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Beta-Alanine supplementation reduces acidosis but not oxygen uptake response during high-intensity cycling exercise

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The oral ingestion of beta-alanine, the rate-limiting precursor in carnosine synthesis, has been shown to elevate the muscle carnosine content. Carnosine is thought to act as a physiologically relevant pH buffer during exercise but direct evidence is lacking. Acidosis has been hypothesised to influence oxygen uptake kinetics during high-intensity exercise. The present study aimed to investigate whether oral beta-alanine supplementation could reduce acidosis during high-intensity cycling and thereby affect oxygen uptake kinetics. 14 male physical education students participated in this placebo-controlled, double-blind study. Subjects were supplemented orally for 4 weeks with 4.8 g/day placebo or beta-alanine. Before and after supplementation, subjects performed a 6-min cycling exercise bout at an intensity of 50% of the difference between ventilatory threshold (VT) and VO(2peak). Capillary blood samples were taken for determination of pH, lactate, bicarbonate and base excess, and pulmonary oxygen uptake kinetics were determined with a bi-exponential model fitted to the averaged breath-by-breath data of three repetitions. Exercise-induced acidosis was significantly reduced following beta-alanine supplementation compared to placebo, without affecting blood lactate and bicarbonate concentrations. The time delay of the fast component (Td(1)) of the oxygen uptake kinetics was significantly reduced following beta-alanine supplementation compared to placebo, although this did not reduce oxygen deficit. The parameters of the slow component did not differ between groups. These results indicate that chronic beta-alanine supplementation, which presumably increased muscle carnosine content, can attenuate the fall in blood pH during high-intensity exercise. This may contribute to the ergogenic effect of the supplement found in some exercise modes.
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ORIGINAL ARTICLE
b-Alanine supplementation reduces acidosis but not oxygen
uptake response during high-intensity cycling exercise
Audrey Baguet Katrien Koppo Andries Pottier
Wim Derave
Accepted: 22 September 2009 / Published online: 16 October 2009
ÓSpringer-Verlag 2009
Abstract The oral ingestion of b-alanine, the rate-limit-
ing precursor in carnosine synthesis, has been shown to
elevate the muscle carnosine content. Carnosine is thought
to act as a physiologically relevant pH buffer during
exercise but direct evidence is lacking. Acidosis has been
hypothesised to influence oxygen uptake kinetics during
high-intensity exercise. The present study aimed to inves-
tigate whether oral b-alanine supplementation could reduce
acidosis during high-intensity cycling and thereby affect
oxygen uptake kinetics. 14 male physical education stu-
dents participated in this placebo-controlled, double-blind
study. Subjects were supplemented orally for 4 weeks with
4.8 g/day placebo or b-alanine. Before and after supple-
mentation, subjects performed a 6-min cycling exercise
bout at an intensity of 50% of the difference between
ventilatory threshold (VT) and _
VO2peak. Capillary blood
samples were taken for determination of pH, lactate,
bicarbonate and base excess, and pulmonary oxygen uptake
kinetics were determined with a bi-exponential model fitted
to the averaged breath-by-breath data of three repetitions.
Exercise-induced acidosis was significantly reduced fol-
lowing b-alanine supplementation compared to placebo,
without affecting blood lactate and bicarbonate concen-
trations. The time delay of the fast component (Td
1
) of the
oxygen uptake kinetics was significantly reduced following
b-alanine supplementation compared to placebo, although
this did not reduce oxygen deficit. The parameters of
the slow component did not differ between groups. These
results indicate that chronic b-alanine supplementation,
which presumably increased muscle carnosine content, can
attenuate the fall in blood pH during high-intensity exer-
cise. This may contribute to the ergogenic effect of the
supplement found in some exercise modes.
Keywords Ergogenic supplements
High-intensity cycling Acidosis _
VO2kinetics
Introduction
Maintaining acid–base balance is a major challenge during
high-intensity exercise, when large amounts of protons are
produced and released as a consequence of the anaerobic
energy delivery in the active musculature (Hultman and
Sahlin 1980). A significant portion of the contraction-
induced protons are rapidly transported out of the active
myocytes and buffered by the circulating buffers, such as
bicarbonate. In addition to the various sarcolemmal ion
transporters that can promote this proton efflux (Juel 1998),
several intramyocellular pH buffers are available as a first-
line defence against exercise-induced acidosis (Parkhouse
and McKenzie 1984). These include free inorganic phos-
phate, creatine phosphate and the histidine residues in both
proteins and in histidine-containing dipeptides (HCD). In
humans, the HCD are exclusively represented by carnosine
(b-alanyl-L-histidine) (Quinn et al. 1992). The relative
importance and quantitative contribution to total buffering
capacity of these skeletal muscle buffering constituents has
been a subject of discussion. Carnosine is likely not the
most important buffer in absolute terms, with a relative
contribution estimated to be approximately 8–15% in
human muscle (Hill et al. 2007; Parkhouse et al. 1985).
However, the molecule draws our special attention because
Communicated by Susan Ward.
