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The Muscle-Building Messenger: Your Complete Guide To Insulin
Furthermore, the sampling site (leg or forearm) is another critical factor because of the possible differential contribution of the nonmuscle tissues (mostly skin and bone) to each limb protein turnover. Skin protein breakdown is decreased and skin protein synthesis is unaffected by insulin (), whereas bone marrow proteins are likely to turn over faster than muscle proteins, given the high turnover rate of bone marrow cells, but no data are available on the effects of insulin on these cells. The choice of the model to calculate muscle protein turnover can also significantly affect the conclusions of the study. In our experiment, we used all of the three models available to measure human muscle protein synthesis: the two arteriovenous balance methods [two-pool () and three-pool () models] and the precursor-product model (). This decision was made because most earlier studies had been performed using the two-pool arteriovenous balance method (, , , , , -, ). Thus the use of that method allowed us to compare previous results with ours. However, that method cannot detect increases in intracellular amino acid recycling from breakdown into synthesis because it measures the utilization of plasma amino acids for synthesis and the release in the plasma of amino acids deriving from proteolysis. The three-pool model allows for the measurement of the total synthesis and breakdown rates, including recycling (), and the precursor-product model provides a measure of the overall effect of the treatment over the entire experimental period.
N2 - 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
Glossary | Linus Pauling Institute | Oregon State University
In human subjects, insulin infusion induces net amino acid uptake across a limb (forearm or leg), an indication of net muscle protein anabolism, but the mechanisms are still unclear (, , , , , , , -, ). About half of these studies reported that this effect was due to an increase in protein synthesis with no major changes or some reduction in proteolysis (, , , , , ). Conversely, the other studies found a significant reduction in protein degradation with no significant changes in protein synthesis (, , , , ). Because these experiments were performed using comparable stable-isotope arteriovenous balance methodologies, it is unlikely that technical or methodological problems were responsible for the conflicting findings. A review of these studies suggests that these apparent discrepancies on the metabolic mechanisms by which insulin stimulates muscle protein anabolism might be explained by differences in amino acid availability for the muscle tissue (). Specifically, all studies in which muscle protein synthesis had been stimulated by insulin also had an increased amino acid delivery to the muscle tissue (amino acid concentration × blood flow) (, , , , , , ), whereas most studies reporting a decrease in muscle protein breakdown with no increase in synthesis during insulin infusion also had a decrease or no change in amino acid delivery (, , , ). The differences in amino acid delivery were mainly due to differences in amino acid concentrations, which, in turn, were determined by the modality of insulin infusion (systemic or local) and/or the concomitant infusion of exogenous amino acids. This is because systemic insulin infusion decreases blood amino acid concentrations (, , , , , ) unless amino acids are replaced by exogenous infusion (-, -). Conversely, local insulin infusion in a leg or a forearm allows for the exposure of the muscle tissue to relatively high insulin levels while avoiding a major reduction in blood amino acid concentration (, ).
We studied 19 young subjects (11 men and 8 women) from the Los Angeles metropolitan area. All subjects were healthy and physically active, but they were not engaged in an exercise training program. 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, TSH, lipid profile, pregnancy test in women, urinalysis, drug screening test, and electrocardiogram. Only subjects with screening results within the normal limits were randomly assigned to one of three groups according to the dose of insulin infused during the experiment: low dose (LD), intermediate dose (ID), and high dose (HD). The doses were chosen to increase insulin concentrations in the femoral vein within the physiological postprandial range (, , ). We did not include a fourth group infused with saline because we have found in a separate experiment that muscle protein metabolism is unaffected by time alone over the short term (6 h; data not shown). The subjects’ characteristics are summarized in .
Protein Synthesis -Translation and Regulation
The protocol was designed to measure muscle protein and amino acid and glucose kinetics in the postabsorptive basal state (0−240 min) and during insulin infusion (240−420 min).
for muscle proteins. Insulin deficiency leads to a protein catabolic state with loss of muscle mass that can only be reversed by insulin therapy (). Nonetheless, the mechanisms by which insulin enhances muscle protein anabolism are still debated. A stimulatory effect of insulin on protein synthesis has been demonstrated in various tissues, including skeletal muscle (, , , ). Furthermore, in vitro animal studies and a recent human experiment have shown that insulin can acutely stimulate muscle protein synthesis by increasing the initiation of mRNA translation (, -). On the other hand, if the physiological increase in insulin secretion is pharmacologically suppressed during feeding in rats, the stimulation of translation initiation is abolished and muscle protein synthesis suppressed (). Insulin can also reduce protein breakdown by stabilizing lysosomes and reducing the activity of the ubiquitin-proteasome pathway (, , ).
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All about protein: What is it and how much do you need?
Phenylalanine was chosen because it is not oxidized by skeletal muscle, and therefore its utilization is a direct measure of muscle protein synthesis. With the two-pool model, phenylalanine enrichments and concentrations in the femoral artery and vein were used to estimate muscle protein synthesis, breakdown, and net balance. These parameters are based on the extraction of the labeled phenylalanine from the femoral artery, the appearance of unlabeled phenylalanine from the muscle in the femoral vein, and the net arteriovenous difference of the phenylalanine concentrations, respectively (). Thus this model provides data regarding the kinetics of plasma phenylalanine across the leg with no consideration for intracellular recycling of the amino acid from breakdown to synthesis. In other words, this method allows for the measurement of the effect of our treatments on the net kinetics of plasma phenylalanine across the leg while not offering any insight into its intracellular kinetics.
13/10/2008 · Which protein is best
A method for perfusion in vitro of a preparation of rat hemicorpus was developed for study of the metabolism of skeletal muscle. The preparation was stable during perfusion, as indicated by maintenance of ATP concentration, perfusion pressure, and oxygen consumption for up to 90 min. The perfused hemicorpus provided the following advantages for study of protein synthesis in skeletal muscle: (a) hormones and substrates reached the muscle cells through an intact capillary bed, and (b) the preparation included the psoas muscle, which was sufficiently large to allow measurements of intermediates in the pathway of protein synthesis and was readily homogenized for preparation of ribosomes and ribosomal subunits. Perfusion of psoas muscle from fasted rats with buffer containing glucose and insulin reduced the concentration of ribosomal subunits and increased phenylalanine incorporation as compared to perfusion with buffer containing glucose alone. In addition, the hormone increased glucose uptake from the perfusate and inhibited release of free fatty acids from the preparation. When the muscle was perfused with buffer that contained glucose and palmitate, the concentration of ribosomal subunits and phenylalanine incorporation were unchanged. Since fatty acid is known to stimulate protein synthesis in heart muscle, these results indicated that rates of protein synthesis in heart, but not in skeletal, muscle would be maintained during fasting or in diabetic animals by increased plasma concentration of fatty acid.
An overview of insulin signaling pathways | Abcam
We also calculated the fractional synthetic rate (FSR) of mixed muscle proteins by measuring the incorporation rate of the phenylalanine tracer into the proteins (ΔEP/t) and using the precursor-product model to calculate the synthesis rate as follows ():
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