422 PHYSIOLOGICAL FACTORS IN MAMMALS 



exchanged and the tissue bridge. In the measured exchange of radio- 

 active sodium (14) it was found that the sodium equihbration between 

 parabiotic animals was 3.5-4.5 times slower than the exchange of labeled 

 red cells. Thus, the distribution of material in the body is another 

 factor which must be considered when using the parabiont as a test 

 animal. Indeed, it must be said that negative results found in the para- 

 biotic state should never be used to indicate an absence of indirect ef- 

 fect of radiation, since only relatively stable, relatively unexcreted, rel- 

 atively slowly metabolized substances have a chance of crossing the 

 parabiotic bridge in concentrations approaching the level in the irradi- 

 ated member of the pair. Van Dyke and Huff (15) have shown that, 

 when a parabiont is irradiated (900 r) with one member shielded, both 

 become epilated and the non-irradiated member is relatively more epi- 

 lated than the other. The effect occurs late, 24 days after irradiation, 

 and reaches a maximum at 30-60 days. If this observation is evidence 

 for a specific substance producing epilation, the substance must have a 

 long latent period or continue to be produced in considerable quantity 

 long after exposure to radiation. The irradiated members of the pairs 

 are obviously stunted, and the observed effects, as the authors (15) 

 point out, may be a "manifestation of loss of critical metabolic material 

 from the non-irradiated to the irradiated member." 



Changes in the Environment 



The effect of work and fatigue upon ability to survive irradiation has 

 been evaluated by Stapleton and Curtis (16) and Kimeldorf et al. (17). 

 Both indicate an increase in the lethality of irradiation when experi- 

 mental animals are worked to exhaustion after irradiation. Probably 

 moderate physical activity does not lessen chance of survival from whole- 

 body irradiation. 



Stapleton and Curtis have evaluated physical state after a median 

 lethal dose (neutron radiation exposure) and find a marked decrease in 

 ability of rats to do forced work (16). Kimeldorf has measured the 

 survival of rats that have been swum to exhaustion once a day after radi- 

 ation exposure. In 860- and 700-r whole-body-irradiation groups, the 

 rats which were swum daily died 2-4 times faster than the non-swum 

 animals. At 600 r half the swum group died but all the non-swum 

 lived (Fig. 4). The approximate influence of this severe exercise seems 

 to have been to produce effects that closely simulate, as seen in survi- 

 val curves, those of radiation dosages 150-200 r greater. These results 

 are for very severe exercise carried to exhaustion. Probably moderate 

 physical activity would not lessen chance of survival significantly. 



