Myofibrillar protein turnover. Synthesis of protein-bound 3-methylhistidine, actin, myosin heavy chain and aldolase in rat skeletal muscle in the fed and starved states

Post-natal muscle growth and protein turnover: a narrative review of current understanding

A model explaining the dietary-protein-driven post-natal skeletal muscle growth and protein turnover in the rat is updated, and the mechanisms involved are described, in this narrative review. Dietary protein controls both bone length and muscle growth, which are interrelated through mechanotransduction mechanisms with muscle growth induced both from stretching subsequent to bone length growth and from internal work against gravity. This induces satellite cell activation, myogenesis and remodelling of the extracellular matrix, establishing a growth capacity for myofibre length and cross-sectional area. Protein deposition within this capacity is enabled by adequate dietary protein and other key nutrients. After briefly reviewing the experimental animal origins of the growth model, key concepts and processes important for growth are reviewed. These include the growth in number and size of the myonuclear domain, satellite cell activity during post-natal development and the autocrine/paracrine action of IGF-1. Regulatory and signalling pathways reviewed include developmental mechanotransduction, signalling through the insulin/IGF-1-PI3K-Akt and the Ras-MAPK pathways in the myofibre and during mechanotransduction of satellite cells. Likely pathways activated by maximal-intensity muscle contractions are highlighted and the regulation of the capacity for protein synthesis in terms of ribosome assembly and the translational regulation of 5-TOPmRNA classes by mTORC1 and LARP1 are discussed. Evidence for and potential mechanisms by which volume limitation of muscle growth can occur which would limit protein deposition within the myofibre are reviewed. An understanding of how muscle growth is achieved allows better nutritional management of its growth in health and disease.

Heat production in quiescent cardiac muscle is length, velocity and muscle dependent: Implications for active heat measurement

NEW FINDINGS

What is the central question of this study? Intracellular energetic processes in quiescent cardiac muscle release 'basal' heat; during contraction, a much larger amount of 'active' heat is also produced. Previously, measurement challenges have constrained researchers to assume that basal heat rate remains constant during contraction and shortening. Is this assumption correct? What is the main finding and its importance? We show that basal heat rate is modulated by the extent and velocity of muscle shortening. Their relative contributions are muscle specific. We apply a method with which researchers can now disentangle, for each experiment, changes in basal heat from active heat production, providing more precise measures of the individual energetic processes underlying cardiac muscle contraction.

ABSTRACT

Separating the variations in cardiac basal heat rate from variations in active heat rate is necessary to determine cardiac muscle energy consumption accurately during the performance of active work. By developing a model of cardiac muscle basal heat rate, we aimed to investigate changes in basal heat rate when cardiac muscle performs work. Experiments were conducted on 10 isolated rat cardiac trabeculae subjected to both active (work-loops) and quiescent (length-change and velocity) interventions. Muscle force, length and heat output rate were measured simultaneously in a flow-through work-loop calorimeter. Quiescent muscle characteristics were used to parameterize muscle-specific models of change in basal heat rate, thereby to predict dynamic changes in basal heat rate during active work-loop contraction. Our data showed that the quiescent heat characteristics of cardiac muscle varied between samples, displaying dependence on both the extent and the rate of change in muscle length. We found a moderate correlation between muscle dimensions (cross-sectional area and volume) and the length-dependent basal heat parameter (P = 0.0330 and P = 0.0242, respectively), but no correlation with the velocity-dependent parameter. These findings lead us to conclude that the heat output of cardiac muscle at quiescence varies with both the extent and the velocity of shortening, to an extent that is muscle specific, and that this variation must be measured and accounted for in each specimen when assessing active energetics.

Interactions between Growth of Muscle and Stature: Mechanisms Involved and Their Nutritional Sensitivity to Dietary Protein: The Protein-Stat Revisited

Childhood growth and its sensitivity to dietary protein is reviewed within a Protein-Stat model of growth regulation. The coordination of growth of muscle and stature is a combination of genetic programming, and of two-way mechanical interactions involving the mechanotransduction of muscle growth through stretching by bone length growth, the core Protein-Stat feature, and the strengthening of bone through muscle contraction via the mechanostat. Thus, growth in bone length is the initiating event and this is always observed. Endocrine and cellular mechanisms of growth in stature are reviewed in terms of the growth hormone-insulin like growth factor-1 (GH-IGF-1) and thyroid axes and the sex hormones, which together mediate endochondral ossification in the growth plate and bone lengthening. Cellular mechanisms of muscle growth during development are then reviewed identifying (a) the difficulties posed by the need to maintain its ultrastructure during myofibre hypertrophy within the extracellular matrix and the concept of muscle as concentric "bags" allowing growth to be conceived as bag enlargement and filling, (b) the cellular and molecular mechanisms involved in the mechanotransduction of satellite and mesenchymal stromal cells, to enable both connective tissue remodelling and provision of new myonuclei to aid myofibre hypertrophy and (c) the implications of myofibre hypertrophy for protein turnover within the myonuclear domain. Experimental data from rodent and avian animal models illustrate likely changes in DNA domain size and protein turnover during developmental and stretch-induced muscle growth and between different muscle fibre types. Growth of muscle in male rats during adulthood suggests that "bag enlargement" is achieved mainly through the action of mesenchymal stromal cells. Current understanding of the nutritional regulation of protein deposition in muscle, deriving from experimental studies in animals and human adults, is reviewed, identifying regulation by amino acids, insulin and myofibre volume changes acting to increase both ribosomal capacity and efficiency of muscle protein synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) and the phenomenon of a "bag-full" inhibitory signal has been identified in human skeletal muscle. The final section deals with the nutritional sensitivity of growth of muscle and stature to dietary protein in children. Growth in length/height as a function of dietary protein intake is described in the context of the breastfed child as the normative growth model, and the "Early Protein Hypothesis" linking high protein intakes in infancy to later adiposity. The extensive paediatric studies on serum IGF-1 and child growth are reviewed but their clinical relevance is of limited value for understanding growth regulation; a role in energy metabolism and homeostasis, acting with insulin to mediate adiposity, is probably more important. Information on the influence of dietary protein on muscle mass per se as opposed to lean body mass is limited but suggests that increased protein intake in children is unable to promote muscle growth in excess of that linked to genotypic growth in length/height. One possible exception is milk protein intake, which cohort and cross-cultural studies suggest can increase height and associated muscle growth, although such effects have yet to be demonstrated by randomised controlled trials.

