Blood flow restriction during low-intensity resistance exercise increases S6K1 phosphorylation and muscle protein synthesis.

Abstract

Low-intensity resistance exercise training combined with blood flow restriction (REFR) increases muscle size and strength as much as conventional resistance exercise with high loads. However, the cellular mechanism(s) underlying the hypertrophy and strength gains induced by REFR are unknown. We have recently shown that both the mammalian target of rapamycin (mTOR) signaling pathway and muscle protein synthesis (MPS) were stimulated after an acute bout of high-intensity resistance exercise in humans. Therefore, we hypothesized that an acute bout of REFR would enhance mTOR signaling and stimulate MPS. We measured MPS and phosphorylation status of mTOR-associated signaling proteins in six young male subjects. Subjects were studied once during blood flow restriction (REFR, bilateral leg extension exercise at 20% of 1 repetition maximum while a pressure cuff was placed on the proximal end of both thighs and inflated at 200 mmHg) and a second time using the same exercise protocol but without the pressure cuff [control (Ctrl)]. MPS in the vastus lateralis muscle was measured by using stable isotope techniques, and the phosphorylation status of signaling proteins was determined by immunoblotting. Blood lactate, cortisol, and growth hormone were higher following REFR compared with Ctrl (P < 0.05). Ribosomal S6 kinase 1 (S6K1) phosphorylation, a downstream target of mTOR, increased concurrently with a decreased eukaryotic translation elongation factor 2 (eEF2) phosphorylation and a 46% increase in MPS following REFR (P < 0.05). MPS and S6K1 phosphorylation were unchanged in the Ctrl group postexercise. We conclude that the activation of the mTOR signaling pathway appears to be an important cellular mechanism that may help explain the enhanced muscle protein synthesis during REFR.

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103:903-910, 2007. First published Jun 14, 2007; doi:10.1152/japplphysiol.00195.2007
J Appl Physiol
Dreyer, Yoshiaki Sato, Elena Volpi and Blake B. Rasmussen
Satoshi Fujita, Takashi Abe, Micah J. Drummond, Jerson G. Cadenas, Hans C.

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Blood flow restriction during low-intensity resistance exercise increases S6K1
phosphorylation and muscle protein synthesis
Satoshi Fujita,1,4Takashi Abe,4Micah J. Drummond,2,3Jerson G. Cadenas,1Hans C. Dreyer,2,3
Yoshiaki Sato,4Elena Volpi,1and Blake B. Rasmussen2,3
Departments of1Internal Medicine and2Physical Therapy; and3Division of Rehabilitation Sciences, University of Texas
Medical Branch, Galveston, Texas; and4Department of Human and Engineered Environmental Studies, Graduate School of
Frontier Sciences, University of Tokyo, Chiba, Japan
Submitted 16 February 2007; accepted in final form 11 June 2007
Fujita S, Abe T, Drummond MJ, Cadenas JG, Dreyer HC, Sato
Y, Volpi E, Rasmussen BB. Blood flow restriction during low-
intensity resistance exercise increases S6K1 phosphorylation and
muscle protein synthesis. J Appl Physiol 103: 903–910, 2007. First
published June 14, 2007; doi:10.1152/japplphysiol.00195.2007.—
Low-intensity resistance exercise training combined with blood flow
restriction (REFR) increases muscle size and strength as much as
conventional resistance exercise with high loads. However, the cellu-
lar mechanism(s) underlying the hypertrophy and strength gains
induced by REFR are unknown. We have recently shown that both the
mammalian target of rapamycin (mTOR) signaling pathway and
muscle protein synthesis (MPS) were stimulated after an acute bout of
high-intensity resistance exercise in humans. Therefore, we hypothe-
sized that an acute bout of REFR would enhance mTOR signaling and
stimulate MPS. We measured MPS and phosphorylation status of
mTOR-associated signaling proteins in six young male subjects.
Subjects were studied once during blood flow restriction (REFR,
bilateral leg extension exercise at 20% of 1 repetition maximum while
a pressure cuff was placed on the proximal end of both thighs and
inflated at 200 mmHg) and a second time using the same exercise
protocol but without the pressure cuff [control (Ctrl)]. MPS in the
vastus lateralis muscle was measured by using stable isotope tech-
niques, and the phosphorylation status of signaling proteins was
determined by immunoblotting. Blood lactate, cortisol, and growth
hormone were higher following REFR compared with Ctrl (P ?
