Effect of Exercise on Skeletal Muscle - Coursework Example

and sur Due I Effect of Exercise on Skeletal Muscle In this study a novel approach has been tried to determine whether there are any alterations in the metabolic and mitogenic pathways in skeletal muscle physiology of normal persons and highly trained athletes. Both groups have been subjected to a well measured and equivalent degree of exercise intensity and appropriate tests conducted after obtaining biopsies from a target skeletal muscle, Vastus lateralis. The stimulus for this study was provided by past research which indicated that exercise had numerous growth as well as metabolic effects on the skeletal muscle (Widegren et al, 1998). Alterations in glycogen metabolism, glucose and amino acid uptake, protein synthesis and gene transcription have been well documented (Widegren et al, 1998), but the mechanisms involved are still unclear. Intracellular signalling mechanisms after a skeletal muscle is subjected to intense exercise result in changes to gene expression which has resulted in many putative elements to be studied, prominent amongst them being the Mitogen activated protein kinase (MAPK) which has two parallel pathways including the extracellular signal regulated kinase (ERK), stress activated protein kinase cascades (SAPK1/JNK) and the extracellular signal regulated kinase 5 (ERK 5) and 5`-AMP- activated protein kinase (AMPK) cascades (Mei Yu et al, 2002). The physiological activation of enzymes downstream of the MAPK cascade has been under intense study (Krook Anna et al). It is a major signalling system by which cells transducer extracellular signals into intracellular responses (Aronson Doron et al, 1997). The overall impact is a marked alteration in the structure and functioning of the human skeletal muscle. Exercise not only affects the upstream cascade of the MAPK cascade but also increases the activity of upstream regulators of MAPK and other associated molecules (Aronson Doron et al, 1997). Numerous studies have been conducted on this aspect and this study (Mei Yu, et al, 2002) in particular focuses on MAPK and AMPK cascades by comparing them in volunteers chosen from two groups, one group involving highly trained cycling athletes and the other one of normal/control persons, by subjecting them to a pre calculated equivalent exercise stress according to their capacities, to further differentiate if there are any changes in the cascades between the two groups. The oxygen uptake of the two groups has also been strategically pre defined in order to provide similar inputs to both groups in order to avoid abnormality in result due to this factor. Although the trained subjects completed eight five minute bouts of exercise with adequate rest (60 seconds) in between and the control group completed just four of them, the relative intensity of exercise was well balanced and matched to be identical in both groups. Furthermore the selection of the volunteers was made by carefully considering and matching their ages and body mass index. The test protocol was rigorously set up according to previously determined standards and the cycle ergometer and the bag gas analysis system used to measure the oxygen consumption, Carbon dioxide production and minute ventilation were standardised and appropriately calibrated prior to use. Muscle and liver glycogen stores were standardised 24 hours prior to the experiment and then the subjects fed a standardised diet containing 63% carbohydrates, 20% fat and 17% protein calculated according to body mass with instructions to avoid any strenuous activity prior to the day of the experiment. On the day of the experiment muscle biopsies for analysis were collected prior to and after the completion of the total exercise protocols assigned to the two groups with similar levels of peak Oxygen consumption (~85% VO2 peak) with a work: rest ratio of 5:1 (5 minutes of intense cycling followed by a 1 minute rest). Drinking water was provided ad libitum (at liberty) to both groups. Trained subjects cycled at a workload of ~100 Watts while the untrained subjects did it at ~50 Watts, with similar intensity expected due to difference of exercise tolerance levels between the two groups. Blood samples were collected after the warm up and at the end of the final bout of exercise to measure the blood lactate concentration. Muscle biopsy was collected from the Vastus lateralis muscle from two different sites prior to and after the exercise bouts. Approximately 125 mg of muscle tissue was collected and immediately stored in liquid nitrogen for analysis later. Immunological tests were then employed to analyze and quantify the mediators of the specific cascades under study. The cascade elements measured in this particular study are ERK1/2 MAPK phosphorylation, p38 MAPK phosphorylation, MSK1 activity, nuclear protein histone H3 phosphorylation, AMPKα1 activity and AMPKα2. The authors have found significant differences in the levels in control and trained subjects and arrived at some significant conclusions. Results and Discussion This particular study has highlighted the marked difference in physiological changes in the skeletal muscle between the two groups under investigation in eliciting responses which were variable between the two. Exercise itself has an effect on the mitogen and stress activated signal transduction in human skeletal muscle as it is a powerful stimulator of mitogen activated protein kinase cascades (Krook Anna, et al, 2000). As expected although the relative exercise intensity was similar in both groups, the blood lactate concentration, which corresponds to the muscle lactate concentration was less (4.7± 1.7 mmol l-1) in the