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When patients are confined to bed for long stretches, they run the risk of muscle atrophy due to the extended period of inactivity. Muscular proteins are degraded and their synthesis rate is diminished, leading to wasting, reduced muscle tone and decreased locomotor capacity. The same results have been seen in lab studies on rodents, when their limbs were suspended for several weeks. Muscle mass fell by up to 50% and the levels of contractile and stress proteins fell by 40-70%. So why do hibernating mammals like bats and bears suffer minimal muscle loss over several months of inactivity? Bats, for instance, are dormant for 4-6 months during winter, then they stir and launch more or less straight into their normal activities of flying, feeding and breeding. The clue to their muscular success lies in a true understanding of the term hibernation. Hibernation is not sleep, as some mistakenly think. It is a state of torpor, or suspended animation, during which the body temperature drops and the metabolic rate slows right down. These conditions are conducive to retarded protein degradation as well as protein synthesis, and would appear to be beneficial for energy conservation and survival. However, this is not the only factor thought to prevent muscle atrophy. During hibernation, bats undergo brief but regular periods of arousal, typically for several hours every few days. These periods are accompanied by muscle tremors and shivering which quickly raise the metabolic rate and increase the body temperature by as much as 25 °C in the first 30 minutes. This activity might prevent breakdown of the myofibril matrix, the arrays of thick and thin filaments that allow the muscle to contract. In a similar way, black bears raise their neck surface temperatures periodically during hibernation, with the same aim of preventing muscle degeneration. Although it is recognised that these hibernation activities are related to muscle function, the mechanism involved is unknown. In the first reported study of skeletal muscles in hibernating creatures by two-dimensional gel electrophoresis (2-DE), scientists from Korea have attempted to shed some light on the molecular processes behind atrophy prevention. Inho Choi and colleagues from Yonsei University and the Korea Atomic Energy Research Institute studied the pectoral muscle of the Korean great tube-nosed bat (Murina leucogaster ognevi). They dissected the muscles from bats captured in a natural cave in three different states: summer active (SA), winter hibernation (HB) and the early phase of arousal from hibernation (AR). The myofibrillar structures of the muscles were studied by electron microscopy and their contractile properties were also measured. These revealed that the muscles showed no signs of atrophy or tension reduction following three months of winter hibernation. The proteins were extracted from the muscles for 2-DE, for quantification by image analysis and identification by digestion with trypsin for mass spectrometric analysis. This proteomic analysis yielded 109 protein spots following 2-DE but only 38 were identified by database searching because there is no bat protein database: the rat database gave matches with the highest scores. Of these, 28 were deemed to be relevant to the study and they were separated into metabolic, stress and sarcomeric proteins. (The sarcomer is the basic unit of the myofibril). The levels of most of the proteins in each class were maintained during hibernation, with a few notable exceptions. For instance within the structural proteins, one isoform of actin and voltage-dependent anion channel 1 were up-regulated 1.6-1.8-fold in the HB and AR groups compared with the SA group. Similarly, two isoforms of the metabolic protein ATP synthase beta-subunit were increased in the HB group compared with the others. Within the stress proteins, heat shock protein 70 was increased in the HB and AR groups compared with SA, while the GRP75 precursor and the other heat shock proteins identified maintained their SA levels. GRP70 is associated with the survival and protection of cells in response to stresses such as oxygen deficiency. The heat shock proteins act as chaperones during protein fold and repair under oxidative and heat stress. The levels of most of these proteins should have fallen during dormancy. The fact that they remained steady or were increased supports the theory that periodic bouts of arousal help to prevent muscle atrophy in the bat by boosting the cellular and biochemical processes. The stress proteins will help to protect and retain myofibrillar proteins during rewarming. These results are also relevant to spaceflight. Hibernation, or suspended animation, has been considered as one way to transport astronauts on long flights, perhaps to the more distant planets. The bat study implies that the astronauts could be kept healthy by waking them up regularly for exercise sessions before reintroducing a period of further dormancy. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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Image: courtesy US Fish & Wildlife Service |
