448 ADVENTURES IN RADIOISOTOPE RESEARCH 



gastrocnemius. After the lapse of 4 days, the corresponding figures are 

 233 and 1653 per cent, respectively. The rate of penetration of phosphate 

 ions into the muscle cells of the frog is, thus, a very slow one even at 

 22° and still slower at lower temperature. After the lapse of 10 hours 

 at 0", the apparent phosphorus space of the gastrocnemius was found 

 to be 18 per cent ; thus, only about i/. as much labelled phosphate 

 diffused into the cells at 0° than 22°. 



As mentioned above, we arrive at the values stated for the amount 

 of labelled phosphate which migrated into the muscle cells by sub- 

 tracting from the total amount of ^2? found in the muscle the amount 

 of 32p present in the interspaces. The accuracy of the figures obtained 

 depends largely upon the accuracy of the figures assumed for the size 

 of the interspaces. 



In determining the extracellular volume we make two assumptions : 

 a) We assume that all sodium or chlorine present in the muscle cells 

 is exclusively found in the interspaces ; b) we assume the concen- 

 ration of sodium and chlorine, respectively, to be the same in the plasma 

 water and the extracellular fluid. Much evidence is available that 

 these assumptions are essentially correct. It is possible, however, that 

 a small amount of sodium or chlorine penetrates into the cells (Hastings 

 and EiCHELBERGER, 1937) and also that the extracellular fluid does 

 not show exactly the same sodium or chlorine content as the plasma 

 (comp. Manery et al. 1938). The size of the interspaces calculated 

 from the distribution figures of sodium and chlorine, respectively, is, 

 however, about the same. Fenn and Cobb (1936) found the average 

 sodium space and the average chlorine space of the rats' muscle to be 

 12.6 and 11.4 per cent, respectively. Manery and Hastings (1939) state 

 the apparent extracellular space of the gastrocnemius of the rabbit to be 

 11.3 per cent, calculated from the distribution of chlorine, and 11.0 

 per cent from the distribution of sodium, while for the abdominal 

 muscle they give the figures 16.3 and 13.9 per cent, respectively. 



We assume, furthermore, that an equal distribution of the labelled 

 phosphate between plasma and extracellular fluid takes place at an 

 early stage in the experiment. This assumption involves some uncer- 

 tainty. After the lapse of 3 minutes (see Table 1), an equal distribution 

 is far from being reached by either sodium or phosphate ; after 1 hour 

 equality may be reached also by the phosphate, but the possibility can- 

 not be excluded that the equality of the sodium space and the phosphate 

 space found after the lapse of 1 hour is a fortuitous one and is due to the 

 fact that some sappenetrates into the cells before the equipartition mention- 

 ed above was obtained, the sum of the cellular and extracellular P present 

 in 1 gm muscle making out 14 per cent of the ^ap content of 1 gm plasma. 

 In the case of the brain tissue, the capillaries of which are only at a slow 

 rate permeable to phosphate, we found obvious indications of a pene- 



