1. Rat brain-cortex mitochondria were incubated in media containing 1, 5 or 100mm-K+ in the presence of ADP, uncoupler (FCCP, carbonyl cyanide p-trifluoro-methoxyphenylhydrazone) or valinomycin while metabolizing pyruvate and malate, or acetylcarnitine and malate or glutamate and malate as substrates. Both the uptake of oxygen and disappearance of substrate were measured under these conditions. 2. With pyruvate and malate as substrate in the presence of both ADP and valinomycin, both the uptake of oxygen and disappearance of pyruvate increased markedly on increasing the K+ content of the incubation medium from 5 to 100mm-K+. However, in the presence of uncoupler (FCCP), although the oxygen uptake doubled little change was observed in the rate of disappearance of pyruvate on increasing the K+ concentration. 3. Only small changes in uptake of substrate and oxygen were observed in the presence of ADP, uncoupler (FCCP) or valinomycin on increasing the K+ concentration when acetylcarnitine+malate or glutamate+malate were used as substrates by brain mitochondria. 4. Further, increasing the K+ concentration from 1 to 20mm when rat brain mitochondria were oxidizing a mixture of pyruvate and glutamate in the presence of malate and ADP caused a 30% increase in the respiration rate, 50% increase in the rate of disappearance of pyruvate and an 80% decrease in the rate of disappearance of glutamate. 5. Investigation of the redox state of the cytochromes and the nicotinamide nucleotides in various conditions with either pyruvate or acetylcarnitine as substrates suggested that the specific stimulation of metabolism of pyruvate by K+ could not be explained by a general stimulation of the electron-transport system. 6. Low-amplitude high-energy swelling of rat brain mitochondria was investigated in both Na+- and K+-containing media. Swelling of brain mitochondria was much greater in the Na+-containing medium and in this medium, the addition of Mg2+ caused a partial reversal of swelling together with an 85% decrease in the rate of utilization of pyruvate. However, in the K+-containing medium, the addition of Mg2+, although also causing a reversal of swelling, did not affect the rate of disappearance of pyruvate. 7. Measurements of the ATP, NADH/NAD+ and acetyl-CoA/CoA contents were made under various conditions and no evidence that K+ concentrations affected these parameters was obtained. 8. The results are discussed in relationship to the physiological significance of the stimulation of pyruvate metabolism by K+ in rat brain mitochondria. It is proposed that K+ causes its effects by a direct stimulation of the pyruvate dehydrogenase complex.

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