Articles in PresS. Am J Physiol Endocrinol Metab (July 12, 2011). doi:10.1152/ajpendo.00276.2011
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Effects of hypoxia on muscle protein synthesis (MPS) and anabolic signaling at rest and
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in response to acute resistance exercise
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Timothy Etheridge1*, Philip J Atherton1*, Daniel Wilkinson2, Anna Selby1, Debbie Rankin1,
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Nick Webborn2, Kenneth Smith1, Peter W Watt2
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DE22 3DT. 2University of Brighton, Chelsea School, Eastbourne, UK, BN20 7SP.
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*These authors’ contributed equally.
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Running head: Effects of hypoxia and exercise on protein synthesis
University of Nottingham, School of Graduate Entry Medicine and Health, Derby, UK,
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Correspondence:
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Dr Timothy Etheridge
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School of Graduate Entry Medicine and Health
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Division of Clinical Physiology
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University of Nottingham
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Derby Royal Hospital
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Uttoxeter Road
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Derby
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DE22 3DT
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[email protected]
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Tel: +44 (0)1332 724685
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Fax: +44 (0)1332 724611
Copyright © 2011 by the American Physiological Society.
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ABSTRACT
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Background: Chronic reductions in tissue O2 tension (hypoxia) are associated with muscle
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atrophy and blunted hypertrophic responses to resistance-exercise (RE) training. However,
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the effect of hypoxia on muscle protein synthesis (MPS) at rest and after RE is unknown.
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Methods: In a crossover study, seven healthy men (21.4±0.7 y) performed unilateral leg RE
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(6×8 repetitions at 70%-1RM) under normoxic (20.9% inspired O2) and normobaric hypoxic
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(12% inspired O2 for 3.5 h) postabsorptive conditions. Immediately after RE the rested leg
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was biopsied and a primed-continuous infusion of [1,2-13C2]leucine was maintained for 2.5 h
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before final biopsies from both legs to measure tracer incorporation and signaling responses
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(i.e., ribosomal S6 kinase 1). Results: After 3.5 h hypoxia, MPS was not different from
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normoxia in the rest leg (normoxia 0.033±0.016 vs. hypoxia 0.043±0.016 %h.-1). MPS
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significantly increased from baseline 2.5 h after RE in normoxia (0.033±0.016 vs.
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0.104±0.038 %h.-1) but not hypoxia (0.043±0.016 vs. 0.060 ± 0.063 %h.-1). A significant
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linear relationship existed between MPS 2.5 h after RE in hypoxia and mean arterial blood O2
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saturation during hypoxia (r2= 0.49; P= 0.04). Phosphorylation of p70S6KThr389 remained
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unchanged in hypoxia at rest but increased after RE in both normoxia and hypoxia (2.6±1.2-
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fold and 3.4±1.1-fold, respectively). Concentrations of the hypoxia responsive mTOR
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inhibitor, Regulated in Development and DNA damage-1 (REDD1) were unaltered by
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hypoxia or RE. Conclusion: Normobaric hypoxia does not reduce MPS over 3.5 h at rest but
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blunts the increased MPS response to acute RE to a degree dependent on extant SpO2.
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Word count: 249
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Key words: Muscle, SpO2, p70S6K, REDD1
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INTRODUCTION
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Hypoxia is the result of a decrease in arterial O2 availability and may be induced by exposure
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to high altitude or in pathological conditions such as chronic obstructive pulmonary disease
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(COPD (20)), obstructive sleep apnoea (OSA (9)), anaemia and chronic heart failure (1).
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Tolerance to hypoxia differs in health and disease. Acclimatized healthy highlanders living
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above 4000 m with a resting PO2 of 6.5-8 kPa (17) have a relatively normal life expectancy
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and suffer few medical problems (although they do have reduced lean mass (11)).
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Furthermore, patients experiencing chronic hypoxemia (i.e., lung disease PO2<8 kPa) have a
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46% 5 y mortality and associated muscle wasting (26). Thus, chronic reductions in tissue O2
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supply, whether under environmental or pathological conditions, are associated with reduced
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muscle mass (21). Although the mechanisms for this are unclear, because reductions in MPS
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underlie muscle atrophy in a number of other wasting conditions (e.g. immobilization (10),
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aging, slow cancer), it is likely that reduced MPS and/or increases in protein degradation also
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contributes to hypoxia-induced muscle atrophy.
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Increases in MPS after RE underpin chronic adaptations (i.e., hypertrophy) to RE training.
