Model to assess muscle protein turnover: domain of validity using amino acyl-tRNA vs. surrogate measures of precursor pool

2003 ◽  
Vol 285 (5) ◽  
pp. E1142-E1149 ◽  
Author(s):  
Gianna Toffolo ◽  
Robert Albright ◽  
Michael Joyner ◽  
Niki Dietz ◽  
Claudio Cobelli ◽  
...  
Keyword(s):  
Amino Acids ◽  
Protein Turnover ◽  
Muscle Protein ◽  
Compartment Model ◽  
Tissue Fluid ◽  
Protein Breakdown ◽  
Muscle Protein Turnover ◽  
Three Compartment Model ◽  
Surrogate Measures ◽  
Domain Of Validity

Current models to measure protein turnover across muscle bed are based on many surrogate measures of amino acyl-tRNA. We measured muscle protein turnover based on tracer-to-tracee ratios of the stable isotopes of leucine, phenylalanine, and ketoisocaproate (KIC) in artery and vein and muscle amino acyl-tRNA and muscle tissue fluid (TF) in 26 healthy subjects. A three-compartment model calculation based on arteriovenous and tRNA measurements was first performed and its domain of validity assessed. The results were then compared with those using simpler approaches based on surrogate measures of tRNA such as those of TF and KIC and a one-compartment model based on arteriovenous amino acids. In 96% of cases, the model using tRNA was applicable, but only in a lower percentage of cases were the results using surrogate measures applicable. Protein breakdown, protein synthesis, and shunting of amino acids from artery to vein were consistently underestimated, and fluxes of amino acid from artery to intracellular compartment and from intracellular compartment to vein were overestimated, when surrogate measures were used. The one-compartment model also underestimated protein breakdown and synthesis. Measurements using tissue fluid gave results closer to those based on tRNA. In conclusion, a three-compartment model using arteriovenous samples and amino acyl-tRNA provides measurements of muscle protein turnover of acceptable precision in 96% of cases. The precision was unacceptable in a substantial percentage of cases, and the accuracy of the estimation of protein fluxes was significantly affected when surrogate measures were used.

Exercise training and protein metabolism: influences of contraction, protein intake, and sex-based differences

2009 ◽  
Vol 106 (5) ◽  
pp. 1692-1701 ◽  
Author(s):  
Nicholas A. Burd ◽  
Jason E. Tang ◽  
Daniel R. Moore ◽  
Stuart M. Phillips
Keyword(s):  
Signaling Pathways ◽  
Protein Turnover ◽  
Muscle Hypertrophy ◽  
Muscle Protein ◽  
Muscle Protein Synthesis ◽  
Protein Breakdown ◽  
Muscle Protein Turnover ◽  
Muscle Protein Breakdown ◽  
Response To Exercise ◽  
Net Protein

Muscle contraction during exercise, whether resistive or endurance in nature, has profound affects on muscle protein turnover that can persist for up to 72 h. It is well established that feeding during the postexercise period is required to bring about a positive net protein balance (muscle protein synthesis − muscle protein breakdown). There is mounting evidence that the timing of ingestion and the protein source during recovery independently regulate the protein synthetic response and influence the extent of muscle hypertrophy. Minor differences in muscle protein turnover appear to exist in young men and women; however, with aging there may be more substantial sex-based differences in response to both feeding and resistance exercise. The recognition of anabolic signaling pathways and molecules are also enhancing our understanding of the regulation of protein turnover following exercise perturbations. In this review we summarize the current understanding of muscle protein turnover in response to exercise and feeding and highlight potential sex-based dimorphisms. Furthermore, we examine the underlying anabolic signaling pathways and molecules that regulate these processes.

