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INFLUENCE OF RESISTANCE EXERCISE INTENSITY AND METABOLIC
STRESS ON ANABOLIC SIGNALING AND EXPRESSION OF MYOGENIC
GENES IN SKELETAL MUSCLE
DANIIL V. POPOV, PhD,
1
EVGENY A. LYSENKO, PhD,
1
ANTON V. BACHININ,
1
TATIANA F. MILLER, PhD,
1
NADEZDA S. KUROCHKINA,
2
IRINA V. KRAVCHENKO, PhD,
3
VLADIMIR A. FURALYOV, PhD,
3
and
OLGA L. VINOGRADOVA, PhD
1
1
Laboratory of Exercise Physiology, Institute of Biomedical Problems, Russian Academy of Sciences, 76A Khoroshevskoe Shosse,
Moscow 123007, Russia
2
Faculty of Fundamental Medicine, M.V. Lomonosov Moscow State University, Moscow, Russia
3
Laboratory of Enzyme Engineering, A.N. Bach Institute of Biochemistry, Russian Academy of Sciences, Moscow, Russia
Accepted 4 June 2014
ABSTRACT: Introduction: We investigated the effect of resist-
ance exercise intensity and exercise-induced metabolic stress
on the activation of anabolic signaling and expression of myo-
genic genes in skeletal muscle. Methods: Ten strength-trained
athletes performed high-intensity [HI, 74% of 1-repetition maxi-
mum (RM)], middle-intensity (MI, 54% 1RM), or middle-intensity
(54% 1RM) no-relaxation exercise (MIR). Kinase phosphoryla-
tion level and myogenic gene expression in muscle samples
were evaluated before, 45 min, 5 h, and 20 h after exercise.
Results: The lactate concentration in MI was approximately 2-
fold lower than in the 2 other sessions, and was highest in MIR.
The phosphorylation level of extracellular kinase 1/2
Thr202/Tyr204
after exercise was related to metabolic stress. Metabolic stress
induced a decrease in myostatin mRNA expression, whereas
mechano-growth factor mRNA level depended on exercise
intensity. Conclusions: This study demonstrates that both inten-
sity and exercise-induced metabolic stress can be manipulated
to affect muscle anabolic signaling.
Muscle Nerve 51: 434–442, 2015
An increase in muscle mass and strength is an
important goal in rehabilitation medicine, sarcope-
nia, and protection from inactivity in elderly sub-
jects and for athletic training. According to the
recommendations of Spiering et al.,
1
the intensity
of resistance exercise should be >70% of 1-
repetition maximum (1RM). It is known that the
mechanical load affects the extracellular matrix of
muscle fibers and regulates muscle mass via
integrin-associated focal adhesion kinase (FAK),
2
generation of phosphatidic acid,
3
activation of
mitogen-activated protein kinases (MAPK),
4
and
expression of mechano-growth factor (MGF or
insulinlike growth factor-1Ec).
5
Several studies
have investigated the effect of resistance exercise
intensity on anabolic signaling, regulation of myo-
genic gene expression, and myofibrillar protein
synthesis rate after acute exercise sessions.
6–9
Inter-
estingly, it was impossible to unambiguously evalu-
ate the effects of resistance exercise intensity,
because the training loads in those studies differed
in intensity, total number of contractions, and total
tension time.
High-intensity resistance exercise may be exces-
sively strenuous, especially for the elderly, and it
may lead to exercise-induced damage. Therefore,
the anabolic potential of low- and middle-intensity
resistance exercise (20250% of 1RM) with blood
flow restriction in working muscles has been inves-
tigated intensively during the last decade. Blood
flow restriction during an exercise bout may be
achieved by vascular occlusion
10
or with a special
type of “no-relaxation” exercise of trained
muscles.
11–13
The essence of no-relaxation exercise
is that, during rhythmic extension2flexion move-
ments, the muscles do not relax at the end of each
movement cycle (i.e., the next extension is started
immediately after flexion). Previous studies have
shown that several weeks of low- or medium-
intensity resistance training (20250% of 1RM)
with blood flow restriction led to a greater increase
in muscle mass and strength compared with tradi-
tional training at the same exercise intensity.
10,14
Skeletal muscle hypertrophy induced by low-
intensity resistance exercise with blood flow restric-
tion is associated with substantial intramuscular
metabolic stress, which stimulates growth hormone
and insulin-like growth factor-1 (IGF-1) secretion
11,15
and activates mammalian target of rapamycin com-
plex 1 (mTORC1), extracellular kinase 1/2 (ERK1/
2) signaling,
16,17
and expression of myogenic regula-
tory genes.