A. Baguet K. Koppo A. Pottier W. Derave (&)
Department of Movement and Sports Sciences,
Ghent University, Watersportlaan 2, 9000 Ghent, Belgium
e-mail: wim.derave@ugent.be
123
Eur J Appl Physiol (2010) 108:495–503
DOI 10.1007/s00421-009-1225-0
of two reasons: (1) its imidazole group has an optimal pKa
value of 6.83 (Bate Smith 1938), knowing that intramyo-
cellular pH at rest is 7.0–7.1 and can drop during high-
intensity contractions to values as low as 6.3–6.5, and (2)
because of the flexibility of its myoplasmic concentration.
Recent studies (Harris et al. 2006; Hill et al. 2007; Derave
et al. 2007) have shown that oral supplementation with
b-alanine, the rate-limiting precursor of the dipeptide
synthesis, can elevate muscle carnosine content by 40–80%,
depending on the dose (usually between 3.2 and 6.4 g/day)
and duration (4–10 weeks). Noneof the other skeletal muscle
buffer pools is nearly as expansible.
Carnosine is a pleiotropic molecule, with pH buffering
capacity in muscle as only one of several possible other
physiological functions (Begum et al. 2005). Carnosine can
act as metal chelator, an anti-oxidant and an antiglycation
agent. In skeletal muscle, where the majority of the body’s
carnosine is found, carnosine can act as a Ca
??
sensitizer
for the sarcomeres (Dutka and Lamb 2004; Lamont and
Miller 1992) and by this mechanism possibly protect
against fatigue (Rubtsov 2001). Additionally, exercise-
induced oxidative stress is involved in contractile muscle
fatigue (Powers and Jackson 2008) and can supposedly be
antagonised by the antioxidative effects of carnosine
(Kohen et al. 1988). Several lines of evidence suggest that
carnosine is ergogenic. b-Alanine supplementation resulted
in performance enhancement in some exercise modes, such
as a single high-intensity exercise bout (Hill et al. 2007;
Stout et al. 2007), in sprint exercise at the end of an
endurance cycling race (Van Thienen et al. 2009) and in
repeated maximal contraction bouts (Derave et al. 2007). In
vitro work on isolated frog muscles has shown that addition
of carnosine to the incubation medium can potently an-
tagonise contractile fatigue (Severin et al. 1953; Boldyrev
and Petukhov 1978). It remains to be established by which
mechanism these ergogenic effects are established. An
improved pH buffer capacity certainly is a candidate,
because alternative ways to improve the buffer capacity
(albeit in blood instead of muscle) by acute oral bicar-
bonate ingestion have shown to improve performance in
exercise modes of similar duration and intensity, i.e. single
or repeated maximal exercise bouts of 30 s to 7 min of
duration (Linderman and Gosselink 1994). A first aim of
the current study was to explore the potential of carnosine
to attenuate exercise-induced acidosis in high-intensity
exercise of fixed intensity and duration.
The low blood pH resulting from anaerobic work is
thought to affect the oxygen uptake ( _
VO2) kinetics of high-
intensity exercise. In the transition from rest to exercise at
an intensity above the ventilatory threshold (VT) three
phases can be distinguished: (1) a short cardio-dynamic
phase (15–25 s), (2) a fast component (2–3 min) charac-
terised by an exponential rise in _
VO2and (3) a slow
component where _
VO2shows a slow but gradual increase
towards a steady-state or peak oxygen uptake ( _
VO2).
Several studies have shown that pre-exercise alkalosis,
either elicited by hyperventilation (Hayashi et al. 1999;
Ward et al. 1983) or by bicarbonate ingestion (Kolkhorst
et al. 2004), slows the fast component of _
VO2, probably by
inducing a leftward shift of the oxygen–haemoglobin dis-
sociation curve and reducing the O
2
delivery to the work-
ing muscles. However, others (Berger et al. 2006) did not
observe an effect of pre-exercise metabolic alkalosis on
_
VO2kinetics, and even others (Zoladz et al. 2005) found a
speeding of the fast component.
With regard to the nature of the slow component of _
VO2
kinetics, i.e. why _
VO2/W values are higher above than
below the VT, several physiological mechanisms have
been investigated, such as elevation in body and/or muscle
temperature, cardiac and ventilatory muscle work, auxil-
iary muscle work, recruitment of fast-twitch fibres and
metabolic factors (reviewed in Gaesser and Poole 1996;
Zoladz and Korzeniewski 2001). A possible candidate for
the latter is the acidosis, because a slow component only
occurs at exercise intensities above VT, i.e. where lactate
accumulation occurs. In order to investigate this more
causally, several interventions with enhancement or atten-
uation of exercise-induced acidosis have been performed.
Zoladz et al. (1998) observed that the magnitude of the
slow component is increased following acute pre-exercise
acidification induced by ingestion of ammonium chloride.
The effects of pre-exercise alkalinisation by oral sodium
bicarbonate ingestion are more equivocal. Some authors
found a significant reduction of the slow component
(Kolkhorst et al. 2004; Berger et al. 2006), whereas several
others observed no effect of bicarbonate (Heck et al. 1998;
Santalla et al. 2003; Zoladz et al. 1997).
The purpose of the present study was to explore whether
4 weeks of b-alanine supplementation can attenuate exer-
cise-induced acidosis during a fixed 6-min exercise bout at
an intensity calculated as 50% of the difference between
VT and _
VO2peak. In order to investigate the effect of the
possibly suppressed acidosis on the fast and slow compo-
nent of _
VO2kinetics, the exercise bout was repeated three
times in each condition on separate days, allowing optimal
bi-exponential modelling of breath-by-breath data.
