an alle, die Interesse an Kraft und Muskelmasse haben

[kurt]

Regulation of Satellite Cell Proliferation and Differentiation

Muscle fibers are permanently differentiated; therefore, they are incapable of mitotic activity to produce additional myonuclei in times of increased protein synthesis and muscle growth (88). Yet, myonuclear number increases during skeletal muscle hypertrophy, thereby maintaining the myonuclear domain (the amount of sarcoplasm managed by a single myonucleus) (89). The predominant source of the additional myonuclei is satellite cells that are localized in indentations in the sarcolemma beneath the basal lamina (90). Less commonly, satellite cells may possibly fuse with each other to form new fibers (hyperplasia) (90).

The requirement of satellite cell activation for muscle hypertrophy was first demonstrated by a "nontransgenic knockout" approach in which mild -irradiation (which damages DNA while leaving other cellular machinery intact) was employed to block satellite cell proliferation. In response to functional overload, myonuclear number or muscle size was not increased in irradiated rat muscles (91). Adams and coworkers found that most of the hypertrophy potentially achievable during mechanical overload was prevented by similar treatment for four months (92). Thus it is likely that neither endogenous mesenchymal stem cells nor extramuscular (e.g., bone marrow) stem cells contribute much to the stem cell population of overloaded muscles, and the proliferation and fusion of existing satellite cells are responsible for the full load-induced increases observed in the muscle mass.

The limited proliferative capacity and the decrease in satellite cell number during normal aging may be implicated in atrophy and poor regeneration in elderly subjects (93). The number of satellite cells is thought not to be limiting to hypertrophy in normal human skeletal muscle, even in the elderly, although it may be in chronic users of anabolic steroids (94). Numerous growth factors have been shown to increase satellite cell proliferation. Here we focus on the key muscle growth regulators IGF-1 and myostatin. The effects of IGF-1 on muscle growth are pleiotrophic, activating satellite cell proliferation by spurring progression through G1 to S phase, increasing protein synthesis, decreasing protein degradation, and decreasing apoptosis. The mechanism by which IGF-I signals satellite cells to proliferate is by a decrease in p27Kip1 protein concentrations via activation of the phosphatidylinositol 3'-kinase (PI3K)/protein kinase B (PKB/Akt) signaling (95). As a result, increased p27Kip1 inhibits cyclin-dependent kinase 2 (cdk2), producing a late G1 arrest in the satellite cell cycle.

Myostatin inhibits both satellite cell proliferation and differentiation. Myostatin halts the satellite cell cycle by upregulating p21, which inactivates cyclin-dependent kinase activity so that retinoblastoma protein is particularly dephosphorylated (96). Myostatin also regulates satellite cell differentiation by inhibiting the expression of the myogenic growth factor, MyoD, via Smad 3 signaling (97). Reciprocally, MyoD upregulates myostatin to control myogenesis during the G1 phase of the cell cycle (at least in C2C12 myoblasts) (98).


SUMMARY OF REGULATORY MECHANISMS PRODUCING MUSCLE HYPERTROPHY Resistance exercise and most other muscle growth factors lead to the activation of a signal transduction network that will regulate the expression of the muscle growth factors IGF-1, MGF, and myostatin. The activated signal transduction pathways and the changed receptor binding of IGF-1, MGF, and myostatin lead to the activation of translation or protein synthesis and satellite cell proliferation and differentation, resulting in muscle growth. Blocking any one of these signal transduction pathways or factors may limit hypertrophy. However, it is incorrect to conclude that the blocking of hypertrophy by a single pathway or factor supports the conclusion that only one mechanism accounts for all of the resistance exercise or strain-induced hypertrophy of skeletal muscle. Likewise, enhancement of resistance exercise or strain-induced hypertrophy by a single factor should not be interpreted to mean that this factor alone is the means whereby increased mechanical load signal are transmitted to muscle growth physiologically. Indeed, the normal physiological response to produce the full potential hypertrophy during a load-induced growth of skeletal muscle involves the orchestration of multiple, simultaneous, and temporarily related sequential signals.