18
Themechanismsunderlyingmuscle
Abbreviations: 1RM, 1-repetition maximum; AMPK, 5’-AMP-activated
protein kinase; ATP, adenosine triphosphate; ERK1/2, extracellular kinase
1/2; FAK, focal adhesion kinase; FOXO, forkhead box protein; HI, high-
intensity resistance exercise session; IGF-1, insulin-like growth factor 1;
MAPK, mitogen-activated kinase; HIF-1, hypoxia-inducible factor 1; MGF
(IGF-1Ec) mechano-growth factor (insulin-like growth factor 1, isoform 1Ec);
MI, middle-intensity resistance exercise session; MIR, middle-intensity no-
relaxation resistance exercise session; mTORC1, mammalian target of
rapamycin complex 1; MF, muscle fiber; MyoD1, myogenic differentiation
1; p21, cyclin-dependent kinase inhibitor 1A; p70S6K, p70 ribosomal S6
kinase; p90S6K, p90 ribosomal S6 kinase; PCR, polymerase chain reac-
tion; ROS, reactive oxygen species; SSC, stretch2shortening cycle.
Key words: gene expression; metabolic stress; protein kinase; resistance
exercise; skeletal muscle
This work was supported by the M.V. Lomonosov Moscow State Univer-
sity Program of Development; the program of the Presidium of the Russian
Academy of Sciences, “Molecular Mechanisms of Physiological
Functions”; and by a grant from the Russian Foundation for Basic
Research (12-04-01668-a).
Correspondence to: D.V. Popov; e-mail: danil-popov@yandex.ru
V
C2014 Wiley Periodicals, Inc.
Published online 11 June 2014 in Wiley Online Library (wileyonlinelibrary.com).
DOI 10.1002/mus.24314
434 Effects of Resistance Exercise MUSCLE & NERVE March 2015
hypertrophy induced by resistance exercise with
blood flow restriction have been investigated only
for low-intensity exercise (20% of 1RM). Notably,
these studies compared low-intensity exercise with
blood flow restriction with traditional resistance
exercise of equal intensity (20% of 1RM) and work.
It is well known that traditional low-intensity resist-
ance exercise performed without substantial fatigue
does not increase muscle protein synthesis,
6,17
growth of muscle mass, or strength.
14
Suga et al. used
31
P-spectroscopy to demonstrate
that, to achieve intramuscular metabolic stress
comparable to high-intensity resistance exercise
(65% of 1RM), the exercise intensity with blood
flow restriction should be >30% of 1RM.
19
More-
over, a middle-intensity (50% of 1RM) no-
relaxation exercise session induced more lactate
accumulation in the blood compared with a tradi-
tional high-intensity (80% of 1RM) session, and
several weeks of middle-intensity training led to an
increase in muscle mass and strength that was
comparable to that of high-intensity training.
11
To
date, the molecular mechanisms of adaptation to
middle-intensity exercise with blood flow restric-
tion remain unclear.
The goal of this study was to evaluate the
effects of resistance exercise intensity and meta-
bolic stress on the activation of intramuscular ana-
bolic signaling and on the expression of myogenic
regulatory genes. We investigated the effects of a
high-intensity (75% of 1RM) resistance exercise
session (HI), a middle-intensity (50% of 1RM)
exercise (MI) session, and a middle-intensity (50%
of 1RM) no-relaxation exercise session (MIR),
which had equivalent total tension time, total num-
ber of contractions, and range of motion. We used
the exercise of small muscle mass (bilateral leg exten-
sion2flexion), because the secretion of anabolic hor-
mones depends on the muscle mass involved.
20,21
This approach allowed us to avoid exercise-induced
blood hormone increases that could influence ana-
bolic signalling, specifically insulin and IGF-1 affect-
ing Akt-mTORC1 and testosterone influencing
protein synthesis rate.
22
We hypothesized that
increasing the exercise intensity from 50% of 1RM
(MI) to 75% of 1RM (HI) would induce activation of
intramuscular anabolic signaling and expression of
myogenic regulatory genes. In addition, we hypothe-
sized that MIR, even for middle-intensity exercise,
would activate the mechanisms that underlie muscle
protein synthesis and myogenesis by producing
strong metabolic stress.
METHODS
Ethics Approval. This study was approved by the
human ethics committee of the Institute of Bio-
medical Problems. All participants provided written
consent to take part in the study. The investigation
complied with the guidelines set forth in the Dec-
laration of Helsinki.