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Indeed, under normoxic conditions, we have recently shown that there is a sigmoidal dose-
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response relationship between MPS at 1-2 h post-exercise and exercise intensity after
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unilateral resistance-type exercise in the fasted condition (15). Accordingly, we and others
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have shown that increases in MPS are associated with a stimulation of phosphorylation(s)
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events on numerous kinases known to regulate mRNA translation such as Akt (protein kinase
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B)/mammalian target of rapamycin (mTORC1)/p70S6K signaling (15; 29; 31). Although
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human MPS has not been measured directly during hypoxia, either at rest or after exercise,
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Rennie et al. showed nearly 30 y ago that forearm leucine non-oxidative disposal was
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reduced in subjects breathing 12% O2 under hypobaric conditions (22) suggesting that MPS
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was reduced in response to hypoxia. Similar findings have been obtained in cell culture and
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whole animal studies where reductions in anabolic signaling during hypoxia (1 – 5% O2) via
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increased activity of inhibitory hypoxia-responsive signaling proteins (e.g. AMPK, Regulated
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in development and DNA damage responses (REDD1) (14)) have been suggested as a
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mechanism to blunt MPS. As such, reductions in MPS may explain blunted hypertrophic
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responses to RE-training at altitude (19) and in pathological hypoxia (7).
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Simulation of altitude in healthy humans under artificial conditions of hypoxia (i.e., breathing
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O2 at reduced concentrations) not only contribute to understanding the molecular and
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systemic mechanisms involved in O2 sensing and the ensuing adaptive responses, but also
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help in the understanding pathological conditions associated with hypoxia. Due to the
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catabolic effects of hypoxia on muscle mass, our aim was to conduct preliminary
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investigations to determine the effects of exposing healthy young men to physiologically and
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clinically important hypoxia (12% O2 to achieve 80-90% SpO2) on myofibrillar protein
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synthesis and associated anabolic signaling. Moreover, we chose to determine these
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responses not only under resting conditions, but also under conditions of anabolic challenge
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(i.e., a single bout of resistance exercise) since there have been reports of poorer adaptive
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capacity to resistance exercise training in hypoxic conditions (19). We hypothesized that
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hypoxia would blunt MPS both at rest and in the period after resistance exercise and this
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would be associated with increases in REDD1 protein concentrations and blunted
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p70S6K1Thr389 phosphorylation.
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MATERIALS AND METHODS
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Subjects
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Seven healthy males (age 21.4±0.7 years; BMI 24.9±1.5 kg/m2) were recruited for the study.
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Volunteers were recreationally active (≤ 3 d.wk-1), had no respiratory ailments and had not
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been exposed to any form of hypoxic atmosphere for >6 months. All subjects were instructed
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to refrain from heavy exercise for 72 h, and caffeine and alcohol for 48 h, before the study
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day. Prior to testing subjects provided written informed consent. Ethical approval was
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attained from the University of Brighton Research Ethics and Governance Committee and
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studies complied with standards set by the Declaration of Helsinki.
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Study design
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At least two weeks before the study, subjects reported to the laboratory for familiarization of
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all procedures. Subjects were tested under two experimental conditions in a crossover fashion:
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normoxia (normal atmospheric conditions, 20.9% inspired O2 concentration) and normobaric
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hypoxia (12% inspired O2 concentration). Trials were ordered and separated by a minimum of
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3 months, with the normoxic condition always being performed first to avoid any long lasting
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effect of the hypoxic bout.
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Normoxic trial; Participants arrived at the laboratories at ~0800 h in the postabsorptive state
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(~12 h overnight fast) for insertion of 18-g catheters into antecubital veins of both arms for
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blood sampling and tracer infusion. Participants were then placed in custom built isometric
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force apparatus for performance of unilateral RE. Maximum isometric voluntary contraction
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(MVC) of the quadriceps was determined by fixing subjects’ knee joint angle at 90° (1.57
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rad). A strap was attached to the ankle of the dominant leg, connected via chain to a strain
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gauge (Tedea Ltd., Huntleigh, UK) that measured the amount of isometric force developed in
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the contracting quadriceps performing the knee extension. The strain gauge signal (sample
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rate = 0.1 kHz) was amplified, filtered, and converted from analog to digital (model Micro
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1401, CED). Results were recorded and analysed using Spike II software. Subjects performed
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three maximal 5 s efforts with the peak value taken as the highest value achieved. Following
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2 min recovery, RE consisted of 8 sets of 6 repetitions at 70% MVC, with 2 minutes recovery
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between sets. Immediately after RE a baseline muscle biopsy was taken from the non-
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exercised leg and a primed, constant intravenous infusion (priming dose 7.8 μmol·kg body
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wt-1, infusion rate 0.13 μmol·kg body wt-1·min-1) of 1,2-[13C2]leucine tracer (99 Atoms %,
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Cambridge Isotopes, Cambridge, MA, USA) was started and maintained for 2.5 h. Blood
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samples were collected at 20 min intervals throughout the study to determine plasma α-
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ketoisocaproic acid (KIC) enrichment. Further muscle biopsies were collected from both legs
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at 2.5 h post-exercise. Fingertip measures of blood oxygen saturation (SpO2) were recorded
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using pulseoximetry (9500 Onyx, Nonin) every 10 min for the duration of the study except
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during exercise. The O2 / CO2 concentration of inhaled air was also monitored every 10 min
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using a Servomex Xentra 4100 Gas Purity Analyzer (Servomex, Crowborough, England).. A
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schematic representation of the study protocol can be viewed in Figure 1.