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[kurt]

ADAPTATION OF THE HUMAN MUSCLE MASS

The second part of this review is focused on observed changes in human MPS and breakdown and on translational and transcriptional control mechanisms, insofar as they act in adult human skeletal muscle, with particular reference to the effects of nutrition and physical activity.


EFFECTS OF NUTRITION ON SKELETAL MUSCLE PROTEIN MASS

The influences of food on protein metabolism are separable into two parts: those brought about through an increase in amino acid availability (e.g., the amino acidactivation of translational regulators via the Raptor-mTOR complex discussed above) and those resulting from increases in the concentration of hormones and growth factors (principally insulin and growth hormone/IGF-1) produced after stimulation by dietary secretogogues (e.g., glucose and amino acids).

Effects of Amino Acids

The discovery of the Raptor-mTOR complex and its likely function as an amino acid sensor (see above) has provided a likely explanation for the known stimulatory effect of amino acids on translation and protein synthesis. Here we review human studies in which the authors investigated this relationship in order to provide information that can be practically applied by those that wish to increase MPS in athletes, the elderly, or patients in whom muscle atrophy has occurred because of diminished protein synthesis. We aim to inform about effective amino acid concentrations, timings of ingestion, and the combination of amino acid feeding with resistance exercise.


MUSCLE PROTEIN SYNTHESIS An increase in the supply of amino acids to skeletal muscle (in many in vitro and in vivo models) stimulates the incorporation of tracer amino acids into protein (74). This effect can be observed independently of any hormones, although insulin may enhance it (see below). In human subjects, intravenous infusion of mixed amino acids doubles the incorporation of stable-labeled tracer amino acid into anterior tibialis muscle without any increase in the availability of insulin (99). Modulation of MPS via availability of amino acids appears to show a sigmoidal relationship; rises and falls in amino acid availability cause rapid changes in MPS in the basal to postprandial range, with a shallower slope at the upper and lower concentration limits (100, 101). In humans, the upper limit of amino acid concentrations at which MPS appears to be saturated is about 50% greater than the blood amino acid concentrations normally achieved after a meal (101). These and other recent data (109112) make a powerful point: The amount of amino acids necessary to stimulate MPS in the resting state and after exercise is in fact small (<10 g) compared with the accepted whole-body protein requirements (>70 g for most men).

In animal muscle, it is easily demonstrated that the branched chain amino acids (and leucine in particular) stimulate MPS in muscle cells in tissue culture, in perfused systems, and in intact mice and rats (102106). A similar effect has been observed in whole human beings; administration of boluses of single essential amino acids (including threonine, valine, phenylalanine, and leucine) but not non-essential amino acids (such as proline, glycine, serine, and alanine) markedly stimulated the incorporation of tracer-labeled amino acids into muscle protein (107). Could such a stimulation could be sustained in vivo? When large amounts of leucine were infused in human subjects, the intramuscular concentrations of other amino acids fell (108), presumably owing to stimulation of MPS (together with the possible inhibition by leucine of MPB). However, synthesis of protein requires all 20 physiological amino acids, and those that have the highest concentration ratio between muscle protein and the free pool will have the largest fall under situations of net anabolism unless transport from the blood occurs sufficiently quickly. Unfortunately, the capacity for muscles to continue to produce protein when the supply of all 20 amino acids is limited is not known.