Initial Study. Ten amateur athletes (sprinters and
middle-distance runners) and physically active men
with a median weight of 76 (interquartile range
72–80) kg, height 1.78 (1.74–1.88) m, and age 23.1
(21.7–24.6) years participated in this study. All par-
ticipants usually perform 1 or 2 strength training
sessions per week. During the first 2 weeks of the
study (4 visits to the laboratory), the subjects were
familiarized with the 1RM test and exercise proto-
cols. For simultaneous loading of both legs during
exercise (bilateral knee extension) 2 dynamome-
ters (Pro System 3; Biodex, USA) were mounted
on 1 bed. The levers of the dynamometers were
secured, and equal ranges of motion were set for
both. The torque and angle analog signals were
converted by an analog-to-digital converter (Model
E-440; L-card, Russia) and recorded by Power-
Graph 3.3 software (DISoft, Russia). The signals
from both dynamometers were averaged and dis-
played online. After warm-up 1RM was determined;
each participant performed a single bilateral knee
extension in isotonic mode with a torque of 140
N•m. After a 1-min rest, the torque was increased
by 10 N•m. The 1RM was determined as the maxi-
mal torque when a subject could turn the dyna-
mometer levers from the initial position (knee
joint angle 90)toa50
angle.
Primary Study. All subjects performed 3 separate
resistance test sessions, each once per week in a
randomized order. All participants were instructed
to refrain from resistance exercise for 1 week and
to refrain from all exercise 36 h before the test.
The test session consisted of a warm-up (10 min of
cycling, workload 1 W/kg of body weight) and 8
sets of 12 bilateral knee extensions2flexions sepa-
rated by 6-min rest periods. The dynamometer tor-
que levels were set at 50% of 1RM for MI and MIR
and at 75% of 1RM for HI. The knee extensions
and flexions were performed in isotonic concentric
and eccentric modes, respectively. The rhythm of
the movements was set by visual and sound signals
using custom software. The extension and flexion
times were 0.4 s and 1.9 s, respectively. The total
tension time was equal in all training sessions (Fig.
1). During MI and HI, the rest period between
extension2flexion cycles was 3 s. During MIR,
each extension was started immediately after a flex-
ion. Therefore, the knee extensor muscles continu-
ously produced tension during a set so that the
torque during a set was maintained at an approxi-
mately constant level (Fig. 1). The knee angle and
the real torque produced by the knee extensors
during exercise were recorded as described above.
Effects of Resistance Exercise MUSCLE & NERVE March 2015 435
The participants arrived at the laboratory at
9:40 A.M. and consumed a standard breakfast (3624
kJ; 24 g of protein, 157 g of carbohydrates, and
15 g of lipids). The exercise session started 1 h 40
min after breakfast. All participants consumed 24 g
of carbohydrates, 5 g of branched-chain amino
acids (2.5 g of leucine, 1.25 g of isoleucine, and
1.25 g of valine), and ad libitum water during the
second part of the exercise session and also con-
sumed a standard lunch (3650 kJ; 29 g of protein,
116 g of carbohydrates, and 43 g of lipids) 60 min
after termination of the exercise. All participants
had their usual dinner at home prior to 9:00 P.M.
and returned to the laboratory the next morning
at 8:00 A.M. in the fasted state for muscle biopsy.
Venous blood was drawn before and 15 min
after exercise for evaluation of testosterone and
insulin using an immunoassay system (DxI 800;
Beckman Coulter, UK) and of IGF-1 using another
immunoassay system (Immulite 1000; Siemens,
Germany). Blood was drawn from fingertip capilla-
ries 30 s after 3, 5, and 8 sets for determination
of lactate and glucose concentrations (Super GL
Easy Analyzer; Dr. Mueller Geraetebau GmbH,
Germany). Biopsies were taken from the vastus lat-
eralis muscle using the microbiopsy technique
23
before and at 45-min, 5-h, and 22-h intervals after
exercise under local anesthesia (2 ml of 2% lido-
caine). The muscle samples were quickly blotted
with gauze to remove superficial blood, frozen in
liquid nitrogen for 20 s, and stored at 280C until
analysis. The first biopsy was taken 12 cm proximal
to the lateral femoral condyle. Subsequent biopsies
were taken 2 cm proximal to the previous biopsy.
The biopsies were taken from the right leg during
the first and third test sessions and from the left
leg during the second test session.
