532 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 28 



the vertebrate, the essential nature of strontium as a trace element has not 

 been established. 



Studies with Sr 85 have shown that, when injected, the element rapidly 

 concentrates in the skeleton. In the mouse the radioactivity per unit wet 

 weight of skeleton is about 100 times that of the soft tissues in 24 hr. Of 

 the soft tissues the liver and especially fat show the lowest activity at this 

 tirne [Srl9,20]. 



Studies on bone-fracture healing in normal rats and in rats on abnormal 

 vitamin balance have shown that recalcification is less active in vitamin A- and 

 D-deficient and D-hypervitaminotic rats. In normal rats the callus under- 

 goes most active calcification within from 8 to 16 days, normal strength of the 

 broken bone being recovered at 12 to 16 days [Sr4]. In another experi- 

 ment it was found that hypophysectomized rats treated with growth hor- 

 mone deposited in a short period essentially the same amount of Sr 85 in 

 the femur and mandible as did untreated hypophysectomized rats. This 

 suggests that mineral deposition in bone is independent of growth hormonal 

 control [Srl7]. 



For the therapeutic application of Sr 85 to bone malignancy see Chap. 29. 



i. Columbium. There is suggestive evidence that columbium is an essen- 

 tial trace requirement in certain plants. Tracer work has, however, been 

 very limited and so far carried out with Cb 95 (37-day half-life). It has been 

 used in radioactive colloids that localize in the reticuloendothelial system 

 [Cbl, see Chap. 29] and in one experiment with the tracer-free element [Cb2] 

 in which it was shown that in the rat parenterally administered columbium 

 accumulated about 40 per cent in the bone and was lost therefrom with a 

 half-time of about 100 days. (For further discussion of the uptake in bone 

 of fission elements see Sec. 28.5.) 



j. Tin. Although tin has been found as a trace element rather widely 

 in animals, no essential physiological role has been demonstrated. Tracer 

 work is yet to be done. There are at least three potentially useful tracer 

 isotopes: the radioactive species Sn 113 (70- to 105-day half-life), Sn 121 (60-hr 

 half-life), and Sn 123 (10-day half-life). 



k. Tungsten. Tungsten has been thought to be essential as a micro- 

 nutrient of certain plants, but no tracer work has yet been attempted with it. 

 There are available two potentially useful radioactive species: W 185 (77 day 

 half-life) and W 187 (24.1-hr half-life). 



28.3. Elements of Importance in Pharmacology and Toxicology. The 

 common elements of toxicological and pharmacological importance not 

 already considered are arsenic, silver, antimony, gold, mercury, lead, bismuth, 

 and radium. Tracer studies have been carried out for all of these except 

 silver. 



