Muscle Adaptations to Exercise
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Muscle Adaptations to Exercise
Learn about what changes take place in the muscle, when activated during exercise, and ultimately changes in the expression and, or, activity of key proteins that affect the size and functional properties of skeletal muscle. This includes adaptations as a result of exercise intensity and stimuli, signaling, gene, mRNA, protein regulation, muscle temperature, muscle tension, changes in metabolites, and changes in circulating hormones.
In our final lecture on muscle, we’re going to look at what changes take place in muscle in response to various types of exercise. When muscles are activated during exercise, we need to think of some of the changes that are taking place, how they’re detected, and what might be some of the consequences that mediate these adaptations to exercise. So, if we think about the various stimuli, that might activate pathways that lead to muscle adaptation there are a number of obvious candidates. A number of times, we’ve spoken about calcium. Calcium is fundamental to muscle contraction and it activates a number of processes within the muscle cell. Changes in energy status as the muscle break down ATP and other important fuels. That is, they are potential signals that could lead to adaptation. Changes in the redox state, the oxidation and the reduction reactions that occur within the muscles. Changes in muscle temperature, muscle tension, changes in metabolites, changes in circulating hormones. All of these can change during exercise and impact on the muscle.
In recent years, there’s been a lot of work on the various key enzymes and kinases phosphatase and various proteins that detect these signals. The calcium-sensitive enzymes include calcium calmodulin-dependent kinase, Calcineurin (CN). Energy status is monitored by the AMP-activated protein kinase. There are mitogen-activated protein kinases that seem to play a role in growth adaptations. An important enzyme involved in protein synthesis known as mTOR. Certainly with the advances in molecular biology and cell biology we understand a lot more about these molecular responses. So how do these signaling pathways impact ultimately on gene expression, messenger RNA expression, protein expression? Various transcription factors MEF2, GLUT-4 enhancement factor, PGC-1 alpha, which is thought to play an important role in mitochondrial biogenesis, are all transcription factors that are been in implicated in the adaptive response of muscles to exercise. More recently interest in small non-coding pieces of RNA called micro RNAs, which have been shown to influence the expression and transcription of genes and the expression of proteins. Ultimately, all of these changes are coordinated so that there is either or there’s a change in either the expression and or the activity of various proteins, which are fundamental to muscle performance, and effect, ultimately, the functional properties of that muscle.
Signals Mediating Muscle Adaptation to Exercise
So, here in graphical form and nicely summarized we can see various signals that are acting on a contracting muscle. Either in an endurance type prolonged exercise, high-intensity exercise but with dynamic exercise. Through two more resistance type exercises where the adaptations perhaps are more related to muscle mass or what we can refer to hypertrophy. You can see all of the signals here, the circulating factors in the bloodstream, the metabolites that we’re going to talk about in the fuel lectures and how these can impact various signaling kinases to either increase mitochondria or to increase muscle mass.
Muscle Adaptation to Endurance Exercise
If we focus on some of the adaptations to endurance-type exercise, these are largely focused on increasing the oxidative capacity of the muscle. The mitochondria who are the oxidative powerhouse of the muscle cell. They increase their total volume in response to exercise and that has important metabolic consequences. We see large increases in mitochondrial density and oxidative enzymes to response to endurance-type exercise. We also see an increase in capillary density and you can see in this slide here, the capillaries that circle muscle fibers and there are more of them after training and that facilitates the delivery of oxygen to the muscle and needs to occur in parallel with the increases in oxidative capacity within the muscle. There’s an increase in the glucose transport protein GLUT 4, and that helps facilitate the storage of glycogen. There’s an increase in the sodium-potassium ATPase activity. This is an enzyme that’s critically involved in the activation of the muscle in limiting the loss of potassium from contracting muscle. Question mark on fiber type changes? I mentioned in a previous lecture, it’s been ongoing discussion whether prolonged endurance-type exercise can induce fiber type changes, and there are some studies suggesting that that can occur, and certainly in animal studies with chronic long term electrical stimulation. You see quite a profound transformation of the characteristics of skeletal muscles. In terms of the functional consequences, we see reduced reliance on carbohydrates, reduced lactate production and increased fat oxidation during exercise after endurance type training. And we’ll talk more about this in the next module on fuels for exercise. From a health perspective, endurance training also improves the action of insulin. And that’s thought to be very important in minimizing the risk and indeed managing metabolic diseases such as type II diabetes.
Training Status, Muscle Oxidative Capacity & GLUT4
In terms of specific adaptations, let me give you just one example. This is one of our research interests here at the University of Melbourne. How does endurance training impact on the expression of the GLUT4 transport protein? Here’s one of our early studies where we examined cross-sectionally the differences between untrained and trained subjects in terms of their oxidative capacity as measured by the maximal activity of an enzyme in the mitochondria called citrate synthase and the glucose transport protein GLUT4. You can see that the trained subjects had higher muscle oxidative capacity and higher levels of GLUT4 compared with the untrained subjects. When we asked these trained subjects to stop their regular training for 10 days and report back to the lab. You can see that there was a reduction in their oxidative capacity and a reduction in their GLUT4 expression. So, cessation of training reduced or diminished some of these adaptive responses. You’ll note that they didn’t fall all the way down to untrained levels. Whether that would take a longer period of detraining, or whether these athletes have some genetic characteristics, which endow them with slightly higher muscle oxidative capacity or GLUT4 again, is one of those questions we may ask later in the course.