Immunoblotting. Frozen samples (10 mg) were sec-
tioned at 20 mm by an ultratome (Leica, Germany)
and homogenized in ice-cold radioimmunoprecipita-
tion buffer containing protease and phosphatase
inhibitors (50 mM b-glycerophosphatase, 50 mM
NaF, 1 mM Na
3
VO
4
,20lg/ml aprotinin, 50 lg/ml
leupeptin, 20 lg/ml pepstatin, and 1 mM phenylme-
thylsulfonylfluoride). Samples were then centrifuged
for 10 min at 10,000 3gand 4C. Protein content
was analyzed by the bicinchoninic assay. The samples
(20 lg protein per lane) were mixed with Laemmli
buffer and loaded onto a 10% T polyacrylamide
gel. Electrophoresis was performed using the Mini-
Protean Tetra Cell system (Bio-Rad, USA) at 20 mA
per gel. The proteins were transferred onto a nitro-
cellulose membrane using the Mini Trans-Blot system
(Bio-Rad) in Towbin buffer for 3 h at 300 mA. The
membrane was stained with Ponceau S to verify con-
sistent loading of proteins, which was followed by
washing and incubation in 5% nonfat dry milk for
1 h. The membrane was then incubated at 25Cwith
anti2phospho-p70S6K
Thr389
(Santa Cruz Biotechnol-
ogy, Germany) for 2 h or at 4C with anti-p70S6K,
anti-phospho-Akt
Thr308
,anti-Akt1,anti-phospho-Erk1/
2
Thr202/Tyr204
, anti-Erk1/2 (all from Cell Signaling
Technology, USA), anti2phospho-AMPKa1/2
Thr172
,
and anti-AMPKa1/2 (Santa Cruz Biotechnology,
Germany) overnight. On the next day, the mem-
brane was incubated with anti-rabbit secondary
antibody (Cell Signaling, USA)for1h.Aftereach
FIGURE 1. (A)Representative dynamics of torque and the knee angle during high-intensity resistance exercise (HI), middle-intensity
resistance exercise (MI), and middle-intensity resistance no-relaxation exercise (MIR). During MIR, the next knee extension was
started immediately after flexion so the torque and the muscle tension were practically constant, which produced a restriction of blood
flow in the working muscles. (B) Average torque during the exercise sessions. (C) Average tension time during the exercise sessions.
Values are expressed as median and interquartile range.
**
P<0.01 difference vs. MIR and MI.
436 Effects of Resistance Exercise MUSCLE & NERVE March 2015
step, the membrane was washed with PBS-Tween
20 (3 washes for 5 min each). The membrane was
incubated with enhanced chemiluminescence sub-
strate (Bio-Rad), luminescent signals were cap-
tured with X-ray film (Kodak, USA), and band
intensities were densitometrically scanned with
ImageJ software (National Institutes of Health,
USA). All data are expressed as the ratio of phos-
phorylated to total protein.
RNA Extraction. RNA was extracted from approxi-
mately 20 mg of wet muscle using TRIzol (Invitro-
gen, USA). RNA concentration was measured by
spectrophotometry (BioPhotometer, Eppendorf,
Germany) at an absorbance of 260 nm, and RNA
purity was assessed by the A260/A280-nm absorp-
tion ratio. cDNAs were obtained by annealing 1.5
lg of denatured (70C for 5 min) total RNA with
oligo (dT)
15
at 40C for 60 min (Sileks, Russia).
Real-Time Polymerase Chain Reaction. Real-time
polymerase chain reaction (PCR) was carried out
using the Rotor-Gene Q cycler (Qiagen, Germany).
The annealing temperature for each primer set
was optimized in trial PCR runs. The thermal pro-
file included an initial heat-denaturing step at
95C for 5 min followed by 40 cycles of denatura-
tion at 95C for 15 s, annealing at 60C for 30 s,
and extension at 72C for 45 s. Amplified genes
were quantified by fluorescence using the
SYBR-Green master mix (Syntol, Russia). After
amplification, the specificity of the amplification
was monitored using melting curves and agarose gel
(1%) electrophoresis. Each sample was run in tripli-
cate, and a non-template control was included in
each run. The target gene mRNA expression level
was calculated by the efficiency-corrected DDCt
method. PCR efficiency was calculated using stand-
ard curves corresponding to reference and target
genes. The primer sequences are listed in Table 1.
Statistics. Sample volumes were small with non-
normal data distributions, and thus the data are
expressed as median and interquartile range. The
Wilcoxon signed rank test was used to compare
the fold change of protein and gene expression
with the initial level, and the remaining matched
samples were compared using the Wilcoxon
matched-pairs test. The relation between samples
was evaluated by the Spearman rank correlation
test. Level of significance was set at 0.05.