mTORC1 signalling is not essential for the maintenance of muscle mass and function in adult sedentary mice

BACKGROUND

The balance between protein synthesis and degradation (proteostasis) is a determining factor for muscle size and function. Signalling via the mammalian target of rapamycin complex 1 (mTORC1) regulates proteostasis in skeletal muscle by affecting protein synthesis and autophagosomal protein degradation. Indeed, genetic inactivation of mTORC1 in developing and growing muscle causes atrophy resulting in a lethal myopathy. However, systemic dampening of mTORC1 signalling by its allosteric inhibitor rapamycin is beneficial at the organismal level and increases lifespan. Whether the beneficial effect of rapamycin comes at the expense of muscle mass and function is yet to be established.

METHODS

We conditionally ablated the gene coding for the mTORC1-essential component raptor in muscle fibres of adult mice [inducible raptor muscle-specific knockout (iRAmKO)]. We performed detailed phenotypic and biochemical analyses of iRAmKO mice and compared them with muscle-specific raptor knockout (RAmKO) mice, which lack raptor in developing muscle fibres. We also used polysome profiling and proteomics to assess protein translation and associated signalling in skeletal muscle of iRAmKO mice.

RESULTS

Analysis at different time points reveal that, as in RAmKO mice, the proportion of oxidative fibres decreases, but slow-type fibres increase in iRAmKO mice. Nevertheless, no significant decrease in body and muscle mass or muscle fibre area was detected up to 5 months post-raptor depletion. Similarly, ex vivo muscle force was not significantly reduced in iRAmKO mice. Despite stable muscle size and function, inducible raptor depletion significantly reduced the expression of key components of the translation machinery and overall translation rates.

CONCLUSIONS

Raptor depletion and hence complete inhibition of mTORC1 signalling in fully grown muscle leads to metabolic and morphological changes without inducing muscle atrophy even after 5 months. Together, our data indicate that maintenance of muscle size does not require mTORC1 signalling, suggesting that rapamycin treatment is unlikely to negatively affect muscle mass and function.

Myosin heavy chain 2A and α-actin expression in human and murine skeletal muscles at feeding; particularly amino acids

BACKGROUND

Protein dynamics during non-steady state conditions as feeding are complex. Such studies usually demand combinations of methods to give conclusive information, particularly on myofibrillar proteins with slow turnover. Therefore, time course transcript analyses were evaluated as possible means to monitor changes in myofibrillar biosynthesis in skeletal muscles in conditions with clinical nutrition; i.e. long term exposure of nutrients.

METHODS

Muscle tissue from overnight intravenously fed surgical patients were used as a model combined with muscle tissue from starved and refed mice as well as cultured L6 muscle cells. Transcripts of acta 1 (α-actin), mhc2A (myosin) and slc38 a2/Snat 2 (amino acid transporter) were quantified (qPCR) as markers of muscle protein dynamics.

RESULTS

Myosin heavy chain 2A transcripts decreased significantly in skeletal muscle tissue from overnight parenterally fed patients but did not change significantly in orally refed mice. Alpha-actin transcripts did not change significantly in muscle cells from fed patients, mice or cultured L6 cells during provision of AA. The AA transporter Snat 2 decreased in L6 cells refed by all AA and by various combinations of AA but did not change during feeding in muscle tissue from patients or mice.

CONCLUSION

Our results confirm that muscle cells are sensitive to alterations in extracellular concentrations of AA for induction of protein synthesis and anabolism. However, transcripts of myofibrillar proteins and amino acid transporters showed complex alterations in response to feeding with provision of amino acids. Therefore, muscle tissue transcript levels of actin and myosin do not reflect protein accretion in skeletal muscles at feeding.

