56 ABSORPTION AND RETENTION OF POTASSIUM 



SUMMARY. 



1. Individual variations in the potassium content of the fresh 

 muscles of frogs are notable even.when computed as percentages of 

 the dry solids. The potassium content averaged higher in freshly 

 collected summer frogs than in winter frogs after a period of captivity. 



2. Muscles show a loss of from 8 to 15 per cent of their potassium 

 during perfusion with potassium-free Ringer solution but tena- 

 ciously hold the remainder. 



3. Muscles, stimulated to contract under conditions that do not 

 produce irreversible stages of fatigue, show losses of potassium no 

 greater than those attributable to the presence of a potassium-free 

 medium. 



4. A condition favorable to the taking up of potassium probably 

 occurs in a contracting muscle because rubidium and cesium, 

 substances very similar to potassium in chemical and physiological be- 

 havior, are absorbed in retainable form by a contracting muscle but 

 not by a resting one. 



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