induced metabolic acidosis. Am. J. Physiol. Regul. Integr. Comp. Physiol. 289,902 -903.
Brooks, G. A. (1985). Anaerobic threshold: review of the concept and directions for future research. Med. Sci. Sports. Exerc. 17,22 -34.[Medline]
Brooks, G. A. (1986). The lactate shuttle during exercise and recovery. Med. Sci. Sports. Exerc. 18,360 -368.[Medline]
Brooks, G. A. (2002a). Lactate shuttles in nature. Biochem. Soc. Trans. 30,258 -264.[CrossRef][Medline]
Brooks, G. A. (2002b). Lactate shuttle
between but not within cells? J. Physiol.
541, 333.
Brooks, G. A. and Mercier, J. (1994). Balance
of carbohydrate and lipid utilization during exercise: the `crossover'
concept. J. Appl. Physiol.
76,2253
-2261.
Brooks, G. A., Dubouchaud, H., Brown, M., Sicurello, J. P. and
Butz, C. E. (1999). Role of mitochondrial lactate
dehydrogenase and lactate oxidation in the intracellular lactate shuttle.
Proc. Natl. Acad. Sci. USA
96,1129
-1134.
Butz, C. E., McClelland, G. B. and Brooks, G. A.
(2004). MCT1 confirmed in rat striated muscle mitochondria.
J. Appl. Physiol. 97,1059
-1066.
Chatham, J. C., Des Rosiers, C. and Forder, J. R. (2001). Evidence of separate pathways for lactate uptake and release by the perfused rat heart. Am. J. Physiol. 281,E794 -E802.
Clapham, D. E. (2003). TRP channels as cellular sensors. Nature 426,517 -524.[CrossRef][Medline]
Coles, L., Litt, J., Hatta, H. and Bonen, A.
(2004). Exercise rapidly increases expression of the
monocarboxylate transporters MCT1 and MCT4 in rat muscle. J.
Physiol. 561,253
-261.
Conley, K. E. and Lindstedt, S. L. (1996). Minimal cost per twitch in rattlesnake tail muscle. Nature 383,71 -72.[CrossRef][Medline]
Conley, K. E. and Lindstedt, S. L. (2002).
Energy-saving mechanisms in muscle: the minimization strategy. J.
Exp. Biol. 205,2175
-2181.
Connett, R. J., Gayeski, T. E. J. and Honig, C. R.
(1986). Lactate efflux is unrelated to intracellular
PO2 in a working red muscle in situ. J. Appl.
Physiol. 61,402
-408.
Constant, J. S., Feng, J. J., Zabel, D. D., Yuan, H., Suh, D. Y., Scheuenstuhl, H., Hunt, T. K. and Hussain, M. Z. (2000). Lactate elicits vascular endothelial growth factor from macrophages: a possible alternative to hypoxia. Wound Repair Regen. 8,353 -360.[Medline]
Corbett, J., Fallowfield, J. L., Sale, C. and Harris, R. C. (2004). Relationship between plasma lactate concentration and fat oxidation. Proc. 9th Annu. Congr. Eur. Coll. Sports Sci. 107,P172 .
Cori, G. T. and Cori, C. F. (1929). Glycogen
formation in the liver from D- and L-lactic acid.
J. Biol. Chem. 81,389
-403.
Cori, G. T. and Cori, C. F. (1933). Changes in
hexosephosphate, glycogen, and lactic acid during contraction and recovery of
mammalian muscle. J. Biol. Chem.
99,493
-505.
Davis, J. A. (1985). Anaerobic threshold: review of the concept and directions for future research. Med. Sci. Sports. Exerc. 17,6 -18.[Medline]
Deuticke, B. (1982). Monocarboxylate transport in erythrocytes. J. Membr. Biol. 70, 89-103.[CrossRef][Medline]
Donovan, C. and Brooks, G. A. (1983). Endurance training affects lactate clearance, not lactate production. Am. J. Physiol. 244,E83 -E92.[Medline]
Drew, L. J., Rohrer, D. K., Price, M. P., Blaver, K. E.,
Cockayne, D. A., Cesare, P. and Wood, J. N. (2004).