Acute molecular response of mouse hindlimb muscles to chronic stimulation

Stimulation of the mouse hindlimb via the sciatic nerve was performed for a 4-h period to investigate acute muscle gene activation in a model of muscle phenotype conversion. Initial force production (1.6 +/- 0.1 g/g body wt) declined 45% within 10 min and was maintained for the remainder of the experiment. Force returned to initial levels upon study completion. An immediate-early growth response was present in the extensor digitorum longus (EDL) muscle (FOS, JUN, activating transcription factor 3, and musculoaponeurotic fibrosarcoma oncogene) with a similar but attenuated pattern in the soleus muscle. Transcript profiles showed decreased fast fiber-specific mRNA (myosin heavy chains 2A and 2B, fast troponins T(3) and I, alpha-tropomyosin, muscle creatine kinase, and parvalbumin) and increased slow transcripts (myosin heavy chain-1beta/slow, troponin C slow, and tropomyosin 3y) in the EDL versus soleus muscles. Histological analysis of the EDL revealed glycogen depletion without inflammatory cell infiltration in stimulated versus control muscles, whereas ultrastructural analysis showed no evidence of myofiber damage after stimulation. Multiple fiber type-specific transcription factors (tea domain family member 1, nuclear factor of activated T cells 1, peroxisome proliferator-activated receptor-gamma coactivator-1alpha and -beta, circadian locomotor output cycles kaput, and hypoxia-inducible factor-1alpha) increased in the EDL along with transcription factors characteristic of embryogenesis (Kruppel-like factor 4; SRY box containing 17; transcription factor 15; PBX/knotted 1 homeobox 1; and embryonic lethal, abnormal vision). No established in vivo satellite cell markers or genes activated in our parallel experiments of satellite cell proliferation in vitro (cyclins A(2), B(2), C, and E(1) and MyoD) were differentially increased in the stimulated muscles. These results indicated that the molecular onset of fast to slow phenotype conversion occurred in the EDL within 4 h of stimulation without injury or satellite cell recruitment. This conversion was associated with the expression of phenotype-specific transcription factors from resident fiber myonuclei, including the activation of nascent developmental transcriptional programs.

How nutrition and exercise maintain the human musculoskeletal mass

In this article we review some of our recent work concerning the effects of nutrition and exercise on protein synthesis and signal transduction in human musculoskeletal tissues. A great deal of new information is being generated by the application of recently refined techniques for measuring protein turnover. The field remains one that is largely descriptive but increasingly we are beginning to discern mechanisms underlying lean tissue maintenance, growth and wasting especially as multidisciplinary tools are applied to its study. Several types of exercise and nutrition are potent stimuli for protein synthesis in skeletal muscle. By contrast, collagen in the extracellular matrix in muscle and tendon appears to be mechanically but not nutritionally sensitive. The rates of collagen turnover in a variety of tissues are sufficiently high to account for a sizeable proportion of whole body protein turnover. One of the most recent surprises is the high turnover rate of human bone collagen and its anabolic response to feeding. As our understanding of the normal physiology of these processes advances, we become better able to construct testable hypotheses concerning the effects of ageing and disease on the musculoskeletal mass. Current evidence suggests that one of the major problems with loss of muscle during ageing is an inability of the tissue to respond adequately to increased availability of nutrients.

Skeletal muscle glutamate metabolism in health and disease: state of the art

PURPOSE OF REVIEW

Glutamate is an amino acid of interest because it participates in many metabolic pathways. However, there is evidence that skeletal muscle glutamate metabolism is disturbed in disease. This review presents current knowledge regarding the metabolic function and regulation of glutamate in skeletal muscle under physiological and pathophysiological circumstances. Furthermore, several options for modulating muscle glutamate concentration in order to improve glutamate metabolism are discussed.

RECENT FINDINGS

The high correlation between muscle glutamate concentration and muscle glutathione concentration suggests that glutamate plays a determining role in the glutathione synthesis pathway. During exercise, glutamate plays a central role in energy provision because it participates in the tricarboxylic acid and the purine nucleotide cycles. However, a consistent finding in several diseases is reduced skeletal muscle glutamate. Remarkably, only few studies focused on modulation of muscle glutamate status either by exercise or by nutritional supplementation. There are several options for modulating glutamate metabolism, but the specific effects of the individual options require further elucidation. Nutritional supplementation of glutamate or its precursors glutamine, (ornithine) alpha-ketoglutarate, or the branched chain amino acids can influence muscle glutamate status.

SUMMARY

Specific intervention studies must be conducted to investigate the effect of supplementation on skeletal muscle glutamate turnover and its related metabolic and functional consequences in healthy individuals and in patients with acute or chronic disease.

Mechanical ventilation depresses protein synthesis in the rat diaphragm

Prolonged mechanical ventilation results in diaphragmatic atrophy and contractile dysfunction in animals. We hypothesized that mechanical ventilation-induced diaphragmatic atrophy is associated with decreased synthesis of both mixed muscle protein and myosin heavy chain protein in the diaphragm. To test this postulate, adult rats were mechanically ventilated for 6, 12, or 18 hours and diaphragmatic protein synthesis was measured in vivo. Six hours of mechanical ventilation resulted in a 30% decrease (p < 0.05) in the rate of mixed muscle protein synthesis and a 65% decrease (p < 0.05) in the rate of myosin heavy chain protein synthesis; this depression in diaphragmatic protein synthesis persisted throughout 18 hours of mechanical ventilation. Real-time polymerase chain reaction analyses revealed that mechanical ventilation, in comparison with time-matched controls, did not alter diaphragmatic levels of Type I and IIx myosin heavy chain messenger ribonucleic acid levels in the diaphragm. These data support the hypothesis that mechanical ventilation results in a decrease in both mixed muscle protein and myosin heavy chain protein synthesis in the diaphragm. Further, the decline in myosin heavy chain protein synthesis does not appear to be associated with a decrease in myosin heavy chain messenger ribonucleic acid.

