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3.1 Introduction
Subjecting animals to sustained acceleration in a centrifuge (fig. 1), leading to an increase in the gravity force (hypergravity or HG), results in changes to various parts of the organism. This chapter reviews the results of experiments performed to study the effect of hypergravity on body weight, bones and muscles, the cardiovascular system, reproduction and the vestibular system, with the emphasis on studies concerning small rodents (mice, hamster and rat).
Fig. 1. Animal centrifuge of the vestibular department (ENT) at the Academic Medical Center in Amsterdam.
The effect of weightlessness (microgravity) on these systems will not be discussed here. Not much research is performed in space (space shuttle missions, biosatelite and spacelab). Moreover, the available results show that these experiments lasted for a shorter period (microgravity lasted 7 - 12 days) than for the hypergravity experiments discussed in this section (Kozlovskaya et al. 1990, Ross et al. 1985; Ross, 1987, 1993).
Often a clear comparison and evaluation of the results obtained from different hypergravity studies were impossible, either caused by lack of information about the number of animals (Lychakov, 1988; Krasnov 1991, Daunton, 1991), testing procedure (Matthews, 1953; Oyama and Platt, 1965; Clark, 1973, 1974) or statistic procedure (Krasnov, 1991; Fox et al. 1992). By omitting these data, the authors made it in impossible to repeat their experiments or to confirm their conclusions. Although the results have been summarized with great care, some of the conclusions can be regarded as preliminary because of the problems mentioned above.
3.2 Body weight
In 1953, Matthews reared rats under continual centrifugation in 3 G for 1.5 years. The body weight of these rats remained below that of control rats reared under normal gravity. Later studies with hamsters (Wunder et al. 1960; Briney and Wunder, 1962), mice (Oyama and Platt, 1967; Keil, 1968) and rats (Oyama and Platt, 1964; Oyama and Platt, 1965; Oyama et al. 1971; Pitts et al. 1972; Lim et al. 1974; Amtmann and Oyama, 1976; Martin, 1980; Economos et al. 1982; Serova, 1992) showed similar body weight decreases in rodents exposed to a hypergravity condition ranging from 2 G to 4.7 G. In the first days of hypergravity exposure, the rodents lost weight and gained weight again in the following days but stayed at a lower weight level than control animals (Wunder et al. 1960). Moreover, at higher G loads (2.5 G and higher) adult rodents were generally not able to fully restore their body mass to their precentrifugation level. The weight decrease was linearly related to the increase in G load (Smith and Burton, 1967). Steel (1962) investigated the growth of rats of different ages when exposed to 3 G for the same period (42 to 47 days) and concluded that the effect of hypergravity on body weight is stronger the earlier exposure begins.
What causes this decrease in body weight in rodents? A decrease in food and water intake is only observed during the first days, with a minimum intake in the first 24 hours (Oyama and Platt, 1964; Pitts et al. 1972). The imposed stress of hypergravity could be responsible for this reduced intake (Oyama and Platt, 1964). After the first days the daily food intake of HG rats per kg body weight increased again and exceeded that of controls (Steel, 1960; Oyama and Platt, 1965; Economos et al. 1982). Pitts et al. (1972) suggested that animals exposed to hypergravity were recovered from stress effects by the 10th - 15th day of centrifugation and that the sustained decrease in body weight was probably caused by a physiological regulation to adjust the organism to longterm increased G-load.
Anatomical examination of the HG animals showed that body fat was sensitive to increases in gravity. When gravity increased from 1 to 4 G, a pronounced decrease in body fat had been found, particularly in the abdominal fat depots and in the lipid concentration of liver, kidney, and adipose tissues of HG rats (Briney and Wunder, 1962; Oyama and Zeitman, 1967). Feller et al. (1968) suggested that the energy demands increased in animals exposed to hypergravity resulting in decreased body fat. Rats adapted to hypergravity showed higher respiratory metabolism rates which were necessary for the greater energy requirements (Oyama and Chan, (1973). When the animals were switched from a hypergravity condition to a normal gravity environment, the decrement in body weight was almost undone in the first 2 or 3 weeks immediately after ending centrifugation (Smith and Burton, 1967; Pitts et al. 1972). Rotation without the increased gravity component (when the animals were placed at the center of the centrifuge) did not induce a decrease in body weight; rats subjected to rotation for 2 - 16 weeks had the same body weight as controls (Martin, 1980).
