Sec. 26.3] MAJOR MINERAL METABOLITES 521 



mal. The time of equilibration of sodium injected in man has been found 

 with Na 24 to be between 9 and 12 hr [Na51]. Initially the spread is rapid — ■ 

 for 2 or 3 hr — to a volume of fluid representing about one-fourth of the body 

 weight. It has been suggested that this represents the extracellular fluid 

 volume. 



Na 24 has had wide application in studies of vascular and cellular permea- 

 bility. It has been possible to demonstrate that the turnover between the 

 blood sodium and that of the intestinal lumen is very large [Na85,86], being 

 equal in the dog to the total plasma sodium in about 83 min. In connection 

 with the passage of sodium across the placental barrier in several species, 

 including man, it has been found that the transfer per unit weight of pla- 

 centa increases greatly as gestation proceeds [Nal8-24, 27-30]. The pene- 

 tration of Na + into erythrocytes has been investigated by a number of 

 workers [Na9,56,90] and a considerable species difference shown to exist in 

 the rate of this process. In general, however, sodium can be demonstrated 

 to diffuse in and out of the red cell. 



The role of the mammalian adrenal in sodium metabolism has also been 

 investigated with Na 24 [Nal-5]; and Na 24 has been used in the experimental 

 therapy of human and mouse leukemia (see Chap. 29). 



With Na 22 much longer term experiments are of course possible than with 

 Na 24 . So far this isotope has had relatively limited application. It has 

 been used to demonstrate the increase in the sodium space of the rat body 

 under conditions of dietary chloride deficiency [Nail] and to show the great 

 depression of sodium excretion in congestive heart failure in man — as much 

 as /50 °f the normal clearance [Na71]. 



26.3. Potassium. Potassium at the present time has but one useful species 

 for biological tracer work, K 42 (12.4-hr half-life), produced in the cyclotron 

 largely by the reaction K 41 (d, p)K 42 and in the nuclear pile by the reaction 

 K 41 (n, 7)K 42 . K + is an almost universally essential intracellular cation; in 

 the mammal it serves as the main cation within the cells in contrast with 

 Na + in the extracellular fluids. In ascending the periodic system potassium 

 is the first element that has a naturally occurring, rare, long-lived radioactive 

 isotope; this species, K 40 (about 10 9 -year half-life), has been speculated upon 

 as conferring on potassium some of its physiological properties [K18,36,42,51], 

 but there has been no adequate confirmation of any of these views [K18]. 

 Some evidence exists, however, that biological systems may deal with the 

 isotopes of potassium in a slightly different manner [K16,35]. 



The half-life of K 42 limits it to short-term experiments. Nevertheless, a 

 number of studies of potassium uptake [K18,24] and distribution [K15,17,21, 

 28,52] have been carried out. In mammals injected with labeled K + , the 

 ion penetrates rapidly into most tissues of the body, the testes, brain, and 

 erythrocytes being slowest to exchange intracellular with plasma potassium 



