Hibernation allows animals to reduce their energy needs during winter, when food may be scarce and high amounts of energy would be required to maintain body temperature. These animals prepare for winter by storing body fat that can be used as fuel during hibernation. Skeletal muscles in particular have high energy needs as they make up nearly half of an animal’s body mass. They also play important roles in regulating body temperature.
When small mammals hibernate, they typically undergo repeated cycles of torpor interrupted by short periodic arousals. During torpor, their metabolism and body temperature declines. Periodic arousals allow them to consume foods and eliminate bodily wastes.
Larger mammals, on the other hand, experience lower body temperatures during hibernation and skeletal muscle becomes very important for regulating body temperature as well as fuel utilization. Even a relaxed muscle requires energy. Myosin is a major protein responsible for muscle contractions. The structure of myosin includes many small “heads” that, when activated, attach to actin filaments and pull them closer together to produce muscle contractions.
Interestingly, myosin can be in varying states of activity even in relaxed muscles. Scientists call these states “super-relaxed” (SRX: using not much energy at all) and “disordered-relaxed” (DRX), which actually uses energy 5-10x faster than the SRX state. In a new study, scientists examined the ratio of myosin heads found in the SRX vs DRX state in small (thirteen-lined ground squirrels and garden dormouse) and large (American black bear and brown bear) hibernating mammals.
Comparing muscle fibers sampled from bears during the summer to those collected in the winter showed no differences in the ratio of myosin states or the use of energy by their muscles, which may be related to their stable body temperatures during hibernation. The researchers suspect that keeping the muscles active during hibernation may also help prevent bears from losing muscle mass.
Similar to bears, there were no differences in the ratio of myosin in the SRX and DRX state in active vs hibernating small mammals. In contrast to bears, the turnover of energy took longer during torpor and arousals than during their active seasons. This extended turnover means that the smaller mammals use more energy during these periods. The researchers hypothesized that this increase in energy utilization may be used to fuel non-shivering thermogenesis, which creates heat from metabolic processes as opposed to muscle contractions. They also suspect that changes in the myosin protein Myh2 may be involved in helping to support the increased use of energy by myosin during torpor in these small mammals.
Source:
CTA Lewis, EG Melhedegaard, MM Ognjanovic, MS Olsen, J Laitila, RAE Seaborne, MN Grønset, C Zhang, H Iwamoto, AL Hessel, MN Kuehn, C Merino, N Amigó, O Fröbert, S Giroud, JF Staples, AV Goropashnaya, VB Fedorov, BM Barnes, Ø Tøien, KL Drew, RJ Sprenger, J Ochala. Remodelling of skeletal muscle myosin metabolic states in hibernating mammals. eLife. preprint. https://doi.org/10.7554/eLife.94616.1
Categories: Comparative Physiology, Environment, Extreme Animals, Hibernation and Hypoxia, Nature's Solutions
Tags: ATP, fitness, health, metabolism, muscle, thermogenesis, torpor, winter
Life Lines by Dr. Dolittle 