Mechanism of insulin's anabolic effect on muscle: measurements of muscle protein synthesis and breakdown using aminoacyl-tRNA and other surrogate measures

2006 ◽  
Vol 291 (4) ◽  
pp. E729-E736 ◽  
Author(s):  
Lisa S. Chow ◽  
Robert C. Albright ◽  
Maureen L. Bigelow ◽  
Gianna Toffolo ◽  
Claudio Cobelli ◽  
...  
Keyword(s):  
Protein Synthesis ◽  
Muscle Protein ◽  
Muscle Protein Synthesis ◽  
Physiological Role ◽  
Tissue Fluid ◽  
Protein Breakdown ◽  
Anabolic Hormone ◽  
Precursor Pool ◽  
Healthy Humans ◽  
Surrogate Measures

Despite being an anabolic hormone in skeletal muscle, insulin's anticatabolic mechanism in humans remains controversial, with contradictory reports showing either stimulation of protein synthesis (PS) or inhibition of protein breakdown (PB) by insulin. Earlier measurements of muscle PS and PB in humans have relied on different surrogate measures of aminoacyl-tRNA and intracellular pools. We report that insulin's effect on muscle protein turnover using aminoacyl-tRNA as the precursor of PS and PB is calculated by mass balance of tracee amino acid (AA). We compared the results calculated from various surrogate measures. To determine the physiological role of insulin on muscle protein metabolism, we infused tracers of leucine and phenylalanine into 18 healthy subjects, and after 3 h, 10 subjects received a 4-h femoral arterial infusion of insulin (0.125 mU·kg−1·min−1), while eight subjects continued with saline. Tracer-to-tracee ratios of leucine, phenylalanine, and ketoisocaproate were measured in the arterial and venous plasma, muscle tissue fluid, and AA-tRNA to calculate muscle PB and PS. Insulin infusion, unlike saline, significantly reduced the efflux of leucine and phenylalanine from muscle bed, based on various surrogate measures which agreed with those based on leucyl-tRNA (−28%), indicating a reduction in muscle PB ( P < 0.02) without any significant effect on muscle PS. In conclusion, using AA-tRNA as the precursor pool, it is demonstrated that, in healthy humans in the postabsorptive state, insulin does not stimulate muscle protein synthesis and confirmed that insulin achieves muscle protein anabolism by inhibition of muscle protein breakdown.

Decrease in human quadriceps muscle protein turnover consequent upon leg immobilization

1987 ◽  
Vol 72 (4) ◽  
pp. 503-509 ◽  
Author(s):  
J. N. A. Gibson ◽  
D. Halliday ◽  
W. L. Morrison ◽  
P. J. Stoward ◽  
G. A. Hornsby ◽  
...  
Keyword(s):  
Protein Turnover ◽  
Muscle Protein ◽  
Fibre Diameter ◽  
Quadriceps Muscle ◽  
Fibre Volume ◽  
Type I ◽  
Protein Breakdown ◽  
Muscle Protein Turnover ◽  
Muscle Protein Breakdown ◽  
Synthetic Rate

1. Quadriceps muscle protein turnover was assessed in the post-absorptive state in six men immediately after the end of unilateral leg immobilization (37 ± 4 days) in a plaster cast after tibial fracture. A primed-constant intravenous infusion of l-[1-13C]leucine was administered over 7 h. Quadriceps needle biopsies, taken bilaterally at the end of the infusion, were analysed for muscle protein leucine enrichment with 13C. 2. Quadriceps muscle protein synthetic rate, calculated from the fractional incorporation of [13C]leucine into protein compared with the average enrichment of blood α-ketoisocaproate, was 0.046 ±0.012%/h in the uninjured leg, but was only 0.034 ±0.007%/h in the quadriceps of the previously fractured leg (P > 0.05, means ± sd). 3. Muscle RNA activity (i.e. protein synthetic rate per RNA) fell from 0.27 ±0.08 μg of protein synthesized h−1 μg−1 of RNA in the control leg to 0.14 ±0.03 μg of protein synthesized h−1 μg−1 of RNA in the immobilized leg (P > 0.02). 4. Immobilization was associated with a significant atrophy of type I muscle fibres (mean diameter 69.5 ±21 μm immobilized, 81.1 ±18 μm control, P > 0.05), but no significant change occurred in type II fibre diameter. Mean quadriceps fibre volume calculated from the values for fibre diameter and percentage of each fibre type, was smaller in the injured leg by 10.6%; this value was near to the calculated difference in muscle thigh volume (calculated from thigh circumference and skin-fold thickness) which was less by 8.3%. 5. From estimated mean daily values for quadriceps protein synthetic rate (1.65 ±0.44%/day in the control legs and 1.22±0.28%/day in the injured legs) and change in fibre volume, mean daily muscle protein breakdown rates were calculated as 1.65%/ day and 1.53%/day respectively, suggesting that muscle protein breakdown was not enhanced and may have fallen. 6. The results suggest a decrease in muscle protein turnover during limb immobilization in man, with the decrement in muscle mass being due mainly to a substantial (25%) depression of muscle protein synthesis.