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Hypoxic trial; The experimental conditions for the hypoxic trial were similar to that of the
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normoxic trial; the difference being that volunteers breathed 12% O2 for 1 h before RE,
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during RE and for the remainder of the study to total 3.5 h hypoxic exposure. Hypoxic
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conditions were delivered using a custom-made face mask, adapted to allow gas sampling of
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the internal mask environment via sealed capillary tubing, and connected to a nitrogen
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generator set to reduce the O2 concentration of inspired air to 12%.
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Evaluation of myofibrillar protein synthesis
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Muscle preparation for leucine tracer analysis; Briefly, muscle tissue (~ 25 mg) was
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homogenized with scissors in ice cold homogenization buffer containing 50 mM Tris HCl
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(pH 7.4), 1 mM EGTA, 0.1% 2-mercaptoethanol, 0.1% Triton X-100, 1 mM EDTA, 10 mM
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β-glycerophosphate, 50 mM NaF, 0.5 mM activated sodium orthovanadate (all Sigma-
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Aldrich, Poole, UK) and a complete protease inhibitor cocktail tablet (Roche, West Sussex,
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UK). The resulting homogenate was centrifuged at 13,000 × g for 20 min to precipitate the
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myofibrillar fraction. The myofibrillar pellet was solubilized with 0.3 M NaOH and
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centrifuged at 3,000 × g for 20 min to separate it from the insoluble collagen fraction. The
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solubilized myofibrillar protein was precipitated using ice cold 1 M perchloric acid, the
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resulting pellet washed twice with 70% ethanol and collected by centrifugation. Protein-
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bound amino acids were released by acid hydrolysis in a Dowex H+ resin slurry (0.05M HCl)
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at 110° C overnight. The amino acids were purified by ion exchange chromatography on
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Dowex H+ resin. The amino acids were derivatized as their n-acetyl-N-propyl esters as
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previously described (12). Incorporation of [1, 2-13C2]leucine into protein was determined by
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gas chromatography- combustion-isotope ratio mass spectrometry (Delta plus XP,
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Thermofisher Scientific, Hemel Hempstead, UK) using our standard techniques (16).
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Calculation of myofibrillar protein synthetic rates; The fractional synthetic rate (FSR) of
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myofibrillar proteins was calculated based on the incorporation rate of [1,2-13C2] leucine into
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muscle proteins using a standard precursor-product model as follows: FSR = ΔEm / Ep (1 / t)
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× 100, where ΔEm is the change in enrichment (APE) of protein-bound leucine in two
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subsequent biopsies, Ep is the mean enrichment over time of the precursor for protein
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synthesis (plasma KIC was chosen to represent the immediate precursor for muscle protein
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synthesis, i.e. leucyl-t-RNA (8; 28), and t is the time between biopsies. Values for FSR are
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expressed as percent per hour (%·h-1).
