title: The hematological responses of rats exposed to conditions of simulated microgravity and acute exercise.
abstract: BACKGROUND
  Although humans have experienced microgravity since 1961, it is unknown whether PaO2 or PaCO2 will change in humans or in animals exposed to similar conditions. Reports from subjects participating in long-term head down tilt studies indicate that PAO2 will decrease and PaCO2 will increase, presumably because of impairments in oxygen delivery and carbon dioxide transport. To investigate this topic with suspended (HDS) rats, we hypothesized that 14 d of HDS would lower resting PaO2 and elevate PaCO2 pressures when compared with control 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 PaO2 and elevations in hydrogen ions.
  
  
  RESULTS
  Blood gas results during HDS indicated PaO2 and PaCO2 partial pressures were significantly decreased during the first week of suspension. Maximal exercise significantly increased PaO2 pressures in both animal groups, but during exercise the suspended rats exhibited significant increases in lactic acid and hydrogen ion concentrations when compared with control animals.
  
  
  CONCLUSIONS
  The suspended rat model effectively characterized PaO2 changes that have been reported for humans exposed to conditions of simulated microgravity. However, the decreases in VO2max reported for exercising humans and animals could not be explained by PaO2 changes and the rat model was not effective in predicting changes in blood PaCO2.
  
  
  CONCLUSIONS
  NASA should encourage and support studies that characterize PaO2 and PaCO2 change in humans and animals, in space and after they return to a 1-G environment.
authors.affiliation: Department of Physiology, University of Arizona, Tucson 85721, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Tipton
authors.forename: K A
authors.initials: KA
authors.lastname: Monnin
authors.forename: M C
authors.initials: MC
authors.lastname: Hall
authors.forename: L A
authors.initials: LA
authors.lastname: Sebastian
mesh_terms: D000818:Animals; D001784:Blood Gas Analysis; D002245:Carbon Dioxide / Q000097:blood*; D003430:Cross-Sectional Studies; D017079:Exercise Tolerance / Q000502:physiology; D019417:Hindlimb Suspension; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000097:blood; D008137:Longitudinal Studies; D008297:Male; D010100:Oxygen / Q000097:blood*; D010805:Physical Conditioning, Animal / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley / Q000097:blood*; D012146:Rest / Q000502:physiology; D018474:Weightlessness Simulation / Q000009:adverse effects*
chemical_list: D002245:Carbon Dioxide; D019344:Lactic Acid; D010100:Oxygen