RESULTS
The 1RM was 235 (2142253) N•m. The average
exercise intensities for HI, MI, and MIR were 74%
(72275%), 54% (52255%), and 54% (52255%),
respectively. The average tension time did not differ
among groups and was 253 (2252268) s, 262
(2422278) s, and 252 (2432259) s for HI, MI, and
MIR, respectively (Fig. 1).
After the third set, blood glucose decreased,
but after the sixth and eighth sets, it did not differ
from the initial level in all groups. Blood lactate
rose (P<0.05) during all exercise sessions, but the
increments by which the levels increased were dif-
ferent. Lactate concentration during HI was 2-fold
higher (P<0.05) than during MI, but it was
greater during MIR than during HI and MI
(P<0.05; Fig. 2). Blood insulin, IGF-1, and testos-
terone levels did not change after any of the exer-
cise sessions (Table 2).
The phosphorylation level of protein kinase B
(Akt
Thr308
) did not change after any of the exer-
cise sessions (Fig. 3). The phosphorylation level of
ribosomal protein S6 kinase (p70S6KThr
389
)
increased 1.3-fold (P50.048) at 22 h after termi-
nation of MI and did not increase after MIR or
HI. HI induced a 1.4-fold increase of 50-AMP-
activated protein kinase (AMPK)
Thr172
phosphoryl-
ation levels 45 min and 22 h after termination of
exercise (P<0.05), whereas MIR led to a 1.6-fold
increase of ERK1/2
Thr202/Tyr204
phosphorylation
levels 45 min and 22 h after termination of
exercise (P<0.01 and P<0.05, respectively). The
phosphorylation levels of AMPK
Thr172
45 min after
HI and of ERK1/2
Thr202/Tyr204
45 min and 22 h
after MIR were higher (P<0.05) than at those
times after MI. A weak but significant correlation
was found between peak blood lactate content
Table 1 . Primer sequences.
Gene Forward (50–30) Reverse (50–30)
p21 CCTCATCCCGTGTTCTCCTTT GTACCACCCAGCGGACAAGT
MyoD1 GGTCCCTCGCGCCCAAAAGAT CAGTTCTCCCGCCTCTCCTAC
IGF-1Ea ATGCTCTTCAGTTCGTGTGTG GCACTCCCTCTACTTGCGTTC
MGF (IGF-1Ec) ACCAACAAGAACACGAAGTC CAAGGTGCAAATCACTCCTA
Myostatin CATGATCTTGCTGTAACCTTCC CGATAATCCAATCCCATCC
RPLP0 CACTGAGATCAGGGACATGTTG CTTCACATGGGGCAATGG
ACTB CGTGACATTAAGGAGAAGCTGTGC CTCAGGAGGAGCAATGATCTTGAT
p21, cyclin-dependent kinase inhibitor 1A; MyoD1, myogenic differentiation 1; IGF-1Ea, insulin-like growth factor-1, splice variant Ea; MGF or IGF-1Ec,
insulin-like growth factor-1, splice variant Ec; RPLP0, ribosomal protein, large, P0; ACTB, actin, beta.
Effects of Resistance Exercise MUSCLE & NERVE March 2015 437
during exercise and phosphorylation levels of
ERK1/2
Thr202/Tyr204
45 min after termination of
exercise sessions (r50.38, P<0.05). The data
from all exercise sessions were included in the cor-
relation analysis.
The mRNA expression levels of cyclin-
dependent kinase inhibitor 1A (p21) and myo-
genic differentiation 1 (MyoD1), which are
markers of satellite cell activation and differentia-
tion, did not change, neither did expression of
IGF-1Ea mRNA (Fig. 4). HI induced a 2-fold
increase (P<0.05) in MGF (IGF-1Ec) mRNA
expression 22 h after termination of the exercise
session. Myostatin mRNA expression markedly
decreased 22 h after both HI and MIR, by 20-fold
(P<0.01) and 6-fold (P<0.05), respectively. After
HI and MIR, myostatin mRNA levels were lower
(P<0.05) than levels measured after MI.
DISCUSSION
In this study, blood lactate was increased in all
exercise sessions, but the increments by which the
levels increased were different. The lactate concen-
tration in MI was approximately 2-fold lower than
in the 2 other sessions, and lactate concentration
was highest in MIR. The latter finding is in agree-
ment with a previous study that compared intramus-
cular metabolic stress (pH, phosphocreatine, and
inorganic phosphate) during MIR (40% of 1RM)
and HI (65% of 1RM).
