THE HEMATOLOGICAL RESPONSES OF RATS EXPOSED TO CONDITIONS OF SIMULATED MICROGRAVITY AND ACUTE EXERCISE

Background: Although humans have experienced microgravity since 1961, it is unknown whether Pao(2) or Paco(2) will change in humans or in an imals exposed to similar conditions. Reports from subjects participati ng in long-term head down tilt studies indicate that PAO(2) Will decre ase and Paco(2) will increase, presumably because of impairments in ox ygen delivery and carbon dioxide transport. To investigate this topic with suspended (HDS) rats, we hypothesized that 14 d of HDS would lowe r resting Pao(2) and elevate Paco(2) pressures when compared with cont rol rats. Since returning astronauts, previously bed rested subjects, and suspended rats had exhibited decreases in maximum aerobic capacity , we predicted after 2 weeks of HDS, rats performing maximum exercise would demonstrate significant decreases in Pao(2) and elevations in hy drogen ions. Results: Blood gas results during HDS indicated Pao(2) an d Paco(2) partial pressures were significantly decreased during the fi rst week of suspension. Maximal exercise significantly increased Pao(2 ) pressures in both animal groups, but during exercise the suspended r ats exhibited significant increases in lactic acid and hydrogen ion co ncentrations when compared with control animals. Conclusions: The susp ended rat model effectively characterized Pao(2) changes that have bee n reported for humans exposed to conditions of simulated microgravity. However, the decreases in Vo(2)max reported for exercising humans and animals could not be explained by Pao(2) changes and the rat model wa s not effective in predicting changes in blood Paco(2). Recommendation s: NASA should encourage and support studies that characterize Pao(2) and Paco(2) change in humans and animals, in space and after they retu rn to a 1-G environment.