Myosin localization during meiosis I of crane-fly spermatocytes gives indications about its role in division

We showed previously that in crane-fly spermatocytes myosin is required for tubulin flux [Silverman-Gavrila and Forer, 2000a: J Cell Sci 113:597-609], and for normal anaphase chromosome movement and contractile ring contraction [Silverman-Gavrila and Forer, 2001: Cell Motil Cytoskeleton 50:180-197]. Neither the identity nor the distribution of myosin(s) were known. In the present work, we used immunofluorescence and confocal microscopy to study myosin during meiosis-I of crane-fly spermatocytes compared to tubulin, actin, and skeletor, a spindle matrix protein, in order to further understand how myosin might function during cell division. Antibodies to myosin II regulatory light chain and myosin II heavy chain gave similar staining patterns, both dependent on stage: myosin is associated with nuclei, asters, centrosomes, chromosomes, spindle microtubules, midbody microtubules, and contractile rings. Myosin and actin colocalization along kinetochore fibers from prometaphase to anaphase are consistent with suggestions that acto-myosin forces in these stages propel kinetochore fibres poleward and trigger tubulin flux in kinetochore fibres, contributing in this way to poleward chromosome movement. Myosin and actin colocalization at the cell equator in cytokinesis, similar to studies in other cells [e.g., Fujiwara and Pollard, 1978: J Cell Biol 77:182-195], supports a role of actin-myosin interactions in contractile ring function. Myosin and skeletor colocalization in prometaphase spindles is consistent with a role of these proteins in spindle formation. After microtubules or actin were disrupted, myosin remained in spindles and contractile rings, suggesting that the presence of myosin in these structures does not require the continued presence of microtubules or actin. BDM (2,3 butanedione, 2 monoxime) treatment that inhibits chromosome movement and cytokinesis also altered myosin distributions in anaphase spindles and contractile rings, consistent with the physiological effects, suggesting also that myosin needs to be active in order to be properly distributed.

Development and application of a compartmental model of 3-methylhistidine metabolism in humans and domestic animals

Measurement of urinary 3-methylhistidine (3MH) excretion is the primary in vivo method to measure skeletal muscle (myofibrillar) protein breakdown. This method requires quantitative collection of urine and is based on the assumption that no metabolism of 3MH occurs once it is released from actin and myosin. This is true in most species, but in sheep and swine a proportion is retained in muscle as a dipeptide, balenine. In neither of these species does urine 3MH yield any data on the metabolism of 3MH. We have conducted studies that propose that 3MH metabolism in humans, cattle, dogs, swine, and sheep can be defined from a single bolus infusion of a stable isotope 3-[methyl-2H3]-methylhistidine. Following the bolus dose of the stable isotope tracer, serial blood samples and/or urine was collected over three to five days. A minimum of three exponentials were required to describe the plasma decay curve adequately. The kinetic linear-time-invariant models of 3MH metabolism in the whole animal were constructed by using the SAAM/CONSAM modeling program. Three different configurations of a three-compartment model are described: (A) A simple three-compartment model for humans, cattle, and dogs, in which plasma kinetics (3-[methyl-2H3]-MH/3MH) are described by compartment 1 and with one urinary exit from compartment 1. (B) A plasma-urinary kinetic three-compartment model with two exits was used for sheep with a urinary exit out of compartment 1 and a balenine exit out of a tissue compartment 3. (C) A plasma three-compartment model was used in swine with an exit out of a tissue compartment 3. The kinetic parameters reflect the differences in known physiology of humans, cattle, and dogs as compared to sheep and swine that do not quantitatively excrete 3MH into the urine. Steady-state model calculations define masses and fluxes of 3MH between three compartments and, importantly, the de novo production of 3MH. The de novo production of 3MH for humans, cattle, dogs, sheep, and swine are 3.1, 6.0, 12.1, 10.3, and 7.2 mumol x kg-1 x d-1, respectively. The de novo production of 3MH as calculated by the compartmental model was not different when compared to 3MH production as calculated via traditional urinary collection. Additionally, data suggest that steady-state compartment masses and mass transfer rates may be related to fat free mass and muscle mass in humans and swine, respectively. In conclusion, models of 3MH metabolism have been developed in numerous species, and these models can be used for the assessment of muscle proteolysis and 3MH kinetics without the collection of urine. This methodology is less evasive and will be useful in testing further experimental designs that alter myofibrillar protein breakdown.

Cellular adaptations in the diaphragm in chronic obstructive pulmonary disease

BACKGROUND

In patients with severe chronic obstructive pulmonary disease, the diaphragm undergoes physiologic adaptations characterized by an increase in energy expenditure and relative resistance to fatigue. We hypothesized that these physiologic characteristics would be associated with structural adaptations consisting of an increased proportion of less-fatigable slow-twitch muscle fibers and slow isoforms of myofibrillar proteins.

METHODS

We obtained biopsy specimens of the diaphragm from 6 patients with severe chronic obstructive pulmonary disease (mean [+/-SE] forced expiratory volume in one second, 33+/-4 percent of the predicted value; residual volume, 259+/-25 percent of the predicted value) and 10 control subjects. The proportions of the various isoforms of myosin heavy chains, myosin light chains, troponin, and tropomyosin were determined by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis. We also used immunocytochemical techniques to determine the proportions of the various types of muscle fibers.

RESULTS

The diaphragm-biopsy specimens from the patients had higher percentages of slow myosin heavy chain I (64+/-3 vs. 45+/-2 percent, P<0.001), and lower percentages of fast myosin heavy chains IIa (29+/-3 vs. 39+/-2 percent, P=0.01) and IIb (8+/-1 vs. 17+/-1 percent, P<0.001) than the diaphragms of the controls. Similar differences were noted when immunohistochemical techniques were used to compare the percentages of these fiber types in the two groups. In addition, the patients had higher percentages of the slow isoforms of myosin light chains, troponins, and tropomyosin, whereas the controls had higher percentages of the fast isoforms of these proteins.

CONCLUSIONS

Severe chronic obstructive pulmonary disease increases the slow-twitch characteristics of the muscle fibers in the diaphragm, an adaptation that increases resistance to fatigue.