Studies of the latency and duration of the effect of amino acids on MPS suggest that it takes 30 min for a stimulatory effect to be detected; thereafter the rate of increase is rapid, and peak rates are obtained within 60 to 90 min (101). Then MPS falls back to basal levels despite the continued abundant availability of amino acids, suggesting that the system is full of protein and is no longer responsive to nutritional stimulation. The extent of the refractory period before restimulation can occur and the identity of the mechanisms involved in the switch-off are unknown. When the limb arterio-venous exchange methods are applied, after increases in blood amino acid concentration, the apparent increase in net amino acid balance (and in model-derived values for MPS) is greater and occurs more quickly than the increase of the rate of tracer incorporation into muscle protein (13, 109, 110). There may be two reasons for this: First, amino acids may in fact be inhibiting MPB more rapidly than simulating MPS (although this seems unlikely given the results of previous studies) (111); or second, the apparent increase in the net protein balance may be because the muscle amino acid pool is overfilled. This latter possibility is acknowledged in a recent paper from the Galveston group (112). In any case, attributing arterio-venous concentration difference flow as signifying apparent increase in net protein balance should be regarded with caution under circumstances in which blood amino acid concentrations are changing; when arterial blood amino acid concentrations are rising (as after an oral dose of amino acids), there is a tendency to overestimate net muscle balance.

The cellular mechanisms involved are likely to be similar in human and animal muscle with stimulation of p70S6k and 4E-BP1 activation by amino acids (113).


MUSCLE PROTEIN BREAKDOWN Although there is no doubt that increasing amino acid concentrations by intravenous infusion, meal feeding, or ingestion of free amino acids increases MPS, inhibitory effects of amino acids on MPB, which are relatively easy to detect in animal muscles (70, 114), are, in human, beings absent or at least much less evident than those on MPS (115, 116). Some of the protein anabolic effects of a protein meal in vivo may possibly be modulated through the stimulation of insulin, with consequent inhibitory effects on muscle protein breakdown, but in our experience they are slight. Certainly, in the postexercise period increased availability of amino acids enhances MPS without having an effect on protein breakdown (111).

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[kurt]

Effects of Insulin

The effects of insulin on MPS in animals and humans may be different in terms of sensitivity and responsiveness. Much of the work demonstrating a marked stimulatory effect of insulin on MPS has been carried out in immature rodents or in only partially differentiated muscle cultured in vitro (117119), and it has been much harder to obtain consistent results demonstrating a coherent pattern of responsiveness in adult (especially human) MPS to insulin. Two main areas of contention concern the question of the extent of the human MPS response to insulin and the dose response characteristics of the system.


MUSCLE PROTEIN SYNTHESIS Barrett and coworkers (120, 121) first raised questions about the efficacy of insulin in human muscle when insulin was supplied to the forearm by close arterial infusion. No effects of insulin could be discerned on the disappearance of tracer into protein, i.e., protein synthesis, although there was a dose-dependent inhibition of protein breakdown. In this experimental model, protein metabolism in the forearm was assessed on the basis of arterio-venous balance of amino acids and the dilution across the arm of radio-labeled phenylalanine, an amino acid that is not subject to intermediary metabolism in muscle. However, the mathematical formula used by these workers produced results that may be underestimates of the rate of synthesis (see 122, 123 for discussion of this point). Also, Barrett and colleagues did not take muscle biopsies to check that the intramuscular concentration of amino acids was sufficient to sustain protein synthesis. Furthermore, as shown by Biolo and coworkers, when a different mathematical modeling approach was used for lysine and phenylalanine (a three- rather a than a two-pool model) (124), insulin could be shown to stimulate MPS. This conclusion was supported by independent data showing increased incorporation of stable tracer-labeled leucine into muscle protein sampled in the same period. Other workers have also demonstrated that insulin will stimulate MPS measured by incorporation or by limb arterio-venous difference exchangebut only when sufficient amounts of amino acids are present (116, 125127).

Nevertheless, there are still no data that adequately describe the dose-response relationship between MPS, measured unequivocally by means of tracer incorporation into protein, and the availability of insulin in blood. There is a pressing need for construction of a dose-response curve (carried out using somatostatin to inhibit basal insulin and with insulin added back systematically) that will simultaneously measure amino acid balance across the limb and tracer incorporation into muscle protein.