3.3 Bones and muscles
The shape and size of bones and muscles can be modified by external factors, especially during growth. Altered gravity is one of these external factors. Alterations in bones and muscles compensate for the changes in loads and forces on these structures. The femur and the hind leg muscles located near the femur received most attention in the prolonged hypergravity experiments.
Femur
Concerning femur mass, Oyama and Zeitman (1967) found that the absolute femur bone mass was reduced but femur mass to body mass ratio was increased in the HG rats exposed to 1.0, 3.5 or 4.7 G for 1 year when compared to controls. This could indicate an increase in bone tissue. However, the body mass and the fat depots in HG rats were markedly decreased. Thus, the ratio between body mass and bone structures was altered accordingly. Therefore, these authors questioned the validity of the comparison of the relative femur mass with respect to body mass in the hypergravity experiments.
Concerning femur length, Wunder et al. (1960) observed that femurs in 5 week old mice exposed to 4 G for 1 to 8 weeks grew just as fast, if not faster, than the ones of control mice, especially during the first week of hypergravity exposure. A relative increase in femur length was also found in hamsters exposed to 4 or 5 G for four weeks (Briney and Wunder (1962). However, Amtmann and Oyama (1973) found a decrease in the relative length of femurs of rats exposed to either 1.0, 2.76 or 4.15 G for 810 days after birth. They also evaluated the work of Wunder et al. (1960) and Briney and Wunder (1962) and could demonstrate by recalculations of their data that the femurs of their mice and hamsters showed a decrease in length instead of the increase reported by the original experimenters. Decreases in femur length of rats conceived and born in hypergravity (2 G) were also reported by Smith (1975), thus showing that hypergravity inhibits the longitudinal growth of the femur.
Jaekel et al. (1977) found that rats centrifuged at 2.76 G for 810 days had a higher bone density than controls when cross-sections of equal size were compared. A higher density results in higher stability and capability to withstand higher mechanical stress (Amtmann, 1971). Jaekel et al. (1977) concluded that the changes in shape, size and density of the femur represented a functional adaptation to increased mechanical stress resulting from sustained centrifugation. The bones, by becoming thicker, shorter and denser, are better adapted to increased G-loads and the accompanying mechanical stress. Furthermore, femurs of HG adapted mice, and even more of HG adapted rats, have a greater ability to oppose bending than femurs of control animals and thus capable of sustaining greater forces as during hypergravity exposure (Wunder et al. 1977; Wunder and Welch, 1977; Kimura et al. 1979; Wunder et al. 1987). Jaekel et al. (1977) reject the suggestion made by Amtmann and Oyama (1976), that the observed femur changes could be attributed to immobilization effects (less locomotor activity). If immobilization was the cause of these changes, Jaekel and colleagues would have expected lower bone densities in HG animals, which was not so. The effects of long term hypergravity on bone tissue was less pronounced in adult animals subjected to the altered gravity condition then in young animals in which the bone material matured during hypergravity (Wunder et al. 1977; Smith, 1977). In addition, Serova (1992) showed that bone development was delayed in fetuses subjected to hypergravity (2 G).
