Logan Steele
Skeletal muscle, the largest organ system in the body, is responsible for movement, posture, temperature regulation, soft tissue support, and
| Characteristics | Values |
|---|---|
| Skeletal muscle glucose role | Regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from circulation |
| Skeletal muscle composition | Striated muscle tissue attached to bones via tendons |
| Skeletal muscle fibre types | Slow-twitch Type 1 and fast-twitch Type 2 |
| Skeletal muscle and insulin | Insulin resistance is caused by desensitization of muscle to insulin, leading to elevated blood glucose levels |
| Exercise and glucose | Exercise increases glucose uptake by up to 50-fold through delivery, transport across the muscle membrane, and intracellular flux |
| Exercise types | Aerobic and resistance training are two main forms with distinct effects on glucose homeostasis |
| Diabetes and exercise | Exercise is beneficial for diabetes patients, acting as a cornerstone therapy by increasing insulin sensitivity |
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What You'll Learn
Skeletal muscle and glucose uptake
Skeletal muscle, the largest organ system in the body, is essential for glucose clearance and is responsible for over 80% of glucose uptake from an oral glucose load, postprandial. It is also the regulator of glucose homeostasis.
Insulin resistance is caused by the desensitization of muscle to the insulin released by the pancreas to elicit glucose uptake, leading to elevated blood glucose levels. Skeletal muscle insulin resistance can appear decades before the onset of β-cell failure and symptomatic Type 2 diabetes. As the principal site of insulin-stimulated glucose uptake, skeletal muscle is also considered the primary driver of whole-body insulin resistance. When the primary defect is in skeletal muscle, remediating insulin resistance in the muscle alone is sufficient to restore whole-body glucose homeostasis.
Under normal conditions, postprandial glucose uptake into muscle increases linearly with time. However, with insulin resistance and Type 2 diabetes, there is a delay in insulin action and glucose uptake, causing diminished overall glucose uptake by the skeletal muscle.
Exercise-stimulated glucose uptake is preserved in insulin-resistant muscle, emphasizing exercise as a therapeutic cornerstone among patients with metabolic diseases such as diabetes mellitus. Exercise increases glucose uptake by up to 50-fold through the simultaneous stimulation of three key steps: delivery, transport across the muscle membrane, and intracellular flux through metabolic processes (glycolysis and glucose oxidation). Exercise training is the most potent stimulus to increase skeletal muscle GLUT4 expression, an effect that may partly contribute to improved insulin action and glucose disposal.
Glucose enters the muscle cell via facilitated diffusion through the GLUT4 glucose transporter, which translocates from intracellular storage depots to the plasma membrane and T-tubules upon muscle contraction. Contraction-induced molecular signaling involves a variety of signaling molecules, including AMPK, Ca2+, and NOS in the proximal part of the signaling cascade, as well as GTPases, Rab, and SNARE proteins and cytoskeletal components in the distal part.
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Insulin resistance
Muscle glucose refers to the glucose that is extracted by skeletal muscle from the blood to maintain the demand for carbohydrates as an energy source during exercise. The skeletal muscle organ system is the largest in the body, comprising about 40% of the body weight of a young man and 80% of postprandial glucose uptake from the circulation.
Several factors can increase the risk of developing insulin resistance, including excess body fat, physical inactivity, age, and family history. Certain medications, such as steroid use, can also cause temporary insulin resistance. Maintaining a healthy weight, engaging in physical activity, and stopping smoking can help prevent insulin resistance.
Exercise is an effective therapeutic strategy for individuals with insulin resistance and metabolic diseases. Exercise-stimulated glucose uptake is preserved in insulin-resistant muscle, and exercise can increase glucose uptake by up to 50-fold through simultaneous stimulation of delivery, transport across the muscle membrane, and intracellular metabolic processes.
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Diabetes and muscle glucose
Skeletal muscle, which makes up about 40% of the body weight of a young man, is the largest organ system in the body. It is essential for movement, posture, temperature, soft tissue support, metabolism, and glucose homeostasis. Skeletal muscle is responsible for over 80% of glucose uptake from an oral glucose load.
Diabetes mellitus, often referred to simply as diabetes, is a group of metabolic disorders that impair glucose homeostasis and cause persistently high blood glucose levels. This impaired glucose homeostasis increases the risk of heart disease, stroke, neuropathy, and other complications. There are two main types of diabetes: Type 1 diabetes (T1D) and Type 2 diabetes (T2D). T1D is an autoimmune disorder that leads to pancreatic β-cell dysfunction, impeding their ability to produce insulin. Insulin is necessary for glucose uptake in peripheral tissues, including skeletal muscle. Therefore, the lack of insulin in T1D results in high blood glucose levels.