Acid-sensing ion channels ASIC2 and ASIC3 do not contribute to mechanically
activated currents in mammalian sensory neurones. J.
Physiol. 556,691
-710.
Dubouchaud, H., Butterfield, G. E., Wolfel, E. E., Bergman, B. C. and Brooks, G. A. (2000). Endurance training, expression, and physiology of LDH, MCT1, and MCT4 in human skeletal muscle. Am. J. Physiol. 278,E571 -E579.
Escoubas, P., De Weille, J. R., Lecoq, A., Diochot, S.,
Waldmann, R., Champibny, G., Mionier, D., Menez, A. and Lazdunski,
M. (2000). Isolation of a tarantula toxin specific for a
class of proton gated Na+ channels. J. Biol.
Chem. 275,25116
-25121.
Fabiato, A. and Fabatio, A. (1978). Effects of pH on the myofilaments and the sarcoplasmic reticulum of skinned cells from cardiac and skeletal muscles. J. Physiol. 276,233 -255.[Abstract]
Fattor, J. A., Miller, B. F., Jacobs, K. A. and Brooks, G. A. (2005). Catecholamine response is attenuated during moderate-intensity exercise in response to the `lactate clamp'. Am. J. Physiol. 288,E143 -E147.
Fitts, R. H. (1994). Cellular mechanisms of
muscle fatigue. Physiol. Rev.
74, 49-94.
Fitts, R. H. (2003). Mechanisms of muscular fatigue. In Principles of Exercise Biochemistry. 3rd edition (ed. J. R. Poortmans), pp. 279-300. Basel: Karger.
Fletcher, W. M. and Hopkins, F. G. (1907). Lactic acid in amphibian muscle. J. Physiol. 35,247 -309.
Gao, J., Islam, M. A., Brennan, C. M., Dunning, B. E. and Foley, J. E. (1998). Lactate clamp: a method to measure lactate utilisation in vivo. Am. J. Physiol. 275,E729 -E733.[Medline]
Garcia, C. K., Goldstein, J. L., Pathak, R. K., Anderson, R. G. and Brown, M. S. (1994). Molecular characterization of a membrane transporter for lactate, pyruvate, and other monocarboxylates: implications for the Cori cycle. Cell 76,865 -873.[CrossRef][Medline]
Gargaglioni, L. H., Bicego, K. C., Steiner, A. A. and Branco, L. G. (2003). Lactate as a modulator of hypoxia-induced hyperventilation. Respir. Physiol. Neurobiol. 138, 37-44.[CrossRef]
Gitterman, D. P., Wilson, J. and Randall, A. D.
(2005). Functional properties and pharmacological inhibition of
ASIC channels in the human SJ-RH30 skeletal muscle cell line. J.
Physiol. 562,759
-769.
Gladden, L. B. (2004). Lactate metabolism: a
new paradigm for the third millennium. J. Physiol.
558, 5-30.
Gladden, L. B., Crawford, R. E., Webster, M. J. and Watt, P. W. (1995). Rapid tracer lactate influx into canine skeletal muscle. Am. J. Physiol. 78,E205 -E211.
Green, H. and Goldberg, B. (1964). Collagen and cell protein synthesis by established mammalian fibroblast line. Nature 204,347 -349.[Medline]
Green, H., Halestrap, A., Mockett, C., O'Toole, D., Grant, S. and Ouyang, J. (2002). Increase in muscle MCT are associated with reductions in muscle lactate after a single exercise session in humans. Am. J. Physiol. 282,E154 -E160.
Green, S., Langderg, H., Skovgaard, D., Bulow, J. and Kjaer,
M. (2000). Interstitial and arterial-venous [K+]
in human calf muscle during dynamic exercise: effect of ischemia and relation
to muscle pain. J. Physiol.
529,849
-861.