scholarly journals A comparison of methods for the measurement of protein turnover in vivo

1979 ◽  
Vol 184 (2) ◽  
pp. 473-476 ◽  
Author(s):  
M L MacDonald ◽  
S L Augustine ◽  
T L Burk ◽  
R W Swick
Keyword(s):  
Amino Acids ◽  
Steady State ◽  
Protein Turnover ◽  
Muscle Protein ◽  
Comparison Of Methods ◽  
Ornithine Aminotransferase ◽  
Protein Breakdown ◽  
Muscle Protein Breakdown ◽  
Fractional Rate

Steady-state rates of turnover of two single proteins were measured in vivo by two independent methods. The fractional rate of synthesis of liver ornithine aminotransferase, measured by a continuous infusion of L-[2,6-3H]tyrosine, was 0.42 day-1, whereas in the same animals the fractional rate of degradation measured by loss of radioactivity from amino acids labelled via [14C]bicarbonate was 0.40 day-1. The agreement between methods confirms the reliability of each method for the study of hepatic protein turnover. In contrast, [14C]bicarbonate-labelled amino acids are extensively reutilized in muscle, and are therefore unsuitable for measuring rates of muscle protein breakdown.

scholarly journals Myostatin Inhibition-Induced Increase in Muscle Mass and Strength Was Amplified by Resistance Exercise Training, and Dietary Essential Amino Acids Improved Muscle Quality in Mice

Nutrients ◽  
2021 ◽  
Vol 13 (5) ◽  
pp. 1508
Author(s):  
Jiwoong Jang ◽  
Sanghee Park ◽  
Yeongmin Kim ◽  
Jiyeon Jung ◽  
Jinseok Lee ◽  
...  
Keyword(s):  
Amino Acids ◽  
Muscle Mass ◽  
Mitochondrial Biogenesis ◽  
Protein Turnover ◽  
Muscle Protein ◽  
Essential Amino Acids ◽  
Muscle Quality ◽  
Resistance Exercise Training ◽  
Muscle Protein Turnover ◽  
Myostatin Inhibition

It has been frequently reported that myostatin inhibition increases muscle mass, but decreases muscle quality (i.e., strength/muscle mass). Resistance exercise training (RT) and essential amino acids (EAAs) are potent anabolic stimuli that synergistically increase muscle mass through changes in muscle protein turnover. In addition, EAAs are known to stimulate mitochondrial biogenesis. We have investigated if RT amplifies the anabolic potential of myostatin inhibition while EAAs enhance muscle quality through stimulations of mitochondrial biogenesis and/or muscle protein turnover. Mice were assigned into ACV (myostatin inhibitor), ACV+EAA, ACV+RT, ACV+EAA +RT, or control (CON) over 4 weeks. RT, but not EAA, increased muscle mass above ACV. Despite differences in muscle mass gain, myofibrillar protein synthesis was stimulated similarly in all vs. CON, suggesting a role for changes in protein breakdown in muscle mass gains. There were increases in MyoD expression but decreases in Atrogin-1/MAFbx expression in ACV+EAA, ACV+RT, and ACV+EAA+RT vs. CON. EAA increased muscle quality (e.g., grip strength and maximal carrying load) without corresponding changes in markers of mitochondrial biogenesis and neuromuscular junction stability. In conclusion, RT amplifies muscle mass and strength through changes in muscle protein turnover in conjunction with changes in implicated signaling, while EAAs enhance muscle quality through unknown mechanisms.

scholarly journals Acute Effects of Peritoneal Dialysis with Dialysates Containing Dextrose or Dextrose and Amino Acids on Muscle Protein Turnover in Patients with Chronic Renal Failure

2001 ◽  
Vol 12 (3) ◽  
pp. 557-567
Author(s):  
GIACOMO GARIBOTTO ◽  
ANTONELLA SOFIA ◽  
ALBERTO CANEPA ◽  
STEFANO SAFFIOTI ◽  
PAOLO SACCO ◽  
...  
Keyword(s):  
Amino Acids ◽  
Peritoneal Dialysis ◽  
Protein Turnover ◽  
Muscle Protein ◽  
Muscle Protein Synthesis ◽  
Cumulative Effect ◽  
Insulin Levels ◽  
Muscle Protein Turnover ◽  
Combined Use ◽  
High Insulin