Methods
Subjects
Fourteen male physical education students volunteered to
participate in this study. All subjects were physically
active, but not involved in sports competition or organised
training. The subjects’ age, weight, height and maximal
496 Eur J Appl Physiol (2010) 108:495–503
123
oxygen uptake were 21.9 ±1.5 years, 74.9 ±8.3 kg,
1.80 ±0.05 m and 55.5 ±3.6 mL/(kg min) for placebo
and 21.1 ±0.7 years, 71.8 ±8.8 kg, 1.78 ±0.07 m and
57.1 ±4.7 mL/(kg min) for b-alanine group, respectively
(NS). Subjects reported that they did not take any other oral
supplement during the study nor had taken nutritional
supplements in the 3 months prior to the study. Subjects
were asked to abstain from exercise 24 h before each test
and to maintain their normal physical activity during the
study. During this study they did not participate in regular
or organised training. The subjects gave their informed
consent and the study was approved by the local Ethics
Committee (Ghent University Hospital, Belgium).
Experimental protocol
The subjects of this placebo-controlled, double-blind study,
were randomised, based on their ( _
VO2peak) and blood pH at
the end of a 6-min cycling exercise at a power output
equivalent to 50% of the difference between VT and
_
VO2peak (50% D) into a control and experimental group.
They were supplemented for 4 weeks with either placebo
(maltodextrine) or b-alanine (Carnosyn
TM
, National
Alternatives International, San Marcos, USA). Supple-
ments were provided in capsules of 400 mg and were
administered each day as six divided doses, with at least
2 h in between ingestions. Daily doses consisted of 2.4 g/
day during the first 2 days, 3.6 g/day during the subsequent
2 days, and from then on 4.8 g/day until the end of the
supplementation. In a recent study on a similar study
population, we have shown that this supplementation
schedule leads to significant elevations in the carnosine
content of both slow-twitch and fast-twitch muscle types
(Baguet et al. 2009). Each subject performed a maximal
ramp exercise test on an electromagnetically braked cycle
ergometer (Lode, Excalibur sport; Groningen, The Neth-
erlands) to determine _
VO2peak and ventilatory threshold
(VT). Pedalling frequency was kept between 75 and
80 rpm. After a warm-up of 3 min at 50 W, the work load
was increased by 30 W/min to the point the subjects failed
to continue to pedal at 75 rpm. The gas exchange threshold
(GET) was determined as the point at which there was the
beginning of a systematic increase in _
VE_
VO2, but not in
_
VE_
VCO2, by two independent experienced researchers.
For all subjects, work rates equivalent to 50% Dwere
calculated.
There were two test periods (Pre and Post) with 4 weeks
of supplementation with b-alanine or placebo in between.
Each test period lasted 1 week and included on Monday,
Wednesday and Friday a 6-min cycle exercise bout at a
power output equivalent to 50% D. The 50% Dtest was
preceded by 3 min of baseline pedalling at 10 W and was
followed by 4 min of unloaded pedalling. The exercise test
sessions were conducted at the same time of the day for
each subject to account for any possible circadian rhythm
effects.
Measurements
Before and during the exercise bouts, _
VO2was measured
continuously on a breath-by-breath basis by means of a
computerised O
2
–CO
2
analyser-flowmeter combination
(Jaeger Oxycon Pro, Germany). Prior to each exercise
test, the gas analysers (an O
2
-analyser with functioning
based on the differential-paramagnetic principle and an
infrared CO
2
-analyser) were calibrated and the volume
calibration (‘triple V’ transducer) was conducted. Capil-
lary blood samples (150 lL) were taken from a hype-
raemic ear lobe in order to determine blood gas analysis
(GEM, Premier
TM
3000, Instrumentation Laboratory) at
rest (following warm-up), after 3 min cycling at 50% D,
after 6 min cycling at 50% Dand after 4 min recovery.
The blood samples were taken in two of the three tests
per condition (Wednesday and Friday) and the data of the
two samples were averaged. In the blood samples pH
and lactate were measured and bicarbonate and base
excess were calculated (base excess =(1 -0.014[Hb]) 9
([HCO
3-
]-24 ?(1.43[Hb] ?7.7) (pH -7.4))). In sub-
jects where blood lactate exceeded 15 mmol/L during
exercise, Lactate Pro strips (Arkray Inc, Kyoto, Japan)
were used for blood lactate determination.
Analysis
The breath-by-breath _
VO2data from each test were initially
examined to exclude errant breaths caused by coughing,
swallowing, sighing, etc., and those values exceeding local
mean by more than 4 standard deviations were deleted. The
breath-by-breath _
VO2data from each test were subsequently
linearly interpolated to give 1 s-values. For each subject and
each condition, the three identical repetitions were time-
aligned to the start of exercise, superimposed, and ensemble
averaged to reduce the breath-to-breath noise and enhance the
underlying physiological response characteristics. The base-
line _
VO2was defined as the average _
VO2measured during
baseline pedalling between 150 and 30 s before the start of the
50% Dbout. The initial cardiodynamic component was
ignored by eliminating the first 20 s of data after the onset of
exercise. Subsequently, each averaged response was descri-
bed using a bi-exponentialmodel with the following equation:
_
VO2tðÞ¼ _
VO2baseline þA11etTd1ðÞ
=s1

þA21etTd2ðÞ
=s2

This model includes amplitudes (A), time constants (s)
and delay times (Td) for the _
VO2fast (subscript 1) and the
Eur J Appl Physiol (2010) 108:495–503 497
123
_
VO2slow (subscript 2) component which were determined
using a non linear least-square algorithm. Because the
asymptotic value A
2
may represent a higher value than that
actually reached at the end of the exercise, the value of the
_
VO2slow exponential term at the end of exercise was
defined as A
2
0.