19
The total tension time,
total number of contractions, and range of motion
were the same in all sessions. Therefore, during MI,
the mechanical load and metabolic stress were low.
During MIR, the mechanical load was low, but the
metabolic stress was the highest and, during HI, the
mechanical load was high under pronounced meta-
bolic stress. Thus, the experimental design allowed
us to evaluate the effects of resistance exercise
intensity and metabolic stress on the activation of
intramuscular anabolic signaling and on the expres-
sion of myogenic regulatory genes.
Exercise-induced secretion of anabolic hor-
mones depends on the muscle mass involved in
exercise. In this study, the testosterone and IGF-1
levels did not change after the exercise sessions,
because the working muscle mass was not large. In
addition, no changes in insulin level were recorded
after the sessions. This finding allowed us to
exclude blood hormones as a potential regulator
of anabolic signaling and gene expression after the
exercise sessions.
Akt is one of the upstream proteins of the Akt-
mTOR1 pathway. Previous studies have shown
that the Akt
Thr308
phosphorylation level transiently
increased,
24–26
did not change,
27,28
or decreased
29
after resistance exercise. Phosphorylation of Akt is
regulated by insulin and IGF-1. In our study, the
absence of changes in the phosphorylation of
Akt
Thr308
after exercise may have been associated
with the lack of changes in insulin and IGF-1
levels.
One of the mTOR1 downstream targets, phos-
phorylated p70S6K
Thr389
, was shown to correlate
directly with an increase in skeletal muscle mass
after resistance exercise.
30,31
In our study, phos-
phorylation of p70S6K increased 1.3-fold
(P50.048) 22 h after termination of the MI.
Increases in exercise intensity and metabolic stress
during HI and MIR did not induce an increase of
FIGURE 2. Blood lactate levels during high-intensity resistance
exercise (HI), middle-intensity exercise (MI), and middle-
intensity no-relaxation exercise (MIR). Number of sets is shown
on the abscissa. Values expressed as median and interquartile
range.
*
P<0.05 difference vs. HI and
§§
P<0.01 difference vs.
MIR and HI.
Table 2. Blood hormones before and 10 min after high-intensity resistance exercise (HI), middle-intensity exercise (MI), and
middle-intensity no-relaxation exercise (MIR).
HI MIR MI
Before After Before After Before After
Insulin (mkU/ml) 20.7 24.0 18.3 19.3 21.6 20.3
(10.9–39.2) (16.3–32.0) (11.4–47.3) (16.8–28.7) (9.6–27.9) (12.3–29.6)
IGF-1 (ng/ml) 205 234 227 216 219 212
(161–293) (146–270) (175–286) (165–280) (152–278) (143–291)
Testosterone (nmol/L) 12.9 12.1 12.5 10.0 11.8 11.8
(10.6–15.9) (10.0–15.1) (10.8–14.5) (9.4–14.0) (10.6–15.6) (10.1–14.9)
Values expressed as the median (interquartile range). IGF-1, insulin-like growth factor-1.
438 Effects of Resistance Exercise MUSCLE & NERVE March 2015
p70S6K
Thr389
phosphorylation compared with MI.
This finding may be related to the training status
of our subjects,
27,32
because an increase of phos-
phorylated p70S6K
Thr389
content 3 h after HI was
found in endurance-trained athletes but not in
strength-trained athletes.
33
Chronic strength train-
ing in rats also confirmed the assumption that a
higher training status leads to a lower increase of
the p70S6K
Thr389
level in response to an acute
training session.
32
However, the absence of an
increase in p70S6K
Thr389
phosphorylation level
after HI may be connected with activation of
AMPK. Indeed, pharmacological activation of
AMPK after maximal electrically evoked contrac-
tions suppresses mTOR1 signaling in rat skeletal
muscle.
34
In our study, the increase in phosphoryl-
ated AMPK
Thr172
level was found only after HI.
This finding coincides with the increased activity
of AMPKa2
25
and the phosphorylation level of
AMPK
Thr172
during the first hour after HI.
35,36
Adenosine triphosphate (ATP) and phosphocre-
atine levels are diminished immediately after heavy
multiple-set HI.
37
According to the Henneman size
principle, the first movements of MI and MIR
bouts presumably involve mainly slow-twitch muscle
fibers (MFs), because the relative load is substan-
tially lower than 1RM. On the contrary, an HI
bout leads to recruitment of both slow- and fast-
twitch MFs even during the first movements.