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Immunoblotting
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Immunoblotting was performed similar to previously described (4; 5; 15). Briefly, frozen
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muscle tissue (~20 mg) was rapidly homogenised with scissors in ice-cold buffer (50 mM
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Tris-HCL pH 7.5, 1 mM EDTA, 1 mM EGTA, 10 mM glycerophosphate, 50 mM NaF, 0.1%
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Triton-X, 0.1% 2-mercaptoethanol, 1 complete protease inhibitor tablet [Roche Diagnostics
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Ltd, Burgess Hill, UK]) at 6 μl·mg-1 tissue. Proteins were extracted by shaking for 15 min at
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4ºC and samples were then centrifuged at 13000×g for 10 minutes at 4ºC and the supernatant
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containing the proteins was collected. Bradford assays were used to determine sarcoplasmic
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protein concentration (B6916, Sigma-Aldrich, St. Louis, MO) and adjusted to 1 mg·ml-1 in
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3×Laemmli buffer in order to measure relative phosphorylated protein concentrations of
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P70S6KThr389, PKBSer473, mTORSer2448, 4EBP1Thr37/46, eIF4ESer209, eEF2Thr56, ERK1/2Tyr202/204
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and ACC-βser79 and total protein concentration of REDD1. After heating at 95°C for 7 min,
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fifteen micrograms of protein (15μl) from each sample were loaded onto 12% NuPAGE
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Novex Bis-Tris gels (Invitrogen, Paisley, UK), separated by SDS PAGE, and transferred on
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ice at 100 V for 45 min to methanol pre-wetted 0.2 μm PVDF membranes. Blots were then
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blocked in 5% (w/v) BSA in TBS-T (Tris Buffered Saline and 0.1% Tween-20; both Sigma-
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Aldrich, Poole, UK) for 1 h, then incubated in primary antibodies (1:2000) overnight rotating
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at 4ºC. Membranes were then washed (3×5 min) with TBS-T and incubated for 1 h at room
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temperature with HRP-conjugated anti-rabbit secondary antibody (1:2000; New England
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Biolabs, UK), before further washing (3×5 min) with TBS-T. Membranes were developed by
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incubation with ECL reagents for 5 min (enhanced chemiluminescence kit, Immunstar; Bio-
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Rad Laboratories, Richmond, CA) and the protein bands were visualized and quantified by
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assessing peak density on a Chemidoc XRS system (Bio-Rad Laboratories, Inc. Hercules, CA)
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ensuring no pixel saturation. Data were expressed in relation to β-actin loading controls.
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Statistical analysis
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Paired sample t-tests were performed on MVC and SpO2 data to determine any differences in
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these measures between the normoxic and hypoxic conditions. Data were tested for normality
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and, if normally distributed, two-way repeated measures analysis of variance (ANOVA) was
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used to evaluate possible differences between normoxia and hypoxia in MPS and intracellular
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signaling elements between unilateral exercised leg and contralateral control leg with post
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hoc comparisons using GraphPad software (La Jolla, San Diego CA). If failing the normality
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test, data were log transformed prior to analyses. Pearson’s product moment correlation was
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performed to determine relationships between FSR and SpO2. A P value of <0.05 was
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considered statistically significant. Data are presented as mean±SEM.
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RESULTS
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Mean MVC and SpO2 responses
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Peak MVC, and thus subsequent exercise intensity, was not different between the normoxic
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and hypoxic conditions (629±63 vs. 613±60 N for normoxic and hypoxic MVC, respectively;
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P>0.05, Figure 2A). Throughout the normoxic trial SpO2 remained unchanged from baseline
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(98.2±0.3 vs. 97.4±0.5% for SpO2 at baseline and the average of all subsequent time points,
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respectively; P>0.05). In hypoxia SpO2 decreased significantly from baseline within 10 min
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to 86.4±1.7% (P<0.01) and the average of all subsequent time points remained depressed
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throughout hypoxia for all subjects (82.7±0.5%; P<0.001, range 75-89%, Figure 2B), as did
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all individual 10 min time points (P<0.01, Figure 2C).
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MPS at rest and after RE in normoxia and hypoxia
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After 3.5 h hypoxia, MPS was not different from that in normoxia in the rest leg
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(0.033±0.016 vs. 0.043±0.016 %h.-1; P>0.05, Figure 3A). In normoxia MPS was significantly
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increased from baseline 2.5 h after RE (0.033±0.016 vs. 0.104±0.038 %h.-1; P<0.01) but not
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in hypoxia (0.033±0.016 vs. 0.060 ± 0.063 %h.-1; P>0.05, Figure 3A). A significant
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relationship existed between MPS 2.5 h after RE and mean hypoxic SpO2 (r2 = 0.49, P<0.05;
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Figure 3B).
