Hypergravity (HG), i.e., gravity levels higher than 1 g, the Earth gravity level, can be considered as a stress because the animal subjected to a high g-load has to adapt to a higher weight: a 3 g level means that the weight of the animal is magnified thrice. Contrarily to temperature, HG is not a natural stress, because the gravity level is constant on Earth even if one can experience slight and short HG episodes in everyday life, for instance in cars or elevators during strong braking. Therefore, while species can detect the direction of the Earth’s gravity vector, for instance by using the vestibular system (see Sondag 1996), they have probably not evolved specific defense mechanisms against HG. However, even if the analogy is of a limited value, it could be said that subjecting a man weighing 70 kg to a 2 g level is like subjecting him to a 70 kg extra-weight in a backpack. This analogy is of a limited value because, in HG, the increased weight is not confined to the back but is spread on each cell of the body. Thus, even if specific defense mechanisms against HG do not exist, animals can adapt to an increased weight, as it is the case in females of mammals during pregnancy, and it can be hypothetized that they would react in HG conditions as if they were carrying an extra-weight.
Beyond the study of mild stress in flies (see below), HG has been used in mammals, mainly at the US National Aeronautics and Space Administration (NASA) during the 1960s and 1970s (Miquel and Economos 1982). In one of these studies, Economos et al. (1982) reported that rats kept at 3.14 g for life had a shorter, albeit not significant, longevity than rats kept at 1 g. Using HG in mammals is however not an easy procedure, due to technical problems. The centrifuge of the NASA, designed to study aging in rats, had a 16 m diameter (for a picture, see Oyama 1982), while that used to study the vestibular system in hamsters (for a picture, see Sondag 1996) had a 3.5 m diameter. Only a very few laboratories can use such centrifuges, due to their size. Beyond technical problems, it is worthy of note that rodents, after a severe adaptation phase during which the body weight decreases (review in Le Bourg 1999), have still some difficulty to cope with a high g-load. Hypergravity attenuates body mass gain (e.g., Kita et al. 2006) and rats exposed to 3.6 g for 21 months or to 4.7 g for 25 months show a deformation of vertebra of the thoracic cervical region with a marked lordosis (Oyama 1982). Therefore, the use of mammals to study the effect of HG on aging and longevity may face various technical and physiological issues. These physiological issues can explain why HG has not been considered in mammals as a mild stress which could have beneficial effects on aging and longevity but, rather, as a strong stress with deleterious effects. In such conditions, turning our attention to other species than mammals can be of interest.
This article summarizes the experiments conducted in D. melanogaster flies subjected to HG. These results have been published between 1989 and 2005. As a first step of these studies, flies were subjected throughout life to HG. Thereafter, flies were subjected to HG only at a young age.
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Le Bourg, E. (2008). Hypergravity in Drosophila melanogaster. In: Le Bourg, E., Rattan, S.I.S. (eds) Mild Stress and Healthy Aging. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-6869-0_4
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