Effect of functional overload on substrate oxidation capacity of skeletal muscle

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Abstract
The effects of surgically induced functional overload (O) on rodent fast-twitch plantaris (P) and slow-twitch soleus (S) skeletal muscle substrate oxidative capacity were determined. Compared with normal control muscles of wt-matched rats, bilateral overload produced 68 and 23% increases in the wet wt of OP and OS muscles, respectively (P < 0.05). Total protein concentrations of the O muscles remained unchanged relative to controls. The enzymatic capacity to oxidize pyruvate, palmitate and .alpha.-glycerophosphate was unchanged in OP muscles relative to controls. Certain ketone oxidative enzyme markers were increased in the whole and in the inner red and outer white regions of the OP muscle. Citrate synthase activity (a marker for tricarboxylic acid cycle oxidative capacity) was decreased in the whole and in the red region but unchanged in the white region of OP muscles. The above measurements were significantly decreased in the OS muscles compared with controls (P < 0.05). There apparently is both an expansion and a change in composition of the mitochondrial pool of the enlarged P muscle. The effects on OS muscle suggest the possibility that the oxidative capacity is not altered parallel with the enlargement, although fiber-typing (fast-twitch to slow-twitch) changes and altered mitochondrial composition could contribute in part to the change.

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