Abstract. Whether changes in substrate and insulin levels that occur during peritoneal dialysis (PD) have effects on muscle protein dynamics was evaluated by studying muscle protein synthesis (PS), breakdown (PB), and net protein balance (NB) by the forearm perfusion method associated with the kinetics of 3H-phenylalanine in acute, crossover studies in which PD patients served as their own controls. Studies were performed (1) in the basal state and during PD with dialysates that contained dextrose alone in different concentrations (protocol 1: eight patients), (2) during PD with dialysates that contained dextrose alone or dextrose and amino acids (AA) (protocol 2: five patients), and (3) in time controls (five patients). PD with dextrose alone induced (1) a two- to threefold increase in insulin, as well as a 20 to 25% decrease in AA, mainly BCAA, levels; (2) an insulin-related decline (-18%) in forearm PB (P<0.002); (3) a 20% decrease in muscle PS (P<0.04), which was related to arterial BCAA and K+ (P<0.02 to 0.05); (4) a persistent negative NB; and (5) a decrease in the efficiency of muscle protein turnover, expressed as the ratio NB/PB. PD with dextrose+AA versus PD with dextrose induced (1) similarly high insulin levels but with a significant increase in total arterial AA (+30 to 110%), mainly valine; (2) a reduced release of AA from muscle (P<0.05); and (3) a decrease in the negative NB observed during PD with dextrose, owing to an increase (approximately 20%) in muscle PS, without any further effect on muscle PB. This study indicates that in PD patients in the fasting state, the moderate hyperinsulinemia that occurs during PD with dextrose alone causes an antiproteolytic action that is obscured by a parallel decrease in AA availability for PS. Conversely, the combined use of dextrose and AA results in a cumulative effect, because of the suppression of endogenous muscle PB (induced by insulin) and the stimulation of muscle PS (induced by AA availability). The hypothesis, therefore, is that in patients who are treated with PD, when fasting or when nutrient intake is reduced, muscle mass could be maintained better by the combined use of dextrose and AA.

Exercise- and nutrient-controlled mechanisms involved in maintenance of the musculoskeletal mass

2007 ◽  
Vol 35 (5) ◽  
pp. 1302-1305 ◽  
Author(s):  
M.J. Rennie
Keyword(s):  
Amino Acids ◽  
Protein Synthesis ◽  
Protein Turnover ◽  
Muscle Protein ◽  
Muscle Protein Synthesis ◽  
Mammalian Target Of Rapamycin ◽  
Inhibitory Effect ◽  
Bone Collagen ◽  
Disuse Atrophy ◽  
Protein Breakdown

The mechanisms of maintenance of the protein mass of muscle and associated connective tissue and bone are becoming more accessible as a result of the use of a combination of well-established techniques for measurement of protein turnover and measurement of protein expression and phosphorylation state of signalling molecules involved in anabolic and catabolic responses. Amino acids, hormones and physical activity appear to be the major short-term physiological regulators of muscle mass, mainly through their actions on protein synthesis and breakdown, on a time scale of minutes to hours, with duration of changes in gene expression up to weeks. Amino acids are the main components in the diet regulating protein turnover, having marked effects in stimulating muscle protein synthesis and with almost no effect on muscle protein breakdown. Branched-chain amino acids, and in particular leucine, simulate protein synthesis via signalling pathways involving mTOR (mammalian target of rapamycin) in a dose–response manner. Insulin has little effect on protein synthesis in human muscle, but it has a marked inhibitory effect on protein breakdown. The amino acid simulation of anabolism is not dependent on the presence of insulin, IGF-1 (insulin-like growth factor-1) or growth hormone. Exercise not only stimulates protein synthesis in muscle, but also in tendon; and disuse atrophy is accompanied by marked decreases of both muscle and tendon collagen protein synthesis. Bone collagen synthesis appears to be nutritionally regulated by the availability of amino acids, but not lipid or glucose.

