522 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 26 



[K17,52]. In tissues with rapid penetration — liver, heart, kidney, etc. — the 

 total potassium has a relative activity higher than the simultaneous plasma 

 value for the first 1 to 2 hr after administration, particularly after intra- 

 peritoneal injection. 



The penetration of K + into erythrocytes has been investigated by several 

 workers [Kll-13,40,45,46,50,59]. As with Na + a considerable species 

 difference in permeability and exchangeability has been shown. 



The physiology of the adrenal cortex has also been studied with K 42 

 [Kl-5]. 



Long-term tracer studies of potassium metabolism may become possible 

 now that the rarer naturally occurring species, K 40 (radioactive) and K J1 

 (stable), can be concentrated in a practical manner, or if an artificial species 

 of long half-life is discovered. 



26.4. Calcium. Calcium, the most abundant positively ionized mineral 

 element of the mammalian body, is largely found in bone, but the small 

 amount occurring in the plasma and inside cells is of great physiological 

 significance — in neuromuscular irritability, blood coagulation, cellular per- 

 meability, and certain enzymatic systems. In tracer work the one radio- 

 active species so far used, Ca 45 (180-day half-life), has had rather limited 

 application because of poor cyclotron yields from the reaction Ca 44 (d, p)Ca 45 . 

 It is now, however, available in much larger quantities from the nuclear pile 

 by the reaction Ca 44 (n, 7)Ca 45 and should have greatly increased utilization. 

 The cyclotron reaction producing Ca 45 also results in some Ca 41 (8.5-day 

 half-life) by Ca 40 (d, p)Ca 41 . However, this latter species decays by K cap- 

 ture and emits only feeble x-rays, which are difficult of detection. Ca 45 can 

 presumably be prepared in a carrier-free state in the nuclear-pile reactor by 

 the reaction Sc 45 (n, p)Ca 45 . 



Studies with Ca 45 have so far been limited to a few investigations, pri- 

 marily on the uptake of calcium by calcified tissues. Ca 45 is stored almost 

 entirely in bone [Ca2,7,9] with traces appearing throughout the soft tissue. 

 Vitamin D promotes the absorption of calcium from the digestive tract and 

 the mineralization of the bones of rachitic rats [Ca4]. 



Much work on bone metabolism has been done by substituting radioactive 

 strontium for calcium by reason of the similar metabolic behavior of these 

 two elements. For citations to this work see Sec. 28.2 h. 



26.5. Magnesium. Despite the important biological role of magnesium, 

 as an essential element for all known forms of life, it has scarcely been studied 

 by tracer isotopes. The radioactive species of longest half-life Mg 24 , pro- 

 ducible in the cyclotron by the reaction Mg 26 (d, p)Mg 27 , has a half-life of 

 only 10.2 min. It has been used in but one biological experiment [Mgl] in 

 which its incorporation into chlorophyll by an alga and by barley was shown. 

 By the time chlorophylls a and b could be separated, however, the activity 