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Anabolic signaling at rest and after RE in normoxia and hypoxia
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Protein concentrations of REDD1 were not altered 2.5 h after RE in normoxia (0.3±0.2-fold
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increase, P>0.05). After 3.5 h hypoxic exposure REDD1 concentrations were also unchanged
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at rest (0.2±0.2-fold increase, P>0.05) and 2.5 h after RE in hypoxia (0.1±0.2-fold increase,
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P>0.05, Figure 4A). In normoxia the phosphorylation of p70S6KThr389 increased from
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baseline values 2.6±1.2-fold 2.5 h after RE (P<0.05). After 3.5 h exposure to hypoxia,
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phosphorylation of p70S6KThr389 increased from normoxic basal values 2.3±1.0-fold in the
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rest leg and 3.4±1.1-fold 2.5 h after RE (both P<0.05). There were no differences in the level
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of p70S6KThr389 phosphorylation between conditions (P>0.05; Figure 4B). No correlation was
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present between p70S6KThr389 phosphorylation and MPS (r2 = 0.23, P>0.05) or SpO2 (r2 =
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0.18. P>0.05) 2.5 h after RE in hypoxia. Phosphorylation of PKBSer473, mTORSer2448,
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4EBP1Thr37/46, eIF4ESer209, eEF2Thr56, ERK1/2Tyr202/204 and ACC-βser79 were unchanged from
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normoxic baseline values in all conditions (all P>0.05; data not shown), and did not correlate
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with the level of SpO2 2.5 h after RE in hypoxia (P>0.05).
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DISCUSSION
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Falls in SpO2 to between 80-90% are frequently experienced even at moderate altitudes
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(~4300 m; ~12% O2; SpO2 81±1% (32)) and in various clinical situations (9; 20). We report
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new findings on the response of MPS and associated anabolic signaling to 3.5 h exposure to
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simulated hypoxia (breathing 12% O2) at rest and after acute RE. The principle findings are (i)
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MPS is unchanged following 3.5 h hypoxia at rest, (ii) hypoxia blunts the rise in MPS after
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acute RE in normoxia, despite maintained increases in p70S6K phosphorylation, and (iii)
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suppression of MPS after RE in hypoxia correlates with extant SpO2.
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Muscle mass is reduced in native highlanders residing at altitude (11), in lowlanders exposed
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to low atmospheric O2 conditions for >3 wks (24) and in various clinical situations where
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tissue hypoxia is a feature (26). The primary cause of atrophy in other clinical scenarios is
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reductions in resting rates of MPS (i.e., disuse atrophy, cancer (23)) and as such we
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hypothesized that MPS would be suppressed during acute hypoxia exposure. In contrast, we
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show for the first time that 3.5 h exposure to hypoxia is insufficient to blunt resting MPS, or
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to induce expression of hypoxic responsive signaling proteins that suppress MPS such as
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REDD1. Although these findings contrast to early reports of reduced MPS during acute
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hypoxia (22) this may be explained by a number of key differences between the studies. First,
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the techniques utilised to measure MPS were different. Whereas the previous study used
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leucine uptake into muscle which provides only an indirect index of MPS, in the present
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study we used accurate methods to precisely determine MPS. Second, the duration of the
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hypoxic exposure was almost twice as long in the previous study (6.5 h exposure vs. 3.5 h
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presently). Finally, in the earlier study, hypobaric hypoxia was used versus normobaric
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hypoxia in the present study. This is a key difference since at equivalent PO2 hypobaric
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hypoxia induces greater reductions in SpO2 compared to normobaric hypoxia (25). Following
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on, Rennie et al. observed greater falls in SpO2 (from 97 to ~65%) than those reported
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presently (from 97 to ~82.7%). As such, a greater hypoxaemic stress with either hypobaric
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hypoxia or more prolonged exposure may be required to inhibit MPS whilst in the rested state.
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Nevertheless, falls in SpO2 to 65% as in Rennie et al. are uncommon, requiring rapid ascent
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to altitudes of 5000 m and above (3; 13) and have not been reported in any hypoxic pathology.
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Finally, we acknowledge interpretative limitations of these ground based studies in relation to
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chronic exposure to altitude i.e., short term exposure to low O2 and discriminating the effects
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of normobaric vs. hypobaric hypoxia.