0.05). Ribosomal S6 kinase 1 (S6K1) phosphorylation, a downstream
target of mTOR, increased concurrently with a decreased eukaryotic
translation elongation factor 2 (eEF2) phosphorylation and a 46%
increase in MPS following REFR (P ? 0.05). MPS and S6K1
phosphorylation were unchanged in the Ctrl group postexercise. We
conclude that the activation of the mTOR signaling pathway appears
to be an important cellular mechanism that may help explain the
enhanced muscle protein synthesis during REFR.
mammalian target of rapamycin; ischemia; hypertrophy; protein me-
tabolism; postexercise recovery
RESISTANCE EXERCISE is a potent stimulus for an increase in
muscle protein synthesis and subsequent muscular hypertro-
phy. After an acute bout of high-intensity resistance exercise,
muscle protein synthesis increases significantly within 1–2 h
and remains elevated for up to 48 h (34, 36, 37). The mam-
malian target of rapamycin (mTOR) signaling pathway plays a
significant role in stimulating translation initiation and muscle
protein synthesis (50). Recent studies have shown that the
activation of mTOR signaling pathway is gradually activated
during the recovery phase of resistance exercise (5, 9, 20). In
fact, mTOR signaling to its downstream effector, ribosomal S6
kinase 1 (S6K1), is involved in the regulation of mRNA
translation initiation and appears to be a critical regulator of
exercise-induced muscle protein synthesis and training-in-
duced hypertrophy (3, 4, 39). More recently, we have shown
that following an acute bout of resistance exercise, the mTOR
signaling pathway is activated in association with an increase
in protein synthesis in human skeletal muscle (9). Furthermore,
the phosphorylation of Akt/protein kinase B (PKB) increased
and the phosphorylation of eukaryotic translation elongation
factor 2 (eEF2) decreased during postexercise recovery (9).
Akt activation promotes mTOR phosphorylation and signaling
(50). Additionally, mTOR signaling to S6K1 inhibits eEF2
kinase, which reduces eEF2 phosphorylation and thus pro-
motes translation elongation (50).
According to the size principle (9, 14, 15), motor units with
a smaller cell size, i.e., slow-twitch type I muscle fibers, are
recruited first at lower exercise intensities, whereas at higher
exercise intensities, larger motor units and their associated type
II muscle fibers are recruited. These fast-twitch type II fibers
respond to high-intensity resistance exercise with a greater
amount of hypertrophy than type I fibers (28, 31). Therefore,
the American College of Sports Medicine (22) recommends
that resistance exercise be performed at an intensity of at least
70% of an individual’s one repetition maximum (1-RM) to
achieve the maximal hypertrophy. Although these guidelines
are optimal for healthy people, there are numerous circum-
stances in which it would be extremely difficult to achieve such
a high exercise intensity level in populations such as the frail
elderly, in patients with osteoarthritis, or in patients undergoing
the immediate rehabilitation phase following surgery. There-
fore, interventions designed to prevent muscle atrophy and/or
enhance muscle hypertrophy using exercise protocols consist-
ing of lower intensities may be useful for counteracting a
variety of muscle wasting conditions, such as sarcopenia,
cancer cachexia, AIDS, chronic obstructive pulmonary disease,
stroke, trauma/surgery, etc. Low-intensity resistance exercise
during blood flow restriction may be a useful intervention to
enhance muscle growth in these clinical conditions.
A series of recent studies have shown that a low-intensity
(20?50% of 1-RM) resistance exercise training combined with
a moderate reduction of blood flow to the working muscle
(REFR) produces similar increases in muscle size and strength
compared with traditional high-intensity resistance training (1,
Address for reprint requests and other correspondence: S. Fujita, Univ. of
Tokyo, Graduate School of Frontier Sciences, 5-1-5, Kashiwanoha, Kashiwa-
shi, Chiba 277-8563, Japan (e-mail: fujita@k.u-tokyo.ac.jp).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
J Appl Physiol 103: 903–910, 2007.
First published June 14, 2007; doi:10.1152/japplphysiol.00195.2007.
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42, 46, 47). However, the metabolic and molecular mecha-
nisms responsible for inducing the increase in muscle hyper-
trophy during REFR are currently unknown.
Therefore, the purpose of the present study was to determine
the effect of an acute bout of low-intensity resistance exercise
(with or without blood flow restriction) on the muscle mTOR
signaling pathway (an important regulator of translation initi-
ation, elongation, muscle protein synthesis, and muscle cell
growth) and muscle protein synthesis. We hypothesized that an
acute bout of REFR would enhance mTOR signaling and
stimulate muscle protein synthesis.
