1. 1. Conditions for controlled K+ depletion of rat-liver mitochondria are described. These depleted mitochondria will reaccumulate K+ when supplied with an energy source and external K+. 2. 2. Determination of mitochondrial water and K+ showed that K+ was transported against a concentration gradient. 3. 3. It was demonstrated that K+ uptake was accompanied by H+ efflux. There appeared to be a 1:1 exchange of K+ for H+. 4. 4. No evidence for K+-Na+ exchange was obtained. No inward shift of anions was necessary for K+ uptake. 5. 5. ATP and Mg2+, but not ATP alone, energised K+ transport. 6. 6. Glutamate together with Pi also energised K+ transport. Other substrates tested were not effective with Pi alone. In the presence of a system capable of supporting oxidative phosphorylation, all the substrates were effective.
Bibliographical noteFunding Information:
We are indebted to Miss S. ELSEY and Mr. W. GARRETT for skilled technical assistance. This work was supported by grants from the U.S. Public Health Service (AM 7226-o2), the Life Insurance Medical Research Fund, the Otho S. A. Sprague Memorial Institute and the Burroughs-Wellcome Fund. G. S. C. was in receipt of a travel grant from the Anti Cancer Council, Victoria, Australia. A. E. M. McL. was in receipt of a U.S. Public Health Service International Fellowship (FF 492).