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The Journal of Experimental Biology 204, 4107-4113 (2001)
© 2001 The Company of Biologists Limited

Simulated weightlessness alters the nycthemeral distribution of energy expenditure in rats

Stéphane Blanc1, Alain Géloën2, Sylvie Normand3, Claude Gharib1,* and Laurence Somody1

1 Laboratoire de Physiologie de l’Environnement, Faculté de Médecine Lyon Grange-Blanche, 8 Avenue Rockefeller, 69373 Lyon Cedex 08, France,
2 CNRS UMR 5578, Faculté de Médecine Lyon Nord, 8 Avenue Rockefeller, 69373 Lyon Cedex 08, France and
3 Centre de Recherche en Nutrition Humaine de Lyon, Faculté de Médecine Laënnec, 8 Rue Guillaume Paradin, 69372 Lyon Cedex 08, France

*Author for correspondence

Accepted September 17, 2001

The energy metabolism adaptations to simulated weightlessness in rats by hindlimb tail suspension are unknown. 12 male rats were assigned to 7 days of isolation, 7 days of habituation to the suspension device, 10 days of simulated weightlessness, and 3 days of recovery. The 24-hour energy expenditure was measured by continuous indirect calorimetry. We calculated the 12-hour energy expenditure during the active (night) and inactive (day) periods, the minimal observed metabolic rates with the day values taken as an index of the basal metabolic rate, and the non-basal energy expenditure representing the cost of physical activity plus the diet-induced thermogenesis. Suspension did not change the mean 24-hour energy expenditure (360.8±15.3 J min–1 kg–0.67), but reduced the night/day difference by 64 % (P<0.05) through a concomitant drop in night-energy expenditure and increase in day values. The difference between night and day minimal metabolic rates was reduced by 81 % (P<0.05), and the transient rise in day values suggests an early and moderate basal metabolic rate increase (9 %). An overall 19 % reduction in non-basal energy expenditure was observed during simulated weightlessness (P<0.05), which was mainly attributable to a reduction in the cost of physical activity. 3 days of recovery restored the night/day differences but increased the 24-hour energy expenditure by 10 % (P<0.05). In conclusion, hindlimb tail suspension in rats did not alter the 24-hour energy expenditure, but it transiently increased the basal metabolic rate, and altered both the energy expended on physical activity and the nycthemeral distribution of motor activity. These data suggest that the circadian rhythms of energy expenditure are affected during simulated weightlessness.

Key words: indirect calorimetry, microgravity, rat, energy expenditure, physical activity.


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© The Company of Biologists Ltd 2001