Smith (1975, 1977) was interested in the role of the rotation element during centrifugation on femur length and growth. Rats were either centrifugated at 1.03 G (Rotation at the center of the centrifuge) or 2 G for 3 months. Afterwards, the femurs of the third generation of these groups of rats were examined. He found that hypergravity and rotation have differential effects on femur length and growth. Femurs of animals subjected to rotation were longer and larger in diameter than femurs of HG animals, while the relative cortex (diameter femur/cortical thickness) was thinner in rotation rats than in HG rats. According to the author, rotation advances the formation of secondary ossification centres and enhances growth, whereas hypergravity represses the function of the epiphyseal plates and produces more "robust" femurs.
Hind leg muscles
Briney and Wunder (1962) found increases in muscle size to body size ratio of the gastocnemii muscles in hamsters after being exposed to hypergravity (4 - 5 G) for 5 weeks. Burton et al. (1967) investigated the effect of long term hypergravity ( 1.5 - 3 G for periods ranging from 3 months to a year) on the size of a hip extensor (m. adductor) and a hip flexor (m. sartorius) in chickens. They found a differential effect of hypergravity on these antagonistic muscles; the extensor (an antigravity muscle) increased in muscle size while the flexor size reduced. Pitts et al (1972) found a reduction in the mass of total muscle in the limb extensors of rats exposed to either 2.5 or 4.15 G for 60 days. Amtmann and Oyama (1976) found that rats exposed to 2.76 G for 810 days had the same absolute hind leg muscle weights (m. triceps, m. semimembranosus and m. gluteus medius) as control animals of the same total body size. Smith (1972) postulated in his review that muscles generally hypertrophy because of increasing gravity, but the effect may be quite local and selective.
Concerning changes in the muscle fibre populations of HG animals, research has been done in the hind leg muscles such as the plantaris and the soleus muscles (Martin, 1978). The plantaris muscle contains 2 regions with different muscle fibre populations. The deep region contains more slow oxidative (SO) fibres with higher oxidative activity and fast oxidative glycolytic (FOG) fibers while the superficial region contains more fast glycolytic (FG) fibres. In the soleus muscle, the predominant fibre type is the SO fibre, with a smaller proportion of FOG fibres. The SO fibre is capable of contraction over longer periods without fatigue and is used in maintenance of posture. The FG fibre fatigues rapidly and is utilized during strong locomotory movements. The FOG fibre is intermediate in becoming fatigued and is employed in both functions. In skeletal muscle, oxidative enzyme shows resistance to fatigue and oxidative activity can be changed in response to experimental stress such as exercise. Exercise programs that demand strength plus resistance to fatigue, like in the hypergravity condition, accelerates the changeover of FOG to SO fibres in soleus muscle. Martin (1978) found an increase in SO fibres and a decrease in the number of FOG fibers in the soleus muscle in a third generation of 90 day old HG rats, conceived and born in a centrifuge under conditions of 2 G and in animals which were 60 days old at the onset of the hypergravity condition. The increase in SO population resulted from conversion of the FOG fibres into the more fatigue resistant SO fibres. In 1980, Martin studied the effects of hypergravity on the soleus and plantaris muscle in young developing rats (30 days old) who were centrifugated at 2 G for 2 - 16 weeks or subjected to continuous rotation (1.05 G) for the same period. After 4 weeks the number of SO fibres in both muscles of the HG rats was increased relative to control rats, both in males and in females. The muscle fibre composition in the rotation rats was the same as in controls. Martin (1980) suggested that the body weight of HG animals was higher during the hypergravity condition and this stimulated the transformation of more fatigue-resistant fibres. In the rotation group the proportion of SO fibers was reduced in males when compared to controls, but not in females. These results of hypergravity on muscle fiber population were further supported by Krasnov et al (1992) who found an increase in SO fibres and a lower content of FOG fibres in the soleus muscle of 60-day old rats pre and post natally developed in 2 G. Transition of rats from 2 G to 1 G caused an increase in the FOG fibres and a decrease in SO fibres after 15 days, which could reflect a growth of the animals motor activity.
