114 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



mitochondrial potassium exchanges slowly and that this reaction is metaboli- 

 cally dependent, while Bartley and Davies (i), in independent studies with 

 mitochondria derived from sheep kidney cortex, reported an extremely rapid 

 exchange which appeared to be independent of metabolic activity. Since it is 

 the present writer's opinion that the different results obtained by these two 

 studies are in large measure due to differences in the experimental conditions, 

 some of the experimental variables will be described in detail as illustrative of 

 difficulties inherent to this type of in vitro study. 



Potassium exchange was studied by Stanbury and jNIudge by incubating 

 the mitochondria in a medium contaning small amounts of a-ketoglutarate as 

 substrate, 20 mM/1. of 'Tris' buffer, KCl in a final concentration of 25 mEq/L, 

 and approximately an equal amount of NaCl. Incubation was at 25° C with 

 oxygen in the gas phase. After incubation, the mitochondria were washed three 

 times with large volumes of cold 0.15 m/1. NaCl. Under these conditions the 

 level of mitochondrial potassium remained constant for as long as 100 minutes of 

 incubation. When radioactive potassium was added to the suspending medium, 

 as much as 80 per cent of the mitochondrial potassium was found to have ex- 

 changed with that of the medium and the half-time for exchange was approx- 

 imately 30 minutes. Bartley and Davies employed an incubation medium con- 

 taining a small concentration of sodium a-ketoglutarate, and potassium in a 

 final concentration of about 140 mEq/1. After incubation the mitochondria were 

 separated from the suspending medium by a single centrifugation at 20° C. 

 They found that mitochondrial potassium exchanged completely with that of 

 the medium within less than two minutes. 



Whether there is a species difference in the electrolyte metabolism of mito- 

 chondria has not been systematically examined. Preliminary studies in our 

 laboratory have failed to demonstrate significant differences between par- 

 ticulates obtained from kidney and those from liver. 



As shown in table i, as the potassium concentration in the external medium 

 is increased, there is a moderate increase in thelevel of mitochondrial potassium, 

 but a much more striking increase in its rate of exchange as estimated from the 

 amount exchanged in a fixed interval of time. This latter effect might be due 

 in part to contamination of the mitochondria by radioactive potassium from 

 the medium due to incomplete washing. That this is a negligible factor is in- 

 dicated by estimations of the mitochondrial potassium which had not yet ex- 

 changed by the end of the experiment; since this value decreases, in absolute 

 terms, as the external potassium is raised, it follows that the high ambient 

 potassium level has a real effect on the rate of potassium exchange. Under 

 these conditions variations in the level of potassium are not associated with any 

 consistent change in oxygen consumption. 



Bartley and Davies recovered their mitochondria from the incubation me- 

 dium by a single centrifugation at high speed without the addition of any wash- 



