We learned earlier this semester that the endocrine and the nervous system work together to maintain homeostasis within the body. The nervous system as we learned in the previous chapters uses electrical impulses to communicate with different muscles in the body to produce responses. The endocrine system on the other hand uses chemical signals called hormones that also produce responses. In this essay we will be focusing on the main hormones during exercise metabolism, so mainly on the endocrine system.
One of the very first things we learned during this unit in class is the definition of metabolism which is the sum of all chemical processes in the body. Now before jumping into discussing the different hormones during exercise metabolism we need to know more about hormones and how they work. There are many hormones in the body and each is released from a specific gland in the body which then are circulated in the blood where they bind to specific receptors where they produce a lot of chemical signaling and depending on the hormone that was released this may speed up or slow down enzyme activity. Enzymes are controlled by hormones and these control the speed of activity. There are two parts to metabolism. We have the breakdown of molecules which is catabolism and synthesis of molecules which is called anabolism. You will see later that usually the catabolic reactions usually have the word-lysis which means to split or to break down while the synthesis reactions usually not always will have the word genesis which basically means to make. During exercise our body produces work and in order to produce work we need energy which comes from adenosine triphosphate (ATP). By breaking down ATP we can get enough energy to supply our demands. We also need to consider how to replenish this ATP that was used to make energy and this is done through the phosphogen, glycolysis, and oxidative system.
As a review the phosphogen system is the fastest way to resynthesize ATP and is used for very intense exercise lasting up to 10 seconds and relies on creatine phosphate (enzyme) and when this has run out then the body has to use other systems. The glycolysis system is the next fastest and it provides ATP for high intense 1-3 minutes of exercise. Very quickly, as from what I said earlier glycolysis as the ending lysis so it is the breakdown of glucose to provide energy and it uses glucose directly from the blood at first. The oxidative system takes more time to create the ATP but it produces more and this is usually used for low intense exercise. In prolong exercise where intensity is low the body will probably use fat as the energy source instead of muscle glycogen (storage form of glucose) and blood glucose as kind of like a back up plan.
The catabolic hormones that were discussed in class that occur during endurance exercise were epinephrine, norepinephrine, glucagon, thyroxine, and cortisol (which also has some anabolic reactions). These hormones are called catabolic hormones because they break down molecules. Epinephrine and noepinephrine are very important hormones. Both are extremely important in helping the sympathetic nervous system also known as the fire-and-flight to produce energy and maintain body function during endurance exercising. Both of these hormones are called catecholamines and they are very similar. Epinephrine also sometimes referred to adrenaline is produced by the adrenal medulla and this hormone increases blood glucose levels by breaking down glycogen stores in the liver and muscle to produce energy and this is called glycogenolysis. Both epinephrine and norepinephrine functiona as neurotransmitters which are chemical messengers that communicate with the brain and they relay signals between neurons. As for acting as hormones the effects last longer because it is slow to remove from the blood compared to quickly being reuptaken. When the exercise has ended the epinephrine returns to normal concentrations within a few minutes, on the other hand norepinephrine doesn’t return to normal until several hours.
In specific epinephrine and norepinephrine binds to a specific receptor on the plasma membrane and this activates an enzyme called adenyl cyclase which breaks down ATP to cyclic AMP. Cylic AMP then activates protein kinase and this specific molecule causes a phosphate (high energy bond) to be added to phosphorylase and synthase. When a phosphate group is added to phosphorylase this will cause phosphate to be activated and when activated this enzyme acts on glycogen to take off a glucose molecule in the muscle or liver and this glucose will be used by energy. On the other hand phosphate is also added to synthase and this acts on glycogen to add a glucose molecule together for glycogen storage. During exercise the amount of epinephrine increases and this speeds up phosphorylase and slows down synthase so more energy is made during exercise. So when there is more epinephrine this an increase of glucose in the blood. Carbohydrate metabolism burns slow at first and as exercise duration increases the carbohydrates burn faster. The glucose that is available in the blood, released by kidneys, and the glucose released by the liver enters the cell by a muscle transporter protein GLUT 4 where it is phosphorylated once inside the muscle and this causes trapping. From here if the muscle is not active it will be stored as glycogen, when active the glucose will go through glycolysis where the end product will be 2 pyruvate and this will make atp that can be used in the cross bridges and pumps. On the other hand when glucose in blood runs low then the muscle glycogen will be phosphorylated and also creates pyruvate to go through glycolysis to make energy. In addition, in fat metabolism we have lipolysis which is the break down of triglycerieds into 3 free fatty acids and glycerol using the enzyme lipase so this is increased when epinephrine is increased. Then the free fatty acids can be oxidized to make ATP to produce energy, or it can be used in the synthesis of glucose from non-carbohydrates (gluconeogenesis) which again can act as an energy source raising blood glucose levels. Epinephrine and norepinephrine builds muscle and burns fat for hours after your workout and assuming you have a good diet could be around five days and is used best with high intense activities (). Finally, when you exercise you are stressing the body and with this stress you have an increase of epinephrine in the blood to speed up heart rate, respiration, blood pressure, and metabolism so there is an increase in metabolic rate and the more calories you burn.