3.4 Cardiovascular system and blood
Briney and Wunder (1962) found increases in the organ to body weight ratio of the heart and diaphragm of 9 week old hamsters kept under 4 G or 5 G for 4 weeks. Oyama and Platt (1965) found no significant increases in organ to body weight ratio of the heart and lung of weanling and adult HG rats after 4.5 months of centrifugation at 3.5 G or 4.7 G when compared with control rats. Plasma cholesterol was only significantly increased in the 3.5 HG group while plasma calcium was significantly increased in the 4.7 HG group when compared to age-matched and weight-matched controls after 1 year of centrifugation (Oyama and Zeitman, 1967). The authors concluded that hypergravity, even as high as 4.7 G for one year does not cause any serious damage to the animal. Except for a sharp decrease in body fat only minor changes were found in the chemical composition of blood and visceral tissues of these animals. Furthermore, they found that the life span of HG animals was not decreased when compared to controls. However, Economos et al. (1982) found an enlargement of the mitochondria (for production of energy) in heart tissue and a reduction in the number of mitochondria in adult rats subjected to sustained centrifugation (3.15 G) for 8 months. Based on these findings they concluded that hypergravity induced an increase in the ageing rate of the myocardium; since normal ageing results in a decreased number of mitochondria present in the myocardium or liver. Furthermore, they observed a trend for heart and kidneys of HG rats to have more lipofuscine. That supported their conclusion that the faster ageing rate of organs of the HG animals was due to the increased specific metabolic rate (rate of living) of these animals.
Concerning the cytoskeleton of the heart muscle, rats born and reared in 1.7 G showed a decrease in the number of microtubules and a disruption of their normal arrangement pattern. These changes appeared related to hormonal responses, regulated by catecholamines, since a number of microtubule-associated proteins were phosphorylated by cyclic AMP-dependent protein kinase. Cyclic AMP in mammals is known to take part in the regulation of the contraction-relaxation cycle of the heart muscle (Mednieks et al. 1987).
Burkovskaya and Krasnov (1991) examined the blood and bone marrow composition in rats conceived, born and reared in 2 G up to 60 or 75 days and controls. They found that the bone marrow of 60-day old HG rats contained less neutrophyles, more eosinophyles, more erythrocaryocytes and more reticulocytes in the blood. The results were interpreted as an increase in the formation of red blood cells (erythropoiesis) arising from increased metabolic processes during hypergravity. In 75-day old HG rats the values tended to be the same as for control rats, but a decrease in the number of lymphoid and plasmatic cells in the bone marrow showed that the adaptation to hypergravity was not yet completed. After two days in normal gravity, the 60 day old HG rats showed a further intensification of the erythropoiesis. According the authors, this intensification is necessary because of the higher motor activity of the rats after the hypergravity condition. After 15 days, the composition of both blood and bone marrow was almost the same as for control rats showing an adaptation to normal gravity.
3.5 Reproduction
In 1967, Oyama and Platt showed that rats exposed to 2.5 or 3.6 G mated and gave birth to pups during these hypergravity conditions. No pups were born when the animals were exposed to 4.7 G. The authors also found that average litter size decreased by higher G loads. Franciscis et al (1990) showed that even a few hours (3 times 45 minutes) of hypergravity (3 G) a day can result in deformalities of the embryos. Rats centrifuged at the fifth or sixth day of pregnancy had embryos with smaller length. These embryos also showed serious morphological anomalies that resulted in less surviving embryos. Serova et al (1985, 1991, 1992) observed that the period before copulation increased from 5.5 days to 15 days in rats exposed to 2 G. Furthermore, the total embryonic mortality and the number of living fetuses in rats exposed to 2 G during the 14 and 21 day of their embryonic development were the same as for controls. In pregnant rats exposed to 2 G from the seventh gestational day, one third of the pregnancies was interrupted. The remaining pregnancies developed normally. Oyama et al. (1992) also studied the prenatal and postnatal growth and development of mice and rats during 1.71 G or 2.03 G. They found no prenatal growth impairments in the living pups but the survival rate of neonatal rats was reduced at these G-values (at 2.03 G only 37.1% survived). Postnatal growth was the same in 1.71 G rats but decreased at 2.03 G, especially in males. In mice, differences between HG animals and controls were hardly observable if present at all.