EXPERIMENTAL PROCEDURES
Subjects
We studied six young male subjects on two separate occasions. All
subjects were healthy and physically active but were not currently
engaged in an exercise training program. All subjects gave informed
written consent before participating in the study, which was approved
by the Institutional Review Board of the University of Texas Medical
Branch. Screening of subjects was performed with clinical history,
physical examination, and laboratory tests, including complete blood
count with differential, liver and kidney function tests, coagulation
profile, fasting blood glucose, and oral glucose tolerance test (OGTT),
hepatitis B and C screening, HIV test, thyroid-stimulating hormone,
lipid profile, urinalysis, drug screening, and ECG. The subjects’
characteristics are summarized in Table 1. The subjects were initially
randomized to an infusion study in which they performed resistance
exercise during blood flow restriction (REFR) or a control group in
which the subjects performed resistance exercise with no restriction of
blood flow (Ctrl).
Study design
Each subject was admitted to the General Clinical Research Center
of the University of Texas Medical Branch the day before the exercise
study, and a dual-energy X-ray absorptiometry (DEXA) scan (Hologic
QDR 4500W, Bedford, MA) was performed to measure body com-
position and lean mass. The subjects were then fed a standard dinner,
and a snack was given at 2200. The subjects were studied following
an overnight fast under basal conditions and refrained from exercise
for 24 h before study participation. The morning of the infusion study,
at 0600, an 18-gauge polyethylene catheter was inserted into an
antecubital vein for tracer infusion. Another 18-gauge polyethylene
catheter was inserted retrogradely in a hand vein of the opposite arm,
which was kept in a heated pad for arterialized blood sampling. After
drawing a background blood sample, a primed continuous infusion of
L-[ring-13C6]phenylalanine (Cambridge Isotope Laboratories, An-
dover, MA) was begun (time ? 0 h at 0800) and maintained at a
constant rate until the end of the experiment (Fig. 1). The priming
dose for the labeled phenylalanine was 2 ?mol/kg, and the infusion
rate was 0.05 ?mol?kg?1?min?1.
Two hours following the initiation of the tracer infusion, the first
muscle biopsy was obtained from the lateral portion of the vastus
lateralis of the leg with the biopsy site between 15 and 25 cm from the
midpatella. The biopsy was performed using a 5-mm Bergstro ¨m
biopsy needle under sterile procedure and local anesthesia (1% lido-
caine). Muscle tissue was immediately blotted and frozen in liquid
nitrogen and stored at ?80°C until analysis.
REFR group. After the second biopsy was obtained, a lower-
extremity pressure cuff (Kaatsu-Master Mini, Sato Sports Plaza,
Tokyo, Japan) was placed around the most proximal portion of each
leg. While the subject was seated on a chair, the pressure cuff was
increased to 120 mmHg for 30 s, and the air pressure was released.
The pressure cuff was then inflated four more times with each period
being increased by 20 mmHg. Each period lasted 30 s, and then the
cuff was released for 10 s between periods until a final pressure of 200
mmHg was reached. With the pressure maintained at 200 mmHg, the
subjects then performed a set of 30 repetitions of bilateral leg
extension exercise at 20% of 1-RM, followed by a 30-s rest period.
Subsequently subjects then performed three more sets of 15 repeti-
tions with 30-s rest intervals for a total of four sets and 75 repetitions.
Immediately after the fourth set, the pressure was released from the
cuff. Total time for the exercise period was approximately 4–5 min.
Resistance exercise without blood flow restriction: Ctrl group. The
subjects in the Ctrl group performed the identical exercise protocol as
the REFR group except that the cuff was not inflated and no pressure
was applied to the legs.
Infusion study 2. Three weeks after the first visit, subjects assigned
to the REFR group during infusion study 1 repeated the protocol
without blood flow restriction (Ctrl). Subjects assigned to the Ctrl
group during infusion study 1 then completed the blood flow-re-
stricted exercise protocol (REFR). Subjects were initially randomized
to either the Ctrl or the REFR group, and therefore six subjects were
studied for both groups.
Hormones and Glucose/Lactate
Serum concentrations of growth hormone (GH), IGF-1, total tes-
tosterone, and cortisol were determined by chemiluminescent enzyme
immunoassay using the Immunolite Automated Analyzer (Diagnostic
Products, Los Angeles, CA). Plasma glucose and lactate concentration
was measured using an automated glucose and lactate analyzer (YSI,
Yellow Springs, OH).
