Metabolic Activity

The cellular mechanisms underlying the adaptation to altered gravity require energy consumption. This consumption can directly be determined by glucose utilization. Marker enzymes can also be used to study the energy demands during adaptation. For example, Ghicose-6-Phosphate Dehydrogenase (G6P-DH) and Succinate Dehydrogenase (SDH), which are the limiting enzymes of the Krebs' cycle, are important to maintain energy availability in the cells. There is also the creatine kinase involved in the mechanism of ATP-regeneration, or the cytochrome oxidase that characterizes basic metabolic activity.

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Figure 5-13. Developmental characteristics of fictive swimming demonstrated for the parameters "burst duration " and "episode duration ". Fictive swimming in Xenopus laevis can only be induced up to developmental stage 47. Note the increase in burst duration (A) and the decrease in episode duration (C) up to this stage. Similar characteristics exist for cycle length and rostrocaudal delay. The effects of microgravity on burst and episode durations are shown in B and D, respectively. Recordings started 1-2 days after landing of the Soyuz capsule, or several days later (3-6). The embryos were at stage 24/27 at launch of the spacecraft. Adapted from Böser (2003) and Böser et al. (2002).

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Figure 5-13. Developmental characteristics of fictive swimming demonstrated for the parameters "burst duration " and "episode duration ". Fictive swimming in Xenopus laevis can only be induced up to developmental stage 47. Note the increase in burst duration (A) and the decrease in episode duration (C) up to this stage. Similar characteristics exist for cycle length and rostrocaudal delay. The effects of microgravity on burst and episode durations are shown in B and D, respectively. Recordings started 1-2 days after landing of the Soyuz capsule, or several days later (3-6). The embryos were at stage 24/27 at launch of the spacecraft. Adapted from Böser (2003) and Böser et al. (2002).

Using these markers, space studies revealed that brain metabolism was strongly affected by a change in gravity level. In immature and adult aquatic animals, modifications occurred mostly in vestibular related structures of the developing brain or in the sense organs. After the 20-day Cosmos-782 biosatellite mission, creatine kinase activity was increased in the cortex of the vestibular cerebellum of killifish Fundulus heteroclitus that hatched during the flight, with respect to the ground controls (Krasnov 1977). Cytochrome oxidase activity was decreased in the sensory epithelia of the utricle but not in the saccule after spaceflight exposure (Anken et al. 1998).

In some instances, hypergravity can induce an effect opposite to that of microgravity. In fact, hypergravity increases and microgravity decreases energy consumption, or vice versa. For example, mammary metabolic activity in pregnant rats was significantly increased in response to spaceflight (STS-70) but decreased under conditions of hypergravity (Plaut et al. 1999, 2003) (Figure 5-14, right). The reactivity of G6P-DH in the whole brain of young fish Oreochromis was increased after development in 3 g and decreased after development in simulated microgravity. Similar observations were made for the reactivity of SDH in brain nuclei connected to the vestibular system, such as the nucleus magnocellularis of larval fish.

On the other hand, altered gravity was ineffective at this period of development in nuclei that were not connected with the vestibular input, such as the pretectal nucleus (Figure 5-14, left). In the nucleus magnocellularis, cytochrome oxidase was also positively correlated with gravity and followed the relation: 0 g in orbit < 1 g in orbit and 1 g on the ground < 1.4 g < 3 g (Ankenetal. 1996, 1998).

Gravity Load (g) Gravity Load (g)

Figure 5-14. Sensitivity of metabolic activity to microgravity exposure. Left: SDH reactivity in the brain of young cichlid fish after 10-day exposure to microgravity, in normal gravity, and after exposure to 1.4 g and 3 g. The optical density of brain sections was used as a measure of SDH reactivity. Total brain sections (Brain) were compared with the area of the N. magnocellularis (Nm) that receives input from the vestibular (especially utricular) endorgan and the pretectal N. corticalis (Nc) of the retinohypothalamic system that is not a primary vestibular center of the brain. Note the strong effects of altered gravity in the Nm and the absence of any effect in the non-vestibular nucleus, and the opposite efficiency of microgravity and hypergravity (modifiedfrom Anken et al. 1996, 1998). Right: Mammary metabolic activity in relation to altered gravity. Rats were exposed to 9-day of microgravity during spaceflight (STS-70) or hypergravity up to 2 g on a centrifuge from day 11 to 20 of gestation. Within 2 h after the centrifuge was stopped, the abdominal mammary glands were removed, and metabolic activity was measured as oxidation of [U-14C] glucose to C02 (open symbols) or incorporation into lipid (closed symbols). The ratio between data from animals exposed to altered gravity and the 1-g controls was plotted against g-load. Note the opposite effects of microgravity and hypergravity. Adapted from K. Plaut et al. (2003).

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