- Regulation of Muscle GLUT4 Expression by Exercise
- High Intensity Training is Effective
- Increased Muscle Mass with Resistance Exercise
- Muscle Mass and Disuse
- mTOR Signaling & Protein Synthesis
Regulation of Muscle GLUT4 Expression by Exercise
If we look at the factors that regulate GLUT4 expression, and this seems like a complex slide. But it really reprises the thing in the very first slide about signals, enzymes that detect those signals and functional consequences. So, in the case of GLUT4 expression, there are two main signals that are thought to be important. One is the increase in calcium, which activates the calcium calmodulin independent protein kinase and the other changes in the high energy metabolites, the change in the energy status of the muscle cell, activates the AMP-activated protein kinase. Both of these enzymes can target various transcription factors, which bind to the GLUT4 gene. Ultimately changes in their localization and their activity increase GLUT4 transcription so that you see increase GLUT4 mRNA. After an exercise in skeletal muscle and ultimately this increased in mRNA is translated into an increased GLUT4 protein. This increased GLUT4 protein means that you’re better able to take up glucose in response to insulin and in response to contractions.
High Intensity Training is Effective
In our next module, we’ll talk about glucose uptake during exercise and the importance of GLUT4 for that process. An interesting observation that’s been made for a number of years is that intensity can often be a very powerful stimulus. Here’s a recent study looking, comparing two types of exercise training, designed to increase the functional capacity of muscle. They measure the oxidation capacity again using the enzyme the activity of an enzyme, the activity of the enzyme citrus synthesize. There are two types of exercise training implied one was a very high-intensity exercise that you can see here, involved repeated 30-sec sprints, 3 per week over about a 6 week period. The other was a more traditional endurance type training program, 40 to 60 minutes of cycling, 5 times per week. You can see here, over the entire training period, the high-intensity exercise for a total of about 10 minutes at a very high intensity and the endurance training group for a total of 4 1/2 hours, yet when you looked at the increases, they were somewhat similar. There was no statistically significant difference between the increase. Whether you would see changes in the long term remains to be seen, but it makes the point that intensity can be a powerful stimulus. And in time, when people are increasingly time-poor, there has been some interest in using these high-intensity training programs in health and disease to try and promote these muscle adaptations.
Increased Muscle Mass with Resistance Exercise
The other important adaptation that it can occur in muscle is an increase in its size and one of the hallmark adaptations to prolonged resistance-type exercise, weight lifting type exercise, is an increase in muscle mass. You can see in this slide a relatively short exercise period quite a large increase in maximal voluntary contraction, the force-generating capacity of the muscle with relatively modest changes in the muscle cross-sectional area of the mark of the size of the muscle. So, if you measure the EMG or the electrical activity, you can see that much of the early increase in strength, functional strength, following resistance training is due to neurological adaptation. But over time you do see increases in muscle mass.
Muscle Mass and Disuse
Conversely, if a muscle is subject to disuse because of injury, sickness, subjects that confined to bed, like have a cast, a range of reasons that might mean they’re unable to fully activate the muscles fully on a regular basis. You can see that the myofibrillar protein synthesis decreases quite quickly and in times this results in a cross-section of muscle cross-sectional surface area.
Again, it has been a lot of interest in the regulation of muscle mass because of the importance of muscle mass in health and disease. For athletes, it is important because it’s the force-generating tissue. In normal, healthy individuals an adequate muscle mass is important because of the role that skeletal muscle has in supporting the movement, supporting body functions and supporting metabolism. And one of the consequences of aging and a number of disease states is a loss of muscle mass, and that can impact on functional capacity and the quality of life. So, there’s been a lot of interest in the regulation of muscle mass. In terms of building muscle, an important enzyme appears to be mTOR, and this has an important role in protein synthesis. It’s influenced by a number of growth factors acting through insulin-like pathways.
On the other side, factors that influence the degradation of muscle or muscle atrophy. There’s been a lot of interest in the so-called ubiquitin ligases, which are involved in breaking down muscle and how that might be regulated.
mTOR Signaling & Protein Synthesis
In relation to exercise effects on one of those signaling proteins, mTOR what we see is that exercise activates that enzyme. At least as measured by its phosphorylation status, which is the usual way that, we measure the activation of these proteins. If you look at protein synthesis, and here is the measure, the fractional synthetic rate for protein, during exercise the protein synthesis rate actually goes down. Given that exercise is an energy-demanding process and protein synthesis requires energy, it makes sense to perhaps turn that off while you are exercising. But you can see after exercising, an increase in protein synthesis. And so, with regular bouts of exercise. Regular activation of mTOR and various other proteins synthetic pathways. You get an increase, a cumulative increase of protein, myofibrillar protein. Muscle gets bigger and the muscle gets stronger.
Finally on an interesting aspect of muscle adaptation exercise that’s really been studied in great detail in the last decade or so. That’s the observation that contracting muscles, in fact, release biologically active substances, termed myokines, and don’t worry about the detail in this slide in terms of specific myokines, but you can see a number of them are released and they have a fix on adipose tissue, on the liver, perhaps on the pancreas, on the bone, and on blood vessels. This raises the interesting possibility and likelihood that these biologically active compounds that are released from contracting muscles have effects on tissues distant from the muscle and may account at least partly for the systemic benefits of regular muscular exercise in promoting health and well-being..
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