554 ADVENTURES IN RADIOISOTOPE RESEARCH 



70 kgm. Urine samples were collected at intervals, and the activity of 



42K 



the samples was determined. As the ratio of can be 



total potassium 



assumed to be about the same in the plasma and the urine, the deter- 

 mination of the activity of the urine potassium informs us on the ^^K 

 content of the plasma potassium at the time of the formation of the urine. 



The urine samples were ashed below 400° and their activity was 

 compared with the acitivity of a standard preparation. This was prepared 

 by adding an aliquot part of the solution of labelled purified potassium 

 chloride taken by mouth to non-active urine. The ash samples weighed 

 about 300 mgm. They were placed under the Geiger counter in aluminium 

 dishes of 1.2 cm diameter. The potassium content of the samples was 

 kindly determined by Dr. L. Hahn, using Sohl and Bennet's method. 



We shall first consider the case that, in the course of the experiment, 

 no interchange takes place between the extracellular and the cellular 

 potassium. Then, the total ^-K absorbed from the intestine should 

 be present in the extracellular fluid of the body except for the 

 *2K excreted and the amount taken up by the corpuscles. The amount 

 of labelled potassium excreted in the course of 24 hours was found to 

 be about 7 per cent of the amount present in the body, while as was 

 found in the case of the rabbit and the rat, the corpuscles can be assu- 

 med to contain about as much ^^K as is present in the extracellular fluid. 

 Assuming the potassium content of the plasma to be 20 mgm per cent 

 <Wechselbaum et al. 1940) and the extracellular space to be 19 litres 

 {Brodie et al. 1939; Ivaltreider et al. 1940) in a human subject 

 weighing 70 kgm, the extracellular fluid can be estimated to contain 

 3800 mgm potassium. Since about !/> of the ^-K administered should 

 in the above case be present in the corpuscles, 1 mgm plasma (ex- 



46.5 „ 



tracellular) potassium will contain about = 0.012 per cent ot 



' ^ 3800 



the amount of ^^j^ administered. 



Let us now consider the other extreme case, viz. full interchange in 

 the course of 24 hours between extracellular and cellular potassium. 

 To estimate the percentage of ^^K administered which will be present 

 in 1 mgm plasma (urine) potassium, we have to estimate the potassium 

 content of the tissue of the human subject in question. The major part 

 of tissue potassium is found in the muscles. The average potassium 

 content of human muscles is stated (Cuming 1939; Mangus and Myers 

 1940) to be 330 mgm per 100 gm fresh weight. The weight of the muscles 

 was estimated, following a suggestion of Dr. Brandt Rehberg, from 

 the amount of creatinine excreted through the kidneys in the course of 

 24 hours. Dr. Rehberg most kindly determined the creatinine content 

 of the urine and found a daily excretion of 1300 mgm. Prior to these 

 determinsitions, the subject was kept on a vegetarian diet. In the deter- 