trained subjects as compared to the untrained ones (8.0± 0.4 mmol l-1 ), although the respiratory exchange rates did not display much difference. This can be explained on the basis of better exercise tolerance of the trained individuals. ERK1/2 MAPK phosphorylation was significantly greater (2.6 fold) in the control subjects as compared to the trained ones (1.5 fold) from resting stage to the post exercise stage. Similarly there was a 2.1 fold increase in p38 MAPK phosphorylation in the control subjects as compared to only 1.6 fold increase. MSK1 activity was increased by 2-fold in the control subjects and 1.4-fold in the trained subjects. Nuclear protein histone H3 phosphorylation was increased by almost equivalent levels in both the control and trained subjects although it was originally higher at the resting level stage in the trained subjects. As far as the AMPK activity is concerned the results indicated no change in the AMPKα1 activity in both control and the trained subjects, but there was a remarkable change of a 4.2-fold increase in AMPKα2 levels in the control subjects as against the 2.3 fold in trained subjects after the exercise. As AMPK phosphorylation and ACC phosphorylation had shown to be correlated, it was observed that there was a 1.9-fold increase in the control subjects which was lesser than that of trained subjects which was 2.8-fold. The authors of this study expected to find parallel activation of MAPK kinase cascades and AMPK transduction as a response to intense muscular activity as both MAPK and AMPK cascades ‘constitute molecular targets involved in the regulation of exercise induced responses on gene expression.’(Mei Yu et al, 2002) Therefore they measured and compared the results with control subjects in order to find whether there were any significant differences within as well as between the two groups. In the trained subjects the authors have proven with statistically significant evidence that MAPK signaling in response to intense exercise is increased via ERK1/2, p38and MSK1 pathways which were interestingly proven to be coupled with the phosphorylation of the nuclear protein histone H3. There was a significant increase in AMPKα2 level in the trained subjects which correlated with the increase in the phosphorylation of the downstream substrate ACC. The increase in AMPKα2 was much more in the control subjects. The level of activation of these signaling intermediates was found to be much more in the skeletal muscle obtained from the control subjects who were subjected to relatively the same level of exercise intensity, which prompted the authors to conclude that activation of both the MAPK and AMPK pathways requires a greater stimulus in trained individuals to activate signal transduction. Exercise induced MAPK signaling has already been proven to depend on the level of exercise intensity (Krook Anna, et al, 2000) as mitogen and stress activated kinases (MSKs I & II) were found to increase by a level of 400-500% and 200-300% respectively (Krook Anna, et al). Krook et al (2000) have also reported a dramatic increase in MAPK levels as a response to exercise. Their experiments suggested that these actions took place at a local level inside the skeletal muscle independent of the systemic and hormonal activities of the individual. As a result the exercise induced changes are associated with the changes in the expression of key proteins involved in metabolism. Exercise induced increase in muscle mass and an increased uptake of glucose is well documented (Krook Anna, et al, 2000). These specific signaling molecules and pathways which enable exercise to modulate cellular processes in the skeletal muscle cause marked alterations in the structure and functioning of the human skeletal muscle (Aronson Doron et al, 1997). In the study under review the authors claim that the p38 response of the MAPK signaling in untrained subjects was significantly higher than the trained ones indicating that high intensity exercise leads to partial adaptation of p38 MAPK signaling in trained individuals. The authors have substantiated this fact by a similar study in rats. The authors have suggested further research in this matter by evaluating the p38 levels in untrained individuals before and after intensive muscle training exercises. Similar results were obtained in the MSK1 activity which was much less in the highly trained subjects. MAPK substrates also mediate alterations in the chromatin environment of specific genes. The nuclear protein histone H3 is a potential MSK1 substrate and its phosphorylation is related to the expression of immediate early (IE) response genes. Physiological induction of histone H3 in mammalian skeletal muscle has been demonstrated for the first time in this study and the authors believe that it has direct correlation to MSK1 activity which however they admit cannot be correlated in this study. Further the increase in AMPKα2 levels in both the untrained and the trained subjects suggests that it ‘may be an important molecular target mediating metabolic and mitogenic events in skeletal muscle in response to exercise’. The study concludes that both the MAPK and AMPK pathways occur concurrently and in a parallel manner when the skeletal muscle is subjected to intense exercise, and even if the responses are low in trained subjects they do occur in the same manner as in untrained subjects. Works Cited Aronson Doron , Violan Mariona A., Dufresne Scott D. , Zangen David, Roger Fielding A.and Goodyear Laurie J., Exercise Stimulates the Mitogen-activated Protein Kinase Pathway in Human Skeletal Muscle, J. Clin. Invest., Volume 99, Number 6, March 1997, 1251–1257 Krook Anna , Widegren Ulrika , Jiang