RATE OF PENETRATIOX OF PHOSPHATE INTO MUSCLE CELLS 449 



tration of ^^P into the cells before an equipartition of ^^P between plasma 

 and extracellular fluid was obtained. It is, therefore, of importance to 

 find a method which permits us to determine the amount of ^^p pene- 

 trating into the cells without having to make any assumption regarding 

 the size of the interspaces and the time involved in obtaining an equal 

 distribution of ^sp between plasma and extracelkilar fluid. Such a 

 method will be described in Ihe following section. 



DESCRIPTION OF THE MODIFIED METHOD 



When applying the modified method, we compare the activity of the 

 inorganic P of a plasma sample of known weight with the activity of 

 the organic phosphorus extracted from the muscle sample of the same 

 weight. This method is based on the assumption that the organic phos- 

 phorus compounds present in the muscle tissue are formed in the muscle 

 cells from inorganic phosphate and that, correspondingly, all active 

 P atoms present in the organic constituents of the muscle are such 

 which passed from the plasma into the cells as inorganic ^^^p. Since 

 some of the active phosphate penetrated into the cells will not have 

 had opportunity to be incorporated into organic molecules, but will 

 remain in inorganic state, the method here outlined will give a lower 

 limit for the extent of phosphorus exchange between plasma and muscle 

 cells. By adding to the activity of the organic P of the muscles that of 

 the cellular inorganic P we arrive at the total cellular activity. 



In experiments of several hours' duration or more, we can estimate 

 the amount of cellular inorganic ^ap by the following consideration. 

 Let us consider an experiment taking 10 hours. We find that, in this 

 experiment, 66 per cent of the ^sp content of the acid soluble organic P 

 of the muscle is present as creatinephosphoric acid P and that the 

 amount of creatinephosphoric P makes out 2.4 times that of the in- 

 organic P present as such in the muscle. The last mentioned data are 

 obtained by the usual chemical determination of creatine P and in- 

 organic P, respectively. Since from activity data we know that, in the 

 course of 10 hours, almost all the creatinephosphate molecules present 

 in the muscle get renewed, the ^^p content of 1 mgm inorganic P will 

 be about equal te the ^^p content of 1 mgm creatine P. From 

 these data it follows that the activity of the cellular inorganic P makes 

 out 66 : 2.4 = 28 per cent of that of the cellular acid soluble organic 

 P. We have, thus, to add to the values obtained for the acid soluble 

 organic cellular ^^p content of the muscle (see Table 3, column 2) 0.28 

 times the value obtained in order to get the value of the total ^^P mig- 

 rated into the muscle cells during 10 hours. In an analogous way the 

 other figures seen in column 2 of Table 3 were obtained. The correction 



29 Hevesy 