Therefore, during the HI bout, fast-twitch MFs
recruit earlier than during MI and MIR. The
increase in phosphorylation of AMPK 45 min after
HI may be connected with fatigue of fast-twitch
MFs and decreased ATP content in the muscle
immediately after termination of the exercise.
Notably, in our study the increase in phosphoryla-
tion of AMPK during late recovery may have been
connected with more pronounced muscle glycogen
depletion after HI when compared with MIR and
MI, because total work in HI was approximately
25% higher than in MIR and MI. The decreased
muscle glycogen content in the resting state leads
to an increase in AMPK activity.
38,39
It is possible
that presumed lower muscle glycogen content after
HI stimulated AMPK phosphorylation for the
recovery period, which lasts up to 22 h.
The ERK1/2 pathway can activate several sub-
strates, such as p90 ribosomal S6 kinase (p90S6K)
and MAPK-interacting kinase 1, which leads to acti-
vation of the ribosomal subunit S6 and transcrip-
tion factors.
40
Most studies have reported an
FIGURE 3.
FIGURE 3. (A)Representative immunoblot. The fold change of
phosphorylation levels (phosphorylated to total protein) of (B)
Akt, (C) p70S6K, (D) AMPK, and (E) ERK1/2 before and after
high-intensity resistance exercise (HI), middle-intensity exercise
(MI), and middle-intensity no-relaxation exercise (MIR) normal-
ized to the initial level. The time of exercise session termination
is 0 h. Values are expressed as median and interquartile range.
*
P<0.05 difference vs. the initial level;
**
P<0.01 difference vs.
initial level; and
§
P<0.05 difference vs. MI.
Effects of Resistance Exercise MUSCLE & NERVE March 2015 439
increase of ERK1/2
Thr202/Tyr204
phosphorylation
immediately after or during the first hour after
HI.
41,42
However, 1 study indicated an increase at
6 h and 24 h after exercise.
26
The increase of
ERK1/2
Thr202/Tyr204
and p90S6K
Thr573
phosphoryla-
tion was shown to not depend on the degree of
muscle glycogen depletion
24
or on the adaptation
of muscle to strength training.
27,32
We showed that
the increase of exercise intensity from 54% to 74%
1RM did not induce change in the phosphoryla-
tion level of ERK1/2
Thr202/Tyr204
. This finding may
be connected with the AMPK activation, because
the pharmacological increase of AMPKa1/a2 activ-
ity in the myotube blocked the increase of the
ERK1/2
Thr202/Tyr204
phosphorylation level, presum-
ably through Raf1.
43
However, the substantial met-
abolic stress increased the ERK1/2
phosphorylation level 45 min and 22 h after MIR.
This finding coincides with the increase found in
ERK1/2
Thr202/Tyr204
and p90S6K
Thr573
phosphoryla-
tion levels at 4 h and 24 h, respectively, after a low-
intensity (30% 1RM) exercise that was performed
without pause until volitional failure.
6,28
Interest-
ingly, this type of exercise was similar to the MIR
exercise regime in our study. The phosphorylation
of ERK1/2 is expected to be sensitive to the num-
ber of contractions performed during an exercise
bout.
42
In our study, the number of contractions
was the same in all sessions. Therefore, activation
of ERK1/2
Thr202/Tyr204
depended on metabolic
stress (comparison of the MIR and MI). This find-
ing is consistent with the result of a previous study
in isolated rat skeletal muscle.
4
The MIR-induced
increase of the ERK1/2 phosphorylation level may
be connected to increased reactive oxygen species
(ROS) production, because MIR induced repeated
episodes of ischemia2reperfusion. ROS may then
have interacted with acidosis and mechanical ten-
sion to cause a greater response to the ERK1/2
phosphorylation level than in the other protocols.
4
In the MIR, the working muscle oxygenation index
is substantially decreased in comparison to HI and
MI.
13
A myoblast study showed that hypoxia enhan-
ces and prolongs ERK1/2
Thr202/Tyr204
activation in
a hypoxia-inducible factor-1 (HIF-1)-dependent
fashion.
44
Based on our study findings, we suggest
that the increase in ERK1/2 phosphorylation level
at the later stage of recovery after MIR was related
to activation of HIF-1. To our knowledge, there
are no existing studies on HIF-1 protein expression
after resistance exercise. However, Larkin et al.
45
showed that MIR (40% of 1RM) induced expres-
sion of HIF-1 mRNA, whereas traditional resistance
exercise with the same intensity did not induce
changes in expression of this gene.
Satellite cells are important for resistance exer-
cise2induced muscle hypertrophy.