Groussard, C., Morel, I., Chevanne, M., Monnier, M., Cillard, J.
and Delamarche, A. (2000). Free radical scavenging and
antioxidant effects of lactate ion: an in vitro study. J.
Appl. Physiol. 89,169
-175.
Gunthorpe, M. J., Smith, G. D., Davis, J. B. and Randall, A. D. (2001). Characterisation of a human acid-sensing ion channel (hASIC1a) endogenously expressed in HEK293 cells. Pflügers Arch. 442,668 -674.[CrossRef][Medline]
Halestrap, A. and Meredith, D. (2004). The SLC16 gene family from monocarboxylate transporters (MCTs) to aromatic amino acid transporters and beyond. Pflügers Arch. 447,619 -628.[CrossRef][Medline]
Halestrap, A. and Price, N. T. (1999). The proton-linked moncarboxylate transporter (MCT) family: structure, function and regulation. Biochem. J. 343,281 -299.[CrossRef][Medline]
Hardarson, T., Skarphedinsson, J. O. and Sveinsson, T.
(1998). Importance of the lactate anion in control of breathing.
J. Appl. Physiol. 84,411
-416.
Harris, R. C. and Foster, C. V. L. (1990). Changes in muscle free carnitine and acetylcarnitine with increasing work intensity in the thoroughbred horse. Eur. J. Appl. Physiol. 60,81 -85.[CrossRef]
Hashimoto, T., Masuda, S., Taguchi, S. and Brooks, G. A. (2005). Immunohistochemical analysis of MCT1, MCT2 and MCT4 expression in rat plantaris muscle. J. Physiol. 597,121 -129.[CrossRef]
Haydon, C. E., Watt, P. W., Morrice, N., Knebel, A., Gaestel, M. and Cohen, P. (2002). Identification of a phosphorylation site on skeletal muscle myosin light chain kinase that becomes phosphorylated during muscle contraction. Arch. Biochem. Biophys. 397,224 -231.[CrossRef][Medline]
Hildebrandt, A. L., Pilegaard, H. and Neufer, P. D. (2003). Differential transcriptional activation of select metabolic genes in response to variations in exercise intensity and duration. Am. J. Physiol. 285,E1021 -E1027.
Hill, A. V. (1932). The revolution in muscle
physiology. Physiol. Rev.
12, 56-67.
Hill, A. V. and Meyerhof, O. (1923). Ueber die vorgange bei der muskelkontraktion. Ergeb. Physiol. Biol. Chem. Exp. Pharmakol. 22,299 -344.[CrossRef]
Hill, A. V., Long, C. N. H. and Lupton, H. (1924a). Muscular exercise, lactic acid, and the supply and utilisation of oxygen. Parts I-III. Proc. R. Soc. Lond. B 96,438 -475.
Hill, A. V., Long, C. N. H. and Lupton, H. (1924b). Muscular exercise, lactic acid, and the suppply and utilisation of oxygen. Parts IV-VI. Proc. R. Soc. Lond. B 97,84 -138.
Hochachka, P. W., Beatty, C. L., Burelle, Y., Trump, M. E., McKenzie, D. C. and Matheson, G. O. (2002). The lactate paradox in human high-altitude physiological performance. News Physiol. Sci. 17,122 -126.[Medline]
Ide, K. and Secher, N. H. (2000). Cerebral blood flow and metabolism during exercise. Prog. Neurobiol. 61,397 -414.[CrossRef][Medline]
Immke, D. C. and McCleskey, E. W. (2001). Lactate enhances the acid-sensing Na+ channel on ischemia-sensing neurons. Nat. Neurosci. 4, 869-870.[CrossRef][Medline]
Immke, D. C. and McCleskey, E. W. (2003). Protons open acid-sensing ion channels by catalyzing relief of Ca2+ blockade. Neuron 37,75 -84.[CrossRef][Medline]
Issekutz, B., Jr, Shaw, W. A. and Issekutz, T. B.
(1975). Effect of lactate on FFA and glycerol turnover in resting
and exercising dogs. J. Appl. Physiol.
39,349
-353.