MUSCLE PROTEIN BREAKDOWN The effect of insulin on MPB has been well defined in terms of a decrease in the appearance of amino acids from preparations of muscle in tissue culture, in isolated whole muscles in perfused systems, and in measurement of arterio-venous tracer exchange in humans (116, 128). The major effect of insulin in inhibiting proteolysis appears to be modulated by effects on the proteasome, the ATP-ubiquitin-dependent proteolytic system that is responsible for myofibrillar protein breakdown in mammals (129). Despite a wealth of information describing alterations in mRNA and proteasome protein concentrations through manipulations of nutritional status, it has often been difficult to match up alterations in skeletal muscle balance or in measured protein breakdown with changes in the mRNA or proteasome content, e.g., in animals (72). In fact, no such parallel can be found from results of studies in human subjects as far as we are aware. In one well-designed study in which protein breakdown (measured as loss of amino acids from the limb) was elevated by three days of cortisol infusion, no changes occurred in the components of the proteasome pathway or their mRNA (130). A similar lack of correspondence between mRNA and protein for proteasome components and changes in net protein loss has been observed in muscle of lung cancer patients and in patients with acidosis due to renal disease (131, 132).

In short, after a meal and probably after exercise, the insulin-mediated decrease in MPB appears to be less important for the attainment of net anabolism than the stimulation of MPS.


Growth Hormone and Insulin-Like Growth Factor-1

Growth hormone has a number of metabolic actions on salt and water balance; fat metabolism; and, in growing animals and children, muscle and bone growth. Rennie recently reviewed the metabolic effects of growth hormone on human skeletal muscle and concluded that the balance of evidence suggests there are no major anabolic effects of exogenous rhGH in stimulating muscle protein accretion, muscle size, muscle strength, or muscle fiber characteristics in normal, healthy adult men or women, including the elderly (133). The published data, which have contributed to this conclusion, include information on incorporation of stable isotope-labeled amino acids into muscle and measurements of muscle mass and muscle fiber type using modern imaging and immunohistochemical methods.

There are, nevertheless, strong indications that IGF-1 involvement in metabolism may be locally important in skeletal muscle in humans and may modulate some of the effects of contractile activity in maintaining, or even increasing, muscle mass.

MGF is elevated in human muscle after exercise (134) but only in young (30-year-old) and not in old (75-year-old) subjects. One puzzling feature of this finding is that the MGF transcripts appear at concentrations that are very much lower than those of the IGF-1, so the effects of MGF must be because of different targeting or because the MGF is much more potent than IGF-1.

Although administration of IGF-1 seems to have acute stimulatory anabolic effects (135, 136) in muscle, long-term systemic administration of IGF-1 without its binding protein has no anabolic effect on lean body mass in elderly women (137), whereas a combination of IGF-1 with its binding protein 3 is markedly anabolic even in burn patients (138).


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[kurt]

Effects of Resistance Training on the Response of Muscle Protein Turnover

Many of the muscle metabolic systems show adaptations with habitual physical activity. Whether habitual physical activity results in a chronically altered rate of muscle protein turnover is currently the subject of some interest. In diabetic rats trained to perform resistance exercise, Farrell and coworkers demonstrated a reduced response of MPS to exercise after training (161). However, obtaining a clear answer to this question for human muscle is difficult. First, the residual effects of a previous bout of exercise, which may last up to 72 h, depend on intensity. Second, there is the problem of the habitual dietary intake of athletes who are subjected to much marketing and coaching information suggesting that they need to eat large amounts of protein in order to maintain or build muscle mass; this is a problem because habitually high rates of dietary protein intake lead to the induction of amino acid catabolic enzymes (particularly of the branched chain and aromatic amino acids) that decrease the deposition of dietary protein (162, 163). Until this effect abates (after reducing protein intake), there will be a tendency to exhibit negative nitrogen balance, so studies should not be conducted with rapid variation in dietary protein contents.

