Dominic Stone
Hormones are chemical messengers that are vital for maintaining countless bodily functions and processes. They are produced by the endocrine system, which consists of tissues (mainly glands) that create and release them directly into the bloodstream. While hormones are typically produced by endocrine glands, such as the pituitary, thyroid, and pancreas, they can also be synthesized by other tissues throughout the body, including muscle tissue. The hormonal response to muscle contraction and exercise has been well-studied, with testosterone and growth hormones playing a significant role in muscle growth and repair. Additionally, skeletal muscles have been found to produce myokines, which are molecules that act in an endocrine hormone-like manner and are involved in inflammatory processes.
| Characteristics | Values |
|---|---|
| Muscles that synthesize hormones | Skeletal muscles |
| How hormones are synthesized | Hormone synthesis is controlled by various biochemical and neural signals that induce (sub-)acute fluctuations or periodic rhythms, leading to hourly, circadian, monthly or seasonal variations. |
| Hormones synthesized by muscles | Cytokines, peptides, and myokines (IL6, IL8, IL15, Brain-derived neurotrophic factor, and leukaemia inhibitory factor) |
| Role of hormones synthesized by muscles | These hormones have autocrine, paracrine, or endocrine actions and are involved in inflammatory processes. |
| Hormones affecting muscle growth | Testosterone, IGF-I, thyroid hormones, estrogen, and growth hormone |
| Effect of exercise on hormone synthesis | Appropriately structuring a resistance training program can minimize catabolic responses and optimize anabolic hormone signaling, contributing to an increase in muscle size. |
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What You'll Learn
Testosterone and muscle growth
Testosterone is the major sex hormone in males and is essential for male growth and the development of masculine characteristics. It is produced in the testes when the pituitary gland relays signals to them to do so. The amount of testosterone in the blood is closely regulated by a "feedback loop".
Testosterone plays a role in muscle growth. A decrease in testosterone levels can cause a decrease in muscle mass, and an increase in testosterone levels can increase muscle mass. This is why athletes who want to increase muscle mass and improve athletic performance may use anabolic steroids, testosterone, or related hormones. However, artificially high testosterone levels in men can cause several problems, including low sperm counts, shrinking of the testicles, impotence, heart muscle damage, and an increased risk of heart attack.
Testosterone therapy can slow the loss of muscle mass and improve bone density. However, it may also stimulate the growth of prostate cancer cells. Research does not indicate that testosterone therapy improves muscle strength, muscle mass, or physical performance measurements.
To counteract age-related losses in muscle mass related to lower testosterone levels, one can engage in strength training and multi-joint exercises, such as squats, deadlifts, and chest and shoulder presses. These exercises activate large muscle groups, releasing growth hormones that help stimulate the production of proteins in a similar way to testosterone. Eating a healthy diet with adequate animal or non-animal protein can also help maintain muscle mass.
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Hormonal response to exercise
The endocrine system regulates the production of hormones, which are chemicals that control cellular functions. Hormones can affect a number of different cells, but they only influence those with specific receptor sites. They control a number of physiological reactions in the body, including energy metabolism, reproduction, tissue growth, hydration levels, and the synthesis and degradation of muscle protein.
Hormones can be either anabolic or catabolic. Anabolic hormones help build new tissue, while catabolic hormones break it down. Anabolic hormones, such as testosterone, growth hormones, and insulin, stimulate muscle protein synthesis by sending signals to produce proteins, regenerate, and grow muscle. Testosterone is a steroid hormone produced by the Leydig cells of the testes in males and the ovaries of females, with small amounts produced by the adrenal glands of both genders. It is responsible for muscle protein resynthesis and the repair of muscle proteins damaged by exercise. Resistance exercise increases the concentration of anabolic hormones in the blood during exercise and for approximately one hour afterward. This helps signal the body to rebuild and repair body tissue, including muscle.
Cortisol is a catabolic steroid hormone produced by the adrenal gland in response to stress, low blood sugar, and exercise. It supports energy metabolism during long periods of exercise by facilitating the breakdown of triglycerides and proteins to create the glucose necessary to help fuel exercise. However, exercising for too long can elevate cortisol levels, causing muscle protein to be used for fuel instead of repairing damaged tissues.
The initial response to the onset of exercise is enhanced sympathoadrenal activity and the secretion of pituitary hormones, resulting in a reduction in insulin levels and a rise in other hormones. This shift in hormone balance modifies the metabolism of intra- and extra-muscular triglycerides and glycogen as fuels for muscular exercise. The variation in the mobilisation of one fuel source may influence its combustion, along with the mobilisation and combustion of the other fuel source. When exercise is prolonged, the hormonal response is influenced by additional factors such as temperature, glucose availability, oxygen tension, and changes in plasma volume.
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Muscle atrophy causes
Muscle atrophy, or muscle wasting, is when muscles waste away. It is usually caused by a lack of physical activity or inability to move, often due to an injury or underlying health condition. Astronauts, for example, experience muscle atrophy after a few days of weightlessness.
There are three types of muscle atrophy: physiologic, pathologic, and neurogenic. Physiologic atrophy is caused by not using the muscles enough, often due to seated jobs, health problems that limit movement, or decreased activity levels. This type of atrophy can often be reversed with exercise and better nutrition. Pathologic atrophy is associated with aging, starvation, and diseases such as Cushing's disease, which is caused by the overuse of corticosteroids or overactive adrenal glands. The third type, neurogenic atrophy, is the most severe form of muscle atrophy. It occurs when there is an injury to, or disease of, a nerve that connects to the muscle. This type of atrophy can happen suddenly and can be caused by amyotrophic lateral sclerosis (ALS), carpal tunnel syndrome, or Guillain-Barré syndrome.