Measurement of tissue protein synthesis rates in vivo: a critical analysis of contrasting methods

The extension of the flooding method for measuring the rate of protein synthesis, from animal to human tissues, has led to criticism. This is based on the observation that in human muscle, unlike animal tissues, the rate of synthesis in the fasting state measured with constant infusion is lower than that obtained with the flooding technique. Moreover, incorporation of infused tracer can be enhanced with a simultaneous flood, although an inhibition of incorporation has also been reported. Explanations for these observed discrepancies are explored. Evidence from studies in human muscle both with flooding and with a nonisotopic technique have given no indication of a stimulation of protein synthesis during flooding. It is therefore concluded that the most likely explanation for the discrepancy between methods is that changes in the isotopic enrichment of the precursor amino acid, which are minimized by the flooding procedure, are not adequately accounted for with the constant infusion method.

Effects of fibric acid derivatives on accumulation of actin in myocardiocytes

We used sodium dodecylsulphate polyacrylamide gel electrophoresis and immunoblotting to analyze the effects of the fibric acid derivatives bezafibrate, fenofibrate and gemfibrozil on the accumulation of actin in the cytoplasmic and cytoskeletal fraction of cultured myocardiocytes. All three drugs tested modified cellular and subcellular actin in different ways, and the findings are thought to be related with the secondary effect of arrhythmia known to be caused by these drugs. Bezafibrate and gemfibrozil more markedly affected accumulation of actin by myocytes, while fenofibrate interfered less notably with the accumulation of this protein.

Effects of brief starvation on brain protease activity

Changes in the activity of proteases (cathepsin D and calpains) caused by 48-h food withdrawal were studied in the brain, liver, kidney, spleen, and heart of 3-, 12-, and 24-month-old Fischer rats. Cathepsin D activity was similar in brain, liver, and heart of control animals; in kidney it was 5-fold higher and in spleen about 10-fold higher. With age, activity increased in all organs tested except spleen. Brief starvation caused no change of cathepsin D activity in brain, but caused an increase in liver and a decrease in spleen. Neutral proteolytic activity in control was highest in the pons-medulla-cerebellum fraction of brain, and activity in liver and heart was below that in brain. Activity increased with age in brain and decreased in other organs. Brief starvation in young animals caused an increase in activity in brain, and a decrease in liver and spleen. Isolated calpain II activity was high in control brain. It increased with age in the cerebrum. Brief starvation resulted in a decrease in the brain. The results indicate that the protease content of the brain is altered with age and in malnutrition, with changes not being the same for all proteases, and changes in brain being different from those in other organs.

Regulation of myosin and overall protein degradation in mouse C2 skeletal myotubes

We compared the breakdown of total cellular protein with that of the contractile protein, myosin, in cultured C2 mouse skeletal myotubes. The degradation of long-lived cellular proteins (which comprise the vast majority of myotube proteins) was inhibited by serum, insulin, and rat insulin-like growth factor-2. A physiological concentration of insulin was effective, but most of the effect of insulin occurred at concentrations well above the physiological range. IGF-2 inhibited protein breakdown at concentrations well within the range of total IGF-2 known to be present in the serum of fetal and neonatal rats. The breakdown of short-lived proteins was not altered by insulin or serum. We measured myosin degradation using a monoclonal antibody directed against myosin heavy chain. The half-life of myosin was 27 hours, and myosin breakdown was not altered by serum withdrawal applies to certain proteins, but not to others.

Whole body protein synthesis: studies with different amino acids in the rat

These studies were undertaken to determine to what extent constant infusion measurements and plasma sampling could provide sensible answers for rates of whole body protein turnover and also which amino acid would be the most representative probe of whole body protein turnover. Whole body protein synthesis rates were estimated in 70-g rats with L-[U-14C]threonine, L-[U-14C]lysine, L-[U-14C]tyrosine, L-[U-14C]phenylalanine, and L-[1-14C]leucine by either simultaneous tracer infusion of four amino acids or by injections of large quantities of 14C-labeled amino acids. In the infusion experiment, indirect estimates of whole body protein turnover based on free amino acid specific radioactivity and stochastic modeling were compared with direct measurement of the incorporation of the tracer into proteins. These two methods of analysis provided similar results for each amino acid, although in each case fractional synthesis rates were lower (by between 26 and 63%) when calculations were based on plasma rather than tissue specific radioactivity. With the flooding-dose method, whole body fractional protein synthesis rates were 41.4, 25.6, 31.1, and 31.4% with threonine, lysine, phenylalanine, and leucine, respectively. These values were similar to those obtained by the continuous infusion method using tissue specific radioactivity for threonine and lysine. For leucine, however, the flooding-dose method provided an intermediate value between the two estimates derived either from the plasma or the tissue specific radioactivity in the infusion method.(ABSTRACT TRUNCATED AT 250 WORDS)

In-vitro stimulation of the rat epitrochlearis muscle. I. Contractile activity per se affects myofibrillar protein degradation and amino acid metabolism

The influence of contractile activity on protein degradation and amino acid metabolism in skeletal muscle was investigated by utilizing an in-vitro electrical stimulation model with the rat epitrochlearis muscle preparation. Graded decreases in contraction force and in the muscle content of ATP and PCr, and increases in lactate were recorded with different rates of stimulation (1 h) and with both isometric twitches and tetanic contractions. 3-Methylhistidine and phenylalanine were chosen as indicators of myofibrillar and total protein degradation, respectively. The release of 3-methylhistidine was significantly stimulated by contractile activity, but a significant increase in the total amount of this amino acid (released amount + tissue content) occurred only at the most intense contraction rates. The release rate, tissue content and total amount of phenylalanine were not influenced by the contractions. Glutamate formation was generally inhibited, but its release was increased. Alanine synthesis was increased in moderately and intensely stimulated muscles. Glutamine and glycine were not influenced by the contractions, however. Inhibition of protein synthesis did not significantly influence protein degradation or amino acid release. The data suggest that in the absence of anabolic factors in the medium, myofibrillar protein degradation is increased in heavily activated muscle. This takes place without total protein breakdown being affected.