Jeukendrup, A. E. (2002). Regulation of fat
metabolism in skeletal muscle. Ann. NY Acad. Sci.
967,217
-235.
Jobsis, F. F. and Stainsby, W. N. (1968). Oxidation of NADH during contractions of circulated mammalian skeletal muscle. Respir. Physiol. 4,292 -300.[CrossRef][Medline]
Juel, C. (1988). Intracellular pH recovery and lactate efflux in mouse soleus muscles stimulated in vitro: the involvement of sodium/proton exchange and a lactate carrier. Acta Physiol. Scand. 132,363 -371.[Medline]
Juel, C., Honig, A. and Pilegaard, H. (1991). Muscle lactate transport studied in sarcolemmal giant vesicles: dependence on fibre type and age. Acta Physiol. Scand. 143,361 -365.[Medline]
Karelis, A. D., Marcil, M., Peronnet, F. and Gardiner, P. F.
(2004). Effect of lactate infusion on M-wave characteristics and
force in the rat plantaris muscle during repeated stimulation in situ.J. Appl. Physiol. 96,2133
-2138.
Kemp, G. (2005). Lactate accumulation, proton buffering, and pH change in ischemically exercising muscle. Am. J. Physiol. 289,E895 -E901.
Kemper, W. F., Lindstedt, S. L., Hartzler, L. K., Hicks, J. W.
and Conley, K. E. (2001). Shaking up glycolysis:
Sustained, high lactate flux during aerobic rattling. Proc. Natl.
Acad. Sci. USA 98,395
-397.
Kim, C. M., Goldstein, J. L. and Brown, M. S.
(1992). cDNA cloning of Mev, a mutant protein that facilitates
cellular uptake of mevalonate, and identification of the point mutation
responsible for its gain in function. J. Biol. Chem.
267,23113
-23121.
Knebel, A., Morrice, N. and Cohen, P. (2001). A
novel method to identify protein kinase substrates: eEF2 kinase is
phosphorylated and inhibited by SAPK4/p38. EMBO.
J. 20,4360
-4369.
Krishtal, O. (2003). The ASICs: signalling molecules? Modulators? Trends Neurosci. 26,477 -482.[CrossRef][Medline]
Krishtal, O. and Pidoplichko, V. L. (1980). A receptor for protons in the nerve cell membrane. Neuroscience 5,2325 -2327.[CrossRef][Medline]
Kristensen, M., Albertsen, J., Rentsch, M. and Juel, C.
(2005). Lactate and force production in skeletal muscle.
J. Physiol. 562,521
-526.
Laughlin, M. R., Taylor, J., Chesnick, A. S., DeGroot, M. and Balaban, R. S. (1993). Pyruvate and lactate metabolism in the in vivo dog heart. Am. J. Physiol. Heart Circ. Physiol. 264,2068 -2079.
Lazarow, P. B. and de Duve, C. (1976). A fatty
acyl-CoA oxidizing system in rat liver peroxisomes: enhancement by clofibrate,
a hypolipidemic drug. Proc. Natl. Acad. Sci. USA
73,2043
-2046.
Lindinger, M. I., McKelvie, R. S. and Heigenhauser, G. J.
(1995). K+ and Lac distribution in
humans during and after high-intensity exercise: role in muscle fatigue
attenuation? J. Appl. Physiol.
78,765
-777.
MacLean, D. A., Bangsbo, J. and Saltin, B.
(1999). Muscle interstitial glucose and lactate levels during
dynamic exercise in humans determined by microdialysis. J. Appl.
Physiol. 87,1483
-1490.
MacRae, H. S.-H., Dennis, S. C., Bosch, A. N. and Noakes, T.
D. (1992). Effects of training on lactate production and
removal during progressive exercise in humans. J. Appl.
Physiol. 72,1649
-1656.
Margaria, R., Edwards, R. H. T. and Dill, D. B. (1933). The possible mechanisms of contracting and paying the oxygen debt and the role of lactic acid in muscular contraction. Am. J. Physiol. 106,E689 -E715.