Xin Jian , Henriksson Jan , Wallberg-Henriksson Harriet , Alessi Dario and Zierath Juleen R., Effects of exercise on mitogen- and stress-activated kinase signal transduction in human skeletal muscle, Am J Physiol Regulatory Integrative Comp Physiol 279:1716-1721, 2000 Widegren Ulrika ,Jiang Xin Jian , Krook Anna , Chibalin Alexander V., Marie Rnholm Bjo Rnholm, Tally Michael, Roth Richard A. Henriksson H Jan, Wallberg-Henriksson Harriet and Zierath Juleen R., Divergent effects of exercise on metabolic and mitogenic signaling pathways in human skeletal muscle, The FASEB Journal, Vol. 12 October 1998 Widegren U., J. Ryder W. & Zierath J. R., Mitogen-activated protein kinase signal transduction in skeletal muscle: effects of exercise and muscle contraction, Acta Physiologica Scandinavica Volume 172 Issue 3, Pages 227 - 238 Yu Mei , Stepto Nigel K. , Chibalin Alexander V. , Fryer Lee G. D. , Carling Dave , Krook Anna , Hawley John A. and Zierath Juleen R. , Metabolic and mitogenic signal transduction in human skeletal muscle after intense cycling exercise, J Physiol (2003), 546.2, pp. 327–335 Your first name and surname Instructor’s name Course title Due date Skeletal Muscle Plasticity Harridge Stephen (2007) in his review article entitled ‘Plasticity of human skeletal muscle: gene expression to in vivo function’ has elaborated on the role of ‘mechanical, hormonal and nutritional’ signals influencing the regulation of skeletal muscle and sought to clarify the mechanisms which have an overlapping and rather complex mode of regulation and action. The skeletal muscle as such is a highly adaptable specialized tissue which has to play the vital role of supporting the skeleton and providing mobility to an individual and is subject to great variations in size and composition based upon the physical stress, hormonal influences and genetic triggers regulating it. Most of these aspects have been covered in this review on skeletal muscle physiology. The focus of the review has successfully dealt with the following factors: 1. Satellite cell mediated mechanisms. 2. Influence of growth factors like insulin-like growth factor I (IGF-I) & Testosterone. 3. Negative Regulators like Myostatin 4. Role played by myosin heavy chains – MHC-I, MHC-IIa & MHC-IIx 5. Role of fast and slow gene programs The complex mechanisms of action of the above factors on the composition and functioning of the skeletal muscle have been discussed with citations from pertinent in-vitro as well as in-vivo studies. At the outset the author has stressed on the basic functional unit of the skeletal muscle, the sarcomere which under the influence of various regulatory mechanisms attains ‘multiplicity of isoform expression’. The skeletal muscle responds to the functional tasks and challenges placed before it by either increasing the number of sarcomeres thereby undergoing hypertrophy or by altering its protein isoform expression. These two capabilities of the skeletal muscle help it to increase the force of action and the speed of movement respectively. The author has elaborated more on the mechanisms involved in muscle contractility rather than going into the details of the metabolic adaptations which are currently under intensive investigation due to the availability of modern tools in cell and molecular biology. According to Flück Martin (2006), the skeletal muscle shows remarkable malleability in adjusting to altered functional demands by adjusting its metabolic pathways and contractility and is therefore a prime model for studying the regulatory mechanisms which regulate its phenotypic adaptations. Physical stress of a striated muscle initiates a complex physiological response which occurs up to transcriptional level of protein synthesis. The ability of skeletal muscle adaptation in response to altered load states by the interplay between specific genes and their protein products, and the alterations they undergo has been studied extensively in relation to the quantities and types of myosin isoforms(Baldwin & Haddad, 2002). Harridge (2007) has initially focused on the classical studies on the force-velocity and length-tension relationship in the striated muscle. The strength and the ability to contract are dependent upon the number of sarcomeres working together in a parallel manner and the force is measured by the physiological cross sectional area of the muscle. The speed or velocity of action is determined by isoform of the molecular structure expressed in humans primarily by three myosin heavy chains, MHC-I (slow), MHC II-a (fast) and MHC II-x (fastest). The human skeletal muscle besides its mechanical role of movement is also in a dynamic state of metabolic activity. It has the largest store of protein in the body which can be easily utilized in conditions of starvation and critical illness. Muscle protein synthesis and degradation are in a physiological equilibrium state during normal health. Muscle hypertrophy can occur only when either there is an increased level of protein synthesis or a decrease in the rate of degradation. When excess demand or stress is put on the skeletal muscle there is always an initial increase in protein breakdown which is followed by increased protein synthesis if adequate protein diet is ingested by initially recovering the initial protein loss and subsequent increase and net gain in protein mass. Synthesis of new protein is ‘dependent upon transcription of DNA into mRNA, followed by translation of mRNA into protein’. This activity is controlled by interplay between nutrition, exercise and hormones. This activity is regulated by phosphorylation and dephosphorylation by the signal transduction proteins. Myogenic precursor cells