GILBERT II. MUIK;K II7 



amined above have a real effect on potassium exchange is indicated by changes 

 in the values for unexchanged potassium. Thus, in table i, when the medium 

 concentration of potassium is raised from o to 125 mEq/1., there is an increase 

 in both the absolute level of mitochondrial potassium and in its specific activity. 

 However, in the experiment with high external potassium the absolute amount 

 of potassium which has not yet exchanged is significantly reduced. 



Active aerobic metabolism has been considered to play a significant role in 

 determining the ionic composition of the mitochondria. Under optimal aerobic 

 conditions of incubation the concentration of potassium in the particles was 

 about twice that obtained when either oxygen or substrate was omitted. In 

 another type of experiment mitochondria were labeled with radioactive potas- 

 sium during incubation at 25° C, and were then washed in the cold, one-half 

 with solutions of sodium chloride, the other with an equal amount of potas- 

 sium chloride. The same amount of isotope remained in both sets, thus demon- 

 strating the failure of mitochondrial potassium to exchange with that of the 

 cold washing fluid which represented about a thousand-fold excess of potassium 

 for each wash (17). 



In attempting to characterize the nature of mitochondrial potassium, the 

 type of experiment must constantly be kept in mind. Although it can not be 

 claimed that the experiments reported above have completely resolved the dif- 

 ferences between the studies cited, these results nevertheless indicate the role 

 played by some of the variables. At present the findings indicate that the 

 mitochondria contain a rather small amount of residual potassium. The ab- 

 solute level and rate of exchange of this fraction are at least in part metaboli- 

 cally determined. In addition, the mitochondria appear to be freely permeable to 

 the potassium ion so that the mitochondrial level increases as the external 

 concentration is elevated. This type of reaction is particularly evident from 

 the studies of Bartley and Davies (i) who found mitochondria medium ratios 

 of potassium slightly greater than unity^ over a wide range of external potas- 

 sium concentrations and under extremes of metabolic activity. 



The role of metabolic activity in determining the mitochondrial electrolyte 

 content remains diflicult to evaluate in precise terms. Some of the correlations 

 have been summarized above, but they fail to describe the mechanisms by 

 which the electrolyte content is related to any metabolic event. Evidence has 

 been reported from studies on intact cells that metabolism may be concerned 

 with the generation or orientation of a hypothetical ion-carrier complex of un- 



^ The use of concentration ratios as a basis for expressing the experimental data may at 

 times be somewhat misleading. For examjjle, in all the experiments reported from our labo- 

 ratory in which the mitochondria were washed with potassium-free solutions, the concentra- 

 tion ratio of potassium between the mitochondria and the final wash solution would be im- 

 measureably high in all instances, regardless of the absolute concentration of electrolyte in 

 the mitochondria. 