Mazzeo, R. S., Brooks, G. A., Schoeller, D. A. and Budinger, T.
F. (1986). Disposal of blood [1-13C]lactate in
humans during rest and exercise. J. Appl. Physiol.
60,232
-241.
McClelland, G. B., Khanna, S., Gonzalez, G. F., Butz, C. E. and Brooks, G. A. (2003). Peroxisomal membrane monocarboxylate transporters: evidence for a redox shuttle system? Biochem. Biophys. Res. Commun. 304,130 -135.[CrossRef][Medline]
McCullagh, K. J. A., Juel, C., O'Brien, M. and Bonen, A. (1996). Chronic muscle stimulation increases lactate transport in rat skeletal muscle. Molec. Cell. Biochem. 156, 51-57.[CrossRef][Medline]
McGroarty, E., Hsieh, B., Wied, D. M., Gee, R. and Tolbert, N. E. (1974). Alpha hydroxyl acid oxidation by peroxisomes. Arch. Biochem. Biophys. 161,194 -210.[CrossRef]
Miller, B. F., Fattor, J. A., Jacobs, K. A., Horning, M. A.,
Navazio, F., Lindinger, M. I. and Brooks, G. A.
(2002). Lactate and glucose interactions during rest and exercise
in men: effect of exogenous lactate infusion. J.
Physiol. 544,963
-975.
Miller, B. F., Lindinger, M. I., Fattor, J. A., Jacobs, K. A.,
Leblanc, P. J., Duong, M., Heigenhauser, G. J. and Brooks, G. A.
(2005). Hematological and acid-base changes in men during
prolonged exercise with and without sodium-lactate infusion. J.
Appl. Physiol. 98,856
-865.
Mills, S. E., Foster, D. W. and McGarry, J. D. (1984). Effects of pH on the interaction of substrates and malonyl-CoA with the mitochondrial carnitine palmitoyltransferase 1. Biochem. J. 219,601 -608.[Medline]
Mitchell, J. H. and Blomqvist, G. (1971). Maximal oxygen uptake. New Engl. J. Med. 284,1018 -1022.[Medline]
Moon, B. R., Hopp, J. J. and Conley, K. E.
(2002). Mechanical trade-offs explain how performance increases
without increasing cost in rattlesnake tailshaker muscle. J. Exp.
Biol. 205,667
-675.
Nielsen, O. B., de Paoli, F. and Overgaard, K.
(2001). Protective effects of lactic acid on force production in
rat skeletal muscle. J. Physiol.
536,161
-166.
Noakes, T. D. (1998). Maximal oxygen uptake: `classical' versus `contemporary' viewpoints: a rebuttal. Med. Sci. Sports Exerc. 30,1381 -1398.[Medline]
Noakes, T. D., Peltonen, J. E. and Rusko, H. K. (2001). Evidence that a central governor regulates exercise performance during acute hypoxia and hyperoxia. J. Exp. Biol. 204,3225 -3234.
Noakes, T. D., St Clair Gibson, A. and Lambert, E. V.
(2004). From catastrophe to complexity: a novel model of
integrative central neural regulation of effort and fatigue during exercise in
humans. Br. J. Sports. Med.
38.511
-514.
Pedersen, T. H., Clausen, T. and Nielsen, O. B. (2003). Loss of force induced by high extracellular [K+] in rat muscle: effect of temperature, lactic acid and beta2-agonist. J. Physiol. 551,277 -286.
Pedersen, T. H., Nielsen, O. B., Lamb, G. D. and Stephenson, D.
G. (2004). Intracellular acidosis enhances the excitability
of working muscle. Science
305,1144
-1147.
Pellerin, L. and Magistretti, P. J. (2003). How
to balance the brain energy budget while spending glucose differently.
J. Physiol. 546,325
.