SDS PAGE and Western Blot Analysis
Details of the immunoblotting procedures have been previously
published (9). Briefly, ?30–50 mg of frozen tissue was homogenized
Table 1. Subject characteristics
CharacteristicValue
Age, yr
Weight, kg
Height, m
BMI, kg/m2
Leg lean mass, g
Left leg
Right leg
32?2
84?4
1.70?0.02
29?1
9,840?447
9,974?541
Values are means ? SE; n ? 6 subjects. Each subject was tested once during
blood flow restriction (REFR) and a second time without the pressure cuff
(Ctrl). REFR, resistance exercise with blood flow restriction; Ctrl, resistance
exercise without blood flow restriction. BMI; body mass index.
Fig. 1. Study design. Blood and muscle samples were taken at the times
indicated by the arrows. Exercise was performed immediately after the second
biopsy.
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(1:9, wt/vol), centrifuged for 10 min at 4°C, followed by the removal
of the supernatant. Total protein concentrations were determined by
using the Bradford assay (Bio-Rad, Smartspec Plus spectrophotome-
ter). The supernatant was diluted (1:1) in a sample buffer mixture
containing 125 mM Tris (pH 6.8), 25% glycerol, 2.5% SDS, 2.5%
B-mercaptoethanol, and 0.002% bromphenol blue and then boiled for
3 min at 100°C. Fifty micrograms of total protein was loaded into each
lane, and the samples were separated by electrophoresis (100 V for 60
min) on a 7.5% polyacrylamide gel (Bio-Rad, Criterion). A molecular
weight ladder (Bio-Rad, Precision Plus protein standard) was also
included on each gel. Following electrophoresis, protein was trans-
ferred to a polyvinylidene difluoride membrane (Bio-Rad, Hercules,
CA) at 50 V for 60 min. Blots were incubated in a single primary
antibody overnight at 4°C (antibody concentrations are described
below). The next morning, blots were incubated in secondary anti-
body for 1 h at room temperature. Chemiluminescent solution (ECL
plus, Amersham BioSciences, Piscataway, NJ) was applied to each
blot. After a 5-min incubation, optical density measurements were
obtained with a phosphorimager (Bio-Rad) and densitometric analysis
was performed using Quantity One software (version 4.5.2) (Bio-
Rad). Preliminary experiments were performed to assess if protein
abundance changed over the 7 h of the experiment. We found that
protein abundance did not change over the short time frame of the
study; therefore, the data were expressed as the change in phosphor-
ylation (in arbitrary units) normalized to a rodent standard.
Antibodies
The primary antibodies used were all purchased from Cell Signal-
ing (Beverly, MA): phospho-mTOR (Ser2448; 1:1,000), phospho-p70
S6K1 (Thr389; 1:500), phospho-Akt (Ser473; 1:500), and phospho-
eEF2 (Thr56; 1:1,000). Anti-rabbit IgG horseradish peroxidase-con-
jugated secondary antibody was purchased from Amersham Bio-
science (1:2,000).
Muscle Fractional Synthetic Rate
Muscle intracellular free amino acids and muscle proteins were
extracted as previously described (51, 52). Muscle intracellular free
concentration and enrichment of phenylalanine was determined by gas
chromatography-mass spectrometry (GCMS, 6890 Plus GC, 5973N
MSD, 7683 autosampler, Agilent Technologies, Palo Alto, CA) using
appropriate internal standard (L-[15N]phenylalanine) (51, 52). Mixed
muscle protein-bound phenylalanine enrichment was analyzed by
GCMS after protein hydrolysis and amino acid extraction (51, 52),
using the external standard curve approach (6). We calculated the
fractional synthetic rate of mixed muscle proteins (FSR) by measuring
the incorporation rate of the phenylalanine tracer into the proteins
(?Ep/t) and using the precursor-product model to calculate the syn-
thesis rate:
FSR ? (?Ep/t)/[(EM1? EM2)/2]?60?100
where ?Ep is the increment in protein-bound phenylalanine enrich-
ment between two sequential biopsies, t is the time between the two
sequential biopsies, and EM1 and EM2 are the phenylalanine enrich-
ments in the free intracellular pool in the two sequential biopsies. Data
are expressed as percent per hour.
Statistical Analysis
All values are expressed as means ? SE. Comparisons were
performed using ANOVA with repeated measures, the effects being
group (REFR, Ctrl) and time (baseline, postexercise). Post hoc testing
was performed using Tukey-Kramer when appropriate. Significance
was set at P ? 0.05.
RESULTS
Blood pH and Lactate
There were no significant differences in blood pH and
plasma lactate between groups before performing the bout of
resistance exercise. However, peripheral blood pH decreased
immediately after exercise in both groups (P ? 0.05, Fig. 2A)
and gradually returned to baseline levels. There were no
differences between groups in the blood pH response to exer-
cise; however, blood pH tended to return to the baseline value
more slowly in the REFR group (P ? 0.10).
