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IS ANIMAL AGE A FACTOR IN THE RESPONSE OF BONE TO SPACEFLIGHT?  E.R. Morey-Holton¹, L. P. Garetto², S.B. Doty³, B.P. Halloran⁴, and R.T. Turner⁵.  ¹NASA Ames Research Center, Moffett Field, CA, ²Indiana University School of Dentistry, Indianapolis, IN, ³Hospital for Special Surgery, New York, NY, ⁴VAMC, San Francisco, CA, and ⁵Mayo Clinic, Rochester, MN.

   The rodent bone response to spaceflight may be influenced by a multi- tude of factors including flight duration, strain, and housing. Review of bone formation rates during spaceflight suggests that age may also play a role in the response. Weanling rats show fewer bone changes than older rats. To determine if the long bones of weanling rats were insensitive to weight-bearing, a hindlimb unloading experiment was conducted simultaneously with a 9d shuttle flight in 34d old group-housed male rats.  All animals were injected with bone markers 7d and 1d before flight and euthanized at landing, 24hr, and 72hr following recovery.  If no differences in body weight, bone length, or bone formation at the tibiofibular junction were noted at the different time points, data were combined for each group.  No significant differences in body weight were found at any time period among the groups.  The humerus, tibia, and femur elongated significantly during the flight period with no difference in lengths between groups at the end of the flight period.  The group-housed flight rats showed no change in cortical bone formation rate compared to preflight values, flight controls, or vivarium controls.  However, the hindlimb unloading group showed a significant 30% decrease in bone formation rate compared to all other groups. Individually-housed 38d old animals flown for 14d showed ~10% suppression of cortical growth. Older singly-housed flight animals appear to show equal or greater bone changes compared to hindlimb unloaded rats. We speculate that the mechanical threshold required for cross-sectional bone growth is reached in group-house weanling rats during spaceflight, perhaps through physical interactions, and that weanling animals are sensitive to loading.  However, the threshold is not fully reached in either singly-housed flight or hindlimb unloaded weanling rats.  We conclude that age, housing, flight duration, and strain have important roles in rodent skeletal responses to spaceflight.

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