Megory and Oyama (1984) investigated the effects of hypergravity on litter size and on nursing activity of the mother in HG adapted rats. The criterion for nursing ability was the survival of at least 4 pups for more than 48 hrs. No gross impairment of fertility was found in either sex during hypergravity exposure to 2.16 G or 3.14 G. The females went through normal gestation time, and had normal deliveries. The number of fetuses was reduced in the 2.16 G group and declined even further in the 3.14 G group. Undeveloped conceptions were common in the HG groups. A high rate of pathological conditions, such as antepartum death of the pregnant rats and stillbirth, was found in the 3.14 G group. Of the 2.16 G mothers 50% (22 of the 44) nursed their pups. Of the non-nursing mothers, 50% canniba-lized their pups within 24 hrs. The remaining pups, which were not eaten by their mother, died within a few days. Of the nursing rats, 77% continued to nurse 4 - 7 pups until weaning age (survival rate = 36.5% of all pups which were born). None of the 3.14 G mothers nursed their pups. Hypergravity induced in the female rat a hypothyroid state, with low TSH, T3 and T4 before parturition, which could account for the reduced litter size, the increased rate of undeveloped conceptions and stillbirth. It was not possible for the authors to decide the cause of the high mortality of the pups. Both the lack of nursing behaviour of the mothers and the inability of the newborns to endure hypergravity could be responsible for this phenomenon.
3.6 Vestibular system
Structure
Lim et al. (1974) observed changes in the otolith membrane of the saccules of rats exposed to long term hypergravity (2.30 G or 4.15 G) as compared to controls. These changes were a redistribution of otoconia in the direction of the gravitational force, e.g. in the antero-inferior portion of the primary membrane and an "accessory" membrane that appeared to be thicker. The displacement of some otoconia that is also observed under normal gravity conditions was enhanced by hypergravity. According to Money and Correia (1972), the accessory membrane is an anchoring device to hold the otolith membrane in place and to limit its shearing motion against the sensory hairs during rapid accelerations. Lim et al. (1974) speculated that the thickening of the accessory membrane represents an adaption to hypergravity.
Ballarino and Howland (1984) investigated the effect of centrifugation (2 G for 2 weeks) on the otoconial mineral weight in chick embryos. Mineral weight did not change in the HG animals when compared to controls. Thus, the authors concluded that the weight of the otoconia did not regulate calcite deposition and otoconial growth, as was hypothesized in an earlier experiment (Howland and Ballarino, 1981).
Lychakov et al. (1988) exposed rats to 2 G for 1 month and found a decrease in mean size of the otoconia of the utricle. This decrease in otoconial size was partly confirmed by Krasnov (1991) who investigated the effects of hypergravity on the utricle in 60 day old rats. These rats developed pre- and postnatally under 2 G, under conditions of 1.1 G (rotation) or under normal gravity conditions (control). The rotation rats showed no change in otoconial size when compared to controls. However, the author noticed that the large otoconia in the K-zone, the anterior third part of the peripheral utricular patch, were replaced in the HG rats by medium-sized ones. Krasnov suggested that smaller otoconia have sufficient weight under 2 G for the receptor cells of the utricle to receive information during 2 G. According to the author, a change in the size of otoconia is one of the mechanisms underlying alteration in the sensitivity of the otolith organs when the gravity level changes. He suggests that especially the peripheral part of the utricle is subjected to changes during hypergravity since the sensory cells perceiving Earth gravity are located in this area. He further reported that the adaptation process seemed not yet completed at the 60th postnatal day. In contrast with these findings Hara (1993) and Hara et al. (1995) found giant otoconia in the peripheral layer in chick embryos developed in 2 G. These otoconia are probably the result of an alteration in the mechanism that controls otoconial growth. Furthermore, they found that the utricular otoconial formation was delayed in the HG embryos.
