Degenerative Tissue Responses to Space-like Radiation Doses in a Rodent Model of Simulated Microgravity

This study examines acute and degenerative tissue responses to space-like radiation doses in a rodent model of simulated microgravity. We have studied four groups of rats, control (CON), irradiated (IR), irradiated and hindlimb suspended (IR-HLS), and suspended (HLS) that were maintained for two weeks. IR and IR+HLS groups were exposed to five sessions of X-ray irradiation (1.2 Gy each, at 3-4 days intervals). Body weights, soleus muscle weights, and hindlimb bone mineral density (BMD) were measured. Results show that compared to CON animals, IR, HLS, and IR+HLS group reduced the body weight gain significantly. IR-associated growth retardation appeared to be closely linked to acute and transient post-IR 'anorexia' (a decrease in food intake). HLS but not IR induced major changes in the musculoskeletal system, consisting in decreases in soleus muscle mass and bone mineral density of distal femur and proximal tibia. Additional dosimetric studies showed that the effect of IR on weight is detectable at 0.3 Gy X-ray doses, while no threshold dose for the IR-produced decrease in food intake could be observed. This study suggests that space flight-associated anorexia and musculoskeletal degenerative changes may be driven by different, radiation- and microgravity-associated (respectively) mechanisms.