The O
2
deficit was computed by integrating the differ-
ence between the _
VO2requirement for the exercise
(assumed to be represented by the average _
VO2during the
last 30 s of exercise) and the actual measured _
VO2from
t=0tot=360.
Statistics
A292 repeated measures analysis of variance (RM
ANOVA) was used to evaluate pH, lactate, bicarbonate,
base excess, _
VO2kinetic parameters (A, sand Td), _
VO2,
ventilation ( _
VE) and CO
2
output ( _
VCO2) with ‘group’
(placebo vs. b-alanine) as between-subjects factor and
‘time’ (Pre and Post) as a within-subjects factor (SPSS
statistical software, SPSS Inc, Chicago, USA). Values are
presented as mean ±SD and significance was assumed at
p\0.05.
Results
Blood gas analysis
Table 1shows an overview of the blood gas analysis param-
eters. Blood pH at rest was approximately 7.41–7.42 and was
not affected by supplementation. The cycling exercise at an
intensity of 50% Delicited a marked acidosis towards values
around 7.20 at the sixth minute. There was no significant
interaction effect of the absolute pH values at 6 min of exer-
cise (Ex6), but Fig. 1shows that the pH difference between
baseline and Ex6 (i.e. the exercise-induced acidosis) is sig-
nificantly different between b-alanine and placebo group over
the time (interaction effect; p=0.031). As a result of
4 weeks’ supplementation the DpH from baseline to the end of
high-intensity cycling decreased with 0.015 units in the
b-alanine group and increased with 0.012 pH units in the
placebo group. Blood lactate increased to values of
*13 mmol/L at the end of exercise and slightly declined at
4 min into recovery. Bicarbonate and base excess markedly
decreased during high-intensity exercise. None of these
parameters (lactate, bicarbonate and base excess) showed a
significant group effect or interaction.
Pulmonary gas exchange
Figure 2shows a typical graph of the _
VO2before (Pre) and
after (Post) b-alanine and placebo supplementation. As
shown in Table 2, the _
VO2profile contains a clear slow
component (A
2
0)of*500 mL, as can be expected for
exercise intensities above the VT. For both groups there
were no significant differences (p[0.05) in _
VO2
throughout exercise prior to or after supplementation. As
shown in Table 3the _
VE was *22 L/min after warm up
and increased fivefold throughout the 6-min intensive
cycling, with no differences between both groups. For both
groups the _
VCO2was *750 mL/min after warm-up and
increased to *4,100 mL/min after a 6-min cycling exer-
cise at 50% Dbefore and after supplementation. Also for
_
VCO2there were no significant differences between
b-alanine and placebo (Table 3).
Table 1 Capillary pH, lactate, bicarbonate and base excess before (Pre) and after (Post) supplementation with b-alanine or placebo at rest, after
3 min exercise (Ex3), after 6 min exercise (Ex6) and 4 min into recovery (R4)
b-Alanine Placebo
Rest Ex3 Ex6 R4 Rest Ex3 Ex6 R4
pH
Pre 7.419 ±0.009 7.284 ±0.042 7.214 ±0.058 7.251 ±0.057 7.425 ±0.026 7.270 ±0.039 7.203 ±0.062 7.229 ±0.067
Post 7.411 ±0.016 7.274 ±0.032 7.221 ±0.054 7.251 ±0.048 7.411 ±0.024 7.244 ±0.046 7.177 ±0.070 7.216 ±0.059
Lactate (mmol/L)
Pre 1.16 ±0.62 9.89 ±1.90 13.41 ±2.16 11.31 ±2.87 1.01 ±0.38 10.26 ±2.06 13.64 ±1.07 11.89 ±2.09
Post 1.36 ±0.66 10.69 ±1.97 13.82 ±2.13 12.24 ±2.63 1.06 ±0.35 10.16 ±1.78 13.91 ±0.98 11.97 ±1.80
Bicarbonate (mmol/L)
Pre 26.55 ±1.27 20.39 ±2.03 15.37 ±2.30 15.71 ±2.72 26.49 ±2.29 20.62 ±2.72 15.85 ±2.49 15.44 ±3.11
Post 26.21 ±1.02 19.99 ±2.07 15.74 ±2.57 15.58 ±2.50 26.76 ±1.91 20.69 ±1.83 16.36 ±2.54 15.43 ±2.98
Base excess (mmol/L)
Pre 1.82 ±1.15 -6.19 ±2.29 -11.72 ±3.05 -10.59 ±3.41 1.91 ±2.41 -6.31 ±2.84 -11.63 ±3.30 -11.31 ±3.98
Post 1.39 ±1.06 -6.71 ±2.20 -11.29 ±3.17 -10.66 ±3.06 1.84 ±2.06 -6.89 ±2.27 -11.84 ±3.46 -11.64 ±3.69
Data are mean ±SD of 7 subjects per group
498 Eur J Appl Physiol (2010) 108:495–503
123
_
VO2kinetic parameters
In the fast component of the _
VO2kinetics, a significant
interaction (p=0.007) in the time delay (Td
1
) was observed,
which resulted from both a decrease over time (-2.2 s post
vs. pre) in the b-alanine group and an increase (?3.9 s) in the
placebo group. The time constant (tau
1
), however, tended to
display an opposite pattern (p=0.088 for the interaction
effect), i.e. a slowing in the b-alanine group (?1.2 s) and a
speeding (-4.8 s) in the placebo group. Therefore, the oxy-
gen deficit, which is influenced by both the Td
1
and tau
1
,was
not affected by either intervention (p=0.937). The ampli-
tude of the slow component (A
2
0) was not affected by
b-alanine supplementation. For the time constant (tau
2
)and
time delay (Td
2
) of the slow component, there was a tendency
for an interaction effect (p=0.082 and p=0.068, respec-
tively) (Table 2).
