Electrolyte Transport in Isolated Mitochondria^ 



GILBERT H. MUDGE 



Department of Medicine, College of Physicians and Surgeons, Columbia 

 University, and the Presbyterian Hospital, New York City 



FOR MANY YEARS biologists havc considered the possibility that the intra- 

 cellular particles known as mitochondria might play a role in secretory 

 processes. Much of the supporting evidence has been based on cytological ex- 

 periments in which changes in the morphological characteristics of the mito- 

 chondria have been correlated with various types of cellular activity. It is not 

 the purpose of this paper to review these cytological studies, but rather to con- 

 sider a different type of approach employed in recent years — namely, the direct 

 determination of the chemical composition of mitochondria which have been 

 isolated from tissue homogenates and which have then been subjected to con- 

 trolled experimental procedures under /// vitro conditions. Such an approach 

 permits the examination of the behavior of the mitochondria in a two-phase 

 system — the particles being one phase and the suspending medium the other. 

 These two phases can be separated by centrifugation and then analyzed, and 

 some of the problems inherent to histological techniques are thus avoided. 

 However, as shall be emphasized in considerable detail, the preparation of 

 tissue particulates for analysis raises so many technical problems that the re- 

 sults at present must be interpreted quite tentatively. 



Studies on the electrolyte metabolism of tissue slices obtained from a variety 

 of mammalian organs (especially kidney, liver and brain), have revealed several 

 phenomena which have directed attention to the mitochondria. First, the main- 

 tenance of a high level of intracellular potassium by the intact cell is dependent 

 upon aerobic metabolism and the generation of phosphate bond energy (6, 9). 

 During recent years the capacity to catalyze the reactions of aerobic phosphory- 

 lation has been localized for the most part in the mitochondrial fraction of tissue 

 homogenates (4). And, secondly, isotope techniques applied to a number of 

 tissues have shown that intracellular potassium is heterogenous in terms of 

 its exchangeability (10). In the kidney cortex a fraction of cell potassium has 

 been identified which is non-exchangeable under anaerobic conditions, but which 

 rapidly exchanges aerobically (8). The relation between metabolic activity 



1 The original investigations reported herein were supported by a grant from the Rocke- 

 feller Foundation. 