46
We did not
FIGURE 4. The fold change of the mRNA level of (A) p21, (B)
MyoD, (C) MGF (IGF-1Ec), (D) IGF-1Ea, and (E) myostatin,
before and after high-intensity resistance exercise (HI), middle-
intensity exercise (MI), and middle-intensity no-relaxation exer-
cise (MIR). The time of exercise session termination is 0 h. Val-
ues expressed as median and interquartile range.
*
P<0.05
difference vs. initial level;
**
P<0.01 difference vs. initial level;
and
§
P<0.05 difference vs. MI.
440 Effects of Resistance Exercise MUSCLE & NERVE March 2015
observe changes in the mRNA expression of p21
and MyoD1, which are the markers of satellite cell
activation and differentiation. It has been sug-
gested that, in mature skeletal muscle, MGF (IGF-
1Ec) is responsible for satellite cell activation and
proliferation, whereas IGF-1Ea is responsible for
differentiation.
47
We found that only HI induced
MGF (IGF-1Ec) mRNA expression 22 h after termi-
nation of the exercise. This finding allows us to
conclude that mRNA expression of MGF (IGF-1Ec)
in trained muscle depends on the exercise inten-
sity (comparison of MI and HI) and does not
depend on the metabolic stress (comparison of
MIR and MI). We have shown previously that
myofibrillar proteins (such as myomesin 1, myosin-
binding protein C, and titin) released from dam-
aged cells stimulate MGF expression at both the
mRNA and protein levels in primary murine myo-
blasts or differentiated in vitro myotubes.
48
It is
possible to speculate that, in our study, HI induced
degradation of myofibrillar proteins, which may
stimulate MGF gene expression. This suggestion is
supported by our previous data. HI leg-press exer-
cise sessions increased blood creatine kinase activ-
ity (marker of muscle cell damage) at 20 h of
recovery more than MIR.
11
In skeletal muscle,
resistance exercise2induced expression of MGF
(IGF-1Ec) mRNA occurs earlier (at 24 h) com-
pared with IGF-1Ea mRNA (at 72 h).
49
This find-
ing may explain partially the lack of changes in
IGF-1Ea mRNA expression in our study.
Myostatin inhibits muscle stem cell prolifera-
tion
50
and differentiation by down-regulating
MyoD expression,
51
and it activates the forkhead
box protein (FOXO)-E3 ligase2proteasome system
through down-regulation of Akt.
52
We found that
high-intensity exercise under metabolic stress (HI)
decreased (20-fold) the myostatin mRNA level.
Comparison of MI and MIR allowed us to con-
clude that substantial metabolic stress is a suffi-
cient stimulus to decrease (6-fold) myostatin
mRNA expression.
Limitations. There are some notable limitations to
this study. In the MIR, each repetition was followed
immediately by another, so an eccentric phase pre-
ceded a concentric phase for each new repetition as
in a stretch-shortening cycle (SSC). Potentially it
may increase skeletal muscle mechanical efficiency
during the concentric phase. Also, eccentric knee
angle velocity was relatively low (30/s) in all proto-
cols, and therefore potentiation effects of the slow
SSC movements may be minimal due to the pro-
longed eccentric phase.
53
The mTORC1, MAPK, and FAK signaling path-
ways and strength training2induced muscle hyper-
trophy have been shown to depend on different
contraction variables such as peak torque, the
time2torque integral, and rate of strain.
54,55
Our
study design did not allow us to evaluate the influ-
ence of these variables on muscle anabolic
signaling.
The expression of myogenic factors was investi-
gated at the mRNA level only. Despite the lack of
change in p21 gene and MyoD1 gene expression, it
is possible that satellite cell activation and prolifer-
ation occurred (but this was not detected) along
with possible differences between the protocols.
CONCLUSION
Our study and other recent works
6,7,28
have
demonstrated that contractile variables, such as
intensity, duration, work, and exercise-induced
metabolic stress, can be manipulated to affect the
responses of muscle anabolic signaling. We showed
that, in trained skeletal muscle, the phosphoryla-
tion level of ERK1/2
Thr202/Tyr204
after resistance
exercise was related to metabolic stress and did
not depend on exercise intensity. Metabolic stress
itself induced a decrease in myostatin mRNA
expression, whereas MGF (IGF-1Ec) mRNA level
depended on resistance exercise intensity and not
on metabolic perturbations.
The authors thank Dr. Dmitriy Perfilov (Institute of Biomedical
Problems, Russian Academy of Sciences, Moscow) for medical
assistance, and Dr. Anatoly Borovik for excellent technical
support.
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