Discussion
The primary goal of the present study was to investigate the
role of muscle carnosine in the acid–base balance during
high-intensity exercise. According to the current working
hypothesis, an increased intramyocellular content of car-
nosine would attenuate the drop in intracellular pH during
high-intensity contractions. The smaller transsarcolemmal
concentration gradient of [H
?
] decreases the acid efflux
from the active muscle cells and results in less pronounced
circulating acidosis. Evidence for this hypothesis is now
presented in the current results. A 6-min exercise bout at a
fixed intensity above the VT (50% of the difference
between VT and _
VO2peak) in healthy male subjects resulted
in a decline in capillary blood pH from *7.4 to *7.2, yet
this acidosis, when expressed as the difference between
baseline and end-exercise, but not in absolute values, was
less pronounced after subjects were supplemented with
b-alanine for 4 weeks compared to placebo. Post-supple-
mentation, the exercise-induced acidosis was 19% smaller
in the b-alanine group compared to the placebo group
(Fig. 1). Although we did not measure the carnosine con-
centration, our two previous studies demonstrated that all
subjects had increased muscle carnosine content in 4–5
weeks of b-alanine supplementation (4–6 g/day) (Baguet
et al. 2009; Derave et al. 2007). This suggests that the
difference between groups is related to the presumable
elevation in muscle carnosine content (Harris et al. 2006;
Baguet et al. 2009). This supports the earlier suggestion of
Hill et al. (2007) that the physicochemical buffer property
of the dipeptide probably in part underlies the ergogenic
potential of b-alanine supplementation. However, it does
not exclude the additional contribution of other factors.
Traditionally, the importance of carnosine as a physi-
cochemical buffer in human skeletal muscle has been
largely ignored, because various calculations and mea-
surements have designated its relative contribution to only
8–15% of total buffer capacity (Hill et al. 2007; Parkhouse
et al. 1985; Hultman and Sahlin 1980). Indeed, in various
other vertebrates the HCD contribute more in both absolute
and relative terms (Abe 2000). In the middle gluteal muscle
of the thoroughbred horse, for example, the carnosine
concentration is 6.7-fold higher than in human vastus
lateralis, increasing its relative contribution to total buffer
capacity to 30.6% (Harris et al. 1990; Sewell et al. 1992).
The significant reduction in exercise-induced acidosis,
observed in the present study following b-alanine supple-
mentation (and a presumable increase in muscle carnosine
content of 40–50%), illustrates that the importance of
carnosine as a pH buffer should not be dismissed. The
cause for the discrepancy between its calculated small
importance and its observed physiological larger impor-
tance remains to be established. Part of the explanation
may be found in the fact that most changes in contracting
muscle occur in the narrow range of pH around the value of
6.8, where carnosine exerts its maximal effect, leading to
an underestimation of carnosine’s relative contribution.
In the present study the circulating bicarbonate and
lactate concentrations were not different between condi-
tions, which allows for the following interpretations. Given
the identical lactate levels, the lower degree of acidosis
evident in the b-alanine supplemented group, is not
caused by a lower anaerobic component of total energy
delivery. Hence, smaller acidosis genuinely represents
better buffering capacity, and not smaller acid production.
The identical bicarbonate levels suggest that the enhanced
intracellular buffering capacity (by b-alanine-induced
carnosine loading) is not compensated by sparing of
Fig. 1 DpH from baseline to the end (6 min) of high-intensity
cycling before and after 4 weeks’ supplementation of b-alanine or
placebo. Data are mean ±SD of seven subjects per group. *Signif-
icant interaction effect (p=0.03)
Eur J Appl Physiol (2010) 108:495–503 499
123
extracellular buffering capacity (mainly bicarbonate).
Carnosine has been implicated as an activator of carbonic
anhydrase activity (Temperini et al. 2005), but since in the
current study neither the circulating bicarbonate level and
pCO2, nor the pulmonary CO
2
output differed between
groups, it seems unlikely that b-alanine supplementation
attenuated acidosis through carbonic anhydrase activity.