There is, in fact, little data on the subject in respect to MPS or even muscle mass. Studies of military recruits undergoing intense physical training suggest that there is a loss of body protein over the first few days of training but that adaptation rapidly occurs and nitrogen balance is restored, all at the same rate of dietary protein intake (164). Butterfield & Calloway found that in young men undergoing physical training, exercise increased the efficiency of protein utilization (165), i.e., trained subjects would require less protein. Partial validation of this position was provided by the first of two studies by Phillips and colleagues (166, 167). When two groups of subjects, one strength-trained and the other sedentary, were compared, there were no differences in resting post-absorptive MPS or MPB; also when the post-exercise responses to a single bout of pleiometric exercise at 120% of each subjects concentric 1 RM were compared, the rise in MPS in the trained subjects was 50% less than in the sedentary group, and there was no rise in MPB, which increased by about 40% in the untrained group. Thus net muscle balance (MPS minus MPB) was improved to the same extent in each group. However, a different result was obtained in a second longitudinal study of the effects of 8 weeks of resistance training in young previously untrained men, studied in the fed state at rest and also after a bout of exercise at 80% of their pretraining 1 RM (166). These results suggested that there was no difference in the response of the subjects in the trained and untrained state to acute exercise; also, rather oddly, the trained subjects did now show a marked increase in MPB as a result of exercise. In addition, basal rates of MPS and MPB were in fact now higher in the trained state; one consequence of this was that the effect of training seemed to decrease the relative response to exercise, a result that was consonant with the earlier findingsbut by a different mechanism! All in all, the data on net balance suggest that there was no effect of training tending to confirm the settled views of the present authors (143, 148) (although resisted by many athletes, their trainers, and, of course, sports nutrition companies) that habitual physical activity imposes no greater demands on protein requirements. As Phillips and coworkers (166) point out in their discussion, they did not test whether the same relative workload might affect protein turnover in trained and untrained subjects: It may be that if the above longitudinal studies had been conducted at the same relative intensity, a different result might have been obtained.

In the elderly, the rejuvenating effect of training may confound the issue. There is considerable controversy about whether aging is associated with a fall in muscle protein turnover [see (168) for review of this topic, which will not be dealt with further here]. However if it is true that the frail (as opposed to healthy) elderly show a fall in MPS, as seems likely, then exercise training may normalize it (169). The mechanism may be by decreasing the amount of TNF- in muscle (170).


Effects of Creatine on Human Muscle Protein Turnover

Dietary supplements containing creatine have become popular with athletes and trainers hoping to promote greater increase in muscle mass and strength in resistance training programs (171173). Measurements of myofibrillar protein synthesis (as incorporation of 13C leucine) and forearm protein breakdown (as dilution of deuterated phenylalanine) were unable to discern any differences in subjects studied before and after creatine supplementation, either in the post-absorptive or the fed state, at rest, or immediately after acute exercise (174, 174a). These results appear to rule out any acute effect of creatine alone on translation of pre-existing mRNA or on MPB but do not invalidate the possibility of transcriptional changes or satellite cell activation stimulated by creatine and physical activity.


Effects of Intensity of Contraction and Metabolic Power Output on Muscle Protein Turnover

It seems clear that maneuvers resulting in a relatively rapid rise in muscle mass are all associated with substantial increases, albeit after a short latency, possibly of about one hour, in MPS as a result of translational stimulation produced by changes in 4E-BP1 and p70S6k phosphorylation (176). These changes are followed, probably shortly thereafter, by transcriptional changes associated with intense exercise. Thus questions of the extent and temporal pattern of disturbance need to be addressed.

In human muscle, our group (M.J. Rennie, D.J.R Cuthbertson, K. Esser & M. Fedele, unpublished work) consistently observe a long-lasting rise in p70S6k phosphorylation after acute, high-intensity exercise, with smaller transient rises in PKB (Akt) phosphorylation, which are associated with a consistent rise in incorporation of tracer-labeled amino acid into muscle protein, whether myofibrillar or sarcoplasmic. We find no difference in the extent of stimulation of p70S6k or MPS in different quadriceps in which the same amount of force is applied during stepping exercise (one leg up, one leg down, while carrying 20% of body weight) to exhaustion (81). Because concentric exercise is energetically much less efficient than eccentric exercise and normally requires a higher rate of ATP turnover, this suggests that the crucial factor in determining the extent of the rise of MPS is force or intensity rather than ATP turnover, unless there is some threshold effect beyond which the rise in MPS remains constant.

