Oxidation of acetate in rabbit skeletal muscle: Detection by ¹³C NMR spectroscopy in vivo

The results of a proton-decoupled and Overhauser-enhanced ¹³C NMR study of acetate metabolism in skeletal muscle are reported. [2-¹³C]acetate was infused intravenously over 2 h into anesthetized rabbits, and skeletal muscle in the lateral thigh was monitored by ¹³C NMR spectroscopy at 4.7 T. Stable ¹³C enrichment in carbons 2, 3, and 4 of glutamate was observed at the end of the infusion, and the half-time for enrichment was 17 min for glutamate C4 and 50 min for glutamate C2 and C3. The contribution of exogenous acetate to acetylcoenzyme A was nearly equal in skeletal muscle and heart in vivo (83-87%, measured in tissue extracts), comparable with earlier perfused heart studies in which acetate was the sole available substrate. Although relative flux through the combined anaplerotic pathways (relative to citric acid cycle flux) was higher in quiescent skeletal muscle (26%) compared with hearts (3%) from the same animals, actual anaplerotic flux was estimated to be substantially higher in heart than in skeletal muscle after correcting for differences in citric acid cycle flux in the two tissues.