Thus, the buffering actions of circulating bicarbonate and
intracellular carnosine are additive in order to better protect
the ‘milieu interieur’ against the homeostatic perturbation
of (extreme) acidosis. This is somewhat different from the
effects observed by Suzuki et al. (2006) during repeated
sprint exercise after subjects were acutely supplemented
with a 1.5 g carnosine/anserine or placebo mixture. In
that study, the enhanced circulating buffering capacity
by dipeptide supplementation was compensated by a
decreased utilisation (sparing) of bicarbonate (Suzuki et al.
2006). Therefore, acute dipeptide supplementation enhan-
ces the relative contribution of non-bicarbonate buffering
with only little effect on total blood buffer capacity. The
emerging conclusion of the latter and the current study is
that carnosine and bicarbonate can work as additive in
Fig. 2 Pulmonary oxygen
uptake before (Pre) and after
(Post) placebo (a) and b-alanine
(b) supplementation in a
representative subject
500 Eur J Appl Physiol (2010) 108:495–503
123
physicochemical buffering, provided they are located in
different compartments, i.e. carnosine intracellularly and
bicarbonate in the circulation.
An additional goal of the current study was to explore
the role of acidosis in the _
VO2kinetics during high-
intensity exercise. The Td
1
of the fast component was
significantly shorter following b-alanine compared to pla-
cebo supplementation, which suggests faster kinetics. The
physiological significance of this finding, however, is
probably limited because the effect was not sufficient to
alter the calculated oxygen deficit. The latter is probably
due to the fact that the time constant (tau
1
) changed, albeit
not significantly (p=0.088), in the direction of slower
kinetics (larger tau) following b-alanine supplementation.
Taken together, the effect of reduced exercise-induced
acidosis following b-alanine supplementation on the fast
component of the _
VO2kinetics is very limited. This is in
agreement with the recent study by Berger et al. (2006) that
reported no alteration in the fast component following
induced metabolic alkalosis. The fact that other studies did
find a significantly faster (shorter tau
1
, Zoladz et al. 2005)
or slower (longer tau
1
, Kolkhorst et al. 2004) kinetics
following metabolic alkalosis, may be caused by the
shortcoming that kinetic modelling was based on only one
repetition per condition, rather than several like transitions
in the current study and that of Berger et al. (2006).
As outlined in the introduction, some but not all studies
that have experimentally altered pre-exercise blood pH
(alkalinisation or acidification) support a role for proton
accumulation in the slow component of _
VO2. In the study by
Berger et al. (2006), the appearance of the slow component
(TD) is significantly delayed and the absolute _
VO2above
baseline at the end of exercise was significantly reduced
following sodium bicarbonate ingestion. In the current study,
the absolute end-exercise _
VO2and A
2
0were not different
between conditions. Therefore, the present study does not
support a role for acidosis in the physiological basis of the
slow component. At first sight, our results appear to be in
contradiction with the study from Berger et al. (2006)and
others (Kolkhorst et al. 2004;Forbesetal.2005). However,
the magnitude of experimental alteration of pH is less pro-
nounced in the current study compared with the latter studies.
Furthermore, the experimental intervention is essentially
different in nature. In the bicarbonate supplementation
studies, the resting pH is substantially elevated beyond values
that lie within the physiological variation range. In the
present study, however, the chronic b-alanine supplementa-
tion has no direct effect on resting blood pH, but the enhanced
buffer capacity suppresses the acidosis that results from high-
intensity work. Thus, although the absolute pH cannot be
excluded as a contributing factor, the magnitude of the
decline in pH during exercise is not a factor involved in the
slow component of _
VO2kinetics.
Conclusions
It can be concluded from the current data that chronic
b-alanine supplementation can reduce acidosis during
Table 2 _
VO2kinetic parameters: amplitude, tau and time delay of
the fast and slow components before (Pre) and after (Post) b-alanine
or placebo supplementation
b-Alanine Placebo Interaction effect
A
1
(mL/min), fast component
Pre 2,350 ±442 2,365 ±298 p=0.113
Post 2,462 ±670 2,311 ±259
Tau
1
(s), fast component
Pre 21.0 ±4.3 25.7 ±9.8 p=0.088
Post 24.2 ±6.8 20.9 ±4.9
Td
1
(s), fast component
Pre 14.7 ±1.9 11.5 ±3.9 p=0.007
Post 12.5 ±3.1 15.4 ±1.4
A
2
0(mL/min), slow component
Pre 601 ±198 632 ±103 p=0.194
Post 508 ±167 684 ±136
Tau
2
(s), slow component
Pre 179 ±97 241 ±121 p=0.082
Post 119 ±48 318 ±207
Td
2
(s), slow component
Pre 107 ±30 116 ±30 p=0.068
Post 131 ±42 98 ±34
O
2
deficit (L)
Pre 3.01 ±0.34 3.43 ±0.80 p=0.937
Post 3.06 ±0.53 3.47 ±0.69
Data are shown as mean ±SD
Table 3 Absolute oxygen uptake ( _
VO2), ventilation ( _
VE) and CO
2
output ( _
VCO2) after warm-up at 10 W (0 min) and after 6 min
intensive cycling at 50% Dbefore (Pre) and after (Post) supplemen-
tation with b-alanine or placebo
b-alanine Placebo
0 min 6 min 0 min 6 min
_
VO2(mL/min)
Pre 1,030 ±225 3,713 ±887 978 ±248 3,831 ±364
Post 937 ±128 3,844 ±702 972 ±226 3,876 ±410
_
VE (L/min)
Pre 23.6 ±3.4 111.7 ±39.6 22.4 ±3.5 106.1 ±23.3
Post 21.3 ±2.8 113.9 ±37.7 20.9 ±3.4 101.5 ±20.9
_
VCO2(mL/min)
Pre 808 ±102 4,090 ±815 781 ±86 4,256 ±551
Post 742 ±66 4,138 ±766 745 ±103 4,189 ±455
Data are shown as mean ±SD. There were no significant differences
Eur J Appl Physiol (2010) 108:495–503 501
123
high-intensity exercise. This indicates that carnosine may
act as a physiologically meaningful physicochemical buffer
in human skeletal muscle and may provide at least a part of
the explanation for the ergogenic effect of the b-alanine
supplement found in some exercise modes. Additionally,
these data do not support an important role for acidosis in
the oxygen deficit or in the origin of the slow component of
_
VO2kinetics during high-intensity exercise.
