PHILIP H. MITCHELL AND J. WALTER WILSON 53 



In addition, the experiment reported on the second Hne in Table 

 III included stimulation by the same method so that the muscles in 

 the course of 8 hours of perfusion gave 330 contractions of 1 second 

 each. The results of this experiment do not fall out of line with 

 those including no stimulation. Results therefore give no indication 

 of loss of potassium while the muscle is contracting, other than the 

 loss attributable to the presence of a potassium-free medium. 



Such results are in marked contrast to those obtained when muscle 

 is stimulated enough to cause loss of irritability. The considerable 

 difference between the potassium content of the rested and the 

 exhausted muscle as found in the following experiment seemed too 

 great to be explained on the basis of change in muscle weight. Both 

 legs were perfused with potassium-free Ringer solution during 2 

 hours. The muscles of one leg were given prolonged tetanizing stimuli 

 at j&rst through the nerves until they failed to respond and then by 

 direct stimulation until exhausted. The average potassium content 

 of the muscles of this leg was 0.141 per cent and that of the rested 

 muscles, 0.266 per cent. A similar experiment to determine the 

 potassium content of the muscle solids after exhaustive stimulation was 

 made. 1600 cc. of potassium-free Ringer solution were perfused 

 through both legs in 6j hours. The muscles of one leg were given 

 direct tetanizing stimuli for varying periods with intervening rest. 

 The total time of stimulation was 98 minutes. The average potassium 

 content of the muscles of this leg was 0.102 per cent; of the others, 

 0.221 per cent. Computed as percentages of the solids there was 

 0.89 per cent in the fatigued and 1.70 per cent in the rested muscles. 

 The stimulated muscles lost about half of their potassium. 



The development of acidity in a fatigued muscle suggests that 

 hydrogen ion concentration is a factor in the control of the processes 

 by which potassium is retained or set free in the cells. Experiments 

 to test this possibility will be the subject of a later communication. 

 For the experiments reported in this paper perfusion solutions were 

 in general adjusted with sodium bicarbonate, using Clark and Lubs 

 (13) indicators, to a pH of approximately 7.3. 



That muscular contraction does enable the cells to lake in potassium 

 is indicated by experiments on the absorption by muscles of rubidium 

 and cesium, substances very similar to potassium in chemical and 