Insulin resistance, which occurs in T2D, is caused by the desensitization of muscle to the insulin released by the pancreas to facilitate glucose uptake. This results in elevated blood glucose levels. Skeletal muscle insulin resistance can precede the onset of T2D by decades and is often observed in lean nondiabetic, normoglycemic individuals with a high risk of developing T2D, such as children with diabetic parents. As the primary site of insulin-stimulated glucose uptake, skeletal muscle dysfunction plays a crucial role in the development of whole-body insulin resistance.
Additionally, diabetes, particularly in older individuals, is associated with a decline in skeletal muscle mass. Research has revealed that elevated blood sugar levels trigger muscle atrophy, and two proteins, WWP1 and KLF15, play key roles in this process. Specifically, elevated blood sugar levels decrease the amount of WWP1, a protein that promotes the degradation of KLF15. This deceleration of KLF15 degradation leads to an increased abundance of this protein, contributing to muscle atrophy.
Exercise is an essential therapeutic intervention for individuals with diabetes. It increases glucose uptake by up to 50-fold through the simultaneous stimulation of delivery, transport across the muscle membrane, and intracellular flux through metabolic processes. Exercise training, in particular, enhances skeletal muscle GLUT4 expression, contributing to improved insulin action and glucose disposal.
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Exercise-stimulated glucose uptake
Muscle glucose, or glucose uptake in skeletal muscle, is essential for human metabolism. Skeletal muscle is the largest organ system in the body, comprising about 40% of the body weight of a young man. It is responsible for movement, posture, temperature, soft tissue support, metabolism, and
- Delivery: Exercise increases glucose delivery by increasing muscle blood flow.
- Transport across the muscle membrane: Exercise increases the muscle membrane's glucose transport capacity.
- Intracellular flux through metabolic processes: Exercise increases enzymatic activity related to glucose metabolism, such as glycolysis and glucose oxidation.
The regulation of exercise-stimulated glucose transport involves two major pathways:
- Intracellular metabolic milieu alterations: This pathway is likely mediated by AMPK, a heterotrimeric protein composed of catalytic and regulatory subunits. AMPK activation is positively correlated with increased skeletal muscle glucose uptake.
- Mechanical stress: This pathway is partly mediated by RAC1.
In summary, exercise-stimulated glucose uptake in skeletal muscle is a complex process involving multiple signaling pathways and metabolic processes. This process is essential for maintaining muscle energy supply during physical activity and has important therapeutic implications for metabolic diseases.
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Muscle glucose transport
Glucose transporters, known as GLUTs (facilitative glucose transporters), are integral to muscle glucose transport. These transporters are a family of proteins that facilitate the movement of glucose across cell membranes. The two primary GLUTs involved in muscle glucose transport are GLUT-1 and GLUT-4. GLUT-1 is found in the plasma membrane of skeletal muscle cells, while GLUT-4 is responsible for transporting glucose from intracellular storage vesicles to the plasma membrane, where it can then be utilised by the cell.
Regulation of muscle glucose transport is a complex process influenced by various factors, including physical activity, metabolic stimuli, and pathophysiological conditions such as insulin resistance and type 2 diabetes. Exercise, in particular, plays a significant role in enhancing muscle glucose transport. Both aerobic exercise training and resistance exercise training have been shown to improve muscle glucose transport and overall glucose homeostasis. Exercise stimulates glucose transport through distinct molecular signalling pathways that increase glucose uptake, even in cases of insulin resistance.
Additionally, muscle glucose transport is regulated by specific molecular mechanisms. SNARE proteins, for instance, are essential regulators of glucose transport into skeletal muscles. Other molecular players, such as AMPK, CaMKII, and RabGAP proteins, are also involved in the intricate process of muscle glucose transport. These molecules interact with GLUTs to facilitate glucose uptake and maintain energy homeostasis in the body.
Understanding muscle glucose transport and its regulatory mechanisms is crucial in developing therapeutic interventions for metabolic diseases, including diabetes. By targeting these transporters and regulatory pathways, researchers aim to improve glucose uptake and management, ultimately enhancing metabolic health and reducing the risk of diabetes-related complications.
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Frequently asked questions
Muscle glucose refers to the glucose that is extracted and used by the muscles during exercise. Skeletal muscle, which is the largest organ system in the body, is responsible for regulating glucose homeostasis and is essential for glucose clearance.
Exercise increases the uptake of glucose by up to 50-fold. This is due to the simultaneous stimulation of three key steps: delivery, transport across the muscle membrane, and intracellular flux through metabolic processes (glycolysis and glucose oxidation). Exercise training, especially aerobic exercise, is known to improve human health, particularly for those with type 2 diabetes.
Insulin stimulates glucose uptake in the muscles. Insulin resistance, which is caused by the desensitization of muscles to insulin, can lead to elevated blood glucose levels and increase the risk of developing type 2 diabetes. Exercise can improve insulin sensitivity, thereby reducing the risk of insulin resistance.
