Substrate oxidation specificity in different types of mammalian muscle

The maximal capacity of homogenates prepared from cardiac, fast-oxidative-glycogenolytic (FOG) vastus lateralis, fast-glycogenolytic (FG) vastus lateralis, and slow-oxidative (SO) soleus muscle to oxidize pyruvate, palmitate, alpha-glycerophosphate, and acetoacetate was assessed by measuring oxygen consumption under conditions of nonlimiting substrate and cofactors. Pyruvate oxidation varied eight-fold among the muscle types and was highest in cardiac, followed by FOG, SO, and FG muscle. Palmitate was oxidized at 97%, 85%, 77%, and 57% of the relative rate for pyruvate in cardiac, FOG, SO, and FG muscle, respectively. In contrast, alpha-glycerophosphate oxidation rates were highest in FG muscle, followed by cardiac, FOG, and SO muscle. Although cardiac muscle possessed the highest absolute rate for acetoacetate oxidation, it had the lowest capacity relative to pyruvate (19%), whereas SO muscle possessed the highest (61%). FOG and FG muscle had similar relative capacities for this substrate (30%). These results provide further evidence to suggest that mammalian muscle types are differentiated in terms of both mitochondrial mass and substrate oxidation specificity.