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Hibernation is a strategy employed by many animals to survive harsh winters. During this dormant, inactive state, animals reduce their metabolic rate and limit body processes such as heart rate and body temperature to minimize their energy use. An important organ that changes during hibernation is skeletal muscle, which typically uses large amounts of energy, making up around 45-55% of body mass. Despite this, hibernators emerge from winter with very little atrophy, indicating that skeletal muscles are remarkably well preserved during hibernation. This is due to the down-regulation of glycolytic pathways and an increased reliance on lipid metabolism.
| Characteristics | Values |
|---|---|
| Muscle mass | Remains unchanged before and after hibernation |
| Muscle strength | Remains unchanged before and after hibernation |
| Muscle morphology | Well-preserved during hibernation |
| Protein synthesis | Reduced during hibernation |
| Protein degradation | Reduced during hibernation |
| Energy use | Reduced during hibernation |
| Metabolic fuel preference | Lipids over glucose oxidation |
| Glycolysis | Suppressed during hibernation |
| Myosin | Regulates energy use |
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What You'll Learn
Hibernators can maintain muscle mass
Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. It is characterised by low body temperature, slow breathing and heart rate, and a low metabolic rate. It is most commonly used to pass through the winter months. Obligate hibernators, such as ground squirrels, enter hibernation annually and regardless of the ambient temperature or access to food. Facultative hibernators, such as chipmunks, enter hibernation only when they are cold-stressed, food-deprived, or both.
Despite long periods of inactivity and fasting, hibernating animals are remarkably adept at maintaining skeletal muscle mass. This is particularly true of bears, which exhibit atrophy resistance and emerge from hibernation with very little atrophy. Studies have shown that the ratio of skeletal muscle mass to body mass is maintained in hibernating animals.
The preservation of muscle mass in hibernating animals is due to a balance between protein synthesis and protein degradation. During torpor, there is a depression of both protein synthesis and protein degradation. In bears, protein synthesis is depressed during deep torpor, but this is matched by a corresponding depression of protein degradation, leading to a net balance of protein synthesis and degradation.
Hibernating animals also exhibit a down-regulation of glycolytic pathways and an increased reliance on lipid metabolism. This may be important for maintaining skeletal muscle mass and avoiding alterations in fiber type associated with low activity during hibernation. Additionally, hibernating animals may undergo daily bouts of skeletal muscle activity that function to maintain muscle mass.
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Hibernation slows muscle atrophy
Hibernation is a state of minimal activity and metabolic depression undergone by some animal species, typically to survive the winter months. It is characterised by low body temperature, slow breathing, a slow heart rate, and a low metabolic rate. Obligate hibernators, such as certain species of ground squirrels, rodents, European hedgehogs, and marsupials, enter hibernation annually and spontaneously, regardless of temperature and food availability.
Hibernation is an intriguing phenomenon in the context of muscle atrophy because it represents a unique form of disuse. Animals abstain from eating or engaging in physical activity for months, yet they experience only minimal muscle loss. In fact, hibernators emerge from winter with very little atrophy, and their skeletal muscles are preserved during hibernation. This is in stark contrast to humans, who experience muscle loss at a rate of 0.2–2.3% per day during periods of bed rest and immobilisation.
The ability of hibernating animals to maintain muscle mass is especially interesting given the conditions of prolonged inactivity and malnutrition they endure. While muscle mass and protein content are reduced during hibernation in most rodent and bat species studied, the loss is not as severe as in traditional models of disuse or starvation. For example, hibernating bears have been found to maintain bone mass and avoid osteoporosis, as well as increase the availability of certain essential amino acids in the muscle.
The preservation of skeletal muscle strength and fatigue resistance in hibernating animals enables them to resume critical activities such as predator avoidance, foraging, and mating immediately after hibernation. This has led to scientific investigations into the mechanisms behind this preservation, with the hope of developing treatments to prevent disuse-induced skeletal muscle atrophy in humans.
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Muscle strength is preserved
Hibernation is a state of minimal activity and metabolic depression experienced by some animal species, typically during winter. It is characterised by low body temperature, slow breathing and heart rate, and a low metabolic rate. Despite this, hibernators emerge from their hibernation with very little muscle atrophy.
Hibernating animals exhibit an unexplained physiological characteristic of skeletal muscles being atrophy-resistant, meaning muscle mass and strength remain almost unchanged before and after hibernation. Studies have shown that muscle mass and strength are well preserved in hibernating animals, despite prolonged periods of inactivity and malnutrition. This is supported by research that found no significant alterations in body weight, muscle fibre size, or fibre type composition during active and hibernating periods.