Acknowledgments This study was financially supported by grants
from the Research Foundation—Flanders (FWO 1.5.149.08 and
G.0046.09). Audrey Baguet is a recipient of a PhD-scholarship from
the Research Foundation—Flanders (FWO). We thank Dr. John Wise
and Natural Alternatives International (San Marcos, CA) for gener-
ously providing the b-alanine (CarnoSyn) and placebo supplements.
We thank Peter Van Mossevelde and Tim Decleir for their practical
contributions and Dr. Jacques Bouckaert for his valuable advice. The
experiments of this manuscript comply with the current laws of
Belgium.
Conflict of interest statement The authors declare that they have
no conflict of interest.
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Thesis
Hintergrund und Ziele: Jucken ist ein häufiges klinisches Symptom, welches insbesondere in dessen chronischer Form von den betroffenen Patienten als äußerst quälend beschrieben wird. Die zugrundeliegenden Mechanismen des chronischen nicht Histamin-vermittelten Pruritus sind weitestgehend ungeklärt. In den letzten Jahren wurden deshalb viele Experimente auf zellulärer Ebene und am Tier durchgeführt, wobei zahlreiche neue Mediatoren und deren Rezeptoren identifiziert werden konnten. Aktuell ist jedoch noch nicht geklärt, inwieweit diese Ergebnisse auf den Menschen übertragbar sind. Eine Gruppe von Erkrankungen, bei denen die Patienten oft an chronischem Pruritus leiden, sind cholestatische Lebererkrankungen. Aufgrund mangelnder Informationen über den Entstehungsmechanismus dieses Symptoms existiert bisher noch keine kausale Therapie. In der Vergangenheit wurden verschiedene Theorien diskutiert, die erklären könnten, wie es zur Empfindung Jucken kommt und wie diese von der Empfindung Schmerz zu unterscheiden ist. Ein Erklärungsansatz ist die „labeled-line theory“ von MÜLLER (1837), die besagt, dass für jede Sinnesmodalität – also auch für Jucken – ein separater neuronaler Weg existiert. Im Gegensatz dazu behauptet die „spatial contrast theory of pain and itch“, dass die Empfindung Jucken durch einen starken Kontrast von aktivierten und nicht aktivierten Nozizeptoren zustande kommt (Namer und Reeh 2013). Vor diesem Hintergrund untersuchten wir zunächst die „spatial contrast theory of pain and itch“ genauer. Hierfür testeten wir an verschiedenen Pruritogenen, ob die Applikationsarten Injektion und fokale Applikation unterschiedliche Juck- und Schmerzempfindungen hervorrufen. Ebenfalls wurden diese Experimente für potenzielle Pruritogene des cholestatischen Pruritus wie Lysophosphatidsäure (LPA) und verschiedene Gallensalze analysiert. Zusätzlich wurde bei Patienten mit cholestatischer Lebererkrankung untersucht, ob eine veränderte Erregbarkeit der Nervenfasern das Jucken verursachen könnte. Material und Methoden: Diese Arbeit kann in drei Versuchsteile unterteilt werden: Bei allen drei Versuchsteilen wurden die Reize an der volaren Seite des Unterarms gesetzt. Nach der Applikation wurden Juck- und Schmerzratings anhand einer numerischen Ratingskala (NRS) alle 10 Sekunden über 7 Minuten abgefragt. Die Substanzen wurden zum einen fokal mithilfe winziger Härchen einer tropischen Bohne (Mucuna pruriens) und zum anderen großflächig mit einer Injektion in die Haut eingebracht. Bei allen drei Versuchsteilen wurde die Größe des Axonreflex-Erythems, das durch den jeweiligen Reiz ausgelöst wurde, mithilfe des Laserdoppler-Imagings (LDI) bestimmt. Im ersten Teil wurde an insgesamt 19 gesunden Probanden getestet, inwieweit die unter-schiedliche Applikationsart (fokale Applikation versus Injektion) die Juck- bzw. Schmerzempfindung beeinflusst. Als Testsubstanzen wurden die Pruritogene β-Alanin, BAM8-22, Chloroquin und Cowhage verwendet. Der zweite Teil der Dissertation beschäftigte sich mit der Empfindung gesunder Probanden bei der Injektion und fokalen Applikation verschiedener Gallensalze bzw. Gallensalz-Kombinationen. An insgesamt 19 Probanden wurden folgende Substanzen ausgetestet: Desoxycholat (DC), Glykodesoxycholat (GDC), Ursodesoxycholat (UDC), Glykochenodesoxycholat (GCDC), Gallensalzpool aus DC, UDC und GDC, Kombination aus dem Gallensalzpool und LPA, LPA, dem semi-synthetischen farnesoid X receptor (FXR)-Agonisten