If epinephrine and norepinephrine were high enough this could stimulate glucagon which would increase (kind of take over) glucose to help fuel anaerobic metabolism(). This hormone comes from alpha cells in the pancreas and is basically an antagonist of insulin (which is controlled by the beta cells in pancreas). Glucagon is important in the liver and it keeps from blood glucose from dropping too low. This hormone will do this by converting stored glycogen in the liver to glucose which can be released in the blood (liver glycogenolysis). Remember from earlier that we talked about phosphorylase in which a phosphate group is added to phosphorylase this in return makes phosphorylase active and will act on glycogen and take off a glucose so when glucagon is increased this enzyme phosphorylase will increase to increase blood glucose levels. Not only does it break down glycogen but it can also stop the amount of glucose entering the liver and this can be very helpful in maintaining glucose levels in the blood. Next, glucagon increases blood glucose levels by gluconeogenesis and it does this by breaking down amino acids into glucose. Glucagon can also break down stored fat triglycerides into free fatty acids where it can be converted to glucose and produce energy. With exercise lasting thirty minutes or longer the body somewhat tries to maintain glucose concentrations while insulin concentrations decline. During exercise glucagon will increase throughout the exercise and it does this by liver glycogenolysis. This happens because there is an increase ability to bind to receptors on muscle cells and this increases during exercise due to increase in blood flow. During exercise lasting several hours there is a significant increase in glucagon which uses gluconeogenesis to provide more energy during carbohydrate metabolism but there is a huge depletion in glucose during late in activity as liver glycogen depletes.
Thyroxine was mentioned a little but not much in class but this hormone is released by the thyroid gland and it releases two nonsteroid hormones triiodothyronine and thyroxine T4. Both of these hormones increase metabolic rate of all tissues and increases it a lot to 60-100%. (105).These hormones not only do this but they increase protein synthesis, mitochondria, uptake glucose, glycolysis, gluconeogenesis, and free fatty acids availability. When exercising this increases thyroxine but during prolong exercises thyroxine increases sharply and remains constant.
As for cortisol this steroid hormone is produced by the adrenal cortex and in class we learned that it treats inflammation. Cortisol increases fuels by stimulating gluconeogenesis in the liver, and increases triglyceride breakdown to free fatty acids and glycerol in which can be used to convert to glucose. There is also protein catabolism and as the name suggest this is the break down of proteins into amino acids and these amino acids can be used to convert to glucose and are used to repair and create more enzymes. There is one exception where cortisol acts as a anabolic hormone and this is to decrease glucose uptake and one of the main reasons why it does this is to spare some of the glucose for the brain. Cortisol not only does gluconeogenesis which is the production of new glucose but it also plays a role in using free fatty acids for energy in performing endurance exercises. Cortisol peaks thirty to forty-five minutes of exercise and then will decrease to normal levels.
The next three hormones that I will talk about are insulin, growth hormone, and testosterone all of which are anabolic hormones and insulin and growth hormone have exceptions.
Testosterone was not mentioned that much in class, but this is a steroid hormone that is produced in the ovaries in women and testes in men along with the adrenal gland in both sexes. This hormone simulates protein synthesis which makes proteins from amino acids. In acute training there are small increases with exercise and with prolong exercise resting levels are decreased in male runners.
The next hormone that is important in exercise metabolism is the growth hormone. This hormone is basically an anabolic hormone and this hormone basically is a substance that builds up in organs and tissues that will produce growth and increase size of tissues. Its job is to promote growth and helps amino acids transport to cells (amino acid uptake). So this hormone directly stimulates lipolysis breakdown of triglycerides to free fatty acids and glycerol. These hormones are said to increase during exercise and stay elevated for awhile after exercise. This hormone is mediated by the insulin-like growth factor which is a hormone that comes from the liver. In protein metabolism, the growth hormone stimulates protein anabolism and increases amino acid uptake and increases protein synthesis. In fat metabolism the hormone stimulates triglyceride breakdown. In carbohydrate metabolism this hormone maintains glucose levels and suppresses insulin to take up glucose. The free fatty acids continues to increase throughout endurance training and this means that lipase continues to be activated by this hormone. In response to acute exercise this increases with increasing work. This hormone also decreases the rate of carbohydrate use and is inhibited by somatostatin. The growth hormone increases glucose in muscle indirectly by somatomins which are released by liver and stimulate protein synthesis.
Last but not least we have insulin. Insulins main function is to decrease the amount of glucose in the blood by taking up glucose. This hormone is released from the beta cells of the pancreas. It is basically a anabolic hormone but does have some catabolic reactions. If there is too much sugar in the body then this hormone will store glucose in the form of glycogen in the liver and will release it when blood sugar is low so this hormone is used for balancing sugar levels. As we learned earlier this semester this is a very important hormone with people who have type 1 diabetes where the beta cells are damaged and these are the people that need injections of insulin while type 2 diabetes people are resistant to insulin. Insulin has a glucose transporter called GLUT 4 where the glucose is transported into the muscle. By doing this this increases phosphofructokinase an enzyme that breaks down glucose and stimulates glycogen synthase. Insulin in the liver will activate an enzyme called hexokinase which phosphorylates glucose and basically traps it in the cell. This is where insulin inhibits glucose-6-phosphotase. When the liver is overwhelmed with the glycogen the extra glucose is taken up by liver cells and released as lipoproteins which are used to make free fatty acids to synthesize triglycerides. On the other hand insulin will inhibit breakdown of fat by inhibiting lipase. From this insulin helps glucose enter fat cells so glucose can synthesize to glycerol and this glycerol added to the free fatty acids from liver will synthesize triglyceride. During exercise we could see in class that basically all the hormones are increasing and the ones decreasing are insulin. As exercise duration increases insulin decreases at almost a constant rate and will plateau out around 90 minutes and continue to drop while blood glucose intake increases rapidly and plateaus.
In conclusion all of these things are happening in our body and it is very complex and sometimes overwhelming. Many of these hormones do different things but it is nice that we can somewhat distinguish them from eachother in that some are anabolic and some are catabolic. One hormone may do twenty different things they are widely used in the body and they are changing and adjusting especially when we are exercising.
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