called satellite cells and are basically stem cells which can differentiate into myoblasts which ultimately fuse with the existing myofibres in order to repair the damage done by excessive stress to the muscle. Some of these differentiated cells do not differentiate and revert back to replenish the activated satellite cells. Strength training for years in some individuals creates a higher pool of satellite cells as compared to the muscles of persons with normal activity. Sex and age also influence the number of satellite cells in response to exercise as they increase to a greater extent in young men (42%), as compared to females (29%) and older males (16%). This is due to the greater capacity of muscles in young males to incorporate new nuclei. Insulin-like growth factor (IGF-I) induces both proliferation and differentiation of satellite cells. It is the mediator through which the growth hormone (GH) exerts its anabolic effect on muscle mass. Local IGF-I rather than the circulating IGF-I plays an important role in increasing the muscle mass. IGF-I exerts its action by increasing the diameter of myotubules, suppression of protein degradation, increasing amino acid uptake and stimulating protein synthesis. The IGF-IEc variant of the spliced variant of the spliced mature mRNA transcripts has been termed as ‘mechano growth factor’ MGF and is responsible along with IGFI-Ea to increase muscle IGF-I expression without increasing circulating IGF-I. This may kick start the repair of muscle cells by activating satellite cells. The role of hormones especially Testosterone which is the predominant hormone in males has also been studied for its positive effect on myogenesis and is responsible for the increased muscle mass in adolescent males as compared to females. The circulating level of Testosterone is 10 fold higher in males than in females. Endogenous testosterone is intensified as a result of increased exercise and muscular activity. Testosterone combined with exercise results in greater muscle mass as compared to exercise alone. Exercise training promotes the action of testosterone by increasing the number of the androgen receptors. The testosterone induced increase in protein synthesis has also been hypothesized to be mediated through the stimulation of IGF-I system. Suppression of Testosterone production and the artificial blockade of testosterone synthesis result in reduction of the hypertrophic effect of exercise on the skeletal muscle indicating the presence of androgen receptors on the satellite cells. Myostatin is a negative regulator of muscle mass and acts by inhibiting satellite cell proliferation. It is down regulated during strength training. This has opened up a potential approach to promote muscle mass by inhibiting Myostatin. However experiments have suggested that this approach may not be useful as it results in hypertrophied muscles in experimental animals which are intrinsically weaker. Hormones which have a negative effect on the skeletal muscle by causing atrophy include Cortisol and atrogin. Glucocorticoids like dexamethasone induce atrophy of muscle. Atrogin I is a gene which is induced during fasting, diabetes and cancer. Such genes are up regulated during muscle atrophy induced by lower limb immobilization. Such catabolic effects are however neutralized in the presence of IGF-I and Insulin. Modern techniques including ultrasound and Magnetic Resonance Imaging (MRI) have been used to study the effects of exercise on skeletal muscle where increase in the cross sectional area and the muscular tissue differentiation can be precisely visualized. According to the study under review in vitro techniques have been used during the last fifteen years to visualize the effect of various mediators of muscle contraction and their cascades more precisely. The role played by the skeletal muscle myosin proteins has been elaborated in these studies. Such studies using single fibre segments whose sarcolemma has been removed have shown higher per unit area force exerted by the MHC-IIa and MHC-IIx fibres as compared to the MHC-I fibres. These fibres have the capability of transformation from one type to the other especially in aged people. Slow to fast and fast to slow transformations do occur in the human skeletal muscle and this is brought about by the regulation of the MHC isoform transformations. Fast to slow transformations can be artificially induced in an isolated muscle by providing electrical stimulation mimicking a frequency pattern which is more suitable for slow muscle. The nechanisms involved in fibre switching involve suppression or up regulation of one set of genes or the other. A better understanding of the intricate physiological events in skeletal muscle physiology and its analysis using modern techniques of cellular and molecular biology is throwing up new insights which are under intensive study due to the development of modern in vitro techniques and precise immunologic detection approaches of the various mediators involved. Works Cited Baldwin KM,2002, Haddad F: Skeletal muscle plasticity: Cellular and molecular responses to altered physical activity paradigms. Am J Phys Med Rehabil;81(Suppl):S40–S51. Flück Martin, 2006, Functional, structural and molecular plasticity of mammalian skeletal muscle in response to exercise stimuli, The Journal of Experimental Biology 209, 2239-2248 Harridge Stephen D. R.,2007, Plasticity of human skeletal muscle: gene expression to in vivo function, Exp Physiol 92.5 pp 783–797 Hood David A., 2001, Plasticity in Skeletal, Cardiac, and Smooth Muscle Invited Review: Contractile activity-induced mitochondrial biogenesis in skeletal muscle, J Appl Physiol 90: 1137–1157 Read More