Pellerin, L., Pellegri, G., Bittar, P. G., Charnay, Y., Bouras, C., Martin, J.-L., Stella, N. and Magistretti, P. J. (1998). Evidence supporting the existence of an astrocyte-neuron lactate shuttle. Dev. Neurosci. 20,291 -299.[CrossRef][Medline]
Pilegaard, H. and Juel, C. (1995). Lactate transport studied in sarcolemmal giant vesicles from rat skeletal muscles: effect of denervation. Am. J. Physiol. 269,E679 -E682.
Pilegaard, H., Domino, K., Noland, T., Juel, C., Hellsten, Y., Halestrap, A. P. and Bangsbo, J. (1999). Effect of high-intensity exercise training on lactate/H+ transport capacity in human skeletal muscle. Am. J. Physiol. 276,E255 -E261.
Posterino, G. S. and Fryer, M. W. (2000).
Effects of high myoplasmic L-lactate concentration on E-C coupling in
mammalian skeletal muscle. J. Appl. Physiol.
89,517
-528.
Posterino, G. S., Dutka, T. L. and Lamb, G. D. (2001). L(+)-lactate does not affect twitch and tetanic responses in mechanically skinned mammalian muscle fibres. Pflügers. Arch. 442,197 -203.[CrossRef][Medline]
Randle, P. J., Garland, P. B., Hales, C. N. and Newsholme, E. A. (1963). The glucose fatty acid cycle: its role in insulin sensitivity and the metabolic disturbances of diabetes mellitus. Lancet 1,785 -789.[Medline]
Rasmussen, H. N., Van Hall, G. and Rasmussen, U. F.
(2002). Lactate dehydrogenase is not a mitochondrial enzyme in
human and mouse vastus lateralis muscle. J. Physiol.
541,575
-580.
Rauch, H. G., St Clair Gibson, A., Lambert, E. V. and Noakes, T.
D. (2005). A signalling role for muscle glycogen in the
regulation of pace during prolonged exercise. Br. J. Sports.
Med. 39,34
-38.
Richards, J. G., Mercado, A. J., Clayton, C. A., Heigenhauser, G. J. F. and Wood, C. M. (2002). Substrate utilization during graded aerobic exercise in rainbow trout. J. Exp. Biol. 205,2067 -2077.
Richardson, R. S., Noyszewski, E. A., Leigh, J. S. and Wagner,
P. D. (1998). Lactate efflux from exercising human skeletal
muscle: role of intracellular PO2. J. Appl.
Physiol. 85,627
-634.
Robergs, R. A., Ghiasvand, F. and Parker, D. (2004). Biochemistry of exercise-induced metabolic acidosis. Am. J. Physiol. Regul. Integr. Comp. Physiol. 287,502 -516.
Roberts, T. J., Weber, J.-M., Hoppeler, H., Weibel, E. R. and
Taylor, R. C. (1996). II. Defining the upper limits of
carbohydrate and fat oxidation. J. Exp. Biol.
199,1651
-1658.
Roef, M. J., de Meer, K., Kalhan, S. C., Straver, H., Berger, R. and Reijngoud, D.-J. (2003). Gluconeogenesis in humans with hyperlactatemia during low-intensity exercise. Am. J. Physiol. 284,E1162 -E1171.
Roepstorff, C., Halberg, N., Hillig, T., Saha, A. K., Ruderman, N. B., Wojtaszewski, J. F. P., Richter, E. A. and Kiens, B. (2005). Malonyl-CoA and carnitine in regulation of fat oxidation in human skeletal muscle during exercise. Am. J. Physiol. 288,E133 -E142.
Rooyackers, O. (2005). Microdialysis to investigate tissue amino acid kinetics. Curr. Opin. Clin. Nutr. Metab. Care 8,77 -82.[Medline]
Rosdahl, H., Ungerstedt, U., Jorfeldt, L. and Henriksson, J. (1993). Interstitial glucose and lactate balance in human skeletal muscle and adipose tissue studies by microdialysis. J. Physiol. 471,637 -657.[Abstract]
Roth, D. A. and Brooks, G. A. (1990). Lactate transport is mediated by a membrane-bound carrier in rat skeletal muscle sarcolemmal vesicles. Arch. Biochem. Biophys. 279,377 -385.[CrossRef][Medline]
Ruderman, N. B. and Dean, D. (1998). Malonyl CoA, long chain fatty acyl CoA and insulin resistance in skeletal muscle. J. Basic. Clin. Physiol. Pharmacol. 9, 295-308.[Medline]
Sahlin, K., Fernstrom, M., Svensson, M. and Tonkonogi, M.