The preservation of muscle morphology is combined with the down-regulation of glycolytic pathways and an increased reliance on lipid metabolism. While rates of protein synthesis are reduced during hibernation, this is balanced by correspondingly low rates of protein degradation. Potential mechanisms for this include a number of signalling pathways and transcription factors that lead to increased oxidative fibre expression, enhanced protein synthesis, and reduced protein degradation.
The functional significance of these outcomes is the maintenance of skeletal muscle strength and fatigue resistance. This enables hibernating animals to resume active behaviours such as predator avoidance, foraging, and mating immediately after hibernation.
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Energy use in muscles is regulated
Hibernation is a state of minimal activity and metabolic depression undergone by some animal species, primarily to survive the winter months. Obligate hibernators include many species of ground squirrels, rodents, European hedgehogs, insectivores, monotremes, and marsupials. Hibernating animals exhibit an unexplained physiological characteristic of skeletal muscles being atrophy resistant, allowing them to maintain muscle mass and locomotor function despite prolonged periods of inactivity.
Energy use in muscles is a highly regulated process, especially during intense exercise. Adenosine triphosphate (ATP) is the source of energy for all muscle contractions, and it is derived from both anaerobic (not requiring oxygen) and aerobic (requiring oxygen) processes. The primary energy source depends on the intensity and duration of muscle contractions. During high-intensity exercises, the rate of ATP demand increases significantly, and ATP needs to be regenerated at a rate that complements this demand. The three energy systems that function to replenish ATP in muscles are:
- Phosphagen system: This system provides energy for rapid, high-intensity contractions but is depleted within 30 seconds. Phosphocreatine (PCr) is broken down to replenish ATP, and it is the primary energy source for activities like a 100-meter sprint.
- Glycolytic system: This system uses carbohydrate metabolism to generate ATP. During high-intensity exercises, carbohydrates in the form of glucose or glycogen are partially broken down or oxidized, producing pyruvate. To prevent a reduction in ATP regeneration, the pyruvate is either removed from the cell or converted to lactate.
- Mitochondrial respiration: This system is responsible for complete oxidation when the glycolytic system cannot keep up with the demand. Carbohydrates are fully oxidized, and this system contributes to ATP regeneration during sustained exercises.
The interaction and relative contribution of these energy systems vary depending on the intensity and duration of the physical activity. For example, endurance athletes may experience glycogen depletion, commonly known as "hitting the wall." To counter this, athletes often manipulate their carbohydrate intake to maximize glycogen stores before an event. Additionally, fat breakdown, or lipolysis, is the most abundant energy source available to muscle fibers, but its contribution decreases as contraction intensity increases.
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Hibernation affects muscle metabolism
Hibernation is a state of minimal activity and metabolic depression experienced by some animals, primarily to pass through the winter months. During hibernation, animals experience a decrease in body temperature, breathing rate, heart rate, and metabolic rate.
Studies have shown that hibernating animals exhibit an unexplained physiological characteristic of skeletal muscle atrophy resistance. In Asiatic black bears, for example, muscle mass and strength remain almost unchanged before and after hibernation. Similarly, studies on brown bears have shown that muscle mass and strength are preserved during hibernation, with only moderate hypothermia and a drop in metabolic rate to about 25% of basal metabolism.
The preservation of muscle morphology in hibernating animals is accompanied by a down-regulation of glycolytic pathways and an increased reliance on lipid metabolism. While rates of protein synthesis are reduced during hibernation, this is balanced by correspondingly low rates of protein degradation. In addition, hibernating animals may increase the availability of certain essential amino acids in the muscle and regulate the transcription of genes that limit muscle wasting.
Overall, the hibernation state allows animals to minimize the wasting of energy and nutrients, enabling them to withstand unfavorable environmental conditions and food scarcity.
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Frequently asked questions
Hibernation muscles are skeletal muscles that undergo changes during hibernation to help conserve energy.
During hibernation, skeletal muscles reduce energy loss by decreasing myosin ATPase activity. This is in addition to the general reduction in metabolic rate that occurs during hibernation.
Hibernation muscles exhibit an unexplained physiological characteristic of being atrophy-resistant. Muscle mass and strength remain almost unchanged before and after hibernation.
No, the level of muscle preservation varies between species. For example, studies on rodents and bats show a reduction in muscle mass and protein content during hibernation, but the reduction is less severe than in traditional models of disuse or starvation.
Examples of animals that exhibit muscle preservation during hibernation include Asiatic black bears, ground squirrels, and dormice.
