68 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



clear that a given dosage of an alpha-emitting bone seeker such as radium 

 would produce more radiation response in the tissue than would a dosage 

 of the beta-emitting bone seeker Sr^^. It is interesting to note that, 

 although Sr^^ was less effective lethally than P^^^ ^^g former produced 

 more response in the tissues studied. Observe that radiosodium, which 

 is generally distributed throughout the body, had the lowest lethal action. 

 It must be remembered that these are histologic observations only, and it 

 is probable that physiologic dysfunctions could occur at lower levels. 

 Although the time factor is an important one, it is not practical to discuss 

 the observations of Bloom as a function of time after dosage, which varied 

 from a day up to several months. In any event, it would be wisest, where 

 possible, to use amounts of radioisotopes well below those known to have 

 produced injury, as reported in Table 2-1. Care must be taken partic- 

 ularly in connection with bone studies if the experiments are to be pro- 

 longed. In this laboratory there was a suggestion of bone pathology in a 

 700-lb steer sacrificed 1 yr after having ingested about 600 nc Sr^°. 



Bloom and Bloom (18) have described definite bone overgrowths in 

 mice at 5 months after intraperitoneal injection of 0.03 fxc radium per 

 gram body weight. Similarly, as little as 0.003 /xc plutonium per gram 

 body weight administered intravenously produced marked bone changes. 

 Moses et al. (19) reported that ingestion by rats of 0.1 mc P^" per gram 

 body weight caused a temporary decrease in total white count but pro- 

 duced no histologic or rate-of-growth changes in the kidneys, liver, adre- 

 nals, and testes. Warren, MacMillan, and Dixon (20, 21), using mice, 

 have described the effects of subcutaneous injection of various levels of 

 P^-. Their observations on the lowest level, about 0.8 ixc/g body weight, 

 showed significant changes in lymphoid tissue, spleen, lymph nodes, thy- 

 mus, and ovaries. It has been reported that in man the threshold dose 

 of P^- is 0.006 to 0.009 fxc/g body weight so far as effects on the formation 

 of blood corpuscles are concerned (22). 



Considerable attention has been given to responses to P^^ on account 

 of the importance of this isotope in medical and physiological studies. 

 Maloof, Dobyns, and Vickery (23) have reviewed earher work and dem- 

 onstrated that as little as 0.045 ixc/g body weight administered to rats 

 caused a definite impairment of function even though there was no his- 

 tologic evidence of radiation damage in the thyroids of these animals. No 

 functional abnormalities were noted in animals receiving about 0.009 fxc/g. 

 The functional changes with the dose of 0.045 fxc/g were as follows: (a) 

 The thyroid lost ability to respond to a low iodine diet, (b) The ability 

 of the thyroid to enlarge in response to low iodine stimulus was decreased. 

 (c) The capacity of the thyroid to hypertrophy was impaired. Skanse 

 (24) reported dysfunction of the chick thyroid after administration of 

 10 fjLC radioiodine. As indicated earlier, particularly with iodine, the 