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This study is also the first to show that the normal 2-3-fold increase in MPS after RE is
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blunted during hypoxia. Many scenarios exist whereby the adaptive response to RE training
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are blunted during both environmental (19) and pathological (7; 18; 30) hypoxia. The present
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findings provide evidence that this effect may be a direct consequence of blunted increases in
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MPS after individual RE bouts which, over the course of a training program may attenuate
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increased accretion of muscle proteins and thus hypertrophy. Moreover, we speculate that
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blunted responses to resistance exercise may perhaps also be translated to much lower levels
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of activity that normally stimulate muscle protein synthesis (27) and are important for muscle
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maintenance i.e., we contend it possible that muscles mimic a ‘mild state of disuse’ in low O2
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conditions. The mechanisms regulating this blunted MPS response to RE in hypoxia,
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however, remain elusive. As observed at rest in hypoxia, total protein content of REDD1 (or
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HIF-1/REDD1/2 mRNA; data not shown), and the phosphorylation status of ACC-βser79 (a
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proxy for AMPK activity) were unaffected after hypoxic RE, indicating other mechanisms
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must exist to signal low O2 levels to the muscle after RE in order to attenuate increases in
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MPS. We also observed dissociation between p70S6K1 and protein synthesis where increases
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in p70S6K1 under hypoxic conditions were not translated into elevated MPS. Such
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dissociation between p70S6K1 phosphorylation and MPS suggests that some other signal is
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essentially overriding mTORC1 signaling in hypoxia, and though speculative, it is possible
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that this could be due to induction of an unfolded protein response (which attenuates protein
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synthesis); a common feature of hypoxic tissue (2). Nonetheless, we have previously reported
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dissociation between mTOR signaling and muscle protein synthesis (4; 12). Although the
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reasons for these discrepancies are currently unknown, we are strong advocates that mTOR
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signaling should not be used as a proxy for muscle protein synthesis– measures of which we
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have provided.
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We also report that there is a substantive inter-individual variation in drops in SpO2 after
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breathing 12% O2. Though we are only able to speculate at present, this may indicate the
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existence of a responder and non-responder phenotype. Indeed, such a ‘responder/non-
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responder’ phenomenon relating to falls in SpO2 has been postulated to exist amongst
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mountaineers to explain why some experience symptoms of acute mountain sickness whilst
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others do not (6). Clearly, further work is required to establish the mechanisms underlying
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this variation such as investigating individual differences in O2 carrying capacity of the blood
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(i.e. haemoglobin concentration and/or haematocrit), genetics, or in hyper-ventilatory
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responses. Importantly, MPS after RE in hypoxia was significantly correlated with extant
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SpO2 i.e., subjects with a lesser degree of hypoxaemia had a greater capacity to maintain
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post-RE increase in MPS. At present we cannot establish whether this is a true linear
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relationship or whether there exists a specific SpO2 threshold below which increases in MPS
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are blunted which would suggest a ‘reserve’ in the metabolic capacity to sustain anabolic
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responses during mild systemic hypoxemia. Nevertheless, individual variation aside, it is
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likely that the degree of systemic hypoxemia inversely relates to adaptive responses to
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exercise training. Moreover, our findings support observations that the efficacy (i.e.,
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hypertrophic responses) of exercise prescription varies greatly between individuals with
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clinical hypoxia of different severity (7; 18).
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We report for the first time that a short period of normobaric hypoxia is not sufficient to
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induce a blunting in MPS, showing that humans have a robust metabolic reserve against acute
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falls in SpO2. We also report the novel finding that increases in MPS after RE are blunted
15
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under conditions of normobaric hypoxia to a level depending on extant SpO2. In conclusion,
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human muscles are seemingly able to maintain protein synthesis in hypoxia (at least during
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acute exposure). However, when faced with the challenge of an anabolic stimulus such as
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resistance exercise, the capacity to increase protein synthesis is limited by extant SpO2, and
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perhaps ability to resist declines in SpO2.
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ACKNOWLEDGEMENTS
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T. Etheridge is supported by a Medical Research Council grant (G0801271). P.J. Atherton is
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a designated Research Councils UK fellow. Funding for this study was also obtained from the
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Graduate Entry Medical School at the University of Nottingham and from grants by the
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Biotechnology
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BB/C516779/1).
and
Biological
Sciences
Research
Council
(BB/X510697/1
and
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DISCLOSURE
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The authors declare no conflicts of interest.
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Figure 1. Cross over study protocol schematic for investigation of the effects of hypoxia
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and resistance exercise on protein synthesis and translational signalling.
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Figure 2. Maximal isometric voluntary contraction (A), mean SpO2 of all time points (B)
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normoxia. * Significant change from normoxic values (P<0.05).
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Figure 3. (A) Fractional synthesis rate (FSR) of myofibrillar proteins at rest and after
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Figure 4. Changes in REDD1 (A) and p70S6KThr389 (B) phosphorylation after resistance
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exercise (RE) in normoxia and at rest and after RE in hypoxia. * Significantly different
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from normoxia basal values (P<0.05).