Muscle atrophy can also be caused by malnutrition or inadequate nutrition, which can lead to conditions such as cachexia, sarcopenia, and myositis. Cachexia causes extreme weight loss and muscle atrophy and is often a symptom of another underlying condition such as cancer, HIV, or multiple sclerosis. Sarcopenia is a condition caused by the reduced production of proteins that promote muscle growth, resulting in muscle cell shrinkage. Myositis refers to the inflammation of muscles, causing weakness and pain, and can develop after a viral infection or as a side effect of an autoimmune condition.
In addition to these causes, certain medical conditions can directly cause muscle atrophy or make movement difficult, leading to atrophy. These conditions include dermatomyositis, multiple sclerosis, mitochondrial dysfunction, osteoarthritis, and polio. Treatment for muscle atrophy includes physical therapy, ultrasound therapy, functional electrical stimulation, and in some cases, surgery.
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Muscle as a secretory organ
Skeletal muscle, which is the largest organ in the body, has been identified as a secretory organ. This discovery provides a new framework for understanding how muscles communicate with other organs, such as adipose tissue, liver, pancreas, bones, and brain.
Skeletal muscles are primarily characterised by their mechanical activity, which is required for posture, movement, and breathing. This mechanical activity depends on muscle fibre contractions. However, skeletal muscle is not just a component of our locomotor system. It has been found that skeletal muscle can produce, express, and release several hundred secreted peptides, known as myokines. These myokines are cytokines and other peptides that are produced, expressed, and released by muscle fibres. They exert autocrine, paracrine, or endocrine effects. The muscle secretome, consisting of several hundred secreted peptides, has been found to play a role in muscle-liver and muscle-adipose tissue crosstalk.
Myokines may mediate the protective effects of muscular exercise, with regard to diseases associated with a physically inactive lifestyle. For example, muscular force production stimulates the release of various types of GH from the pituitary gland. GH is a primary hormone in a superfamily of various types and forms of primary hormones. Once released, GH binds to various receptors on the membrane of target cells in the body, stimulating the genetic machinery via intracellular signalling processes. Testosterone, another hormone produced primarily in the testes in men and the adrenal glands in women, also plays a role in muscle growth. When testosterone binds to its receptors inside muscle cells, it signals the cell's nucleus to increase protein synthesis, leading to an increase in muscle fibre size.
In summary, skeletal muscle has been recognised as a secretory organ, producing and releasing myokines that play a role in organ communication and the protective effects of exercise. This discovery has provided valuable insights into the complex functions of skeletal muscle beyond just locomotion.
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Hormone synthesis and metabolism
Hormones are chemical messengers that coordinate and control various bodily functions, including metabolism, growth and development, reproduction, and response to injury, stress, and environmental factors. They are synthesized, remain biologically active for a period, and then degrade or are destroyed.
Hormone synthesis can be influenced by various factors, such as changes in the number and size of hormone-producing cells, alterations in the activity of hormone-synthesizing enzymes, the absence of precursors, or interference with enzyme cofactors. For instance, the herbicide glyphosate decreases TSH levels, while pesticides have been linked to changes in FSH and LH levels. The insecticide methoxychlor decreases progesterone production, and cadmium reduces testosterone production by inhibiting cholesterol synthesis, a precursor for all steroid hormones.
The liver, an endocrine organ, plays a crucial role in hormone synthesis, metabolism, and the production of binding proteins to which hormones may bind. Peptide hormones, such as insulin, are synthesized in the endoplasmic reticulum, transferred to the Golgi apparatus, and then packaged into secretory vesicles for export. They can be secreted through regulated or constitutive secretion pathways. Steroid hormones, on the other hand, are lipids derived from cholesterol. They are rapidly secreted from cells and bind to plasma proteins, which influence their half-life and elimination.
Exercise and muscular contraction also induce a hormonal response. Testosterone, produced primarily in the testes in men and adrenal glands in women, binds to androgen receptors in muscle cells, increasing protein synthesis and muscle growth. Resistance training programs can be structured to optimize anabolic hormone signaling and enhance muscle size. Additionally, exercise stimulates the release of growth hormones (GH) from the pituitary gland, which can act directly on skeletal muscle and stimulate the production of IGF-1 in the liver and muscles.
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Frequently asked questions
Hormones are chemical messengers that coordinate and control different functions in the body by carrying messages through the blood to organs, skin, muscles and other tissues.
The endocrine system is a network of tissues (mainly glands) that create and release hormones into the bloodstream. It is responsible for maintaining homeostasis, including metabolism, growth, development, reproduction and response to stress.
Skeletal muscles have been shown to produce molecules called "myokines", which act in an endocrine fashion. Myokines are involved in inflammatory processes and include IL-6, IL-8, IL-15, Brain-Derived Neurotrophic Factor (BDNF), and Leukemia Inhibitory Factor (LIF).
Testosterone and IGF-I are critical for activating satellite cells, which contribute to muscle growth. Exercise stimulates the release of various hormones, including GH from the pituitary gland, which can directly act on skeletal muscle to increase protein synthesis and muscle fibre size.
The endocrine system continuously monitors hormone levels in the blood and adjusts its release accordingly. Hormones act on target tissues with specific receptors, delivering messages that cause the target site to take a specific action.