Concerning the sensory epithelium of the vestibular end organs, Krasnov (1991) found thickened membrane sections of type I hair cells and expansions of intercellular space between receptor cells and dendrites in rats conceived and born in 2 G, while Daunton et al. (1991) reported that the number of synapses on type II hair cells was reduced in rats exposed to 2 G.
In studying the central vestibular system, Johnson et al. (1976) observed the morphology of the lateral vestibular nucleus (VL) in rats, conceived and born in 1 G or 2.76 G, centrifuged under conditions of 2.76 or 4.15 G for shorter (4 days) or longer (16 - 21 months) periods and control rats. Several changes in this nucleus occurred; an increase in the number of filamentous inclusions in the VL (only in the younger animals), alterations in organelles in the cytoplasm of cell bodies, altered mitochondria in the cell-bodies of the VL and alteration in the relative number of synaptic terminals. In rats under 4.15 G conditions for 16 to 21 months, neuro-axonal dystrophy was observed which was characterized by axons filled with reticulated mitochondria. The authors give the following explanations for these changes: i) hypergravity causes compressive loading or changes the hydrostatic pressure gradients within the brain, ii) hypergravity alters the functional activity of the labyrinth resulting in an altered input into the VL, inducing structural changes or iii) hypergravity alters the neuroendocrine function.
Krasnov (1991) studied the granular layer of the cerebellar nodulus (receiving otolith input) in rats prenatally developed in 2 G. He discovered several changes in this layer; i) most glomerules (synaptic connections between tightly grouped clusters of terminals) were filled with synaptic vesicles and have enlarged mitochondria, ii) synaptic vesicles clustered near the presynaptic membrane, iii) post synaptic thickening in axo-dendritic synapses with granular cell dendrites and iv) an increased number of microtubules in most granular cell dendrites. According to the author, these differences at the cell level point to a state of excitation in this part of the brain. After return to normal gravity, these differences decreased thus showing a decrease in impulse from the otolith organs to the glomerules.
Behaviour
In 1953, Matthews observed some abnormal types of movement when rats exposed to hypergravity (3 G for periods ranging up to 1.5 years) were placed back under normal gravity. No detailed information was given about these movements. Oyama and Platt (1965) observed a postrotatory nystagmus along with a characteristic side-to-side rolling movement of the head in rats exposed to 2.5, 3.5 and 4.7 G for longer periods of time. These vestibular reflexes occurred in the first week of centrifugation whenever the centrifuge was stopped and disappeared after this first week. Later experiments do not report these reflexes (Clark, 1973; Clark, 1974; Daunton et al. 1991; Fox et al. 1992).
Clark (1973) studied the effect of 60 days of hypergravity on postrotatory nystagmus in 8 adult HG rats and 6 control rats. He reported that the HG rats showed a reduced sensitivity to head angular acceleration. However, no description was given concerning the methods of inducing rotation or assessing the postrotatory nystagmus. In another experiment, Clark (1973, 1974) investigated the effect of continuous (60 days) 2 G centrifugation on equilibrium behaviour in the rat with a "rail test". This rail could rotate from 0 (stationary rail) - 25 (rotation rail) rpm and was located above an electric shock grid. During 10 days, 10 rats per group were exposed to 10 trials each day on the stationary rail. Hereafter they were subjected to the rotatory rail (10 trials) for 3 consecutive days. Eye movements and head-limb position were observed in the 15 minutes before the rail test. No overt signs of vestibular dysfunction were observed during this period. No difference was found between the HG and control animals in their ability to balance using time spent on the stationary rail. During the first 3 days on the stationary rail, the HG group tended to fall earlier from the rail (not significant). On later days (day 4 to 10), no differences were found between the groups. As far as avoidance behaviour (climbing up the rail) was concerned the HG group demonstrated more avoidance behaviour (to the shock) than the controls. Both groups increased avoidance scores from the performance on the first day. The controls reached maximum avoidance scores on the fourth day while the HG group reached maximum on the second day. The control rats remained longer on the rotatory rail than the HG rats during the 3 testing days. At higher rotation speeds the HG rats spent less trials for the entire time on the rail then controls and the difficulty in balancing increased for the HG rats at higher speeds. The author concluded that the HG animals had more difficulties with maintaining equilibrium and even more on dynamic equilibrium (rotating rail) than on static equilibrium (stationary rail). The author suggested that especially the otolith organs, which are specialized to detect head position and linear acceleration, are responsible for the equilibrium disturbances seen in animals living in hypergravity. The difference in avoidance behaviour could be caused by a higher stress factor for the HG animals, caused by the altering gravity, resulting in a different arousal level.
