RATE OF PENETEATIOK OF PHOSPHATE INTO MUCSLE CELLS 445 



the extracellular volume of the muscle. The amount of extracellular 

 phase (E) in percent of the weight of the muscle is calculated (Manery 

 and Hastings, 1939) from the equation 



^_ rNa)^- 0.97. 100 



(24Na) . 0.99 



in which the subscripts ni and p represent muscle and plasma, respec- 

 tively (i>. 



The size of the interspaces being known, the measurement of the 

 distribution of ^sp between plasma and muscle permits us, as described 

 above, to determine the amount of ^^P which penetrates into the muscle 

 cells. 



A simultaneous measurement of the radioactivity of sodium and 

 phosphorus is made possible by the fact that 24Na decays with a half-life 

 period of 14.8 hours while ^sp decays with a period of 14.5 days. When 

 measuring the activity of the sample, for example, two weeks after the 

 start of the experiment, the 24Na originally present in the tissue and 

 the plasma is entirely decayed and the activity measured is solely due 

 to the 32p content. Let us say we measured at that date 10 counts per 

 minute, then two weeks previously the activity of the ^sp of the prepa- 

 ration was 20 counts. Assuming we measured, two weeks previously, 

 a total of 100 counts, then out of these 80 counts were due the ^^N'd 

 content and 20 counts to the ^ap content of the gastrocnemius sample. 

 The accuracy of the determination can be augmented by administering 

 a preparation which is showing a strong 24>j^a and a comparatively 

 weak =^2p activity. In view of the variability of the size of the interspaces 

 in different muscles and in different frogs (comp. Eggleton ef al. 

 1937), it can be of importance to determine the extracellular volume 

 of the muscle the permeability of which to phosphate ions is to be 

 determined. 



In experiments of short duration, the P activity of the plasma is 

 solely due to the presence of radioactive inorganic P, the amount of 

 radioactive phosphatides present being negligible and the plasma con- 

 taining but an insignificant amount of acid soluble organic P. In ex- 

 periments of very long duration, we are not permitted to consider the 

 total activity of the plasma, but we have to extract the plasma inor- 



^1 When carrying out the calculation mentioned above, we assume that the 

 extracellular phase is identical with the ultrafiltrate of serum. The water content 

 of the extracellular phase is assumed to be 99, that of the plasma 95 per cent. 

 We arrive at the figure 0.97 by taking into consideration that the sodium ion 

 concentration of the plasma and its ultrafiltrate somewhat differs and by 

 calculating the difference from the Gibbs-Donnan equation. 



