p. H. MITCHELL, J. W. WILSON, AND R. E. STANTON 147 



employed. The tissues of this animal showed a high content of 

 cesium. 



The considerable quantity of rubidium or cesium in the rat muscle, 

 as shown in some of the experiments, is striking. Computed in 

 actual concentration, it amounts in three of the experiments to about 

 half the concentration of potassium as given for normal mammalian 

 muscle (0.32 to 0.42 per cent). This looks like an actual replacement 

 of potassium by rubidium or cesium. We have not yet undertaken the 

 difficult and somewhat uncertain methods for determination of potas- 

 sium in the presence of such quantities of the interfering substances, 

 rubidium and cesium. The spectroscope, beyond revealing qualita- 

 tively that some potassium was present, would not serve the purpose. 

 We cannot, therefore, draw any conclusions as to whether rubidium 

 and cesium. are taken into the cell in the place of potassium or in 

 addition to it. It is interesting to note that rubidium and cesium 

 can replace potassium with considerable success in furthering excita- 

 bility as studied by Zwaardemaker (7) and his coworkers, in antago- 

 nistic salt reactions as shown by Loeb (2), and in permitting the develop- 

 ment of Arhacia eggs as shown by Loeb (8). All of these effects 

 may be due, although proof is not entirely adequate, to action at 

 cell surfaces. In contrast to this, a prolonged action of rubidium 

 and cesium, under circumstances permitting their incorporation into 

 the cell would seem from our observations to constitute physiologically 

 a less successful substitution for potassium. 



SUMMARY. 



1. Frog muscles perfused with Ringer solution in which potassium 

 chloride has been replaced by an equivalent amount of rubidium or 

 cesium chloride take up rubidium or cesium and incorporate them 

 into the tissue substance in such form as to be retained during a 

 subsequent perfusion with potassium-free Ringer solution, provided 

 the muscles contract during the first perfusion. Retention of rubid- 

 ium or cesium by a resthig muscle does not occur. 



2. Rats on synthetic diets, adequate in all respects except that 

 potassium was replaced by an equivalent amount of rubidium or 

 cesium, died after a period varying from 10 to 17 days with charac- 

 teristic symptoms including tetanic spasms. Muscle, heart, liver, 