Fox et al. (1993) investigated the effect of 2 G and rotation (1 G) on air-righting, swimming, and locomotion in adult rats (number of rats unknown) during 1 - 16 days. Tests were performed under normal gravity immediately after hypergravity and after 2 to 16 days of normal gravity. They observed that the HG rats were unable to show a correct air-righting response, swam more underwater and showed disruption of normal gait. These disturbances in spatial orientation lasted from 2 to 8 days after HG exposure. The animals subjected to rotation behaved the same as controls thus showing that the altered behaviour found in the HG animals was due to 2 G and not to the rotational component present during centrifugation. According the author especially the air-righting and swimming disturbances are related to changes in vestibular functioning. They concluded that hypergravity changes the anatomy of the vestibular end-organs (although they do not mention how) leading to a reduced vestibular sensory function causing spatial orientation deficits upon return to normal gravity.
3.7 Summary
Decreased body weight is a structural adaptation to the increased gravity forces and is most noticeable in decreased body fat, especially in the abdominal fat depots. The higher energy demands when animals are exposed to hypergravity might be the cause of this decrement. The higher G-load is also responsible for the structural changes observed in the femurs of these animals. Bones become thicker, shorter, denser and with greater ability to oppose bending than the ones of controls. That make them better adapted to mechanical stress caused by the higher gravity forces. Analysis of muscle fibre populations show that in hypergravity fatiguing FOG fibres were conversed to more resistant fatigue SO fibres in order to survive the higher mechanical stress.
The higher energy demands during hypergravity resulted in a decrease in the mitochondria and an increase in lipofuscine in the myocardium and liver suggesting faster ageing organs of the HG animals due to the increased specific metabolic rate (rate of living) of the HG animals. Furthermore, an increase was found in the formation of red blood cells (erythropoiesis) arising from increased metabolic processes during hypergravity. If this leads to a decreasing life span remains unknown.
In the area of reproduction, it was demonstrated that hypergravity during the first part of embryonal development causes deformations of the embryos resulting in a higher mortality especially during higher G loads. Hypergravity during the last part of gestation has less severe effects. At high G loads the number of surviving pups is even more decreased due to the lack of nursing behaviour of the mothers or the inability of the newborns to endure hypergravity.
As far as the otolith organs are concerned longterm hypergravity resulted a redistribution of saccular otoconia and an thicker "accessory" membrane. If the otoconia alter in size, shape and distribution when subjected to hypergravity remains unanswered, the data from the experiments are in conflict with each other. Differences were also found in the lateral vestibular nucleus and cerebellar nodulus of HG rats and in vestibular evoked behaviour. This indicates that structural adaptation to hypergravity seems to take place both at the peripheral and central level of the vestibular system leading to disruption of spatial orientation with return to the 1 G environment. However this last conclusion is based on just a few articles that lacked some necessary information concerning methods of testing. Moreover, none of these experiments combined behavioural tests and anatomical examination of the vestibular system.