(2002). No evidence of an intracellular lactate shuttle in rat
skeletal muscle. J. Physiol.
541,569
-574.
Sidossis, L. S., Wolfe, R. R. and Coggan, A. R. (1998). Regulation of fatty acid oxidation in untrained vs. trained men during exercise. Am. J. Physiol. 274,E510 -E515.[Medline]
Skinner, M. R. and Marshall, J. M. (1996). Studies on the roles of ATP, adenosine and nitric oxide in mediating muscle vasodilation induced in the rat by acute systemic hypoxia. J. Physiol. 495,553 -560.[Abstract]
St Clair Gibson, A., Baden, D. A., Lambert, M. I., Lambert, E. V., Harley, Y. X. R., Hampson, D., Russell, V. A. and Noakes, T. D. (2003). The conscious perception of the sensation of fatigue. Sports Med. 33,167 -176.[Medline]
Starritt, E. C., Howlett, R. A., Heigenhauser, G. J. and Spriet, L. L. (2000). Sensitivity of CPT1 to malonyl-CoA in trained and untrained human skeletal muscle. Am. J. Physiol. 278,E462 -E468.
Street, D., Bangsbo, J. and Juel, C. (2001).
Interstitial pH in human skeletal muscle during and after dynamic graded
exercise. J. Physiol.
537,993
-998.
Trabold, O., Wagner, S., Wicke, C., Scheuenstuhl, H., Hussain, Z., Rosen, N., Seremetiev, A., Becker, H. D. and Hunt, T. K. (2003). Lactate and oxygen constitute a fundamental regulatory mechanism in wound healing. Wound Rep. Reg. 11,504 -509.[CrossRef]
Van Loon, L. J. C., Greenhaff, P. L., Constantin-Teodosiu, D.,
Saris, W. H. M. and Wagenmakers, A. J. M. (2001). The
effects of increasing exercise intensity on muscle fuel utilisation in humans.
J. Physiol. 536,295
-304.
Waldmann, R. and Lazdunski, M. (1998). H(+)-gated cation channels: neuronal acid sensors in the NaC/DEG family of ion channels. Curr. Opin. Neurobiol. 8, 418-424.[CrossRef][Medline]
Watt, P. W., MacLennen, P. A., Hundal, H. S., Kuret, C. M. and Rennie, M. J. (1988). L(+)-lactate transport in perfused rat skeletal muscle: kinetic characteristics and sensitivity to pH and transport inhibitors. Biochim. Biophys. Acta 944,213 -222.[Medline]
Watt, P. W., Gladden, L. B., Hundal, H. S. and Crawford, R. E. (1994). Effects of flow and contraction on lactate transport in the perfused rat hindlimb. Am. J. Physiol. 267,E7 -E13.[Medline]
Westerblad, H., Allen, D. G. and Lannergren, J. (2002). Muscle fatigue: lactic acid or inorganic phosphate the major cause? News Physiol. Sci. 17, 17-21.[Medline]
Wilson, M. C., Jackson, V. N., Heddle, C., Price, N. T.,
Pilegaard, H., Juel, C., Bonen, A., Montgomery, I., Hutter, O. F. and
Halestrap, A. P. (1998). Lactic acid efflux from white
skeletal muscle is catalysed by the monocarboxylate transporter isoform MCT3.
J. Biol. Chem. 273,15920
-15926.
Zhou, M., Lin, B.-Z., Coughlin, S., Vallega, G. and Pilch, P. F. (2000). UCP-3 expression in skeletal muscle: effects of exercise, hypoxia and AMP-activated protein kinase. Am. J. Physiol. 279,E622 -E629.