The effects of fasting or hypoxia on rates of protein synthesis in vivo in subcellular fractions of rat heart and gastrocnemius muscle

We measured rates of protein synthesis in vivo in subcellular fractions (soluble, myofibrillar and stromal fractions) of the heart and the gastrocnemius from rats after fasting or under hypoxic conditions (i.e. atmospheres containing 5% or 10% O2). Such interventions are known to inhibit protein synthesis under some circumstances. The recovery of tissue protein after fractionation was 80-100%. The proportions of protein present in the soluble and stromal fractions were different in the two muscles. The rates of protein synthesis in the myofibrillar and stromal fractions were less than those for total mixed tissue protein, whereas the rate for soluble protein was greater. Both fasting and moderate hypoxia (10% O2 for 24 h) inhibited protein synthesis in the gastrocnemius. In this tissue, the synthesis of the myofibrillar fraction was apparently the most sensitive to inhibition, and this resulted in some significant increases in the soluble-fraction/myofibrillar-fraction protein-synthesis rate ratios. In the heart, fasting inhibited protein synthesis, but moderate hypoxia (10% O2 for 24 h) did not. The rate of protein synthesis in the cardiac myofibrillar fraction was again more sensitive to fasting than were the rates in the other fractions, but it was not as sensitive as that in the gastrocnemius. Under severely hypoxic conditions (5% O2 for 1 or 2 h), protein synthesis was decreased in all fractions in both tissues. These results suggest that the rates of protein synthesis in these relatively crude subcellular fractions vary.

Actin synthesis rate and mRNA level increase during early recovery of atrophied muscle

The purpose of this study was to correlate actin synthesis rate and alpha-actin mRNA level in the gastrocnemius-plantaris muscles of limbs during their recovery from 7 days of immobilization in 200- to 280-g female rats. The fractional synthesis rate of actin in control muscle was 1%/day. Actin synthesis rate was 33% of control level at the 7th day of hindlimb immobilization, returned to control value at the 2nd recovery day, and was three times higher than control on the 4th day of recovery. The alpha-actin mRNA was 53% of control at the 7th day of immobilization, and its increase during the 1st 2 recovery days paralleled the increase in actin synthesis rate; this suggests that pretranslational mechanisms caused the initial increase in actin synthesis. Further increases in actin synthesis from the 2nd to the 4th day appear to be under translational control, since actin synthesis was 300% of control on the 4th recovery day and alpha-actin mRNA was only 128% of control.

Protein synthesis and degradation during starvation-induced cardiac atrophy in rabbits

To determine the relative importance of protein degradation in the development of starvation-induced cardiac atrophy, in vivo fractional synthetic rates of total cardiac protein, myosin heavy chain, actin, light chain 1, and light chain 2 were measured in fed and fasted rabbits by continuous infusion of [3H] leucine. In addition, the rate of left ventricular protein accumulation and loss were assessed in weight-matched control and fasted rabbits. Rates of total cardiac protein degradation were then estimated as the difference between rates of synthesis and growth. Fasting produced left ventricular atrophy by decreasing the rate of left ventricular protein synthesis (34.8 +/- 1.4, 27.3 +/- 3.0, and 19.3 +/- 1.2 mg/day of left ventricular protein synthesized for 0-, 3-, and 7-day fasted rabbits, respectively). Inhibition of contractile protein synthesis was evident by significant reductions in the fractional synthetic rates of all myofibrillar protein subunits. Although fractional rates of protein degradation increased significantly within 7 days of fasting, actual amounts of left ventricular protein degraded per day were unaffected. Thus, prolonged fasting profoundly inhibits the synthesis of new cardiac protein, including the major protein constituents of the myofibril. Both this inhibition in new protein synthesis as well as a smaller but significant reduction in the average half-lives of cardiac proteins are responsible for atrophy of the heart in response to fasting.

The contribution of small gut to the 3-methylhistidine metabolism in the adult rat

Twenty male adult Sprague-Dawley rats received parenteral nutrition following a duodenoileostomy which left only 8% to 10% of the small gut intact. On the first postoperative day, the urinary 3-methylhistidine (3-MH) excretion rose to 1.5 to 1.7 times the preoperative level but fell again within 12 to 14 days to the basal level. A control group of 10 rats with small gut anastomosis without resection yielded similar results. We conclude that, at least in rats, the small intestine does not make a significant contribution to 24-hour urinary excretion of 3-MH. The transient postoperative rise in 3-MH excretion is probably due to postinjury metabolism.