INT-747 sowie dem semi-synthetischen TGR5-Agonisten INT-777. Im dritten Teil wurde an insgesamt 24 Patienten mit cholestatischer Lebererkrankung (davon 12 mit Pruritus und 12 ohne Pruritus) die Empfindung von verschiedenen Gallensalzen und elektrischer Stimulation mit Sinus- und Halbsinus-Strompulsen untersucht. Es wurde ein Gallensalzpool (aus DC, GDC und UDC), INT-747 und INT-777 appliziert. Anschließend wurde abgefragt, inwieweit verschiedene Sinus- und Halbsinus-Strompulse Schmerz bzw. Jucken auslösen. Statistische Signifikanzen wurden mithilfe des Programms STATISTICA © ermittelt. Ergebnisse: Zusammenfassend wurden in dieser Studie folgende Ergebnisse erzielt. Die psychophysikalische Testung verschiedener Pruritogene (β-Alanin, BAM8-22, Chloroquin, Cowhage) an gesunden Probanden zeigte mit Ausnahme von β Alanin, dass die intradermale Injektion dieser Substanzen sowohl Schmerz als auch Jucken verursacht. Bei der fokalen Applikation hingegen dominierte sehr deutlich das Jucken gegenüber dem Schmerz. Die natürlichen Gallensalze DC, GDC sowie UDC lösten bei gesunden Testpersonen sowohl nach Injektion als auch nach fokaler Applikation leichtes Jucken aus, wohingegen nach der Applikation des Gallensalzes GCDC keine Empfindung beobachtet werden konnte. Die Injektion der minimal modifizierten Gallensalze INT 747 sowie INT 777 verursachte mehr Schmerz und weniger Jucken. Die fokale Applikation der INTs verursachte hingegen leichtes Jucken. Die psychophysikalische Testung verschiedener Gallensalze/Gallensalz-Kombinationen sowie Halbsinus-/Sinus-Strompulse bei cholestatischen Patienten zeigte bei Patienten mit Pruritus mehr Jucken als bei den Patienten ohne Pruritus. Dies konnte sowohl bei der Injektion des Gallensalzpools (aus DC, GDC und UDC) wie auch bei der fokalen Applikation von INT-747 und INT-777 und bei der elektrischen Reizung mit Halbsinus- und Sinus-Strompulsen verschiedener Intensitäten beobachtet werden. Diskussion: Dass die „spatial contrast theory of pain and itch“ ein vielversprechender Ansatz für die Entstehung der Empfindung Jucken ist, bekräftigen die Ergebnisse der von uns durchgeführten Psychophysik-Studie. Vorwiegend Jucken wurde empfunden, wenn die Testsubstanzen (BAM8-22, Chloroquin, Cowhage) fokal über Spicules in die Haut appliziert wurden, wodurch nur vereinzelte Nozizeptoren aktiviert werden. Schmerz hingegen wurde gespürt, wenn die Substanz viele nebeneinanderliegende Nozizeptoren gleichmäßig aktiviert – wie es bei der intradermalen Injektion der Fall war. Lediglich β-Alanin erzeugte unabhängig von der Applikationsart hauptsächlich Jucken. Dies kann ebenfalls mit der „spatial contrast theory of pain and itch“ erklärt werden: Erstens wird der MrgprD, der Rezeptor von β-Alanin, lediglich auf einer sehr kleinen Untergruppe aller DRGs exprimiert (Dong et al. 2001) und zweitens wird nur die Hälfte von den MrgprD-positiven Nervenfasern von β-Alanin aktiviert (Liu et al. 2012). Dass die Gallensalze an der Entstehung des cholestatischen Pruritus beteiligt sind, wurde bereits öfter diskutiert. In unserer Psychophysik-Studie verursachten jedoch nicht alle Gallen-salze gleichermaßen Jucken. Lediglich DC, GDC sowie UDC – bekannte MrgprX4-Agonisten („unpublished“ Daten von Katharina Wolf, AG Kremer) – erzeugten nach deren Applikation Jucken. GCDC, das den MrgprX4 nicht aktiviert, löste hingegen keine juckende Empfindung aus. Das lässt vermuten, dass der MrgprX4 eine entscheidende Rolle in der Vermittlung des cholestatischen Pruritus spielt. Die psychophysikalische Testung verschiedener chemischer und elektrischer Reize bei Patienten, die an einer cholestatischen Erkrankung leiden, zeigte, dass Patienten mit chronischem Pruritus nach Applikation der Reize mehr Jucken spürten verglichen mit Patienten ohne chronischen Pruritus. Das lässt vermuten, dass die Patienten mit Pruritus eine veränderte Empfindung aufweisen. Ursächlich kann sein, dass diese Patienten über erhöhte Konzentrationen bestimmter Substanzen, welche Prurizeptoren aktivieren, verfügen. Es könnten aber auch veränderte Nervenfasern oder ein Nervenfaserverlust zu dieser veränderten Empfindung führen.
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