scholarly journals Disassociation between the effects of amino acids and insulin on signaling, ubiquitin ligases, and protein turnover in human muscle

2008 ◽  
Vol 295 (3) ◽  
pp. E595-E604 ◽  
Author(s):  
P. L. Greenhaff ◽  
L. G. Karagounis ◽  
N. Peirce ◽  
E. J. Simpson ◽  
M. Hazell ◽  
...  
Keyword(s):  
Amino Acids ◽  
Protein Turnover ◽  
Muscle Protein ◽  
Muscle Protein Synthesis ◽  
Serum Insulin ◽  
Human Muscle ◽  
Protein Breakdown ◽  
Ubiquitin Proteasome ◽  
Proteasome Pathway ◽  
Higher Doses

We determined the effects of intravenous infusion of amino acids (AA) at serum insulin of 5, 30, 72, and 167 mU/l on anabolic signaling, expression of ubiquitin-proteasome components, and protein turnover in muscles of healthy young men. Tripling AA availability at 5 mU/l insulin doubled incorporation of [1-13C]leucine [i.e., muscle protein synthesis (MPS), P < 0.01] without affecting the rate of leg protein breakdown (LPB; appearance of d5-phenylalanine). While keeping AA availability constant, increasing insulin to 30 mU/l halved LPB ( P < 0.05) without further inhibition at higher doses, whereas rates of MPS were identical to that at 5 mU/l insulin. The phosphorylation of PKB Ser473 and p70S6k Thr389 increased concomitantly with insulin, but whereas raising insulin to 30 mU/l increased the phosphorylation of mTOR Ser2448, 4E-BP1 Thr37/46, or GSK3β Ser9 and decreased that of eEF2 Thr56, higher insulin doses to 72 and 167 mU/l did not augment these latter responses. MAFbx and proteasome C2 subunit proteins declined as insulin increased, with MuRF-1 expression largely unchanged. Thus increasing AA and insulin availability causes changes in anabolic signaling and amounts of enzymes of the ubiquitin-proteasome pathway, which cannot be easily reconciled with observed effects on MPS or LPB.

Skeletal and cardiac muscle protein turnover during cold acclimation in young rats

2000 ◽  
Vol 278 (3) ◽  
pp. R705-R711 ◽  
Author(s):  
T. A. McAllister ◽  
J. R. Thompson ◽  
S. E. Samuels
Keyword(s):  
Skeletal Muscle ◽  
Protein Synthesis ◽  
Cardiac Muscle ◽  
Cold Acclimation ◽  
Protein Turnover ◽  
Muscle Protein ◽  
Muscle Protein Turnover ◽  
Protein Mass ◽  
The Absolute

The effect of long-term cold exposure on skeletal and cardiac muscle protein turnover was investigated in young growing animals. Two groups of 36 male 28-day-old rats were maintained at either 5°C (cold) or 25°C (control). Rates of protein synthesis and degradation were measured in vivo on days 5, 10, 15, and 20. Protein mass by day 20 was ∼28% lower in skeletal muscle (gastrocnemius and soleus) and ∼24% higher in heart in cold compared with control rats ( P < 0.05). In skeletal muscle, the fractional rates of protein synthesis ( k syn) and degradation ( k deg) were not significantly different between cold and control rats, although k syn was lower (approximately −26%) in cold rats on day 5; consequent to the lower protein mass, the absolute rates of protein synthesis (approximately −21%; P < 0.05) and degradation (approximately −13%; P < 0.1) were lower in cold compared with control rats. In heart, overall, k syn(approximately +12%; P < 0.1) and k deg(approximately +22%; P < 0.05) were higher in cold compared with control rats; consequently, the absolute rates of synthesis (approximately +44%) and degradation (approximately +54%) were higher in cold compared with control rats ( P < 0.05). Plasma triiodothyronine concentration was higher ( P < 0.05) in cold compared with control rats. These data indicate that long-term cold acclimation in skeletal muscle is associated with the establishment of a new homeostasis in protein turnover with decreased protein mass and normal fractional rates of protein turnover. In heart, unlike skeletal muscle, rates of protein turnover did not appear to immediately return to normal as increased rates of protein turnover were observed beyond day 5. These data also indicate that increased rates of protein turnover in skeletal muscle are unlikely to contribute to increased metabolic heat production during cold acclimation.

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