Mobilisation of structural proteins during exercise

In general, the mobilisation of structural proteins is necessary for enzyme synthesis and for renewing cellular structures with amino acids and precursors of nucleotic acids. However, during exercise the adaptive synthesis of proteins occurs only in the liver to some extent. In muscle tissue most protein synthesis is suppressed, although the synthesis of certain proteins in muscle remains unchanged or even increases. The general suppression of protein synthesis in muscle leaves much of the free amino acid pool unused. The breakdown of tissue proteins may also increase in various tissues, but there is no convincing evidence for proteolysis of contractile proteins in active muscle. As a result of these processes, an increased pool of available free amino acids is created. The main use of free amino acids is connected with the energy requirement of muscular activity, through the oxidation of branched-chain amino acids and the use of alanine in gluconeogenesis. In active muscles the output of alanine is increased. It is based on usage of pyruvate, which is produced in increased amounts due to intensified glycogenolysis and glycolysis, and of amino groups, which are liberated in oxidation of branched-chain amino acids. In the liver, alanine is consumed. The carbon skeleton of alanine is required for gluconeogenesis and the liberated amino groups are used in ureagenesis. The branched-chain amino acids are transported from the liver to active muscle for their oxidation. The increases in the free amino acid pool, in the rate of the glucose-alanine cycle, and in the use of amino acids in the liver are stimulated by an increased level of glucocorticoids and a decreased level of insulin during exercise. During recovery after exercise the use of amino acids for adaptive protein synthesis is intensified. This coincides with a persistently high rate of protein breakdown, constituting an increased rate of protein turnover. During recovery, the production of 3-methylhistidine by previously active muscles increases. It results in an increase in urinary output of 3-methylhistidine after exercise. Immediately after exercise the level of free 3-methylhistidine is elevated in the intestine for only a short time and the fact that it does not contribute significantly to the delayed increase in the excretion of 3-methylhistidine excretion after exercise must be considered as a sign of increased turnover of contractile proteins, helping to restore a good contractile function.(ABSTRACT TRUNCATED AT 400 WORDS)

Partial purification and characterisation of S-adenosylmethionine:protein-histidine N-methyltransferase from rabbit skeletal muscle

A new class of protein methylase (S-adenosylmethionine:protein-histidine N-methyltransferase) which methylates histidine residues of protein substrates using S-adenosylmethionine as the methyl donor has been partially purified from rabbit skeletal muscle, 22-fold with a yield of 56%. The enzyme activity was monitored using denatured myofibrils from young rabbit skeletal muscle as the methyl acceptor protein substrate. The enzyme was localised in the myofibrillar fraction and myofibrils isolated in pure form represented the enzyme-substrate complex. The enzyme was solubilised in 0.275 M KCl. The methylase showed no requirement for any metal ion and has a pH optimum of 8.0. It was shown to require a reducing agent like mercaptoethanol for its activity. It was also shown that cardiac and skeletal muscle forms of actins obtained from different species serve as the efficient methyl acceptor protein substrates. Since the enzyme was found to methylate specifically the histidine residues of actin we propose to designate this new methylase as protein methylase IV, to distinguish it from the already known protein methylases I, II and III.

Regulation of myofibrillar protein degradation in rat skeletal muscle during brief and prolonged starvation

Myofibrillar protein breakdown during brief and prolonged starvation was assessed in perfused rat skeletal muscle from 8-week-old fat-fed rats that conserve skeletal muscle protein during starvation and survive for 12 to 15 days and age-matched chow-fed rats that do not conserve protein and survive only five to six days. Following the inhibition of protein synthesis with cycloheximide, myofibrillar proteolysis was assessed by measuring the release of 3-methylhistidine from the perfused rat hindquarter while simultaneous measurement of total protein breakdown was assessed by measuring tyrosine release. Myofibrillar proteolysis progressed through three distinct phases during starvation: an early phase occurring within 24 hours in which proteolysis increased in all rats, a middle phase, which took three to five days to develop and during which proteolysis decreased and was present only in fat-fed rats, and a late phase in which proteolysis again increased. Total protein breakdown (ie, tyrosine release) changed little in phase I, decreased in phase II, and increased in phase III. The release of 3-methylhistidine from the perfused hindquarter reflected changes in muscle and urine of intact rats suggesting that data obtained with the perfused hindquarter reflected the in vivo situation. Insulin, amino acids, high concentrations of glucose, indomethacin, or epinephrine as well as adrenalectomy failed to attenuate the increase in 3-methylhistidine release from the perfused hindquarter during brief and late starvation. Free fatty acids and ketone bodies were also without effect in vitro. Refeeding fasting rats for four hours decreased myofibrillar proteolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of starvation or treatment with corticosterone on the amount of easily releasable myofilaments in rat skeletal muscles

Treatment of isolated myofibrils with an ATP-containing relaxing solution results in the dissociation of a preformed quantity of myofilaments called 'easily releasable myofilaments'. Van der Westhuyzen, Matsumoto & Etlinger [(1981) J. Biol. Chem. 256, 11791-11797] presented experimental evidence that these myofilaments represent intermediate products in the turnover of myofibrillar proteins. To investigate further this question, we measured the size of the fraction of easily releasable myofilaments in three different species of skeletal muscles from rats subjected to well-defined catabolic conditions, namely starvation or chronic glucocorticoid administration. The results were as follows: (1) The amount of easily releasable myofilaments was transiently increased about 2-3-fold during both experiments, and thus paralleled the known alterations in the rate of overall muscle protein breakdown rather than in those of synthesis. (2) These changes were observed in muscles containing predominantly fast-twitch fibres, but not in slow-twitch soleus muscle, a muscle that is known to be more resistant to catabolic conditions. (3) The starvation-induced increase of the size of the fraction of easily releasable myofilaments could be significantly reduced by treatment of the starving animals with the proteinase inhibitor E-64. These results are compatible with the idea that easily releasable myofilaments are intermediates in the degradative pathway of myofibrillar proteins and that a proteolytic step may be involved in the conversion of myofilaments into easily releasable myofilaments.

