The ability of the mitochondria from red and white skeletal muscle of rabbit to oxidize various substrates with coupled phosphorylation was determined to evaluate the relative capacities of the two muscle types to utilize carbohydrate and fat for aerobic energy production. The respiratory rates were highest with pyruvate and were similar with mitochondria of red and white muscle. Compared to pyruvate, respiration with (-)palmitoylcarnitine and (-)-acetylcarnitine was only slightly slower in red but nearly half as great in white muscle mitochondria. The ADP/O values were similar with pyruvate and (-)-acetylcarnitine but lower during (-)-palmitoylcarnitine oxidation. The activities of palmitate-activating enzyme and carnitine acyltransferases were twice as high in red as in white muscle mitochondria. Thus, white muscle, and to some extent red muscle, appears potentially capable of deriving more usable energy by oxidizing carbohydrate than fat, and this correlates with the known observations that in vigorously exercising muscle carbohydrate utilization predominates over that of fat.

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