Maximus Gage
Glucose metabolism in muscle is an important process for maintaining glucose homeostasis in the body. The skeletal muscle is the largest organ in the body by mass and is responsible for regulating glucose homeostasis, accounting for 80% of postprandial glucose uptake from the circulation. Glucose uptake by skeletal muscle occurs via insulin-dependent and -independent mechanisms, involving glucose delivery, glucose transport, and glucose metabolism. Once glucose enters the muscle cell, it is phosphorylated to glucose-6-phosphate, which can then be metabolised or stored as glycogen. Exercise plays a significant role in enhancing glucose uptake by skeletal muscle, with increased skeletal muscle GLUT4 expression facilitating post-exercise glucose uptake and glycogen storage. Understanding glucose metabolism in muscle is crucial for managing metabolic diseases, such as insulin resistance, diabetes, aging, and obesity.
| Characteristics | Values |
|---|---|
| Regulator of glucose homeostasis | Skeletal muscle is the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation |
| Role in metabolism | Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease |
| Glucose uptake | Glucose uptake by contracting skeletal muscle occurs by facilitated diffusion, dependent on the presence of GLUT4 in the surface membrane and an inward diffusion gradient for glucose |
| Glucose transport | Under resting conditions, glucose transport is the rate-limiting step for muscle glucose uptake, since GLUT1 expression is relatively low and the vast majority of muscle GLUT4 resides within intracellular storage sites, excluded from the sarcolemma and T-tubules |
| Glucose metabolism | Once glucose enters the muscle, it is trapped via phosphorylation to glucose-6-phosphate. The fate of this intracellular glucose is to either be metabolized or stored as glycogen |
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What You'll Learn
Glucose uptake by contracting skeletal muscle
With exercise, skeletal muscle hyperemia, capillary recruitment, and GLUT4 translocation to the sarcolemma and T-tubules effectively remove delivery and transport as major barriers to glucose uptake, with glucose phosphorylation becoming potentially limiting, especially at high exercise intensities. Once glucose enters the muscle, it is trapped via phosphorylation to glucose-6-phosphate. The fate of this intracellular glucose is to either be metabolised or stored as glycogen.
The skeletal muscle is the largest organ in the body by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Insulin-dependent and -independent skeletal muscle glucose disposal requires glucose delivery to the muscle from circulation, glucose traversing the extracellular matrix to the cell membrane, uptake via facilitative glucose transporters either constitutively on the cell membrane or translocated in response to insulin or exercise, and a glucose gradient to facilitate glucose transport modulated by intracellular glucose metabolism.
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Insulin-dependent glucose uptake
- Glucose delivery to the muscle from circulation
- Glucose traversing the extracellular matrix to the cell membrane
- Uptake via facilitative glucose transporters either constitutively on the cell membrane or translocated in response to insulin or exercise
- Glucose gradient to facilitate glucose transport modulated by intracellular glucose metabolism
The initial step for skeletal muscle glucose clearance is glucose delivery. Skeletal muscle blood flow and perfusion play a key role in glucose disposal, which is often overlooked. Once glucose enters the muscle, it is trapped via phosphorylation to glucose-6-phosphate. The fate of this intracellular glucose is to either be metabolised or stored as glycogen.
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Glucose transport
The presence of GLUT4 in the surface membrane is important for glucose uptake by contracting skeletal muscle. Under resting conditions, glucose transport is the rate-limiting step for muscle glucose uptake, as GLUT1 expression is relatively low and most muscle GLUT4 is within intracellular storage sites. However, with exercise, skeletal muscle hyperemia, capillary recruitment, and GLUT4 translocation to the sarcolemma and T-tubules effectively remove delivery and transport as barriers to glucose uptake.
Once glucose has been transported across the sarcolemma, it is phosphorylated to glucose 6-phosphate (G-6-P) in a reaction catalysed by HKII. This is the first step in the metabolism of glucose, which can then be metabolised via either the glycolytic and oxidative pathways responsible for energy generation during exercise or converted to glycogen in the post-exercise period.
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Glucose metabolism
Glucose uptake by contracting skeletal muscle occurs by facilitated diffusion, which is dependent on the presence of GLUT4 in the surface membrane and an inward diffusion gradient for glucose. There are three main sites/processes that can be regulated: glucose delivery, glucose transport, and glucose metabolism. Under resting conditions, glucose transport is the rate-limiting step for muscle glucose uptake, since GLUT1 expression is relatively low and the vast majority of muscle GLUT4 resides within intracellular storage sites, excluded from the sarcolemma and T-tubules.
During exercise, skeletal muscle hyperemia, capillary recruitment, and GLUT4 translocation to the sarcolemma and T-tubules effectively remove delivery and transport as major barriers to glucose uptake. With glucose phosphorylation becoming potentially limiting, especially at high exercise intensities. Once glucose enters the muscle, it is trapped via phosphorylation to glucose-6-phosphate. The fate of this intracellular glucose is to either be metabolized or stored as glycogen.
The skeletal muscle GLUT4 level correlates with the capacity for glucose uptake during very intense exercise. Increased skeletal muscle GLUT4 expression would also facilitate post-exercise glucose uptake and glycogen storage. Once glucose has been transported across the sarcolemma, it is phosphorylated to glucose 6-phosphate (G-6-P) in a reaction catalysed by HKII. This is the first step in the metabolism of glucose via either the glycolytic and oxidative pathways responsible for energy generation during exercise or conversion to glycogen in the post-exercise period.
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Glucose phosphorylation
Glucose uptake by contracting skeletal muscle occurs by facilitated diffusion, dependent on the presence of GLUT4 in the surface membrane and an inward diffusion gradient for glucose. The skeletal muscle is the largest organ in the body by mass and is the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation.
Under resting conditions, it is generally believed that glucose transport is the rate-limiting step for muscle glucose uptake, since GLUT1 expression is relatively low and the vast majority of muscle GLUT4 resides within intracellular storage sites, excluded from the sarcolemma and T-tubules. However, with exercise, skeletal muscle hyperemia, capillary recruitment, and GLUT4 translocation to the sarcolemma and T-tubules effectively remove delivery and transport as major barriers to glucose uptake, with glucose phosphorylation becoming potentially limiting, especially at high exercise intensities.
Increased skeletal muscle GLUT4 expression would also facilitate post-exercise glucose uptake and glycogen storage.
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Frequently asked questions
The skeletal muscle is the largest organ in the body and is responsible for regulating glucose homeostasis. It is involved in glucose uptake and is essential for metabolism.
Glucose enters the muscle via insulin-dependent glucose uptake. This requires four steps: 1) glucose delivery to the muscle from circulation, 2) glucose traversing the extracellular matrix to the cell membrane, 3) uptake via facilitative glucose transporters, and 4) glucose gradient to facilitate glucose transport modulated by intracellular glucose metabolism.
Once glucose enters the muscle, it is trapped via phosphorylation to glucose-6-phosphate. It can then be metabolized or stored as glycogen.