Myofibrillar protein turnover. Synthesis rates of myofibrillar and sarcoplasmic protein fractions in different muscles and the changes observed during postnatal development and in response to feeding and starvation

Measurement of rates of synthesis of skeletal-muscle proteins in adult rats shows that the faster overall rate of turnover in diaphragm and soleus muscles compared with several other, more glycolytic, muscles is also exhibited by the myofibrillar proteins, since the ratio of sarcoplasmic to myofibrillar protein synthesis is similar for all muscles. Further, throughout postnatal development, when the overall turnover rate falls with age, parallel changes occur for the myofibrillar proteins, as indicated by a constant ratio of sarcoplasmic to myofibrillar protein synthesis (2.06) in the steady state after overnight starvation. Only in the youngest (4 weeks old) rats is a slightly lower ratio observed (1.72). These results indicate that, when changes in the overall turnover rate of muscle proteins occur, the relative turnover of the two major protein fractions stays constant. However, measurements in the non-steady state during growth and after starvation for 4 days show that the relative synthesis rates of the two fractions change as a result of a disproportionate increase in myofibrillar protein synthesis during growth and decrease during starvation. Thus the synthesis rate of the slower-turning-over myofibrillar protein fraction is more sensitive to nutritional state than is that of the sarcoplasmic protein. It is suggested that such responses may help to maintain constant tissue composition during non-steady-state conditions of growth and atrophy.

3-Methylhistidine turnover in the whole body, and the contribution of skeletal muscle and intestine to urinary 3-methylhistidine excretion in the adult rat

The tissue origin of 3-methylhistidine (N tau-methylhistidine) was investigated in adult female rats. The decay of labelling of urinary 3-methylhistidine was compared with the labelling of protein-bound 3-methylhistidine in skeletal muscle and intestine after the injection of [methyl-14C]methionine. The decay curve for urinary 3-methylhistidine was much steeper than that in muscle or intestine, falling to values lower than those in either tissue after 30 days. The lack of decay of labelling in muscle during the first 30 days is shown to result from the persistence of label in the precursor S-adenosylmethionine. The relative labelling of urinary, skeletal-muscle and intestinal 3-methylhistidine cannot be explained in terms of skeletal muscle accounting for a major proportion of urinary 3-methylhistidine. Measurements were also made of the steady-state synthesis rate of protein-bound 3-methylhistidine in intestinal smooth muscle in vivo in adult female rats. This involved measurement of the overall rate of protein synthesis and measurement of the relative rates of synthesis of 3-methylhistidine and of mixed protein. The synthesis rate of 3-methylhistidine was 29.1%/day, compared with the overall rate of 77.1%/day for mixed, non-mucosal intestinal protein. Measurement of the amount of 3-methylhistidine in skeletal muscle (0.632 +/- 0.024 mumol/g) and in the whole body (0.332 +/- 0.013 mumol/g) indicate that, although the muscle pool is 86% of the total, because of its slow turnover rate of 1.1-1.6%/day, it only accounts for 38-52% of the observed excretion. Measurements of the mass of the intestine (9.95 g/250 g body wt.) and protein-bound 3-methylhistidine content (0.160 mumol/g of tissue) indicate a pool size of 1.59 mumol/250 micrograms rat. Thus 463 nmol of the urinary excretion/day would originate from the intestine, 22% of the total. The tissue source of the remaining urinary excretion is not identified, but other non-muscle sources constituting about 10% of the whole-body pool could account for this with turnover rates of only 6%/day, a much lower value than the turnover rate in the intestine.

Time course of the effect of catabolic doses of corticosterone on protein turnover in rat skeletal muscle and liver

The time course of the response of protein synthesis in muscle and liver to catabolic doses of corticosterone (10 mg/day per 100 g body wt.) was studied in vivo in growing rats over a 12-day period. The rate of protein synthesis in muscle and liver and the rate of actomyosin synthesis in muscle were measured by the phenylalanine-flooding technique, and 3-methylhistidine (N tau-methylhistidine) synthesis was measured by injection of labelled histidine. 3-Methylhistidine concentrations in tissue free pools and urinary excretion were also measured to compare directly with the rate of muscle protein degradation determined as the difference between synthesis and growth each day during the treatment. The overall rate of protein synthesis in muscle fell gradually over the first 4 days, reaching a rate after 5 days that was 36% of the initial rate, and this lower rate was then maintained for the following week. This decrease in the overall rate was accompanied with changes in the relative rate of synthesis in muscle proteins, since during the first 4 days there was a disproportionate decrease in the rate of actomyosin synthesis, and specifically 3-methylhistidine synthesis. In the latter case the synthesis rate was decreased to only 4% of its initial rate after 4 days. These changes in protein synthesis in muscle were accompanied by a transient increase in the rate of protein degradation, which was more than doubled on days 2 and 3 of treatment but which returned to the original rate on day 5, and a similar pattern of response was indicated by urinary 3-methylhistidine excretion, which also exhibited a transient increase. Thus in this case 3-methylhistidine excretion and measured rates of protein degradation in muscle do correlate. The transient effects of the glucocorticoids on degradation compared with the sustained effect on synthesis suggest that these two responses are achieved by different mechanisms. The hepatic size and protein mass were increased by the treatment, and protein synthesis was well maintained until after 12 days, when the rate was suppressed. Although the fractional synthesis rate was transiently increased for 24 h, it is argued that the enlarged liver most likely reflects a decrease in protein degradation resulting from the increased amino acid supply to the liver. This would result from the cessation of muscle growth while dietary supply was maintained.