PHYSICAL AND BIOLOGICAL FACTORS 935 



Evans, 1950), and exhaustive exercise (Stapleton and Curtis, 1946; 

 Kimeldorf et al, 1950) increase susceptibility to radiation as to most 

 noxious stimuli. Some degree of radioresistance may be acquired as a 

 result of previous exposures to /3 or X radiation (Raper, 1947; Cronkite 

 et al, 1950; Bloom, 1950), pretreatment with horse serum (Hektoen, 

 1918; Graham et al, 1950), estrogens (Treadwell et al, 1943; Patt, 

 Straube, et al, 1949), and adrenal cortical extract (Graham et al, 1950), 

 and by immunization (Kohn, 1949). There are no adequate explanations 

 of these phenomena. In the case of the estrogens it is believed that the 

 decreased toxicity is related to stimulation of myeloid activity following 

 the initial estrogen-produced depression since the pretreated animals 

 show rapid recovery from radiation granulocytopenia but not from 

 lymphopenia (Patt, Straube, et al, 1949). Along these lines, an increased 

 resistance of erythropoietic tissues has been observed following the pro- 

 duction of a regenerative anemia by phlebotomy, administration of phen- 

 ylhydrazine (Jacobson et al., 1948), or exposure to high altitude (Schack 

 and MacDuffee, 1949). 



TEMPERATURE AND METABOLIC RATE 



There is general agreement that the changes incidental to irradiation 

 are dependent upon the rate of metabolism. This is consistent with 

 observations made in other types of injury and is not unreasonable in 

 view of the apparent nonspecificity of radiobiological effects. An 

 increased metabolic activity after exposure to ionizing radiation induced 

 by thyroid extract or dinitrophenol (W. W. Smith and F. Smith, 1951; 

 Blount and Smith, 1949), a cold environment in nonacclimatized mammals 

 (W. W. Smith et al., 1949 ; Hempelmann et al., 1949), or exhaustive exercise 

 (Stapleton and Curtis, 1946; Kimeldorf et al, 1950) enhances lethality, 

 while depression of metabolic rate, at least in poikilotherms and isolated 

 tissues (Patt and Swift, 1948; Allen et al, 1950; Schrek, 1946; Cook, 

 1939), decreases the rate of development of radiation damage. Surpris- 

 ingly, administration of thiouracil and thyroidectomy appear to have 

 little influence on lethality in adult mice and rats (W. W. Smith and F. 

 Smith, 1951; Blount and Smith, 1949; Hempelmann et al, 1949; Haley 

 et al, 1950). 



Evidence that relates to the effect of temperature during exposure 

 to radiation is equivocal, and virtually every conceivable effect has been 

 described. Sensitivity of the eggs of Ascaris (Holthusen, 1921) and of 

 Drosophila (Packard, 1930) is increased by elevation in temperature 

 during irradiation. Yet temperature is without influence on E. coli 

 (Lea, 1946), thymic cell suspensions (Schrek, 1946), wheat seedlings 

 (Henshaw and Francis, 1935), or eggs of the frog and hen (Ancel and 

 Vintemberger, 1927). Moreover, cold is reported to increase the sensi- 

 tivity of the roots of Vicia faba and of certain tumor cells to the growth- 