However, paradoxically, when ATP turnover and the extent of quadriceps motor unit recruitment is kept constant during exercise at 60, 75, and 90% of 1 RM for different numbers of repetitions, the stimulation of MPS is constant (175).



CONCLUSION

As we have seen, our current ability to describe the adaptive responses of skeletal muscle to a wide variety of circumstances with changes in mass, composition, and function is impressive. The time resolution of techniques for measuring changes in muscle mass and composition and rates of protein turnover have increased such that we can now make robust measurements of the time courses of, for example, the rate of myofibrillar protein synthesis, which was impossible 10 years ago. Much information about the interrelationships between signaling pathways, which are important for transcriptional and translational regulation, has been accrued, and we have a much better understanding of the importance of satellite cells for growth and regeneration of muscle. There are, however, a substantial number of gaps that need to be filled. We still have no clear idea of the temporal relationship between the components of amino acid sensing and signaling to the processes of protein synthesis and breakdown and how these are affected by individual amino acids, insulin, and IGF-1. The exact pathways by which anabolic and catabolic steroids affect gene transcription and translation of mRNA remain obscure in human muscle despite the existence of response elements predicted for the muscle genes; the commonality (if any) of the pathways between myofibers and satellite cells is not at all well understood. The nature of the dichotomy of the responses to short-term, high-intensity exercise leading to hypertrophy and long-term low-intensity exercise leading to mitochondriogenesis and fast-to-slow fiber type transition remains a mystery. We still require a good description of the dose-response relationship between exercise intensities and the observed changes in mass and protein composition, and until we have these, it will be difficult to sort out the relative contributions of signaling pathways, their commonality, additivity, or independence from each other in controlling the adaptive responses of muscle.

Nevertheless, the increasing power of post-genomic techniques, particularly the use of transcriptional profiling and subsequent bioinformatics, should enable us to identify previously unknown means of controlling transcriptional and translational events. Perhaps some time in the next 10 years, our view will suddenly snap into focus, and it will become obvious how, for example, changes in the concentrations of Ca2+ or AMP can modulate the size and shape of muscle.


(es folgen noch die ACKNOWLEDGMENTS und 176 literaturangaben. ich bitte um verständnis, wenn ich sie nicht mehr hieherkopiere. wenn jemand ein literaturzitat wissen will, kann er es mir ja mitteilen)

gruß, kurt

[moul]

Werd mich durchackern.

[Widar]

Ebenfalls: Danke!

Gruß
Sascha

[Knackar]

And I need some practice anyway for my graduation, so ...

Thanks

Best regards,

markus

[Knackar]

For prints, here´s it as word document!

best regards,

markus

[ThomasB]

Ich beneide alle, die so einen Artikel in englischer Sprache einfach so locker lesen können.

Gruß
Thomas

[hps]

Ich beneide alle, die so einen Artikel in englischer Sprache einfach so locker lesen können.

Gruß
Thomas

Hallo Thomas,

mit dem Englisch habe ich keine Probleme (naja, ich lebe und arbeite schliesslich in den USA) aber mit der Fachsprache kann so eine Lektuere dann doch ein wenig "aufwendiger" werden. Also ich kann sowas nicht gut vor dem Einschlafen lesen... :]

Gruss,
Hans-Peter

[Irongemse]

Dito ! Mußte mich schon genug herumprügeln mit dem technischen Part der Autoindustrie, ich hab ehrlich gesagt keine Lust mehr, jetzt auch noch "Medical-English for runnaways" zu erlernen :-)

[Knackar]

Dito ! Mußte mich schon genug herumprügeln mit dem technischen Part der Autoindustrie, ich hab ehrlich gesagt keine Lust mehr, jetzt auch noch "Medical-English for runnaways" zu erlernen :-)

Bin auch in der Industrie beschäftigt.

Mfg Markus