Plasma lactate concentration increased immediately after
exercise and stayed high for 40 min after exercise in the REFR
group (P ? 0.05, Fig. 2B). Similarly, plasma lactate increased
significantly after exercise in the Ctrl group, but the values
were significantly lower than those of REFR group (P ? 0.05,
Fig. 2B).
Hormonal Response
There were no significant differences in serum GH, cortisol,
IGF-1, or total testosterone between groups at baseline (P ?
0.05, Fig. 3).
Serum GH concentration significantly increased at 10 min
postexercise and remained elevated for 40 min postexercise in
Fig. 2. REFR group, low-intensity resistance exercise training combined with
blood flow restriction; Ctrl group, low-intensity resistance exercise training
without blood flow restriction. A: Peripheral vein blood pH response in REFR
and Ctrl subjects before, immediately after, and 1–3 h postexercise. B: plasma
lactate response in REFR and Ctrl subjects before, immediately after, and 1–3
h postexercise. *Significantly different from baseline (P ? 0.05). #Signifi-
cantly different from Ctrl (P ? 0.05).
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the REFR group compared with both baseline and Ctrl con-
centrations (P ? 0.05, Fig. 3A). GH concentration did not
change following exercise in the Ctrl group (P ? 0.05).
Serum cortisol concentration increased at 10 min postexer-
cise and remained elevated (compared with baseline and Ctrl)
for 30 min postexercise in the REFR group (P ? 0.05, Fig. 3B).
Cortisol concentrations remained significantly higher than Ctrl
at 40, 50, and 60 min postexercise (P ? 0.05). Serum cortisol
concentration did not change during or following exercise in
the Ctrl group.
No significant changes in IGF-1 were observed in either the
REFR or the Ctrl group (P ? 0.05). Similarly, serum total
testosterone concentration did not change in either group fol-
lowing the bout of resistance exercise (P ? 0.05, Fig. 3D).
Phosphorylation of Translation Initiation and Elongation
Regulatory Proteins
Phosphorylation status of Akt, mTOR, S6K1, or eEF2 were
not different between groups before performing the bout of
resistance exercise (P ? 0.05). Therefore, the data in Fig. 4 are
expressed as the percent change in phosphorylation status 3 h
postexercise compared with baseline values for both groups.
Phosphorylation of Akt at Ser473 tended to increase in both
the REFR and Ctrl group (time effect ? 0.054, Fig. 4A);
however, no group differences were observed (P ? 0.05).
The phosphorylation status of mTOR at Ser2448 did not
change significantly 3 h postexercise in either group (Fig. 4B).
Phosphorylation of S6K1 at Thr389 was increased by three-
fold postexercise in the REFR group (P ? 0.05, Fig. 4C). S6K1
phosphorylation at 3 h postexercise was also significantly
higher in the REFR group compared with the Ctrl group (P ?
0.05). S6K1 phosphorylation status in the Ctrl group was not
different from baseline 3 h postexercise (P ? 0.05).
Interestingly, phosphorylation of eEF2 at Thr56 decreased to
a similar extent in both the REFR and Ctrl group (P ? 0.05,
Fig. 4D) in response to exercise.
Phenylalanine Concentration and Enrichment
Phenylalanine concentration within the muscle free pool was
similar between groups at baseline without a significant group
difference (P ? 0.05, Table 2). In addition, muscle phenylal-
anine concentration at 3 h postexercise did not change in either
group (P ? 0.05). Muscle intracellular enrichment of phenyl-
alanine was at a steady state throughout the infusion study with
no significant differences between groups (P ? 0.05, Table 2).
Muscle Protein Synthesis
Whereas the phosphorylation statuses of signaling proteins
are indicators of enhanced translation initiation and elongation,
we also directly measured muscle protein synthesis, the end
product of translation initiation and elongation. We found that
the mixed muscle protein FSR, a direct measure of the incor-
poration of amino acids into protein, was significantly in-
creased 3 h postexercise in the REFR group compared with
Fig. 3. Peripheral vein serum hormone concentrations in REFR and Ctrl
subjects before, immediately after, and for 1–3 h postexercise. A: serum growth
hormone concentration. B: serum cortisol concentration. C: serum IGF-1
concentration. D: serum total testosterone concentration. *Significantly differ-
ent from baseline (P ? 0.05). #Significantly different from Ctrl (P ? 0.05).
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