Amino acids regulate skeletal muscle PHAS-I and p70 S6-kinase phosphorylation independently of insulin
W. Long, L. Saffer, L. Wei, and E. J. Barrett
Department of Internal Medicine, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908
Refeeding reverses the muscle protein loss seen with fasting. The physiological regulators and cellular control sites responsible for this reversal are incompletely defined. Phosphorylation of phosphorylated heat-acid stabled protein (PHAS-I) frees eukaryotic initiation factor 4E (eIF4E) and stimulates protein synthesis by accelerating translation initiation. Phosphorylation of p70 S6-kinase (p70S6k) is thought to be involved in the regulation of the synthesis of some ribosomsal proteins and other selected proteins with polypyrimidine clusters near the transcription start site. We examined whether phosphorylation of PHAS-I and p70S6k was increased by feeding and determined the separate effects of insulin and amino acids on PHAS-I and p70S6k phosphorylation in rat skeletal muscle in vivo. Muscle was obtained from rats fed ad libitum or fasted overnight (n = 5 each). Other fasted rats were infused with insulin (3 µU · min-1 · kg-1, euglycemic clamp), amino acids, or the two combined. Gastrocnemius was freeze-clamped, and PHAS-I and p70S6k phosphorylation was measured by quantifying the several phosphorylated forms of these proteins seen on Western blots. We observed that feeding increased phosphorylation of both PHAS-I and p70S6k (P < 0.05).
Infusion of amino acids alone reproduced the effect of feeding. Physiological hyperinsulinemia increased p70S6K (P < 0.05) but not PHAS-I phosphorylation (P = 0.98). Addition of insulin to amino acid infusion was no more effective than amino acids alone in promoting PHAS-I and p70S6k phosphorylation.
We conclude that amino acid infusion alone enhances the activation of the protein synthetic pathways in vivo in rat skeletal muscle. This effect is not dependent on increases in plasma insulin and simulates the activation of protein synthesis that accompanies normal feeding.
Amino Acids Stimulate Translation Initiation and Protein Synthesis through an Akt-Independent Pathway in Human Skeletal Muscle
Zhenqi Liu, Linda A. Jahn, Liping Wei, Wen Long and Eugene J. Barrett
Division of Endocrinology and Metabolism, Department of Internal Medicine, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908
Address all correspondence and requests for reprints to: Zhenqi Liu, M.D., Division of Endocrinology and Metabolism, Department of Internal Medicine, P.O. Box 801410, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908. E-mail:
zl3e@virginia.edu.
Abstract
Studies in vitro as well as in vivo in rodents have suggested that amino acids (AA) not only serve as substrates for protein synthesis, but also as nutrient signals to enhance mRNA translation and protein synthesis in skeletal muscle. However, the physiological relevance of these findings to normal humans is uncertain. To examine whether AA regulate the protein synthetic apparatus in human skeletal muscle, we infused an AA mixture (10% Travesol) systemically into 10 young healthy male volunteers for 6 h. Forearm muscle protein synthesis and degradation (phenylalanine tracer method) and the phosphorylation of protein kinase B (or Akt), eukaryotic initiation factor 4E-binding protein 1, and ribosomal protein S6 kinase (p70S6K) in vastus lateralis muscle were measured before and after AA infusion. We also examined whether AA affect urinary nitrogen excretion and whole body protein turnover.
Postabsorptively all subjects had negative forearm phenylalanine balances. AA infusion significantly improved the net phenylalanine balance at both 3 h (P < 0.002) and 6 h (P < 0.02). This improvement in phenylalanine balance was solely from increased protein synthesis (P = 0.02 at 3 h and P < 0.003 at 6 h), as protein degradation was not changed. AA also significantly decreased whole body phenylalanine flux (P < 0.004). AA did not
activate Akt phosphorylation at Ser473, but significantly increased the phosphorylation of both eukaryotic initiation factor 4E-binding protein 1 (P < 0.04) and p70S6K (P < 0.001).
We conclude that AA act directly as nutrient signals to stimulate protein synthesis through Akt-independent activation of the protein synthetic apparatus in human skeletal muscle.
