444 ADVEIfTURDS IN RADIOISOTOPE RESEARCH 



DESCRIPTION OF THE METHOD 



Sodium phosphate of neghgible weight containing radioactive P as 

 an indicator is introduced into the circulation of the frog (injected into 

 the lymph sack). After the lapse of, for example, ten hours, we compare 

 the labelled phosphorus (^^P) content of a plasma sample and of a gas- 

 trocnemius sample of the same weight by determining their radio- 

 activity. Let us assume that 1 gm plasma is found to be 5 times more 

 active than 1 gm muscle tissue and the interspaces to make up Vio of 

 the muscle's weight, then the amount of phosphate ions which pene- 

 trated from 1 gm plasma into the cells of 1 gm muscle works out to be Vio 



m 

 of that present in 1 gm plasma or, in general, is — — i p, where m denotes 



the activity of 1 gm muscle, p the activity of 1 gm plasma, and i the 

 size of the interspaces as a fraction of the muscle weight. When carrying 

 out the calculation given above, we assume that ^ap becomes equally 

 distributed between plasma and interspaces in an early stage of the 

 experiment. How far this assumption is justified will be discussed later. 



The experiment described above is carried out under strictly physio- 

 logical conditions ; the phosphorus content of the plasma and the 

 muscle remains practically constant during the experiment and we can, 

 therefore, conclude that the penetration of phosphate ions from the 

 plasma into the muscle cehs was followed by a migration of an equal 

 number of phosphate ions from the muscle cells into the plasma. If 

 an equilibrium is reached with an uptake of less ^sp than corresponding 

 to the total water content of the system, we must assume saturation of 

 a certain fraction of the tissue but, when the relative concentration 

 of ^^P continuously increases in the muscle, we can utiKze the results 

 to measure the rate of exchange between cehular and extracellular 

 phosphorus. 



The application of the method outlined above requires the knowledge 

 of the extent of the interspaces of the muscle tissue. The size of the 

 interspaces can be obtained by comparing the chloride or sodium content 

 of muscle and plasma samples of the same weight or by other methods. 

 When applying the first mentioned method, the assumption is made 

 that all sodium and chlorine present in the tissue is to be found in the 

 interspaces. Fenn and Cobb (1935) state for the chlorine space of the 

 sartorius of rana pipiens values varying between 7.5 and 16.0 per cent, 

 the average being 11.3 per cent. 



To determine the size of the extracellular space of the gastrocnemius 

 of the Hungarian frog (Rana esculenta) used in our experiments, we 

 administered labelled sodium along with the labelled phosphate. By 

 measuring the distribution of the labelled sodium ^^Na between plasma 

 and fresh gastrocnemius of equal weight, we arrive at a figure indicating 



