CIRCULATION OF PHOSPHORUS IN THE FROG 39 IJ 



incorporated inio the organic molecules. For example, creatinephosphoric 

 acid is degraded uikUm- splitting off of phosphate and resynthesized 

 under uptake of phosphate radicals. If labelled phospha1<> ions arc prc- 

 *?enl, they \vill have 1he same chance to be incorporated as have non- 

 labelled ones. J.,et us assume 10*^ free phospha1(> ions pr(\s(>n1 in 1 he 

 muscle cells to conlain 10 ^'^PO^ ions Avhiie from lo« P aloms isolated 

 from hexosemonophosphale of the muscle tissue only 1 is ^'I*, 1hen we 

 have to conclude that 10 per cent of the hexosemonophosphale^ mole- 

 cules were renewed during the experiment under incorporation of fre(> 

 phosphate. The ratios of the specific activities of the inorganic P and 

 the organic P are, thus, a measure of the extent of renewal of the organic 

 P compound which took place during the experimeni . When trying 

 1o arrive at quantitative data we encounter the following difficulties: 

 (a) The free phosphate extracted from the muscle tissue is partly cellular 

 and partly extracellular phosphate; it is, however, the specific activity 

 of the cellular phosphate only which is to be considered when calculating 

 the rate of renewal, (b) The specific activity of cellular phosphate changes 

 during the experiment, the change being due, for example, to an increas- 

 ing influx of labelled phosphate into the muscle cells. In this connection 

 it should also be mentioned that the method permits to distinguish 

 between renewed and non-renewed, between "old" and ''new" molecules,- 

 but no information is supplied on the point whether the molecules are 

 repeatedly renewed in the course of the experiment or not. 



As to point (a), to account for the share of the extracellular phosphate 

 in the total phosphate of the muscle tissue, we must know the specific 

 activity of the plasma phosphate which we assume to be identical with 

 the specific activity of the extracellular phosphate. We must also know 

 the phosphate content of the plasma and that of the muscle tissue and, 

 finally, the size of the extracellular space. The last mentioned magni- 

 tude can be determined in each case by administering simultaneously 

 with the labelled phosphate labelled soclium^^), or it can be assumed that 

 the extracellular space makes out 14 per cent of the weight of the muscles. 

 Another procedure Avhich mc used repeatedly is the following. We remove 

 one leg of the frog 1 hour after the start of the experiment and determine 

 the specific activity of the free muscle phosphate P. After further 3 hours, 

 we extract the phosphate of the other gastrocnemius and determine the 

 specific activity of the free phosphate P. If, within 1 hour, a proportional 

 partition of ^^p between plasma phosphate and the extracellular phos- 

 phate took place, then the increment of the specific activity of the tissue 

 phosphate between 1 hour and 4 hours is solely due to an increase in 

 the specific activity of the cellular inorganic P. By this method, we can 

 determine the percentage of cellular P which was replaccnl in \hr muscle 



fi) 



Comp. G. Heve.sy and (). Rebbe, Acta Physiol. Scand. 2, 171 (1940). 