Effect of carbohydrate intake on net muscle protein synthesis during recovery from resistance exercise
Elisabet Børsheim, Melanie G. Cree, Kevin D. Tipton, Tabatha A. Elliott, Asle Aarsland, and Robert R. Wolfe
Department of Surgery, Metabolism Unit, Shriners Hospitals for Children-Galveston, University of Texas Medical Branch, Galveston, Texas 77550
Submitted 3 April 2003 ; accepted in final form 24 October 2003
The purpose of this study was to determine the effect of ingestion of 100 g of carbohydrates on net muscle protein balance (protein synthesis minus protein breakdown) after resistance exercise. Two groups of eight subjects performed a resistance exercise bout (10 sets of 8 repetitions of leg presses at 80% of 1-repetition maximum) before they rested in bed for 4 h. One group (CHO) received a drink consisting of 100 g of carbohydrates 1 h postexercise. The other group (Pla) received a noncaloric placebo drink. Leg amino acid metabolism was determined by infusion of 2H5- or 13C6-labeled phenylalanine, sampling from femoral artery and vein, and muscle biopsies from vastus lateralis. Drink intake did not affect arterial insulin concentration in Pla, whereas insulin increased several times after the drink in CHO (P < 0.05 vs. Pla). Arterial phenylalanine concentration fell slightly after the drink in CHO. Net muscle protein balance between synthesis and breakdown did not change in Pla, whereas it improved in CHO from -17 ± 3 nmol·ml-1·100 ml leg-1 before drink to an average of -4 ± 4 and 0 ± 3 nmol·ml-1·100 ml leg-1 during the second and third hour after the drink, respectively (P < 0.05 vs. Pla during last hour). The improved net balance in CHO was due primarily to a progressive decrease in muscle protein breakdown.
We conclude that ingestion of carbohydrates improved net leg protein balance after resistance exercise. However, the effect was minor and delayed compared with the previously reported effect of ingestion of amino acids.
Determinants of post-exercise glycogen synthesis during short-term recovery.
Jentjens R, Jeukendrup A.
Human Performance Laboratory, School of Sport and Exercise Sciences, University of Birmingham, Edgbaston, Birmingham, UK.
The pattern of muscle glycogen synthesis following glycogen-depleting exercise occurs in two phases.
Initially, there is a period of rapid synthesis of muscle glycogen that does not require the presence of insulin and lasts about 30-60 minutes. This rapid phase of muscle glycogen synthesis is characterised by an exercise-induced translocation of glucose transporter carrier protein-4 to the cell surface, leading to an increased permeability of the muscle membrane to glucose. Following this rapid phase of glycogen synthesis, muscle glycogen synthesis occurs at a much slower rate and this phase can last for several hours. Both muscle contraction and insulin have been shown to increase the activity of glycogen synthase, the rate-limiting enzyme in glycogen synthesis. Furthermore, it has been shown that muscle glycogen concentration is a potent regulator of glycogen synthase. Low muscle glycogen concentrations following exercise are associated with an increased rate of glucose transport and an increased capacity to convert glucose into glycogen.The highest muscle glycogen synthesis rates have been reported when large amounts of carbohydrate (1.0-1.85 g/kg/h) are consumed immediately post-exercise and at 15-60 minute intervals thereafter, for up to 5 hours post-exercise. When carbohydrate ingestion is delayed by several hours, this may lead to ~50% lower rates of muscle glycogen synthesis. The addition of certain amino acids and/or proteins to a carbohydrate supplement can increase muscle glycogen synthesis rates, most probably because of an enhanced insulin response. However, when carbohydrate intake is high (>/=1.2 g/kg/h) and provided at regular intervals, a further increase in insulin concentrations by additional supplementation of protein and/or amino acids does not further increase the rate of muscle glycogen synthesis. Thus, when carbohydrate intake is insufficient (<1.2 g/kg/h), the addition of certain amino acids and/or proteins may be beneficial for muscle glycogen synthesis. Furthermore, ingestion of insulinotropic protein and/or amino acid mixtures might stimulate post-exercise net muscle protein anabolism. Suggestions have been made that carbohydrate availability is the main limiting factor for glycogen synthesis.
A large part of the ingested glucose that enters the bloodstream appears to be extracted by tissues other than the exercise muscle (i.e. liver, other muscle groups or fat tissue) and may therefore limit the amount of glucose available to maximise muscle glycogen synthesis rates. Furthermore, intestinal glucose absorption may also be a rate-limiting factor for muscle glycogen synthesis when large quantities (>1 g/min) of glucose are ingested following exercise.
If you want to discuss the situation, you need to take into the conditions post workout, not post marathon.
